Performance of a sedimentation pond system at Roko construction concrete batch plant in Kampala- Uganda

Abstract

There is increased demand for concrete in the construction industry worldwide and Uganda in particular. This has increased concrete production leading to high generation of concrete wash water due to cleaning of concrete trucks and other equipment. This water contains significant levels of pollutants which can adversely affect the environment if discharged without proper treatment. One alternative treatment method is the use of sedimentation ponds which has been extensively used by most construction companies in Uganda. However, it is not known whether the use of sedimentation ponds as a standalone technology is able to reduce pollutants to levels that are accepted for disposal to the environment. This study was aimed at evaluating the performance of the sedimentation pond system at Roko construction concrete batch plant to remove pollutants from concrete wash water. The study involved characterizing concrete wash water as it was flowing through the pond system, determining the efficiency of the pond system, and assessing the quality of water in the ponds during plant operation and during off operation. Before the pond system, wash water had a pH of 10.5, TSS concentration of 30 mg/l, TA of 1514 mg/l, TDS concentration of 5028 mg/l and high concentrations of other chemical pollutants including heavy metals. Concentration of TSS significantly reduced as the wash water passed through the pond system. pH, Ca, Fe, Mn, Cr, Ni, Pb, Al, Cl- , EC showed positive correlations with TSS whereas TA, TDS and SO4 2- showed negative correlations. The overall pond system efficiency for reducing different pollutants decreased in the order Ni > TSS > Cl - >Al > Pb > Mn > Zn > Ca > Fe > Cr > TDS > SO4 2- with respective values of 55%, 46%, 44%, 35%, 28.4%, 28.3%, 12%, 11.9%, 9.3%, 1%, -7%, -19%. Pollutant reduction in the ponds was higher during no plant operation. Total suspended solids together with other particle bound substances were reduced by the system. However, the treatment efficiency of different pollutants was low and depended on how much was transported with the solids. Efficiency was low probably due to high concentration of solids in the influent and reduced retention time. Therefore, sedimentation ponds at Roko construction concrete batch plant as a standalone technology for treatment of concrete wash water was not reducing pollutants to accepted levels by NEMA before disposal to the environment. Use of chemical treatment together with sedimentation, increasing wash water retention time by transforming the ponds to operate as a batch system rather than continuous system or increasing on the number of the ponds and wash water recycling was recommended. Further studies were recommended on the efficiency of the sedimentation pond system with addition of chemical treatment.