Vertical flow reedbeds
A major Scottish Water infrastructure development was undertaken in Cowdenbeath including the construction of two new combined sewer overflows (CSOs) which collect additional surface water from within the town. Historically, wastewater from Cowdenbeath had been treated at a wastewater treatment works on the south-east edge of the town, then discharged into the Lochgelly Burn. As the town expanded this WwTW became undersized and was converted to a pumping station, passing flows forward to a larger works at Glenrothes. A holding tank of 1200m3 was retained for this purpose. The increased flows generated by the new CSOs, totalling 229,700m3 per annum, could not be managed by the pumping station or holding tank, so a new treatment facility was required at the site to attenuate and treat the intermittent CSO flows prior to discharge into the Lochgelly Burn.
Scottish Water did not want to re-establish a full WwTW on the site and were therefore looking for a low-maintenance treatment solution which could handle intermittent flows and discharge, within consent, into the Lochgelly Burn. Modelled CSO flow and load data was provided for a 9-year period and provided a basis from which to generate design parameters.
The proposed site for the treatment solution was to the east of the pumping station on an old landfill site which covered mine workings. This meant that significant project costs would be linked to ground remediation, minimising the size of the treatment solution would be critical. Consequently, an aerated vertical flow reedbed was the selected solution as this provides a high treatment capacity per unit area, minimising the total land take. This was enhanced by making the bed much deeper than standard systems reducing the required treatment area further.
The data set was converted into cumulative summaries which determined that achieving complete treatment would not be cost-effective as taking into account the highest flow outliers would greatly increases the size of the treatment area. The optimum treatment capability was concluded to be 79% of the annual BOD load, providing 95% overall compliance, and 100% of the ammonia load. The design solution was a 2m-deep bed with a 4000m2 process area which could treat 4000m3/d.
Flows are fed forward to the bed at a maximum rate of 46 litres/s from a 3000m3 holding tank. On the occasions where flow rates exceed the storage capacity flows will spill to the Lochgelly Burn on the basis that they these will be rare events and the CSO will be significantly diluted.
The aerated reedbed system has provided Scottish Water with an effective, low-maintenance treatment solution at Cowdenbeath, thereby avoiding the need to construct a new WwTW to handle the significantly increased flow of wastewater generated by the new CSOs in Cowdenbeath. Compared to the alternatives, the system has a lower TOTEX, and aesthetically and ecologically enhances the landfill site.