Managing biofilm growth and clogging to promote sustainability in an intermittent sand filter (ISF)

The sustainability of rural sanitation includes the long-term welfare of both rural and urban societies. As a commonly used rural sanitation technology, operation of intermittent sand filters (ISF) is impacted by biofilm clogging inside the ISF. In this study ISF performance is studied at low hydraulic loading rates (HLR) to explore the interaction between biofilm growth and wastewater treatment efficiency. CW2D/HYDRUS, a simulation model which does not include media hydraulic property changes caused by biofilm growth, is utilized as a numerical control to contrast the effects of biofilm growth inside an experimental ISF. A paired experiment with simulation demonstrate that biofilm clogging comprised dominantly of heterotrophs occurred in the top layers of the ISF. Lowered HLR slows clogging development but not final clogging extent. The biofilm clogging development zone offers adequate removal of applied biodegradable COD and NH₄⁺ - N. However, the spatial distribution of heterotrophs and biodegradable COD does not match the denitrification requirement of the resulting NO₃^- - N. A simultaneous nitrification and denitrification (SND) potential is manifested in the clogging development zone, but lowered HLR reduces media moisture level to a less favorable level for denitrification. Furthermore, slowed water movement under lower HLR aggravates the accumulation of NO₃^- - N, which can potentially result in counterproductive salt accumulation. Since biofilm growth is a natural and self-adaptive response to wastewater application, this study suggests accepting limited, managed biofilm growth and clogging in ISFs. In addition, this study calls for further research to manage biofilm growth and clogging for long-term ISF sustainability.