Watershed degradation increases water treatment costs

Watershed degradation increases water treatment costs

A salient example illustrating the societal cost of sediment water pollution is that of the Lukunga water treatment plant. Constructed in 1939, the Lukunga station supplies western Kinshasa with 48,000 m3 of drinking water per annum and serves an estimated population of approximately half a million people. Sandy soils and steep topography make the Lukunga catchment highly vulnerable to soil erosion. Until the 1970s the basin was naturally protected with dense forest cover. Land use changes including unplanned agriculture and urban development have exposed the fragile soils to heavy rainstorms, leading to significantly accelerated soil erosion rates.

According to records, turbidity levels of the Lukunga River in the 1940s were typically less than 15 Nephelometric Turbidity Units (NTU), reaching more than 25 NTUs during rainstorms. Today, average suspended sediment concentrations are in the range of 100-120 NTUs. Moreover, turbidity levels of well over 3,000 NTUs and up to 6,000 NTUs have been recorded following heavy rainfall events. Beyond the 1,000 NTU threshold, water plants are generally obliged to stop their operations.

The exponential increase in the Lukunga River’s turbidity levels over the past 70 years is attributed to forest clearance and the haphazard expansion of agriculture and informal settlements. Formerly situated in a protected forest zone, Lukunga’s water intake is today encircled by vegetable gardens and informal housing settlements. Moreover, the Mbinza water intake, which formerly supplied the Lukunga plant, was abandoned in the 1980s as it was destroyed by gully erosion and landslides. Sedimentation of Lukunga’s riverbed is a growing problem requiring the water plant’s management to conduct frequent dredging. The risks of flooding and geomorphologic changes in the river’s watercourse are additional problems facing the water utility.

Excessive turbidity levels have necessitated the utilization of larger quantities of imported chemical coagulants to precipitate the sediment particulate. As the coagulant agent (aluminum sulphate) used is acidic, it significantly lowers the water’s pH. Therefore, lime is applied to adjust the pH, which is also procured from overseas. Increased use of expensive chemicals to treat the degradation of raw water quality represents a significant financial burden on the cost of water production. To help address the problem, REGIDESO recently secured an official land title for the Lukunga water treatment plant, which should support its case in halting surrounding detrimental activities and help prevent further encroachment. In addition, the plant’s management is considering plans to establish a three kilometer protection zone around the site and undertaking catchment reforestation. Nevertheless, the situation remains difficult and REGIDESO expressed concern that it may have to shut down the Lukunga water plant.

The problem of sediment pollution is not unique to Lukunga but was also observed in Kinshasa’s other water treatment plants of N’Djili, Ngaliema and Lukaya. Elevated turbidity levels above the 1,000 NTU limit reportedly occur in both the N’Djili and Lukaya rivers during the rainy season. While the Ngaliema station abstracts its water from the relatively clear Congo River, it is negatively impacted by the highly turbid waters of Basoko River, which occasionally backflows into its intake point. Similar problems are also impinging on the operation of REGIDESO’s provincial water centres. For example, in Kindu, Maniema Province, the water treatment plant reported frequent stoppages due to elevated turbidity levels (> 1,000 NTU) of the Mikelenge River, which discharges immediately above its water intake. All the aforementioned treatment plants are therefore compelled to use additional coagulants as well as pH adjusters to deal with rising turbidity. Watershed degradation from unplanned agricultural and settlement encroachment was consistently cited as the main cause of elevated sediment pollution.

The increasing cost of drinking water treatment due to diminished water quality is negatively impinging on the operations of many of the DRC’s water utilities, both in terms of rising chemical and dredging expenditures. Concurrently, sediment pollution also highlights the role of ecosystem services, particularly of forests and wetlands, in providing good water quality. Empirical studies measuring the effect of increasing turbidity levels on the chemical cost of water treatment would provide hard economic evidence on the importance of ecosystem services in protecting water infrastructure and achieving national and MDG water supply targets. It would also serve as a valuable reference case for the DRC on the role of natural capital in national development.