18 Anaerobic digesters and Up flow Anaerobic Sludge Blanket (UASB) Reactor

Meenakshi Nandal

Objectives

• To Understand the process of anaerobic digestion and the role of microbes in this process To Explain various types, uses and importance of anaerobic digesters
• To Understand the anaerobic and Up flow Anaerobic Sludge Blanket (UASB) reactor process
• To Explain various advantages & disadvantages of UASB technology and the factors which affect the efficiency of UASB reactor

Introduction:

Anaerobic digester is a promising technology which provide environment and health benefits include reduction of greenhouse gas, organic waste, odor and pathogen destruction. It is an eco-friendly technology for producing biogas which can be used for heating, mechanical energy, generating electricity and for supplementing the natural gas supply. In developing countries, small-scale anaerobic digesters are used to meet the heating and cooking needs.

19.1 Anaerobic digestion:

It is a biologically motivated process in which microorganisms decompose biodegradable material in the oxygen free environment. The product so obtained is called biogas which is used as a fuel, to generate electricity, heat and it can also be processed into transportation fuels and renewable natural gas. Various anaerobic digestion technologies are used for converting municipal wastewater solids, food waste, livestock manure, high strength industrial wastewater and residuals, fats, oils and grease and various other organic waste streams into biogas. Separated digested solids can be composted, utilized for dairy bedding, directly applied to cropland or converted into other products. Nutrients in the liquid stream are used in agriculture as fertilizer.

19.2 Role of microbes in anaerobic process:

The  microbial  digestion  is   four  step  process,  namely,  hydrolysis, acidogenesis, acetogenesis  and methanogenesis . The details of each step are given below:

a) Hydrolysis: This step is the slowest step among all the four processes. In this step, complex organic materials (proteins, carbohydrates and lipids & fats) are converted into simpler forms such as amino acids, monosaccharides and fatty acids respectively by bacteria. The extracellular enzyme converts higher mass organic molecules into basic structural building blocks is very important as particulate organic materials are simply too large to be  Source: Yasar and Tabinda, 2010 directly absorbed and used by microorganisms as substrate/ food source.

b) Acidogenesis: In this step, acidogenic bacteria convert the soluble organic monomers of sugars and amino acids to ethanol and acids (such as propionic and butyric acid), acetate, hydrogen and carbon dioxide. The breakdown of amino acids also leads to production of ammonia.

c) Acetogenesis: In this step, long chain fatty acids & volatile fatty acids and alcohols are converted by acetogenic bacteria into hydrogen, acetic acid and carbon dioxide. During this step, BOD and COD are reduced and the pH is decreased.

d) Methanogenesis: During this final step, methanogenic bacteria convert hydrogen and acetic acid to methane gas and carbon dioxide. Methanogenesis process is affected by several conditions in the digester such as organic loading rate, feed composition and temperature. The gaseous product, biogas, consists mainly of methane and carbon dioxide, but also contains several other gaseous “impurities” such as hydrogen sulphide, nitrogen, oxygen and hydrogen. Biogas with methane content higher than 45% is flammable.

19.3 Types of Anaerobic digesters:

There are various technologies which are used now-a-days to convert waste into useful materials. The various technologies are discussed below:

• 1) Fixed-dome digester: A fixed-dome plant is composed of a closed dome shape digester with an immovable, rigid gas-holder, a feedstock inlet, and a displacement pit, compensation tank. The gas evolved in the digester is stored in the upper part of the reactor.

Fig: 18.3 Scheme of fixed-dome digester Source: www.funscience.in/images/StudyZone

With an outlet gas valve, increasing gas production increases the gas pressure inside the digester thereby forcing the digestate into the compensation tank. When the gas valve is open for gas use, gas pressure lowers and a proportional amount of slurry flows back from the compensation tank into the digester. In this design, gas pressure varies depends on gas production and use. Usually, such plant is constructed underground, so that digester can be protected from low temperatures at night and during cold seasons. Surrounding soil, up to the top of the gas-filled space, counteracts the internal pressure in the digester.

• 2) Floating-drum digester: A floating-drum biogas plant composed of a cylindrical digester and a movable, floating gasholder. The digester is generally constructed underground the floating gasholder is above ground. Smaller household-scale systems may also be fully above ground. The digesting section of the reactor is generally constructed of bricks, concrete or quarry-stone masonry and then plastered. The gas-holder is made up of metal and coated with oil paints, synthetic paints or bitumen paints to protect it from corrosion. Regular de-rusting is usually essential to ensure sustained use and the cover coating should be applied annually. A well-sustained metal gas-holder can be supposed to last from 3-5 years in humid areas and 8-12 years in a dry climate. A suitable alternative to standard grades of steel are fiberglass reinforced plastic or galvanized sheet metal. The gas produced can be accumulated in the gas drum which rises or falls again, depending on the amount of gas produced and used.

• 3) Tubular digester: The tubular biogas plant composed of a longitudinal shaped heat-sealed, weather resistant plastic or rubber bag (balloon) that can be as digester and gas holder in one. The gas is stored in the upper part of the balloon. The inlet and outlet are attached directly to the balloon. Due to of the longitudinal shape, no short-circuiting occurs, but as tubular digesters do not have stirring device, active mixing is limited and digestate flows through the reactor in a plug-flow manner. Gas pressure can be increased by putting weights on the balloon while taking care not to damage it.

The benefits are that that their construction cost is very low. Also, shallow below ground installation makes them appropriate for use in areas with a high groundwater table. But the plastic balloon is quite breakable and vulnerable to mechanical damage and a very short life of 2-5 years.

• 4) Garage-type digester: In contrast to the above mentioned reactors, the garage- type digester is operated in batch-mode with dry digestion process. The organic waste stream is digested in batch mode into a simple garage-like digester having an airtight door. The material does not require to be transported once the door is closed.

The term “dry digestion” can be misleading in this process because water plays a crucial role. The bacteria require a wet environment involved in the anaerobic process and are only active in the liquid phase of the substrate. So, water should be added for proper functioning of the digester from time to time.

19.4 Uses of Anaerobic Digesters:

Agriculture: In agriculture, animal and crop wastes are typically used as a feedstock for anaerobic digesters and energy produced can be used in homes.

Industrial: Organic waste generally from industrial processes, especially waste from the food processing industry, can be used as a feedstock for an anaerobic digester. Food waste has 15 times the methane production potential that dairy cattle manure does, so it makes an excellent feedstock. During the process co-digestion, Food waste substrates may also be combined with manure to improve methane generation. Biogas produced is generally used for heat or other energy production.

Wastewater treatment plants: Wastewater treatment facilities employ anaerobic digesters to digest sewage sludge and eliminate pathogens in wastewater. Often, biogas is collected from digesters and used to heat nearby facilities. Some municipalities have even begun to divert waste from landfills to wastewater treatment plant; this relieves waste burdens placed on local landfills and allows for energy production.

Anaerobic digesters make several contributions to climate change mitigation. Digesters collect biogas or landfill gas that would have been emitted anyway because of the nature of organic waste management at the facility where the digester is in operation. Another benefit of anaerobic digesters is the displacement of fossil fuel-based energy that occurs when biogas is used to produce heat or electricity.

Fig: 19.6 Scheme of a garage-type dry digestion plant Source: www.bekon.eu/wp-content/uploads/2015

UPFLOW ANAEROBIC SLUDGE BLANKET (UASB) REACTOR

Anaerobic digestion is biological breakdown of degradable material in absence of oxygen and comprises of biological phases in which microorganisms break in the absence of oxygen. It is a widely used process used in the treatment of wastewater and organic waste as it provides a significant reduction of the input material. There are seven sub-processes in the process of anaerobic digestion of organic polymeric materials. Firstly complex organic materials are hydrolyzed, secondly amino acids and sugars are fermented and thirdly long chain fatty acids and alcohols are oxidized. In the fourth stage, the anaerobic oxidation of short-chain fatty acids (except acetate) takes place, in the fifth stage, acetate is produced from carbon dioxide and hydrogen, and in the sixth stage the acetate is converted into methane. Finally, methane is produced by reduction of carbon dioxide by hydrogen.

19.5 UASB REACTOR AND PROCESS:

The UASB technology was developed by Lettinga and his co-workers in the late 1970’s. Originally the reactor was designed to treat industrial wastewater but later its application was extended to treatment of sewage. Nowadays, the use of UASB reactor is extensively for the treatment of several types of wastewater, forming a part of the high-rate anaerobic technology. The UASB scheme basically consisted of Influent tank, peristaltic pump, cylindrical UASB reactor, Gas/liquid/solid separator, effluent outlet, gas outlet and collection system. Figure 18.7 showing the basic scheme of the UASB reactor.

Fig: 19.7 Anaerobic treatment setup Source: www.iwapublishing.com/sites/default/files/images/5_3.jpg

The UASB reactor works as a suspended growth system in which microorganisms attach themselves against each other or to small particles of suspended matter to form clusters of highly settleable granules which help in formation of an active sludge blanket at the bottom of the reactor. The formation of granules keeps sufficient agitation so that the bed can be inter- mixed. The wastewater is introduced from bottom of the reactor through the peristaltic pump. The wastewater flows upward through a sludge bed basically composed of biologically activated granules. As the waste comes in contact with the granules, treatment occurs. The gas produced during anaerobic digestion helps in internal circulation, formation and maintenance of the granules. Some of the gas particles gets fixed with the biological granules within the sludge blanket which rise to the top of the reactor. The particles that rise collide the bottom of the degassing baffles which liberate attached gas bubbles and the degassed granules fall back to the surface of the sludge blanket. The gases released from the granules recollec in the gas collection domes located in the top of the reactor.

It should also be recognized that besides the formation of sludge granules, erosion takes place in the sludge bed under the influence of the friction forces to which the sludge flocs are exposed in particular at high mixing intensities. The sludge particles would gain sufficient mechanical strength when proper environmental conditions are kept at the start up. Gas re- circulation provides mechanical agitation at the gas-liquid interface in the digester compartment and is useful to prevent an accumulation of biodegradable waste solids in the lower part of the reactor to ensure a good contact between bacteria and the substrate even at low gas production rates or high hydraulic loading rates. Under proper conditioned active anaerobic sludge can be preserved unfed for many months without deterioration.

Residual solids are separated from biological granules within the liquid when passes through settling chamber. The separated solids fall through the baffle system to the top of the sludge blanket.

 

19.6 FACTORS AFFECTING PERFORMANCE OF UASB REACTOR:

There are various factors which affect the working efficiency of the reactors like pH, temperature, nutrients availability, presence of VFA (volatile fatty acids), influent COD concentration, influent type, sludge retention time (SRT), organic and hydraulic load.

Organic Loading Rate: Organic loading rate (OLR) is an important parameter that considerably affects the performance of a UASB reactor. The fluctuations in organic load depends on the sludge properties, mixing intensity, SRT (sludge retention time), HRT (hydraulic retention time), duration of the variation, bacterial mass and activity. Higher values of OLR can cause reduction in COD removal efficiency in a wastewater treatment system, higher loading rate could cause unrecoverable acidification, suppression of the methanogenic activity due to serious imbalance between the methanogens and the acidogens, as well as inhibition of methanogens by VFA production.

Nutrients: The growth of anaerobic microorganisms depends on the availability of the essential nutrients present in the wastewater. Deficiency of these nutrients could negatively affect their growth of microbes and also the efficiency of the anaerobic degradation. Enzymes which are involved in the process of fermentation and CH4 production need different trace elements which need to be supplied as nutrients.

Hydraulic Retention Time: The hydraulic retention time (HRT) is defined as the average time that wastewater spends inside the reactor. HRT is affected by both the flow rate and composition of wastewater entering the UASB reactor. High HRT increases the contact time of wastewater with the sludge and hence improves the effluent quality and biogas production rate. Therefore, a suitable HRT is very important for proper wastewater treatment in a UASB reactor for better treatment efficiency as well as quality and quantity of biogas concentration.

Volatile Fatty Acids: Volatile fatty acids are important intermediate products in the formation of CH4. The right concentration determines the efficiency of substrate removal from the reactor. For a typical reactor, overloading, or sudden variations in HRT and OLR could cause the accumulation of VFA and stressful conditions in the reactor during the break down of complex organic matter. It can also affect the type of intermediates produced. This might cause a shift between acetogens and acidogens population (VFA producers), nitrogen reducing bacteria, sulphate reducing bacteria and methanogens leading to drastic changes in biogas production rates and compositions. Therefore, VFAs should be monitored and parameters adjusted in order to avoid their accumulation in the UASB reactor to prevent the inhibition of methanogenic organisms, thus reducing biogas production.

Operational Temperature: Operational temperature is an important parameter in anaerobic degradation processes. It determines the dominant bacterial flora and the growth rates of microorganisms present in a reactor. Different species of bacteria can survive at different temperature ranges. Operational temperature greatly affects the biodegradation and the biogas production rate of any anaerobic reactor. The temperatures at which anaerobic reactors could operate include psychrophilic (0-25°C), mesophilic (25-40°C) and thermophilic conditions (40-60°C).

Operational pH: pH is a very important parameter for an anaerobic digester as microbes are highly pH dependent and require suitable conditions of pH to grow optimally. pH far below or higher than the required range could cause an accumulation of acetate, thereby inhibiting the methanogens and leading to conversion of COD to volatile acids instead of CH4 production. Therefore, mostly anaerobic digestion reactors have been operated at pH of between 6.5–7.5. The standard operating method to keep the pH in this range has been found to be the addition of lime and bicarbonate salts or by reusing treated effluent in the reactor. Consequently, maintaining the pH of reactor is an essential factor for the growth of diverse group of microorganisms and high reactor performance.

19.7 ADVANTAGES & DISADVANTAGES OF UASB

So, we can say that UASB technology provides a low-cost system for the direct treatment of municipal wastewater. It has low construction costs, small land requirements, low excess sludge production, smooth operation and maintenance, energy generation in the form of biogas and high organic removal efficiency, pH stability and recovery time. Only one discharge of sludge required per year from a UASB reactor. They are widely used to treat wastewater with a high organic loading rates and no inert material is required for the granules so formed inside the UASB reactor. Treated water can be reused for floor washing, road making, construction sites, flushing in toilets, horticulture purposes, car washing and recreational purposes, so it can reduce load on natural water resources. So, it can be applied in small commodities where the wastewater flow variation is high due population load increases or during tourist season to rainy season.

Summary:

In this lecture we learnt about:

• Anaerobic process for waste treatment Role of microbes in anaerobic treatment Types of anaerobic digesters for biowaste
• Uses of anaerobic digesters and their importance Anaerobic process for wastewater treatment
• Upflow Anaerobic Sludge Blanket (UASB) reactor and its technology Factors affecting performance of UASB reactor
• Various advantages and disadvantages of UASB technology

References:

• Mahvi AH, Mesdaghinia A, Karakani F (2004). Feasibility of continuous flow sequencing batch reactor in domestic wastewater treatment. American J App Sci, 1(4): 348-53.
• Mahvi AH (2008). Sequencing batch reactor: a promising technology in wastewater treatment. Iran J Environ Health Sci Eng, 5(2):79-90.
• Metcalf & Eddy Inc (2003). Wastewater Engineering: Treatment and Reuse. 4th ed. McGraw-Hill Inc, New York, pp.: 6-17.

Web links:

1. http://nptel.ac.in/courses/105104102/Lecture%2024.htm
2. http://wss.hry.nic.in/WriteReadData/Notice/Sewage%20Treatment%20Processes.pdf
3. https://www.vssut.ac.in/lecture_notes/lecture1424353637.pdf