Settling on the Flyde coast
New system takes on primary sedimentationh role in Blackpool debut
Since the works went into service a number of upgrades have been carried out to meet Environmental Agency (EA) discharge consents. The main process elements of the original plant were coarse and fine screening, grit removal, free oil and grease (FOG) separation, submerged biological contactors (SBC) and final settlement (FST). Over time it became clear the secondary-treatment SBCs were under-performing, due partly to problems at the FOG separation stage. In response, United Utilities sought a solution based on a primary treatment stage in order to reduce the overall loading on the SBCs.
Three processes were considered, conventional primary settlement, dissolved air flotation (DAF) and Actiflo. Conventional primary settlement tanks were ruled out due to a high capital cost of approximately £10M. A DAF unit was tested but found to be ineffective as a result of the high sand content in the incoming sewage. A full-scale DAF unit would also mean extending the existing building, creating planning problems.
A pilot of OTVB's Actiflo system was set up in a bid to reduce the organic loading to the SBC's in various modes of operation. The results demonstrated removal efficiencies of 60 to 70% in suspended solids (SS), COD and BOD with up-flow velocities of 100m/h. It was also shown that, after full shutdown, the process could start up again and operate at full efficiency within 30 minutes. An additional benefit was Actiflo's small footprint which allowed retro-fitting to the existing works, thereby reducing capital costs and avoiding the requirement for planning permission.
Following the trials, the company won a £2.2M contract to design, build and commission the full-scale plant. The design principally comprised three Actiflo units, two Multiflo units, ferric chloride storage and dosing, polymer preparation and dosing, microsand storage, batching and transfer system, hydrocyclones, sludge transfer pumps and instrumentation and control.
To minimise capital costs, and avoid the need for planning permission, the three existing FOG tanks were used to provide the feed tanks for the Actiflo units. Modifications to accommodate new penstocks, coagulation and flocculation mixers, baffle arrangements, lamella packs and launders, together with bottom sludge draw-offs were designed into the existing reinforced concrete structures.
The system incorporates enhanced settlement with very small sand particles (microsand) used to add weight to the flocculated organic solids. Following fine screening and grit removal, raw influent enters a coagulation stage where a primary coagulant, ferric chloride or aluminium sulphate, polymer and microsand are injected and mixed rapidly. The influent enters a flocculation stage, via a baffle arrangement, where gentle mixing and further addition of polymer causes the suspended solids and the microsand to combine forming weighted flocs. Lamella plate settlers separate the flocs and the clarified effluent passes to the secondary-treatment stage. Sludge is re-circulated to a hydrocyclone where the sand is recovered for re-injection into the process. Separated sludge from the hydrocyclone, is treated in a Multiflo unit and supernatant is returned to the Actiflo inlet.
The Multiflo process is designed to thicken sludge from the Actiflo before dewatering. It consists of two stages; polymer and ferric chloride are added to the separated sludge and denser flocs are formed with the aid of a picket fence mixer. The sludge then passes through lamella settlers where it is thickened to approximately 4 to 6%. In practice, sludge is consistently being produced at the upper end of the concentration scale and can be sent directly to the belt presses, instead of being mixed with SBC sludge and thickened via gravity belt thickeners. Supernatant, with an SS level below 100mg/l, is discharged from the top of the unit and returned to the inlet of the Actiflo.
The system's operating regime is dictated by seasonal differences in influent characteristics. Maximum flow, biological demand and SS levels occur in winter, whereas significantly higher levels of FOG enter the STW in summer. The Actiflo units were therefore designed to take 50% of the maximum winter flow, 4,140m3/h, during winter - when this is reached flows run directly to the SBCs via a new bypass channel. During summer, when input does not reach 4,140m3/hour, flows are processed entirely by Actiflo.
Take-over tests commenced in June and show the plant should meet United Utilities'
requirements and discharge limits set by the EA. The quality of sludge from
the SBCs was also improved dramatically, affording substantial cost savings
on the operation of the existing odour treatment system.