Anaerobic Water Treatment Technologies

Transcription

1 Anaerobic Water Treatment Technologies Shihwu Sung, Ph.D., PE Department of Civil, Construction & Environmental Engineering Iowa State University National Cheng Kung University July 4 th, 2008

2 Wastewater Treatment Options aerobic treatment for soluble chemical oxygen demand (COD) in 50-40,000 mg/l range anaerobic treatment for high CODs ( ,000 traditionally) alternative processes for CODs < 50 mg/l (e.g., carbon adsorption, ion exchange) and > 50,000 mg/l (e.g., evaporation and incineration)

3 Anaerobic Waste Treatment : An Overview Historical development: Mainly used for reducing mass of high solids wastes, e.g. human waste (nightsoil), animal manure, agricultural waste and sludge. Early applications of anaerobic waste treatment include: Mouras automatic scavenger - cited in French journal cosmos in 1881 Septic tank- developed by Donald Cameron in 1895 (England)

4 Imhoff tank: developed by Karl Imhoff in 1905 (Germany) Imhoff tank is a modified version of septic tank consisted of two - story in which sedimentation was allowed in upper tank and digestion of settled solids in the lower compartment. Upper tank Lower tank Imhoff tank Cont..

5 Cumulative Number of AD Plants for Industrial Applications No. of plants (Source: Frankin,, 2001)

6 AD Waste Treatment and Resources Conservation Renewable Energy Renewable Energy Liquid Liquid Wastes Wastes (Industrial, (Industrial, Domestic Domestic etc.) etc.) Slurries Slurries (Sewage (Sewage sludge, sludge, Liquid Liquid manure) manure) Solid Solid Wastes Wastes (Manure, (Manure, Organic Organic Refuse) Refuse) Agri- Agri-residues residues (Crops (Crops residues residues etc.) etc.) CH CH 4, 4, H 2 2 CH CH 4, 2 S, 4, H 2 S, H 2, CO 2 2, CO 2 Anaerobic Anaerobic bioconversion bioconversion NH 4+, PO 3- NH 4, 4+, PO 3-4, S 2-2- Biosolids Biosolids Scrubbing Scrubbing Micro-aerobic S o o recovery recovery Treated Treated effluent effluent Irrigation Irrigation Food Food Food Food Fish Fish ponds ponds Posttreatment Posttreatment Food Food Organic Fertilizer Organic Fertilizer

7 Anaerobic Digestion (1) Complex Organics Carbohydrates Proteins Lipids 1. Hydrolysis Simple Organics (2) Volatile Organic Acids Propionate, Butyrate, etc. 2. Acidogenesis Acetate H 2 + CO 2 (3) 72% 28% CH 4 + CO 2 3. Methanogenesis

8 Advantages of AD 1. Less energy requirement as no aeration is needed kwh energy is needed for every 1 kg of COD removal by aerobic process 2. Energy generation in the form of biogas 1.16 kwh electricity is produced for every 1 kg of COD removal by anaerobic process 3. Less biomass (sludge) generation Anaerobic process produces only 20% of sludge that of aerobic process

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10 Biodegradable Biodegradable organic organic 1 1 kg kg Aerobic process CO CO H 2 O kg kg New New biomass biomass Waste + O 2 CO 2 +H 2 O + new cells kg kg Biodegradable Biodegradable organic organic 1 1 kg kg Anaerobic process Biogas Biogas > kg kg New New biomass biomass Waste CH 4 + new cells < kg kg

11 Advantages of AD - continued 4. Less nutrients (N & P) requirement Lower biomass synthesis rate also implies less nutrients requirement: 20% of aerobic 5. Application of higher organic loading rate 5-10 times higher organic loading rates than aerobic processes 6. Space saving Higher loading rate requires smaller reactor volume 7. Ability to transform several hazardous solvents including chloroform, trichloroethylene and trichloroethane

12 Limitations of AD 1. Long start-up time Lower biomass synthesis rate -> longer start-up time to attain a biomass concentration 2. Long recovery time Subjected to disturbances and take longer time to return to normal operating condition 3. Specific nutrients/trace metal requirements Methanogens have specific nutrients e.g. Fe, Ni, and Co requirements 4. More susceptible to changes in environmental conditions Methanogens are prone to changes in conditions such as temperature, ph, redox potential, etc.

13 Limitations of AD continued 5. Treatment of sulfate rich wastewater The presence of sulfate reduces the methane yield could also inhibit the methanogens due to sulfide toxicity 6. Effluent quality of treated wastewater May not able to degrade the organic matter to the level meeting the discharge limits 7. Treatment of high protein & nitrogen containing wastewater High ammonia may cause inhibition

14 Comparison between Anaerobic & Aerobic processes Anaerobic Aerobic Organic loading rate: High loading rates:10-40 kg COD/m 3 -day Low loading rates: kg COD/m 3 -day (for high rate reactors, e.g. AF, & UASB) Biomass yield: Low biomass yield: kg VSS/kg COD (biomass yield is not constant but depends on types of substrates metabolized) Specific substrate utilization rate: (for activated sludge process) High biomass yield: kg VSS/kg COD (biomass yield is fairly constant irrespective of types of substrates metabolized) High rate: kg COD/kg VSS-day Start-up time: Long start-up: 1-2 months for mesophilic : 2-3 months for thermophilic Low rate: kg COD/kg VSS-day Short start-up: 1-2 weeks

15 SRT: Microbiology: Anaerobic Longer SRT is essential to retain the slow growing methanogens Multi-step process and diverse group of microorganisms degrade organic matters in a sequential order Aerobic SRT of 4-10 days is enough Mainly a one-species phenomenon Environmental factors: Highly susceptible to changes in environmental conditions Less susceptible to changes in environmental conditions.

16 Biogas Content Methane, CH 4 : 50 75% Carbon Dioxide, CO 2 : 25 50% Nitrogen, N 2 : 1 5% Hydrogen Sulfide, H 2 S < 1% Hydrogen, H 2 -trace

17 Methane Content Carbon Dioxide content can be estimated by reactor ph and bicarbonate concentrations K CO H 2 (g) CO 2(l) + H 2 O H 2 CO 3 K H = Henry s Law constant Ka H 2 CO 1 3 H + + HCO - Ka 2 3 H + + CO -2 3 pka 1 = 6.33 pka 2 = at 20 o C Methane content is the balance of carbon dioxide

18 Relationship between ph, bicarbonate and carbon dioxide at 35 o C and 1 atm pressure

19 Theoretical methane yield per kg COD at STP Assumption: No oxygen demand could be satisfied in an anaerobic reactor but production of methane Step 1: Calculation of COD equivalent of CH 4 CH + 4 2O 2 16 g 64g > CO 2 + 2H 2 O 16 g CH 4 ~ 64 g O 2 (COD) => 1 g CH 4 ~ 64/16 = 4 g COD (1) Step 2: Conversion of CH 4 mass to equivalent volume => 1 Mole CH 4 ~ 22.4 L CH 4 => 16 g CH 4 ~ 22.4 L CH 4 => 1 g CH 4 ~ 22.4/16 = 1.4 L CH (2)

20 Step 3: CH 4 generation rate per unit of COD removed From eqs. (1) and (2), we have => 1 g CH 4 ~ 4 g COD ~ 1.4 L CH 4 => 4 g COD ~ 1.4 L CH4 => 1 g COD ~ 1.4/4 = 0.35 LCH 4 or 1 Kg COD ~ 0.35 m 3 CH (3) Complete anaerobic degradation of 1 Kg COD produces 0.35 m 3 CH 4 at STP

21 Essential Conditions for Anaerobic Treatment Avoid excessive air/o 2 exposure No toxic/inhibitory compounds present in the influent Maintain ph between Sufficient alkalinity present Low volatile fatty acids (VFAs) Temperature around mesophilic range (30-38 o C) or thermophilic range (50-60 o C) Enough nutrients (N & P) and trace metals especially, Fe, Co, Ni, etc. COD:N:P = 350:7:1 (for highly loaded system) 1000:7:1 (lightly loaded system) Microorganism Retention (high-rate anaerobic reactors)

22 Anaerobic Process Configurations Low rate anaerobic reactors High rate anaerobic reactors Anaerobic lagoon Anaerobic contact process Septic tank Imhoff tank Standard rate anaerobic digester Slurry type bioreactor, temperature, mixing, SRT or other environmental conditions are not regulated. Loading of 1-2 kg COD/m 3 -day. Anaerobic filter (AF) Upflow anaerobic slugde Blanket (UASB) Fluidized bed Reactor Hybrid reactor: UASB/AF Anaerobic Sequencing Batch Reactor (ASBR) Able to retain very high concentration of active biomass in the reactor. Thus extremely high SRT could be maintained irrespective of HRT. Load 5-20 kg COD/m 3 -d COD removal efficiency : 80-90%.

23 Anaerobic Lagoon Suspended growth continuously/batch fed system Typical depth of lagoons range from 15 to 35 feet Lagoons can be covered to collect methane and reduce odors, but can also remain uncovered. Lagoons often used in slaughterhouse industry (FDL Foods, Dubuque, IA) Typical detention times of 1 50 days Temperature dependent

24 Ambient Temperature Covered Anaerobic Lagoon

25 Ambient Temperature Covered Anaerobic Lagoon

26 Plug Flow Anaerobic Digester AA Dairy, Candor, NY TS content: 11 13%

27 Traditional AD vs. High-Rate AD Adequate Mixing Temperature Control Effective Way to Retain Biomass - Separate solids retention time (SRT) from hydraulic retention time (HRT). SRT >> HRT SRT = mass of solids in system (g) / daily solids wasted rate (g/day)

28 Hydraulic retention time (HRT) Hydraulic retention time (HRT) is defined: Influent flow rate (Q), m 3 /day V, m 3 Q HRT, days = Volume V (m 3 ) = Flow rate Q (m 3 /day) For a given HRT, the size of reactor can be easily determined since flow rate (Q) is known Digester volume, V (m 3 ) = Flow rate (Q) x HRT (θ H )

29 Anaerobic Contact Process (ACP) Anaerobic contact process is essentially an anaerobic activated sludge process. Hence ACP is able to maintain high concentration of biomass in the reactor and thus high SRT irrespective of HRT. Degassifier allows the removal of biogas bubbles (CO 2, CH 4 ) attached to sludge which may otherwise float to the surface.. Biogas Biogas Settling tank Influent Completely mixed reactor Degassifier Effluent Recycled sludge Waste sludge

30 Cont.. ACP was initially developed for the treatment of dilute wastewater such as meat packing plant which had tendency to form a settleable flocs. ACP is suitable for the treatment of wastewater containing suspended solids. The biomass concentration in the reactor ranges from 4-6 g/l with maximum concentration as high as g/l depending on settleability of sludge. The loading rate ranges from kg COD/m 3 -day. The required SRT could be maintained by controlling the recycle rate similar to activated sludge process.

31 Anaerobic Filter biogas effluent media recirculation flow distribution recirculation pump influent

32 Anaerobic Filter Anaerobic filter: Young and McCarty in the late 1960s to treat dilute soluble organic wastes. The filter was filled with rocks similar to the trickling filter. Wastewater distributed across the bottom and the flow was in the upward direction through the bed of rocks Whole filter submerged completely Anaerobic microorganisms accumulate within voids of media (rocks or other plastic media) The media retain or hold the active biomass within the filter Non-attached biomass contributes significantly to waste treatment

33 Cont.. Originally, rocks were employed as packing medium in anaerobic filter. But due to very low void volume (40-50%), serious clogging problem was witnessed. Now, many synthetic packing media made up of plastics, ceramic tiles of different configuration have been used in anaerobic filters. The void volume in these media ranges from %. Moreover, these media provide high specific surface area typically 100 m 2 /m 3 or above which enhance biofilm growth.

34 Cont.. Typically HRT varies from days & the loading rates varies from 5-15 kg COD/m 3 -day. Biomass wastage is generally not needed. Down Flow Anaerobic Filter (DAF). Down flow anaerobic filter is similar to trickling filter in operation. DAF is closer to fixed film reactor as loosely held biomass/sludge within the void spaces is potentially washed out of reactor. There is less clogging problem and wastewater with some SS concentration can be treated using DAF.

35 Upflow Anaerobic Sludge Blanket Reactor Effluent biogas Influent Diameter mm

36 UASB three phase separator biogas effluent weir effluent gas bubbles baffles granule gas bubble floc particle granular sludge distribution baffle flocculent sludge recirculation pump influent

37 original granule diameter gravitational force upflow velocity buoyancy force gas bubbles Upflow in UASB reactor Upflow in UASB reactor

38 Upflow Anaerobic Sludge Blanket (UASB) UASB was developed in 1970s by Lettinga in the Netherlands. UASB is essentially a suspended growth system in which proper hydraulic and organic loading rate is maintained in order to facilitate the dense biomass aggregation known as granulation. The size of granules is about 1-5 mm diameter. Since granules are bigger in size and heavier, they will settle down and retain within the reactor. The concentration of biomass in the reactor may become as high as 50 g/l. Thus a very high SRT can be achieved even at very low HRT of 4 hours. The granules consist of hydrolytic bacteria, acidogen/acetogens and methanogens. Complex organic degrading granules show layered structure with a surface layer of hydrolytic/fermentative Acidogens. A mid-layer comprising of syntrophic colonies and an interior with acetogenic methanogens.

39 Cont.. Loading rate: kg COD/m 3 -day Important components of UASB: 1. Influent flow distributor 2. Sludge blanket 3. Solid-liquid-gas separator 4. Effluent collector Type of waste treatable by UASB: Alcohol, bakers yeast, bakery, brewery, candy, canneries, chocolate, citric acid, coffee, dairy & cheese, distillery, Domestic sewage, fermentation, fruit juice, fructose, landfill leachate, paper & pulp, pharmaceutical, potato processing, rubber,sewage sludge liquor, slaughter house, soft drinks, starch (barley, corn, wheat), sugar processing, vegetable & fruit, yeast, etc.

40 Important considerations in UASB operation Initial seeding of some well digested anaerobic sludge could be used. The seed occupies 30-50% of total reactor volume. Minimum seeding is 10% of the reactor volume. Provide optimum ph, and enough alkalinity. Supplement nutrients and trace metals if needed. Provide N & P at a rate of COD: N:P of 400:7:1 (conservative estimate). Addition of Ca 2+ at 200 mg/l promotes granulation. Ca 2+ conc. higher than 600 mg/l may form CaCO 3 crystals which may allow methanogens to adhere to and then become washed out of the system.

41 Expanded Granular Sludge Bed (EGSB) Reactor USAB EGSB (Triple Baffle Internal Settler)

42 EGSB

43 Anaerobic Sequential Batch Reactor (ASBR) BIOGAS RECYCLE BIOGAS SUPERNATANT SETTLED BIOMASS DECANT PORTS SETTLE DECANT FEED REACT EFFLUENT FEED

44 Anaerobic Sequencing Batch Reactor (ASBR) Developed at Iowa State University by Dague and Sung Batch fed suspended growth system. ASBR relies on internal clarification for solids retention by allowing biomass to settle. Four distinct phases Feed React Settle - Decant Four phases compose one cycle. Cycle lengths vary from 4 to 24 hours. Good system for wastes with high solids. Studies show that intermittent mixing is as effective as continuous mixing Low F/M at end of react cycle allows low gas production during settle phase. By lowering settle time selection of better settling granules can occur. Granulation is key to system (see UASB).

45 Demonstration at Crawford farm ASBR

46 ASBR Digester - Liquid Swine Waste

47 Temperature-Phased Anaerobic Digestion (TPAD) LABORATORY SETUP Gasmeter Gasmeter H 2 S scrubber H 2 S scrubber Feed Tank Thermophilic at 55 o C Mesophilic at 35 o C

48 Temperature-Phased Anaerobic Digester (TPAD)

49 Fluidized Bed Reactor (FBR) FBR is similar to EBR in terms of configuration. But FBR is truly fixed film reactor as suspended biomass is washed out due to high upflow velocity. The bed expansion is % of the settled bed volume which requires much higher upflow velocity (10-25 m/hr). The bio-carriers are supported entirely by the upflow liquid velocity and therefore able to move freely in the bed. The fluidized bed reactor is free from clogging problem short-circuiting and better substrate diffusion within the biofilm.

50 Hybrid system: UASB/AF Hybrid system incorporates both granular sludge blanket (bottom) and anaerobic filter (top). Such approach prevents wash-out of biomass from the reactor. Further additional treatment at the top bed due to the retention of sludge granules that escaped from the bottom sludge bed. UASB reactor facing a chronic sludge wash-out problem can be retrofitted using this approach. Hybrid system may be any combination or two types of reactor

51 Anaerobic baffled reactor Sludge In anaerobic baffled reactor, the wastewater passes over and under the baffles. The biomass accumulates in Between the baffles which may in fact form granules with time. The baffles present the horizontal movement of of biomass in the reactor. Hence high concentration of biomass could be maintained within the reactor. Biogas

52 Anaerobic Membrane Bioreactor (AMBR) Low energy consumption (no aeration needed) Low sludge production (1/5th of aerobic) Anaerobic bioreactor Membrane Able to maintain long SRT - minimize biomass wash-out Superior effluent quality for water reuse Useful energy byproduct methane Operation at ambient temperature Minimum knowledge of AMBR for low strength wastewater

53 Non-woven Filter (NWF) and Polytetrafluoroethylene (PTFE) NWF: random, entangled and multi-layer assembly of fibers Formation of dynamic membrane by either pore clogging or cake-layer formation 500μm Non-woven filter PTFE laminated non-woven filter

54 25.0 ph ORP Level Sensor 25.0 ph ORP Level Sensor Outlet Pressure Inlet Pressure Pressure Valve Peristaltic Pump P Outlet Pressure Inlet Pressure Pressure Valve Peristaltic Pump P 25.0 ph ORP Level Sensor 25.0 ph ORP Level Sensor Outlet Pressure Inlet Pressure Pressure Valve Peristaltic Pump P Outlet Pressure Inlet Pressure Pressure Valve Peristaltic Pump P AMBR schematic

55 Acknowledgement: These studies were supported by grants from Midwest Grain Processors (MGP); Lakota, IA Western Lake Superior Sanitary District, Duluth, MN Iowa Energy Center, Nevada, Iowa Black & Veatch Corporations, Kansas, KS U.S. Department of Energy U.S. Department of Agriculture through Iowa Biotechnology Byproducts Consortium

56 Question?