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

With the increased use of wastewater for irrigation, there is the need to reduce the contaminant levels in wastewater. The slow sand filtration (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 effective size. Acquiring sand of a particular effective size is becoming increasing difficulty and, therefore, this study sought to investigate over a specified area, the optimal depth and effective 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 effective sizes (0.27 mm and 0.45 mm) were set up to investigate their effectiveness 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 effective size of up to 0.45 mm reduced the contaminants tested significantly, wastewater from the drain can be treated. It must be mentioned that the finer sand (0.27 mm) had a slightly better removal efficiency. This implies that the extra cost of acquiring sand of relatively smaller effective 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 effects of the optimal sand bed depth and effective size.