Effect of operating mode on the performance of sand filters treating greywater.

A long-duration laboratory study spanning more than 6 months was conducted to evaluate the effect of operating mode on the performance of sand filters for greywater treatment. Performance of saturated and unsaturated filters operated in continuous or intermittent mode was evaluated using settled real greywater. Effects of pause period and higher loading rate on their performance were also evaluated. Furthermore, the effect of the depth of the filter medium on the performance of the filters was also assessed. The saturated filters operated continuously or intermittently performed significantly better than the respective unsaturated filters. Saturated continuous (SC) and unsaturated intermittent (UI) filters were the best and worst performers amongst the different filters. SC filter removed on an average 98% turbidity, 76% BOD, 88% COD, 84% ammonia-N (NH-N) and 95% phosphate (PO-P). Up to 99.68% (2.40 log) removal of faecal coliforms was also achieved by this filter. SC filter showed stable and reliable performance as its effluent quality was insensitive to fluctuations in influent quality. The pause period significantly affected the removal of some of the parameters. Doubling the hydraulic loading rate significantly affected the performance of continuous filters. Though most of the pollutant removal occurred in the top 10 cm in all the filters, a depth of 50 cm was found optimum for the removal of different pollutants. Whilst all the filters produced effluent conforming to different reuse standards except microbial standards, saturated continuous filter (SC) could also meet the faecal coliform standards towards the end of the filter operation.