Article

A GIS and Multi-criteria analysis based approach for evaluating suitability of sewage treatment sites in Blantyre City, Malawi

Authors:
Japhet Khendlo at Mzuzu University
Japhet Khendlo
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Abstract

Sewer system is one of the essential utilities systems in the human settlement history. They have been constructed since civilisation started and always provide quick and efficient way of collecting and treatment of sewage (Afshar, 2006). Blantyre city has only 2 working sewage treatment plants among the 5which were designed and constructed in 1950s and 1970s to cater for a 200,000 population (BCC). Recently, Blantyre City has undergone increased urban growth attributed to the urban sprawl with a population of a million people (MNSO, 2015). With the current high population, a lot of sewage is being produced incapacitating the ability of treatment sites to efficiently collect, treat and dispose of the sewage. Hence, this research focused on evaluating the suitability of the sewage treatments sites in Blantyre City. The research aimed at evaluating the suitability of the sewage treatment sites and also finding optimum sites. Data collection involved collecting shape files of the suitability evaluation criterion, which involved data about, Land use, Surface water, Soil classes, Slope, Access network and Population distribution. Blantyre satellite image was digitized to get treatments location coordinates which were used in ArcGIS Software to produce a Map. Multi-Criteria Evaluation Specifically Analytical Hierarchy Process was used in Paired wise comparison matrix in the analysis through ranking and assigning weights. The shapes files obtained were in Polygon and Polyline format, a conversion tool in Arc tool box of ArcGIS software was used to convert them into Raster data set for easy analysis. Spatial Analysis tool in the Arc tool box of ArcGIS was used in the suitability evaluation, in which the weighted overlay tool was used to assign a common scale of importance to the criterion variables for the model builder tool. After running the model, four levels of suitability was produced; unsuitable, less suitable, moderate suitable and most suitable. An overlay tool in the Arc tool box was used to overlay the sewage treatment sites layer of the suitability Map, and it was found that all the sites were located in the unsuitable area. The study recommends relocation of the current sewage treatment sites to most suitable areas basing on the suitability map. Key words: GIS, Suitability, Multi-Criteria, Weighted-overlay, AHP

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Hydrograph-based storm sewer design optimization by genetic algorithm
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A model is developed for the optimal design of storm water networks. The model uses a genetic algorithm (GA) as the search engine and the TRANSPORT module of the US Environmental Protection Agency storm water management model version 4.4H (SWMM4.4H) as the hydraulic simulator. Two different schemes are used to formulate the problem with varying degrees of success in reaching a near-optimal solution. In the first scheme, the nodal elevations and pipe diameters are selected as the decision variables of the problem which were determined by the GA to produce the trial solutions. In the second scheme, only nodal elevations are optimized by the GA, and determination of pipe diameters is left to the TRANSPORT SWMM module. Simulation of the trial solutions in both methods is carried out by the TRANSPORT module of SWMM4.4H. The proposed model is applied to some benchmark examples, and the results are presented and compared with the existing results in the literature.Key words: genetic algorithm, optimal design, sewer network, SWMM.
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