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07/05/2016
1
THERE ARE NO SUBSTITUTES TO WATER FOR LIFE.
ENERGY IS FUNDAMENTAL TO DECENT LIFE. BOTH
WATER AND ENERGY ARE F...
07/05/2016
2
( )
2030
07/05/2016
3
WASTEWATER TREATMENT SYSTEMS ARE TRADITIONALLY
IMPLEMENTED TO REDUCE (POLLUTING) HARMFUL (LIQUID)
EMISSIONS T...
07/05/2016
4
ENERGY USE FACTS IN W&WWT
• 3-4% of electricity in the U.S.A. is used in the water sector (about the same in ...
07/05/2016
5
Targets for electricity consumption at WWTPs
according to North Rhine Westphalia Energy
Manual for WWTPs.
(SO...
07/05/2016
6
x
We cannot solve problems by using the same kind
of thinking we used creating them.
Determination of Energy ...
07/05/2016
7
Determination of Energy Content of Municipal Wastewater Streams
WASTEWATER CONTAINS A SUBSTANTIAL AMOUNT OF H...
07/05/2016
8
Energy self-sufficiency as a feasible concept for wastewater treatment systems: example
(North Toronto TP)
th...
07/05/2016
9
Domestic Wastewater Treatment as a Net Energy Producer.
Can This be Achieved?
Under conventional approaches (...
07/05/2016
10
ANAEROBIC DIGESTION (AD)
• existed as a technology for over 100 years, evolving from airtight vessels and
se...
07/05/2016
11
ANAEROBIC DIGESTION (3)
While on summer holidays, in 1776, on Lake Maggiore, Volta’s boat got stuck
alongsid...
07/05/2016
12
ANAEROBIC DIGESTION (5)
• in contrast to aerobic organics degradation, which is mainly a single step
phenome...
07/05/2016
13
ANAEROBIC DIGESTION (7)
• in countries with warm climate throughout the whole year, meso- and thermo-
philic...
07/05/2016
14
ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY
UASB (UPFLOW ANAEROBIC SLUDGE BLANKET) REACTORS
Are a single stage...
07/05/2016
15
ANAEROBIC WW TREATMENT TECHNOLOGY (3) – UASB REACTORS SUMMARY
Advantages
Good removal efficiency at high loa...
07/05/2016
16
ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (5)
AF (ANAEROBIC FILTER) REACTORS
• AF is a form of anaerobic dig...
07/05/2016
17
ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (7)
AF (ANAEROBIC FILTER) REACTORS
• AF suspended solids and BOD r...
07/05/2016
18
ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (9)
ANAEROBIC BAFFLED REACTORS (ABRs)
• typical inflows range is 2...
07/05/2016
19
ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (11)
ANAEROBIC MEMBRANE REACTORS (AnMBRs)
Combining anaerobic reac...
07/05/2016
20
ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (13)
ANAEROBIC MEMBRANE REACTORS (AnMBRs)
ANAEROBIC SLUDGE DIGESTI...
07/05/2016
21
ANAEROBIC SLUDGE DIGESTION (2)
LARGE vs. SMALL SCALE DIGESTERS
Small-scale digestion is used at the single f...
07/05/2016
22
ANAEROBIC SLUDGE DIGESTION (4)
SMALL-SCALE SLUDGE DIGESTION
• often installed at household/community level i...
07/05/2016
23
ANAEROBIC SLUDGE DIGESTION (6)
LARGE-SCALE SLUDGE DIGESTION
• digestion is either carried out in mesophilic ...
07/05/2016
24
ANAEROBIC SLUDGE DIGESTION (7)
LARGE-SCALE SLUDGE DIGESTION
for domestic wastewater, biogas yield is 15–22 m...
07/05/2016
25
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Lectures Capodaglio - Session 1

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Sustainable Water - Energy - Centric Communities school
May 9 - 13, 2016 – Lake Como School of Advanced Studies

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Lectures Capodaglio - Session 1

  1. 1. 07/05/2016 1 THERE ARE NO SUBSTITUTES TO WATER FOR LIFE. ENERGY IS FUNDAMENTAL TO DECENT LIFE. BOTH WATER AND ENERGY ARE FUNDAMENTAL FOR FOOD. THE FUNDAMENTAL DIFFERENCE BETWEEN WATER AND ENERGY IS THAT ENERGY CAN BE RENEWABLE. WATER RESOURCES ARE NOT.
  2. 2. 07/05/2016 2 ( ) 2030
  3. 3. 07/05/2016 3 WASTEWATER TREATMENT SYSTEMS ARE TRADITIONALLY IMPLEMENTED TO REDUCE (POLLUTING) HARMFUL (LIQUID) EMISSIONS TO RECEIVING WATER BODIES According to current paradigm, fossil energy consumption and greenhouse gas (GHG) emissions into the atmosphere have been out of scope for wastewater utilities, the former being of concern only as far as it influences the economics of WWT With Kyoto and subsequent protocols, CO2 and GHGs emissions will impact significantly on these systems, by gradual introduction of specific regulations and/or penalties associated with methane and nitrous oxide emissions The selection of treatment technology, process operation, post-processing and disposal of residual solids influences GHGs contribution of WWT utilities IT APPEARS NECESSARY NOWADAYS TO INCORPORATE FOSSIL ENERGY USE AND GHG EMISSIONS REDUCTION IN THE DESIGN PARADIGMS OF WW TREATMENT SYSTEMS Keller, J., Hartley, K. (2003). Greenhouse gas production in wastewatertreatment: Process selection is the major factor. Water Science & Technology, 47/12, 43-48
  4. 4. 07/05/2016 4 ENERGY USE FACTS IN W&WWT • 3-4% of electricity in the U.S.A. is used in the water sector (about the same in EU?) • 80% of that energy is used for water transport • U.S. average of power consumption is about 0.2 to 0.6 kWh/m3 treated (depends on type of treatment, plant size, topograhy, etc…) • Energy costs represent 15-30% of O&M budget at a large WWTP, 30-40% at a small one • Aeration devices use more than 50% of the energy in WWT • Oxygen requirements typically vary during the day by a factor of 5 to 7 • Energy expenditures can be significantly reduced by matching oxygen supply to actual requirements (Souce: Moore, Energy Use at Water and WastewaterTreatment Plants, Dept. Of Civil Engrg. University of Memphis, 2012) In most municipalities, WWTPs typically represent one of the highest energy users The aeration systems in a WWTP can account for 54% or more of the overall energy usage at the facility. Specific Energy Efficiency programs may promote: • high efficiency aeration technology (HE Aeration Blowers, Fine or Ultra Fine Bubble Diffusers, Oxygen Sensor Devices), • custom measures to improve WWT (hyperboloid mixers, efficient digestion mixers, sludge thickeners, and high-pressure UV lamp controls) • Real Time Control (RTC) of aeration actuation (Source: Energy Resources Center at the University of Illinois at Chicago, 2016 http://www.erc.uic.edu/ )
  5. 5. 07/05/2016 5 Targets for electricity consumption at WWTPs according to North Rhine Westphalia Energy Manual for WWTPs. (SOURCE: MURL – Ministry for Environment, Nature Protection, Agriculture & Consumer Protection in the German State of North Rhine Westphalia (1999): Energy in WWTPs (in German). Düsseldorf, Germany.) Some standardized approaches for energy optimization, reduction of operating costs and, last but not least, reduction of CO2 emissions have been developed… is this all that can be done?
  6. 6. 07/05/2016 6 x We cannot solve problems by using the same kind of thinking we used creating them. Determination of Energy Content of Municipal Wastewater Domestic wastewater contains energy in different forms: • an intrinsic energy resource embedded within wastewater organics: this is the most direct and most commonly exploited useful energy source (theoretically 3.86 kWh/kg COD oxidized to CO2 and H2O)a • an external fossil-fuel energy equivalent which would be required for the production of equivalent amounts of fertilizing elements N and P, (19.3 kWh/kg N by the Haber-Bosch Process and 2.11 kWh/kg P after Gellings and Parmenter)b, and • energy that might be recovered from wastewater’s thermal content by heat pump operation (7 kWh/m3 per 6oC water temperature drop through heat extraction). (Sources: a Owen, Energy in WastewaterTreatment, Prentice-Hall, Inc., Englewood Cliffs, 1982; b Gellings and Parmenter, Energy efficiency in fertilizer production and use. In Knowledge for Sustainable Development, Gellings & Blok, Eds.; Eolss Publishers, Oxford, 2004
  7. 7. 07/05/2016 7 Determination of Energy Content of Municipal Wastewater Streams WASTEWATER CONTAINS A SUBSTANTIAL AMOUNT OF HIDDEN INTRINSIC ENERGY WW energy content can be measured by bomb calorimetry (same used to measure the energy content of FOOD) For North Toronto Treatment Plant WW: COD = 430 mg/l TOC = 73 mg/l - ΔUc,s = 3.2 kJ/gdry For sludges: (SOURCE: Shizas and Bagley, Experimental Determination of Energy Content of Unknown Organics in Municipal WastewaterStreams. ASCE Journal of Energy Engineering, Vol. 130, No. 2, August 1, 2004) Primary Sludge Secondary Sludge Anaerobically Digested Sludge 16 kJ/g 12.4 kJ/g 12.7 kJ/g WW and sludge energy contents depend on raw WW characteristics, but are fairly consistent on a normalized basis even for different wastewaters. The difference between wastewater and sludge energy content can be explained by their respective Volatile-to-Total Solids ratios. Raw wastewater has a VS:TS ratio of around 0.10, which is about 5 times lower than the ratio observed in primary sludge samples. This corresponds well to the energy content of the raw wastewater being 5 times lower than the energy content of primary sludge. All values shown in kJ/g dry weight With the energy content of the different streams determined, the potential of a wastewater treatment plant to become a net producer of renewable energy can be evaluated. Source Primary Sludge Secondary Sludge Anaerobically digested Sludge Zanoni & Mueller (1982) 15.0 13.5 11.4 Vesilind & Ramsey (1995) 12.6 Shizaz & Bagley (2004) 15.9 12.4 12.7
  8. 8. 07/05/2016 8 Energy self-sufficiency as a feasible concept for wastewater treatment systems: example (North Toronto TP) the relevant energy content terms Ei are defined as Stream Flow Qi (m3/day) Ei (kJ/day) i Raw wastewater 35,700 2.263108 WW Primary sludge 310 1.503108 1S Primary effluent 35,390 7.593107 1E Secondary sludge 810 3.173107 2S Biogas production 3,430 8.583107 BG Treated solids 4,450 ~kg/day 5.653107 TS All data from year 2000 A cogeneration technology providing 28% recovery of biogas energy as electricity would meet the total electricity needs of the plant. Increasing primary settling efficiency, anaerobic digester solids’ destruction, electricity generation and increased aeration efficiency, the North Toronto Treatment Plant could become a net generator of renewable electricity. Total actual energy required to run the plant Estimated energy contentof raw wastewater Potential energy contentof produced biogas kJ/day 2.44x107 2.26x108 8.58x107 Column i/1 1 9.3 3.5 Energy self-sufficiency as a feasible concept for wastewater treatment systems (2) Similar analyses at different municipal WWT facilities could determine their potential to produce renewable energy. Net energy produced (subtracting internal plant usage) will be expected to vary due to changes in raw WW composition and processes. Economic viability of producing renewable energy is a separate issue that will vary from location to location and depend strongly on the price of electricity. Wastewater energy content measurements indicate that the potential energy available in raw wastewater may exceed a facility’s electricity requirements for treatment processes by a factor of almost 10!!! POTENTIAL ≠ ACCESSIBLE !!!! due to thermodinamical, technological and process approach limitations APPLIED TECHNOLOGIES AND PROCESS EFFICIENCIES MUST BE IMPROVED IN ORDER TO FULLY TAP THIS ENERGY SOURCE.
  9. 9. 07/05/2016 9 Domestic Wastewater Treatment as a Net Energy Producer. Can This be Achieved? Under conventional approaches (aerobic treatment) only about ¼-½ of a plants’ energy needs can be satisfied by using biogas produced by anaerobic digestion of organics. Plant modifications might reduce energy needs. Soluble organic fractions are not easily concentrated, and undergo processes designed to treat dilute streams at short detention times. Aerobic processes are very effective, but with high energy requirements. A solution would be to use processes that capture dissolved organics energy potential and also meet effluent standards. Thermal, chemical or electrical processes may be used to condition refractory organics to increase biodegradability (and biogas production), but energy cost for this may offset gains. Thermal processes (incineration) can extract energy from all organic sludge fractions. However, unless humidity is reduced below 30%, more energy is required than is produced by combustion. Thus, these are generally not energy producers. (Source: McCarty, Bae and Kim, Domestic Wastewater Treatment as a Net Energy Producer. Can This be Achieved? Environ. Sci. Technol. 2011, 45, 7100–7106)
  10. 10. 07/05/2016 10 ANAEROBIC DIGESTION (AD) • existed as a technology for over 100 years, evolving from airtight vessels and septic tanks, to temperature-controlled, completely mixed digesters, and then to high rate reactors, with highly active biomass. • anaerobic reactors can be divided into “high-rate” systems, involving biomass recycle (HRT << SRT), and “low-rate” systems without biomass recycle (HRT=SRT) • wet AD systems operate at low total solids (<10–20% TS) and dry systems have high operating solids (20–>40% TS). Biogas calorific value is about 6 kWh/m3, corresponding to about half L of diesel oil ANAEROBIC DIGESTION (2) • was discovered by the Italian polymath Count ALESSANDRO VOLTA Born in Como, 1745 Professor of Physics at the University of Pavia, 1779 Invented the electrophorus Identified methane, 1776 Invented the electric battery
  11. 11. 07/05/2016 11 ANAEROBIC DIGESTION (3) While on summer holidays, in 1776, on Lake Maggiore, Volta’s boat got stuck alongside the reeds near Angera. He started poking the muddy bottom of the water with an oar, and saw lots of gassy bubbles floating up to burst on the surface. He later collected some of this gas, discovered it was flammable, and called it flammable marshland air. In 1778 he isolated it and gave an explanation for its generation. Nowadays it is called methane. The possibility of causing an explosion with a mixture of gas, even in closed environment, led Volta to construct an interesting gadget later called Volta’s Cannon. ANAEROBIC DIGESTION (4) • microbiology of methane digestion has been studied intensively since. It has been established that three physiological groups of bacteria are involved in the anaerobic conversion of organic materials to methane. The first group, of hydrolyzing and fermenting bacteria, converts complex organic materials to fatty acids, alcohols, carbon dioxide, ammonia and hydrogen. The second group of hydrogen producing acetogenic bacteria converts the products of the first group into hydrogen, carbon dioxide and acetic acid. The third group consists of methanogenic bacteria, converting hydrogen and carbon dioxide or acetate to methane.
  12. 12. 07/05/2016 12 ANAEROBIC DIGESTION (5) • in contrast to aerobic organics degradation, which is mainly a single step phenomenon, anaerobic is a chain process, in which several organisms are involved sequentially. Anaerobic conversion of complex substrates therefore requires a synergistic action of involved micro-organisms. In addition to the absence of free (dissolved) oxygen, another factor of utmost importance, in the process is the partial pressure of hydrogen (pH) and its thermodynamics. • anaerobic processes involve electron acceptors other than free oxygen, and bacteria must therefore “work harder” to access them, with the consequence of achieving lower degradation rates in equivalent environmental conditions compared to aerobic ones. Since anaerobic process rates are slower than aerobic’s, these processes require larger reactors to degrade an equivalent amount of organic matter. • a technological advance of utmost importance in anaerobic digestion has been the development of methods to concentrate methanogenic biomass in the reactor. High concentrations of biomass can be achieved by the principle of autoflocculation and gravity settling as in the UASB (Upflow Anaerobic Sludge Blanket) reactor, by attachment to a static carrier (anaerobic filter), or to a mobile carrier (fluidized bed), or by growth in/on a solid matrix. ANAEROBIC DIGESTION (6) • AD is currently used mostly for waste sludge digestion (higher specific recoverable energy content) • conventional operational temperatures of digesters determine the species of methanogens:  mesophilic digestion takes place optimally around 30 to 38°C, (ambient temperatures between 20 and 45°C), mesophilic bacteria are the primary microorganisms present  thermophilic digestion takes place optimally around 49 to 57°C, (and up to 70°C), where thermophiles are the primary microorganisms present. Mesophilic systems are more stable than thermophilic ones, but thermophilic digestion energy output is higher, with more biogas generated from organic matter in a unit time. Increased temperatures facilitate faster reaction rates, and thus faster gas yields. They also facilitate greater pathogen reduction in the digestate. • meso-and thermophilic operation usually requires special construction (insulation) of reactors, and additional heating of their contents
  13. 13. 07/05/2016 13 ANAEROBIC DIGESTION (7) • in countries with warm climate throughout the whole year, meso- and thermo- philic operating conditions can be reached naturally. In addition, high wastewater concentration may favor anaerobic treatment of the entire sewage flow, not only of the sludge • recently, progress has also been made on direct anaerobic treatment of wastewaters at low temperatures (8 - 25°C). In Bolivia, anaerobic digestion was operated at temperature working conditions of less than 10°C (requiring, however, more than threefold normal mesophilic process duration). • digestion systems can be configured with different levels of complexity. Single- stage systems, see all the reactions occur within a unique reactor. A single stage reduces construction costs, but results in lower control of the process: acidogenic bacteria produce acids, reduce the pH in the tank (that may fall out of the prescribed range for methanogens) putting biological reactions of different species in direct competition with each other. • in two- (multi-)stage systems, different reactors are optimised to achieve maximum control over bacterial communities. (eg: Acidogenic bacteria producing organic acids grow more quickly than methanogenic bacteria). ANAEROBIC DIGESTION (8) • under typical circumstances, hydrolysis, acetogenesis, and acidogenesis occur mainly within the first vessel. Organic material is then heated to the required operational temperature (mesophilic or thermophilic) prior to pumping into the methanogenic reactor. Incomplete hydrolysis can be a limiting factor for the process: addition of an unheated, stirred holding tank between 1st and 2nd stage (or increasing 1st stage volume) can improve the level of organics hydrolysation and improve overall gas production. • residence time in digesters varies with the amount and type of feed matter and configuration of the system. Typical two-stage mesophilic digestion requires residence times between 15-40 days, single-stage thermophilic digestion takes around 14 days. In UASB reactors HRT can be as short as 1-24 hrs, with SRT up to 90 days. (UASB systems de-link SRT and HRT through sludge blanket effects). • anaerobic process can be inhibited by ammonia, sulfides, light metal ions (Na, K, Mg, Ca, Al), heavy metals, organics (chlorophenol, halogenated aliphatics, N- substituted aromatics, long chain fatty acids), each affecting one or more of the bacterial groups present, to a degree that depends on inhibitor concentrations.
  14. 14. 07/05/2016 14 ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY UASB (UPFLOW ANAEROBIC SLUDGE BLANKET) REACTORS Are a single stage process for anaerobic industrial and/or blackwater treatment, achieving high removal of organic pollutants. A suspended sludge blanket filters and treats wastewater as it flows through it transforming organics into biogas. Solids are retained by the filtration effect of the blanket. Upflow regime and motion of gas bubbles within, allow liquor mixing without mechanical assistance. Gas escapes at the top of the reactor, baffles preventing an outflow of the sludge blanket. Due to low removal of nutrients, effluent water and stabilized sludge can be used in agriculture. ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (2) UASB (UPFLOW ANAEROBIC SLUDGE BLANKET) REACTORS • UASB reactors can treat all type of high-strength WW. It can be used at large-scale (e.g. agro-industrial wastes) or decentralized treatment systems for domestic WW. • domestic treatment is still relatively new and not always successful as domestic WW has generally lower strength. Main applications of UASB to domestic WW are active in tropical countries (South America, Brazil, India, etc.) • recently, experiences with UASBs treating urban wastewater at low (10-20oC) temperatures have shown promising results
  15. 15. 07/05/2016 15 ANAEROBIC WW TREATMENT TECHNOLOGY (3) – UASB REACTORS SUMMARY Advantages Good removal efficiency at high load/low temperature. Simple construction and operation. Easily applied on either very large or very small scale. At high loading rates, the area needed for the reactor is small, reducing capital cost. If no heating of the influent is needed and plant operations are done by gravity, energy consumption is minimal. Energy is produced in the form of methane. Reduce CO2 emissions due to low demand for input energy and surplus energy production, but may cause CH4 escapes if improperly built. Few bio-solids waste generated since energy in WW is converted mostly to gaseous form. Very little energy is left for new cells growth. Low sludge production, 5-10% compared to aerobic methods, due to low bacterial growth rates. Sludge is well stabilized for disposal, with good dewatering properties. Handle organic shock loads effectively. (Hydraulic shocks less effectively). Low nutrients and chemical requirements. Stable pH can be maintained without addition of chemicals. Disadvantages Pathogens are only partially removed (except helminths eggs, effectively captured by sludge bed). Nutrients removal is not complete: post- treatment is required. Due to the low growth rates of methanogens, longer start-up periods are required to reach steady state operation. Hydrogen sulphide is produced during process, especially if sulphate concentration in the influent is high. (Possible bad smell and corrosion). Post-treatment of effluent generally required to reach surface water discharge standards for organic matter, nutrients and pathogens. Temperature control is required for colder climates. Influent UASB type COD in (g/L) OLR (kgCOD/m3d) HRT (d) Temp oC COD rem % Biogas (l/d) CH4 % in biogas Palm oil ref. single 42.5 10.63 4 35 96 11.5 60 Palm oil ref. dual 30.6 30 1 35 90 10 70 Palm oil ref. single 50 15.5 3.33 28 80.5 14 50 Distillery (recalcitr.) single 10 19 0.5 55 <67 6.4 55 Distill. (whisky) single 21 10 2.1 35 93 4.7 - Distill. (whisky) dual 20.9 17.2 1.22 35 92 310 77 Distill. (cognac) single 33 11 3 36 85 0.08 74 Dairy Batch 13.5 22 2 35 97 - 54 L/d Dairy manure Dual 17.8 8.9 2 35 87.3 - 0.27 L/d Cheese whey Dual 55.1 11.1 4.95 - 95 - 23.4 L/d Fish canning single 2.72 8 0.33 - 80-90 - - Slaughterhouse single 1.2 3.5 0.33 20 70 104 65-70 Slaughterhouse three 4-6 4.6-10.7 0.58 35 86-90 10.6-11.9 - Slaughterhouse five 2.87 30 0.1 33 90 - 280 L/d Piggery single 8.12 1.62 5 30-35 75 4.1 58 Pre-settled piggery dual 2 3.17 0.63 30 91 1.5 40 Municipal landfill leachate dual 20 16 4.5 37 79 9.5 60 Municipal WW single 3.2 1 0.42 20 86 1.97 79 Domestic WW single 0.39 1.2 0.33 30 85 - 26 L/d Sewage single 0.15- 0.5 0.77-2.55 0.2 13-25 68 - 3.5 L/d Source: Latif et al., Integrated application of UASB reactors for the treatment of wastewaters. Wat. Res. 45 (2011)
  16. 16. 07/05/2016 16 ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (5) AF (ANAEROBIC FILTER) REACTORS • AF is a form of anaerobic digester in which the tank contains a filter medium where anaerobic microbial populations can establish themselves. The packing material can consist of high-density PVC or natural materials (stones, etc..). ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (6) AF (ANAEROBIC FILTER) REACTORS • AF processes are based on the combination of physical (settling) and biological treatment. As the wastewater flows through the filter from bottom to top (up- flow), it comes into contact with the biomass growing on the filter, and is subjected to anaerobic degradation. • low cost building & operation process. Often used for first-step sanitation in developing countries. • AFs are used for WW with low percentage of suspended solids and narrow COD/BOD ratio. They are suitable for domestic and all industrial WWs with low content of suspended solids. Pre-treatment may be necessary to eliminate solids of larger size before they are allowed to enter the filter (danger of clogging). • Recovering biogas may be considered in case of BOD concentration >1,000 mg/L, or merely for local daily family use
  17. 17. 07/05/2016 17 ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (7) AF (ANAEROBIC FILTER) REACTORS • AF suspended solids and BOD removal can be as high as 90%, typically 50-80%. Limited N removal, normally not exceeding 15% in terms of TN. Total Coliform reduction is 1 to 2 log units. • HRTs in the range of 1.5-2 days for pre-settled blackwater, 0.7-1.5 days for greywater. For domestic WW, constructed anaerobic total volume (voids plus filter mass) estimated at 0.5 m3/cap., for smaller units up to 1 m3/cap. • construction of AFs is similar to septic tanks, but with more chambers and baffles to ensure upflow pathways. Non-settleable and dissolved solids are treated by coming in contact with active bacteria fixed on the filter. The larger the surface for bacterial growth, the quicker the digestion. Good filter materials provide 90-300 m2 area/m3 filter volume. • upflow mode limits the washout of active biomass from the system ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (8) ANAEROBIC BAFFLED REACTORS (ABRs) • very similar consruction to Afs, but without the filling medium. • treatment performance of ABRs in the range 65-90% COD removal (corresponding to about 70-95% BOD), is far superior to that of conventional septic tanks (30- 50%). Settleable solids are removed in the initial sedimentation chamber (typically 50% of TSS). The design allows for enhanced treatment of non- settleable solids in subsequent chambers. Overall TSS removal up to 90% can be achieved. Tanks in series help digest poorly degradable (in the rear part) organics.
  18. 18. 07/05/2016 18 ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (9) ANAEROBIC BAFFLED REACTORS (ABRs) • typical inflows range is 2-200 m3/day, and OLR below 3 kg COD/m3d. Higher OLRs are possible with higher temperatures/easily degradable substrates. Critical design parameters include HRT between 48-72 hrs, upflow velocity 0.6-2 m/h and a number of upflow chambers between 3-6. For larger inflows, the system becomes anti-economical due to high areal footprint. • usually, biogas produced in an ABR is not collected due to insufficient production, or is collected for local family uses (kitchen). Tanks should be vented to allow controlled release of odorous and potentially harmful (explosive, toxic) gases. ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (10) ANAEROBIC MEMBRANE REACTORS (AnMBRs) AnMBRs have evolved from aerobic MBRs, with membranes either external or submerged in the reactor, and can achieve high COD removals (98%) at HRTs as low as 3 h. Since membranes stop biomass being washed out, they can enhance performance with inhibitory substrates, at psychrophilic/thermophilic temperatures, and enable nitrogen removal via Anammox. • . Membrane pore sizes for WW vary from 0.2 to 20 μm D.O.A.
  19. 19. 07/05/2016 19 ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (11) ANAEROBIC MEMBRANE REACTORS (AnMBRs) Combining anaerobic reactors and membranes is done in three main different ways. • Cross-flow external membrane. This makes membrane cleaning and replacement simple, but involves an external pump which circulates the biomass at high velocity (2–4 m/s), scouring the membrane surface to reduce fouling and provide high pressure to force the liquid through the membrane. Energy costs are therefore high. Correlation between aerobic and anaerobic membrane behavior is still not clear, but they seem to behave similarly. • Vacuum-driven submerged membrane. They use a vacuum (or hydrostatic head) to draw the effluent through the membrane. Is a common method in aerobic plants. The membrane can be immersed in the reactor itself, or in a separate reactor, requiring a pump. An advantage of submerged membrane is that energy required for pumping is eliminated, although biogas needs to be recycled within the reactor to provide bubble shear to avoid fouling. In addition, biomass is subject to less shear, and hence should be less stressed. This can lead to lower shear rates, and hence lower fluxes, requiring greater installed membrane areas. ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (12) ANAEROBIC MEMBRANE REACTORS (AnMBRs) • Sequential membrane reactors. In this recently developed configuration, effluent from one reactor (with larger pore sizes) is treated by another membrane reactor with smaller pore size. This has been used in the treatment of the Organic Fraction of Municipal Solid Waste where the (acid) hydrolysis reactor contains a coarse steel membrane (30–140 micron) and the leachate containing fine colloidal particles is treated in either an anaerobic filter, or in submerged anaerobic reactors. performances and process conditions of AnMBR used in high strength wastewater treatment SOURCES: Stuckey D, (2012) Recent developments in anaerobic membrane reactors. BIORESOURCE TECHNOLOGY, 122, Chang S. (2014) Anaerobic Membrane Bioreactors (AnMBR) for Wastewater Treatment. Adv. Chem. Engin. Sci.,4
  20. 20. 07/05/2016 20 ANAEROBIC WASTEWATER TREATMENT TECHNOLOGY (13) ANAEROBIC MEMBRANE REACTORS (AnMBRs) ANAEROBIC SLUDGE DIGESTION • anaerobic digesters are used for the conversion of the organic fraction of slurries and sludge into biogas through an AD process. Biogas is a green energy that has potential to reduce GHG emission. It is recovered and used either directly for heating the reactors or transformed by combined heat and power (CHP) generators. It can also be upgraded to natural gas quality (biomethane) and fed into the grid or used to power motor vehicles (CNG). • substrates are typically excess sludge from WWTPs or waste slurries from agriculture and industry (manure/dairy-food). Energy crops may be added to increase gas yields (co-digestion). • large-scale digesters have been mainly developed in industrialized countries with different designs (most of them high-tech requiring expert construction and O&M skills). • residuals after digestion are rich in nutrients and can be used as soil amendment in agriculture, generally after anaerobic composting as final treatment step, or can be further processed to extract secondary raw materials and energy (nutrients, biodiesel-like oils).
  21. 21. 07/05/2016 21 ANAEROBIC SLUDGE DIGESTION (2) LARGE vs. SMALL SCALE DIGESTERS Small-scale digestion is used at the single farm’s scale. They are usually fed with combined substrates.Main design elements of small-scale digesters are: inlet, airtight chamber, gas collection vessel and expansion chamber. There are three general types of small reactor designs: rubber-balloon, floating-drum plants and fixed-dome plants. ANAEROBIC SLUDGE DIGESTION (3) LARGE vs. SMALL SCALE DIGESTERS Large-sale digesters are used for WW sludge processing at WWTPs or in decentralized and regional co-digestion facilities. The most common forms of large-scale digesters are batch, plug-flow and continuously stirred tank reactor (PFR and CSTR). Completely mixed and batch systems are generally built vertically, plug-flow reactors are generally horizontal reactors. Horizontal reactors are often constructed similar to floating or expandable plastic dome plants (see biogas digester small scale) but much larger. Fixed dome are also used, but require large volumes of retention tanks for sludge expansion.
  22. 22. 07/05/2016 22 ANAEROBIC SLUDGE DIGESTION (4) SMALL-SCALE SLUDGE DIGESTION • often installed at household/community level in rural areas for co-digestion of animal manure and toilet products. • gas recovered either directly for cooking and lighting or transformed in a gas heater system or CHP. Rarely upgraded to biomethane. • digestate (nutrient-rich sludge) can be used as soil additive in agriculture. Digesting temperature 20 to 35 °C (mesophilic) Retention time 40-100 days Biogas energy 6kWh/m3 = 0.61 L diesel fuel Biogas generation 0.3 – 0.5 m3gas/m3reactor volume-day Human yield 0.02 m3/person-day Cow yield 0.4 m3/Kg dung Gas requirement (cooking) 0.3-0.9 m3/person-day Gas requirement (one lamp) 0.1-0.15m3/h ANAEROBIC SLUDGE DIGESTION (5) SMALL-SCALE SLUDGE DIGESTION • small-scale biogas digesters generally follow a wet anaerobic digestion process with an optimal total solid (TS) content of 5-10%. • fluid properties of the slurry are important for operation, as it is easier for methanogenic bacteria to come into contact with feed material, accelerating the digestion process. Animal dung has generally higher TS content then required: to obtain an optimum TS content, substrates can be diluted (with greywater or toilet wastes for instance). Cow dung contains 18% of TS, is diluted with water in the ration of 1:1 (by weight) to obtain the optimum concentration of 9% TS. Cattle Buffalo Pig Chickens Human Source L Biogas /Kg manure 40 23-40 = 30 = = 60 40-59 = 70 = = 60 20-28 40 MANG (2005) FAO(1996): SASSE (1998) L Biogas /kg TS In faeces 430 L (35°C) 300 L (25°C) GTZ (2009)
  23. 23. 07/05/2016 23 ANAEROBIC SLUDGE DIGESTION (6) LARGE-SCALE SLUDGE DIGESTION • digestion is either carried out in mesophilic (20-35 °C) or thermophilic (50-60°C) range. Thermophilic processes produce more biogas in a shorter time, however, mesophilic processes are often preferred as high temperatures require higher energy inputs, and increase the production of ammonia, which is toxic for the anaerobic microorganism producing biogas • large-scale anaerobic digesters are generally designed according to a wet digestion process with 10-20% TS. Volumes of reactors range from several hundred to several thousand m3. • a small-scale plant cannot ‘simply’ be enlarged to any degree: when a low-tech solution is required, it is better to construct several low-tech small plants instead of one single larger one, to facilitate operation and maintenance. • to optimise large-scale anaerobic digesters, multi-stage digestion is used, which allows to more accurately control pH and temperature. In such systems, stages of digestion (hydrolysis and acidogenesis, acetogenesis and methanogenesis) are separated in different consecutive compartments. Consequently, optimum conditions for each type of bacteria can be maintained in a each volume, resulting in simplified maintenance, and energy savings. ANAEROBIC SLUDGE DIGESTION (6) LARGE-SCALE SLUDGE DIGESTION Advantages • Combined treatment of different organic waste and ww • High reduction of waste volume • Generation of renewable energy (biogas) • Potential for GHG emissions reduction • Remaining sludge could be used as fertilizer • Low space requirements Disadvantages • Experts required for design, construction, O&M • High technical and organizational complexity (normally increasing with scale) • Reuse of produced energy must be planned and established (users available nearby) • High sensitivity of methanogenic bacteria to a large number of chemical compounds • Requires seeding (start-up can be long due to the low growth yield of anaerobic bacteria)
  24. 24. 07/05/2016 24 ANAEROBIC SLUDGE DIGESTION (7) LARGE-SCALE SLUDGE DIGESTION for domestic wastewater, biogas yield is 15–22 m3/103 cap.d. Typical biogas production in secondary treatment plants is increased to about 28 m3/103 cap.d. For other feedstock, values are indicated in the Table (in m3/t) Feedstock Biogas Yield (m3/t) Feedstock Biogas Yield (m3/t) Cattle slurry 15-25 (10% DM) Potatoes 276-400 Pig slurry 15-25 (8% DM) Rye grain 283-492 Poultry 30-100 (20% DM) Clover grass 290-390 Grass silage 160-200 (28% DM) Sorghum 295-372 Whole wheat crop 185 (33% DM) Grass 298-467 Maize silage 200-220 (33% DM) Red clover 300-350 Maize grain 560 (80% DM) Jerusalem artichoke 300-370 Crude glycerine 580-1000 (80% DM) Turnip 314 Wheat grain 610 (85% DM) Rhubarb 320-490 Rape meal 620 (90% DM) Triticale 337-555 Fats up to 1200 Oilseed rape 340-340 Nettle 120-420 Canary grass 340-430 Sunflower 154-400 Alfalfa 340-500 Miscanthus 179-218 Clover 345-350 Flax 212 Barley 353-658 Sudan grass 213-303 Hemp 355-409 Sugar beet 236-381 Wheat grain 384-426 Kale 240-334 Peas 390 Straw 242-324 Ryegrass 390-410 Oats grain 250-295 Leaves 417-453 Chaff 270-316 Fodderbeet 160-180 ANAEROBIC SLUDGE DIGESTION (8)
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