Mesocosm study on the potential use of vertical sub-surface flow constructed wetlands for removal of organic matter in slaughterhouse wastewater

Abstract

The fast pace of economic growth in Kenya has created a large demand for meat products. This stands at an annual average of about 600,000 metric tonnes of red meat which is expected to continue rising according to global animal product consumption trends. Consequent challenges in management of increasing volumes of high strength wastewater have necessitated ardent research into sustainable technologies, for which vertical flow wetlands offer a promising solution. Three month experimentation conducted at Egerton University, explored the potential for use of vertical flow constructed wetlands in removing organic matter from slaughterhouse wastewater. The wastewater used was sourced from a mid-scale size slaughterhouse in Njoro Township. Experimental design consisted of three tanks of 2 mm sand, 8 mm quarry dust and 16 mm gravel at shallow 0.65 m and deeper 0.8 m depths, each with four replicates. Retention times of 1, 3 and 5 days were also investigated. The tanks were operated batch-wise and effluent water samples collected five times for each retention time studied. The water samples were analysed soon after using standard protocols for BOD5, COD, NH4-N and TSS. The untreated slaughter house wastewater characteristics ranged between 28,336-3,2502 mg/L for COD, 2,070-3,653 mg/L BOD5,1,371- 2,160 mg/L TSS and 52.98-52.42 g/L NH4-N. The results from the experimental mesocosm treatment set-up demonstrated that organic matter removal was highest at 5 day retention time, with removals of about 50%, 55% and 82% for BOD5, COD and TSS respectively. Deeper 0.8m mesocosms were noted to have significant differences in treatment for TSS and NH4-N compared to shallow 0.65 m mesocosms. Differences in substrate type were observed to have no significant effect on organic matter removal. In the case of ammonia, increase in substrate size was observed to decrease removal efficiency, although significant nitrification did not occur. NH4-N was observed to fluctuate with removal efficiency averaging at 26.5%. This study demonstrates that vertical flow wetlands operated at longer retention times and by tidal flow pattern facilitate removal of organic matter in slaughter house wastewater. However, a pre-treatment stage is necessary in order to reduce the organic matter load, and ensure lifecycle of the wetland is not threatened. Targeting ammonia reduction at the pre treatment stage can highly increase the overall treatment efficiency