Zeroing in on improving quality
Water companies face increasing pressure from the Environment Agency to improve the quality of final effluent from sewage treatment works, Cross Manufacturing Company believes it has the solution - the Zero Gravity coil.
According to Cross, final effluent quality can often only be improved by the introduction of additional or tertiary treatment equipment, for which various sand filters have traditionally been used.
However, all sand filters are inherently bulky and often expensive, requiring a lot of space and copious amounts of water for backwashing.
For large treatment works serving towns and cities, says Cross, this may not be a problem but consent issues also affect hundreds of small sites in rural areas - there is rarely much space available for new equipment and low flow volumes restrict the potential for backwashing.
Cross says a solution to the problem is its automatic backwashing mechanical filter if a recent application in rural Dorset is anything to go by - a new EA solids consent figure of 45 milligrams/litre was proving difficult to achieve under fluctuating ambient conditions.
Carrying out an on-site test using its portable filtration rig was Cross' first suggestion. This enabled filtration samples to be taken from the final effluent using a range of filter micron ratings, which can be quickly selected by swapping filter coils.
Laboratory analysis subsequently confirmed the most most appropriate filter rating for the application, which was 50 microns.
This information enabled an automatic backwashing filter package to be designed which would accurately and economically suit the maximum operating demands of the works. The four litres/second filter package built for the project is small enough to be housed in a corner of an existing building, although weather-proof housings can be supplied for external locations.
The package draw's from the site's existing effluent sump and pumps the filtered effluent directly into the watercourse. Filtered water is used for automatic backwashing, which is triggered by an adjustable differential pressure switch with set time lapse back-up.
Backwashing does not interrupt forward flow through the filter and uses only minimal quantities of water, which are re-introduced to the works' secondary treatment area.
The technology featured in each Cross Filter is a stainless steel spiral coil with raised nodes on the surface which produce a precise filtration gap.
Called the Zero Gravity coil, the design traps any particles above the required size - these can be selected from seven interchangeable micron ratings, 12, 24, 50, 75, 125, 200 and 400.
The trapped particles are washed away quickly and thoroughly by backwashing, reversing the flow across the coil, forcing it to open slightly and completely flushing any debris out of the system.
Unlike a conventional coil, the Zero Gravity version opens evenly along its length during the backwash process, resulting in complete and through cleaning with the minimum use of liquid.