deep sidewall saves on space
Space constraints at Dunfermline WwTW led Paterson Candy to use a North American final settlement tank design which increases upflow velocity, allowing a reduced tank diameter
In a joint venture with Amec, the company has designed and installed a new
step aeration process combined with deep sidewall final settlement tanks. This
has allowed secondary wastewater treatment at Dunfermline, which now belongs
to Scottish Water, the body created from the amalgamation of Scotland's three
former water authorities. Previously the works provided only primary flow treatment.
Paterson Candy proposed combined process intensity using step aeration and current best practice in North American final tank design to provide a plant which met the space requirement while providing a works which is easy to operate and low in maintenance compared to other process intensive designs. The new treatment process has been designed to treat 506 l/s and handle incoming flows up to 1,800 l/s. The treatment is to Urban Waste Water Treatment Directive standards: biochemical oxygen demand (BOD) 25mg/l and chemical oxygen demand (COD) 125mg/l.
The step aeration process is not new to the UK but Paterson Candy, in partnership with Amec, have laid the process out in a way that minimises the footprint of the unit and reduces construction costs. The process design maintains a mixed liquor concentration of 2,800mg/l across the basins while only feeding 2,000mg/l onto the final tanks. The plant utilises four aeration lanes each fed individually with settled sewage. Return sludge from the final tanks is blended with the settled sewage just prior to the first lane. The design allows the feed streams to be balanced so each lane receives 25% of the incoming sewage, or 30% to each of the first three lanes and 10% to the last. Isolation of individual lanes for maintenance is achieved using stop logs.
The final settlement tank design combines deep side water depth with recirculation baffles, off-centre outlet hopper and a flocculating centre well. These features are an example of some of the most advanced techniques utilised in North American final settlement tank design and demonstrate the diverse expertise available to Paterson Candy in-house and through its parent company Black & Veatch.
The design allows a greater upflow velocity, while still maintaining security of operation, hence providing a significant reduction in tank diameter without compromising process performance. The Dunfermline design allows for the final tanks to operate at up to 1.6m/hr at peak flows, with peak solids loading rates of between 4 and 6kg/m²/hr.
Adding depth to the side wall of final settlement tanks gives several advantages in terms of solids capacity, overflow rate and process integrity. Well operated tanks can be run at significantly higher overflow rates and are able to cope with hydraulic shock loads and the associated shift in solids inventory much more easily. Units are more robust and offer greater process stability, but can give problems if solids are allowed to collect in the tank. The high capacity of the final tanks can however, result in significant quantities of active solids being held up in the settlement process with subsequent effect on the process.
The use of recirculation baffles to prevent short-circuiting up the walls of the clarifier is standard design practice in the US. The baffle is fixed under the outlet trough and extends the hydraulic influence of the trough further into the tank. This acts to direct flows back into the unit and creates a rotating flow in the bulk of the liquid. This in turn increases the retention of the unit.
The tank uses a half-bridge scraper but the bridge supports a further one-third tank radius scraper opposite the half-bridge helix. This gives additional sludge transport close to the hopper, which is located off centre from the tank. This gives the advantage that the whole blade passes over the hopper rather than the sludge being directed into the hopper off the tip of the blade.
The flocculation centre well encourages the formation of larger flocs by providing high energy within the initial reception chamber of the unit. It differs from conventional UK design in that it allows flow to leave the centre well from the side rather than the bottom and does not require any further baffling in the unit. By not directing the flow at the floor of the tank the incoming sewage is not encouraged to impinge on the floor of the tank and disrupt the sludge flow into the centre well. Paterson Candy has found this to be a common problem with conventional UK plants where overflow and solids loading rates have been pushed to limit footprint without compensating for this in any other aspect of the design