| dc.description.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. | en_US |
