USE OF SLOW SAND FILTRATION TO IMPROVE TREATED EFFLUENT AT BOUNDARY SEWAGE TREATMENT PLANT (ELDORET) FOR IRRIGATION

Abstract

Food insecurity in Kenya due to dependence on rainfall, coupled with drying up of rivers and wells during the dry season make it impossible to grow crops throughout the year. These factors have increased the demand on this scarce water and led to people living in urban and peri-urban areas resort to using sewage treated water to irrigate their crops. This informal practice raises concerns on health issues both to the farmers and consumers of the crops. The objectives of the present study were to determine the microbiological and physicochemical parameters at the various stages of sewage treatment at Boundary Sewage Treatment Plant during dry and wet seasons, if the treated water met national and international standards for wastewater to be used in irrigation and if slow sand filters were able to improve the quality of the water for irrigation. The study was undertaken at Eldoret Water and Sanitation Company and University of Eldoret Laboratories in February to early March 2012 for dry season and in June to July 2012 for the wet season. The BOD5 technique was used in determining Biological Oxygen Demand, total coliforms and total aerobic bacteria were determined, COD digestion method and colorimetric method for COD, gravimetric method for both Total Suspended Solids and Total Dissolved Solids, temperature, pH and conductivity were determined. Slow sand filters made of sand sizes of 0.1 mm, 0.05 mm and another one made of 0.1 and 0.05 mm were put in a pipe of 2.6 feet in length. The data was analysed with one way Analysis of Variance (ANOVA), one sample t- test and dependency t- test using SAS 9.2 and 9.3 software and proportions. ANOVA showed that there was significant difference (p < 0.05) in BOD, COD, pH, total coliforms, TDS, TSS, conductivity, temperature during the two seasons at all the stages of treatment however total aerobic bacteria were not significantly different (p > 0.05) during wet season. The one sample t –test showed that the treatment plant was efficient for some of the parameters. Dependent t test analysis revealed that the sand filters improved the final effluent of Boundary Treatment Plant. The treatment Plant reduced COD by 89.23%, BOD by 86.27%, Total Suspended Solids by 58.46%, temperature by 6.73%, total coliforms by 99.68% and total aerobic bacteria by 26.73% however, Total Dissolved Solids, pH and conductivity increased by 79.02%, 3.74%% and 79.12% during dry season. Similar trends were also observed during wet season. Boundary Sewage Treatment Plant and the slow sand filters were capable of improving wastewater quality to the level that can be used for irrigation. Total coliforms was compared with irrigation standard of ≤ 1000 MPN/100 ml, pH with a range of 6.5 – 8.5, BOD with ≤ 30 mg/l, COD with ≤ 100 mg/l, TDS with ≤ 1200 mg /l and TSS with ≤30 mg/l. The means of the influent during dry season were; 4500 for total coliforms, 8.05 for pH, 82.67 for BOD, 169 for COD , 722.7 for TDS and 90 for TSS against the mentioned standards. The findings of the effluent during dry season were 33.33, 27.67, 28.33 for BOD from the 3 filters, 74.33, 62.33, 70 for COD, 81.67, 23.33, 26.33 for TSS and 960, 600, 813 for total coliforms against the standards. Means of the influent during wet season; 28 for BOD, 76.67 for COD, 1600 for total coliforms, 357.3 for TDS, 62 for TSS and 8.03 for pH in comparison with the standards and the means of the effluent were; 706.7, 400 and 440 for total coliforms and 55, 15, and 26 for TSS against the standards for irrigation.