Water/Waste Processing - Feb 2012
The original installation follows a classic and traditional trickling filter design time tested and used in similar treatment plants across the UK A site on the outskirts of town close to the river and adjacent to the railway line where in the 1950s it was thought residential development would never take place features initial screening for solids before passing through a primary settlement tank Secondary treatment occurs in a series of eight bio filter beds before passing the water through humus tanks then gravity feeding to a final sump from which outflow to the river is controlled Storm tanks introduce temporary holding storage for exceptional flows and feedback through the system to reduce the risk of pollution in such cases Wessex Water determined that the flow volume was less of a problem than the need to clean the final effluent so the hydraulic capacity of the system did not need to be increased but its purification effectiveness needed work A study of the alternatives indicated the most efficient compact and cost effective way to achieve this was to install a tertiary treatment system following from the existing primary settlement tanks and biofilter beds We did have options how best to achieve this claims the companys area scientist Pierre Luchon who is responsible for compliance issues on the Gillingham site and many others across the region But in the end aerated sand filters were probably the best way to remove both suspended solids and ammonia and the Dynasand systems had proved themselves on two other sites in the region so we knew how they were installed and operated Four DynaOxy units were installed in a compact group at the lowest point of the site allowing them to receive their input flows from the existing process units Each filter is about six meters high and contains a filtration medium of basalt sand The system operates all units from a central single input to maximize efficiency and filters solid particles as the flow is pumped to the bottom and rises up to the top of the unit The introduction of pumped air to the bottom of the filter helps to lift the dirty sand to the top where the media is washed for continuous reuse Process air is also added within the media bed to provide air for biological treatment to remove ammonia from the effluent A 10 day post installation testing program with the plant working in fully automatic mode confirmed the plant could meet compliance standards at actual and anticipated flow and ammonia load rates and sampling was taken at regular and frequent intervals for evaluation After the 10 days further time was added to look at performance under more widely varying flow conditions Additional ammonia was dosed into the system to ensure the system outputs would remain within the required consent criteria even at the expected future flow levels and all tests produced results within acceptable consent ranges and design limits Each filter is about six meters high and contains a filtration medium of basalt sand 14 February 2012 Water Waste Processing www waterwaste com
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