Vexamus Water has completed a successful pilot trial for United Utilities to investigate the treatment of UIDs (Unsatisfactory Intermittent Discharges, or storm flows) from sewage treatment works.

Vexamus Water has completed a successful pilot trial for United Utilities to investigate the treatment of UIDs (Unsatisfactory Intermittent Discharges, or storm flows) from sewage treatment works. This award-winning process can be considered as an alternative option to that of constructing storm tanks. During the 10-month trial, which took place at Millom WWTW in Cumbria, Vexamus Water developed an innovative flocculation and clarification process that achieved very high levels of treatment.

The new treatment process (designed not to find a cheaper option, but to find a solution that works) involves chemically dosing the screened storm water before transferring it to the flocc/clarification stages of the StormEasy Process.

Flocculation of the chemically dosed feed is achieved within a non-powered FloccEasy tank that creates a spiralling flow pattern to produce readily settleable floccs.

Clarification of the floccs is achieved in a Lamella Plate Separator that comprises a hopper bottomed steel tank containing rows of flat plates inclined at 55 degrees to the horizontal.

The incoming flow is evenly distributed across the plates, promoting settlement of the solids on the plates and a progressive discharge of sludge into the base of the tank.

Clarified water is collected in horizontal launders located above the plates and discharged from the process.

The bacteriological standards worked to during the trial were an overall bacteriological log kill, between crude sewage and receiving water, of 5.4 for bathing water discharges and 5.25 for shellfish waters.

To achieve this, United Utilities required the StormEasy process to produce an effluent with a suspended solids concentration of less than 15mg/l and a % transmissivity greater than 50%.

This would then allow a UV system to provide the additional treatment to ensure that the log kill figures could be met.

Traditionally, the volume of effluent entering a sewage treatment works during a storm event (that cannot be treated by the existing processes) is diverted to storm tank(s) for storage and eventual treatment through the works when influent flows permit.

Typically, the storm tanks are sized to retain 'excess' storm flows for a period of 2 hours and, once full, additional storm water would 'spill' from the tanks to the normal point of discharge from the works.

The availability of more sophisticated software now provides a better understanding of storm events, particularly follow-on storms, with the result that spill frequency targets are likely to become tighter.

United Utilities adopted Continuous Simulation Modelling Techniques to provide a clearer picture of their treatment options for meeting these tighter spill frequency targets.

An internal review of the options concluded that either building additional storm tanks, or ones with larger capacities - normally a cost effective solution - was not always possible, because of operational problems and the difficulty in obtaining more land.

Instead, the ideal solution was to treat the storm effluent on a continuous basis, as it arises, and discharge it to the receiving water.

The combination of the Vexamus Water FloccEasy and Lamella Plate Separator was borne out of the need to develop a process for treating storm water that was efficient, simple to operate, reliable, robust and with low capital and maintenance costs.

At the beginning of the trial, flocculation of the effluent was achieved using a rectangular tank fitted with a variable speed stirrer, which is a standard type used with Vexamus Water Lamella Plate Separators.

Although satisfactory results were obtained, careful consideration was given to improving the overall process by simplifying the operation of the plant and reducing capital and operating costs - without sacrificing plant performance.

The flocculation stage is a key part of the process, since poorly formed floccs will not settle in the Lamella, and it is the only stage that has motorised drives.

Given that the plant is likely to sit idle for long periods and then 'jump' into operation during a storm event, the removal of drives would simplify the process, reduce capital costs and provide a robust plant by reducing the risk of equipment failure at start up.

Thus the development of the FloccEasy began.

On-site colour turbidity and UV transmissivity measurements were made on inlet/outlet samples from the StormEasy process, with the more extensive analysis, to include BOD, SS, faecal coliforms and entroviruses, being undertaken by United Utilities.

The quality of the treated effluent satisfied all requirements.

Increasingly tight discharge consents and improved software further underlines the realisation that the traditional policy of building additional storm tanks, or ones with larger capacities is not always feasible.

This is particularly the case where.

Either the wastewater treatment plant is operating at maximum flow.

Or there are on-site difficulties in operating more storm tanks.

Or when problems arise in obtaining land or permission to construct storm tanks.

The development of the StormEasy process also provides inherent benefits, namely that of saving up to 90% of the area of a conventional settlement tank and therefore conserving the existing valuable land for other processes.

The pilot plant trial at Millom was the first time in the UK that a continuous process had been applied to resolve a UID problem.

Its ability to meet the quality parameters and provide an operator-friendly plant now provides an alternative solution to the building of storm tanks.