NWSC SEWER SERVICES.
Every time a toilet is flushed, no one really cares where it ends up but if left untended to, that flush could lead to serious health issues. That is where NWSC’s Sewerage Services Department (SSD) comes in. The Sewerage Services Department under Kampala Water (KW) is responsible for operation and maintenance of the sewerage network, treatment plants as well as making new sewer connections and/or extensions in Kampala.
The SSD currently operates two Sewage Treatment Plants (STP) in Bugolobi and Lubigi; the former being a conventional treatment plant while the latter is based on Waste Stabilization Ponds (WSP).
The Lubigi STP has a combined capacity to treat 5,400m3 (5.4 million) wastewater a day. It receives and treats wastewater from the piped network as well as faecal sludge that is brought by private cesspool emptier trucks. The Lubigi catchment area consists of Makerere, Katanga, parts of Mulago, Kalerwe, Bwaise and areas along the northern by-pass. In order to ensure compliance to quality of discharge, the Lubigi STP has a fully equipped sewage laboratory that is responsible for quality monitoring at the various stages of treatment as well as the discharge point at all sewage treatment plants.
The Bugolobi STP on the other hand, has a capacity to treat 33,000m3 of wastewater a day and receives piped sewage mainly from the central business district extending to parts of Old Kampala, Mengo, Katwe, Nsambya, Kibuli, Mbuya, Nakawa, Naguru, Bukoto and Kamwokya.
In addition to these, National Water and Sewerage Corporation (NWSC) also operates and maintains satellite Waste Stabilisation Ponds in Bugolobi, Naalya estates and Ntinda Ministers’ village. These areas have their separate sewer networks which collect sewage from the various premises.
There are also several sewer network expansion and rehabilitation projects under the Kampala Sanitation Program that are on-going within Kinawataka and Nakivubo catchments. These are in line with the Strategic Direction 2013-2018 which seeks to increase sewerage coverage. In addition, a new treatment plant is under construction in Bugolobi and Kinawataka geared towards improving treatment efficiency and capacity of the existing plant. This will consequently improve the quality of Nakivubo channel, Lake Victoria inner Murchison bay and the environment in general.
As part of efforts to subsidize the cost of connecting to a sewer line, NWSC put in place a New Sewer Connection Policy which offers free materials and labour within 50m of an existing sewer line.
Sewage Treatment Methods used by NWSC.
Sewage or wastewater is polluted water that has been subjected to various uses and has undesirable physio-chemical and microbiological characteristics. Sewage if not properly collected, transported and treated causes health problems to humans.
NWSC employs two types of Sewage Treatment systems that include Conventional Sewage Treatment Works (CSTW) and /or Sewage Stabilization Ponds (SSP). The former is only applicable at Bugolobi Sewage Treatment Works (BSTW) for Kampala City and one plant in Masaka Sewage Treatment Works.
Conventional Sewage Treatment Works (CSTW)
In a CSTW there are three main stages/units of sewage treatment i.e. preliminary sewage treatment, however, two other stages of sludge treatment, digestion and drying, exist alongside wastewater treatment.
Preliminary Sewage Treatment
Preliminary treatment, this stage deals with removal of suspended solids and organic matter in sewage by use of metallic bar screens, sand trap or detritor. Primary Sewage Treatment, this is a stage where organic solids are separated from liquids, 1st sludge sedimentation tank in the same order. The sewage after removal of most of the suspended matter flows to the secondary purification stage.
Secondary Sewage Purification
Sewage purification in this stage is by bacterial activity. The process is known as bio-filtration because the sewage flows over bacteria growing on a slime film on the surface of a well-aerated stone bed. The bacteria remove organic matter dissolved in sewage for their own growth and replication. After the bio-filters the sewage flows to the 2nd sedimentation (humus) tank where any suspended matter or break off bacterial film is removed as sludge. The treated sewage effluent is then discharged into the receiving environment through an artificial wetland, constructed wetland or directly. It is important to note that before the final sewage effluent is discharged its quality must be in compliance with the national environmental standards if not the effluent is recycled.
The sludge removed in the sedimentation tanks is pumped into sludge digesters where its digested anaerobicallly. After about 8 weeks the purified sludge is pumped into sand drying beds where it is further purified. Any water that drips from the sludge digesters and sand drying beds is pumped back into sewage treatment. Farmers collect the dried sludge for use as manure in crop and flower gardens.
Sewage Stabilization Ponds (SSP)
The SSP are a low cost Sewage Treatment System that use bacterial activity to remove organic matter, nutrients and microbes in sewage. Most of the WSP used by NWSC are usually three in series comprising of anaerobic, facultative and maturation ponds in the same order.The sewage treatment occurs through anaerobic digestion in the 1st pond followed by aerobic degradation in the 2nd pond and finally algal activity and microbes death in the 3rd pond. The ponds are occasionally, 2-3 years, de-sludged when there is sludge accumulation.