Tuesday, 4 March 2008

Alternating twin-pit pour-flush toilets vs. simplified sewerage

India used to have (maybe still has) a policy of no sewerage in cities with a population less than one lakh (100,000); instead alternating twin-pit pour-flush (PF) toilets were to be used (details here). Is this now a policy to stay with? The huge success of ‘Slum Networking’ in India, developed by Himanshu Parikh, would suggest not. Slum networking (details here and here) is slum upgrading focussed on WatSan improvements: ‘The present cost of providing complete physical infrastructure is about INR 16000 per family (i.e., GBP 200). This includes house-to-house sewerage, water supply, roads, stormwater drainage, receptacles for solid waste, and landscaping. Out of this the sewerage component is INR 6000 (INR 4000 for the network and INR 2000 for wastewater treatment per family of 5 people)’ (email from Himanshu Parikh, September 2006); if a ‘pucca’ PF toilet has to be provided as well, this adds a further INR 4000 (email from HP, April 2007). The sewerage is essentially Brazilian-style simplified sewerage (but HP uses the Colebrook-White equation, rather than Manning’s). The Indian Sewerage and Sewage Treatment Manual (2nd ed., Ministry of Urban Development, New Delhi, 1993) does at least mention simplified sewerage (which it calls ‘shallow’ sewerage), but clearly simplified or ‘slum-networking’ sewerage needs to be much more widely understood in India (and elsewhere) than it is at present. The late A. K. Roy, the doyen of low-cost sanitation in India, once said to me that he doubted Brazilian-style simplified sewerage would work in India as “our diet is very different from Brazilians’” − true, but not so different from Sri Lankans’ and there are about 40 simplified sewerage schemes working well in Sri Lanka, with the earliest dating from the mid-1980s.

It wouldn’t take an Indian student long to work out comparative costs of alternating twin-pit PF toilets and simplified sewerage − it’s basically the construction cost of a flow-diversion box plus two 1.5-m deep brick-lined leach pits vs. the cost per household of installing 100-mm diameter sewers (one simple junction box and something like 3−4 m of 100-mm dia. sewer laid at a depth-to-invert of ~350 mm and a gradient of 1 in 200). You could vary the housing density and you’d then get a graph rather like this one to show when sewers become cheaper than on-site sanitation. This sort of work needs to be done in many parts of the developing world, especially in Africa and Asia − and the simplified sewerage costs would be useful in any comparison with periurban EcoSan systems!