Assessment of performance characteristics and applicability of decentralized wastewater treatment systems to PERI urban settlement in Zambia
Sibooli, Humble M.
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In Zambia, sanitation options are still few and inadequate to effectively serve the urban poor in peri-urban settlements. Continued growth of the settlement now calls for alternatives to pit latrines, which have become difficult to manage given the increasing population. Sanitation is poor hence; most people are vulnerable to diarrheal diseases. Decentralised Wastewater Treatment Systems (Dewats) provides an opportunity to improve sanitation for the urban poor. Whilst Dewats have proved effective elsewhere, locally, the concept is new and not fully understood.The objectives of the study were firstly, to investigate effectiveness of Dewats for peri-urban areas, and the impact of treatment on the environment. Secondly, to assess implications of the legal framework on promoting Dewats and lastly, to develop design approaches for peri-urban areas. The relevance of this study revolved around appreciating the applicability and sustainability of Dewats in Zambia within the context of the legal and institutional frameworks as they relate to sanitation. The methodology was centred on scientific experiments from a case study and cross sectional survey on onsite sanitation delivery. Data analyses comprised aggregation, direct interpretation and establishing patterns. Microsoft Excel and SPSS were used in the analyses of quantitative data.Although the desire for Dewats was high, findings also show a wide presence of pit latrines characterised with disproportionate use. However, only one in every six people had access to a pit latrine. From experimental findings, pH of wastewater varied between 6.27 and 6.81, while TSS, COD, NO3 and NO2 treatment efficiencies were above 80 percent, except for BOD and NH4+, which measured below 41 percent. Although, high FC removal (91 percent) was achieved, the 53,000 No/100ml effluent value exceeded the 5,000 No/100ml limit. Major design outputs for a single Dewats needed to serve 3,500 peri-urban users comprises seven-compartment baffled septic tank, four-compartment filter, and 3,400m²-constructed wetland. The questionnaire survey indicated presence of the legal and institutional framework, though it did not adequately provide for onsite sanitation. Financial, technical and environment challenges were the main factors affecting onsite sanitation delivery. Successful removal of TSS, COD, NO3, NO2, and pathogens signalled satisfactory performance of the Dewats for domestic treatment. The variation for pH between 6.5 to 8.5 limit provided optimum anaerobic conditions to degrade organic compounds. High COD removal indicated significant chemical oxidation than biological metabolism. However, because of higher BOD, NH4+ and FC effluent values, occasionally, Dewats would suffer inadequate treatment owing to low nitrification and denitrification. In addition, low temperature conditions and short retention times would intermittently reduce anaerobic digestion. Meanwhile, effluent water with high BOD, NH4+ and FCs values is toxic and harmful, thus, could deplete dissolved oxygen or cause eutrophication, helminthic infection and smells. Disposing such effluent in peri-urban areas with hardly available land and water bodies is an environmental risk likely to render Dewats less sustainable.Despite the effects of climatic changes, improved efficiency is attainable when the reactors have sufficient volumes and retention times to maximise digestion. However, land scarcity in peri-urban areas will limit capacity of Dewats especially, the wetland, hence; 1.0m² of land per capita is required to serve 3,500 users. The design presupposes that 3,500m² of land may be available for a single Dewats. Other conditions favourable to Dewats could be obtained by enforcing existing legal and policy provisions. Generally, the legal framework, although not adequate is enforceable if institutions can demonstrate capacity to implement prescribed functions.Indeed, Dewats showed significant reduction of most organic compounds suggesting capacity for improved performance. Occasionally, the treatment will vary depending on temperature and other design factors; hence, concerns could arise from low effluent qualities. To mitigate such effects, reactors should be designed with longer retention times and low up-flow velocities. Overall, the study generated enough evidence to show satisfactory performance of Dewats as regards providing treatment for peri-urban areas.
- Engineering