Prospect of in - house recycling of gray water

Abstract

This thesis explored present Dhaka city’s situation in order to define the main factors surrounding gray water reuse in urban areas. Suitable treatment technologies that best address the defined gray water reuse factors were identified and clear, standardized and sustainable gray water reuse processes for their application were also established. The major objective of this study was to evaluate the viability of domestic reuse of gray water in Dhaka city for laundry and toilet flushing. The quantification of water uses for different domestic purposes was done mainly by questionnaire survey and by direct measurement in some cases. From the surveys the amount of gray water produced from households has been calculated. The quality of gray water generated from laundry, showers and basins was determined by laboratory analyses of representative samples. Turbidity, suspended solids, BOD, COD and Fecal Coliform (FC) of the collected samples were determined following standard procedure. The viability of reusing untreated gray water for toilet flushing was also evaluated. The treatment requirement of the gray water for in-house recycling for laundry and toilet flushing was determined by comparing its quality with the reuse criteria. A simple gray water treatment system for in-house recycling for laundry and toilet flushing was developed and its performance was tested. For quantification, the questionnaire survey was done on 100 families in different parts of Dhaka city. The average per capita water requirement was estimated at 150 l/day and the average of per capita gray water production was estimated at approximately 95 l/day. The average BOD5 values are 204, 65, 133, 108 and 13 mg/l; the average COD values are 270, 106, 200, 174 and 33 mg/l; the average turbidity values are 200, 98, 135, 86 and 1.5 NTU; the average FC values calculated are 3440, 2350, 2800, 2920 and 180 cfu/100 ml; the average S.S values calculated are 1296, 737, 978, 672 and 17 mg/l for black laundry water, mix laundry water, bath water, basin water and raw water respectively. The quality of gray water with respect to BOD, COD, Turbidity, S.S and F.C indicated that they are very high. But the results varied depending on methods used for washing, people’s choices of soap/detergent/shampoo use, the quantity of soap/detergent/ shampoo used, the time taken for washing or bathing, availability of water and amount of water used. The test results of tap water indicated that the fecal coliform content is very high and thus the quality of DWASA's supply water is very low. A treatment system that is acceptable to urban residents requires the storage of gray water and then simple treatment by disinfection, coagulation, flocculation, settling and sand filtration. The settling time is 30 minutes, optimum coagulant dose is determined 30 mg/l and optimum chlorine dose is determined 3mg/l.

A simple bench scale treatment setup was established. Locally available materials were used to construct the experimental setup and it is very easy to use in household environment. The quality of the treated water sample has been compared with standards and criteria guidelines. This study is targeted towards the building dwellers of Dhaka city having medium income status. This study is only a small step in forming a gray water management system. More studies considering technical feasibility, public health, social acceptability and sustainability should be undertaken. Skill and knowledge are the main concern for the installation and maintenance of gray water treatment systems. The long-term and broad implications of urban gray water systems are not yet fully understood and paramount to its acceptance is the protection of human health as well as community education and participation in community decision processes.