W.N.C. Apsara and B.C. Liyanage
Department of Civil Engineering, The Open University of Sri Lanka
The purpose of a sewerage disposal system is to carry human excreta & urine (Black water) and wastewater (Gray water) back to nature in a proper way without any harm to the environment as well as to human beings. But the implementing and operating costs of the sewerage disposal systems are normally very high which make low income communities unable to reach these techniques.
The aim of this study was to find a proper sewerage system for a low income coastal community, “Pittaniya Watta” in Moratuwa Municipal Council. The community consists of 142 families with about 710 people spreading over a 8092m2 limited area. First the real situation and the problems prevailing were found doing a questionnaire survey, which consisted 105 questions in 10 different categories that covered relevant problems in sewerage disposal according to a proper cross section.
Questionnaires were analyzed and evaluated to find daily water consumption and sewerage disposal capacities. Based on the results, Eco-sanitation and simplified sewerage system were selected as the most economical solution for the problem. The Eco-sanitation is a separate unit, which constructs above the ground level and consists of two compartments that are used only for excreta disposal and could be used alternatively for individual houses. Number of Eco-sanitation units required for the said community was 142 nos.
The simplified sewerage system is the most economical “off-site disposal system” which was designed considering both self cleansing velocity (0.5 m/s) as well as tractive tension (1 Pa). The results revealed that the best fitted minimum sewer gradient and the minimum sewer diameter were 1 in 208 and 100mm respectively. The collected sewage will be treated in primary and secondary stages in anaerobic digestion units. Three units were provided with each dimension length, width and depth of 6.2mx 3mx4m respectively and 0.2 m of free board. Using a lift station a head was added to the effluent to take it from 5m below the ground to the ground surface where the Reed bed was provided before it is released to the existing drains that are directed to the sea. Estimated hydraulic retention time of the reed bed is 6.79 hours while dimensions are: water depth = 0.6m and surface area of the bed =78 m2.
According to the basic cost analysis on two systems Eco-san unit for each house is Rs 47925. When consider the whole community, the total cost for Eco-san was Rs 6,805,350 while the simplified sewerage system was Rs 2,886,000. However, at the operational stage of simplified sewerage system, with maintenance cost, the total cost was estimated as Rs 16,606,390.00. Therefore, based on the cost and considering the most suitable system for the subjected community is Eco-san. But according to communicant’s desire, land scarcity, and population growth as well as future forecast on water consumption, improvement of the living standards etc. simplified sewerage system was selected as the best sewerage disposal system to implement at Pitaniya Watta low income community.