Detergent phosphorous (2000)

Funded by: Water Research Commission

Partner organisations: Partners in Development, eThekwini Water and Sanitation

Project description
In 1986, Heynike & Wiechers of the Water Research Commission (WRC) assessed the contribution made by detergent phosphorus to wastewater phosphorus discharges in South Africa and thus its impact on eutrophication. Their study showed that detergents comprised between 35 and 50 % of the total wastewater phosphorus load and thus presented a significant source of phosphorus to the environment, but the costs associated with eliminating or banning detergent phosphorus outweighed the benefits. The authors indicated that there was lack of suitable data for the study and several assumptions had to be made. They therefore recommended that the WRC continue to keep a watching brief on the situation, while the South African detergent industry investigates and tests cost effective substitutes for phosphate builders.

Towards the latter part of 1990, Lever Brothers (Pty.) Ltd. (Lever) approached the University of Natal, Pollution Research Group (PRG) and the University of Cape Town to discuss sponsoring an investigation into the effect of substitute detergent builders on wastewater treatment. This study was given the go ahead and commenced at UCT in 1991 as a Masters research project. Following this a meeting was held in 1991, with representatives of the WRC, Lever, the PRG and Umgeni Water where it was agreed that the Pollution Research Group in collaboration with Umgeni Water would undertake a WRC funded study which updated the 1986 Heynike & Wiechers study. The latter investigation commenced in 1992, with the principal researcher stationed at Umgeni Water in Pietermaritzburg.

The Mgeni catchment was chosen as the study area because it was relatively well known by the researcher, was easily accessible and water quality data had been collected for the catchment as part of the Umgeni Water routine monitoring programme. It was also an important catchment in that it constituted a major developmental region in South Africa and served the water needs of major urban centres, notably, Pietermaritzburg and Durban, and several surrounding small towns and rural areas.

The topic of detergent phosphorus and its contribution to eutrophication, i.e. nutrient enrichment, is also particularly relevant to South Africa where many impoundments have become eutrophic or are threatened by eutrophication. Walmsley & Thorton investigated the trophic status of 31 major impoundments in South Africa In 1982, and reported that 9 were eutrophic and 13 bordering on eutrophication.

The major consequence of eutrophication is prolific growth of algae or certain rooted macrophytes, which may result in severe water quality problems, including, unpleasant tastes and odours, deoxygenation and fish kills. In the case of the potable user, eutrophication has resulted in considerable expense due to the requirement of sophisticated treatment measures such as the use of powdered activated carbon to treat taste and odour problems. Eutrophication may be controlled by the reduction of the nutrient discharges to the environment. In particular, phosphorus is often singled out for reduction, as it is frequently the growth-limiting nutrient for aquatic plants.

Phosphorus sources include point discharges such as domestic and industrial wastewaters and diffuse sources such as agricultural wastes and fertilizers. In detergents, phosphorus serves as a builder that works in synergy with the surfactant to perform the cleaning function. Phosphorus arising from detergent consumption is usually in the form of orthophosphate - the bio available phosphorus form, and is therefore an attractive option for control. Various authors [e.g. Heynike & Wiechers, 1986; Sas, 1989; Edmondson, 1991] have indicated that about half the phosphorus contained in domestic wastewaters may arise from laundry detergents, which could in turn contribute significantly to the phosphorus loading to impoundments. Should this be the case in the Mgeni catchment, elimination of detergent phosphorus could contribute significantly to improving water quality in impoundments.

Over the years following the Heynike and Wiechers [1986] study, additional data and information have become available, including more detailed water quality and land use data, as well as better data and information on the costs and implications of treating water drawn from eutrophic impoundments. In addition, the number of eutrophic impoundments appears to have increased. The Nagle impoundment on the Mgeni system, previously classified as oligotrophic by Walmsley & Thornton [1982] has since 1989 displayed recurring blooms of Cyanophycae (or blue-green algae), resulting in severe water treatment problems [Umgeni Water, 1994]. In the light of these developments, the contribution of detergent phosphorus to eutrophication of water bodies has been re-examined in the Mgeni catchment.

Project Objectives
  • To assess the contribution of detergent phosphorus to eutrophication in the Mgeni catchment.
  • To determine the economic and water quality consequences of eliminating detergent phosphorus.
  • To provide a methodology for studies in other catchments.