Optimal bed thickness and efective size for improving wastewater quality for irrigation

Abstract

With the increased use of wastewater for irrigation, there is the need to reduce the contaminant levels in wastewater. The slow sand fltration (SSF) is one such method that can be used to improve wastewater quality. However, the treatment quality depends among other factors on the depth of sand bed and the efective size. Acquiring sand of a particular efective size is becoming increasing difculty and, therefore, this study sought to investigate over a specifed area, the optimal depth and efective size that will be able to get rid of contaminants in wastewater. In separate experiments, three depths (30 cm, 40 cm and 50 cm) and two efective sizes (0.27 mm and 0.45 mm) were set up to investigate their efectiveness in removing Faecal coliform, E. coli and heavy metals (Pb, Cu and Fe) for wastewater from a peri-urban drain used for irrigating vegetables. Results showed that a minimum sand bed thickness of 40 cm and an efective size of up to 0.45 mm reduced the contami nants tested signifcantly, wastewater from the drain can be treated. It must be mentioned that the fner sand (0.27 mm) had a slightly better removal efciency. This implies that the extra cost of acquiring sand of relatively smaller efective size and a higher bed depth with the aim of improving wastewater quality can be saved. Further investigations are being carried out on the combined efects of the optimal sand bed depth and efective size.