Institute for Water Quality Studies

ASSESSMENT OF THE IMPLEMENTATION OF THE PHOSPHATE STANDARD AT THE BAVIAANSPOORT AND THE ZEEKOEGAT WATER CARE WORKS

EXECUTIVE SUMMARY

Full text (40k) and graphics (280k)

This report must be cited as:

Hohls, B.C., Quibell, G., Du Plessis, B.J., and Belcher, T. (1998). Assessment of the Implementation of the Phosphate Standard at the Baviaanspoort and the Zeekoegat Water Care Works. Report No. N/A230/01/DEQ0797. Institute for Water Quality Studies, Department of Water and Sanitation, Pretoria, South Africa.

Background

The Department of Water and Sanitation (DWAF) has long recognised eutrophication as one of the major factors affecting the quality of South African water resources (DWAF, 1986). A direct result of eutrophication is an increase in the overall productivity of a water body which may include an increase in algae and aquatic macrophyte biomass, and leads to other specific water quality problems.

The quality of the water in the Roodeplaat Dam has long been a cause for concern and various complaints have been made relating to this. The Northern Province Regional Office of the DWAF, therefore, requested that a study be conducted to determine the water quality status of the impoundment, and that the compliance of the Baviaanspoort and the Zeekoegat Water Care Works (WCWs) to the 1 mg.l-1 phosphate standard should be evaluated.

Eutrophication Management by Means of the Phosphate Standard

Eutrophication management programmes have concentrated on reducing the total phosphorus load to water bodies since phosphorus is generally considered the most manageable of the nutrients, and there are proven and cost effective technologies for the removal of phosphorus from wastewater. An effluent standard of 1 mg.l-1 ortho-phosphate (hereafter referred to as the Standard) was, therefore, promulgated on 1st August 1980 by the DWAF as the first step in a long-term eutrophication control programme. Local authorities responsible for seven sensitive catchments were given five years to upgrade effluent treatment plants in order to comply with the Standard. The DWAF decided to grant local authorities exemption from the Standard for a further three years due to a downturn in the economy. Therefore, the Standard was only implemented in 1988.

Objective

The objective of this report is to assess the extent of compliance to the Standard at the Baviaanspoort and the Zeekoegat WCWs, as well as to assess the effect of possible further water quality management scenarios on ameliorating the eutrophication-related water quality problems in the Roodeplaat Dam.

Discussion

• Compliance to the standard
  The Baviaanspoort and the Zeekoegat WCWs are not complying with the Standard. The Baviaanspoort WCW has shown a decrease in the phosphate concentrations in the treated effluent following the imposition of the Standard, however, 50 % of the samples still exceed the Standard. The Zeekoegat WCW shows an even greater exceedence of the Standard (77 % of the samples).
• Point and Non-Point Source Contributions
  Prior to the imposition of the Standard, and prior to the construction of the Zeekoegat WCW, there was a 50:50 relationship between the point and non-point source contributions to the phosphorus loads to the impoundment. Following the construction of the Zeekoegat WCW this changed to a 70:30 relationship. This is a cause for concern, especially taking into account that the magnitude of the discharges from point sources can be expected to increase in the future.
  The non-point source phosphate load is increasing. Growing informal residential areas appear to be the main cause for the increase.
• Phosphate Load Reduction
  The catchment is strongly point source dominated. This source of nutrients will, due to increases in flow, become more and more significant, and warrants urgent attention.
  The stringent implementation of the 1 mg.l -1 Standard will realise an improvement on the current situation, however, the impoundment's internal load may prolong the effects of the past loads. Taking into account the projected increased flows from the WCWs, within a short period of time the nutrient load to the impoundment will once again be at a high level. For this reason a more stringent phosphate standard will be necessary to bring about an improvement in the longer term. From the simulations it can be seen that a considerable improvement could be expected if a phosphate standard of 0.5 mg.l -1 was implemented.

Recommendations

• Implementation of steps to ensure strict and immediate enforcement of the 1 mg.l-1 phosphate standard at both the Baviaanspoort and the Zeekoegat WCWs.
• A 0.5 mg.l-1 phosphate standard would result in a significant improvement over the long term. In this regard, the cost involved versus the benefits should be assessed.
• In order to quantify non-point sources of nutrients, an investigation should be considered.
• If the control of point sources of nutrients through effluent management alone should prove not to be sufficient in improving the water quality, one of the most obvious options to investigate would be in-lake management.

List of co-workers

• PROJECT MANAGEMENT AND CO-ORDINATION
  • Gavin Quibell
  • Brendan Hohls
  • Kobus Du Plessis
• WATER QUALITY MODELLING
  • Gavin Quibell
  • Brendan Hohls
• DATA PRESENTATION AND GRAPHICS
  • Brendan Hohls
• REPORT
  • Brendan Hohls
  • Gavin Quibell
  • Kobus Du Plessis
  • Toni Belcher

[ IWQS Home Page ]