4.2 How and under which conditions does intermittent sand filtration function?

Intermittent slow sand filtration (ISF) is a relatively low-cost technology characterised by low flow rates, long retention times, and fine sand as a filter medium. This type of sand filter is often combined with all biological digesters, such as biogas digesters, septic tanks, or lagoons, as a secondary or tertiary treatment step after a polishing pond or trickling filter. The ISF can be deployed as a single stage or in series.

The ISF is a viable alternative when soil conditions are not favourable for proper treatment and disposal of effluent through percolative beds/trenches. ISFs can be used on sites with shallow soil cover, inadequate permeability, and high groundwater.

ISF is a fixed film biological treatment system variation and is distinguished by two types of processes: single pass and recirculating. The single pass process is where wastewater travels through the filter once, and the recirculating process is where the wastewater travels through the filter several times. Recirculating filters are in a similar design, except a coarser media is used, and a portion of the effluent is piped back to the dosing tank to be reapplied to the filter.

The influent is applied to ISF periodically in batches through a dosing system. The dosing system provides uniform wastewater distribution (time and volume) across the filter. The influent infiltrates and percolates through the sand bed, displacing supernatant from the previous cycle in the pores and percolating down. This movement allows air to enter the filter bed, which promotes biological activity. The alternating cycles of feeding and aeration create aerobic conditions that effectively decompose contaminants in wastewater. The suspended solids and organic matter are removed through physical straining and biological degradation as the wastewater passes through the sand filter.

The sand in the filter must be clean with a recommended sand size between 0.1 and 1 mm. The recommended optimum for grain size may range between 0.25 and 0.6 mm. Two layers with different sand sizes are possible. If the sand is too coarse, the wastewater will pass through too quickly, and if it is too fine, the flow of wastewater decreases, resulting in the increased likelihood of clogging. The sand should not contain any limestone, organic matter, clay, or other soil particles and no more than 3% fine particles.

Under normal conditions, a filter bed works for some months without clogging. Clogging is evident once the fed batch is not soaking away within the usual time. When a cell clogs, it should be left to dry and subsequently swept and raked. If sweeping and raking are insufficient, the 50 – 100mm top layer needs to be replaced with new and clean sand.

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