The feasibility of wastewater recycling that includes residue from dissolved air flotation within a drinking water treatment plant: case study of Midvaal Water Company, South Africa

Abstract

When purifying water for potable use, wastewater is generated, due to the class of the water treatment plant and the quality of the source water. Midvaal Water Company recycled wastewater that included residue from the dissolved air flotation (DAF), sedimentation and filtration processes in an attempt to save water and reduce costs. The aim of this study was to determine functionality and water quality of such a wastewater recycling system. Samples were collected for analysis, at the sections that contributed to the total wastewater system as well as after various treatment processes. The water quality of these samples was determined, as well as the incidences of water quality failures of the final water, to establish whether the recycle stream that enters the plant together with the source water had any impact on the water quality after the different treatment processes. Data were grouped into periods prior to, during and after recycling to enable comparisons. The water quality of the recycle stream was poorer than that of the source water from the Vaal River with regard to the mean values for total chlorophyll, suspended solids, turbidity and dissolved organic carbon, but the sedimentation process of the wastewater system improved the wastewater quality by drastically reducing total chlorophyll, suspended solids and turbidity. The risk-defined compliance for the final water was excellent (≥95%), despite aluminium, turbidity and total chlorophyll failures of the final water quality during the recycling period. Total chlorophyll was identified as the largest risk during wastewater recycling, especially after the filtration process. It is evident from the data that wastewater recycling, which included wastewater from the DAF, into the main inlet stream of the water treatment plant proved to be effective, based on compliance with national legislation, and had no detrimental impact on overall treatment processes or final water quality.

Keywords: dissolved air flotation, sludge balancing dam, total chlorophyll, wastewater recycling, water treatment