التقنيات الخضراء المتقدمة  AGTs لمعالجة مياه الصرف

ترجمة / أحمد محمد هشام 

ماجستير كيمياء تحليلية 

Ahmedhasham83@gmail.com

أهداف التقنيات الخضراء المتقدمة AGTs

تشير التقنيات الخضراء المتقدمة (AGTs) إلى مجموعة من المواد والمنهجيات العملية القائمة على العمليات الكيميائية غير السامة ، والطاقات النظيفة ، والرصد البيئي لإبطاء أو التخلص من الأثر السلبي الناجم عن الأنشطة البشرية.

تهدف التقنيات الخضراء المتقدمة إلى توفير استدامة أفضل من خلال تلبية احتياجاتنا المجتمعية دون إلحاق المزيد من الضرر بالموارد الطبيعية المتبقية او استنذافها

يمكن تحقيق ذلك من خلال:

  • إعادة تدوير البضائع والمنتجات المصنعة.
  • تقليل النفايات والتلوث عن طريق تحسين طرق الإنتاج والاستهلاك البشري .
  • تطوير تقنيات وطاقات بديلة نظيفة لاستبدال تلك التي ثبت أنها تؤثر سلبًا على الصحة وتلوث البيئة.
  • إعداد نماذج اقتصادية لتنفيذ وتسويق الابتكارات ذات الصلة من خلال تشجيع خلق فرص عمل ومهن جديدة في هذا المجال.

 

مجالات تطبيق AGTs

اليوم ، يتم تنفيذ AGTs في مجموعة متنوعة من المجالات التي تتراوح بين الطاقة المتجددة والبيئة النظيفة الآمنة.

الطاقة :

يعتمد أحد مجالات التطبيق المهمة ل AGTs على تطوير أنواع الوقود البديلة. يجري تطوير وتنفيذ مصادر طاقة جديدة نظيفة ومتجددة وفعالة ، بما في ذلك توربينات الرياح ، والخلايا الشمسية ، والمفاعلات الحيوية.

تنتج هذه المصادر الطاقة دون إطلاق نفايات سامة في البيئة مقارنة بالنفايات التي تنتج عن انتاج واستخدام الوقود الأحفوري التقليدي.

التنظيف البيئي والمعالجة:

التطبيق المهم الثاني للتكنولوجيات الخضراء المتقدمة يتعامل مع التنظيف البيئي والمعالجة. ويشمل ذلك تنقية المياه والهواء ومعالجة مياه الصرف الصحي والمعالجة البيئية وإدارة النفايات. يتم استخدام عدد من العمليات الفيزيائية والكيميائية الخضراء لتنظيف البيئة ومعالجتها دون توليد مواد خطرة أو منتجات سامة.

الرصد البيئي والحفاظ على الطاقة:

يتضمن مجال التطبيق الثالث ل AGTs المراقبة بما في ذلك التنبؤ بالطقس ، والرصد عن بعد عبر الإنترنت لعمليات التصريف مع استخدام هذه المعلومات بشكل اكبر في صنع القرارات.حيث تم استخدام التنبؤ المتقدم بالطقس للتنبؤ بالطقس وتأثيره على البنية التحتية بحيث يمكن ، إلى جانب مراقبة المباني ، تقليل هدر الطاقة وانبعاث الغازات الدفيئة.

تمكّن أنظمة المراقبة عن بُعد عبر الإنترنت المدمجة مع شبكة معلومات البلديات والشركات والهيئات البيئية من تتبع التدفقات السائلة والتصريفات في الوقت الفعلي ، مع القدرة على إجراء العمليات أو التغييرات عليها حسب الضرورة لضمان الامتثال  للإشتراطات لمعالجة مياه الصرف الصحي.  وتشير معالجة المياه العادمة بإستخدام التقنيات الخضراء المتقدمة إلى عملية إزالة الملوثات والمكونات غير المرغوب فيها من المياه المنزلية والصناعية والملوثة لإعادتها بأمان إلى البيئة للشرب والري والاستخدامات الصناعية وغيرها.

اليوم ، أدت الزيادة في الوعي البيئي وتعزيز التنظيم الحكومي إلى جعل بعض أنظمة معالجة مياه الصرف التقليدية محل تساؤل. من أجل الفجوة التي خلفتها  التقنيات التقليدية  الأكثر تلويثاً للبيئة، يتم اختبار AGTs  وتنفيذها كبدائل نظيفة لأغراض معالجة مياه الصرف.

يتم استخدام العديد من الخطوات بشكل أساسي أثناء أي عملية معالجة لمياه الصرف.

أولها : فصل المواد الصلبة عن الماء السائل. يتم تحقيق ذلك من خلال الجاذبية حيث أن المواد الصلبة أثقل من الماء السائل. يمكن إزالة المكونات الصلبة الأخرى مثل الزيوت والأخشاب التي تكون أقل كثافة من الماء السائل من سطح الماء من خلال الطفو.

بعد ذلك ، تتعرض المياه العادمة السائلة لعمليات الترشيح للتخلص من أي معلقات غروانية للمواد الصلبة الدقيقة ، والجسيمات والكيمياويات ، والشوائب.

يتعرض الماء المرشح الناتج في المرحلة الأخيرة للأكسدة لتقليل أو إزالة سمية أي ملوثات متبقية وتطهير المياه العادمة قبل إطلاقها في البيئة.

حاليًا ، يتم اختبار عدد من طرق AGT واستخدامها لمعالجة مياه الصرف الصحي إما بمفردها أو مع طرق تقليدية أخرى.

المفاعلات الحيوية:

تعتمد AGTs الأكثر استخدامًا لمعالجة مياه الصرف على مفهوم المفاعل الحيوي. في الأساس ، المفاعل الحيوي عبارة عن جهاز يحتوي على بكتيريا وكائنات دقيقة موضوعة في / على: غشاء مفاعل بيولوجي متحرك ، أو مودع في طبقة معبأة أو ليفية  لتشكيل غشاء حيوي. عادة ما تكون المفاعلات الحيوية مزودة بفواصل مرتبطة بخزانات متتابعة وفاصل ميكانيكي يهدف إلى تسريع انفصال الماء السائل عن المواد الصلبة الحيوية.

بالإضافة إلى ذلك ، تحتوي أيضًا على أجهزة تهوية لإمداد الأكسجين تهدف إلى تسريع التفاعلات الكيميائية الحيوية التي تقوم بها الكائنات الحية الدقيقة. يؤدي التلامس بين مياه الصرف والبكتيريا / الكائنات الحية الدقيقة الموجودة في منصة المفاعل الحيوي إلى تفاعلات كيميائية حيوية ، والتي تؤدي في النهاية إلى تحول الملوثات / إلى مواد أخرى أقل سمية أو غير سامة.

في حالة مياه الصرف الصحي المحتوية على المعادن ، تنتج المفاعلات الحيوية الملقحة بالبكتيريا التي تقلل الكبريتات (SRB) كبريتيد الهيدروجين الذي يرسب المعادن الذائبة ككبريتيدات معدنية غير قابلة للذوبان يتم استردادها كمنتجات ثانوية ذات قيمة.

 الترشيح الحيوي :

في الترشيح الحيوي ، تزرع بعض الأنواع المختارة من البكتيريا والكائنات الحية الدقيقة على مرشح حيوي لتشكيل فيلم بيوفيلم. ثم يتم تمرير المياه العادمة من خلال الغشاء الحيوي إما التدفق الصاعد أو التدفق الهابط وبطريقة مستمرة أو متقطعة. خلال هذه العملية ، تسارع الكائنات الحية الدقيقة التي تعمل على الحركة في تسريع تحلل المواد العضوية والملوثات الموجودة في المياه العادمة.

تلعب المتغيرات مثل نشاط الكائنات الحية الدقيقة وعمر الأغشية الحيوية ومستويات الأكسجين ودرجة الحرارة وتكوين الماء أدوارًا رئيسية في أداء الغشاء الحيوي ، وبالتالي جودة مياه الصرف الناتجة.

على الرغم من الاستخدام الشائع لهذا النظام في معالجة المياه العادمة المنزلية ، فإن هذا النوع من AGT  قد يستخدم كوسيلة لإزالة  بعض المعادن الثقيلة من مياه الصرف الصناعية.

المعالجة البيولوجية :

تستخدم عملية المعالجة البيولوجية الكائنات الحية الدقيقة لإزالة وتحييد الملوثات والأنواع الخطرة من مواقع مياه الصرف الملوثة لإنتاج مواد أقل سمية أو غير سامة. يمكن تنفيذ العملية إما في الموقع أو خارج الموقع. تضاف الكائنات الحية الدقيقة مباشرة إلى المواقع الملوثة أثناء عمليات المعالجة في الموقع ، وتتم معالجة المواقع الملوثة في مكان آخر أثناء المعالجة خارج الموقع.

بشكل عام ، عندما يتعلق الأمر بمعالجة الملوثات والمخلفات الخطرة فإن الكائنات الحية الدقيقة لها حدودها حيث لا تتم ازالة جميع الملوثات من خلال المعالجة البيولوجية أو الترشيح الحيوي. ومن الأمثلة على ذلك المعادن الثقيلة مثل النحاس والنيكل والكادميوم والرصاص والزئبق وغيرها. وبالتالي ، تم تطوير تكنولوجيات خضراء متقدمة أخرى لهذا الغرض.

مترجم عن موقع: https://blog.emew.com

SOLIDS IN SEWAGE

The solids present in the sewage are of two types viz.,

  1. Organic solids, and
  2. Inorganic solids.

Organic solids are the substances derived from living things like produces from plant and animal. Examples of organic solids are carbohydrate, protein, and fat. The organic solids undergo decomposition by the microorganisms. Inorganic solids are inert materials and they do not undergo decomposition. Examples of inorganic solids are grit, salt etc. Only 0.3 to 0.7% of solids are present in the sewage, if these solids are removed the water can be reclaimed and reused. The purpose of the sewage treatment is to remove the solids present in the sewage.

ROLE OF MICROORGANISMS

Microorganisms are unicellular microscopic living things. They multiply by binary division of cells within 10 to 20 minutes. They require oxygen for their respiration. They decompose the organic matter and convert them into cells. Examples of microorganisms are Bacteria, Fungi, Virus etc. There are two types of microorganisms. They are

  1. Aerobic bacteria, and
  2. Anaerobic bacteria.

Aerobic bacteria use dissolved oxygen (DO) from the water bodies for their respiration. They oxidize organic matter under aerobic conditions. The end products of the decomposition are water, CO2 and Cell tissues. Anaerobic bacteria use oxygen derived from chemical substances for their respiration. They multiply in the absence of DO in the water bodies. They oxidize the organic matter under septic conditions. The end products include fowl smelling gases like H2S, CH, etc.

BIOLOGICAL TREATMENT PROCESS

The overall objectives of the biological treatment of domestic wastewater are:

  1. To oxidize or transform dissolved and suspended biodegradable substances into acceptable end products;
  2. To capture and incorporate suspended non-settleable colloidal solids into biological floc or bio film, and
  3. To transform and remove nutrients such as nitrogen and phosphorous.

The removal of dissolved and suspended carbonaceous BOD and the stabilization of organic matter found in wastewater is accomplished using a variety of microorganisms, principally bacteria. Microorganisms are used to oxidize the dissolved and suspended carbonaceous organic matter into simple end products and additional biomass. This is achieved by providing the favourable environment to microorganisms with food, DO, pH, temperature etc. The organic solids present in the wastewater serve as food for the aerobic microorganisms. The only thing to be provided is the DO, which is essential for the respiration of the aerobic organisms. In the biological treatment processes the DO is supplied either through natural means or by mechanical means by agitation.

Anaerobic organisms can multiply in the absence of DO and do the decomposition, but the end products are undesirable fowl smelling gases like H2S, CH, etc. Hence anaerobic decomposition process is not generally preferred. However, anaerobic treatments are also adopted in certain situations because of certain specific advantages. Examples of anaerobic treatment processes are Septic tanks, UASB, Anaerobic Sludge digesters.

 SECONDARY TREATMENT

The secondary treatment is designed to remove soluble organics from the wastewater. Secondary treatment consists of a biological process and secondary settling is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal and industrial wastewater plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the microorganisms require both oxygen and a substrate on which to live. There are number of ways in which this can be done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc particles.

CLASSIFICATION OF BIOLOGICAL TREATMENT SYSTEM

Biological treatment systems are classified into (a) fixed film or attached growth system and (b) suspended growth systems.

Attached Growth System

In attached growth biological treatment systems the biomass is attached. Trickling filters and biological towers are examples of systems that contain biomass adsorbed to rocks or plastic. Wastewater is sprayed over the top of the rocks or plastic and allowed to trickle down and over the attached biomass, which removes materials from the waste through sorption and biodegradation. A related type of attached-growth system is the rotating biological contactor, where biomass is attached to a series of thin, plastic wheels that rotate the biomass in and out of the wastewater. This coating of microorganisms is able to trap and consume B.O.D. and ammonia in the wastewater.

In attached growth or fixed film systems, the microorganisms responsible for conversion of organic matter are attached to an inert packing material. Packing material used in attached growth processes include rock, gravel, sand and wide range of plastic and other synthetic material. Attached growth system can be operated as aerobic or anaerobic processes. The packing materials can be completely sumersed in liquid or not submerged, with air space above the biofilm liquid layer.

Fixed film systems are more able to cope with shocks in biological loading and provide higher removal rates for BOD and suspended solids than suspended growth systems.

Advantages of attached growth systems include

  • maintain a high density of biomass population,
  • increase the efficiency of the system without the need for increasing the mixed liquor suspended solids (MLSS) concentration, and
  • eliminate the cost of operating the return activated sludge (RAS) line.

Suspended Growth System

In suspended growth systems the microorganisms responsible for treatment are maintained in liquid suspension by appropriate mixing methods. Typically, suspended growth systems require smaller footprints than fixed film systems for an equivalent capacity. There are a number of biological processes. The most common is activated sludge process in which microbes, also known as biomass, are allowed to feed on organic matter in the wastewater and remain in suspension. The make-up and dynamics of the microbial population is a function of how the ASP is operated.

TYPES OF BIOLOGICAL PROCESSES

There are two types of biological treatment process; aerobic and anaerobic. Aerobic process means that oxygen is present for the microbes for respiration. Anaerobic process means that the process proceeds in the absence of DO. Aerobic and anaerobic biological systems are available in both attached and suspended growth configurations. Examples of the aerobic suspended growth systems are trickling filter and RBC. Aerobic suspended growth systems are activated sludge process, waste stabilization ponds etc. Anaerobic attached and suspended growth systems are, respectively, anaerobic filters and upflow anaerobic sludge blanket units.

The end-products of aerobic and anaerobic processes are different. Under aerobic conditions, if completely oxidized, organic matter is transformed into non-hazardous products. But an anaerobic process can produce methane (CH4), which is explosive, and ammonia (NH3) and hydrogen sulfide (H2S), which are toxic. Some materials are better degraded under anaerobic conditions than under aerobic conditions. In some cases, the combination of anaerobic and aerobic systems in a series provides better and more economical treatment than either system could alone.

Because the biomass has a specific gravity slightly greater than that of water, the biomass can be removed from the treated liquid by gravity settling. It is important to note that unless the biomass produced from the organic matter is removed on a periodic basis, complete treatment has not been accomplished because the biomass, which itself is organic, will be measured as BOD in the effluent. Biomass generated during biological treatment is settled in secondary sedimentation tank. This settled biomass or sludge is then piped to sludge-management systems. In activated sludge process part of the settled biomass is returned to the biological reactor in amounts needed to maintain the appropriate biomass level.

Options in the Biological Processes

Available options in the biological treatment processes of domestic sewage options are

  1. Trickling Filter (TF),
  2. Activated Sludge Process (ASP),
  3. Oxidation Ditch (OD),
  4. Aerated Lagoon (AL),
  5. Waste Stabilization Ponds (WSP),
  6. Up flow Anaerobic Sludge Blanket System (UASB),
  7. Moving Bed Biological Reactor (MBBR), and
  8. Membrane Biological Reactor (MBR)

The first two methods are Conventional treatment processes, the next four methods are Low cost methods and the last two methods are emerging technologies.

Trickling filters

Trickling filters are intended to treat particularly strong or variable organic loads. They are typically circular filters filled with open stone or synthetic filter media to which wastewater is applied at a relatively high rate. The design of the filters allows high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant liquor is usually within the normal range for conventional treatment processes.

Activated sludge process

The activated sludge process (ASP) is an aerobic biological wastewater treatment process that uses microorganisms, including bacteria, fungi, and protozoa, to speed up decomposition of organic matter requiring oxygen for treatment. In this process, microorganisms are thoroughly mixed with organics under conditions that stimulate their growth and waste materials are removed. Activated sludge plants use a variety of mechanisms and processes to use dissolved oxygen to promote the growth of biological floc that substantially removes organic material. A portion of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain an optimum concentration of acclimated microorganisms in the aeration tank to break down the organics. It also traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas.

Oxidation ditch

Oxidation ditch is an extended aeration ASP. It is a large holding tank in a continuous ditch with oval shape similar to that of a race-track. The ditch is built on the surface of the ground and is lined with an impermeable lining. With a detention time of more than 24 hours, the wastewater has plenty of exposure to the open air for the diffusion of oxygen. The liquid depth in the ditches is very shallow, 0.9 to 1.5 in, which helps to prevent anaerobic conditions from occurring at the bottom of the ditch.

Aerated lagoon

An aerated lagoon is a suspended-growth process treatment unit. The aerated lagoon system consists of a large earthen pond or basin that is equipped with mechanical aerators to maintain an aerobic environment and to prevent settling of the suspend biomass. Initially, the population of microorganisms in an aerated lagoon is much lower than that in an ASP because there is no sludge recycle. Therefore, a significantly longer residence time is required to achieve the same effluent quality.

Waste stabilization ponds

Waste Stabilization Ponds (WSPs), often referred to as oxidation ponds or lagoons, are holding basins where decomposition of organic matter is taking place naturally. A WSP is a relatively shallow body of wastewater contained in an earthen man-made basin into which wastewater flows and from which, after certain retention time a well-treated effluent flows out. The activity in the WSPs is a complex symbiosis of bacteria and algae, which stabilizes the waste and reduces pathogens. The algae produce oxygen during photosynthesis by utilizing carbondioxide and solar energy derived from sun light. The bacteria utilize oxygen for the biological process to convert the organic content of the wastewater to more stable and less offensive forms and release carbondioxide.

Upflow anaerobic sludge blanket reactor

UASB reactor is an anaerobic treatment system. In a UASB-reactor, the accumulation of influent suspended solids and bacterial activity and growth lead to the formation of a sludge blanket near the reactor bottom, where all biological processes take place. Two main features influencing the treatment performance are the distribution of the wastewater in the reactor and the “three-phase- separation” of sludge, gas and water.

Moving Bed Biological Reactor

Moving Bed Biological Reactor (MBBR) involves the addition of inert media into existing activated sludge basins to provide active sites for biomass attachment. This conversion results in a strictly attached growth system.

Membrane Biological Reactors

Membrane Biological Reactors (MBR) includes a semi-permeable membrane barrier system either submerged or in conjunction with an activated sludge process. This technology guarantees removal of all suspended and some dissolved pollutants. The limitation of MBR systems is directly proportional to nutrient reduction efficiency of the activated sludge process. The cost of building and operating a MBR is usually higher than conventional wastewater treatment.

Secondary sedimentation

The final step in the secondary treatment stage is to settle out the biological floc or filter material in a secondary sedimentation tank (SST) or secondary clarifier and produce sewage water containing very low levels of organic material and suspended matter

Application of Filtration in Wastewater Treatment

Introduction

         Filtration is a fundamental unit operation that separates suspended particle matter from water. Although industrial applications of this operation vary significantly, all filtration equipment operate by passing the solution or suspension through a porous membrane or medium, upon which the solid particles are retained on the medium’s surface or within the pores of the medium, while the fluid, referred to as the filtrate, passes through.

        In a very general sense, the operation is performed for one or both of the following reasons. It can be used for the recovery of valuable products (either the suspended solids or the fluid), or it may be applied to purify the liquid stream, thereby improving product quality, or both. Examples of various processes that rely on filtration include adsorption, chromatography, operations involving the flow of suspensions through packed columns, ion exchange, and various reactor engineering applications. In petroleum engineering, filtration principles are applied to the displacement of oil with gas (Le., liquid-liquid separations), in the separation of water and miscible solvents (including solutions of surface-active agents), and in reservoir flow applications. In hydrology, interest is in the movement of trace pollutants in water systems, the purification of water for drinking and irrigation, and to prevent saltwater encroachment into freshwater reservoirs. [1]

Types of Filtration

Filtration is classified into following three types: [2]

1) Depth filtration

  1. a) Slow sand filtration
  2. b) Rapid porous and compressible medium filtration
  3. c) Intermittent porous medium filtration
  4. d) Recirculating porous medium filtration

2) Surface filtration

  1. a) Laboratory filters used for TSS test
  2. b) Diatomaceous earth filtration
  3. c) Cloth or screen filtration

3) Membrane flirtation

 

DEPTH FILTRATION

      In this method, the removal of suspended particulate material from liquid slurry is done by passing the liquid through a filter bed composed of granular or compressible filter medium.

  • Depth filtration is the solid/liquid separation process in which a dilute suspension or wastewater is passed through a packed bed of sand, anthracite, or other granular media.
  • Solids (particles) get attached to the media or to the previously retained particles and are removed from the fluid. [3]
  • This method is virtually used everywhere in the treatment of surface waters for potable  water supply.
  • Depth filtration is also often successfully used as a tertiary treatment for wastewater.
  • Failure of depth filtration affects the other downstream processes significantly and most of the times results in overall plant failure.
  • Performance of a filter is quantified by particle removal efficiency and head loss across the packed bed.
  • The duration of a filter run is limited by numerous constraints: available head, effluent quality or flow requirement.
  • The head loss and removal efficiency of a filter are complicated functions of suspension qualities (particle size distribution and concentration, particle surface chemistry, and solution chemistry), filter design parameters (media size, type, and depth), and operating conditions (filtration rate and filter runtime).

SURFACE FILTRATION

  • Surface filtration involves removal of suspended material in a liquid by mechanical sieving. In this method, the liquid is passed through a thin septum (i.e., filter material).
  • Materials that have been used as filter septum include woven metal fabrics, cloth fabrics of different weaves, and a variety of synthetic materials. [4]

 

MEMBRANE FILTRATION

  • Membrane filtration can be broadly defined as a separation process that uses semipermeable membrane to divide the feed stream into two portions: a permeate that contains the material passing through the membranes, and a retentate consisting of the species being left behind. [5]
  • Membrane filtration can be further classified in terms of the size range of permeating species, the mechanisms of rejection, the driving forces employed, the chemical structure and composition of membranes, and the geometry of construction . [6]
  • The most important types of membrane filtration are pressure driven processes including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO).

MECHANISMS INVOLVED IN THE FILTRATION PROCESSES

The process of filtration involves several mechanisms listed in the table 1. Straining has been identified as the principal mechanism that is operative in the removal of suspended solids during the filtration of settled secondary effluent from biological treatment processes. Other mechanisms including impaction, interception, and adhesion are also operative even though their effects are small and, for the most part, masked by the straining action.

Table.1 Mechanisms involved in the filtration processes [1]

Mechanism/

phenomenon

Description
Straining

 

a) Mechanical

 

 

b)Chance contact

 

 

Particles larger than the pore space of the filtering medium are strained out mechanically.

Particles smaller than the pore space are trapped within the filter

by chance contact

Sedimentation Particles settle on the filtering medium within the filter
Impaction Heavy particles do not follow the flow streamlines
Interception Particles get removed during contact with the surface of the filtering medium
Adhesion Particles become attached to the surface of the filtering medium as they pass through
Flocculation It can occur within the interstices of the filter medium
Chemical adsorption

 

a)Bonding

b)Chemical interaction

Once a particle has been brought in contact with the surface of the filtering medium or with other particles, either one of these mechanisms, chemical or physical adsorption or both, may occur.
Physical adsorption

a) Electrostatic forces

b) Electrokinetic forces

c) Van der Waals

forces

Biological growth Biological growth within the filter reduces the pore volume and enhances the removal of particles with any of the above removal

mechanisms

Filtration in Wastewater Treatment [1]

             In a very general sense, there are two types of wastewater flows – municipal and industrial. Although municipal wastewaters vary in composition, there are ranges of properties that enable filtration equipment to be readily selected and specified.

        This is not always the situation when treating industrial wastewater streams. The compositions and properties of industrial wastewaters vary significantly, and even within specific industry sectors, these flows can be dramatically different. This is important to realize because although filtration is a physical process, it depends upon and is integrally a part chemical treatment processes such as preconditioning, buffering and filter aid conditioning. These chemical treatment methods must be properly specified along with the filtration equipment itself in order to ensure that a properly designed filtration system is being applied.

      Filtration equipment selection can be complex not only because of the wide variations in suspension properties, but also because of the sensitivities of suspension and cake properties to different process conditions and to the variety of filtering equipment available. Generalities in selection criteria are, therefore, few; however, there are some guidelines applicable to certain classes of filtration applications. One example is the choice of a filter whose flow orientation is in the same direction as gravity when handling polydispersed suspensions. Such an arrangement is more favorable than an upflow design, since larger particles will tend to settle first on the filter medium, thus preventing pores from clogging within the medium structure.

        A further recommendation, depending on the application, is not to increase the pressure difference for the purpose of increasing the filtration rate. The cake may, for example, be highly compressible; thus, increased pressure would result in significant increases in the specific cake resistance. We may generalize the selection process to the extent of applying three rules to all filtration problems:

  1. The objectives of a filtration operation should be defined;
  2. Physical and/or chemical pretreatment options should be evaluated for the intended application based on their availability, cost, ease of implementation and ability to provide optimum filterability; and
  3. Final filtration equipment selection should be based on the ability to meet all objectives of the application within economic constraints.

           In applying these general criteria, one should focus on the intended application. In wastewater treatment applications, filtration can be applied at various stages. It can be applied as a pretreatment method, in which case the objective is often to remove coarse, gritty materials from the waste-stream. This is a preconditioning step for waste waters which will undergo further chemical and physical treatment downstream.

          Filtration may also serve as the preparatory step for the operation following it. The latter stages may be drying or incineration of solids, concentration or direct use of the filtrate. Filtration equipment must be selected on the basis of their ability to deliver the best feed material to the next step. Dry, thin, porous, flaky cakes are best suited for drying where grinding operations are not employed. In such cases, the cake will not ball up, and quick drying can be achieved.

A clear, concentrated filtrate often aids downstream treatment, whereby the filter can be operated to increase the efficiency of the downstream equipment without affecting its own efficiency.

            Filtration may also be applied as a part of the final stages of treatment in the process. This is most commonly referred to as a polishing operation. Indeed, filtration may be applied both as pretreatment and polishing stages, and even as an intermediate stage in the wastewater treatment process. Filtration equipment selection depends upon the specific operation that the equipment must perform.

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Technical Manager Louts for Water Treatment

 

References

[1]   Nicholas P. Cheremisinoff, Handbook of Water and Wastewater   Treatment  Technologies  , Butterworth-Heinemann,2002,p 62.

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

[3]     Cushing, R., Lawler, D. Depth filtration: fundamental investigation through three dimensional trajectory analysis. Environmental Science and Technology, 1998, 32, 3793- 3801.

[4]    Spellman, F. R. “Handbook of water and wastewater treatment plant operations”, CRC  Press, 2nd edition, 2009

[5]       Mallevialle, J., Odendall, P. E., and Wiesner, M. R. “Water treatment membrane  processes”, McGraw-Hill, New York, 1996.

[6]       Zhou, H., Smith, D. W. Advanced technologies in water and wastewater treatment. Canadian Journal of Civil Engineering, 2001, 28, 49-66.

[7] Nicholas P. Cheremisinoff, Handbook of Water and Wastewater   Treatment  Technologies  , Butterworth-Heinemann,2002,p 78.