Looking good and smelling good
Thames' new WwTW at Island Road, Reading, is something of a flagship - with an unusual appeareance, combination of processes and odour management system. WWT reports.
Among the challenges facing Reading Borough Council as it prepared a strategy to regenerate the area around the A33 relief road, dubbed by planners the city's Southern Gateway, was the Manor Farm WwTW. The presence of the works was seen as one stumbling block in drawing the mix of high-grade commercial and residential developments the council was seeking to attract to the area.
The first works at Manor Farm was built in 1875, the last significant upgrade was undertaken in 1963 at a time when the growing city had yet to spread out as far as the site. Since then, Reading's population has grown by 85,000 to 285,000 people, some of whom count Manor Farm among their neighbours.
Odour from the works - the 'Whitley Whiff' - came to be a problem, as did the prospect of achieving new discharge consents, due to come in to force in 2005, and delivering a sludge product suitable for disposal to agricultural land. Although it would have been possible to upgrade Manor Farm further to mitigate the odour problem and meet the new consents, building a new works was deemed to offer the best value - particularly when the development potential of the site was taken into account. A brownfield site at Island Road, 400m from the existing works, was deemed suitable for the new works and purchase by Thames Water from Reading Borough Council was negotiated. As well as the meeting discharge and sludge quality standards, Thames' new works had to achieve what a Chartered Institute of Building (CIOB) report on the project described as: "Synergy with Reading Southern Gateway regeneration through adoption of high-quality architectural standards for the new works." In other words it had to look good. Island Road also had to smell good, very stringent odour limits had to be complied with. A single-project alliance, the Target Alliance, consisting of Thames Water, Taylor Woodrow and Black & Veatch was created to design and build the new works. Design began in January 2001 at the same time as an extensive programme of land remediation at Island Road. Occupying the 10ha site were sewage sludge lagoons from the Manor Farm works across the road as well as municipal and other waste.
Essentially, the land was derelict and approximately 220,000t of sludge and waste had to be removed. There were further environmental concerns - local wildlife had to be preserved, there were archaeological issues and beneath the site was a class one aquifer that had to be protected with the construction of a bentonite cement slurry cut-off wall. These factors, and the need to limit the work's impact on local residents' quality of life, led the alliance to adopt a comprehensive environmental management plan for which ISO 14001 was sought and achieved.Process design
Island Road WwTW is designed to treat a 306,000PE effluent stream and 346,000PE sludge stream. The PE on start-up was 284,000. The design maximum flow to treatment is 1,805 l/s.
Flows are predominantly domestic wastewater, there are no consented combined sewer overflows because most of the catchment has separate storm and foul sewers, but the works receives significant volumes of effluent from a local brewery. As a result of water conservation measures at the brewery, the waste is high-strength and presented designers with treatment and odour management challenges. The design drivers for Island Road WwTW were:
Effluent entering the inlet works is screened to 6mm and undergoes fat, oil, grease and grit removal, this is followed by primary lamella settlement. Settled sewage then enters the BNR bioreactor's anaerobic zone. The BNR step is followed by final settlement, tertiary sand filtration then discharge to the River Kennet via Fobney Brook.
Flow from the brewery enters the works separately and is screened to 2mm. Rapidly biodegradable brewery waste COD is in the wrong form for biological phosphorus removal so the brewery influent enters a fermentation zone, along with the return activated sludge (RAS), to convert sugars to volatile fatty acids before entering the bioreactor's anaerobic zone.The BNR plant
The bioreactor is a concrete basin that has two unaerated lanes, with mechanical mixers, and six aerated lanes, with fine bubble diffuser mixing. The lanes are structural hydraulic boxes baffled into zones and cells by non-structural walls. Zones are defined as a volume with a similar environment or purpose. The bioreactor zones are:
The unaerated lanes have four zones. The fermentation zone reduces dissolved oxygen and nitrate in the RAS and ferments brewery waste. The anaerobic zone stimulates biological phosphorus release. Mixing of the nitrified mixed liquor with the discharge from the anaerobic zone occurs in the anoxic contact zone. The anoxic zone removes BOD using oxidised nitrogen as an oxygen source. Flow leaves the unaerated lane via a culvert running to the aerated lanes. Mixed liquor exits the aerated lane over a weir, to encourage scum floatation, and is recycled to anoxic contact zone. Ferric dosing, trimed to encourage biological phosphorus removal, takes place at the inlet to the bioreactor upstream of the fermentation zone, at the flow splitting weir to the final settling tanks or the last aeration cell in the bioreactor.Sludge treatment
Primary and secondary activated sludge (SAS) is belt thickened then blended before entering the Alpha Biotherm pre-pasteurisation plant. Pasteurisation is achieved through a 1h retention at 70°C in the thermophilic aerobic digester.
The pasteurised sludge is pumped to the four egg-shaped aerobic digesters then dewatered by centrifuge, cake is held in one of three silos. Thickener and dewaterer sludge liquors are returned to the effluent stream for treatment, their phosphorus content has a bearing on the degree of biological phosphorus removal the works can sustain. Biogas drawn off the digesters is held in bags and used as fuel for the CHP engines or dual fuel, biogas/diesel, boilers.Odour control
The site's strict odour limits, five odour units at the nearest receptor as a 98percentile, could only be met if odour management was integrated with architectural and process design - an 'end-of-pipe' solution was deemed inadequate. As a result, a consideration for any plant was its ability to accept odour control covers while retaining acceptable levels of access for operation and maintenance. As well as volatile organic compounds (VOCs) and H2S emissions designers also wanted to limit the dispersion of sweet, earthy odours from volatile compounds generated in the bioreactor or present in the sewage.
Although odour emission per unit area from the bioreactor is low, the large surface area means the mass of odour discharged is not. The inlet works have local odour control and are housed in an odour-controlled building for secondary containment. Iron dosing helps reduce the sulphide emissions in the inlet works and biogas. It was felt covering the primary settlement tanks would hinder operation and maintenance procedures, therefore the lamellas are uncovered but air from the lamella building is changed six times per hour. Air drawn from the inlet, lamella and sludge buildings is treated by a chemical scrubber with standby regenerable carbon filters. Carbon units offer comparable performance with wet chemical units and have the benefit of preventing odour emissions should the works' chemical supply be interrupted. Passing the brewery waste through the primary settlement step would have allowed odorous fermentation by products to enter the atmosphere. By contacting the brewery effluent with the RAS, the odorous components are assimilated by the biomass before they can escape. The bioreactor was covered and foul air from the unaerated lanes, diluted by the flow of process air, receives biological treatment as it passes through the aerated lanes via the fine bubble diffusers. The aerated lane headspace is extracted and dispersed via a high-level stack.
Poor design and control of the sludge stream can be a significant source of H2S emissions. To prevent this at Reading, the time taken for the sludge to pass between primary tanks, bioreactor and silos is carefully controlled. The odour containment capabilities of sludge handling equipment, including thickening belts and centrifuges, was one factor in choice of supplier, as was the ability to deliver a high-quality liquor. Producing a high-quality fresh liquor reduces the risk of uncontrolled odour emissions. Because the disposal of treated sludge is subject to agricultural demand, an onsite sludge cake storage facility was necessary.
The enhanced treated sludge product generated at Island Road is less odorous than sludges treated to lower standards and it was initially planned to store the cake on open pads. Odour modelling concluded the pads would need odour containment and treatment so the design was changed to incorporate silos.
The need for the Island Road site to look good as well as smell good has also been achieved. The numerous buildings, many of which are required to act as part of the odour envelope, have unusual curved profiles and a striking silver finish. The concrete egg-shaped digesters have been clad in aluminium which is both functional and aesthetically pleasing.References: Technical information for this article is drawn from two sources: Patrick Coleman, David Foster and Ian Cranshaw. Biological Nutrient Removal - New Reading Sewage Treatment Works. Ian Cranshaw and Patrick Coleman. Enveloping Odour Emissions, Water & Waste Treatment, September 2003.