1. Introduction:Aneorobic digestion is a biochemical process that organic raw materials are degraded to methane, biogas, new biomass, and mixture of carbon dioxide with existence of other constituents by bacteria in the lack of oxygen. Anaerobic digestion is used to stabilize wastewater sludge and for treatment of industrial and domestic wastewaters. In this process, organics in the mixture of waste activated sludge and primary settled sludge are converted to new cells, oxidized materials, energy, and some gaseous end products (like CH4 , CO2) under anaerobic conditions. Microorganisms are used organic materials as a food source. (Metcalf & Eddy, 2003) The main aim of anaerobic digestion is to transform as much as of the sludge to end products that are liquids and gases, and the other aim is to produce very little residual biomass. Anaerobic digestion process has some advantages. They are; The process is very economic because there is no oxygen need. Operating costs are minimized as much as possible. The amount of producing stabilized sludge is low. Drying this less volume is easy. At the end of the reactions, the methane is produced. Methane is a useful biogas and potential and alternative source of fuel (green solution). High strength industrial wastewaters can be digested. Digester has a resistance to high loading rates. Anaerobic digesters can degrade xenobiotic compounds. The treatment efficiency is high.This process also has some disadvantages. They are followings; Anaerobic digestion is a slower process. It is susceptible to toxic compounds. In the digester, contaminants and heavy metals can be gathered. Temperature range is limited. Maintaining the stability of the process is hard task.(Bitton, 1994)The design of digesters is important. Digester should have suitable conditions for anaerobic microorganisms especially methane producing microbes to grow and for organics can be digested methane, carbon dioxide. The figure shows the general mechanism of the process.

Sludge insoluble organic material

Extracellular enzymesSoluble organic material

Acid producing bacteriaVolatile acids + CO2 + H2 + Other products + Bacterial cells

Bacterial cells

CH4 + CO2 + Endogenous metabolism to end products

Figure 1.1: The mechanism of anaerobic sludge digestion 2. Fundamental Principles:The anaerobic digestion of waste activated sludge process is formed complex biological processes which are hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

Figure 2.1: Subsequent steps in the anaerobic digestion process (Appels et al., 2008)2.1. Hydrolysis: In this step, insoluble organic compounds and large polymers like proteins, polysaccharides, and nucleic acids are reduced to soluble and lower organic compounds such as fatty acids, amino acids. Bonds of insoluble organic substances are broken down by the help of extracellular enzymes. 2.2. Acidogenesis: (or fermentation): In this step, volatile fatty acids, alcohols, carbon dioxide, and hydrogen are produced by the help of fermentative bacteria.2.3. Acetogenesis: In this third step, the products of acidogenesis volatile fatty acids and alcohols are converted to acetic acid, CO2 and H2 with the help of acetogens.2.4. Methanogenesis: In the last step, methanogenic bacteria transform acetic acid, carbon dioxide, and hydrogen to methane and carbon dioxide. (Appels et al., 2008)3. Parameters of Anaerobic Digestion:During anaerobic sludge digestion produce, maintaining of anaerobic condition is essential so that four step can be achieved successfully.3.1. pH: Methanogens work very well at pH range 6.7-7.4. Process is negatively affected if the pH approaches 6.0. In the digester, acidogenic bacteria generate high amounts of acids which cause the decrease in pH. Methanogens can buffer the pH by generating bicarbonate if the process is work smoothly. If there is a problem in the process, methane production will not be observed due to low pH. For this reason, alkalinity and total volatile acids are important parameters to examine the effect of pH. The ratio of total volatile acids to alkalinity should be below 0.1. Solution for equalize the pH of process can be adding alkalinity (lime, sodium hydroxide). Moreover, CO2 gaseous concentration is controlled to balance the pH. (Bitton, 1994)3.2. Temperature: Temperature is an important parameter because it affects the growth of microorganisms in the reactor. Especially, some subgroups of methanogens are negatively impressed very little changes in temperature. In addition to this, temperature has influences on the partial pressures of gaseous in the reactor. According to type of the digesters, the operating temperature should be stabilized. For mesophilic digesters, optimum temperature range is 30-35 oC. However, for thermophilic digesters, optimum temperature range is 50 oC-65 oC. (Appels et al., 2008), (Bitton, 1994)3.3. SRT and HRT: SRT is an important and basic parameter for designing and operating the anaerobic processes. Solid retention time means average existence time of solids in the digester. Hydraulic retention time means average existence time of liquid sludge in the digester. SRT influences the steps of anaerobic sludge digestion process exactly. As SRT reduces, the reactions cannot be completed. To determine optimum value of SRT, the experiments are conducted. According to experimental results, if SRT is higher than 10 days, all reactions are completed and the sludge is stabilized. (Appels et al., 2008)

3.4. Chemical compounds in wastewater: In the process, microorganisms reduced complex substances, lipids, proteins and carbohydrates to generate methane. However, microorganisms cannot degrade some compounds like lignin and n-paraffins. Nitrogen, sulfur, and phosphorus concentrations in the wastewater should be enough for microorganisms to give reactions. Furthermore, the concentrations of trace elements that are cobalt, nickel, iron should be adequate. Concentrations of them should be controlled and standards and regulations should be followed. (Bitton, 1994)

3.5. Toxicity: Toxic substances have negative effects on the growth of microorganisms. This will cause the failure of the process. High amounts of volatile acids can have toxic effects on bacteria if it is not controlled. Ammonia coming from reducing of proteins can have very toxic effects on methanogens. The toxic effect of un-ionized ammonia can be controlled by adjusting the pH value as neutral. In addition to these, sulfide can be very toxic to microbes. It is formed from degradation of proteins. It can be controlled addition of iron (it will precipitate) and adjusting the pH. Also, heavy metals can be toxic even if they are at very low concentrations. They can precipitate with the presence of sulfide. (Bitton, 1994)

Figure 3.1:Conditions for Sludge Digestion

TemperatureOptimumGeneral Range98F85-95F

pHOptimumGeneral Range7.0-7.16.7-7.4

Gas productionPer pound volatile solids addedPer pound volatile solids destroyed8-12 cu. ft.16-18 cu. ft.

Gas CompositionMethaneCarbon DioxideHydrogen Sulfide65-69%31-35%Trace

Volatile Acid Concentration As Ascetic AcidNormalMaximum200-800 mg/L2000 mg/L

Alkalinity Concentration As Calcium CarbonateNormal2000-3500 mg/L

(http://water.me.vccs.edu/courses/env108/anaerobic.htm4. Types of Anaerobic Sludge Digesters:4.1. Standard rate anaerobic digester: (or low-rate): This type of digester is the simplest and oldest type of the anaerobic digestion process. It has a long detention time, 30-60 days. The shape of the tank is cylindrical and tank has a sloping bottom. The volume of the digester should be large due to long detention time. The coming waste activated sludge is not mixed during the digestion process. The external heating may or may not be supplied. During the process, microbes produce biogas and biogas rises to top of the digester. This provides some mixing in the digester. In the digester, four layers are observed. They are a scum layer, a supernatant or liquid layer, an actively digesting sludge layer, and a digested and stabilized sludge layer. Stabilization of sludge is result of this stratification. The collected stabilized sludge is taken from at the bottom of the tank regularly. Collected gas is removed from at the top of the digester. Supernatant is extracted and returned to the wastewater treatment plant as an influent. In recent years, standard rate anaerobic digester is not preferred. However, in some small wastewater treatment plants, it can be used. (Appels et al., 2008)Figure 4.1: Standard rate anaerobic digester (Bitton, 1994)

4.2. High rate digester: This type of digester is formed by improving disadvantages of standard rate digester. In this digester, the coming activated waste sludge is thickened. The inlet sludge is supplied continuously. In the digester, there is a complete mixing. Also, the external heat is provided by heat exchangers. There conditions are supplied so that there can be a uniform environment in the digester. Hence, the tank volume is decreased and the efficiency and stabilization of process are increased with the help of these recoveries. A uniform environment in the tank presents a steady state conditions and it will prevent microbes from shock loads.

Figure 4.2: High rate digester (Appels et al., 2008)

4.3. Two stage anaerobic digester: In this type, one more tank is added near to high rate digester. This second tank is put to stock the digested and stabilized sludge thus; it provides sludge-liquid separation. In the second tank, heating and mixing are not applied. The digesters can be built with floating covers or fixed roof. If the second tank is designed with floating cover, the tank can also stock the digester gas. In general, the first-stage digester is designed with fixed cover and the second-stage digester is designed with floating type cover. In the second tank, sludge digestion and gas production are not observed usually. Two stage anaerobic digesters were preferred in the past however, in today they are not used. (Appels et al., 2008)

Figure 4.3: Two stage high rate anaerobic digester (http://water.me.vccs.edu/courses/env108/anaerobic.htm)

4.4. Mesophilic and thermophilic digestion: Mesophilic digestion shows the temperature range of the process. In this type digestion, the temperature is set to 30-38oC. In this range, mesophilic bacteria survive. And, in most high rate digesters, mesophilic digestion is operated. On the other hand, high temperatures can be preferred to operate the system. Thermophilic bacteria can grow at temperature range 50-57oC. At this range, thermophilic digestion is operated. Thermophilic digestion provides faster reactions than mesophilic digestion. Also, the amount of digested solid is high. Because of high temperature, dewatering is applied successfully. In addition to this, pathogen microbes cannot grow at high temperature. However, thermophilic digestion has some disadvantages. Providing high temperature is increased the energy requirement and cost. The quality of supernatant is not desirable. Supernatant contains large amounts of dissolved substances. Also, during the process, there is a high odor problem. Process stability cannot be achieved successfully because thermophilic bacteria are easily affected changes in temperature. (Appels et al., 2008)5. Design Considerations of Anaerobic Digesters:To build an anaerobic sludge digester, some important parameters are calculated. The type of the digester is determined. Design should be depend on biochemistry and microbiology of the structure. And then, the parameters which are solids retention time, hydraulic retention time, energy recovery, organic loading rate, population basis, chemical addition, gas production, tank configuration and volume, mixing systems, and heating systems are determined by observing whole treatment system. The figure shows some standards for designing a digester.

Figure 5.1: Design parameters for low rate and high rate anaerobic digesters (Reynolds, 1996)6. Pre-treatment:Hydrolysis step is the important step of anaerobic digestion of waste activated sludge. It is the rate limiting step. In hydrolysis, insoluble organic substances become soluble organic compounds by breaking down cell walls using extracellular enzymes. Organics inside and outside the cell are delivered to liquid phase in the sludge. Resulting of this process is decomposition of sludge. Pretreatment processes are applied so that hydrolysis step can be completed easily and effectively. Four groups of pretreatments can be applied. They are mechanical, thermal, chemical and biological pretreatments. Pretreatments help to increase disintegrating rate of sludge and stabilization rate of sludge. (Navaneethen, 2007)6.1. Thermal pre-treatment: The heat pretreatment of waste activated sludge is very useful treatment type. The very high temperatures are supplied to the system (150-200oC). Also, providing pressure to the system is high. High temperature weakens the chemicals bonds of the cell wall hence; organics in the cell become soluble. Usually, thermal pretreatment of waste activated sludge is preferred and used for ease the anaerobic sludge digestion. For low rate system, this pretreatment is lifesaver because no heat is applied to low rate system. In thermophilic and mesophilic digestion, methane generation is raised. The disadvantage of pretreatment of waste activated sludge can be supplying significant amount heat and energy. (Appels et al., 2008)

Figure 6.1: Thermal pretreatments observations (http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322009000300003)6.2. Mechanical pre-treatment: The aim of this pretreatment is to damage the cell walls physically. Some different devices end methods are used. High speed shaker ball mill, grinding glass beads or agitator ball mill are examples of devices. These devices are used to increase shear stress on cell therefore, cell walls are crashed. Generally used method is high pressure homogenization. This method is compacted the sludge with high amounts of pressure hence, the result is decomposing cells. In mechanical pretreatment of waste activated sludge, there is no need to heat and chemicals. However, its efficiency is lower than compared to other pretreatments methods. (Appels et al., 2008)6.3. Chemical pre-treatment: To break the cell walls, several chemical pretreatment methods are improved. Acid and alkaline hydrolysis: (thermochemical hydrolysis): In this method, acid or base is enhanced to system to achieve disintegration of solids. The system reaches high temperature levels because of acid or base. The disadvantage of this method is extreme pH values. pH should be controlled and neutralized after the pretreatment which is a hard task.Oxidative sludge pretreatment: (ozonation): In this method, oxygen or air is provided to the system with high pressures and temperatures. Generally, ozone is preferred to damage cell wall in waste activated sludge. Ozone is an important oxidant and disinfectant which is used to destroy pathogens in water. The disadvantages of this method can be corrosion and odor problem and also system requires high amount of energy. (Appels et al., 2008)6.4. Biological pre-treatment: This method consists of adding specific type of bacteria and extracellular enzymes. With addition of bacteria, sludge is solubilized anaerobically by the help of enzyme reactions. For this method, suitable temperature range should be chosen. (Appels et al., 2008)6.5. Ultrasound: Ultrasound pretreatment method of waste activated sludge is used some range of ultrasound frequencies. The aim of this method is to form cavitation in the sludge. Low frequencies of ultrasound cause cavitation. The pressure of liquid phase decreases and its value becomes smaller than vapor pressure. This results explosive small bubbles. Therefore, explosive bubbles can break the bonds of cell wall easily. Ultrasound pretreatments are used widely in wastewater treatment plants. It can treat high amounts of waste activated sludge and also, it is working with little economic cost. (Appels et al., 2008)

Figure 6.2: Ultrasonic pretreatment studies (Appels et al., 2008)7. Biogas and Stabilized Digestate:When all reactions in the anaerobic digester are completed, biogas and digestate are produced. Using and managing these products are important task. Biogas is composed of 50-60 % methane, carbon dioxide, and other trace gases. This biogas is used to generate electricity. First of all, ammonia, water vapor, and hydrogen sulfide are removed from gas to protect the equipment. Remaining gas mixture which is the mixture of methane and carbon dioxide can be operated as a fuel. 80-150 m3 biogases are generated from one ton of waste activated sludge. In some small treatment plants, produced biogas is used to supply heat requirement. This heat is given to anaerobic digester in the plant for maintaining the process successfully. In larger treatment plants, the produces electricity is used in cogeneration equipment. This results decrease in electricity bill. Advantages of production electricity from biomass are very much. Some of them are that this type of energy is renewable energy and it can also be used when there is black-out. The stabilized sludge that is obtained from the digester is like a milkshake. It consist too much water in it. Dewatering should be applied to sludge. Moreover, the digestate smells bad and for this reason ventilating system should be found in the treatment plant. Digestate can be treated to use as a fertilizer. (Ostrem, 2004)8. Conclusion:Sludge digestion is an important issue because every treatment plant produces some types of sludge. It cannot be thrown to environment before taking precautions. Anaerobic digestion of waste activated sludge is a good topic that shows how sludge can be used effectively? Anaerobic digestion has several advantages. It is widely used in wastewater treatment plants. Operating of the anaerobic process may be a little bit difficult. However, its productions are environmental friendly and renewable. It has different types of treatments, pre or post. This makes the process flexible. It is a microbiologic process and several researches are conducting on it to improve.9. References:1. Appels, L., Bayens, J., Degreve, J., & Dewil, R. (2008). Principles and Potential of the Anaerobic Digestion of Waste Activated Sludge. Progress in Energy and Combustion Science, 34(2008), 755-7812. Bitton, G. (1994). Wastewater Microbiology. John Wiley Sons3. Metcalf & Eddy Inc. (2003). Wastewater Engineering: Treatment, Disposal and Reuse. McGraw- Hill Companies, Inc.4. Navaneethan, N. (2007). Anaerobic Digestion of Waste Activated Sludge with Ultrasonic Pretreatment (Unpublished masters thesis). Asian Institute of Technology5. Ostrem, K. (2004). Greening Waste: Anaerobic Digestion for Treating the Organic Fraction of Municipal Solid Wastes (Unpublished masters thesis). Columbia University

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