Finding a new biodigester, Tanzania

Micheala Chan (2015, Engineering)


Cambridge Development Initiative (CDI)

For two months in the summer of 2018, I volunteered with Cambridge Development Initiative (CDI) in Dar es Salaam, in partnership with KITE Dar es Salaam. KITE Dar es Salaam is a student-run organisation based at Ardhi University and University of Dar es Salaam. I was a member of the WaSH (Water, Sanitation and Hygiene) team, working on the Biogas sub-team.

The Biogas Model

The aim of the Biogas sub-team was to investigate the feasibility of the Biogas Business Model, which can be summarised as follows:

  • Households are connected to simplified sewerage networks (these are sewerage systems with smaller diameter pipes buried less deep than conventional sewerage systems).
  • Sewage flows from the latrines to the biodigester at the end of the route, which digests the sewage to produce biogas.
  • Local entrepreneurs would sell this gas to members of the local community.
  • The funds generated are intended to recuperate the original investment, although this would be expected to take a few years of continuous biogas production. The funds would also provide future funding for scaling the simplified sewerage networks.

The location

Vingunguti is an administrative ward in the Ilala district of Dar es Salaam, Tanzania. According to a 2012 census, the population of Vingunguti is 106,946. Nearly all the settlements in Vingunguti are informal and the area is largely dependent on pit latrines. CDI works in an area close to the Spenco waste settlement ponds.

These waste settlement ponds are the exit points for the sewage from our simplified sewerage routes. Waste from industry and households throughout the city is also treated here.

Scan Tanzania

The team has been in contact with Scan Tanzania Ltd this summer. Scan are a supplier that the KITE decided we would use during the course of this year. We had a few meetings with them and they visited the site in Vingunguti.

The digester that was proposed by Scan is a three-stage 135 m3 (45 m3 per stage) flexible tarpaulin PVC digester. An alternative design proposed was a masonry digester (also three stages of 45 m3 each). The cost of the tarpaulin PVC digester is about 27.1 million TZS per stage (this includes the costs for walls and roofing, etc), while the cost of the masonry digester is about 31 million TZS per stage. However, the lifetime of the tarpaulin PVC digester would be 8 – 10 years based on the grade of UV-treatment given to the material compared to much longer for the masonry digester.

The reason three stages is needed is to fully digest the sewage.

Business Plan

The biogas team has faced many large decisions this year. The first of these was the decision of whether or not to buy a new digester this summer. There were many arguments for both sides, focussing on whether it would be better to prove technical or financial feasibility first. However, ultimately what held us back was that the cost of one stage of digester was greater than the amount available to us in the budget and so the decision was ultimately made not to buy a new digester this summer.

The team has since been focussing on the Biogas Business Plan, including financial modelling and profitability calculations. The variable with the largest effect on profitability was, by far, the gas production rate. Unfortunately, the number that we received from Scan (4.564 m3/day for one stage and 5.6 m3/day for three stages) was much lower than the number estimated by last year’s biogas team (10 m3/day per stage) when collecting biogas produced from the Flexigester, and so the team decided to run a flow rate experiment on the Flexigester gas production, using a flowmeter borrowed from Ardhi University.  However, when the team arrived on site, it was found that that there was no gas coming out of the gas outlet. The team spent about an hour trying to find the reason for this, checking for blockages and leaks. By the time the team left site, the Flexigester had deflated to the point where there was a lot of doubt if there was any gas in the digester at all with which to carry out the planned tests on the calorific content of the produced biogas. No more useful information could be obtained from the Flexigester and it was decided that decommissioning would start as soon as possible.

Another aspect of the business model was questioning the gaps found in the previous HDIF grant application. In an attempt to do this, the biogas team spoke to members of the Entrepreneurship Team about various aspects of the business plan. There was also a plan to do a biogas survey, in collaboration with the M&E and Community Engagement teams, in a community analogous to Vingunguti to gauge community opinions on biogas and whether there was the market for our product. However, this did not take place due to problems obtaining permissions.

Decommissioning the Flexigester

The team started decommissioning in week 7 of the summer. This started with the removal of the roof structure, which was carried out by three local technicians. The sludge and stormwater from within the Flexigester pit and also within the Flexigester were then pumped out, using a maji taka truck. After five trips, only a small amount of sludge remained in the pit. It was decided that we would leave this to dry over the weekend while we decide on the next course of action. The technicians removed the Flexigester from the pit.

The next stage of decommissioning involved returning the site to a state in which it could potentially be used for other purposes (such as constructing a community centre). This meant clearing up the area, filling in the pit and demolishing the walls.

What next?

A major question that has arisen over the course of this summer is what the future of the Biogas Project is. In analysing the profitability, it was found that, with the flow rates given by Scan, the biodigester would not be able to pay itself back within a reasonable amount of time, let alone allow the scaling of simplified sewerage. As such, a decision has been taken to decommission the Biogas Project in its current state (the model being biogas integrated with simplified sewerage) with a plan to propose a new research project into a Clean Energy Project based around biogas generation from biomass sources other than dilute human faeces.

At the end of the summer, the new committee were selected, and I have taken on the role of CDI WaSH Project Director 2018/19, and will be working in collaboration with Amon, the Tanzanian member of the Biogas team, who has taken on the role of KITE WaSH Project Director.

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