Filtration is the separation process in which suspended solid particles separated from liquid. Filtration operates entirely on particle or droplet size; i.e. particles below certain size will pass away through filter media, while larger particles are retained/ hold on or in between filter media. Pressure filters are operated on differential pressure. To operate the filters, it is required to have pressure difference across the filter media and this is done by means of pressure upstream (Pressure filtration) or suction downstream (Vacuum filtration).
Pressure Upstream Filtration:
Pressure upstream filtration or pressure filtration is a process in which liquid is being filtered is passed from top of the media/ vessel by means of pump. The liquid which is being filtered is kept above the minimum pressure of 43.5 psi or 3 bar subject to maximum of 145 psi or 10 bar. To achieve this condition, pump selection is very important criteria for proper working of pressure filter.
Typically pressure filters are designed for filtration velocity of 6 to 15 m3/hr. m2 (m/hr). By fixing the filtration velocity for known flow, vessel diameter can be fixed.
Pressure filters are designed for 50 mg/l of suspended solid loading. If solid loading is beyond design limit, proper pre-treatment is required to reduce solid loading on filter. In pressure filtration, solids with particle size more than 40 microns get separated from liquid results in less than 5 mg/l suspended solid content in outlet of liquid.
Typically pressure filter can hold 3 to 6 kg TSS/ m2 of filtration media surface. For higher dirt holding capacity Anthracite is used which have typical dirt holding capacity of 6 to 10 kg TSS/m2.
Pressure filters are used for many applications, but some of the applications are listed below.
- Cooling water filtration,
- Waste water filtration,
- Drinking water filtration,
- Swimming pools water filtration,
- Pre-filtration for membrane system, etc,…
Filtration cycle consist of
- Service cycle
- Backwash cycle
In this, liquid is passed through top of the filter bed and the suspended particles continuously separated from liquid and gets accumulated on filter bed. Service cycle breaks on pressure drop created due to dirt accumulation on filter bed or once in 24 hrs for back-washing of filter bed.
Typical condition to break service cycle is as follows
If pressure drop across bed increases beyond 10 psi or 0.7 bar. OR
Once in a 24 hours.
When service cycle breaks due to either of the condition. Filter goes into backwash cycle.
Backwash means removal of dirt from filter bed, accumulated during service cycle, by passing cleaned water from bottom of filter and it will carry dirt with water. It is always recommended that, filters must be backwash using filtered water only.
Typical backwash cycle time is in between 15 to 45 minutes depending on feed water quality. Pressure filters are backwash by two ways.
Backwash only with Water
Backwash with air scouring followed by Water
When filter is backwash only with water, the velocity of backwash water is kept at around 36 m3/hr. m2 (m/hr).
For air integrated backwash, velocity of air is kept at 36 m3/hr. m2 (m/hr) and water velocity is in between 15 to 20 m3/hr. m2 (m/hr).
After completion of backwash, filter is ready for next filtration cycle.