Saltend Wastewater Treatment Works – Odour Mitigation

Morgan Sindall Sweco Joint Venture (MS2JV) were commissioned by Yorkshire Water Services to undertake £18m of refurbishment and upgrade works in order to improve the performance of the Saltend Wastewater Treatment Works in Hull.

Key Facts

Client – Yorkshire Water Services
Location – Hull, Yorkshire
Contract Value – £18m


Saltend Wastewater Treatment Works had a history of environmental under-performance.

During the summer of 2015 Yorkshire Water’s Service Incentive Mechanism (SIM) score was directly affected by a significant number of customer complaints due to odour. As a result, the East Riding of Yorkshire Council imposed a Statutory Nuisance Order (SNO) which required the odour to be abated by 1 July 2016.

Saltend WwTW, was built in 2001 and serves a population of over one million, with more than 50% of the incoming load from trade discharges. The works receives flows of up to 22,000l/s to its inlet pumping station via an 8.8 km long 3.6m diameter tunnel.

The works treats a flow to full treatment of 2000l/s utilising small footprint technology, including lamella primary settlers and Sequencing Batch Reactors (SBRs).

There are four lamella settling tanks which utilise inclined plates to increase the settling area. The plant has eight SBRs (each 42m diameter with 10m water depth) as the secondary treatment process. The final effluent is discharged to the River Humber.

An initial odour survey identified that 54% of the odour generated by the works derived from the SBRs and 42% from the inlet pumping station.


Collaborative Investigation Model - This project was different to the conventional Yorkshire Water framework approach which ordinarily involves the award of an investigation contract followed by a risk based delivery contract. Whilst the investigation and delivery phases were retained, the approach was adapted to include three key elements:

1. Problem clarification: The scheme was initiated by Yorkshire Water in August 2015 with a 3-day workshop to understand the problems to be solved using all available data. The workshop drew on Yorkshire Water’s AMP6 Framework partners who provided key expertise and identified a list of remediation measures.
2. Scoping phase: A collaborative scoping team consisting of Yorkshire Water, Arup, Aquability OPS Ltd, MWH, Turner & Townsend, and Yorkshire Water’s Tier 1 Contract Partners (including MS2JV) was established to allow detailed design to progress in advance of a delivery contract. This approach greatly accelerated the speed of construction delivery.
3. Delivery phase: Conventional delivery scheme by MS2JV.

The team was located at the Saltend site to ensure close collaboration, ownership of the problems and to establish a commitment that the team would ‘fix’ Saltend together by 31st May 2016. Key to driving the tight timescale was the acceptance of process risk by Yorkshire Water thus removing the requirement for MS2JV to undertake extensive investigation works.

Collaborative Delivery Model - The collaborative model was taken into the delivery phase of the contract with the client, scoping team, supply chain partners and MS2JV staff all remaining located at Saltend. The MS2JV design team also moved on site such that they could coordinate and develop the design with the supply chain partners and scoping team.

In order to “fast-track” the design and avoid any duplication of effort, MS2JV worked closely with the supply chain partners to define the scope and limits of their design.

Several of the supply chain partners were given early works contracts ahead of their main orders, which allowed them to progress the design in line with the tight timescales. At one stage or another, representatives from MS2JV, Circle, NPS, ATS, Nomenca, Suprafilt, CEMA, TCS, Staptina, Ward & Burke, Waitings, EMS were all based in the same co-located office developing their elements of the design.

Close collaboration allowed the teams to develop the design with continued reference back to the decisions made by the scoping team.

YWS’s operational team were also based in the co-located office, providing continuous input to the design both informally and through the design reviews and the HazOp and HazComm meetings.

Extensive use of CLIP (Construction Lean Improvement Programming) was made to build, refine, improve and communicate the planning of the works.


The scoping phase of the project focused on developing the list of interventions identified at the initial problem clarification workshop into packages of work to be delivered by MS2JV.

The main packages were:

1. Inlet Pumping Station: New Odour Control Unit (OCU).
2. Primary Treatment (Lamellas): New chemical dosing units, new desludge pumps & refurbished / modified lamellas.
3. Secondary Treatment (SBRs): Control philosophy modifications, conversion of jet aeration system to fine bubble aeration, refurbished decant system and improved control and availability of air blowers.
4. Pre-Treatment: Pre-treatment of effluent at a large seasonal trader.

Reducing the odour at the Inlet Pumping Station - To reduce the odour, a carbon filter OCU was installed to treat 20,000m3/hr of air. Hydrogen sulphide concentrations were monitored in the existing vent stack and when levels reached a pre-set limit, the duty/standby OCU vent fans kicked in to force the odorous air through the carbon filter before it was discharged to the atmosphere. This approach prevented the unnecessary treatment of the air, maximising the life of the carbon and reducing power consumption.

Improving lamella performance - In order to improve lamella performance as quickly as possible, and allow the optimum chemical dose rate to be determined, a temporary modular plant was installed. The use of the temporary modular plant allowed for the provision of ferric and polymer dosing ahead of the permanent works installation.

The lamellas had enclosed inlet flumes with small orifices that fed the wastewater into the lamella plates, these orifices were prone to blocking up. The blockages resulted in poor flow distribution and high velocities between the lamella plates which caused solids to carry over with the effluent.

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