Investigation of the Effect of Grain Size, Flow Rate and Diffuser Design on the CAWST Biosand Filter Performance

The effects of grain size, hydraulic loading rate, batch residence time and diffuser design on the performance of the CAWST version 10 biosand filter was investigated. Two types of sand gradations were prepared – fine sand (ES = 0.20, UC = 2.3) and a coarse sand (ES = 0.25, UC = 2.9). The fine sand and coarse grains resulted in initial hydraulic loading rates of 0.3 m3/m2 min and 0.75 m3/m2 min, respectively. Flow restrictions were installed on 2 coarse grain filters such that their initial hydraulic loading rates were 0.3 m3/m2 min. For the range of grain size and flow rates investigated in this paper, coarse grain size leads to lower bacterial removal efficiencies compared to the fine grain size. The addition of flow restrictions on filters with coarse grain size did not result in improvement in bacterial removal efficiencies. Two different diffuser designs were also investigated (hole size of 1/8 inch or 3.2 mm and 0.5 inch or 12.7 mm spacing, hole size of 3/16 inch or 9.5 mm and 0.5 inch or 12.7 mm spacing). These filters were compared to the standard design of CAWST version 10 filter, i.e. control filter, which has a hole size of 1/8 inch (3.2 mm) and 1 inch (25.4 mm) spacing. Diffuser design was found to have an effect on the bacterial removal efficiency. The two diffuser investigated (smaller hole size, tighter spacing) resulted in lower bacterial removal efficiencies, compared to the CAWST specified diffuser design. Batch residence time was found to have a significant effect on bacterial removal. The deficiencies caused by diffuser design, coarse grain size were compensated by a higher batch residence time.