التوجهات الحديثة في تقنيات معالجة مياه الصرف الصحي

تقنية مفاعل الطبقة الحيوية المتحركة الفائق

(Turbo Moving Bed Bio film Reactor ( T-MBBR

 

بقلم

مهندس / محمد عبد الخالق خليفة

مهندس استشاري ودعم فني لمشروعات تكنولوجيا المياه

مؤسس منتدي خبراء معالجة المياه 

 

ما هي تقنية T-MBBR :-

هي تقنية جديدة ومطورة من التقنية المعروفة بمفاعل الطبقة الحيوية المتحركة (MBBR) والتي تقوم بعملية المعالجة الحيوية للمواد العضوية بمياه الصرف عن طريق عملية النمو الملتصق للبكتيريا Attached Growth  على مواد حاملة Carriers  للبكتيريا مكونة طبقة تسمى الفيلم الحيوي     Bio film والتي تزيد من فعالية التحلل البيولوجي للمواد العضوية, وتم تطوير هذه التكنولوجيا من قبل العالم المسلم الألماني اللبناني الأصل هانز بدر الدين أستاذ الكيمياء الصناعية بجامعة آشنAachen  الألمانية, وسجلت هذه التقنية الجديدة ببراءات اختراع في بريطانيا وأمريكا باسم BIOSHAFT  حيث تم ابتكارها من خلال تطوير التقنية التقليدية المعروفة بمفاعل الطبقة الحيوية المتحركة (MBBR) بحيث أصبحت لا تنتج أي رواسب صلبة (حمأة(Sludge .

مزايا التقنية :

  • تقليل استهلاك الطاقة بنسبة كبيرة نتيجة زيادة تركيز الكتلة الحيوية بنسبة عشرة أضعاف عن التقنيات التقليدية.
  • تقليل المساحة المطلوبة لمحطة المعالجة.
  • عدم انتاج أي رواسب صلبة وانعدام الروائح تقريبا نتيجة لذلك.
  • تقليل التكاليف الانشائية والتشغيلية.
  • تقليل الضوضاء المنبعثة من محطة المعالجة.
  • سهولة التطوير والتوسعة نتيجة المرونة الكبيرة في تركيب وحدات نظام المعالجة.

مكونات النظام:

يتكون نظام BIOSHAFT كما هو موضح بالصورة أسفل من مراحل المعالجة الأولية والثانوية والثلاثية حيث تتضمن مرحلة المعالجة الأولية المصافي الخشنة والناعمة ووحدة إزالة الرمال والحصى ووحدة إزالة الزيوت والشحوم يليها مرحلة المعالجة الثانوية والتي تتكون من حوض الهضم والتهوية يليه وحدة العمود الحيويى أو المفاعل الفائق وهي الوحدة الرئيسية في هذا النظام التي تم فيها تطوير التقنية القديمة بحيث لا ينتج أي رواسب صلبة (حمأة  (Sludgeمن عملية المعالجة.

مكونات نظام بيوشافت

العمود الحيويى أو المفاعل الفائق :-

كما هو موضح بالرسم تدخل المياه من حوض التهوية والهضم مشبعة بكمية مناسبة من الأكسجين وبمستوى خلط ملائم للكتلة الحيوية إلى وحدة المفاعل الفائق او العمود الحيوي Turbo Reactor or Bio shaft  حيث تدخل المياه الى ا

لاسطوانة الوسطى من المفاعل والتي بداخلها مصدر للهواء حيث تتشبع المياه بالأكسجين تماما ثم تخرج بفعل قوة الخلط الشديد مع الهواء من أعلى الاسطوانة وكأنها نافورة مياه لتدخل إلى المفاعل الحيويى حول الاسطوانة الوسطى حيث توجد المواد الحاملة Carriers  والتي تم تطويرها وتصميمها بأسلوب خاص بحيث تقوم البكتيريا بعمل تحلل لا أكسجينيAnoxic Decomposition  على السطح الداخلي للحامل ويحدث تحلل هوائي Aerobic  على السطح الخارجي المموج مما ينتج عنه معالجة بدرجة سريعة وفعالة للمياه,  ويمتاز هذا المفاعل بأن تركيز الكتلة الحيوية Biomass يصل  الى (15-30) كجم/م3 مقارنة ب(2.5- 3) كجم/م3 في الأنظمة التقليدية ولايزيد زمن الاختلاط في هذه المرحلة عن (10) ساعات مقارنة ب(24- 36) ساعة في الأنظمة التقليدية.

ثم يتم تجميع المواد الصلبة المترسبة اسفل المفاعل الفائق وإعادتها مرة أخرى إلى حوض الهضم والتهوية وهكذا حتى يتم استهلاك كافة الرواسب الصلبة, ثم تدخل المياه الى مرحلة المعالجة الثلاثية وتبدأ بالمرسب الحيوي حيث يتم ترسيب كل المواد الصلبة المتبقية وإعادتها مرة أخرى لحوض الهضم والتهوية, ثم تأتي مرحلة التصفية والفلترة عن طريق الفلاتر الرملية أو وحدات الترشيح الغشائي الفائق Ultra filtration لتنقية المياه المعالجة من العوالق الصلبة وبعد ذلك تأتي مرحلة التطهير بالأشعة فوق البنفسجية أو بالأوزون أو بمركبات الكلورين, ويمكن بعد ذلك عمل معالجة رباعية عن طريق إضافة وحدات التناضح العكسي للحصول على مياه ذات مواصفات تفوق مواصفات مياه الشرب.

 

المصدر :

موقع شركة بيوشافت Bio shaft المتخصصة في تقنيات معالجة المياه

www.bioshaft.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

Aerobic Treatment with Biofilm Systems

Aerobic Treatment with Biofilm Systems

Biofilms

         Biofilms are small ecosystems usually consisting of three layers of differing thickness,

which change in thickness and composition with location and over time (Meyer-Reil 1996). In the first phase of colonization, macromolecules are adsorbed  at clean solid surfaces (proteins, polysaccharides, lignin; Wingender and Flemming 1999), because they are transported from the bulk liquid to the solid surface faster than the microorganisms are. As a consequence of this adsorption, the coverage of the solid surface with water is reduced. During the second phase, microbial cells attach to this prepared surface. Frequently, they do not form closed layers of uniform thickness, rather they form small attached colonies, which may spread by growth and further attachment. Usually, these cells are supplied with substrate and oxygen and are able to grow at their maximum rate. During this process, they produce organic molecules, which diffuse through the cell wall and to extracellular polymeric substances (EPS) catalyzed by exoenzymes. These EPS molecules are necessary for the formation of a stable biofilm (Wingener and Flemming 1999). In the third phase, the biofilm may consist of bacteria and EPS, the thickness of which is a function of growth rate and depends on the stability of the biofilm and the shear stress of the flowing water (Van Loodsrecht et al. 1995). At lower shear stresses ,eukaryotic organisms (protozoa, insects, their eggs and larvae) typically establish themselves. All these organisms live in a community. Materials such as substrates and oxygen are transported into the biofilm by diffusion and convection and the products are transported out of the biofilm.

          Oxygen may reach only into the exterior part of the biofilm, resulting in a growth of aerobic microorganisms such as nitrifying bacteria and protozoa. Nitrate and nitrite produced in this layer are reduced by anoxic metabolism within a middle layer, resulting in an anaerobic interior layer directly at the solid surface, where acetic acid and sulfate may be reduced (Marshall and Blainey 1991; Fig.1).

Heterogeneous biofilms grow on the sides of ships and on buildings near the water’s edge, inside human and animal mouths and within inner organs. They frequently cause damage to these surfaces (biocorrosion) and must be removed. In the area of environmental biotechnology, however, they can be utilized to advantage in certain bioreactors, such as:

  • trickling filters,
  • submerged, aerated fixed bed reactors,
  • rotating disc reactors.

The formation of biofilms is a requirement for their effectiveness.

Fig.1 Biofilm model (according to Marshall and Blainey 1991).

Trickling Filters

        A trickling filter consists of a layer of solid particles or bundles of synthetic material inside a cylindrical (Fig..2) or prismoid container. Wastewater must be distributed uniformly at the top of the fixed bed – frequently by a rotating system of two or four horizontal tubes equipped with many nozzles.

To compensate for the fact that the area of a circular section of the reactor increases  with distance from the center, the distance between nozzles must decrease the further they are away from the center in order to have an even distribution of water over the surface. Furthermore, the changes in available pressure in the rotating tubes must be considered as a function of the flow rate. Uniform distribution of the wastewater and uniform packing of the reactor with solid substances are of high importance for a high loading and removal rate. It is critical to ensure that two conditions are met:

The downward flowing liquid films must be in direct contact with the biofilm (i.e. the biofilm has to be trickled over all places and at all times) and must be in contact with the upward or downward flowing air (i.e. the trickling filter should not be flooded at any location or time).

              The wastewater must be practically free of solids. It is absolutely necessary that the  wastewater passes a primary settler under controlled conditions which is never overloaded.

We distinguish between:

Natural aeration as a result of density differences between the air saturated with moisture inside the trickling filter and the air outside the trickling filter, and

Forced aeration by a ventilator at the top of the trickling filter. In this case, the reactor may have a height of up to 12 m and is filled with packages of synthetic supporting material.

Fig. 2 Trickling filter, BIO-NET, Norddeutsche

Submerged and Aerated Fixed Bed Reactors

In cases of high hydraulic loading, the trickling filter may be operated as a flooded bed and the pressure differential needed for the downwards flow increases. The level of wastewater necessary to overcome the flow resistance depends on the form of the substance used as support material and the thickness of the biofilm. Aerobic fixed beds must be aerated near the bottom, producing a two-phase flow in a three phase system with an upwards air flow. As a result of friction forces, water is transported upwards in the center of the reactor and flows downwards near its walls.

Biomass is attached at the surface of the support material and is also suspended as flocs. It is not easy to avoid blockages in regions of biofilms with a higher thickness and a lower local flow rate. The fixed bed must be cleaned from time to time by considerably increasing the wastewater flow rate.

Synthetic support materials such as BIOPAC (ENVICON, Germany) have been used successfully, especially where nitrifying bacteria with lower growth rates must be immobilized (Fig..3).

In contrast to fixed beds with solid particles, the flow of water and air are more easily controlled and blockages can be avoided in reactors with suspended particles.

In contrast to trickling filters, their air flow rates can be adjusted to match the loading of organics and ammonia. The specific surface area can be increased to up to 400 m2 m–3 (Schulz and Menningmann 1999). Using membrane-type tubular aerators, fine bubbles are produced and the mass transfer rate is increased remarkably.

The suspended biological sludge detaches from the surfaces as a result of the friction forces of the flow and is   conveyed to the secondary settler. Obviously, blockages do not occur.

Fig. 3 Submerged aerated fixed bed reactor

(a) and BIOPAC (b)(ENVIRON, Germany; Schulz and Menningmann 1999).

Rotating Disc Reactors

In rotating disc reactors (RDR), the principle behind the intense transport of substrates and oxygen to the biofilm is different. In trickling filters and fixed bed reactors, water and air are moved; here, the support material with the biofilm are moved. In rotating disc reactors, circular plates with diameters of 1–2 m are fitted to a horizontal shaft with a spacing of a few centimeters. The system of parallel plates is submerged nearly halfway in a cylindrical tank through which wastewater flows. The packet of plates rotates at a speed of 0.5–5.0 rpm. Bacteria grow on both surfaces of the circular discs. During the portion of the rotation where the biofilm travels through the air, wastewater drips down and oxygen is taken up by convection and diffusion. Parts of the biofilm rinse off from the discs from time to time.

Larger pieces settle in the tank and must be removed as surplus sludge, while smaller parts are suspended and involved in aerobic substrate degradation and further growth (carbon removal and nitrification).

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science

References

  • Marshall, K.C.; Blainey, B. 1991, Role of bacterial adhesion in biofilm formation and bio corrosion, in: Biofouling and Biocorrosion in Industrial Water Systems, ed. Flemming, H.-C.; Geesey, G.G., Springer-Verlag, Heidelberg, p. 8–45.
  • Metcalf, Eddy 1991, Wastewater Engineering: Treatment, Disposal, And Reuse, 3rd edn, McGraw-Hill, New York.
  • Meyer-Reil, L.-A. 1996, ضkologie mikrobieller Biofilme, in: ضkologie der Abwasserorganismen, ed. Lemmer, H.; Griebe, T.; Flemming, H.-C., Springer-Verlag, Berlin, p. 24–42.
  • Wingender, J.; Flemming, H.-C. 1999, Autoaggregation of microorganisms: flocs and biofilms, in: Environmental Processes I, (Biotechnology, Vol. 11a), ed. J. Winter, Wiley-VCH, Weinheim, p. 65–83.