A municipal WWTP in the North of England was experiencing significant problems with accumulations of FOG in the Wetwell at the head of the treatment plant. The FOG would form a solid cap over the well several feet thick. This put considerable strain on the pumping gear, interrupted the performance of the float switches and required regular manual intervention from the staff. High costs were also being incurred in maintaining the equipment. Additionally the volume of FOG in the system was at a level that did not permit adequate degradation to occur within the aerobic basins so that FOG was being carried forward into the clarifiers. Four or five times a year it became necessary to pump out the accumulated FOG in the Wetwell (some 10 tons on each occasion). The FOG had then to be transported to an anaerobic digester for further treatment.
In January Cleveland Biotech were asked to examine the problem and to propose a solution.
The WWTP in question is designed to deal with a flow of 5,800m3/day, giving full treatment to up to three time that amount and also partial treatment to a further three times in the event of storm conditions. The flow enters the Wetwell through an automatic penstock. The sewage is lifted by pumps situated in the Drywell. After passing through Constant Velocity Grit Channels where the grit is settled out, the full treatment flow goes to two Sedimentation Tanks where the suspended solids settle in the form of sludge. The liquor then passes to the Aeration Units where it is mixed with Activated Sludge and oxygenated before being passed to the Final Tanks where the sludge is settled out and returned to the Aeration Units, surplus sludge being diverted to the Wetwell. The treated flow is discharged over the final tank sills to the adjacent river. The total processing time is approximately 20 hours.
The sludge which collects in the Sedimentation Tanks is pumped to Primary Digesters and held for about a month. This process reduces the solids content by about 50% producing methane gas as a by-product. Sludge from the Primary Digesters is removed to secondary digestion lagoons where it is dewatered and the treated effluent eventually used as an agricultural fertilizer.
Cleveland Biotech carried out laboratory tests on a number of samples taken from the WWTP. These tests established that the accumulations of FOG in the Wetwell were substantial, rising to in excess of 80,000mg/l between pumpouts. With the relatively short residence time within the plant the WWTP was not able to cope with the organic loadings and as a result its final effluent had COD levels of 170mg/l or more.
It was decided to carry out bioaugmentation trials by dosing the Wetwell with Amnite S150L, a proprietary biological product containing specially acclimatised Organics Solids and Fat Digesting bacteria. A CBL Activation System was installed in the Wetwell housing and product was added to the Wetwell on a daily basis starting in April. Over the next month samples were taken from points throughout the plant.
The Utility operating the WWTP were more than satisfied with the results achieved. Not only did the bioaugmentation solve the problems of excessive FOG in the Wetwell but it had a beneficial effect throughout the treatment plant. As a result a regular dosing schedule has been instituted for the plant. The operations staff at The WWTP have now more time to attend to their other duties. There is a direct cash saving from the virtual elimination of pump outs of the Wetwell and of the transport and treatment of the excess FOG in the digester. The COD of the final effluent has been significantly reduced so that effluent consent levels present no problems.
Additional benefits which have yet to be quantified but which have been observed at other treatment plants are a reduction in sludge volumes and increased gas output from the anaerobic digester. All told the cost of the biological treatment has been more than covered through cost reduction in operating expenses and improved functioning of the plant.