2.10 Septic tanks high in organic matter.

Multi-step decentraliced wastewater treatment systems

Synonymous terms to biogas sanitation system include does not need electricity if there is suitable slope for
biogas septic tank in which the anaerobic conditions are gravity flow, saving a large amount of investment into
referred to as "septic" giving the tank its name. the sewerage system. Low energy and maintenance
Compared to a proper designed, operated and maintained cost, low total lifetime cost.
biogas sanitation system, a typical septic tank system
If well designed, constructed and operated, calculated
consists of a (baffled) water tight tank and a soak-away drain sewage sludge production is five times less compared
without any reuse of the pre-treated effluent and without to aerobic systems. The sludge yield from anaerobic
capturing the biogas produced. Therefore, depending on soil treatment is approximately 0.1kg VSS/kg COD
conditions and groundwater level, the effluent of a septic tank removed; by contrast aerobic activated sludge treatment
can transport bacteria, viruses, household chemicals, and results in 0.5kg VSS/kg COD removed.
other contaminants into the groundwater causing serious
As the anaerobic treatment alone can not meet the
environmental problems
9
, and the released biogas requirements of direct discharge into water bodies, a
participates in the climate emissions. post-treatment with an aerobic process is necessary.
But even this combination reduces the specific sludge
production by 40% (Gasparikova et al., 2005).
3 Advantages and limitations of biogas Biodigesters offer a variety of benefits in ethical treatment of
sanitation systems human waste. The most important consideration, which has
not necessarily always been effectively managed, is the
3.1 Advantages of biogas sanitation systems danger pathogens in human waste pose to health. These
systems are scalable from the household, community level to
3.1.1 Advantages compared to aerobic wastewater the larger industrial scale applications. Successful
treatment systems applications can be found worldwide and as well as in history.
Best of all, Anaerobic Digestion offers to turn waste into a
The advantages of biogas sanitation systems include: resource. (Appropedia)

Generation of clean energy for household use: after an
initial investment in the system, there is less or no need 3.1.2 Reuse of digested sludge
to spend money on fuel, and no more smoke from wood Sanitation has a strong link to agriculture, as the nutrients
or charcoal in the kitchen. such as nitrogen, phosphorus and potassium contained in

Cooking on biogas is quicker and easier than cooking human excreta are suitable as fertiliser, and the organics as
with firewood. soil conditioner. Biogas sanitation contributes to closing the

Destruction of bacteria, viruses and helminth eggs in nutrient cycle which is a target of sustainable agriculture.
human and animal excreta. A farm with a biogas system Each day, one adult excretes about 30g of carbon (90g of
is a cleaner and safer place. organic matter), 10-12g of nitrogen, 2g of phosphorus and 3g

Production of safe fertilizers for use on the farm of potassium. Studies carried out under Prof. Dr. Ralph
containing plant nutrients in an easy absorbable liquid Otterpohl (Technical University of Hamburg-Harburg)
form. revealed that in human excreta most of the organic matter is

Support the fight against global warming by facilitating contained in the faeces, while most of the nitrogen (70-80%)
to burn methane from organic waste, instead of and potassium are contained in urine. Phosphorus is equally
escaping into the atmosphere where it adds to the distributed between urine and faeces. (Source: Otterpohl,
greenhouse effect; supports also efforts to restrict Ralph 2002)
deforestation.

Cost effectiveness: Biogas septic tanks have at least It has been calculated that the fertilising equivalent of excreta
the same investment as a conventional septic tank, and is nearly sufficient for a person to grow its own food (Drangert
capture the biogas for further use. Operation and 1998). In reality, part of this potential is lost, during storage
maintenance expenses (energy and supplies) are low and treatment (such as nitrogen loss through ammonia
and require only low skilled labour. For financial volatilisation).
consideration the energy source that is replaced by
Treated excreta may serve not only a fertiliser; its organic
biogas is important (wood, kerosene, LPG). matter content, which serves as a soil conditioner and humus

Low-tech system: Anaerobic technology does not rely replenisher – an asset not shared by chemical fertilisers – is
on complex machines and processes (such as aeration
of equal importance. Traditional practices esp. in South Asia
systems); systems, such as the anaerobic pre-treatment
of recycling faecal sludges to agriculture and aquaculture
units (settler, baffled reactors or filters) of a complex have ever since made use of this resource. Urban farmers in
decentralized wastewater treatment system, require low arid and semi-arid zones or during dry seasons are still using
but adequate maintenance.
wastewater, raw or treated, for irrigation to minimise the

Low space requirement: underground construction does
purchase of chemical fertiliser.
not occupy valuable space especially in urban areas;
2 3
only 0.5-1m per m daily flow are needed, compared to Recycled sludge and water might still contain germs. For re-
2 3
25-30 m /m /d flow in aerobic ponds and constructed use in agriculture and gardening specific sanitization perfor-
wetlands (Gutterer, Panzerbieter et al., 2009). The mance of the chosen systems and post-treatments should be
space above a biogas plant could also be built on as considered. Please refer to details in Table 2. Nevertheless,
parking area, as long as the system remains accessible. agricultural and gardening experts’ advice is required to op-

Treatment capability for a wide variety of domestic and timize seasonal application and quantities related to cash
industrial effluents, especially suitable for wastewater crops, flowers and trees.

9
www.waterencyclopedia.com/Re-St/Septic-System-
Impacts.html

Last Updated: 14 July 2010 Page 11

3.2 Disadvantages of biogas sanitation systems Post-
Frame conditions Effect Advantage Disadvantage
treatment

steps for ria, as well as some vi- quality. produced. sions.

3.2.1 Incomplete pathogen removal liquid ef- ruses, need more than 2
fluents days for a 1 log10 reduction. Tempera Mesophilic
Human excreta are contaminated with all kinds of pathogens (AF, ABR) ture/time treatment
Thermophilic digestion of depend- degrades
and hence a reliable technology is necessary for their sewage sludge in a large- ent. organic pol-
inactivation. During anaerobic digestion an inactivation of scale continuous process lutants.
reduces indicator bacteria
most animal and plant pathogens is obtained under and Salmonella sufficiently. The evalua-
thermophillic conditions (>55°C for several days). Several tion of bio-
On the other hand, meso- gas itself
studies on wet fermentation report that also mesophilic and philic digestion has proven always in-
lower temperature operation inactivates pathogens; further to be more efficient in de- dicated a
grading organic pollutants low risk re-
findings indicate that reactors with retention times of at least such as benzoic acid, m- garding
60 days at 20oC to 15 days and 35-55oC reduce significantly and p-cresol compared with disease
thermophilic digestion. transmis-
any type of pathogens (Michael H. Gerardi 2005). sion.
By using a process adapted
Table 1. Effects of anaerobic batch sanitisation on selected for high ammonia content
(8 g L− 1) at a pH close to 8,
pathogens and parasitic ova as well as on E. Coli indicator it is possible to have a sani-
tising mesophilic process.
Ambient
Mesophilic Ammonia is added either
Thermophilic temperature
fermentation (35- as aqueous ammonia solu-
Pathogens & fermentation (53-55°C) fermentation (8-
37°C) tion or as granulated urea.
parasitic ova 25°C)
This treatment is efficient
Fatality Fatality Fatality for inactivation of bacteria, Gives
Days Days Days
(100%) (100%) (100%) parasites and some vi- good
Low-tech
Salmonella 1–2 100 7 100 44 100 ruses. hygienic
equipment
Shigella 1 100 5 100 30 100 quality.
needed.
Poliviruses 9 100 Ammonia Recommended treatment is Low risk for re-
40 - 10 –
-4 treatment pH and
E-Coli titre 2 10-1 – 10-2 21 10-4 either 0.5% NH3 for one Ammonia growth.
60 10-5 un-
week, or 2% urea for two recycled as
Several 7– charged
weeks at temperatures a fertiliser.
Schistosoma ova 100 7 100 100 NH3 de-
hours 22 above 10 °C, or for one
pendent.
Hookworm ova 1 100 10 100 30 90 month at temperatures be-
Ascaris ova 2 100 36 98.8 100 53 low 10 °C.
(source: Yongfu, Y., Yibo, Q., Yunxuan, G., Hui, Z., Covering needed to avoid
Yuansheng, X., Chengyong, X., Guoyuan, F., Jiequan, X. ammonia emissions
Needs storage
and Taiming, Z. (1992), The Biogas Technology in China. capacity.
Agricultural Publishing House, China, ISBN 7-109-01777-X) Studies
Survival of
have
Many studies reveal also that under fully mixed mesophilic pathogens
shown a
Only ap- in soil, grass
conditions, pathogens are not completely inactivated. more rapid
plication and silage for
reduction in
Therefore recommendations on the use of the not post- weeks close to 2
the Soil
before months has
treated slurry should limit irrigation only to fruit trees, and Land ap- May be spread-on, sub- (“worked-
planting been shown
plication surface drainages or in”), in gen-
exclude spray irrigation to vegetables. Effluent water could be or seed- under labora-
worked-in. eral entero-
ing, or tory conditions
post-treated with UV desinfection by natural sunlight in viruses
before and survival on
seem to be
shallow polishing ponds. Post-composting of sludge may be winter. soil and biosol-
reduced
ids for over
required for a one year period. If the effluent is directly faster than
one year has
indicator
worked into the soil as soil conditionner no further restriction been proven
bacteria.
(spread-on,
applies. sub-surface
drainages).
Two main factors regulating the inactivation of pathogens Low-tech
If the slurry is not used di- May give equipment
have been identified, namely the temperature and the con- rectly used, it may be col- good possible. Low risk for re-
centration of free ammonia as a function of the time of treat- lected and treated in sludge hygienic growth.
drying beds. quality. Perimeter
ment/exposure. The post-treatment alternatives presented bunds will Should be
here (composting, anaerobic digestion, ammonia treatment, Sludge Partially dig up the ground Tempera help in rainwater dilu-
drying bed and pile up the excavated ture/time keeping tion protected
land application) all have their advantages and disadvan- soil to earthen bunds. depend- surface
tages depending on local conditions, the material to be ent. run-off wa- Not applicable
Alternate composting plat- ter from en- in monsoon
treated, and the intended use of the end product. Therefore, forms or -pits for batchwise tering the areas
prior to selection of treatment method it is necessary to management. sludge dry-
ing beds
evaluate the specific local conditions, and to define how the
end-product is to be used as a fertiliser, according to hygiene
risk of the crop.
Above table was compiled with information from: (a) Ann
Table 2: Overview on post-treatment options Albihn (National Veterinary Institute, Uppsala, Swede) and
Björn Vinnerås (Department of Biometry and Engineering,
Post-
treatment
Frame conditions Effect Advantage Disadvantage Swedish University of Agricultural Sciences, Uppsala,
Sweden), Biosecurity and arable use of manure and biowaste
The higher the tempera- Labour-
ture, the shorter the time intensive. — Treatment alternatives, Livestock Science 112 (2007)
May give
needed for treatment. good Low-tech 232–239, available online at www.sciencedirect.com and
Eutrophying
hygienic equipment (b) J. Heeb and M. Wafler (2006) Face-to-Face Training
WHO gives a recommenda- emissions.
quality. possible.
tion of a minimum one
Compost-
week of treatment above Risk for re-
Course “Capacity Building for Ecological Sanitation” Small-
ing Tempera May de-
50 °C for composting of ture/time grade or-
growth. Scale Biogas Sanitation Systems, Network for the
faecal matter. depend- ganic pol- Development of Sustainable Approaches for Large Scale
Leaching water
ent. lutants.
Alternate composting plat- effluent (may Implementation of Sanitation in Africa (NETSSAF), EU-
forms or -pits for batchwise be reused for
management. digestion). Coordination Action Proposal/Contract Number: 037099
www.netssaftutorial.com
Further During mesophilic anaero- May give Risk for re-
More valu-
anaerobic bic digestion many patho- good growth and
able energy
digestion genic and indicator bacte- hygienic methane emis-

Last Updated: 14 July 2010 Page 12

3.2.2 Temperature dependence sanitation system is technically more complicated as it
includes more components than just a (urine diversion
Organic material degrades more rapidly at higher dehydration) toilet. Main considerations for a technical design
temperatures because all biological processes operate faster includes substrate’s property (daily flow and peak flows, flush
at higher temperatures up to 65°C. The three ranges of water consumption, type and quantity of feedstock, Chemcial
temperature in which methanogens work are called Oxygen Demand (COD), Dry Matter (DM) and Volatile Solids
psychrophilic (8-25ºC), mesophilic (30-42ºC) and (VS)), expected operation data (temperature, pH, Organic
thermophilic (50-65ºC). Biogas sanitation is often applied in Load Rate (OLR), Hydraulic retention time (HRT), Sludge
countries where the ambient average temperature ranges retention time (SRT)), performance expectation (methane
above 15°C. In temperatures below 8ºC digestion cap ability production, CODt removal, required biogas storage volume),
is very reduced. The process is also sensitive to temperature post treatment requirements (digestate property (DM, COD,
variations of more than 3ºC; therefore variations have to be VS) and sludge disposal or reuse (DM, VS, composting
kept in a limited range to ensure a steady biogas production. temperature, available drying surface, leachate recycling)
The higher the process temperature the more sensitive is the and remaining agricultural nutrients (Nitrogen (N), Phosphour
process (bacteria). (P), Potassium (K). Special training on biogas sanitation
Still very little work has been done regarding the production design and engineering is offered through BORDA (www.borda-
of biogas at psychrophilic temperatures. At 20 day HRT, net.org), Indian Ecosan Service Foundation
propionate concentration has been reported to be about (http://ecosanservices.org), Technology for Economic Development
three times higher than that of acetate, whereas at higher (TED)-Lesotho (www.ted-biogas.org), Centre for Sustainable
HRT acetate and propionate maintained at almost equal Environmental Sanitation (CSES) at the University of Science
concentrations. It is concluded that anaerobic digestion of and Technology Beijing (USTB) (www.susanchina.cn), and at the
human excreat can be carried out at 10°C using adap ted University of Life Science (UMB) Norway (www.ecosan.no).
inoculums. Below 20°C methane production starts only with
the addition of temperature adopted inoculums, as the 3.2.5 Risk of explosion
practical expereicne of TED-Lesotho has shown. Research
results at the Wageneingen Unversity in The Netherlands As methane is flammable, there is always a small but
from 1991 showed a stable digestion process at a process manageable risk of explosion if methane escapes. The
temperature of 15 0C and an HRT of 100 and 150 days. The Flammable Range (Explosive Range) is the range of a
COD reductions were only 14 and 18 percent respectively. concentration of a gas or vapor that will burn (or explode) if
(Balasubramaniyam 2008) an ignition source is introduced. Below the explosive or
flammable range the mixture is too lean to burn and above
To improve biogas sanitation systems the application of the upper explosive or flammable limit the mixture is too rich
insulation (above and below ground), combined greenhouse to burn. The limits are commonly called the "Lower Explosive
application, keeping the plant environment dry, active and/or or Flammable Limit" (LEL/LFL) and the "Upper Explosive or
passive heating of plant and substrate should be considered, Flammable Limit" (UEL/UFL). For methane the LEL is 5%,
depending on available construction material, design the UEL is 15%, H2S- LEL is 4.3%, H2S-UEL is 46%.
experiences, funds and micro-location. Appropriate technical
solution should by done with the support of experienced
experts. The optimal storage design volume (HRT) has to 4 Overview of main types of biogas
consider the coldest winter conditions, but the post-treatment
choice the seasonal reuse options.
sanitation digesters

3.2.3 Variable performance 4.1 Classification of biogas sanitation systems

Performance may be less consistent than in conventional Biogas sanitation systems can be classified according to
(aerobic) treatments. In terms of removal of organic matter various parameters; the three most important design criteria
and nutrients, biogas sanitation is mainly a primary or are (1) hydraulic retention time (HRT in days), (2) organic
3
secondary treatment step, which may need post-treatment load rate (OLR in kg COD/m active fermenter volume), and
depending on the disposal or reuse strategy. The biological (3) sludge retention time (SRT in days or years).
components are sensitive to toxic chemicals, such as Table 3: Overview on biogas sanitation systems
ammonia and pesticides. Flushed pollutants or surges in
water flow could temporarily reduce treatment effectiveness. Type of Expected
OLR
(kg
HRT (days) SRT (days) optimal application
Therefore buffer tanks or biogas-settler as pre-treatment digester BOD COD/
reduction m3)
units for wastewater flow equalisation should be built. minimum 20,
Pretreatment, energy
biogas optimum 60 miniumum 10
optimized with organic
septic (limited by days, maximum
As effluent from anaerobic reactors will usually require further tank construction 7 years (as
wast as coferment,
baffle in BS required if
treatment prior to discharge to surface or underground water (BST) /
biogas
25-60% costs, but
longer HRT
0.5 – 2 higher to lower
sludge volume
built as main treatment
system with post-
bodies, this depends also on the loading rate of the reactors. settler
(BS)
for
sanitization
handling
challenges)
composting, post-
3 wetland, or drying bed
At organic loading rates between 1 and 4 kg COD per m re- required)
Anaerobic
actor and day, the highest removal rate is at 1 and drops to 4. baffled
70-90% 2-4 1 - 12 At least 2 years
Post-treatment after BS
reactor (than without bafle)
Therefore 1 may be selected as the optimum loading rate. (ABR)
The authors have drawn the assumptions, that if the organic Anaerobic
Theorethicall no,
but sludge may Post-treatment after BST
70–95% 0.5 - 4 5 - 15
load can be removed anaerobically, it is almost in any case filter (AF) accumlate at the or after ABR
bottom
more economical to increase the size of the anaerobic reac- Main-treatment after grid
Upflow chamber, energy
tor, rather than to increase the aerobic post-treatment step. anaerobic optimized with organic
more than 365
sludge 55-90% 0.5 - 10 15 - 32 waste as co-ferement, or
days
blanket post-tretament after BS
3.2.4 Experienced constructions, design and (UASB) or BST, with post-
wetland, or post-lagoon
maintenance staff required (source: H.-P.Mang)
People with experience to design, build and maintain biogas
sanitation systems are required at local level. A biogas

Last Updated: 14 July 2010 Page 13

The required hydraulic retention time (HRT) of the substrate combined anaerobic/aerobic multi-step systems and as pre-
in the digester depends on the process temperature and the treatment in combination with constructed wetlands.
type and concentration of the mixed substrate itself. This will
then determine the volume of the digester. Digesters are Generally, the removal of 65% of solids, up to 60% of
designed for an optimum economic balance between gas biochemical oxygen demand (BOD) and a 1-log removal of E.
coli can be expected in a well designed biogas septic tank
yield and volume (HRT). Therefore the retention time is
chosen as the total time required to produce a certain amount although efficiencies vary greatly depending on operation,
of the total gas (to size the digester to obtain all possible maintenance, and climatic conditions. BSTs can be installed
biogas is often not economic). In a biogas-sanitation system in any type of climate although the efficiency will be less in
the overall HRT should not be less than 20 days due to cold climates. Although a BST is gas and watertight, it should
methanogenic bacterial reproduction time, but from a health not be constructed in areas with frequent flooding.
point of view the HRT should be extended to at least 60 days, In a biogas settler (BS), to achieve that the sludge retention
if no adwquate post-treatment is foreseen. In this case the time (SRT) is longer than the hydraulic retention time (HRT),
3
volumetric load may not exceed 2 kg of COD per m of active a baffle or a separation wall should be added when it is
digester volume. operated as stand alone system. The accumulated settled
A sludge retention time (SRT) of at least 10 days is sludge must be removed from the base of the BS
necessary to promote methanogenesis in the anaerobic periodically, based on experiences this will be nessesary
treatment of primary sludge at a process temperature of 25°C every 5-7 years. For domestic wastewater and blackwater the
(Miron 2000) while a SRT of 15 days is necessary for typical specific settled sludge generation is about 0.0037 l/g
sufficient hydrolysis and acidification of lipids. For of Biological Oxygen Demand (BOD) reduction, under the
temperatures as low as 15°C, a SRT of at least 75 da ys has assumption that the supposed ratio COD/BOD is 2:1.
to be considered to achieve methanogenic conditions
(Zeeman 2001). As longer the SRT as better will be the
degradation and stabilization of the sludge and less sludge
will remain. This is important for bigger units (at prisons or
schools); the handling of sludge can be also a problem in
dense populated areas.
Different types of biogas sanitation units could be combined
with each other (so called combined systems, multi-step
systems or decentralized wastewater treatment systems) in
order to benefit from the specific advantages of the different
systems. The quality of the final effluent from the systems
improves with the multiple steps of the treatment facility.
Design information of the recommended four design
variations are available through many websites and literature
(see Section 6) and expert consultations.

Figure 6: combination example of biogas settler (without

baffle), anaerobic baffled reactor and anaerobic filter, where Figure 7: Linear overlay of the circular coils advancement
blackwater flows in the biogas settler and greywater in the from above in biogas digesters with flat bottomand without
anaeribic baffled reactor (source: teaching material of the baffle (source: University of Oldenburg 2004)
Indian Ecosan Service Fundation www.ecosanservices.org)
Many different forms of constrution are known, for example:
Local design and engineering adaptations to local human diet fixed-dome plant, bag digester, glass fiber (half) bowl plant,
(organic load and biogas potential), hygienisation needs water jacked floating drum, PE or PVC predesigned tanks,
(household or community) and effluent reuse (energy plants, covered anaerobic lagoons. Main parameters for the choice
tree nursery, grasland, vegetable, grain) are always of construction material and for the basic design are
nessesary. Therefore the following sectors can only provide (Kossmann 1996):
an introduction to the different biogas sanitation systems.
· Technical suitability (stability, gas- and liquid tightness);
· cost-effectiveness;
4.2 Biogas settler (BS) or biogas septic tank
· availability in the region and transport costs;
(BST) · availability of local skills for working with the particular
The biogas settler (BS) (with or without baffle(s), depend of building material.
the further treatment step choosen) or biogas septic tank
(BST) (always with integrated baffles) is mainly applied as
on-site household based system with secondary treatment of
effluents in compost (solids) and drainages/subsurface
irrigation (liquid). The direct effluent from the reactor, a dark
slurry, is a nutrient-rich fertiliser for agriculture and
aquaculture, due to the conservation of nitrogen during the
anaerobic process. Any kind of suitable organic waste (for
example kitchen waste) could be added to increase the
biogas yield. BS are also applied as a pre-treatment step in

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