Financially sustainable processing of fecal sludge : feasibility study

Main findings of the research report:

1. From a systems perspective, looking at collecting and processing of fecal matter, the using of pit latrines is inefficient. Larger containers such as 5 to 10 latrines connected to one septic tank should be considered. The collection cost of obtaining fecal sludge from pit latrines, is expensive and prevents the cost effective and safe processing of fecal sludge.

2. Nonetheless, by using anaerobic processing of agri-waste combined with fecal sludge the process can be made financially sustainable. This requires that three products related to anaerobic processing are sold. These products are: (i) electricity, (ii) heat generated from a gas engine fueled with biogas that drives the generator for the production of electricity, and (iii) the digestate, processed into an organic fertilizer. The income from these three outputs should be documented in a legally binding manner before investments are made in the plant. Electricity is sold through a Power Purchase Agreement either to the Rural Electricity Board or through an agreement with an individual off-taker, such as a garment factory. Heat in the form of hot water is sold to either an icemaking factory or a cold storage using an absorption cooling system, while the organic fertilizer is sold as part of a barter agreement in which large farms supply corn-stovers and get a high quality fertilizer with a guaranteed chemical composition. In case the barter system is not possible as the suppliers of corn stovers are different than the clients for the fertilizer, separate contracts need to be put in place. Only when there is certainty regarding the income from the proposed unit, should investment in developing the unit be considered.

3. One important parameter is that such a unit should be located very near or adjacent to the source of agricultural waste, which is the major feedstock. This can be corn stovers, rice straw, water hyacinth, or a combination of products.

4. Our research shows that a biogas unit using corn stovers and chicken manure only can be a profitable investment in Bangladesh. The profit from this unit will cross subsidize the logistical cost of collecting the fecal sludge from pit latrines. Our recommendation is therefore that the unit will be designed and constructed for processing corn stovers (or other agri-waste or even water hyacinth) with chicken manure, and fecal sludge (which requires the installation of a “hygienic unit” or helminth killer), but that the unit starts up only with corn stovers/chicken manure. This would allow for an organic growth of the fecal sludge collection from pit latrines, which would gradually replace part of the corn stovers. In the model presented in the report, the initial feedstock would be 7,000 Metric Ton (MT) of corn stovers/year, 3,000 MT of chicken manure. However this would gradually (over a period of 3 to 4 years) change to 1,000 ton fecal sludge (content of 40,000 pit latrines), 6,000 MT of corn stovers and 3,000 ton of chicken manure.

5. In spite of the fact that biogas or gobar gas is well known, the modern application to produce gas or electricity at a commercial scale, has been made possible only recently through an advanced engineering solution. There are a few manufacturers with solid track records in the form of plant availability for 5 years or longer for 95% of the time. Non-professional experiments have given modern anaerobic digestion a bad reputation. These engineering solutions have gone hand-in hand with new developments in controlling the process through specially designed enzymes. These are produced in highly advanced chemical companies such as DSM Netherlands and Novazyme.

6. Enzymes would be an interesting solution for the feedstock in Bangladesh. The expectation is that the corn stover will be quite dry and fibrous when harvested. This will result in lower gas production as can be expected from similar biomass, which is harvested at the best moment (freshest) for biogas production. For the corn stover it would therefore be advisable to use enzymes to ensure a better breakdown of the biomass and thus an increase of the production of biogas.

7. To fully check the advantages and disadvantages of using enzymes for this project, can only be done when the plant is in operation to measure the real effect. This is because it is very difficult to use the results from other tests on other feedstock. The advantages for corn stover could be, lower amounts of biomass needed for the same production of biogas. A lower overall energy consumption due to a decrease in viscosity and as a result less energy needed for agitating the digester. Also the nutritious value of the digestate might be influenced, as more biomass will be broken down, which will result in more nitrogen being detached from the biomass and thus be more easily available for the plants direct uptake. The disadvantage will be the costs of the use of enzymes. It is not possible to measure the effect of the positives against the negatives and give a final statement on the feasibility of the use of enzymes now without a test on the actual biomass, we therefore have not taken the enzyme costs into account in the financial model.

8. These innovations have made it possible for several companies to guarantee plant availability for over 95% of the time for five or more years. From a few plants in 2002, ENVITEC has now built 467 plants with a combined capacity of 366 MW.