LITERATURE REVIEW CHAPTER Introduction. 2.2 Substrate removal in attached growth treatment process

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1 LITERATURE REVIEW 2.1 Introduction CHAPTER 02 In attached growth processes the micro organisms responsible for the conversion of organic material or nutrients are attached to an inert packing material. The organic materials and nutrients are removed from the waste water flowing past the attached growth also known as a biofilm. Attached growth process can be operated as aerobic or anaerobic process. Excess biomass sloughs from the attached growth periodically and clarification is required for liquid solid separation to provide an effluent with an acceptable suspended solids concentration. During the last few years attention has began to be paid to the use of submerged media beds as opposed to trickling filters or percolating beds. In submerged contact beds, the direction of waste water flow may be upwards or downwards. The type and size of packing is a major factor that affects the performance and operating characteristics of submerged attached growth processes. The major advantage of submerged attached growth processes are their relatively small space requirement, the ability to effectively treat dilute waste waters, less sludge issues and aesthetics. Their disadvantages include a more Cbmplex systems in terms of media arrangement, limitations of economies of scale for application to larger facilities, and generally higher capital cost than activated sludge systems 2.2 Substrate removal in attached growth treatment process In an attached growth treatment process, a biofilm consists of microorganism, particulate material, and extracellular polymers are attached and cover the support packing material. The growth and substrate utilization kinetics described for the suspended growth process were related to the dissolved substrate concentration in the bulk liquid. For attached growth processes, substrate is consumed within a biofilm. Depending on the growth conditions and the hydrodynamics of the system, the biofilm thickness may range from I OOum to I 0 mm. A stagnant liquid layer (diffusion layer) separates the biofilm from the bulk liquid that is flowing over the surface of the biofilm or is mixed outside of the fixed film. Substrates, oxygen, and nutrients 12

2 diffuse across the stagnant liquid layer to the biofilm, and products of biodegradation from the biofilm enter the bulk liquid after diftusion across the stagnant film. Bulk l1quid Filter / Stagnant packing. liquid film B1omass layer (Source: MetCalfand Eddy, 2003) Liquid layer Fig 2.1- Configuration ofbio film The substrate concentration at the surface of the biofilm, decreases with biofilm depth as the substrate is consumed and diftuses into the biolilm layers. As a result. the process is said to be diffusion limited. The substrate and oxygen concentration within the film are lower than the bulk liquid concentration and change with biofilm depth and the substrate utilization rate. The overall substrate utilization rate is less than would be predicted based on the bulk liquid substrate concentration. The biofilm layer is not simply a planar surface as depicted on the above figure. The biofilm layers are in fact very complex non uniform structures with uneven protrusions much like peaks and valleys, and are believed to have vertical and horizontal pores through which liquid flows. The biomass can be very dense in biofilms, and may also vary in density and depth. Biofilm YSS concentrations may range from 40 to loog/l. Uniform growth across the support packing also does not occur, because of periodic sloughing, as well as the hydrodynamics and media configuration (Hinton and Stensel, 1991 ) II ow

3 Mechanistic models have been developed by a number of investigators to describe mass transfer and biological substrate utilization kinetics in biofilms and provide useful tools for the evaluation of biofilm processes. However, because of the complexity of attached growth reactors and the inability to define accurately the physical parameters and model coefficients, empirical relationships, based on observed performance, are used for design. (MetCalf and Eddy, 2003) Attached growth process can be operated as aerobic or anaerobic process known as (a) Aerobic attached growth process (b) Anaerobic attached growth process 2.3 Aerobic attached growth process A wide variety of microorganisms are found in aerobic attached growth treatment process used for removal of organic matters. Aerobic heterotrophic bacteria found in these processes are able to produce extracellular biopolymers that result in the formation of bio film that can be separated from the treated liquid by gravity settling. Protozoa also play an important role in aerobic biological process by consuming free bacteria and colloidal particles. Protozoa requires longer retention time (SRT) than aerobic heterotrophic bacteria. Rotifers as well as nematodes and other multicelluler microorganisms can also be found in bio film. Aefobic attached growth process depending on the bio film thickness, generally have much more complex microbial ecology. Excess sludge production of about kg per kg of BOD applied was reported (Clark et al., 1978). The sludge production is lower than the conventional activated sludge systems. Excess sludge has also been shown to exhibit good settling characteristics. Zone settling occurs and settling is completed within very short time. Aerobic attached growth process can be grouped into three general classes named as non submerged attached growth, Suspended growth with fixed film packing and submerged attached growth. Since this study is on submerged attached growth processes, literature review is performed only for the submerged attached growth process. 14..

4 2.3.1 Submerged Attached Growth Processes Beginning in the 1970s and extending into the 1980s, a new class of aerobic attached growth processes became established alternatives for biological wastewater treatment. These are up flow and down flow packed-bed reactors and fluidized-bed reactors that do not use secondary clarification. Their unique advantage is the small footprint with an area requirement that is a fraction (one-fifth to one-third) of that needed for activated-sludge treatment. Though they are more compact, their capital costs are generally higher than that for activated-sludge treatment. In addition to BOD removal, submerged attached growth processes have also been used for tertiary nitrification and denitrification following suspended or attached growth nitrification. However in recent past various modifications improvements for submerged attached growth processes have been introduced to improve the performance. Mass Transfer Limitations A significant process feature of attached growth processes in contrast to activatedsludge treatment is the fact that the performance of biofilm processes is often diffusion- limited. Substrate removal and electron donor utilization occur within the depth of the attached growth biofilm and subsequently the overall removal rates are a function of diffusion rates and the electron donor and electron acceptor concentrations at various locations in the biofilm. By comparis(ij1, the process kinetics for the activated-sludge process is generally characterized by the bulk liquid concentrations. The diffusion-limited concept is especially important when considering the measurable bulk liquid DO concentrations on attached growth process biological reaction rates. Where a DO concentration of 2 to 3mg/L is generally considered satisfactory for most suspended growth aerobic processes, such low DO concentrations can be limiting for attached growth processes. For uninhibited nitrification in the biofilm a much higher DO concentration may be required, depending on the ammonia concentration. The concept of diffusion limitations on nitrification rates and the ability to develop anaerobic layers within the biofilm may be exploited to accomplish both nitrification and denitrification in attached growth processes with positive bulk liquid DO concentrations. 15.

5 2.4 Anaerobic attached growth process Anaerobic technologies have been successfully used for well over a century in the field of wastewater treatment. The application of anaerobic processes for the treatment of industrial and municipal wastewaters has grown along 20 years as an emerging practical alternative to aerobic treatment techniques. Anaerobic treatment is the use of biological processes in an environment devoid of molecular Oxygen to stabilize organic material by conversion into simpler compounds. Anaerobic processes have several predominant advantages over aerobic treatment techniques. Treatment in an anaerobic reactor reduces a major part of carbonaceous oxygen demand from water with lower energy input as well as with an energy recovery from wastewater in the form of biogas comprising 50-60% of methane. Lower nutrient requirements, ability to withstand higher organic loadings and higher concentrations of wastewater arc among other comparative advantages. Moreover sludge production and subsequent disposal problems are low in comparison with sludge produced from aerobic reactions such as activated sludge and Rotating Biological contactor systems. Recently anaerobic treatment technologies have been given more attention due to the energy crisis in the seventies and implementation of stringent legislative requirements by governments for releasing high strength industr~l eftluents into municipal sewer networks even. Various novel techniques were developed and established in industrial scale during last few decades with reduced reactor volumes and lower hydraulic retention times. Enhancement of the knowledge on process microbiology and kinetics on microbial growth through res6arch work has encouraged the use of anaerobic wastewater treatment technologies for low strength wastewaters as well as at lower temperatures and lower ph and alkalinity levels. Anaerobic reactors present a unique ecosystem in which a diverse group of bacteria carry out the whole process of converting complex organic compounds to methane, carbon dioxide and other products. In a reactor several interdependent reactions are take place to complete biomethanation process. Four main stages of the anaerobic digestion have been recognized as illustrated below in Fig 2.2 (Mara and Horan, 2003). 16 '

6 COMPLEX ORGANIC MATIERS Carbohydrates INTERMEDIARY PRODUCTS / Mothoo I / Step I - Hydrolysis Step 3 - Acitogenesis Step 2- Acidogenesis Step 4 - Methanogenesis Fig 2.2- Four main stages of Anaerobic digestion process Hydrolysis and fermentation Complex organic molecules such as proteins, carbohydrates, lipids are depolymerised in to simpler soluble compounds such as sugars, amino acids and short chain fatty acids with the addition of hydroxyl groups in hydrolysis. The hydrolyzed compounds are further broken down into simpler molecules producing propionate, butyrate and 17...

7 other fatty acids and ammonia, Carbon dioxide and hydrogen sulphide as by-products by fermentative bacteria. Acetogenesis Acetogenesis is the next step of anaerobic process, carried out by three main groups of bacteria. Homoacetogens, syntrophes and sulforeductors are these three groups. Homoacetogenic group of bacteria uses H2 and C02 for their metabolic activities and produce acetate. The intermediary metabolites produced in earlier step are metabolized into acetate, hydrogen and carbon dioxide by syntropic acetogens. Methanogenesis Methanogenesis, the final step of anaerobic respiration is the predominant step of anaerobic digestion of organic matter. The terminal electron acceptor in methanogenesis is Carbon. Methanogenesis utilize C02 and acetic acid as terminal electron acceptors and use carbon from other small organic compounds such as formic acid, methanol and methylamines. Methanogens are obligate anaerobes belonging to the group of archaebacteria. Most methanogens are very specialized for their growth substrate and environmental conditions and can be classified accordingly into acetotrophic methanogens, which disproportionate acetate into methane and carbqp dioxide, and unicarbanotropic methanogens, which oxidize hydrogen gas, methanol or formate and methylamines as electron donors and reduce carbon dioxide and activated methyl group of methane. Rational of the use of anaerobic treatment process can be explained by considering the advantages and disadvantages of the process. Advantages of anaerobic process over the aerobic process Less energy required Less biological sludge production, energetic of the process results in lower. biomass production by a factor of about 6-8 times, sludge processing and disposal costs are reduced. Fewer nutrient requirement 18

8 UBRARY UlfVERSITY OF MOitATUWA.... MORATUWA Methane production from high strength effluent, possible energy source Higher volumetric organic loading can be applied, smaller reactor volume Rapid response to substrate addition after long period without feeding Disadvantages of anaerobic process over the aerobic process Longer startup time to develop necessary bio mass inventory May require alkalinity addition Require further treatment with an aerobic treatment to meet the discharge requirement Biological nitrogen, phosphorous removal is not possible ' Much more sensitive to the adverse effect on low temperature on reaction rates Potential for production of odors and corrosive gasses Types of attached growth anaerobic reactors are as follows (a) Up flow packed bed attached growth reactor (b) Up flow attached growth expanded bed reactor (c) Attached growth anaerobic fluidized bed reactor (d) Down flow attached growth processes The type of anaerobic reactor used in this stutly is a combination of anaerobic up flow and down flow attached growth system. Therefore the literature review is more focused on the up flow and down flow anaerobic processes. In up flow packed bed attached growth reactors the most common packing materials are corrugated plastic cross-flow or tubular modules and plastic rings. A large portion of the biomass responsible for treatment in the up flow attached growth anaerobic processes is loosely held in the packing void spaces and not attached to the packing material. Advantages of up flow attached growth anaerobic reactors are high COO loading, relatively small reactor volume, and operational simplicity. The main 19 S4856 "> *

9 limitations are the cost of packing materials and operational and maintenance issues associated with solid accumulations and possible packing plugging. The process is best suited for the waste water with low solid concentrations. Anaerobic down flow attached growth reactors are generally used for easily degradable waste water. The major advantages of down flow attached growth processes over up-flow systems are usability of higher void space packing material no plugging problems and simple in operation. (MetCalf and Eddy, 2003) Lower bio mass yield in anaerobic process Because of the energetic of anaerobic process, result in a lower biomass production than the aerobic process. Anaerobic metabolism is a less efficient eoergy yielding process than respiratory metabolism. Heterotrophic organisms. that are strictly fermentative (anaerobic bacteria) are characterized by lower growth rate and lower cell yield than respiratory heterotropes (Metcalf & Eddy, 2003, 4th ed.). The fact that the less sludge is produced in anaerobic process is a significant advantage over the aerobic process