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Delivering a better environment in Scotland

Major investment to tackle odour and improvements to management and operations is showing its reward at Seafield Wastewater Treatment Works in Edinburgh. Natasha Wiseman reports

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Scottish Water’s Seafield site is perhaps the utility’s most notorious, hitting the headlines internationally in April 2007 when the failure of both its Archimedes screw pumps caused some 100,000 tonnes of untreated effluent to be dumped into the Firth of Forth. In the wake of the incident, a report by WRc identified multiple failures at the Thames Water Services operated site, which is situated in north-east Edinburgh, near the densely populated district of Leith.

The close proximity to housing is less than ideal and means odour issues have also dogged the site and local population historically. Leith Links Residents Association is one organisation that has been particularly vocal, even pursuing Scottish Water in an attempted class action for compensation.

When Veolia bought out Thames’ services division in November 2007 it acquired a Private Finance Initiative (PFI) contract for the Seafield site. The company has worked hard to improve both the treatment processes and reputation of the site. Another key challenge has been to implement a culture change at the site: improving the working environment, health and safety statistics, and moving from reactive to proactive site maintenance.

Inlet works

A major part of the £26M investment has been the £10M Seafield Inlet Improvement Project (SIIP) for the renewal of the inlet works, which started in 2009 and reached completion last summer. Seafield sewage works processes some 110Mm3 of wastewater from 850,000 population equivalent a year.

The inlet works and preliminary treatment processes are designed to handle everything from low flow during dry weather, when some two tonnes per second may arrive for treatment, to storm flows when up to 19t/s can arrive. Rags, sticks, stones, sanitary products, plastic, grit, sand, old tights can all be flushed into a sewer and arrive at the works.

The project has delivered a doubling of the screenings capture rate, which not only protects downstream processes, but also reduces the potential for odour issues related to the primary treatment processes. Eight new screens sourced by contractor MWH Biwater from Longwood Engineering are now removing rags far more efficiently through a two-stage screening process and the effectiveness of stage one means that the secondary screens no longer get damaged.

Before SIIP, an average of 140t of screenings were collected and removed from site every month. Since the project, this has risen to an average of 300t. At the end of the inlet works, the grit removal system has also been redesigned by the contractor, with new ram pumps fitted to the four existing detritors. The flow entering from much narrower channels slows down to about 300mm/s and the grit and sand settle to the floor of the tank. Scrapers push the material to small sumps at the side of the tank and it is passed through Eimco classifiers to remove organic matter, before being collected in skips for collection by a waste contractor for possible reuse as backfill. At Seafield about 18 tonnes of grit can be removed each week.

The improved screenings and grit removal at the plant have significantly benefitted site operations, resulting in considerable reduced blockages of pumps, desludging equipment and pipelines. Through delivery of the SIIP, there has been a 50% reduction in the number of blockages around the site.

The need for expensive ad hoc interventions such as pipe jetting and tank cleaning has also been significantly reduced. Vacuum tanker usage, removing blockages in the preliminary stages, has reduced from a weekly occurrence to a twice-a-year event.

Blockages within the sludge treatment branch have reduced by some 80% so sludge levels in primary processes, such as the primary settlement tanks, can be much more effectively controlled. Within the storm tanks, SIIP has meant a 60% reduction in blockages to the Amajet mixing units. Wear and tear on pumps downstream of the inlet works has also been significantly reduced.

Odour

As well as better handling of screenings, covering over the inlet channels has also helped to deliver a significant win on the battle against odour. Screenings stored at the site used to be a source of odour, but are now so thoroughly cleansed by washwater effluent, prior to compacting, that storage in open skips presents no odour risk. Further, the washwater itself is reprocessed.

The working environment of the inlet house, which used to be a high risk area, is also much improved. The air is clean and workers at the site no longer need to wear masks.

A huge amount of the project work had to be carried out with the site still operational. Most spectacular is the high level pipe work that was a key strand of the £15M Seafield Odour Improvement Programme, involving the installation of a number of odour control units (OCUs) using a twin-channel, single carbon filter comprised of treatment by sodium hydroxide and sodium hypochlorite.

For every 1m3 of water, the site treats 5.3m3 of air. The air is extracted and treated using a number of odour control units (OCUs), and the cleaned air is finally discharged through stacks. The system can handle some 15,000m3/h or air and a 365-day proving test is now nearing completion. The issue of odour is still a sensitive one for the local community and Veolia says it is “working hard to be good neighbours”.

Treatment

Initial treatment is comprised of six primary settlement tanks, of which only four are usually in use, with those nearest the community rested, again, to help mitigate odour issues. The designers rejected the idea of covering the tanks to reduce odour early on, although there is still pressure from residents for this more expensive option to be implemented.

The designers believed that covering the inlet to the tanks and the weirs that the water passes over at the edge of the tanks would be the most effective way of reducing smells. This means that air entering the channels is not released.

Improvements to the secondary settlement tanks were made to increase the amount of sludge being collected. Counter eddy current killers were installed, slowing the momentum and delivering better quality water.

Aeration

The view from the aeration tanks at Seafield must be one of the best at any UK sewage works, taking in the Firth of Forth to the east and with the backdrop of the city’s famous extinct volcano, Arthur’s Seat, to the west.

Underfoot, though, there had been a much needed overhaul as the process was not getting enough well-oxygenated bacteria, which meant the aeration lanes were often suffering from high foaming and a risk of bacterial issues. Over the course of a year, the contractor removed and replaced the existing floor nozzles in all six lanes. Underwater leaks were also repaired.

The result is that the process is much more efficient, with only four lanes now needed. A new stage will see trials with smaller blowers and variable speed drives to help optimise the air delivered to the system and possibly reduce energy costs further. While not everyone will be rushing for a dip in chilly waters of the Firth of Forth, the Scottish bathing season began on 1 June, which is when UV treatment, consisting of some 2,000 tubes, is switched on. Outfall to the Firth of Forth is then treated to the higher standard until mid-September each year, in accordance with Scottish Environmental Protection Agency requirements.

Sludge

The site has six digesters producing methane and three combined heat & power (CHP) plants, the most recent of which was built last year by Cogenco. The site can return some 3.2MW of power to the national grid, which is helping reduce its carbon footprint.

The finished process is producing sludge at 26% dry solids, which can be used for land restoration. However, tendering is currently under way for the thermal hydrolysis process to be introduced at Seafield by early 2013, with three bids under consideration. This would deliver a higher quality digestate with lower carbon footprint than driers. It would also mean that the sludge cake could be used in agriculture.

Engineers are also interested in Veolia’s ‘bio-refinery’ reference site in Brussels, where wastewater is treated as a resource, with recovery of chemicals like phosphorous built into processes.

The journey for Scottish Water and Veolia has been a tough one. It cannot be obscured that the legacy at Seafield had been very damaging. However, a significant distance has been covered on the journey to bring the site up to scratch.

This has meant close liaison with the community and a complete change in the culture of how the site is maintained, operated and managed. The measures that have been taken have already made a significant impact on the environment, with the improvements at Seafield having already delivered an ‘excellent ‘ranking in the SEPA annual environment and performance assessment. Project manager Alex McTear told WWT: “The environment’s much better for the people here”.

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