4.3 How is the treatment performance of ISFs?

The performance of sand filters correlates to the hydraulic charges, organic loadings, pre-treatment steps, sizing, retention and aeration time, environmental parameters, such as temperature, quality of the components, including filter media, and operation and maintenance.

ISFs rely on the alternating cycles of feeding and aeration. The aeration time especially needs to be respected to ensure treatment efficiency. If the supernatant is continuously present, the process will turn anaerobic, and biological activity in the filter medium of the ISF cannot take place as designed and will lead to rapid clogging. Aeration enables the availability of oxygen for the aerobic decomposition of wastewater.

Temperature affects the rate of microbial growth, chemical reactions and other factors impacting the stabilisation of wastewater. Filter performance is typically higher in areas with warmer climates than in areas with colder climates. Lower hydraulic loading rates are recommended in colder climates or during colder months to ensure desired treatment performances.

Hydraulic loading rates are also dependent on the total treatment process. Loading rates for ISF used as tertiary instead of secondary treatment steps can usually be significantly higher due to a more stable influent that contains fewer solids.

The batch that is fed is recommended to be measured. This helps to determine suitable aeration times. Further, it avoids inadequate wastewater distribution when hydraulic loads are too low, which can create preferential infiltration ways. It is recommended to have hydraulic loads per cell that correspond to 10 – 20 cm per batch.

As a secondary and tertiary treatment step, the ISF can reduce organic matter (BOD) by up to 95%, up to 93% for Total Suspended Solids (TSS), and the reduction of faecal coliform by up to 99%. Nitrogen is primarily converted into nitrates through oxidation, and the overall amount of nitrogen is not significantly decreased. ISFs can cope with influent quantity and strength fluctuations while assuring treatment efficiency.

More information on the design and performance of an intermittent sand filter can be found in the EPA onsite wastewater treatment systems manual which is linked under the Materials tab.