"Fecal Sludge Management: The Costly Alternative to Sewer Systems"

“What is the BOD value of fresh sewage coming straight from the toilet, without passing via the septic tank? Can septic tanks be excluded in sewer systems?”

I find the answers to these questions very crucial. The general calculations and design incorporate the BOD values of 350 mg/l to 700 mg/l while designing wastewater treatment plants. I am curious how this value dramatically increases from 350 mg/l in fresh sewage to 7000 - 60,000 mg/l in septic sludge, which is encountered while designing a fecal sludge treatment unit. This curiosity is followed by the interesting debate of Fecal Sludge Management (FSM) versus sewered Wastewater Treatment Plant (WWTP) system.

A surficial calculation on a recent design of a sewered wastewater treatment system benefitting 21,862 households yielded the project cost of USD 12 million; which aimed to treat 350 mg/l of BOD and other contaminants present in fresh sewage. In contrast, the cost of serving a similar-sized beneficiary with FSM services amounted to USD 2 million only; which aimed to treat 7000 mg/l BOD. Apparently, it seems the ratio of cost-effectiveness of the FSM: WWTP system is nearly 1:6 despite the severe pollution load ratio of 20:1. Doesn't it feel Wow! Modern days scientists and researchers are always in the hunt for such technological solutions having 6*20 = 120 times better performance. There ain't no such thing as a free lunch. This calculation is only the tip of an iceberg, in terms of FSM. In the world of limited resources, the hidden cost of FSM can be a major challenge on the road to safely managed sanitation!!!

Yes, the case is reversed if you incorporate the underlying cost of FSM that aims to achieve safely managed sanitation; by preventing leakage of unwanted waste into the environmental sink. So the hidden cost in the above calculation is the cost of septic tanks for entire beneficiary households that should be built according to its basic design principles. In the above calculation, if the average cost of constructing a septic tank equivalent to USD 1,500 is considered, the additional figure of USD 32 million for constructing 21862 septic tanks would be topped on FSM, and the actual cost of implementing FSM project will be USD 34 million. It is 17 times more than the initial value of only USD 2 million, which was calculated by excluding the cost of 'building storage' mentioned in the FSM chain and in the figure below. My reservation is here- can we simply forget or hide such a 17 times bigger monetary value while implementing any project? Can it give the desired outcome? Of course not. Assuming 50% of the septic tanks are working well, the cost of FSM will be reduced to USD 18 million. Further assuming 75% of septic tanks are working well, the new figure of FSM cost would be 10 million, 2 million less than sewerage. This sums up that in an ideal case where 75% of septic tanks constructed are as per design norms and fully functional,  then only FSM outsmarts sewered wastewater management system. Otherwise, in general, the corrected cost-efficiency ratio of sewered wastewater management system to FSM is 1:3. In other words, the cost of FSM in the road to safely managed sanitation is three times the cost of sewered wastewater management services.

The considered case is of Hetauda Sub-metropolitan city, and the calculations are referenced from Integrated Water Supply and Sewerage Management Project (IWSSMP), Department of Water Supply and Sewerage management (DWSSM), Panipokhari, Kathmandu, Nepal.

You might still be pondering how the first two questions at the beginning of this article relate to this whole story. As we know things are not always such black and white. I still don’t know if the BOD value of 350mg/l of fresh sewage is that of the waste coming from toilets via septic tanks or directly. In case, directly, the case would be further influenced, but at least not reversely as above. Your suggestions will be heartily appreciated.