Eastney - Horsham, West Sussex - Case Study
Project summary: ERG was contracted by Southern Water to provide increased air extraction and upgraded odour control at the Eastney Headworks site in Portsmouth.
Key project data: The upgrade works
- more than doubled the extracted air flowrate to the odour control unit from 20,700m3/hr to 48,500m3/hr
- was completed with live sewage flow throughout
- maintained air extraction from the underground screens area and detritors throughout
- minimised the duration of shut‐downs
- optimised the re‐use of existing assets to keep the project cost as low as possible
ERG project value: £970k
- Feasibility and FEED: 3 months (2015)
- Detailed design and engineering: 3 months (2015)
- Site works: 9 months (2015‐2016)
- All matching Southern Water’s requirements
Eastney Headworks receives sewage and storm water flows from Portsmouth and provides primary solids removal using screens and detritors prior to pumping the screened sewage to Budds Farms WwTW for treatment. Storm flows are directed to Fort Cumberland for storage and reintroduction to the sewer network post storm event back at Eastney. The sewage flow and pumps are housed underground with only the washpactors, skips, MCCs, facilities block and odour control housed above ground. The original parts of the site date back to 1886 with a significant upgrade (including addition of the odour control block) in the late 1990s.
In 2014, in response to an updated DSEAR assessment, Southern Water upgraded some of the electrical installation and drives in the underground works. The risk mitigation included additional ventilation rates in the detritor and screens areas and so Southern Water engaged ERG, as the framework odour control supplier, to investigate options for improving the ventilation and associated odour control.
Fig 1 – Scrubbers and demister. Temporary / bypass scrubber on left; new main odour scrubber on right
This case study explains the challenges associated with design and subsequent project implementation, and how ERG successfully installed a system which delivered everything Southern Water needed.
Feasibility and Front‐End Design
As part of the Front‐End Engineering Design (FEED), ERG’s design team worked in collaboration with Southern Water to establish the requirements of their DSEAR report and agree the design extraction rate from each area of the works required to meet the new design parameters. ERG then surveyed the site to establish the currently‐achieved extraction rate and operating approach and the state of repair of the existing extraction ductwork. Within the headworks, there were two generations of extraction ductwork, some of which worked adequately, although parts were in a poor condition and provided no extraction.
ERG had been maintaining the odour control unit for over a decade under a framework contract with Southern Water (SW). Therefore, our maintenance and operations staff knew the SW site team well, and had a good understanding of the existing odour control package.
The output from the FEED was a report of recommended works which included current and future air flow diagrams, ductwork P&IDs, and upgraded odour control unit outline design. Following review and approval by Southern Water’s engineers, this plan formed the base‐line for a subsequent proposal and contract award.
ERG implemented the project successfully to six key criteria:
1. Increase the extraction rate from site to meet the DSEAR report requirements
This meant an increase from the detritors from 2,400 to 8,900m3 /hr and from the headworks screen room from 5,600 to 20,400m3 /hr. The total extraction from the combined areas therefore increased 365%.
A practical limitation for the new design was the size and position of the ductwork penetration through the screen area ceiling slab. Lack of available civils drawings for the structure meant that new openings in the slab were risky and so ERG re‐used the two existing duct sections passing from the underground area into the existing ductwork network. And new, dedicated detritor area extraction ductwork was routed above ground to combine with the balance of the screen area extracts.
Fig 2 – Detritor extraction and air admittance dampers and ductwork and screens area
In normal operation, balance air cascades from the screen area into the detritors. But during storm events, the higher liquid level blocks this air path, and so to ensure continued detritor extraction, ERG installed air admittance ductwork with weighted air inlet dampers. These inlet and exhaust ducts to the sealed detritor area made use of existing penetration through the 1100mm thick detritor structure wall in order to reduce the scope of new civils works, although one new penetration was required.
Fig 3 – New penetration through the detritor wall for the new odour control extraction duct
The storm pumping station was also ventilated at a higher rate, increasing from 12,700 to 19,200m3 /hr. By careful design of the new ductwork system, ERG was able to retain the existing ductwork between this PS and the odour control building without modification.
The combined air flow to odour control therefore increased 235% from 20,700 to 48,500m3 /hr. Within the underground detritor and screen areas, ERG designed and installed all new PVC extraction ductwork with room grilles and droppers to specific areas, with all parts designed to robustly resist storm event surcharging, while being free‐draining, easy to clean and still capable of providing extraction during the storm events. Above ground, ERG’s new ductwork system was PVC/GRP, and took into account all building and road access requirements, with high‐level supports and a pipe bridge across a road. This above ground system also included bespoke connections at the screen area floor slab penetration, and allowed for the existing power and lighting services within the odour control building.
Fig 4 – New pipe bridge and ductwork from detritor and screens area
ERG’s scope also included for the refurbishment or replacement of Air Handling Units (AHUs) to provide fresh‐air ventilation into certain of the buildings and underground areas, ensuring positive air pressure to prevent fugitive leaks of odorous air and associated H2S corrosion of electrical components.
2. Treat the extracted air to at least the same odour discharge standard
The original (20,700m3 /hr) odour control unit was a two‐stage chemical scrubber. From our maintenance activities and site monitoring, ERG knew that the first stage acid scrubber was not required since the ammonia levels from the site were very low. All of the odour performance could therefore be achieved using a caustic‐bleach scrubber for H2S and mercaptans removal. The existing second stage scrubber performed well, but was undersized for the new, higher airflow and was mechanically failing. ERG therefore agreed a new caustic‐bleach scrubber approach with Southern Water, including supply of a new scrubber vessel.