DESIGN OF SEDIMENTATION TANK TO RESUSCITATE PHYTOREMEDIATION IN OVERLOADED OBAFEMI AWOLOWO SEWAGE TREATMENT PLANT

The discharge of raw sewage or improperly treated sewage into water bodies has always had a threatening effect on aquatic lives and that of human that uses the water on the downstream. Such is the case of Awba Lake which is currently being polluted by effluents from Obafemi Awolowo sewage treatment plant. The treatment plant is currently overloaded and macrophyte (water hyacinth) used for phytoremediation in the treatment plant can no longer survive under such high organic condition.

This project aims to resuscitate phytoremediation and help with overloading of the plant by designing a sedimentation tank that retains sewage to a particular physio-chemical quality (pH, Total Suspended Solids, Biochemical Oxygen Demand, Total phosphorus and Total Kjeldahl Nitrogen) before it flows into the treatment plant. The daily sewage discharge into the plant was determined using population data from halls discharging into the treatment plant. Laboratory scale experiments using prototype sedimentation tanks and plastic containers were used to study the effect of sedimentation (both aerobic and anaerobic) on physio-chemical qualities of sewage, retention period needed in (aerobic and anaerobic) sedimentation tank and the threshold sewage physio-chemical quality to support the growth of water hyacinth.

From the results of this study, sedimentation process was found to reduce the organic load, with the anaerobic condition of sedimentation performing better than aerobic. Overall of 72% and 70% reduction in Biochemical Oxygen Demand was achieved in anaerobic and aerobic condition respectively after day 7.

Four days anaerobic retention period is the threshold to control the 395.3m3 daily discharge to maximum organic load that will allow the use of water hyacinth for secondary treatment. Anaerobic Sedimentation tank with a capacity of 1600m3 was designed to resuscitate phytoremediation using the parameters gotten from this study.

Keywords: Sewage, phytoremediation, sedimentation tank, water hyacinth, BOD