Assessing the possibility of water reclamation for concrete production in Maputo, Mozambique

Abstract

The rapid economic and population growth recently drives Maputo to become more metropolitan, with construction sector as one of the pillars to support its transformation. This progressive move is, however, hindered by one of the most crucial problems: water scarcity. Water is essentially required by construction sector, especially in concrete production. It needs a proper quantity and quality to meet the expected strength. While the quantity is determined by its ratio with cement, the quality varies for plain, structural and high-strength concrete. Drinking or tap water is commonly used in Maputo but previous experiments worldwide show that there is a possibility to use non-drinking water for concrete production. One of the emerging alternatives is treated sewage. This thesis aims to assess whether reclaimed water from the sewage can be used as a source for concrete production in Maputo in order to save tap water for domestic purposes. To determine water demand from the concrete companies, 156 concrete companies in Maputo and its surrounding, Matola and Machava, were listed and 11 of them were interviewed with the variance of locations, types of products, production methods, existing water source, number of employees, monthly water tariff, current practices and willingness to cooperate for a preliminary water reclamation project. The average amount of water consumption for 11 interviewed companies is 311 ± 36 m3/day and for the actual population of 156 companies is 4411 ± 257 m3/day. This value contributes to 12% - 14% from the total tap water use in Maputo. The water price is approximately €0.76/m3 for the small companies and €0.38/m3 for both the medium and large companies. The potential supply sources were investigated from different types, flow rates and locations, such as the outlet endpoints from houses, the wastewater treatment plant and the effluent from other industries. The influent of the wastewater treatment plant was considered the best source for the preliminary plant design with a flow rate of 5682 ± 1196 m3/day. Available standards worldwide and previous works were studied to find the safe ranges of water quality for concrete. Field and laboratory measurements were also conducted to provide preliminary information. The results were used as the basis for designing a treatment plant from the demand of 11 interviewed companies, with lower effluent quality than tap water, yet sufficient for concrete production. Recommendations of a large scale plant for all 156 companies were also presented. The system is designed as a modular scheme to provide different effluent qualities and potential changes, such as the increase of the influent flow rate, the option of combining the new system with the existing plant, etc. The system is divided into four phases. Phase 1, consisting of a coagulation tank (1 x 0.6 x 0.5 m), four flocculation tank (1.5 x 1 x 1 m) and a dissolved air flotation tank (1.6 x 1.4 x 1 m), removes fat, oil and grease, phosphorus and suspended solids. Phase 2 removes remaining suspended solids by a rapid sand filtration (1.50 m bed depth x 0.56 m diameter x 5.45 m height). Phase 3, comprising nine units of nanofiltration in one skid, removes dissolved solids to meet the demand of higher strength concrete. The last phase is disinfection which were excluded since the removal capacities from the previous phases are already sufficient. The cost of the design is divided into three packages depending on the expected effluent quality from each company. The basic package only consists of phase 1 and costs €0.30/m3. The intermediate package, comprising phase 1 and 2, costs €0.40/m3. The advanced package, consisting of phase 1 and 3, costs €0.80/m3. Only the price of the advanced package is higher than the actual tap water price. Pumps are installed before the units of rapid sand filtration and nanofiltration. Valves are incorporated to divide the flow for different treatment packages. The delivery of the effluent is performed by renting trucks since it is less expensive than constructing new pipelines. The design shows that producing concrete with reclaimed water from treated sewage is possible to be performed in Maputo and the sewage does not need to reach drinking water quality. It is the starting point to increase the availability of usable water and the opportunity of supplying tap water as much as possible for domestic purposes.