deep sidewall saves on space

Paterson Candy has undertaken two innovative process designs for the former

Scottish water authority East of Scotland Water (ESW) at Dunfermline WwTW.

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