The sewer system of urban Maputo

Abstract

During three months information about the sewer system of Maputo was gathered, mostly at DNA, DAS, CRA, AdeM, AIAS and at the Municipality of Maputo. The information, consisting of reports, papers, maps, presentations and websites, was used to estimate the potential amount of wastewater in the sewer system of Urban Maputo. This wastewater could be available for reuse in Maputo, at the WWTP, being this the main purpose of the project “Sustainable freshwater supply in urbanizing Maputo, Mozambique” led by TU Delft, UNESCO-IHE and the Mozambican University UEM.
The sewer network consists of system one and system two. System one was built by the Portuguese in the 40s as a drainage system, but nowadays it functions as a combined sewer and it discharges directly into the bay. System two, built by DHV, a Dutch consultancy firm, in the 80s consists of sewer lines, a WWTP and two pumping stations. These pumping stations are also supposed to pump a part of the water of system one to the WWTP. However, because of sand in the pipes the pumping stations are not being operated.
The billed amount of drinking water was used to calculate the flow in systems one and two. These data were obtained per neighbourhood and multiplied by 0.8, a guideline in Maputo for the amount of drinking water ending up in the sewers. For the water flowing in the sewer network, three cases are estimated, the actual status, system two completely working and the total volume of system one and two. The actual flow into the WWTP is 3957 m3/day with 20,665 m3/day being directly discharged into the bay. If the pumping stations of system two were operating, 10,266 m3/day would flow to the WWTP and 14,357 m3/day would be directed into the bay (Figure 1). By measuring the amount of influent at the WWTP, using the existing Venturi meter, the calculations were validated. The measurements show a flow arriving to the WWTP in the order of the magnitude of the calculations .This influent is generated by approximately 38,000 users that are connected to the sewer system.

The sewer network of Maputo has a few critical parts which should be repaired as soon as possible, and better maintained in the future. First of all the pumping station of system two should be turned on. Before this is possible the sand in the sewers in front pumping station two must be removed and the pipes should stay clean. Sand and plastic bags ends up in the sewer system through drains or open manholes. To overcome clogging, drains and manholes have to be better maintained.
Another recommendation is to collect the wastewater being discharged by system one and convey it to the existing WWTP or to a new one. The municipality has plans for this but lacks financing.
All the water which is collected by system two is conveyed to the WWTP, but the WWTP is not functioning well. There is white slime in the effluent and colourful tarnish, which is a sign of bacteria being present in the effluent. The effluent is either directly used for irrigation of crops, which poses a risk for human health, or directed to the estuary.
At the moment there are detailed plans to introduce a sanitation fee, which is necessary to improve, operate and maintain the system. CRA has been working on introducing the fee for several years already and they expect to introduce it within the coming years.