Secondary treatment works

Project manager Bill Williamson on the £70M Howdon STW upgrade.

A third of Northumbrian Water's 2.6M customers are served by Howdon STW. Commercial use adds the equivalent of 200,000 more people, making Howdon the largest estuarial discharge between the Forth and the Thames. Work at the site is a result of the European Commission's (EC) urban wastewater treatment directive (UWWTD), under which secondary treatment must be provided for estuarial sewage discharges from populations in excess of 15,000 by 31 December 2000.

Operational since 1979, Howdon receives raw sewage via the Tyne interceptor sewers. Flows from north Tyneside are screened, de-gritted and undergo primary settlement. Flows from south Tyneside receive preliminary treatment before transferring to Howdon for primary treatment. The treated wastewater streams then combine before discharge to the river. Sewage sludge, both imported from outlying works and produced at Howdon, is taken by ship for drying at Northumbrian's regional sludge treatment centre (RSTC) at Bran Sands.

Howdon will be expanded into three parts;

  • the existing western site with primary treatment works, sludge storage tanks, jetty and offices, will be augmented by a new sludge thickening facility,
  • a new eastern site, adjacent to a chemical works which divides the STW, to provide the secondary treatment units,
  • a link corridor to the north of the chemical works with wastewater transfer pipelines and a permanent access route between the treatment sites.

Because of the attenuation available within the interceptor sewer system, the new works will be given consent to treat flows up to the maximum daily peak flow of 4,500l/s.

South and north bank flows will combine for primary treatment in existing northern settlement tanks. The remaining tanks will be retained to provide additional storm protection for flows up to 7,200l/s. Primary-treated wastewater will be pumped to the eastern site for secondary treatment using activated sludge technology, with fine-bubble diffused air. Following final settlement in new circular clarifiers, the wastewater will return under gravity to the western site where it will combine with storm overflows before discharging to the tidal estuary. A new pumping station will transfer settled sludge to the western site for thickening to 5% dry solids before transportation to Bran Sands.

The environmental assessment identified a chance to enhance an area of the eastern site designated as a site of nature conservation interest (SNCI). Developed by Northumbrian's environment department, with help from the local authority ecologist and the Wildfowl and Wetlands Trust, a new wetland has been constructed.

Following a fee-bid competition, Entec UK was appointed to carry out feasibility and design, geotechnical, construction management and planning and environmental assessments. During the feasibility phase, in spring 1998, it became apparent the most significant challenge was the December 2000 deadline.

The first step was to determine the preferred treatment process. In addition to activated sludge, technologies such as sequencing batch reactors (SBRs) and high-rate filters - with or without primary treatment or chemically-aided settlement - were considered. Value management workshops were held to assess the options and consider the risks to the business in meeting its statutory obligations. On the basis of the lowest whole-life cost and minimum risk to the business in terms of process, the preferred solution was conventional primary treatment and conventional activated sludge secondary treatment.

Having determined the treatment process, attention turned to developing the most appropriate procurement strategy to meet Northumbrian's objectives - a best-value, minimum risk solution which ensured maximum competition in tendering, implemented a culture of teamwork and met the December 2000 deadline.

Constraints and risks to be considered included;

  • the earliest planning permission could be obtained was December 1998,
  • confirmation was required that the land to the east of the existing site could be used for the secondary treatment works
  • confirmation was needed of flow volumes to be treated.

Possible procurement strategies included: Traditional procurement with earthworks undertaken in advance; traditional procurement with earthworks and piling undertaken in advance; a team-working approach involving Northumbrian, the lead designer, an electrical designer and a main contractor with earthworks undertaken early; a teamworking approach involving Northumbrian, the lead designer, an electrical designer and a main contractor with earthworks and piling undertaken early; a design-and-build approach; procuring work in packages rather than as a single main contract.

Packaging the deal
Due to the timescale a packaged approach was chosen because it would allow design of later packages to be undertaken while the earlier packages were under construction. In all 16 work packages were initially identified. To achieve the team-working culture, engineering and construction contract (ECC) conditions of contract were deemed appropriate for the major packages, with G90 used for smaller mechanical and electrical packages with minimal site works.

The three main civil works contracts were tendered on an ECC option B contract with a bill of quantities. The partnering philosophy was fostered with risk sharing and the sharing of any savings achieved through value engineering.

Risk sharing meant the contractors agreeing to absorb defined risks in exchange for a lump sum adjustment to the tender. These risks included ground conditions, interfaces with others and the measurement risk associated with the bills of quantities.

The contractors and their sub-contractors were encouraged to contribute to the partnering process through partnering workshops, cascade workshops and think-tanks. Significant savings have been achieved for all parties.

Faithful & Gould were appointed to develop and implement a risk-management strategy for the project, effective risk management being fundamental to the procurement strategy.

Reducing risk
Preliminary commercial risk assessment was taken during the feasibility stage, a more detailed assessment then determined the contingency allowances to be included in the budget presented to Northumbrian's board. This meant identifying and quantifying key risks, which were modelled using @Risk software to determine the range of likely outcomes of the combined effect of the risks.

Similar techniques determined the value of the risks transferred to the contractors under the partnering approach. Separate risk-management plans have been developed for each of the main civil works contracts together with a project-wide risk management plan. Regular reviews are undertaken to ensure contingency allowances remain adequate and the recorded risk mitigation strategies and fallback plans remain valid.

Site work began in August 1998 with advanced earthworks undertaken by Hall Construction Services to clear the derelict eastern site and prepare the landform for major construction in April 1999.

Good working relations were established swiftly and everyone in the extended team understands the need to keep the existing works in operation. Crucial to the success of the delivery team is the integration of operative staff. Throughout the scheme they have had input and are a fundamental part of the decision making process.

Partnering has encouraged a non-adversarial way of operating and promoted a culture of team-working. Potential problems are recognised at a stage where they can be rectified without losing time. Early into the two-year construction programme, value improvements identified by the combined team began to result in significant cost savings.

The effectiveness of team-working was demonstrated in the building of the 37m diameter final settling tanks. The design was re-appraised by Ultrastore, sub-contractors to Alfred McAlpine, and replaced with one taking advantage of ring stresses in the circular structures. This reduced wall thickness and concrete volume significantly, vertical joints have been eliminated, resulting in quicker construction.

Concerns over the control of sludge age in these tanks have been allayed by suction removal technology from Degremont UK. Settled sludge is carried from the base of the tank via vertical draught tubes into a collection box beneath the rotating bridge. Sludge flow is hydrostatically generated by siphoning sludge from the collection box. This reduces tank floor slope and provides a lower sludge age, which ensures more viable biomass is returned to the activated sludge lanes. In turn, this cuts the potential of denitrification and floating sludge.

To cut top-soil costs, it is intended to use biopellets produced at the RSTC as pre-planting soil conditioner. Such innovations have only been possible by the commitment of the alliance - team building is maintained through briefings, newsletters, social events and shared offices.

With construction on a live treatment works, safety is a high priority. Alfred McAlpine is principal contractor, managing the interfaces between contracts and carrying out worker inductions. Interface with existing Northumbrian operations is handled by a liaison engineer who identifies potential hazards and ensures appropriate risk assessments and method statements are produced.

The project is a registered member of the working well together campaign led by the Health and Safety Commission's (HSC) Construction Industry Advisory Committee (CONIAC). This helps to create a team-based approach to health and safety matters. In-keeping with the partnering philosophy, the campaign seeks to ensure continuous improvement through;

  • commitment to high standards of health and safety,
  • competence - ensuring everyone is trained and competent to do their work,
  • communication - ensuring health and safety permeates throughout the project,
  • cooperation - building-up relationships of trust and partnership.

This work is the latest in a series of improvements in a 'building block' approach started in Northumbrian's first wave. As part of the second wave, work will carry on at Howdon to provide tertiary treatment in the form of ultraviolet disinfection. Scheduled for completion by 2005, this and similar work within Northumbrian's coastal programme will result in 70% of Northumbrian's customers being served by this advanced treatment - the highest proportion in the country.

Project team    

 

Alfred Mc Alpine Civil Engineering:
The single largest contract, £14.3M, to construct the aeration tank (eight lanes of 97.3m x 7.5m x 7.5m), 16 final clarifiers (37m diameter and 4.5m deep), the return activated sludge / surplus activated sludge (RAS/SAS) pumping station (15m x 18m x 9m deep), flow mixing and distribution chambers and transfer pipelines including two tunnel drives (1.8m diameter x 300m and 1.5 diameter x 100m).

Edmund Nuttall:
Two contracts worth £5.9M for constructing the sludge thickening building and associated holding tanks and pipework, plus the two major wet well / dry well pumping stations to handle flows of 4,500l/s together with culvery works and pressure pipelines.

Northern Utility Services:
Supply, install and commission ten-MVA electrical distribution system including switchgear, transformers and cables.

Aston Dane:
Design, manufacture and installation of the supervisory control and data acquisition (SCADA) and programmable logic controller (PLC) equipment including all necessary system software.

Weir Engineering Services:
Design, manufacture and installation of fixed and variable speed pumps including pipework, valves, motors and inverters.

 

Alstom Drives and Controls:
Design, manufacture and installation of the single-stage centrifugal air blowers, air distribution main, distribution lines with actuated control valves and the air diffuser discs mounted on the floor of the aeration layers.

Environmental Resource Group:
Design, supply and install odour abatement systems, involving extraction and treatment of odorous air in gas scrubbing columns.

Alfa Laval:
Design and installation of the sludge thickening equipment including rotating drum filters and polymer preparation and dosing plant.

Blackburn Starling & Co:
Design, manufacture and installation of 11 motor control centres including PLC and instrumentation sections and ancillary equipment.

Crown House Engineering:
Design and establishment of new site-wide cable routes and support systems, termination and earth bonding, field instrumentation, lighting systems and gas monitoring equipment.

Other contractors and consultants include B&W Tunnelling, Hydraulic Models, Value Management Expert Services, Taylor Woodrow, ESP, Cowen Construction Engineering, Waterlink, NJ Doyne.



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