Wastewater & nanotechnology

Marwa Mohamed Faisle

M.Sc. Chemistry

          Wastewater discharge from agricultural, domestic or industrial source with a negative effect on the water, health, and the environment. The disposal of untreated wastewater into water stream cause much pollution. Due to the fast growing of the global population and the elaboration of living standard there are demands continuously driving. However, in present wastewater treatment technologies are reaching their end for achieving appropriate water quality to attain human and environmental requirements. Nanotechnology based treatment has offered very efficient and ecofriendly access. The benefits of nanotechnology in wastewater treatment technology focused in specific areas: sensing and detection, pollution control and treatment, this achieving through nano-filtration technique, adsorption of pollutants on nano-adsorbents and the contaminants breakdown by nano-catalyst.

                 Rapid increase an interest in the use of nano-materials, well developed internal pore structures of nano-particles and their tendency to functionalize with various chemical surface groups increase their affinity towards target contaminants. Recent studies suggest that many of the recent wastewater problems could be solved using nanomaterials.

               Nano particles have great interacting, absorbing and reacting abilities due to their very small size and elevated proportion of atoms at their surface, they can achieve energy conservation due to their small size which may lead in the end to cost saving. The unique properties of Carbon Nano Tubes CNTs have attracted the attention of many researchers, their high strength, and resistance to acidic and basic media, high surface area good thermal, electrical, and conductive properties brought up the possibility of a novel structure with extra -ordinary properties. The CNTs consist of very thin honeycomb structures of graphene sheets rolled up in cylindrical shape with a few nanometer diameter and many micron or even centimeters length as shown in figure, including single walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) distinguished by the layers numbers. Due to the hexagonal network of carbon atoms and strong van der Waals interaction forces between the nanotubes, they form tight bundles as show in figure (1).

Figure (1): Schematic diagram of graphene sheet layers of CNTs.

The CNTs have presented excellent adsorption efficiencies for various organic pollutants, heavy metals, i.e. lead, cadmium, Application of CNTs in wastewater treatment is not restricted to adsorption and filtration; CNTs have strong antimicrobial properties that control microbial pathogens. They are not strong oxidants and relatively inert in water resulting in avoiding the formation of carcogenic disinfection byproducts (DBPs)

The effect of metal and metal oxide nano particles, such as nano silver, nano copper  ,nano gold, titanium dioxide , aluminum oxide, silicon dioxide and zinc oxide on wastewater treatment plants (WWTPs) have been widely investigated. However, few studies have investigated the effect of CNTs on WWTPs.

[References]

  1. M. Faisle, (2017):” Preparation of nanomaterials and its application for wastewater chemical and biological treatment. “M. Sc. Thesis, Chemistry Department, Science faculty, Fayoum University, Egypt.

الإستفادة من الروبة الناتجة عن عمليات تنقية المياه في معالجة مياه الصرف الصناعي

بقلم / أحمد محمد هشام

ماجستير كيمياء تحليلية

كبير مراجعين لنظم إدارة الجودة والبيئة والسلامة والصحة المهنية

Ahmedhasham83@gmail.com

مقدمة:

في عصر التكنولوجيا والأقمار الصناعية لا تزال بعض المشكلات تشكل تحدياً كبيراً علي الصعيدين البيئي والإقتصادي , من هذه المشكلات مشكلة التخلص من مخلفات عمليات معالجة المياه. ويعد الترويب والتنديف أحد أكثر الطرق شيوعًا في عمليات معالجة المياه بسبب بساطتها في التطبيق ، وفعاليتها وغير مكلفة من الناحية الإقتصادية ولكن العيب الرئيسي في تطبيق تقنيات الترويب والتنديف هو التخلص من المواد الصلبة الناتجة عن هذه العملية والتي تعرف ب”الروبة”. ويعتبر التخلص من الروبة   الناتجة عن عمليات معالجة المياه المعضلة الكبيرة لما تحمله من معادن ثقيلة وممرضات وبقايا مواد الترويب1.

الجدير بالذكر أن هذه الروبة تحتوي علي 40-50% من الشبة الغير مستخدمة من الجرعات الأولية المستخدمة في عملية المعالجة مما يجعل التفكير في إعادة إستخدامها أمر حتمي 2.

علي صعيد أخر تشكل مياه الصرف الصناعي بما تحمله من أصباغ معضلة أخري حيث أنها تحتاج إلي عمليات معقدة ومكلفة لمعالجتها. الأصباغ هي مواد كيميائية ملونة ، تتكون غالبا من مركبات عضوية اروماتية (تحتوي في تركيبها علي حلقة بنزين) ملونة (كما يتضح في الرسم التوضيحي رقم 1 – مثال للتركيب الكيميائي لأحد الصبغات) . يتم استخدام الأصباغ الصناعية بشكل متزايد في صناعات النسيج والصباغة بسبب سهولة تطبيقها وفعاليتها من حيث التكلفة، وثبات عالٍ ضد تأثير الضوء ودرجة الحرارة والمنظفات. يتم تصنيع أكثر من 10000 صبغة مختلفة كيميائيا. يقدر الإنتاج العالمي للأصباغ  بحوالي  70 الف طن سنويًا3.

معظم الأصباغ مركبات ضارة للإنسان والحيوان بسبب سميتها وتأثيرها السلبي علي الخلايا  حيث يعتقد بأن بعضها قد يصل تاثيره الي ان يحدث طفرات جينية وغيرها من الخصائص التي تؤثر سلبًا على صحة الإنسان . تصنف لجنة التجارة الدولية الأمريكية الأصباغ إلى 12 نوعا. من بين هذه الأصباغ التفاعلية ، تستخدم الأصباغ الحمضية والأصباغ المباشرة بشكل شائع ، والتي تتواجد بشكل واضح في معظم مياه الصرف الصناعي الناشئة عن صناعات النسيج4.

العرض:

تتوفر العديد من الطرق الفيزيائية والكيميائية والبيولوجية لمعالجة المخلفات السائلة حيث قام غويندي وفريق عمله بإجراء دراسات لإزالة الأصباغ الحمضية الحمراء باستخدام مزيج من عمليات الترويب والامتزاز حيث  كشفت نتائجهم أن تطبيق عملية الترويب قبل الامتزاز كان فعالاً بشكل واضح لإزالة اللون. إذ استخدمت الشبة (كبريتات الألمونيوم )وكلوريد الحديديك كمخثرات للصبغة لترسيبها ، وتم استخدام الكربون المنشط الحبيبي (GAC) كعامل امتزاز لإزالة ما تبقي من لون 5.

ومن الدراسات الجيدة التي قام بها دانشفار وفريق عمله إجراء تجارب باستخدام الحفز الضوئيOptical catalysis  باستخدام أكسيد الزنك لتحلل صبغة الحمض الأحمر 14 (AR 14) في وجود الضوء الفوق بنفسجي UV. ولقد أشارت نتائج بحثهم  إلى أنه يمكن استخدام عملية UV / ZnO بكفاءة لتكسير صبغة (AR14). كما قاموا بدراسات حول التحلل الحيوي الهوائي لصبغة azo Acid Red 151 (AR 151) بواسطة استخدام مرشح بيولوجي ذو تدفق دفعي متسلسل. وتشير النتائج إلى إزالة لون تصل إلى 99 ٪ من تركيز أولي 50 ملغم / لتر من 6AR 151.

وأيضا ما قام به بهادير وفريق عمله لدراسة إمكانية إزالة صبغة النسيج وإزالة أيونات المعادن باستخدام مخاليط ثنائية من Acid Blue 29 و Reactive Red 2 و Acid Red 97 وتطبيق أنودات حديدية ومحلول الكتروليت من كبريتات الصوديوم في مفاعل كهروكيميائي. اعتمادًا على ظروف التفاعل الكهروكيميائي ، لوحظ أن النسب المئوية لإزالة الصبغة وإزالة أيون المعادن تتراوح ما بين 70.6 –   %96.7  لإزالة الأصباغ النسيجية و 64.9 – 100 ٪ لإزالة أيونات الحديد7.

بينما قام فورلان وفريق عمله بدراسة استخدام المخلفات الزراعية في إزالة الأصباغ التفاعلية مثل (Reactive Black 5 RB 5) و (Reactive Orange 16 RO 16) من خلال معالجة تجمع بين تقنيات الترويب والامتزاز. حيث تم استخدام الكربون المنشط المستخلص من قشر جوز الهند كعامل إمتزاز وكلوريد الألومنيوم كمادة ترويب. تم العثور على كفاءة إزالة ما يقرب من 90 ٪ ل RB 5 و و84 % 8 RO 16.

من الدراسات المذكورة أعلاه ، يمكن أن نستنتج أن تطبيق عمليات مثل الامتزاز ، والحفز الضوئي ، والترشيح الحيوي أثبتت نجاحها في علاج المخلفات السائلة الملونة ، وتبين أن الإزالة تصل إلى 100 ٪. ولكن قد لا يوفر تطبيق هذه الطرق حلاً مجديًا اقتصاديًا للصناعات الصغيرة والمتوسطة بسبب قيود مثل ارتفاع التكاليف والمشاكل في الحفاظ على المعايير التشغيلية للمعالجة. لذلك أجريت بعض الدراسات باستخدام تقنية الترويب مع روبة محطات معالجة المياه كمروب.  حيث تنتج محطات معالجة المياه كميات كبيرة من الروبة أثناء عمليات الترويب مما يشكل تحديًا في عملية التخلص من هذه الروبة. علي صعيد أخر يمكن استغلال هذه الروبة لتحقيق توفير كبير محتمل في جرعة المواد الكيماوية المروبة من خلال استرداد عوامل الترويب من الروبة  أو إعادة استخدام الروبة  نفسها في المعالجة2.

 

تركز الأبحاث الحالية على إعادة استخدام روبة محطات تنقية مياه الشرب لمعالجة مياه الصرف الصناعي المحمل بالصبغات. نظراً لأن مياه الصرف الصناعي من المصابغ تلوث المسطحات المائية القريبة.  وذلك هناك حاجة ماسة لتطوير طرق منخفضة التكلفة لمعالجة مياه الصرف الصناعي خاصة الناتج عن أنشطة المصابغ  حيث أن الطرق المتبعة عادةً مكلفة والتي لا يمكن أن توفر خيار معالجة اقتصادية لمثل هذه الصناعات الصغيرة والمتوسطة2.

لذلك يمكن أن يكون استخدام مواد منخفضة التكلفة مثل روبة محطات معالجة المياه للمعالجة المسبقة لمياه الصرف الصناعي خيارًا ممكنًا لتحقيق النتائج المرجوة للناتج النهائي من المياه المعالجة. ولقد استخدمت بعض الدراسات  هذه الروبة  لمعالجة صرف محمل بأصباغ  Acid Red 94, Acid Yellow 1, Direct Green 26, Reactive Blue 21 لأن هذه كانت تستخدم بشكل متكرر في وحدات الصباغة. تحققت أقصى إزالة لـ   Acid Red 94  (41.5 %) ، Acid Yellow 1 (27 %) ، Direct Green 26  (43.5 %) و Reactive Blue 21(26.2%)2.

رسم توضيحي 2شكل يوضح فعالية الروبة المعاد استخدامها مقارنة كمروبات أخري في إزالة ملوثات مياه الصرف

تم التوصل لطرق لاستعادة مواد الترويب من الروبة الناتجة عن عمليات معالجة المياه باستخدام طرق مثل المعاملة بالأحماض والقلويات وتبادل الأيونات ( ion exchange ) والفصل بالأغشية( membranes )مما يقلل من تكاليف تشغيل محطة معالجة المياه. أجريت دراسات لاستعادة مواد الترويب باستخدام حامض الكبريتيك وحمض الهيدروكلوريك. وتشير النتائج إلى أن الحفاظ على الرقم الهيدروجيني منخفض يمكننا من استعادة بين 70 و 90 ٪ من مواد الترويب. في حالة الاستعادة باستخدام عملية المعاملة بالقلويات ، أجريت الدراسات باستخدام هيدروكسيد الصوديوم وهيدروكسيد الكالسيوم وأظهرت النتائج أنه يمكن إستعادة ما يصل إلى 90 ٪ عند الرقم الهيدروجيني 12 باستخدام هيدروكسيد الصوديوم. ويمكن تعزيز فعالية هذه العمليات من خلال إستخدام الأغشية ، ولكن إنسداد الأغشية بسبب الجسيمات يفرض قيودا على إستخدام هذه التقنية. أكدت الدراسات أيضًا وجود مواد قابلة للذوبان إلى جانب مادة الترويب المستعادة التي تؤثر على نوعية المياه إذا تم استخدامها لتنقية مياه الشرب. وبالتالي  تم اقتراح تطبيق المواد المروبة المسترجعة في معالجة مياه الصرف الصحي والصناعي11-9.

ومن الجدير بالذكر في هذا الصدد الدراسة القيمة التي قام شوا بدراسة إزالة الصبغة على نطاق واسع من مياه الصرف في صناعة النسيج باستخدام  روبة  الشبة المعاد تدويرها. وتبين أن  روبة  الشبة المعاد تدويرها تعتبر مادة جيدة لإزالة الأصباغ من مياه الصرف، وكذلك تقلل جرعة الشبة الحديثة بمقدار الثلث12.

ومن الأمثلة الرائعة علي ذلك هي الدراسة التي قام بها إيشيكاوا وفريق عمله  حيث استخدموا  الشبة المسترجعة باستخدام حمض الكبريتيك لمعالجة مياه الصرف الصحي. وأظهرت النتائج أن إزالة الطلب على الأكسجين الكيميائي COD والنيتروجين الكلي والفوسفور الكلي مع المروبات المسترجعة كانت ذات قسمة اقتصادية وفعالة من الناحية العملية مقارنة بكبريتات الألومنيوم التجارية أو بولي كلوريد الالومينيوم10 كما يتضح من الرسم التوضيحي رقم 2. ولمعرفة نسب الإزالة التي تحققت بكل متغير في هذه هذه الدراسة يمكن الرجوع لجدول110.

 

جدول 1 نسب الإزالة التي تحققت لملوثات مياه الصرف  بإستخدام الروبة المعاد إستخدامها مقارنة بمروبات أخري

الخلاصة :

يمكن اعتبار الروبة الناتجة عن عمليات معالجة المياه كمادة فعالة وغير مكلفة لإزالة اللون من الصرف الصناعي بالصناعات النسيجية مما يقلل من تكلفة المعالجة، يمكن استخدام هذه الروبة   بالاقتران مع طريقة معالجة أخرى مناسبة لتقليل جرعة المروبات الكيميائية.

 

المراجع:

  1. Gastaldini, A.L.G., Hengen, M.F., Gastaldini, M.C.C., Amaral, F.D., Antolini, M.B., Coletto, T.: The use of water treatment plant sludge ash as a mineral addition. Constr. Build. Mater. 94, 513–520 (2015).
  2. Shankar, Y. S., Ankur, K., Bhushan, P., & Mohan, D. (2019). Utilization of Water Treatment Plant (WTP) Sludge for Pretreatment of Dye Wastewater Using Coagulation/Flocculation. In Advances in Waste Management (pp. 107-121). Springer, Singapore.‏
  3. Fatih, D., Sengul, K.: Removal of basic red 46 dye from aqueous solution by pine tree leaves. Chem. Eng. J. 170,67–74 (2011).
  4. Uygur, A.: An Overview of Oxidative and Photooxidative Decolorisation Treatments of Textile Waste Waters. J. Soc. Dyers Col. 113, 211–217 (1997).
  5. Guendy, H.R.: Treatment and reuse of wastewater in the textile industry by means of coagulation and adsorption techniques. J. Appl. Sci. Res. 6, 964–972 (2010).
  6. Daneshvar, N., Salari, D., Khataee, A.R.: Photocatalytic degradation of azo dye acid red 14 in water on Zno as an alternative catalyst to Tio. J. Photochem. Photobiol. A: Chem. 162, 317–322 (2004).
  7. Bahadir, K.K., Kahraman, A., Cihan, G., Ayla, O.: Electrochemical decolourization of textile dyes and removal of metal ions from textile dye and metal ion binary mixtures. Chem. Eng. 173, 677–688 (2011).
  8. Furlan, F.R., Silva, L.G.D.M.D., Morgado, A.F., de Souza, A.A.U., de Souza, S.M.A.G.U.:Removal of reactive dyes from aqueous solutions using combined coagulation/flocculation and adsorption on activated carbon. Resour. Conserv. Recycl. 54, 283–290 (2010).
  9. Evuti, A.M., Lawal, M.: Recovery of coagulants from water works sludge: a review. Adv. Appl. Sci. Res. 2(6), 410–417 (2011).
  10. Ishikawa, S., Ueda, N., Okumura, Y., Iida, Y., Baba, K.: Recovery of coagulant from water supply plant sludge and its effect on clarification. J. Mater. Cycles Waste Manag. 9(2), 167–172 (2007).
  11. Joshi, S., Shrivastava, K.: Recovery of alum coagulant from water treatment plant sludge: a greener approach for water purification. Int. J. Adv. Comput. Res. 1(2), 101–103 (2011).

Oxidation ponds for municipal wastewater treatment

By:Dr. Mohamed Elsofy Zain Elabedien Ezz Eldien

Microbiology Lab Manger – Reference Lab of Waste Water (RLWW) Holding Company for Water and Wastewater (HCWW)

Introduction:

Oxidation ponds are known as stabilization ponds which provide greater advantages in treatment over mechanically based units. Ponds can be described as self-sufficient treatment units, because the efficacy of treatment is contingent upon the maintenance of the overall microbial communities of bacteria, viruses, fungi, protozoa, and the proper balance of organics such as (light, dissolved oxygen, nutrients, algal presence and temperature) (Amengual-Morro et al., 2012). Ponds are self-sufficient; there is a reduction of operator responsibilities to manage treatment, reduction in labor costs, and increase in the potential fiscal returns from the tangible products generated by the treatment unit (Hosetti and Frost 1998).

Subject:

Ponds can be used for the purpose of ‘polishing,’ or providing additional treatment to what has been found within conventional treatment methods (Veeresh et al.,2010). Ponds simplify the treatment process by reducing the need for multiple treatment units. So oxidation ponds are a treatment processes that can be used in regions where treating of wastewater using conventional treatment methods are expensive. Indeed, oxidation ponds are commonly used in many regions around the world, specifically in places with year-round mild to warm climates.

Types of oxidation ponds:

There are four major types of oxidation ponds: aerobic (high-rate), anaerobic, facultative, and maturation ponds.

1- Aerobic (high-rate) ponds:

Aerobic ponds are known as high-rate algal ponds that maintain dissolved oxygen throughout a depth of 30–45 cm because of algal photosynthetic activity. Photosynthetic activity supplies oxygen during the day, while at night the wind creates aeration due to the shallow depth of the pond (Davis and Cornwell, 2008).

Aerobic ponds are well known for having high biochemical oxygen demand (BOD) removal potential and are ideal for areas where the cost of land is not expensive. Other characteristics of these ponds include a detention time of 2–6 days and a BOD removal efficiency reach to 95 %.

2- Anaerobic ponds:

Anaerobic ponds operate without the presence of dissolved oxygen, the major products are carbon dioxide and methane (Quiroga, 2011), Typically, these ponds are designed to have a depth of 2–5 m, with a detention time between 1 and 1.5 days, an optimum pH less than 6.2, temperatures greater than 15ºC (Kayombo et al., 2010).

Anaerobic ponds can remove 60 % BOD. However, this efficiency is climate dependent, the driving force behind treatment is sedimentation where Helminthes settle to the bottom of the pond, and bacteria and viruses are removed by attaching to settling solids within the pond or die with the loss of available food or by the presence of predators. In practice, anaerobic ponds are usually incorporated alongside facultative ponds (Martinez et al., 2014).

3- Facultative ponds:

A facultative pond is a treatment unit with anaerobic and aerobic conditions. A typical pond is divided into an aerobic surface region consisting of bacteria and algae. Anaerobic bottom region, consisting of anaerobic bacteria, and a region in between anaerobic and aerobic conditions, where bacteria can thrive in both conditions, if used in series, effluent from a previously treated source enters the pond. Facultative ponds treat BOD, typically within a range of 100–400 kg BOD/ha/day, by removing BOD by 95 %. Because facultative ponds employ algae as decomposers, the treatment time can range between 2 and 3 weeks, which are attributed to the photosynthetic processes that occur within the unit. A facultative pond on average has a depth of 1–2 m.

4-Maturation ponds:

Similar to facultative ponds, maturation ponds use algae as a primary driving force in the treatment. Nevertheless, while facultative ponds typically treat BOD, maturation ponds remove fecal coliform, pathogens, and nutrients (Cinara, 2004). In comparison with the other pond types, the characteristics of the maturation pond include a depth range between 1 and 1.15 m, which makes it shallower than all of the ponds besides the aerobic.

Arrangement of ponds:

There are two arrangements for a multiple pond system; series and/ or parallel. In the series arrangement, wastewater is treated in the initial and subsequent ponds and then polished in the final pond; while wastewater flow is evenly divided in the parallel pond arrangement each multiple pond arrangement has its benefits and therefore an operator can change the pond arrangement depending on the situation. For example, ponds operating in parallel prevent interruption of treatment during the cooler months of the year. This is when a pond can experience low biological activity. Low biological activity can create anaerobic conditions within a pond. In addition, the application of ponds in parallel can reduce problems related to periodic low dissolved oxygen concentrations, particularly in the morning hours, On the other hand, ponds in series are ideal during the summer months and also during periods of low biological loading, Nevertheless, the choice of applying multiple ponds can be beneficial for treatment as compared to a single pond arrangement.

Design factors:

Understanding design factors is important in controlling pollutants such as BOD5. There are many factors that affect the efficiency of BOD5 removal in waste stabilization ponds. These factors include raw wastewater strength, food-to-microorganism ratio (F/M), organic loading rates, pH, and hydraulic detention time (HRT).

We will explain in detail these factors in the next issue.

 

References:

Amengual-Morro C, Moya Niell G, Martinez-Taberner A (2012) Phytoplankton as bioindicator for waste stabilization ponds. J Environ Manag 95:S71–S76.

Hosetti B, Frost S (1998) A review of the control of biological waste treatment in stabilization ponds. Crit Rev Environ Sci Techno l28:193–218.

Veeresh M, Veeresh AV, Huddar BD, Hosetti BB (2010) Dynamics of industrial waste stabilization pond treatment process. Environ Monit Assess 169:55–65.

Quiroga FJ (2011) Waste stabilization ponds for waste water treatment, anaerobic pond.

Kayombo S, Mbwette TSA, Katima JHY, Ladegaard N, Jorgensen SE (2010) Waste stabilization ponds and constructed wetland design manual. UNEP International Environmental Technology Center..

Martinez FC, Cansino AT, Garcia MAA, Kalashnikov V, Rojas RL (2014) Mathematical analysis for the optimization of a design in a facultative pond: indicator organism and organic matter. Math Probl Eng 1–12.

Cinara Columbia (2004). Waste stabilization ponds for wastewater treatment: FAQ sheet on waste stabilization ponds.

المعالجة البيولوجية اللاهوائية في المفاعل ذي الجريان الصاعد عبر طبقة الحمأة المعلقة UASB (Upflow Anaerobic Sludge –Blanket Process)

الكاتب : الدكتور المهندس عبد الله صغير

مقدمة :

لقد أصبحت معالجة مياه الصرف الصحي والصناعي من أولويات الدول والحكومات وذلك لحماية البيئة من التلوث المتوقع حدوثه نتيجة صرف هذه المياه، بما قد تحتويه من سموم وجراثيم ومواد غير متحللة ذات تأثير تراكمي، وما قد يتخلف عنها من مخاطر صحية وبيئية، وتلويث مصادر المياه السطحية والجوفية والأوساط الأحيائية فيها.

وإن معالجة مياه الصرف الصحي والصناعي يؤدي توفير استخدام المياه النقية للاستهلاك العام، وحفظ موارد المياه النقية واستخدام المياه المعالجة في ري الأراضي الزراعية.

لقد تطورت في العقود الثلاث الأخيرة وبشكل ملحوظ تقنيات معالجة مياه الصرف الناتجة عن النشاطات الصناعية وبالأخص تقنيات معالجة مياه الصرف ذات الأحمال العضوية العالية والتي تنتج عادة عن الصناعات الغذائية كصناعة الخميرة وصناعة السكر و وصناعة النشاء وصناعة الألبان ….الخ وتتميز مياه الصرف عالية الحمل العضوي بقيم مرتفعة جداً لـ COD والتي قد تصل قيمتها إلى 25000 ملغ/ل وبارتفاع قيمة الـ BOD5 والتي قد تصل قيمتها إلى 10000 ملغ/ل.

تعتبر طرق المعالجة اللاهوائية وخصوصا طريقة المفاعل UASB من الطرق الحديثة في معالجة مياه الصرف الصحي والصناعي لما يمتاز به هذا المفاعل من سهولة في التصميم وقلة تكاليف الاستثمار والتشغيل وكذلك ينتج غاز المتان الذي يمكن أن يستخدم توليد الكهرباء والطاقة.

1- عملية المعالجة في المفاعل UASB:

في مفاعل UASB يدخل الماء المطلوب معالجته من قاع المفاعل ويجري باتجاه الأعلى عبر طبقة الحمأة المؤلفة من حبيبات أو جزيئات متشكلة بيولوجياً,حيث يمكن أن يوصف المفاعل UASBكنظام تمر فيه مياه الصرف أولاً عبر سرير حمأة متمدد يحتوي على تركيز كبير من الكتلة الحيوية,ويمكن أن توجد هذه الحمأة في المفاعل بشكل حبيبات وإن القسم الأعظم من المعالجة يحدث في سرير الحمأة هذا, وإن القسم المتبقي من الملوثات في الماء يمر بعد ذلك عبر ما يدعى بطبقة الحمأة المعلقة والتي هي أقل كثافة من سرير الحمأة كما في الشكل (1)

الشكل (1) :مقطع شاقولي لمفاعل UASB

2-تصميم نظام دخول المياه الخام إلى المفاعل UASB:

من الضروري في المفاعل UASB الحصول على تماس أمثل بين الحمأة الموجودة ضمن المفاعل والمياه الخام الداخلة إلى المفاعل كما هو مبين في الشكل (2) وكذلك أيضاً من الضروري تجنب تشكل أقنية تمر فيها المياه بدون معالجة عبر سرير الحمأة لذلك يجب تصميم نظام دخول وتوزيع المياه الخام ضمن المفاعل بشكل جيد.

وإن تصميم نظام دخول وتوزيع المياه الخام ضمن المفاعل يتعلق بالعوامل الطبوغرافية وبتصميم محطة الضخ واحتمال انسداد أنابيب دخول و توزيع المياه الخام إلى داخل المفاعل .

وبغض النظر عن عدد فتحات التغذية والتوزيع فإن السرعة الأصغرية و الأعظمية للتدفق الخارج من فوهات التوزيع يجب أن يؤخذ بعين الاعتبار في التصميم حيث أن السرعة الأعظمية لخروج المياه الخام من فوهات التوزيع يجب أن لا تزيد عن 4 م/ ثانية والسرعة الأصغرية يجب أن لا تقل عن 0.5 م/ ثانية .

3-تصميم نظام جمع المياه المعالجة في المفاعل UASB:

إن تدفق المياه المعالجة يجب أن يخرج من المفاعل عبر عدة أقنية موزعة في منطقة تفريغ المياه المعالجة,وإن تصميم أقنية جمع المياه المعالجة لا يختلف عن تصميم الهدارات بحيث يصمم بناء على معدل التحميل على هدارات المخرج ويجب أن لا يتجاوز القيمة 185م3/ م.يوم,إن عرض أقنية الجمع يجب أن لا يقل عن 20 سم وذلك من أجل تسهيل عمليات الصيانة.

4- إقلاع المفاعل UASB :

إن من إحدى مساوئ المعالجة البيولوجية اللاهوائية هي زمن الإقلاع الكبير بالمقارنة مع المفاعلات الهوائية,وذلك بسبب صغر معدل الاصطناع الحيوي (إنتاج الحماة), وبالتالي تحتاج المفاعلات اللاهوائية إلى زمن كبير من أجل تحقيق الحالة المستقرة وقد يستغرق زمن إقلاع المفاعلات اللاهوائية حتى 3 أشهر,وهذا يتعلق بشكل أساس بدرجة الحرارة والحمل الهيدروليكي فمثلاً عندما تكون درجة الحرارة أكبر من 20 درجة مئوية فمن المتوقع أن يتم إقلاع المفاعل خلال فترة لا تزيد عن 3- 4 أسابيع أما في حال انخفاض درجة الحرارة فقد يستغرق إقلاع المفاعل 3- 4 أشهر.

5- العوامل التصميمية والتشغيلية المؤثرة على كفاءة المعالجة في المفاعل :UASB

5-1- تأثير التحميل العضوي في واحدة الحجوم ((Organic Load Rate على كفاءة المعالجة في المفاعل UASB :                               

إن كلاً من التحميل الهيدروليكي والتحميل العضوي الملائم للمفاعل UASB يتعلق بخصائص مياه الصرف ونوعية وكمية الأحياء الدقيقة كما هو مبين في الجدول (1) و يوجد ارتباط كبير بين زمن المكوث الهيدروليكي ومعدل الحمولة العضوية في واحدة الحجوم.

وتجدر الإشارة إلى أن هناك علاقة مباشرة بين ثلاث متحولات وهي : درجة الحرارة ضمن المفاعل وزمن المكوث الهيدروليكي فيه ومعدل التحميل العضوي (Organic Load Rate ) والذي يرمز لهOLR فلكل زمن مكوث هيدروليكي في درجة حرارة ثابتة هناك معدل مثالي للتحميل العضوي.

5-2- تأثير زمن المكوث الهيدروليكي (Hydraulic Retention Time) والذي يرمز له بـ HRT على كفاءة المعالجة في المفاعل :UASB

يعتبر زمن المكوث الهيدروليكي من أهم العوامل التصميمية التي تحكم كفاءة المعالجة

فزمن المكوث الهيدروليكي يرتبط بالتدفق الهيدروليكي بالعلاقة:

زمن المكوث الهيدروليكي= التدفق/حجم المفاعل

التدفق = مساحة مقطع المفاعل x السرعة الشاقولية

وبالتالي فإنه من الضروري عند تصميم المفاعل UASB اختيار زمن المكوث الهيدروليكي المناسب الذي يحقق سرعة شاقولية مناسبة من أجل تأمين زمن ملائم للتماس بين المياه الخام وكريات الحمأة الموجودة ضمن المفاعل.

 

الجدول (1) : أسس تصميم المفاعل UASB

تصنيف مياه الصرف تركيز COD للمياه الخام

ملغ/ل

معدل التحميل العضوي

Kg COD / m3.day

زمن المكوث الهيدروليكي

(ساعة)

السرعة الشاقولية للجريان

م/ساعة

الكفاءة المتوقعة

%

منخفضة التلوث أقل أو يساوي 750 1-3 6-18 0.25-0.7 70-75
متوسطة التلوث 750-3000 2-5 6-24 0.25-0.7 80-90
شديدة التلوث 3000-10000 5-10 6-24 0.15-0.7 75-85
فائقة التلوث أكبر من 10000 5-15 أكبر من 24 —- 75-80

 

 

5-3- تأثير درجة حرارة المياه ضمن المفاعل على كفاءة المعالجة في المفاعل :UASB

 إن لدرجة الحرارة دوراً هاماً جداً في عملية المعالجة إذ أن تعداد ونوع البكتريا التي تنمو في المفاعل ومدى نشاطها يرتبطان بإذنه تعالى بشكل وثيق بدرجة الحرارة,وتقسم البكتريا حسب درجة الحرارة المثالية لنموها إلى محبات البرد Pcychrophilic ومحبات الدفء Mesophilic ومحبات الحرارة العالية Thermoohilic,وبشكل عام فإن العمليات الحيوية تتضاعف لكل ارتفاع 10 درجات مئوية في المجال (5-35) درجة مئوية.

5-4- تأثير قيمة الـ pH ضمن المفاعل على تشغيل و كفاءة المعالجة في المفاعل : :UASB

إن وجود نوعين من البكتريا ضمن المفاعل البكتريا المنتجة للحموض والبكتريا المنتجة للميتان يتطلب وجود قيمتين لـ pH ضمن المفاعل حتى يعمل كلا النوعين بشكل فعال فقيمة الـ pH المثالية لعمل البكتيريا المنتجة للحمض هي 5.5-6.5 وقيمة الـ pH المثالية لعمل البكتيريا المنتجة للميتان هي 7.8-8.2.

6- التوصيات:

  1. يعتبر مفاعل UASB من أهم المفاعلات الخاصة بمعالجة مياه الصرف الصحي والصناعي عالية الحمل العضوي .
  2. يمكن استخدام مفاعل UASB في لمعالجة مياه الصرف الصحي والصناعي عالية الحمل العضوي في أغلب بلدان الوطن العربي وخصوصاً دول الخليج العربي وذلك لأن درجة الحرارة في هذه الدول مناسبة لتشغيل هذا المفاعل.
  3. استخدام المعالجة البيولوجية اللاهوائية في معالجة مياه الصرف الصحي والصناعي عالية الحمل العضوي والاستفادة من الغاز الحيوي الناتج في توليد الكهرباء والطاقة الحرارية.

 أهم المراجع المستعملة:

  1. صغير عبد الله , معالجة مياه الصرف الصناعي في الوطن العربي ,2017 , الدار العربية ناشرون- بيروت- الطبعة الأولى.

2- Amin, M. M and Movahedian. H, 2005-“Performance evaluation of UASB system treating slaughterhouse wastewater“, Sharif University of Technology And Esfahan University of Medical Sciences.

3- Ghangrekar Makarand M .and Tom Keenan .2005- “Design of an UASB Reactor,Indian Institute of Technology Kharagpur,.at http:// www.waterandwastewater.com/AskTom! Column.htm.

3- Khanal, S. K. and Huang, J.-C. 2001- anaerobic treatment of industrial wastewater, Part-1 at www.public.iastate.edu

5-Lettinga, G and Tom Keenan. 2002- anaerobic treatment\Anaerobic Biodegradability وNational Environmental Services Agency (NESA)at http://www.uasb.org.

6-Lettinga, G and Tom Keenan of. 2002- “anaerobic treatment \Anaerobic Toxicity” ,National Environmental Services Agency (NESA) ,at http://www.uasb.org.

7- Lettinga, G., A. F. M. van Velsen, S. W. Hobma, W. De Zeeuw , A. Klapwijk 1980- “Use of upflow sludge blanket reactor concept for biological waste water treatment, especially for anaerobic treatment. Biotechnol. Bioengineer..

8-Warren Viessman ,Jr marks J Hammer 1993 “water supply and pollution control “ Fifth Edition Harper Collins collage publishers .

9- Nguyen Tuan Anh and Tom Keenan, 2004 “Methods for UASB Reactor Design,National Environmental Services Agency (NESA). at http:// www.uasb.org/discover/agsb.htm ,.

 

 

عمليات تداول ومعالجة حمأة الصرف الصحي

1.مقدمة

عمليات المعالجة داخل محطات المعالجة لمياه الصرف الصحي تهدف الي فصل السوائل (المياه)  عن المواد الصلبة العالقة، وبعد المعالجة يتم التخلص من السيب النهائى بأحد الطرق المناسبة لظروف البيئة المحيطة بمواقع محطات معالجة الصرف الصحى. أما معالجة الحمأة (الرواسب الصلبة) العالقة مع المحتوى المائى والتى تتجمع بعد رسوبها فى أحواض ترسيب منفصلة، أو مع المواد الطافية (الخبث) فيتم التخلص منها مباشرة (بدون معالجة) أو بعد معالجتها. وبالتالى فإن الحمأة السائلة عبارة عن المواد العضوية العالقة التى ترسبت بأحواض الترسيب المختلفة ممزوجة بكمية كبيرة من المياه تختلف نسبتها باختلاف نوعية الصرف الصحى الخام وخصائصه وكذلك نظم معالجته

  1. الغرض من معالجة الحمأة

الرواسب الصلبة الناتجة عن منظومة المعالجة يجب معالجتها والتخلص منها بعد المعالجة بطريقة امنة بيئيا وصحيا , وبصفة عامة يتم معالجة الحمأة للاغراض الأتية:

  • تثبيت المواد العضوية Stabilize Organics
  • إزالة الروائح Eliminate Odors
  • تدمير الممرضات Destroy Pathogens
  • خفض حجم المواد الصلبة Reduce Amount of Solids
  • تحفيز نزع الماء Enhance De-watering
  • امكانية الاستخدام الامن للحمأة Safe Use of Sludge

3.طرق معالجة الحمأة

تشمل عمليات معالجة الحمأة الخطوات التالية:

  • عمليات تكثيف وتغليظ الحمأة
  • عمليات تثبيت (تكييف) الحمأة
  • عمليات نزع الماء من الحمأة (التجفيف)
  • عمليات التخلص من الحمأة
  • عمليات تكثيف وتغليظ الحمأة

عمليات تكثيف الحمأة تهدف الي زيادة محتواها من المواد الصلبة وتؤدّي زيادة بسيطة في محتوى المواد الصلبة (من ٣ إلى  ٦) في المائة  إلى تقليص هام في حجم الحمأة (حتى ٥٠ في المائة)، وبالتالي تقليص الحجم المطلوب لوحدات المعالجة  اللاحقة .

ويجري تكثيف الحمأة عادة بطرق فيزيائية منها:

  • الترسيب بفعل الجاذبية في احواض التكثيف (المكثفات)
  • التكثيف باستخدام التعويم بالهواء المذاب
  • التكثيف باجهزة الطرد المركزي.

 

  • عمليات تثبيت (تكييف) الحمأة

تثبّت الحمأة تمهيدًا لتقليص محتواها من العوامل الممرضة وإزالة الروائح المزعجة وتخفيف أو إزالة  احتمال التعفن.  كما تهدف عمليات تثبيت الحمأة بيولوجيا الي اختزال جزء من المواد العضوية الموجودة بالحمأة عبر الانشطة البيولوجية للكائنات الحية الدقيقة. وغالبا ما تنقسم عمليات تثبيت الحمأة الي :

  • عمليات تثبيت الحمأة كيميائيا
  • عمليات تثبيت الحمأة بيولوجيا
  • عمليات تثبيت الحمأة حراريا

  • عمليات نزع الماء من الحمأة (التجفيف)

نزع الماء من الحمأة هي الخطوة النهائية قبل التخلص من الحمأة , والهدف منها ازالة اكبر قدر ممكن  من الماء من الحماة أو من الحمأة المهضومة قبل التخلص منها .

وهناك ستة طرق شائعة للتجفيف وهي كالاتي:

  • التجفيف علي اسطح من الرمال     Sand Sludge Drying Beds
  • التجفيف في برك الاكسدة Drying in Oxidation ponds
  • التجفيف بكبس الحمأة في قوالب Sludge Pressing in Cakes
  • التجفيف بخلخلة الهواء   Vacuum Filtration
  • التجفيف بالمرشحات المضغوطة ( المرشحات الحزامية) Belt press Filters
  • التجفيف بقوة الطرد المركزية  Centrifugal Sludge Drying
  • التجفيف بمرشحات الألواح المرصوصة المجوفة (Filter press)

 

  • عمليات التخلص من الحمأة

عمليات وطرق التخلص من الحمأة في العمليات الاتية:

  • الاختزال الحراري للحمأة
  • التخلص من الحمأة بالحرق
  • الدفن الصحي في الأرض
  • قذف الحمأة في البحر
  • إعادة إستخدام الحمأة في إستخدامات مفيدة

 

أحــمــد السـروي

إستشاري معالجة المياه والبيئة

 

المصادر والمراجع

  1. دليل المتدرب ,البرنامج التدريبي لمشغلي محطات معالجة مياه الصرف الصحي المستوي د , برنامج اعتماد مشغلى مرافق مياه الشرب والصرف الصحى , الوكالة الأمريكية للتنمية الدولية, 2012.
  2. احمد السروي , عمليات المعالجة البيولوجية لمياه الصرف الصحي , موسوعة معالجة الصرف الصحي, دار الكتب العلمية , 2017 .
  3. احمد السروي , عمليات معالجة حمأة الصرف الصحي , موسوعة معالجة الصرف الصحي, دار الكتب العلمية , 2018 .

 

جمع العينات في محطات معالجة مياه الصرف الصناعي

1.مقدمة

يغطي الرصد البيئي للمنشأة الصناعية الإنبعاثات إلى الهواء  والصرف السائل والمواد الصلبة والنفايات الخطرة , ويشمل الرصد البيئي تحديد الملوثات والإنبعاثات الخطرة من كل صناعة, وتحديد أوجه التلوث لكل خط إنتاج , وتحديد أوجه التلوث من الوحدات الخدمية في كل قسم , وتسجيل مخرجات القياسات والتحاليل لمعاملات التلوث في السجل البيئي .

ويتطلب رصد الملوثات والإنبعاثات إعطاء عناية خاصة للأساليب المستخدمة وذلك نتيجة للتأثير المتوقع لها على تفسير النتائج، ومن ثم على دقة البيانات الناتجة ومدى الاعتماد عليها.

  1. الرصد البيئي للصرف السائل للمنشأة الصناعية

وتضع اللوائح الحدود الخاصة بتركيزات ملوثات محددة في مياه الصرف عند إلقائها في المسطحات المائية المستقبلة. ولأغراض الرصد، فإن القيم الملقاة لمواد أو ملوثات محددة يتم التعبير عنها غالبا بالكميات الكلية لكل وحدة زمنية. وفي بعض الأحيان يتم التعبير عن هذه القيم بكميات محددة لكل طن من المنتج أو ككفاءة تنقية. وقد تم وضع حدود قصوى للعديد من معاملات التلوث: الأكسجين الكيميائي/الأكسجين الحيوي ، المواد الصلبة العالقة الكلية، الفوسفور والنيتروجين .

معاملات التحكم المرصودة: تشمل المعاملات النمطية للتحكم في مياه الصرف ما يلي:

  • دفق(تدفق) مياه الصرف م3/يوم
  • المواد الصلبة العالقة الكلية مجم/لتر
  • درجة الحرارة (درجة مئوية )
  • الطلب الكيميائي على الأكسجين COD ( الاكسجين الكيميائي المستهلك ) مجم اكسجين/لتر
  • الطلب البيوكيميائي على الأكسجين BOD   () مجم اكسجين/لتر
  •  النتروجين الكلي مجم/لتر
  •  الأس الهيدروجيني
  • التوصيل الكهربي ميكروسيمنز/سم
  1. جمع العينات

تعرف عملية جمع العينات بانها اخذ جزء من كل بحيث ان يكون هذا الجزء ممثلا ومعبرا عن الكل  , فالعينة التحليلية هي العينة التي يتم الحصول عليها من مكونات الكمية الكلية (مجتمع البحث) بعد القيام بعمل ما عليها (استخلاص- تحلل ….) والتخفيف أو التركيز اذا كان ذلك ضروريا , ومن ثم تستخدم في عملية قياس حقيقية.

إن أخذ العينات بأسلوب جيد هو عامل أساسي لتحديد مواصفات مياه الصرف التي يتم التخلص منها. وهناك تعليمات عامة لأخذ عينات مياه الصرف. ومع ذلك فإن المشاكل المرتبطة بأخذ عينات مياه صرف الصناعات النسيجية والناتجة عن التغير في نوعية مياه الصرف أو معدلات الدفق يجب حلها كل على حدة، آخذين في الاعتبار الأسباب الناتجة عن التشغيل.

وقد تكون العينات مفردة grab sample أو عينات مركبة Composite Sample ، أو عينات مركبة متناسبة مع الدفق. وتوضح العينة المفردة تركيبة مياه الصرف عند لحظة أخذ العينة. وعند أخذ مجموعة من العينات المفردة يمكن متابعة الأحمال القصوى، والتغير في نوعية مياه الصرف وكذلك مجال تغير معاملات التلوث المعنية. وتوضح العينة المركبة متوسط التركيب خلال الفترة الزمنية المختارة. وعادة ما يتم أخذ عينة مركبة على مدى ٢٤ ساعة متناسبة مع الدفق بحيث يكون جهاز أخذ العينات محكومًا بمقياس الدفق.

والغرض من وضع برنامج لأخذ وتجهيز عينات مختلفة للتحليل وإجراء مختلف الأختبارات عليها هو :

  • التأكد ان عمليات معالجة تنقية مياه الصرف قد تمت بنجاح .
  • مدي الكفأءة التي تعمل بها كل وحدة من وحدات المعالجة .
  • الحصول علي سجلات وبيانات تبين اذا كانت وحدات المعالجة تعمل وفقا لتصميمها ام لا .
  • ظبط عملية التحكم في المعالجة وتكاليفها .
  • اكتشاف الاسباب التي تؤدي الي متاعب ومشاكل التشغيل التي تؤثر علي كفاءة التنقية وبالتالي وضع الحلول المناسبة لعلاجها وتلافيها في المستقبل .
  • جمع المعلومات اللازمة للتخطيط المستقبلي لعمل توسعات في محطة المعالجة .

ويتم تحديد فترة أخذ العينة وحجم العينة لكل حالة على حدة على أساس التحاليل المستخدمة وكذلك الأسباب التي تؤثر على دقة أخذ العينات ودقة التحاليل. وغالبا  ما يتم أخذ عينات تحاليل مياه الصرف لمدة ٢٤ ساعة لمدة من 5 الي 7 ايام  , وفي بعض الأحيان يتم تجميد العينات وجمعها معا لتغطي فترة أطول. ويتم اخذ العينات لتحديد الأكسجين الكيميائي المستهلك والمواد الصلبة العالقة TSS,COD يوميا أو باستمرار ويتم تحليلها يوميا. وعادة ما  يتم أخذ العينات لتحديد الاكسجين الحيوي المطلوب BOD والمغذيات أسبوعيا. ويتم القياس المستمر لكل من الأس الهيدروجيني والتوصيل الكهربي.

ومن المحتمل أن يحتاج كل مصنع لبرنامج التحليل الخاص به. ويغطي هذا البرنامج عادة مدى واسع من القياسات والتحاليل، طبقا لما تم وضعه مسبقا في خطة الرصد الذاتي. ويجب إجراء القياسات والتحاليل طبقا للمعايير القياسية التي توصي بها السلطات.

تشمل عملية جمع العينات العناصر الهامة الاتية:

  • الهدف من تحليل العينات
  • طريقة اخذ العينات
  • ادوات اخذ العينات
  • أنواع العينات
  • طريقة اخذ عينات الحمأة
  • مراقبة الجودة في جمع العينات وتحليلها.

الحسابات

يتم حساب كميات مياه الصرف وتسجيلها طبقا للمواصفات الموضوعة في خطة الرصد. ويتم حساب الصرف غالبا كما يلي:

ويتم أيضا التحكم في كفاءة المعالجة البيولوجية لمياه الصرف بواسطة حساب النقص في المادة العضوية (BOD, COD) بين مياه الصرف غير المعالجة قبل الترسيب الأولي ومياه الصرف المعالجة بعد الترويق الثانوي.

ويشمل التقرير النمطي لرصد التخلص من مياه الصرف قيم المتوسطات الشهرية والتغيرات في مياه الصرف عند نقاط الرصد قبل وبعد المعالجة، وقيم الحدود طبقا للقانون وأيضا بعض بيانات الإنتاج.

 

أحمد أحمد السروي

إستشاري الدراسات البيئية

 

المراجع العلمية

  1. احمد السروي , طرق معالجة المخلفات الصناعية السائلة , دار الكتب العلمية , 2017.
  2. احمد السروي , معالجة مياه الصرف الصناعي , دار الكتب العلمية , 2007.
  3. احمد السروي , اساسيات الجودة في المختبرات البيئية , دار الكتب العلمية , 2012.

4.دليل الرصد الذاتي, وزارة البيئة المصرية , 2003.

Sludge Dewatering by Solid Bowl Centrifuges

Dewatering

             Dewatering is the physical operation of reducing the moisture content of sludge and biosolids to achieve a volume reduction greater than that achieved by thickening. Dewatering, because of the substantial volume reduction, decreases the capital and operating costs of subsequent handling of solids.

            Dewatering sludge and biosolids from a solids concentration of 4 to 20% reduces the volume to one-fifth and results in a nonfluid material.

The dewatering processes that are commonly used include mechanical processes such as centrifuges, belt filter presses, and pressure filter presses; and natural processes such as drying beds and drying lagoons. The main variables in any dewatering process are solids concentration and flow rate of the feed stream, chemical demand and solids concentration of dewatered sludge cake, and side stream. The selection of particular process is determined

by the type and volume of sludge to be dewatered, characteristics such as dryness required of the dewatered product, and space available. Table 1 presents a comparison of the most commonly used dewatering processes. [1]

  The solid-liquid separation could be classified into the following four categories: (1) pretreatment; (2) thickening; (3) filtration, and (4) post-treatment. Sludge dewatering reduces the sludge volume to facilitate the subsequent treatment/disposal processes. Inefficient sludge dewatering could significantly increase transportation, handling and final disposal costs. [2]  

Centrifugal Dewatering

          Dewatering of municipal sludge by centrifugation has been widely used in both the United States and Europe. Similar to its application in thickening, it is the process in which a centrifugal force of 500 to 3000 times the force of gravity is applied to sludge to accelerate the separation of the solids and the liquid.

          Two basic categories of centrifuges are used for municipal wastewater sludge dewatering: imperforate basket and solid bowl. A third type, the disk nozzle centrifuge, has been used for thickening sludge but has seldom been used for dewatering. Because of the improved design and efficiency of the solid bowl centrifuges in the past few years, imperforate basket centrifuges have fallen out of favor in the municipal market and are being  replaced by solid bowl machines.

         The main components of a solid bowl centrifuge (also known as continuous decanter scroll or helical screw conveyor centrifuge) are the base, cover, rotating bowl, rotating conveyor scroll, feed pipe, gear unit, backdrive, and main drive . The base provides a solid foundation to support the centrifuge. Vibration isolators below the base reduce the transmission of vibration from the centrifuge to its foundation. The cover that encloses the

rotating bowl assembly completely serves as a safety guard. It also helps contain odors and dampens the noise.

         The rotating bowl of the centrifuge consists of a cylindrical-conical design; the proportion of the cylindrical to conical shape varies depending on the manufacturer and the type of centrifuge. The conveyor scroll fits inside the bowl with a small clearance between its outer edge and the inner surface of the bowl. The conveyor rotates, but at a slightly lower or faster speed than the bowl. This difference in speed between the bowl and the conveyor scroll allows the solids to be conveyed from the zone of the stationary feed pipe where the sludge enters the bowl, to the dewatering beach, where the sludge cake is discharged. The dilute stream called centrate is discharged at the opposite end of the cake discharge port. The differential speed is controlled by the gear unit and the backdrive. Depending on the type of sludge, cake solids concentration varies from about 15 to 36%. Centrifuges are available with capacities as low as 40 L/min (10 gpm) to more than 3000 L/min (800 gpm).

        Solid bowl centrifuges are available in both countercurrent and concurrent bowl designs (see Figure 1). In the countercurrent design, the sludge feed enters through the small-diameter end of the bowl, and the dewatered sludge cake is conveyed toward the same end. In the concurrent fl ow design, the sludge feed enters through the large-diameter end of the bowl, and the sludge cake is conveyed toward the opposite end.

        Because of improvements in the design of the solid bowl centrifuges, cake solids concentrations in excess of 40% have been reported. These machines, known as high-solids (also called high-torque) centrifuges or centripresses, have a slightly longer bowl length, a reduced differential speed, higher torque, and a modified conveyor that presses the solids within the beach end of the centrifuge. These centrifuges may require higher polymer dosages to achieve higher cake solids concentrations.

.bowl centrifuge dewatering. [Parts (a) and (b) from Metcalf & Eddy, 2003

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Technical Manager Louts for Water Treatment

  

References

[1]   Wastewater sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006, p 106.

[2]   L. K. Wang, Y. T. Hung, and N. S. Shammas (eds.), Physicochemical Treatment Processes. Humana Press, Totowa, NJ (2005) ,p 684.

[3]   Metcalf & Eddy, Tchobanoglous, G., Burton, F. L., Stensel, H. D. “Wastewater engineering: treatment and reuse/Metcalf & Eddy, Inc.”, Tata McGraw-Hill, 2003.

Wastewater Treatment Plant

  1. Wastewater Characteristics

            Wastewater is another term for sewage; water that has been used in homes, industries, institutions, and businesses that is not for reuse and is generally collected in a sewage collection or drainage system .In general, raw wastewater is 99.9% water and 0.1% impurities.

             However, the impurities in wastewater can cause damage to our environment, create odors and pose significant risks to human health, if the wastewater is not treated properly.

           Organic matter comprises approximately 75% of the impurities in wastewater; it is predominantly human and food waste.

Nitrogen, phosphorus and trace levels of other nutrients are present in wastewater. Nutrients encourage plant growth that can generate excessive plant and algae growth in water and can be detrimental to the natural ecosystem. Therefore, it is essential that excessive nutrients be removed from the water source prior to disposal.

      Industrial wastewater may have toxic elements that must be removed prior to discharge. The main concerns are heavy metals, organic compounds, oils and fats.

Heavy metals including arsenic, cadmium, cobalt, chromium, copper, iron, lead,manganese, nickel and zinc can be found in wastewater .Most of the metals are removed in the treatment process and end up in the solids. Therefore, most heavy metal concerns deal with the disposal or reuse of the sludge.

         All wastewater contains microorganisms that are beneficial to wastewater processing and others that can be harmful.

Aerobic and anaerobic bacteria carry out decomposition of the organic matter into more stable forms that are more easily disposed of in the environment.

         Pathogens are microorganisms that can cause disease in plants, animals and humans. The processed water is disinfected prior to discharge to kill microorganisms that may be detrimental to the ecosystem.

 

  1. Wastewater Collection

          The wastewater treatment process begins with the collection of waste streams from homes, businesses and industrial complexes.

These streams feed into what is known as the “collection” or “drainage” system which transports the wastewater to the wastewater treatment plant for processing. The collection system is typically operated as a separate department within the municipality.

       The collection system is comprised of pipes, junction boxes, lift stations and associated equipment that channel raw wastewater to the plant. In many cases, the collection system

will also serve to collect storm run off. Systems that convey storm run off and waste streams are known as combined sewer overflow (CSO) systems.

         Individual homes are connected to the collection system through a lateral sewer to the main sewer line. Sewer lines come together from different directions into a junction box.

Junction boxes combine the flow from main lines into a much larger flow heading towards the wastewater treatment plant.

         Wherever possible, the design of the collection system and the location of the facility will utilize gravity to move the wastewater to the wastewater treatment plant. However,

where this is not feasible, because of the municipality’s location, elevation changes and system design, lift stations will be used to pump the wastewater to the plant.

         The size and pumping capacity of each lift station will be dependent on the maximum estimated flow rates at each station. Where flow rates are relatively low, lift stations are quite small and will typically have two small submersible pumps to move the stream. As the collection system gets closer to the treatment plant, flow rates will increase. Lift stations closer to the plant can become quite large, requiring several large capacity pumps to provide adequate flow capacity.

         All lift station designs must consider flow rate changes due to demand variations, such as the time of day and storm surges.

People take showers in the early morning, which corresponds with the highest daily flow rates a treatment facility experiences.

     If the collection system is configured for CSO, there may be a series of large auxiliary storm pumps in these pumping stations.

From the collection system, the wastewater enters the  Wastewater Treatment Plant.

  1. Wastewater Processing

    A Wastewater Treatment Plant (WWTP) is a facility designed to receive the wastewater from primarily domestic, commercial, and industrial sources and to remove materials that damage water quality and threaten public health and safety when discharged into receiving streams or bodies of water.

     Most facilities employ a combination of mechanical removal steps and bacterial decomposition to achieve the desired results. Chlorine is often added to discharges from the plants to reduce the danger of spreading disease by the release of  pathogenic bacteria.

   Figure 2 illustrates the processes of a typical wastewater treatment plant. The top half illustrates the Water Processing flow chart. Raw wastewater is pumped to the wastewater treatment plant through lift or pumping stations.

Lift stations are required when a sewage system serves a community or area lower than the plant, where an uphill shortcut will significantly decrease the total length of pipe required to tie into the plant, or where existing structures or other constraints require an uphill route to the WWTP.

The wastewater enters the plant at the head works where processing starts.

The typical water processing steps include:

  • Preliminary Treatment
  • Primary Treatment
  • Secondary Treatment
  • Tertiary Treatment

Preliminary Treatment

       The head works include the influent channel, coarse and fine screens and aerated grit chambers where preliminary treatment occurs. Flow measurement, screening, pumping, and

grit removal are the typical steps in preliminary treatment.

Wastewater enters the influent channel into the coarse screens. The screens remove large debris that enters the sewage collection system such as rags, tramp metal, sticks, broken glass, rocks, sand and the vast variety of other materials.

Screens are utilized early in the wastewater treatment process to minimize pump and equipment damage within the facility. In many wastewater treatment plants, fine screens are utilized to remove smaller debris. All screened debris is removed and disposed as landfill.

 

     The wastewater is then pumped into grit removal chambers. Air is introduced into the chamber to scour the organic materials from the grit before the grit settles to the bottom of the chamber. The settled grit or sand is delivered by a screw conveyor to a pit at one end of the chamber. From there, it is pumped by a grit pump to a grit/water separator .This debris

is also disposed as landfill. Liquid separated from the grit is returned to the grit chamber. Wastewater from the grit chamber then flows to the primary clarifiers.

 

Primary Treatment

         The primary treatment process reduces the solids content of wastewater through sedimentation. Wastewater slowly flows into large tanks called primary clarifiers where heavier particles are allowed to settle at the bottom of the clarifier. Scrapers move the settled solids (primary sludge) to sumps at one end of the clarifier. From there, the primary sludge is pumped into a holding tank where solids processing commence.

Solids lighter than water float to the top and are skimmed from the top of the primary clarifier and pumped to a thickener for solids processing. The greases and fats skimmed from the top of the clarifier are called scum. Primary treatment removes approximately 30 – 50% of the suspended solids. The remaining clarified liquid, containing mostly dissolved materials, flows to the secondary treatment stage.

Secondary Treatment

         During secondary treatment, organic material is removed through biological treatment. The most widely used biological treatment method is the activated sludge process. The activated sludge process requires an aerated tank containing bacteria that break down the organic materials. The bacteria use the organic material in the liquid and clump together to

form a microbial floc, which is also known as activated sludge. This liquid flows into the secondary clarifiers where the activated sludge is allowed to settle. In some wastewater treatment plants, ferric chloride is added after biological treatment to cause precipitation of phosphate materials remaining in the liquid.

Flow enters the clarifiers from the bottom of the tank through a pipe located at the center of the tank. The clarifiers are designed to direct the flow from the center of the clarifier in a downward direction to encourage the solids to settle. The activated sludge settles at the bottom of the secondary clarifier.

           Some of the settled activated sludge is collected and is returned to the aeration tank to insure sufficient bacteria and organic waste supply to maintain the biological process. This material is called Return Activated Sludge (RAS).The activated sludge not needed for the biological process is called Waste Activated Sludge (WAS) and will be pumped to the sludge conditioning stage for further processing.

The clarified liquid, with over 95% of the organic materials removed, flows to the tertiary treatment stage. Scum, formed on the top of secondary clarifiers is sent to a thickener for solids processing.

Tertiary Treatment

          The tertiary treatment stage normally starts with the filtering of the clarified liquid that flows from the secondary clarifiers. The liquid is processed through a bed of sand or other filtering device that removes additional pollutants from the liquid.

The water then moves to the disinfection tank. Water enters the disinfection tank where chlorine gas or sodium hypochlorite is metered in the tank. The water slowly moves through the tank to enable the chlorine to kill the microorganisms remaining in the wastewater that may be harmful to fish life. The disinfected water is then passed on to a dechlorination stage to remove the chlorinated materials that also could be harmful to fish life. Sulfur dioxide or sodium metabisulfate are the most cost effective chemicals utilized to neutralize chlorine.

           Another disinfection method that eliminates a dechlorination stage is called ultraviolet disinfection. Ultraviolet light sources are submerged in a holding tank. The ultraviolet lamps emit radiation that penetrates the cell wall of the microorganism and is absorbed by cellular materials, which either prevents replication or causes death of the cell. As a result, pathogenic microorganisms are almost entirely inactivated or killed .The UV light disinfection technology is considered to have no adverse environmental impact.

The water or effluent can now be discharged into the ecosystem.

  1. Wastewater sludge

When wastewater is treated using various mechanical, biological, and physiochemical methods to remove organic and inorganic pollutants to levels required by the permitting authority, the sludge produced will also vary in quantity and characteristics from one treatment plant to another.

4.1Types of sludge

    Types of sludge and other solids, such as screenings, grit, and scum, in a wastewater treatment plant vary according to the type of plant and its method of operation. The sources and types of solids generated in a treatment plant with primary, biological, and chemical treatment facilities are illustrated in Figure 5.

        Wastewater sludge can be classified generally as primary, secondary (also called biological), and chemical. Sludge contains settleable solids such as (depending on the source) fecal material, fibers, silt, food wastes, biological flocs, organic chemical compounds, and inorganics, including heavy metals and trace minerals. The sludge is raw sludge when it is not treated biologically or chemically for volatile solids or pathogen reduction. When the sludge is treated, the resulting biosolids can be classified by the treatment, such as aerobically digested (mesophilic and thermophilic), anaerobically digested (mesophilic and thermophilic), alkaline stabilized, composted, and thermally dried. The treated sludge can be only primary, secondary, or chemical, or a mixture of any two or three of the sludge’s.

4.1.1 Primary Sludge

Most wastewater treatment plants use the physical process of primary settling to remove settleable solids from raw wastewater. In a typical plant with primary settling and a conventional activated sludge secondary treatment process, the dry weight of the primary sludge solids is about 50% of that for the total sludge solids. The total solids concentration in raw primary sludge can vary between 2 and 7%. Compared to biological and chemical sludges, primary sludge can be dewatered rapidly because it is comprised of discrete particles and debris and will produce a drier cake and give better solids capture with low conditioning requirements. However, primary sludge is highly putrescible and generates an unpleasant odor if it is stored without treatment.

4.1.2 Secondary Sludge

Secondary sludge, also known as biological sludge, is produced by biological treatment processes such as activated sludge, membrane bioreactors, trickling filters, and rotating biological contactors. Plants with primary settling normally produce a fairly pure biological sludge as a result of the bacteria consuming the soluble and insoluble organics in secondary treatment system. The sludge will also contain those solids that were not readily removed by primary clarification. Secondary sludge generated in plants that lack primary settling may contain debris such as grit and fibers. Activated sludge and trickling filter sludge generally contain solids concentrations of 0.4 to 1.5% and 1 to 4%, respectively, in dry solids weight. Biological sludge is more difficult to dewater than primary sludge because of the light biological flocs inherent in biological sludge.

4.1.3 Chemical Sludge

Chemicals are used widely in wastewater treatment, especially in industrial wastewater treatment, to precipitate and remove hard-to-remove substances, and in some instances, to improve suspended solids removal. In all such instances, chemical sludges are formed. A typical use in removing a substance from wastewater is the chemical precipitation of phosphorus. The chemicals used for phosphorus removal include lime, alum, and “pickle liquors” such as ferrous chloride, ferric chloride, ferrous sulfate, and ferric sulfate. Some treatment plants add the chemicals to the biological process; thus, chemical precipitates are mixed with the biological sludge. Most plants apply chemicals to secondary effluent and use tertiary clarifiers or tertiary filters to remove the chemical precipitates. Some chemicals can create unwanted side effects, such as depression of pH and alkalinity of the wastewater, which may require the addition of alkaline chemicals to adjust these parameters.

4.1.4 Other Wastewater Residuals

In addition to sludge, three other residuals are removed in wastewater treatment process: screenings, grit, and scum. Although their quantities are significantly less than those of sludge in volume and weight, their removal and disposal are very important.

Screenings include relatively large debris, such as rags, plastics, cans, leaves, and similar items that are typically removed by bar screens. Quantities of screenings vary from 4 to 40 mL/m3 (0.5 to 5 ft3/MG) of wastewater. The higher quantities are attributable to wastes from correctional institutions, restaurants, and some food-processing industries. Screenings are normally hauled to a landfill. Some treatment plants return the screenings to the liquid stream after marcerating or comminuting. This is not recommended because many of the downstream pieces of equipment, such as mixers, air diffusers, and electronic probes, are subject to fouling from reconstituted rags and strings.

Grit consists of heavy and coarse materials, such as sand, cinders, and similar inorganic matter. It also contains organic materials, such as corn, seeds, and coffee grinds. If not removed from wastewater, grit can wear out pump impellers and piping. Grit is typically removed in grit chambers. In some treatment plants, grit is settled in primary clarifiers along with primary sludge and then separated from the sludge in vortex-type grit separators. The volume of grit removed varies from 4 to 200 mL/m3 (0.5 to 27 ft3/MG) of wastewater. The higher quantities are typical of municipalities with combined sewer systems and sewers that contribute excessive infiltration and inflow.

Grit is almost always landfilled.

Scum is the product that is skimmed from clarifiers. Primary scum consists of fats, oils, grease, and floating debris such as plastic and rubber products.

It can build up in piping, thereby restricting flow and increasing pumping costs, and can foul probes, flow elements, and other instruments in the waste stream. Secondary scum tends to be mostly floating activated sludge or biofilm, depending on the type of secondary treatment used. The quantity and moisture content of scum typically are not measured. It may be disposed of by pumping to sludge digesters, concentrating, and then incinerating with other residuals, or drying and then landfilling.

  1. Sludge Processing

The purpose of primary and secondary treatment is to remove as much organic solids from the liquid as possible while concentrating solids in a much smaller volume for ease of handling and disposal. Primary sludge has a typical solids content of 4 – 6%. Sludge processing reduces the solids content of this sludge through biological processes and removes more of the liquid content of it prior to disposal.

The overall sludge processing investment cost at the typical wastewater treatment plant is about one-third of the total investment in the treatment plant. However, based on the individual wastewater treatment plant’s processing system, operating expenses in sludge processing typically amount to even a larger portion of the total plant operating costs. To reduce plant operating costs, it is essential to have a properly designed and efficiently operated sludge processing stage.

The design options for each process will be dependent on the type, size, and location of the wastewater treatment plant, and the solid disposal options available. The design must be able to handle the amount of sludge produced and converted economically to a product that is environmentally acceptable for disposal.

As with water processing, sludge process methods will be determined by the specific constraints and requirements of the individual wastewater treatment plant. Our schematic

covers the general processing steps found in a typical plant.

There will be many plant-to-plant variations that are not illustrated in our article highlights the processes of a typical wastewater treatment plant. The bottom half illustrates the Sludge Processing flowchart.

The typical sludge processing steps include:

  • Sludge Thickening
  • Sludge Conditioning
  • Dewatering
  • Disposal

 

 

Sludge Thickening

To optimize the sludge conditioning stage, it is important to maximize the solids content of the materials decanted from the water processing stages. The waste activated sludge, scum, and primary sludge can be thickened to reduce the liquid content prior to sludge conditioning. Due to the varying physical nature and liquid content of these materials, facilities may use different thickening processes and equipment for these three materials. In some cases, the primary sludge may not even be thickened and will be pumped directly to

sludge conditioning.

The intent is to optimize the downstream processing capabilities.

The four most common thickening methods include gravity settling, gravity belt thickening, dissolved air flotation, and centrifuge thickening.

The recovered liquid or supernatant from thickening is pumped back into the aeration tank or to the beginning of the water processing stage and is reprocessed.

 

Sludge Conditioning

Sludge conditioning is a key stage in the reduction of solids prior to disposal. Based on the size and location of the facility five common methods are typically utilized; chemical

treatment, anaerobic digestion stabilization, aerobic digestion stabilization, lagoon storage, and heat treatment.

Many facilities will have some type of aerobic or anaerobic digestion stage prior to dewatering. The purpose of sludge digestion is to convert bulky odorous sludge into a relatively inert material that can be rapidly dewatered without obnoxious odors.

Thickened waste activated sludge, scum, and primary sludge are pumped into the digester. In anaerobic digestion, the digester uses the naturally occurring anaerobic microorganisms

to break down organic materials into methane and carbon dioxide gases.The sludge is heated to 37°C (100°F) and agitated continuously in the digester to improve the rate of digestion.

There are two different anaerobic processes, single stage and two-stage. Single stage digesters utilize one digester (tank) to digest the sludge, capture methane gas and store the sludge until it is transferred to the dewatering process

Two-stage anaerobic digestion uses a primary and secondary digester. The primary digester is heated and utilizes mixers to completely agitate the sludge, which maximizes sludge digestion. The secondary digester is not agitated and is utilized for gravity thickening and storage of the digested sludge. The secondary digester typically incorporates a floating

gas dome for methane gas collection and supernatant is pumped out to increase solids content.

Anaerobic digestion is a biological process that breaks down a significant amount of organic solids in the sludge and produces methane gas that is utilized as a fuel for the plant.

Consequently, the volume of final sludge is greatly reduced, which in turn reduces the cost for sludge disposal.

The process also reduces the level of pathogenic microorganisms enabling digested sludge to be classified as biosolids that can be utilized as a soil conditioner or fertilizer.

Sludge can also be stabilized by long-term aeration that biologically destroys volatile solids. An aerobic digester is normally operated by continuously feeding raw sludge with intermittent supernatant and digested sludge withdrawals.

The digested sludge is continuously aerated during filling and for the specified digestion period after the tank is full.

Aeration is then discontinued to allow the stabilized solids to settle by gravity. Supernatant is decanted and returned to the head of the treatment plant, and a portion of the gravity thickened sludge is removed for dewatering.

The next step for the stabilized sludge is dewatering.

 

Dewatering

Dewatering is the final stage prior to sludge disposal. The goal is to economically remove as much liquid as possible from the sludge or digested sludge prior to disposal. The most common method of dewatering utilizes a belt filter press. The belt filter press has two continuous porous belts that pass over a series of rollers to squeeze water out of the sludge that is compressed between the two belts. Polymers are typically added to the process to enhance dewatering capabilities. Centrifuges are also used for dewatering, typically, in larger wastewater treatment plants.

Any supernatant that is removed in the dewatering process is returned to the beginning of the treatment plant for reprocessing.

Disposal

Digested sludge that is processed into biosolids can be used to spread on farmland as a soil conditioner or can be further processed as fertilizer. It can also be disposed as landfill.

Sludge can also be incinerated and the remaining ash is disposed as landfill. Economics and environmental regulations will be the primary drivers in what disposal method an individual wastewater treatment plant uses.

 

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science

 

References

METCALF & EDDY (1991). Wastewater engineering: treatment, disposal and reuse.

.

 

Aeration in Wastewater Treatment

Aeration in Wastewater Treatment

  1. Introduction [1]

        Aeration is a unit operation of fundamental importance in a large number of aerobic wastewater treatment processes. When a liquid is deficient in a gas (oxygen, in this case), there is a natural tendency of the gas to pass from the gas phase, where it is present in sufficient concentrations, to the liquid phase, where it is deficient.

       The major purpose of dissolving air is to provide oxygen to be used by microorganism in the process of wastewater treatment.

      Oxygen is a gas that dissolves poorly in the liquid medium. For this reason, in various wastewater treatment systems it is necessary to accelerate the natural process, in such a way that the oxygen supply may occur at a higher rate, compatible with the biomass utilisation rate. Among the wastewater treatment processes that use artificial aeration are aerated lagoons, activated sludge and its variants, aerated biofilters and other more specific processes. In terms of sludge treatment, aerobic digesters also use artificial aeration.

There are two main forms of producing artificial aeration:

  • introduce air or oxygen into the liquid (diffused air aeration)
  • cause a large turbulence, exposing the liquid, in the form of droplets, into the air, and also permitting the entrance of atmospheric air into the liquid medium (surface or mechanical aeration).

Figure 1 presents schematically the principles of aeration by diffused air and mechanical aeration.

There is another way of transferring oxygen to the liquid medium is described: gravity aeration (steps, weirs, cascades).

Figure 2. Shows some of Aeration units like turbine aerator with an air sparger; porous ceramic diffuser; and surface aerator.

  1. Gas transfer mechanisms

       There are two basic mechanisms for the transfer of oxygen from the gas phase to the liquid phase:

  • Molecular diffusion
  • Turbulent diffusion

2.1 Molecular diffusion

        Molecular diffusion can be understood as the tendency of any substance to spread itself uniformly in the space available.

For a water body of unlimited depth, exposed to the gas phase through a surface A, the mass transfer rate dM/dt due to the diffusion of the gas molecules in the liquid phase is defined by Fick’s law (P¨opel, 1979): [2]

        It is important to note that, for a certain gas, only the concentration gradient determines the diffusion rate per unit area. The negative sign indicates that the direction of diffusion is opposite to the positive concentration gradient.

For oxygen, the values of the diffusion coefficient are presented in Table 1.

 

 Two theories frequently used to explain the gas transfer mechanism are: [2]

  • Two-film theory. In the gas–liquid interface there are two films, a gas film and a liquid film. The gas is absorbed and transported by molecular diffusion and mixing (convection) by the gas film and subsequently by the liquid film. The films are considered as stagnant and with a fixed thickness.

The two-film theory is simpler but provides a good answer in most cases (Metcalf & Eddy, 1991). [3]

  • Penetration theory. The penetration theory does not assume stagnant films, but fluid elements that are momentarily exposed to the gas phase in the liquid interface. During this exposure time the gas diffuses in the fluid elements penetrating the liquid. Differently from the two-film theory, the penetration process is described by an unsteady diffusion. The exposure time is considered very short (< 1 s) for steady diffusion conditions to prevail. The penetration theory is more soundly theoretically based.

2.2 Turbulent diffusion

          In sewage treatment with artificial aeration, the main gas transfer mechanism occurs through the creation and renewal of the interfaces

The turbulent flux generated by artificial aeration consists of a complex secondary movement that surpasses the primary movement of the liquid mass. The turbulence is characterized by oscillations and eddies that transport fluid particles from one layer to another, with variable velocities. The turbulent movement, which is erratic in direction, magnitude and time, can be defined only probabilistically (O’Connor and Dobbins, 1958). [4]

            As mentioned, gas transfer by turbulent diffusion is much higher than by molecular diffusion. The basic structure of the gas transfer formulation can be maintained, with adaptations only in the sense of simplifying its presentation.

 

  1. FACTORS OF INFLUENCE IN OXYGEN TRANSFER

          The oxygen transfer rate of the aeration equipment to be installed in a wastewater treatment plant is frequently determined in different conditions under which it will operate (operating conditions). Therefore, it is important to be able to quantify the factors that influence the oxygen transfer rate, to allow the estimation of the transfer rate under operating conditions, based on results obtained in tests undertaken under standardized conditions.

The factors of major influence on the oxygen transfer rate are:

  • temperature
  • atmospheric pressure (altitude)
  • dissolved oxygen concentration
  • characteristics of the wastewater
  • characteristics of the aerator and the geometry of the reactor

  1. MECHANICAL AERATION SYSTEMS

The main mechanisms of oxygen transfer by mechanical surface aerators are (Malina, 1992): [5]

  • Atmospheric oxygen transfer to the droplets and the fine films of liquid sprayed in the air
  • Oxygen transfer at the air-liquid interface, where the falling drops enter into contact with the liquid in the reactor
  • Oxygen transfer by air bubbles transported from the surface to the bulk of the liquid medium

The more commonly used mechanical aerators can be grouped according to:

– Classification as a function of the rotation shaft:

  • vertical shaft aerators
  • low speed, radial flow
  • high speed, axial flow
  • horizontal shaft aerators
  • low speed

-Classification as a function of the supporting:

  • fixed aerators
  • floating aerators

Figure 3. Shows schematically mechanical aerators with vertical and horizontal shafts.

         The power of mechanical aerators usually varies between 5 HP and 100 HP, although, in special conditions, lower and higher values can be found.

In mechanical aerators, the submergence of the impellers in relation to the water level is a very important aspect in terms of oxygen transfer and energy consumption. The following situations can occur:

  • Adequate submergence. The performance is optimal. There is good turbulence and absorption of air with relation to the oxygen consumption.
  • Submergence above the optimal. The unit tends to function more as a mixer than as an aerator. The energy consumption increases without being accompanied by a substantial increase in the oxygen transfer rate.
  • Submergence below the optimal. Only a surface spray is formed in the vicinity of the aerator, without creating an effective turbulence. The energy consumption and the oxygen transfer rate decrease.

The installation of the aerator must follow the manufacturer’s instructions. Besides this, local tests should be carried out in order to obtain the optimal submergence in the reactor in question.

In many activated sludge plants, the oxygen transfer rate can be varied in such a way as to adjust itself to the variations in the oxygen utilization rate. The variation can be manual or automated, by means of timers or sensors for dissolved oxygen in the reactor. Listed below are some of the most common forms of varying the oxygen  transfer rate in mechanical aerator systems:

  • switch on and off certain aerators
  • vary the rotation speed of the aerators
  • vary the submergence of fixed aerators through the alteration of the level of the outlet weir (change in the water level)
  • vary the submergence through the change of the level of the aerator shaft

 

  1. DIFFUSED AIR AERATION SYSTEMS

The diffused air aeration system is composed of diffusers submerged in the liquid, air distribution piping, air transport piping, blowers, and other units through which the air passes. The air is introduced close to the bottom of the tank and the oxygen is transferred to the liquid medium while the bubble rises to the surface.

The main diffused air systems can be classified according to the porosity of the diffuser and the size of the bubble produced:

  • porous diffuser (fine and medium bubbles): plate, disc, dome, tube (ceramic, plastic, flexible membrane)
  • non-porous diffuser (coarse bubbles): nozzles or orifices
  • other systems: jet aerator, aspirating aerator, U-tube aerator

    Figure 4 presents schematics of the aeration by porous diffusers and aspirating devices. Aspirating devices have an impeller at the lower end (immersed in the liquid), which, when rotating, create a negative pressure, sucking in atmospheric air through a slot situated at the upper end (outside the liquid). Air is diffused into the liquid medium in the form of small bubbles, which are responsible for the oxygenation and mixing of the liquid mass. The aspirating aerators are presented in some texts as mechanical aerators, since they have motors that rotate outside the liquid, and in other texts as diffused air aerators, because they generate air bubbles in the liquid medium

The diameters of the bubbles considered in the classification of the aeration type are (ABNT, 1989): [6]

  • fine bubble: diameter less than 3 mm
  • medium bubble: diameter between 3 and 6 mm
  • coarse bubble: diameter greater than 6 mm

     In general, the smaller the size of the air bubbles, the greater the surface area available for gas transfer, that is, the greater the oxygenation efficiency. For this reason, aeration systems with fine bubbles are the most efficient in the transfer of

oxygen.

   The oxygen transfer efficiency of the porous diffusers decreases with the use ,due to the internal or external clogging. The internal clogging is due to impurities in the air that are not removed by the filter. The external clogging is due to bacterial growth on the surface, or the precipitation of inorganic compounds.

The oxygen transfer rate can be changed to adjust itself to the oxygen consumption through the control of the blowers and the air distribution system, thus allowing energy savings.

 

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science

 

References

[1]  Marcos von Sperling ,2007. Basic Principles of Wastewater Treatment ,IWA Publishing, London, UK,page 161.

[2]  PO¨ PEL, H.J. (1979). Aeration and gas transfer. 2. ed. Delft, Delft University of Technology. 169 p.

[3]  METCALF & EDDY (1991). Wastewater engineering: treatment, disposal and reuse.

Metcalf & Eddy, Inc. 3. ed, 1334 pp.

[4]  O’CONNOR, D.J., DOBBINS, W.E. (1958). Mechanism of reaeration in natural streams. Journal Sanitary Engineering Division, ASCE, 123. p. 641–666.

[5] MALINA, J.F. Biological waste treatment. In: Semin´ario de transferˆencia de tecnologia.Tratamento de esgotos. ABES/WEF. Rio de Janeiro, 17–20 Aug 1992. p. 153–315.

[6]  ABNT (1989). Projeto de esta¸c˜oes de tratamento de esgotos. NBR-570 (in Portuguese). ARCEIVALA, S.J. (1981). Wastewater treatment and disposal. Marcel Dekker, New York.892 p.

 Wastewater of Food Industry

 Introduction

               The wastewater from industries varies so greatly in both flow and pollution strength. So, it is impossible to assign fixed values to their constituents. In general, industrial wastewaters may contain suspended, colloidal and dissolved (mineral and organic) solids. In addition, they may be either excessively acid or alkaline and may contain high or low concentrations of colored matter. These wastes may contain inert, organic or toxic materials and possibly pathogenic bacteria. These wastes may be discharged into the sewer system provided they have no adverse effect on treatment efficiency or undesirable effects on the sewer system. It may be necessary to pretreat the wastes prior to release to the municipal system or it is necessary to a fully treatment when the wastes will be discharged directly to surface or ground waters [1].

              The development of industries and extensive urbanization means increased water consumption and pollution resulting from problems of waste disposal. Unfortunately, in most developing countries, effluent quality standards imposed by legislation (where they exist) are sometimes easily flouted. Industrial effluents are liquid wastes which are produced in the course of industrial activities. Over the years, the improper disposal of industrial effluents has been a major problem and a source of concern to both government and industrialist. In most cases the disposal or discharges of effluents, even when these are technologically and economically achievable for particular standards, do not always comply with pretreatment requirement and with applicable tonic pollutant effluent limitations or prohibitions. The consequence of these anomalies is a high degree of environmental pollution leading to serious health hazards [2].

            Whereas the nature domestic wastewater is relatively constant, the extreme diversity of industrial effluents calls for an individual investigation for each type of industry and often entails the use of specific treatment processes. Therefore, a thorough understanding of the production processes and the system organization is fundamental .A long-term detailed survey is usually necessary before a conclusion on the pollution impact from an industry can be reached. Typical pollutants and BOD range for a variety of industrial wastes are given in Table-1. The values of typical concentration parameters (BOD5, COD, suspended solids) and pH for different industrial effluents are given in Table-2 [3].

Table1: Wastewater characteristics for typical industries

SS: suspended solids


Table2:
Comparative strengths of wastewaters from industrySS: suspended solids


COD: Chemical Oxygen Demand

BOD5: Biochemical Oxygen Demand in five days

SS: suspended solids

COD as KMnO4 mgO2/l

             By a general viewpoint, food wastewaters are the best type of contaminated water when speaking of industrial activities because of the low amount of toxic compounds normally related to the industry of metals or intermediate chemicals (petroleum, plastics, etc.) [4, 5].

 

Food Industry Wastewater

             Industrial wastewaters are considerably diverse in their nature, toxicity and treatability, and normally require pre-treatment before being discharged to sewer. Food processing in particular is very dissimilar to other types of industrial wastewater, being readily degradable and largely free from toxicity. However, it usually has high concentrations of biological oxygen demand (BOD) and suspended solid [6].Compared to other industrial sectors, the food industry uses a much greater amount of water for each ton of product [7].

             Industrial wastewater characteristics vary not only between the industries that generate them, but also within each industry. These characteristics are also much more diverse than domestic wastewater, which is usually qualitatively and quantitatively similar in its composition. On the contrary, industry produces large quantities of highly polluted wastewater containing toxic substances, organic and inorganic compounds such as: heavy metals, pesticides, phenols and derivatives thereof, aromatic and aliphatic hydrocarbons, halogenated compounds, etc., which are generally resistant to destruction by biological treatment methods. Food industry uses large amounts of water for many different purposes including cooling and cleaning, as a raw material, as sanitary water for food processing, for transportation, cooking and dissolving, as auxiliary water etc. In principle, the water used in the food industry may be used as process and cooling water or boiler feed water. As a consequence of diverse consumption, the amount and composition of food industry wastewaters varies considerably. Characteristics of the effluent consist of large amounts of suspended solids, nitrogen in several chemical forms, fats and oils, phosphorus, chlorides and organic matter [8].

 

      Food and beverage industry is one of the major contributors to growth of all economies. In EU it constitutes the largest manufacturing sector in terms of turnover, value added and employment. However, the sector has been associated with various environmental issues including water usage and wastewater treatment. Food processing industry wastewater poses pollution problems due to its high COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand).Compared to other industrial sectors, food industry requires great amounts of water, since it is used throughout most of plant operations, such as production, cleaning, sanitizing, cooling and materials transport, among others. The wastewater streams with different levels of pollution load (low, medium and high contamination) are collected and treated in an on-site installation or in a municipal sewage treatment plant. Increasing food production will increase the volume of sewage and the cost of disposal for food processing plants and present difficult challenges for municipal wastewater treatment plant operators [9, 10]. Currently, in accordance with the legislation of the European Union introduced more stringent controls and rules concerning pollution of industrial wastewater [11, 12].

          Wastewater produced by food Industry is a potential hazard to the natural water system .This wastewater contains many inorganic and organic matters, which are toxic to the various life forms of the ecosystem [13].   The majority of food processing industries are considered by very high water intake and high organic compounds rich wastewater generation.

 

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Technical Manager Louts for Water Treatment

 

References

[1] ABDULRZZAK ALTURKMANI, DAIRY INDUSTRY EFFLUENTS TREATMENT, Ph.D Thesis, Sanitary Engineering and Water Protection Department, Technical University of Civil Engineering of Bucharest,2007,p1.

[2]   Emmanuel A. Echiegu* Jacob T. Liberty (2013)., “Effluents Characteristics of Some Selected Food Processing Industries in Enugu and Anambra States of Nigeria”. Journal of Environment and Earth Science. Vol. 3, No.9.

[3]   Wang & Howard (2004).”Handbook of Industrial and Hazardous Wastes Treatment”.USA.

[4]  Anonymous (1997) Wastewater reduction and recycling in food processing operations. State of the Art Report— Food Manufacturing Coalition for Innovation and Technology Transfer. R. J. Philips & Associates, Inc., Great Falls

[5]   Nini D, Gimenez-Mitsotakis P (1994) Creative solutions for bakery waste effluent. American

Institute of Chemical Engineers. Symposium Series 300, 90:95–105

[6]. Gray, N. F.(2005). “Water Technology: An Introduction for Environmental Scientists and Engineers”.Oxford : Elsevier Butterworth-Heinemann.

[7]. Mavrov, V., and Beleires, E. (2000). “Reduction of Water Consumption and Wastewater Quantities in The Food Industry by Water Recycling using Membrane Processes”. Desalination; 131, 75-86.

[8]. DorotaKrzemińska, Ewa Neczaj1, Gabriel Borowski (2005). “Advanced Oxidation Processes For Food Industrial Wastewater Decontamination”. Journal of Ecological Engineering.Volume 16, Issue 2, Apr. 2015, pages 61–71. DOI: 10.12911/22998993/1858.

[9]. Mavrov V., Be1ieres E. (2000). “Reduction of water consumption and wastewater quantities in the food industry by water recycling using membrane processes”. Desalination, 131, 75–86.

[10]. Cicek N. (2003). “A review of membrane bioreactors and their potential application in the treatment of agricultural wastewater”. CSBE, 43, 37–49.

[11]. Marcucci M., Ciardelli G., Matteucci A., Ranieri L., Russo M. (2002). Experimental campaigns on textile wastewater for reuse by means of different membrane processes. Desalination, 149 (1-3), 137–143.

[12]. Mason T.J. (2000). Large scale sonochemical processing: aspiration and actuality. Ultrason.Sonochem., 7(4), 145–149.

[13]. F.Spina, A.Anastasi, V.Prigione, V. Tigini and G. C. Varese, “Bi ological treatment of industrial wastewaters: a fungal appro ach”Chem Eng Trans,2012.