Best technology for household sanitation? FINISH Mondial technical sanitation experts weigh-in on the septic tank vs biodigester debate

Ecosans, Aberloos, toilet-linked biogas biodigesters…A great debate wages on in the sanitation sector, each technology and modification striving to be the next great innovation for mass uptake. When it comes down to on-the-ground efficacy however, what are the actual technologies that sanitation experts rely on for sustainable designs?

Generally, one would be hard pressed to find a practitioner in the sanitation field ‘singing the praises’ of the traditional septic tank. Why is this? The FINISH Mondial (FM) Technical Working Group (TWG) set out to settle the matter: Is the septic tank really an archaic, useless technology? Or perhaps, is there simply a fascination with newer, shiner product innovations today?

The TWG supports FINISH Mondial by uniting our key technical experts across 3 continents, to support the entire 6-country programme in:

• Developing technical construction manuals for on-site sanitation systems,
• Institutionalising mason/artisan trainings in various countries, and
• Learning and sharing innovations in design, construction and rehabilitation of sanitation systems considering the possible reuse of faecal sludge.

The FINISH Mondial design philosophy therefore emphasizes: (i) design of technologies that allow for effective removal of the contained waste and therefore, reuse of faecal sludge, (ii) adaptation to different geophysical constrains (e.g. drought-prone areas, rocky grounds, etc.), (iii) use of locally available materials and, (iv) ergonomically and socio-culturally acceptable designs. At its core, the FM programme and Diamond approach does not believe in ‘reinventing the wheel’ but partnering with and improving existing approaches that work and are accessible to the most amount of people.

Technical Working Group

What is a septic tank?

A septic tank is a watertight chamber made of concrete, fibreglass, PVC or plastic, through which blackwater and greywater flows for primary treatment. Settling and anaerobic processes reduce solids and organics, but the treatment is only moderate.

What is a biodigester?

A biodigester is a decomposition mechanized toilet system which decomposes human excretory waste in the digester tank using specific high graded bacteria further converting it into methane and water, discharged further to the desired surface.

We asked some of our FINISH Mondial Technical Experts to weigh in on two prominent ends of the debate spectrum. Which are their preferred systems to implement? The SEPTIC TANK or BIODIGESTER (considering no harvest of the biogas):

G. Anand, Solutions Centre (India) – easy & reliable

Septic Tanks would be my first choice since they are (a) easy to manage with lower frequency of cleaning, (b) is flexible in handling hydraulic and organic load variations, and (c) is generally less affected by fugitive solid wastes than, for example, a biodigester.

Both septic tanks and biodigesters have issues regarding disposal and reuse of the collected matter. In twin leach pits, we can get it in a dry form which is easier to handle, dispose and reuse. Septic Tanks yield sludge or septage which needs further de-watering. Our model of co–composting may not necessarily be practicable in all contexts. However, when you have potential for water pollution, septic tanks are better than leach pits since the discharge from is partially treated unlike the leach pit, where only untreated effluent is discharged in the environment.

John Harrison, FINISH Mondial Technical Adviser (South Africa) – tried & tested solution

A septic tank is a tried and tested, simple solution, also ideal for African conditions if designed and installed correctly. 

Biodigesters only make sense if one intends on harvesting the gas, but then you require additional biomass as fuel, which in turn will probably increase the frequency of emptying. If the gas is not being harvested, then a circular reactor does not make any sense as you are either adding significant complexity, and/or creating significant shortt-circuiting, and/or require significant extra wasted volume in the reactor to achieve the equivalent actual residence time. In short, I see no point in going down the route of biogas reactors if the gas won’t be harvested—making the device suitable for harvesting the gas makes it less efficient as a sanitation system. A biodigester is primarily a gas harvesting device not a sanitation system.

Saurabh Agnihotri, FINISH Society (India) – largely unsuccessful for high density areas

Technologies like biodigesters can be instrumental, particularly in places where households cannot afford standalone treatment and community-linkages can be explored, especially in areas where septic tanks may not be feasible. In the Indian context, septic tanks have been largely unsuccessful, especially regarding emptying them in areas which are densely populated. We often face challenges in some communities where it is not viable to link this with one tank. Several toilets can be linked to this kind of system as large septic tanks are not often feasible and do not function properly.

It is worth noting that for septic tanks, the issue of black water discharge is concerning. If we can identify the best solutions to combine with other systems, we may be able to recycle the water which is also instrumental in low water table areas. In my view, we must increasingly give more weight to technologies which consumes less water or are water-positive in nature.

Henock Belete Asfaw, WASTE (Netherlands, Ethiopia) – cost & environmental implications

An important aspect necessary to evaluate septic tanks vs biodigesters is the investment required by households. The construction costs of septic tanks and biodigesters can be evaluated by considering the typical hydraulic retention times (HRT). HRT is the average amount of time a daily wastewater inflow spends in the tank. Typical HRTs of septic tanks and biodigesters are 2 and 20 days respectively, meaning that tank sizes of biodigesters are about 10x bigger than septic systems. For example, if a household generates an average daily wastewater flow of 1m3, a septic tank size of 2m3 is required, while a 20m3 tank size would be required for a biodigester. This implies the construction cost of a biodigester is significantly higher than a septic system.

Domestic wastewater has low bio–degradable organic matter, meaning the biogas produced by an expensive biodigester is very little. In addition, when the biogas is not harvested and used, which is the case here, biodigesters become a system that significantly pollutes the environment since methane is 30X more potent than CO2. In general, a biodigester system, where biogas is not captured and used (or at least burned), suggests the technology does not align with FINISH Mondial’s philosophy, which prioritizes affordability and access but not at the cost of circular economy and climate resilience (considering methane is 30x more potent than CO2).

Where do you stand on this debate? You can join the conversation on our social media pages where our technical experts can answer any questions.