service conditionsReading time:
Almost all granular material filters work in cycles that include a filtration period followed by a final wash. The essential parameter that restricts the duration of these cycles is head loss. However, below maximum acceptable head loss, preference may be given to other parameters:
- the volume filtered or the cycle time (e.g. 8 hours, 24 hours, 48 hours);
- turbidity that can be verified using a turbidity meter sensor, or even a particle counter.
The maximum pressure drop will be determined by:
- the available hydraulic loading (gravity feed height or pump curves, the pumps having been selected on the basis of acceptable energy consumption parameters), and/or the filter floor mechanical strength (case of pressure filters);
- maintaining filtered water quality throughout the cycle, essential when filtering make-up or drinking water. With some types of industrial water, this condition may not be compulsory and pressure filters can be used with high head losses (0.5 to 1.5 bar); the only important consideration in such cases is average water quality.
When a washed filter is returned into service in a filter battery, we have to avoid any excess output from the filter that has just been washed. The smaller the number of filters, the higher this risk becomes; uniform distribution upstream or downstream will provide the best quality of filtered water.
In some cases, even without an excess output from the washed filter, it is advisable to discharge the initial, more highly laden, water to the drains (very rapid filtration, water that has not been clarified first, stringent quality requirements such as: guaranteed turbidity levels of less than 0.2 or even 0.1 NTU, SDI …). This phase is often designated ‘ripening’.