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Course 2 Unit 4 Lecturer: Mariska Ronteltap Introduction to anaerobic treatment technologies.

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1 Course 2 Unit 4 Lecturer: Mariska Ronteltap Introduction to anaerobic treatment technologies

2 In this file: Part A – Fundamentals of anaerobic digestion Part B – Anaerobic treatment technologies relevant for ecosan concept In a separate file: Part C – Examples and case studies

3 This unit deals with which part of the sanitation system?  Anaerobic digestion can be used to treat faeces, greywater and other organic waste with the aim to produce biogas and a fertiliser  A certain degree of pathogen kill can be achieved through raised temperatures and/or extended digestion times in the anaerobic digester

4 Reminder: Overview of ecosan technology components (where does anaerobic treatment come in?) organic solid waste rainwater greywaterurinefaeces collection treatment (see Course 2) utilisation (see Course 3) Vacuum toilets and vacuum sewerage UD toilets - Storage Fertilizing with urine Soil conditioning with treated excreta and solid biowaste Anaerobic Digesters Composting Rainwater harvesting Urine processing Dehydration Toilet Reuse of wastewater e.g. in agriculture, aquaculture Gravity Sewerage (conv. or small-bore, central or decentral) - Composting toilet Wastewater treatment (centralised or decentr.) Marked in red are those technologies covered in this course unitSource: based on GTZ-ecosan project Resource Book (UD = urine diversion) Disinfection (if required) Reuse: irrigation, cleaning, toilet flushing Greywater separation Constructed wetlands, ponds, trickling filters, septic tanks, soil filters,… Prolonged storage Waterless urinals, UD toilets Reuse: irrigation, toilet flushing from Course 1 Unit 3

5 Reminder: Important treatment technologies often used as part of ecosan concepts ProcessTechnical optionsReason for popularity in ecosan CompostingComposting plants for secondary treatment Composting toilet Suitable for faecal matter and organic solid waste treatment Produces valuable end product (compost) Low energy demand Pathogen destruction (if thermophilic) Anaerobic treatment Septic tanks UASB Anaerobic ponds Anaerobic digesters Suitable for faecal sludge, blackwater, faeces (e.g. together with manure), organic solid waste Preserves nitrogen (unlike aerobic wastewater treatment) Produced biogas for cooking, lighting, heating “Natural systems” (low- rate biological systems) Constructed wetlands Aerobic or facultative ponds/lagoons Waste stabilisation ponds Suitable for greywater treatment Low energy use Cheap if land available Can have aesthetic and environmental benefits (e.g. increased bird life) High-rate biological or physical systems Package plants using attached growth processes Membrane bioreactor Trickling filter Suitable for greywater treatment in urban areas (limited space) High quality effluent is produced from Course 1 Unit 3

6 Part A: Fundamentals of anaerobic digestion Mantopi Lebofa (from NGO TED) lighting the biogas flame (Lesotho, Dec 2006) Course 2 Unit 4

7 Overview about anaerobic treatment in general (this is not specific to ecosan)  Anaerobic treatment works with organic input materials, such as: - liquid organic material - solid organic material (provided it is has a water content of ~ 50% or more), i.e. - slurries/sludges - organic kitchen waste - greywater together with excreta  Anaerobic treatment is not so suitable for: - individual houses, unless animal excreta is available too (farmers)  Anaerobic digestion may be a direct alternative to UDD toilets, e.g. for public toilets, institutions (schools, hospitals, prisons) - biogas used for lighting, cooking  The end product (digested material) is not pathogen-free but still fit for reuse

8 Reminder: what is “organic”? –An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; therefore, carbides, carbonates, carbon oxides and elementary carbon are not organic (see below for more on the definition controversy for this word). The study of organic compounds is termed organic chemistry, and since it is a vast collection of chemicals (over half of all known chemical compounds), systems have been devised to classify organic compounds.chemical compoundsmoleculescarbonhydrogen carbidescarbonatesoxideselementaryorganic chemistry –The name "organic" is a historical name, dating back to 19th century, when it was believed that organic compounds could only be synthesised in living organisms through vis vitalis - the "life-force". The theory that organic compounds were fundamentally different from those that were "inorganic", that is, not synthesized through a life- force, was disproved with the synthesis of urea, an "organic" compound by definition of its known occurrence only in the urine of living organisms, from potassium cyanate and ammonium sulfate by Friedrich Wöhler in the Wöhler synthesis.historicalurea Friedrich WöhlerWöhler synthesis (Source:

9 Substrates (input materials) on which anaerobic treatment processes are used in ecosan context  High-strength greywater (as a pre-treatment step), rule of thumb: BOD > 400 mg/L  Blackwater with or without urine (blackwater: faeces, urine, small amount of water – e.g. from vacuum toilets) – as a pre- treatment step  Human excreta together with animal excreta and greywater, followed by reuse in agriculture “Blackwater” from vacuum toilets in Sneek, the Netherlands (see also Part C of this presentation) High-strength greywater (example from Jordan, see Course 2 Unit 1 Part D)

10 Basic anaerobic digestion (AD) terminology TermDescription AnaerobicWithout oxygen AerobicWith oxygen, e.g. in activated sludge plants or in aerobic ponds Anaerobic digestion / degradation / treatment These terms are all used interchangeably, and mean “breaking down of organic matter” Digestate / digester residue / digested organic matter The effluent from a digester; the liquid product of the anaerobic digestion process BiogasGas produced by microorganisms in anaerobic process (typically 66% methane content) Biogas digester / anaerobic digester A covered vessel (or reactor) in which anaerobic digestion occurs Course 2 Unit 4

11 Just as an aside: Another note on terminology In Germany (and perhaps other countries, too) there is currently still an unwritten convention:  Plants/processes where the input is mainly agricultural waste are called biogas plants  Plants/processes where the input is mainly municipal organic solid waste (“green waste”) are called fermenters or anaerobic digestion plants

12 Basic schematic representation of how dry solids content is determined in the laboratory Sample to be analysed for total solids content Sample is either filtered (schematic above) or, if it is too thick for filtering, it is dried (at 105ºC) without filtering Further drying of solid residue (at 105ºC), then weighing of the dried mass Final result: grams of dry solids per L of sample

13 Convention for the unit of dry solids (d.s.) The measurement result is commonly expressed as % d.s. Example: 1% d.s. is equal to 10,000 mg/L of solids in water  This means that 99% of the sample consists of water Another example: 100% d.s. = 1,000,000 mg/L = 1 kg/L = no water in the sample

14 The total dry solids consist of two parts: volatile solids and inert solids Total solids (TS) –= (organic solids + inorganic solids) –Measured after drying at 105°C –“Dry solids” is another word for total solids Volatile solids (VS) –Also called “organic solids” –That fraction of the total solids which can be burnt (volatilised) in the muffle oven at 520°C –Only the volatile solids can be broken down by anaerobic digestion Inorganic or inert solids (e.g. grit, sand)

15 Anaerobic digestion process overview  In the anaerobic digestion process, micro-organisms convert complex organic matter to biogas, which consists of methane (CH 4 ) and carbon dioxide (CO 2 )  Some organic matter remains even after the digestion step, and this is called digestate or digester residue or digested organic matter  Anaerobic digestion is used to treat high-strength wastewater, organic solid waste, sewage sludges, blackwater, faecal sludge, agricultural waste, food industry waste (e.g. breweries, slaughter houses, dairy), manure,....  Anaerobic digestion with biogas production also occurs in landfills, septic tanks, cows’ rumen, natural or constructed wetlands, dams where vegetation was flooded  all these sites produce methane gas! Course 2 Unit 4

16 As an aside: significant methane releases from other human-influenced processes  Rice production  Thawing permafrost in Siberia (due to climate change)  Bio-industry

17 Remember: Methane is a dangerous (potent) greenhouse gas Methane is a greenhouse gas with a global warming potential over 100 years of 23 i.e. when averaged over 100 years each kg of CH 4 warms the earth 23 times as much as the same mass of CO 2 Source:

18 Some facts about methane  Methane: CH 4  Methane is the major component of “natural gas”*, about 97% by volume  At room temperature and standard pressure, methane is a colorless, odorless gas (the smell characteristic of natural gas is an artificial safety measure caused by the addition of an odorant)  Methane has a boiling point of −162°C at a pressure of one atmosphere  As a gas it is flammable** only over a narrow range of concentrations (5–15%) in air  Methane has a calorific value of 10 kWh/Nm 3 or 35,900 kJ/Nm 3  Hence, biogas with 65% methane has a calorific value 6.5 kWh/m 3 (23,300 kJ/m 3 )  * Natural gas is a gaseous fossil fuel consisting primarily of methane but including significant quantities of ethane, butane, propane, carbon dioxide, nitrogen, helium and hydrogen sulfide. It is found in oil fields and natural gas fields, and in coal beds.  ** Flammability or Inflammability is the ease with which a substance will ignite, causing fire or combustion. Materials that will ignite at temperatures commonly encountered are considered flammable. Source:

19 Anaerobic digestion process schematic Organic matter (energy-rich) Example: Liquid flowrate: 10 m 3 /d Mass flowrate: 1 ton VS/d Digestate (energy- poor; can be used as fertiliser; includes anaerobic biomass) Liquid flowrate: 10 m 3 /d Mass flowrate: 0.3 ton VS/d Biogas (methane): “Green energy” Example: Gas flowrate: 665 Nm 3 /d * Nm 3 stands for normal cubic metre, meaning a measurement at STP or standard temperature and pressure (absolute pressure of 100 kPa (1 bar) and a temperature of K (0 °C)) Anaerobic digester (biological reactor) * Calculated by using 0.95 Nm 3 /kg VS destroyed - see next slide

20 Some guidelines for amount of biogas produced per amount of organic material digested  Sewage sludge: 0.75 – 1.12 Nm 3 per kg of volatile solids destroyed (typical value: 0.95 Nm 3 /kg)  Organic solid waste : –0.38 – 0.42 Nm 3 per kg of volatile solids added (at a retention time of 14 days) for single-stage processes –Up to 0.6 Nm 3 per kg of VS added for two-stage processes (two-stage: a process whereby step 1 & 2 is separated (in separate reactors) from step 3 & 4 as shown in slide 24) Course 2 Unit 4

21 Example: Standard design of household biogas plants in Nepal Waste (water) Digester residue ~ 1 million of these in Nepal (in 2006)

22 Note: At many landfill sites around the world, the biogas produced is now being captured and used (can be with high-tech or low-tech methods) Landfill on island of Maui - Source

23 Anaerobic digestion (AD) microbiology fundamentals  Under anaerobic conditions, organic substances are not aerated (oxidised), but are fermented (reduced) (Reduction = assimilation of electrons)  Energy-rich end products, like organic acids or alcohols are electron acceptors  It is quite a “slow” process (low growth rate of methanogens) compared to aerobic processes  relatively long sludge retention times are required  Like all biological processes, it is temperature dependent (higher conversion rates at higher temperatures)  digesters are typically heated / insulated or below ground  The process occurs as a four-step process (see next slide) Course 2 Unit 4

24 4 steps in anaerobic conversion 24 Remember: this is not a complete conversion - some organic matter will remain (digestate) Note that biogas is a mixture – not only the useful CH 4 Depending on the substrate there can be other gases too (slide 27)

25 Additional explanations on the 4-step process shown on previous slide 1.Volatile fatty acids (VFAs) are an intermediate product: –They should not accumulate under normal operation –VFAs (e.g. acetic acid) accumulate if step 4 is inhibited  In that case, pH value will drop (e.g. to pH of 4.8) and the digestion process will stop (no more gas production)  This is also called a “sour” digester, and is usually very smelly (a well operating digester produces almost no odours)

26 Some information on methanogens (they belong to the group of microorganisms called archaea)  Methanogens are archaea that produce methane as a metabolic byproduct. They are common in wetland, where they are responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they are responsible for flatulence. They are also common in soils in which the oxygen has been depleted.archaeamethanewetlandmarsh gas ruminantshumansflatulenceoxygen  Methanogens are anaerobic. All methanogens are rapidly killed by the presence of oxygen.anaerobic  Archaea are a major division of microorganisms. Like bacteria, Archaea are single-celled organisms lacking nuclei and are therefore prokaryotes, classified as belonging to kingdom Monera in the traditional five-kingdom taxonomy.microorganismsnucleiprokaryotesMonerakingdomtaxonomy  Note: the methanogens are a type of microorganism, but do not belong to the group of bacteria.  Source:

27 Biogas composition CompoundVol % Methane50-75 Carbon dioxide25-50 Nitrogen< 7 Oxygen< 2 Hydrogen sulfide< 1 Ammonia< 1 The methane fraction produced in the biogas varies with the input material; as a rule of thumb: carbohydrates:approx. 50 vol.-% methane fats:approx. 70 vol.-% methane proteins:approx. 84 vol.-% methane Course 2 Unit 4

28 Biogas uses 1. Biogas can be burnt and used for cooking or lighting 2. Biogas can also be converted to electricity and heat (part of the heat is often used to heat the digester)  “Combined heat and power plants” (CHP), or co-generation plants  If biogas is not used it should be flared* because methane is a greenhouse gas  Biogas from individual septic tanks is normally not flared (assumption is that volume is negligible – but is that a fair assumption?) * see next slide for explanation of a flare Top photo: hands of Mantopi Lebofa, Lesotho, Dec. 2006

29 Explanation for previous slide: What is a flare (for biogas) exactly? (slide 1 of 2)

30 What is a flare (for biogas) exactly? (slide 2 of 2)  There are many companies who can provide the equipment for a flare (e.g. for landfill gas flares) - Just as an example, you can look at this website (photos from the previous slide are from their website): This supplier states (for more information, see their website): Our flare systems can also be equipped with: Knockout drums Single or multiple blower arrangements Paperless chart recorders Methane monitors

31 Rule of thumb for removal of different compounds by anaerobic digestion (AD) CompoundRemoval Organic matterHigh level of removal (but not good enough for direct discharge to surface waters; would need aerobic post-treatment) Nitrogen and phosphorus No removal PathogensNot much removal unless operated at thermophilic* temperatures and very long retention times (see next slide) Heavy metalsNo removal * Thermophilic (~55° C) anaerobic digestion will achieve more pathogen removal than mesophilic (~ 35° C) anaerobic digestion

32 Pathogen removal in AD processes  In small biogas digesters, the process is operating at ambient or mesophilic temperatures, and is difficult to control –Temperature and retention time therefore vary and sufficient pathogen reduction is difficult to achieve even at long retention times  Example research results for pathogen removal in AD (Heeb et al., 2007): PathogensTermophilic (53-55°C) Mesophilic (35-37°C) Ambient (8-25°C) fatalityHRTfatalityHRTfatalityHRT Salmonella100 % %7 44 Shigella100 % Polivirus--100 %9-- Schistosoma ova 100 %<1100 % Hookworm100 % %30 Ascaris ova100 %298.8 %3653 %100

33 Important design parameter: residence time  The residence time in a digester is also called hydraulic residence time (HRT), or retention time (  )  It is the length of time that the liquid stays in the reactor  Once you know the design residence time for your process, you can calculate the required volume of the digester V = Q ·  design With: Q: flowrate (m 3 /d), e.g. 0.5 m 3 /d  design : design residence time, e.g. 30 days Then required volume is: 15 m 3  Examples (see also Part B): –Anaerobic baffled reactor: HRT = 2-3 days –Sewage sludge digestion: HRT = 15 – 20 days Course 2 Unit 4

34 Degradability of organic materials Easy to degrade Hard to degrade Examples: Sugar Vegetables Fats Faeces Grass Leaves Wood chips Lots of biogas in short time (short residence time) Not so much biogas and long residence times needed

35 Yield of biogas from different sources (1/2)

36 Yield of biogas from different sources (2/2)

37 Advantages of “biogas toilets” (anaerobic treatment of mixed toilet waste) compared to UDD toilets  No need to separate urine, hence easier for the toilet user, no extra piping, no extra tank  Can receive toilet flushwater - hence no need to abandon habit of flushing with water  Can receive greywater  Biogas can be used for cooking and lighting  Can take animal manure and organic solid waste  Can have the image of a “high- tech” solution Household biogas digester (fixed dome) during construction in Lesotho (note gas outlet at the top) – Photo by Mantopi Lebofa

38 Typical applications for “biogas toilets”  Public toilets in slums, e.g. in India; Kibera slum in Nairobi  Toilets at schools, universities, prisons and other institutions (e.g. India, Rwanda)  Situations where animal waste is available and can be combined with human waste (e.g. Nepal, India, China)  Regions where pour-flush toilets are commonly used (also in combination with anal washing with water)  See Part C for examples

39 Disadvantages of “biogas toilets” compared to UDD toilets Biogas toilets...:  Are not suitable for individual households unless the toilets can also receive animal waste (e.g. from cows)  Have higher capital cost – depending on the number of people served  Require more know-how for construction (higher safety precautions)  Produce digestate which can be relatively high in pathogens –OK for use as fertiliser but needs further safety barriers for safe reuse  You don’t have your nutrients available in a high-concentrated stream  You need to decide on a case-by-case basis which type of toilet is better suited

40 Advantages of anaerobic wastewater treatment (for greywater) compared to aerobic* treatment  Production of energy-rich methane  No energy demand for aeration  No removal of nitrogen and phosphorus (this is an advantage if effluent is to be reused in agriculture)  High organic loading rates can be applied - Suitable for high-strength wastewater (high BOD)  Low production of excess sludge; the digestate is highly stabilised and can easily be dewatered * Examples for aerobic wastewater treatment: activated sludge plants, trickling filter plants (see Course 2 Unit 1 Part D) Course 2 Unit 4

41 Disadvantages of anaerobic wastewater treatment (of greywater) compared to aerobic treatment  Effluent from anaerobic treatment has higher COD concentration than from aerobic treatment - If better effluent quality is required then a second (aerobic) treatment step may be required  Does not remove nutrients (this is a disadvantage if effluent is discharged to receiving water body)  Start-up of the process may take long time (slow growth of methanogens)  Anaerobic microorganisms are sensitive to some toxic compounds  Can cause odour problems if not operated properly  Only limited pathogen removal

42 Classification of anaerobic digestion processes  By temperature: - Mesophilic (35°C) - hermophilic (55°C)  By operation: - Batch - Continuous - Fed-batch or semi-continuous  By water content of input material: - Wet systems: TS content < 15% d.s. - “Dry” systems: TS content 25-50% d.s. -  rule of thumb: AD does not work if all input material has TS > 50% d.s (too dry) Course 2 Unit 4 Remember: 15% d.s. means 150,000 mg/L dry solids content and TS stands for total solids (same as d.s. which stands for dry solids)

43 Example for anaerobic digestion operating and performance parameters Operating parameters  Hydraulic retention time in digesters (also called treatment time): 15 – 20 days  Operating temperature: –Ambient –Mesophilic (35°C) –Thermophilic (55°C)  Type and composition of feed (input material) –TS and VS content of feed –Degradability Performance parameters  VS loading rate: 1.6 – 4.8 kg/m 3 /d  VS destroyed: 56 – 66%  Methane content in biogas (%) – expect 50 – 75%  Gas production per kg VS destroyed (m 3 /kg VS destroyed) Values provided on this slide are for high-rate complete-mix mesophilic anaerobic digestion (Metcalf & Eddy, page 1513 and 1514)

44 How to detect a failing anaerobic treatment process  Odour  Explosion (worst case !! – extremely rare) – see next slide  Foaming  Low pH value (step 4 of 4-step process on slide 24 is inhibited)  No or low biogas production  Low methane content in biogas  Volatile solids (VS) fraction in effluent close to the VS fraction in the influent, indicating no VS removal

45 How could an explosion of an anaerobic digester occur?  If a vacuum develops in the digester (e.g. leaks of liquid):  air is sucked in  if methane content is 5-15% in air, and there is a spark, then there could be an explosion  If digester is in an enclosed building and biogas leaks out:  if there is a lack of ventilation and a spark, then there could be an explosion  Checking for liquid and gas leaks is an important operational maintenance task  Having said all this, I have never heard of such an explosion actually having taken place (have you?)

46 Main possible causes of process failure  Organic overload (too much BOD added per m 3 and day) - This applies particularly to easily degradable substrate, e.g. brewery wastewater  Insufficient alkalinity and therefore a drop in pH (could add alkalinity, e.g. lime)  Toxic substances in influent are inhibiting methanogens (this applies only to industrial wastewater)

47 Course 2 Unit 4 Part B: Anaerobic treatment technologies relevant for ecosan concept Household biogas plant (fixed dome) in Maseru, Lesotho (at the end of construction) Course 2 Unit 4

48 Two principal types of construction to deal with gas development  Fixed dome in which a pressure builds up (see Lesotho example in Part C) –Common for small-scale plants –Needs skilled workers for construction but less attention during operation (no moving parts)  Floating or moveable dome/cover which allows an expansion of the gas volume in the digester (see examples in Part C) –A “gas bubble” can be used –This type is more common for large-scale plants

49 Overview of commonly used anaerobic treatment technologies # Process nameMechanical mixing Covered reactor Biogas collection Scale 1, 2 Septic tanks, anaerobic baffled reactors NoYesNoHousehold or neighbourhood 3 Household biogas plants* NoYes Household, neighbour-hoods, institutions 4 Anaerobic pondsNo Community 5 Upflow anaerobic sludge blanket reactor (UASB) Yes Neighbourhood or community Course 2 Unit 4 * Also called household biogas digesters or decentralised biogas plants (i.e. not just limited to households)

50 1- Septic Tanks  Very common on-site sanitation system for excreta and greywater  Relatively common also in some high-income countries: Australia, USA  In most cases, biogas is not collected (amount is small unless animal manure is digested as well; in that case it is no longer called a septic tank) * Septic is a word used for sewage that has gone anaerobic, but it is not really a scientific term Maseru, Lesotho, Dec 2006 (septic* tanks are always underground)

51 Septic tank process principles  Combined settling, skimming and anaerobic digestion  Commonly followed by filtration of effluent (e.g. sub-surface soil disposal field)  No mechanical equipment (no moving parts) Ground level

52 Reminder: How can septic tanks affect the groundwater? Ground level Effluent to soil infiltration (normal) Groundwater (aquifer) Wastewater from house Soil Soil (unsaturated zone) Faecal sludge (if “leaking septic tank”) The effluent from septic tanks is commonly infiltrated into the ground (on purpose). But faecal sludge is NOT meant to leak out from the septic tank (but often does if not designed properly). from Course 1 Unit 3

53 Septic tank design and advantages and disadvantages  Design:  Sedimentation tank  Settled sludge partially stabilised by anaerobic digestion  Almost no removal of dissolved and suspended matter  1-3 compartments  → look for national design standards!  Advantages:  Simple technology for on-site treatment  Little space required (underground)  Institutional acceptance is high  Disadvantages:  Low treatment efficiency (COD removal approx. 50%; almost no nitrogen removal)  O&M often neglected (desludging) or unknown!  Relies on water for toilet flushing  Effluent quality is difficult to monitor  Requires periodical removal of faecal sludge (every years, depending on tank size)  Faecal sludge management is often not carried out properly (often just dumped in environment) This slide and the next four slides were provided by Dr. Doulaye Koné from SANDEC/Eawag, Switzerland

54 Septic tank design schematic (2 compartments) Aim is to achieve some mixing and contact of influent with sludge layer which contains the anaerobic digestion micro-organisms Wastewater (solids settling) Wastewater effluent (partially treated) Faecal sludge Toilet wastewater, greywater Course 2 Unit 4

55 Septic tank design aspects  Mainly rectangular (some exceptions if prefabricated)  Length to width ratio: 3 to 1  Depth: 1 to 2.5 m  First chamber is at least 50% of the total volume (2 chambers → 1st chamb. = 2/3; 3 chambers → 1st chamb.= 1/2)  Manholes in the cover slab: one each above inlet and outlet and one at each partition wall  Tank must be watertight and stable → construction material: reinforced concrete (most common), steel (corrosion problems), polyethylene, fibreglass or plastic. Cheap solution: bricks Course 2 Unit 4 Are there any national design standards in your country?

56 2 - Anaerobic baffled reactor (baffled septic tank)  Improved septic tank with 2 to 3 chambers in series (up to 5)  Intensive contact between resident sludge and fresh influent  Treatment efficiency: 65 to 90% COD removal; HRT = 2-3 days  Advantages:  Higher treatment efficiency than septic tanks, hardly any blockages  High removal efficiencies, also for suspended and dissolved solids  Disadvantages:  Construction and maintenance more complicated than for conventional septic tank Faecal sludge Wastewater influent Effluent

57 Anarobic baffled reactors during construction

58 3 – Household* biogas plants  Household biogas plants produce a continuous flow of digested material (liquid sludge), which is used as fertiliser (despite not being free of pathogens)  Desludging (removal of sludge) is only necessary if there is a build- up of inert material (e.g. sand; lack of mixing) - Expectation is to “never” have to desludge them (> 15 years)  These plants do not aim for solids settling but rather good mixing - Therefore, they have their reactor outlet at the bottom (rather than at the top like a septic tank)  * The word “household” is a bit misleading  they can also be used for institutions, businesses, hotels etc.

59 Household biogas plants are common world- wide, particularly in Asia  Millions of plants worldwide, particularly in China, India, Nepal –Example rural Nepal: about 1 million biogas plants in 2006 Often users only apply manure but no human excreta nor greywater  but this could change  Work best in conjunction with animal manure –Sufficient biogas for cooking and lighting needs of one family (if they have one cow for example) –Rule of thumb: 1 cow equals 17 people with respect to biogas production from excreta (Ralf Otterpohl, Ecosanres Discussion Forum on 4 July 2006) Course 2 Unit 4

60 Household biogas plant schematic (fixed dome) Removable cover for occasional desludging (rare)

61 4 - Anaerobic ponds  Also called lagoons (in the US) or waste stabilisation ponds  Low-rate anaerobic process (e.g. 1 – 2 kgCOD/m 3 /d)  Solids settling and anaerobic decomposition  Depth: 5-10 m  Could be covered for odour control and gas collection (but most of them are not covered)  Usually several ponds in series (last pond: aerobic maturation pond with algae; pathogen kill by sunlight) Influent (faecal sludge, greywater or conventional ww.) Effluent Sludge layer (increasing over time) Course 2 Unit 4 This slide was provided by Peter van der Steen (UNESCO-IHE)

62 Anaerobic ponds This pond is not covered A sludge crust may form and act as a cover

63 Limitations of anaerobic ponds  Large land area required - Potentially high costs for covers, if these are used  Needs desludging after years (this is often forgotten!) - e.g. By stopping inflow, then settling and drying for 2 months then manual emptying  reuse in agriculture?  Feed flow distribution inefficiencies  Poor contact between substrates and biomass (see schematic below) Influent Effluent Sludge layer (biomass) Substrates

64 The issue of methane emissions and covering of ponds  Anaerobic ponds emit biogas which contains the greenhouse gas methane  It is possible to cover the ponds and to collect the biogas (for energy generation or flaring)  Floating cover systems: floating membrane made of lined PVC or High Density Polyethylene - Covers needs to be durable, UV protected, chemically resistent to biogas; support foot traffic and rainwater loads  New: emission reduction contracts can be signed based on capturing the methane gas from anaerobic lagoons  sale of biogas emission reduction possible - First example in developing country: Santa Cruz, Bolivia in 2006 (Source: Menahem Libhaber in Huber’s Symposium Water Supply and Sanitation for All, Sept. 2007, Berching, Germany) Are you aware of anaerobic ponds (waste stabilisation ponds/lagoons) in your city? Could they be covered?

65 5 - Principles of UASB reactors  UASB = Upflow anaerobic sludge blanket reactor –Inflow flows in vertical direction (from bottom up – upflow)  A high sludge concentration is maintained in the reactor, which results in long solid retention times  Short hydraulic retention times  Good contact between substrates (COD) and the sludge (bacteria)  High-rate system (high organic loading rates, e.g kgCOD/m 3 /d)  UASBs can treat: –Blackwater (faeces and urine), manure –Conventional wastewater (high strength), greywater –Industrial effluent –Agricultural organic waste

66 A UASB reactor for the treatment of 6000 PE (person equivalents) domestic wastewater The biogas contains sulphide, which can be removed in iron filters (FeS precipitation)

67 UASB reactor components – slide 1 of 3 In the sludge bed biogas bubbles are produced that rise through the sludge bed and mix it. There is good contact between the dense sludge bed and the upflowing substrate Influent Effluent Biogas Course 2 Unit 4

68 UASB reactor components – slide 2 of 3 The biogas bubbles are directed into a separator Effluent Influent Biogas

69 UASB reactor components – slide 3 of 3 In the settler compartment there is no turbulence since the bubbles have been removed. Ideal conditions for settling. Solids settle onto the settler and periodically slide back into the sludge bed. Effluent Influent Biogas

70 Concluding remarks regarding anaerobic digestion  Great potential: provides biogas for cooking, lighting and heating; and provides (liquid) fertiliser -Is increasing in importance in the light of climate change (need for alternative energy sources)  Most interesting for: -Combination with animal waste -Institutions with lots of people, e.g. prisons, public toilets, schools, universities  Anaerobic digestion as part of a sanitation system can help to close the loop of nutrients otherwise wasted to the environment, and ensure recycling of valuable wastes in a sustainable manner  Remaining issues: - Quality of digestate not well documented - Pathogen removal in mesophilic AD is quite low  but digestate is widely used in agriculture anyway  use multiple-barrier approach (see Course 3)

71 References  Butare, A and Kimaro, A (2002) Anaerobic technology for toilet wastes management: the case study of the Cyangugu pilot project, World Transactions on Engineering and Technology Education, Vol.1, No.1. AbstractsVol1No1/Microsoft%20Word%20-%2032_Butare.pdf * AbstractsVol1No1/Microsoft%20Word%20-%2032_Butare.pdf  Heeb, J., Jenssen, P., Gnanakan, K. & K. Conradin (2007): ecosan curriculum 2.0. In cooperation with: Norwegian University of Life Sciences, ACTS Bangalore, Swiss Agency for Development and Cooperation, German Agency for Technical Cooperation and the International Ecological Engineering Society. Partially available from and human-dignity-lecture-2006.ppt  Tchobanoglous, G., Burton, F.L., Stensel, H.D. (2003) Wastewater Engineering, Treatment and Reuse, Metcalf & Eddy, Inc., McGraw- Hill, 4th edition. This is a good book on conventional wastewater treatment  Zhang Wudi et al. (2001): Comprehensive utilization of human and animal wastes. Proceedings of the First International Conference on Ecological Sanitation in Nanning 2001,EcoSanRes, China Course 2 Unit 4 * Also under Extra Materials on the I-LE

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