Anaerobic Digestion of Wastewater Treatment Plant Biosolids (Sludge)

Transcription

1 Anaerobic Digestion of Wastewater Treatment Plant Biosolids (Sludge) CIVE 1199 Water and Wastewater Treatment Civil, Environmental and Chemical Engineering RMIT 21 September 2007 Prof. Dr. Göksel N. Demirer Department of Environmental Engineering Middle East Technical University Ankara, Turkey

2 Anaerobic Treatment Anaerobic treatment can be defined as the use of microbial organisms, in the absence of molecular oxygen, for the stabilization of organic materials by conversion to methane and some inorganic end products. Organic matter + H 2 O anaerobes CH 4 + CO 2 + NH 3 + H 2 S + New cells Anaerobic treatment of wastes results in conversion of readily biodegradable organic matter into biogas (20-30% CO 2, 60-79% CH 4, 1-2% H 2 S and other gasses) and water. Using anaerobic treatment, it is possible to convert municipal, agricultural and industrial wastes into useful by-products mainly methane which may be used to provide heat or electrical power.

3 BENEFITS OF ANAEROBIC TREATMENT production of usable energy in the form of methane no need for aeration and associated energy costs Low production of stabilized sludge Very low nutrient requirements Little if any energy requirement Reduction of green house gas emissions, up to 4 levels! Very high loading rates (up to 35 kg COD/m 3.day) Plain technology (relatively simple in operation and maintenance) biodegradation of aerobic non-biodegradables such chlorinated organics Anaerobic sludge can be stored unfed (provision of seasonal treatment important especially for campaign industries ) Start up with granular sludge in 1 week

4 WHY ANAEROBIC TREATMENT? Heat Loss 100 kg COD Aeration (100 kwh) AEROBIC Sludge kg Effluent 2-10 kg COD Biogas 35 m 3 or 285 kwh 100 kg COD ANAEROBIC Effluent kg COD Sludge 5 kg

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8 Relative growth rate of psychrophilic, mesophilic and thermophilic methanogens. (From Van Lier et al, 1997).

9 EFFECT OF SOLIDS RETENTION TIME ON PERFORMANCE

10 Advantages Table 1.1. Advantages of anaerobic over aerobic digestion no need for aeration and associated energy costs production of usable energy in the form of methane less biomass production and biomass disposal costs reduction in nitrogen and phosphorous supplementation costs elimination of off-gas air pollution less foaming problems Disadvantages long start-up times insufficient inherent alkalinity generation for some wastewaters heating requirement insufficient effluent quality for surface water discharge potential odor problems longer retention times biodegradation of aerobic nonbiodegradables reduction of installation space requirements provision of seasonal or intermittent treatment slightly higher initial costs

11 Role of AD on GHG Mitigation A Case Study for the Langerwerf (CA, USA) Dairy Waste Management System A 50-year full life cycle analysis (LCA) of GHG emissions was conducted on the AD System operated by the Langerwerf family of Durham, California. The GHG emissions from the AD System were compared with those that would have been released without the use of that system on the dairy farm. The livestock waste management system prior to installation of the AD System was considered the Reference System. The Langerwerf family currently operates a 400-cow dairy farm with a plug-flow AD System that produces electricity and hot water through the use of a biogas enginegenerator equipped with a heat recovery unit. The effluent from the digester is separated into solids and liquid fractions. The solids are used as bedding in freestalls and calf barns and as a soil conditioner. The wastewater is applied as fertilizer to a field of corn. The hot water from the engine is used for maintaining a temperature of 95 to 100 degrees F.in the anaerobic digester and for farm and domestic applications. Key components of the AD System are given in the following figure.

12 Operation of the System

13 GHG Emissions from AD and Reference Systems Comparison of GWP by type of Greenhouse Gas Total reduction achieved

14 Reactor Types Various reactor types and configurations are available ranging from very simple small scale batch digesters and lagoons to very advanced engineered reactors such as UASB, EGSB, FB, etc. reactors. Small scale batch anaerobic digesters commonly used in rural areas of China and India

15 ANAEROBIC REACTOR TYPES

16 FACTORS THAT AFFECT THE APPLICABILITY OF ANAEROBIC TREATMENT Waste characteristics, strength and fluctuations Composition and fluctuations of organic compounds (SS, biodegradability) Temperature + fluctuations Availability of nutrients (N, P, micro-nutrients) Buffer capacity / ph Presence of alternative electron acceptors (SO4, NO3, etc.) Risk of formation of inorganic precipitates Risk of formation of scum layers and/or flotation layers Presence of toxic compounds Odor problems

17 MICROBIOLOGY OF ANAEROBIC DIGESTION During anaerobic digestion, a mixture of complex compounds are converted to a range of simple compounds. A mixed microbial culture is needed. Different species are dependent on each other for growth in an interacting serial metabolism. Methane fermentation is a versatile biotechnology capable of converting almost all types of polymeric materials to methane and carbon dioxide under anaerobic conditions. This is achieved as a result of the consecutive biochemical breakdown of polymers to methane and CO2 in an environment in which a variety of microorganisms which include fermentative microbes (acidogens); hydrogen-producing, acetate-forming microbes (acetogens); and methane-producing microbes (methanogens) harmoniously grow and produce reduced end-products. See Reynolds/Richards pages

18 Metabolic steps in anaerobic digestion Microbial groups involved Fermentative bacteria H 2 -producing acetogenic bacteria, H 2 -consuming acetogenic or homoacetogenic bacteria, CO 2 -reducing methanogenic bacteria, and Acetoclastic methanogenic bacteria. The first group of microorganisms secretes enzymes which hydrolyze polymeric materials to monomers such as glucose and amino acids, which are subsequently converted to higher volatile fatty acids, H2 and acetic acid. In the second stage, hydrogenproducing acetogenic bacteria convert the higher volatile fatty acids e.g., propionic and butyric acids, produced, to H2, CO2, and acetic acid. Finally, the third group, methanogenic bacteria convert H2, CO2, and acetate, to CH4 and CO2.

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20 Disintegration Carbohydrates Com pos ite Pa rticu la te Ma teria l (100%) 10% 30% Inerts 10% 30% 30% 30% Protein s 30% Fa ts 30% Hydrolysis 2% 29% MS 32% AA 30% LCFA 29% Acidogenesis 13% 16% 2% HPr, HBu, HVa 13% 6% 20% 29% 12% 20% 9% Acetogenesis 9% Acetic 64% H 2 26% Methanogenesis CH 4 90% COD flux for a particulate composite comprised of 10% inerts, and 30% proteins, carbohydrates and fats (in terms of COD).

21 Anaerobic Digestion of Treatment Plant Biosolids (Sludge) What is biosolids? Semisolid product originated from different points in a treatment plant. See below for its sources in a conventional WWTP Unit operation or process Screening Grit removal Preaeration Primary sedimentation Biological treatment Secondary Sedimentation Types of solids or sludge Coarse solids Grit and scum Grit and scum Primary sludge and scum Suspended solids Secondary sludge and scum Remarks Coarse solids are retained on the surface of the bar screens that could be removed mechanically or manually. Grit settles to the bottom of grit chambers and is collected mechanically or manually. Scum formed at the surface may have to be removed by scrappers. If preaeration tanks are not preceded by grit removal facilities, grit deposition may occur in preaeration tanks. Scum formed at the surface may have to be removed by scrappers. Quantities of sludge and scum depend on the nature of the collection system and whether industrial effluents are discharged to the system. Suspended solids are produced by the biological conversion of biochemical oxygen demand (BOD). Suspended solids are mainly of organic origin. Sludge-processing facilities Sludge, compost, and ashes The characteristics of the end products depend on the characteristics of the sludge being treated and the operations and processes used.

22 Typical chemical composition of sludge Parameter Untreated primary sludge Activated sludge Range Typical Range Total dry solids-ts (%) Volatile solids (% of TS) Grease and fats soluble in ether (% of TS) Protein (% of TS) Nitrogen (N, % of TS) 1, Phosphorus (P 2 O 5, % of TS) Potash (K 2 0, % of TS) Cellulose (% of TS) Silica (SiO 2, % of TS) ph Alkalinity (mg/l as CaCO3) Organic acids (mg/l as Hac) Energy content (kj/kg)

23 Sludge processing and disposal methods and their functions Unit Process Preliminary operations Thickening Stabilization Conditioning Disinfection Dewatering Drying Thermal reduction Ultimate disposal Methods Sludge grinding, Sludge blending, Sludge storage, Sludge degritting Rotary drum thickening, Gravity thickening, Flotation thickening, Centrifugation, Gravity Belt Thickening Chlorine oxidation, Lime stabilization, Heat Treatment, Anaerobic digestion, Aerobic digestion, Composting, Irradiation, Lagooning Chemical conditioning, Elutriation, Heat treatment Pasteurization, Long term storage Vacuum filter, Pressure filter, Horizontal belt filter, Centrifugation, Drying beds Lagoons Multiple effect evaporator, Flash drying, Spray drying, Rotary drying Multiple hearth dryer Multiple hearth incineration, Fluidized bed incineration, Flash combustion Co-incineration with solid wastes, Vertical deep well reactor, Wet air oxidation Landfill, Land application, Reclamation, Reuse

24 In sludge stabilization, chemical, biological and/or heat processes are applied to make the sludge suitable for reuse or disposal. Sludge stabilization results in reduced pathogens, elimination of offensive odors, reduction or prevention for the potential of putrefaction (contamination, pollution, decay). The mechanisms of eliminating negative conditions associated with sludges are (1) biological reduction of volatile content, (2) the chemical oxidation of volatile matter, (3) the addition of chemicals to the sludge to make it unsuitable for the survival of microorganisms, and (4) the application of heat to disinfect or sterilize the sludge. The design of the stabilization process must be in agreement with the final disposal method(s) as well as existing regulations. For instance, if the sludge is going to be applied on land, pathogen reduction has to be considered.

25 Process Description There are two types of anaerobic digesters that are commonly used: standard (low) rate and high rate processes. Conventional Standard (Low) Rate Digestion The standard rate process which is usually a single-stage process, does not employ sludge mixing, but rather the digester contents are allowed to stratify in zones. See Figure 19.4a in Reynolds/Richards (page 578). Sludge feeding and withdrawal are intermittent (batch) rather than continuous. The digester is generally heated to increase the rate of fermentation and therefore decrease the required retention time that ranges between days for heated digesters and this may be higher for unheated digesters. The organic loading rate for standard rate digestion is between kg/m3/day. As a result of the stratification and the lack of mixing, not more than 50% of the volume of a standard-rate (single-stage) digester is used. In other words, large volume requirement is the major drawback of these systems. Because of this, the standard-rate process is used principally for small installations.

26 High Rate Digestion The high-rate digestion can be performed both in single- and two-stage digesters. In single-stage digestion, sludge is mixed by gas recirculation, mechanical or draft tube mixers (separation of scum and supernatant layer does not take place). Sludge is heated to achieve optimum digestion rates. The average detention time is about 20 days. The organic loading rate for singlestage high-rate digestion is between lb volatile solids per cubic foot of digester volume per day ( kg/m3/day). The digester can be operated continuously or intermittently. Because there is no supernatant separation in the high-rate digester and the total solids are reduced by 45-50% and given off as gas, the digested sludge is about half as concentrated as the untreated sludge. Digestion tanks may have fixed roofs or floating covers. See Reynolds/Richards Table 19.1 for comparison (page 579).

27 Process Design Biological solids retention time (mean cells residence time), c, is normally the most important parameter to be used in the design of anaerobic digesters. However, there are also some emprical methods used for anaerobic digester design. Therefore, the common methods used in design of anaerobic digesters are as follows: (1) the concept of biological solids retention time, (2) the use of volumetric loading factors, (3) observed volume reduction, and (4) loading factors based on population. Temperature is also very important since it affects the rate of anaerobic fermentation and thus must be considered in design (See Reynolds/Richards Figure 19.5, page 581).

28 Design Based on Biological Solids Retention Time (or Mean Cell Residence Time) Degister design based on biological solids retention time involves application of general biological treatment principles. The first-stage digester of a high-rate system approximates a completely mixed reactor without recycle. Hence the solids retention time and hydraulic retention time are equal for this system. Biological solids retention time ( c) is the factor that determines the degree of volatile solids reduction during digestion. A schematic of a completely mixed reactor without recycle is given below: Q, S 0 X, Q, S X, V, S Q S 0 X V S : flowrate : influent substrate concentration : concentration of biomass in the reactor : volume of the reactor : substrate concentration in the reactor Schematic of of completely mixed reactor without recycle

29 Assuming that substrate utilization follows Monod relationship, the following equations can be derived for such a system; V Y ( S S) K ( 1 k ) c X 0 Q 1 c k S s c d d ( c Yk k ) 1 d where, K s = half velocity coefficient (mass/volume) k d = endogenous decay coefficient (time -1 ) Y = yield coefficient (mass/mass) k = rate of substrate utilization (time -1 ) There is a certain value of c below which waste stabilization does not occur. This critical value of c is called minimum mean cell residence time or cm. Physically, m c is the residence time at which the cells are washed out from the reactor faster than they can reproduce or grow. The minimum mean cells residence time can be calculated using: 1 k S 0 m Y k K S d c s 0. Suggested mean-cell residence times for use in the design of completely mixed digester are given below: Operating temperature ( C) c m (days) c m suggested for design (days)

30 The quantity of methane gas produced from a digester can be calculated by: V ( 562. ) ( S S) ( Q) ( 8. 34) 142. P CH 0 x 4 (3.18) where, V CH4 Volume of methane gas produced at standard conditions of 0 C and 1 atm (ft 3 /day) 5.62 Theoretical conversion factor for the amount of methane produced from the complete conversion of one pound of BOD L to methane and carbon dioxide (ft 3 CH 4 /lb BOD L oxidized) See Example 3.8. Q S 0 S Flowrate (MGD) Ultimate BOD in the influent (mg/l) Ultimate BOD in the effluent (mg/l) 8.34 Conversion factor P x lb MG. (mg / l) net mass of cell tissue produced per day (lb/day)

31 For a completely mixed high-rate digester without recycle, the mass of biological solids synthesized daily, Px, can be estimated by using: Y ( S S)( Q)(8.34) P x 0 1 k d c Typical kinetic coefficients for anaerobic digestion of different substrates Domestic sludge Fatty acids Carbohydrate Protein Coefficient Basis Value* Y k d Y k d Y k d Y k d * Reported values are for 20 C mg VSS/mg BOD 5 d -1 mg VSS/mg BOD 5 d -1 mg VSS/mg BOD 5 d -1 mg VSS/mg BOD 5 d -1 Range Typical