Lecturer: Mariska Ronteltap

Lecturer: Mariska Ronteltap m.ronteltap@unesco-ihe.org
Course 2 Unit 4 Introduction to anaerobic treatment technologies Lecturer: Mariska Ronteltap

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

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

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

from Course 1 Unit 3 Reminder: Important treatment technologies often used as part of ecosan concepts Process Technical options Reason for popularity in ecosan Composting Composting 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

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

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

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. 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. (Source:

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)

Basic anaerobic digestion (AD) terminology
Course 2 Unit 4 Basic anaerobic digestion (AD) terminology Term Description Anaerobic Without oxygen Aerobic With 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 Biogas Gas produced by microorganisms in anaerobic process (typically 66% methane content) Biogas digester / anaerobic digester A covered vessel (or reactor) in which anaerobic digestion occurs

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

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

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

The total dry solids consist of two parts: volatile solids and inert 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 Total solids (TS) = (organic solids + inorganic solids) Measured after drying at 105°C “Dry solids” is another word for total solids Inorganic or inert solids (e.g. grit, sand)

Anaerobic digestion process overview
Course 2 Unit 4 Anaerobic digestion process overview In the anaerobic digestion process, micro-organisms convert complex organic matter to biogas, which consists of methane (CH4) and carbon dioxide (CO2) 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!

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

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 CH4 warms the earth 23 times as much as the same mass of CO2 Source:

Some facts about methane
Methane: CH4 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/Nm3 or 35,900 kJ/Nm3 Hence, biogas with 65% methane has a calorific value 6.5 kWh/m3 (23,300 kJ/m3) * 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:

Anaerobic digestion process schematic
* Calculated by using 0.95 Nm3/kg VS destroyed - see next slide Biogas (methane): “Green energy” Example: Gas flowrate: 665 Nm3/d * Digestate (energy-poor; can be used as fertiliser; includes anaerobic biomass) Liquid flowrate: 10 m3/d Mass flowrate: 0.3 ton VS/d Anaerobic digester (biological reactor) Organic matter (energy-rich) Example: Liquid flowrate: 10 m3/d Mass flowrate: 1 ton VS/d Nm3 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))

Course 2 Unit 4 Some guidelines for amount of biogas produced per amount of organic material digested Sewage sludge: 0.75 – 1.12 Nm3 per kg of volatile solids destroyed (typical value: 0.95 Nm3/kg) Organic solid waste: 0.38 – 0.42 Nm3 per kg of volatile solids added (at a retention time of 14 days) for single-stage processes Up to 0.6 Nm3 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)

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

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

Anaerobic digestion (AD) microbiology fundamentals
Course 2 Unit 4 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)

4 steps in anaerobic conversion
Remember: this is not a complete conversion - some organic matter will remain (digestate) Note that biogas is a mixture – not only the useful CH4 Depending on the substrate there can be other gases too (slide 27)

Additional explanations on the 4-step process shown on previous slide
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)

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. Methanogens are anaerobic. All methanogens are rapidly killed by the presence of oxygen. 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. Note: the methanogens are a type of microorganism, but do not belong to the group of bacteria. Source:

Course 2 Unit 4 Biogas composition 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 Compound Vol % Methane 50-75 Carbon dioxide 25-50 Nitrogen < 7 Oxygen < 2 Hydrogen sulfide < 1 Ammonia

Biogas uses Biogas can be burnt and used for cooking or lighting
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

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

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

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

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): Pathogens Termophilic (53-55°C) Mesophilic (35-37°C) Ambient (8-25°C) fatality HRT Salmonella 100 % 1-2 7 44 Shigella 1 5 30 Polivirus - 9 Schistosoma ova <1 7-22 Hookworm 10 90 % Ascaris ova 2 98.8 % 36 53 % 100 In small biogas digesters, the process is operating at ambient or mesophilic temperature and is difficult to control. Temperature and retention time therefore vary and sufficient pathogen reduction is difficult to achieve even at very long retention times (see graph below). Post-treatment like drying or thermophilic composting with organic material is advisable.

Important design parameter: residence time
Course 2 Unit 4 Important design parameter: residence time The residence time in a digester is also called hydraulic residence time (HRT), or retention time (t) 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 · tdesign With: Q: flowrate (m3/d), e.g. 0.5 m3/d tdesign: design residence time, e.g. 30 days Then required volume is: 15 m3 Examples (see also Part B): Anaerobic baffled reactor: HRT = 2-3 days Sewage sludge digestion: HRT = 15 – 20 days

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

Yield of biogas from different sources (1/2)

Yield of biogas from different sources (2/2)

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

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

Disadvantages of “biogas toilets” compared to UDD 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

Course 2 Unit 4 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)

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

Classification of anaerobic digestion processes
Course 2 Unit 4 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) 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)

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/m3/d VS destroyed: 56 – 66% Methane content in biogas (%) – expect 50 – 75% Gas production per kg VS destroyed (m3 /kg VS destroyed) Values provided on this slide are for high-rate complete-mix mesophilic anaerobic digestion (Metcalf & Eddy, page 1513 and 1514)

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

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?)

Main possible causes of process failure
Organic overload (too much BOD added per m3 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)

Course 2 Unit 4 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)

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

Overview of commonly used anaerobic treatment technologies
Course 2 Unit 4 Overview of commonly used anaerobic treatment technologies # Process name Mechanical mixing Covered reactor Biogas collection Scale 1, 2 Septic tanks, anaerobic baffled reactors No Yes Household or neighbourhood 3 Household biogas plants* Household, neighbour-hoods, institutions 4 Anaerobic ponds Community 5 Upflow anaerobic sludge blanket reactor (UASB) Neighbourhood or community * Also called household biogas digesters or decentralised biogas plants (i.e. not just limited to households)

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) Maseru, Lesotho, Dec 2006 (septic* tanks are always underground) * Septic is a word used for sewage that has gone anaerobic, but it is not really a scientific term

Septic tank process principles
Ground level Ground level 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)

Reminder: How can septic tanks affect the groundwater?
from Course 1 Unit 3 Reminder: How can septic tanks affect the groundwater? Ground level Ground level Effluent to soil infiltration (normal) Wastewater from house Soil Soil Soil (unsaturated zone) Soil Faecal sludge (if “leaking septic tank”) Soil Groundwater (aquifer) 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).

Septic tank design and advantages and 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) 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 This slide and the next four slides were provided by Dr. Doulaye Koné from SANDEC/Eawag, Switzerland

Septic tank design schematic (2 compartments)
Course 2 Unit 4 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 Toilet wastewater, greywater Wastewater effluent (partially treated) Wastewater (solids settling) Faecal sludge

Septic tank design aspects
Course 2 Unit 4 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 Are there any national design standards in your country?

2- Anaerobic baffled reactor (baffled septic tank)
Wastewater influent Effluent Faecal sludge 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

Anarobic baffled reactors during construction

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.

Household biogas plants are common world-wide, particularly in Asia
Course 2 Unit 4 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)

Household biogas plant schematic (fixed dome)
Removable cover for occasional desludging (rare) “Small biogas digesters serving one household (as shown above) are increasingly popular especially in Asia, but also in other parts of the world. The main goal of the household digesters is to produce biogas and provide the family with energy. The basis input is animal manure from a small household livestock, but human excreta and other organic wastes can be used in addition. The digester is generally built subsurface for protection from temperature changes and to save space.” Source: (29)

4 - Anaerobic ponds Course 2 Unit 4 Also called lagoons (in the US) or waste stabilisation ponds Low-rate anaerobic process (e.g. 1 – 2 kgCOD/m3/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) This slide was provided by Peter van der Steen (UNESCO-IHE)

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

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) Effluent Influent Substrates Sludge layer (biomass)

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?

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/m3/d) UASBs can treat: Blackwater (faeces and urine), manure Conventional wastewater (high strength), greywater Industrial effluent Agricultural organic waste

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)

UASB reactor components – slide 1 of 3
Course 2 Unit 4 UASB reactor components – slide 1 of 3 Biogas 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 Effluent Influent

Biogas The biogas bubbles are directed into a separator Effluent Influent

Biogas 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

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)

Course 2 Unit 4 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. * 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 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 * Also under Extra Materials on the I-LE

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