Yorkshire overhauls its largest works

The upgrade of Yorkshire Water's largest WwTW, Knostrop, posed many design challenges, including the existence of an old mineshaft on site. Burak Sencer, Gavin Stonard, Peter White and Mark O'Brien of Halcrow explain

Yorkshire Water's obligation to treat flows to a higher biological oxygen demand (BOD) and ammonia final effluent standard at Knostrop WwTW near Leeds will bring together two process streams, the high level and low level, downstream of the existing primary tanks.

The scheme will also provide secondary treatment in the form of nitrifying activated sludge plant lanes (ASP) for 100% of the combined flow.

Additionally, new technology at Knostrop will enable the operators to monitor and optimise the performance of the plant in real time, using a PC running a dynamic model of the works. The on-site Scada system will enable the real-time transfer of all operating data to allow the model to simulate the conditions at site.

The overhaul for Knostrop includes the following elements:
  • New grit removal plant at high level to protect the downstream processes
  • Three fully nitrifying ASPs to provide secondary biological treatment - provided by converting the existing sequencing batch reactor (SBR) to conventional activated sludge, converting the existing surface ASP into a nitrifying ASP by formation of an anoxic zone and improvements to the aeration system and constructing an additional eight-lane ASP
  • Final settlement tanks: 11 new tanks and refurbishment of four existing tanks
  • Numerous sizeable pumping stations and flow distribution chambers
Site limitations
One of the many challenges of the scheme was fitting the proposed ASP structure into the space available on site. The main location available was surrounded by multiple high-voltage (HV) cables on three sides. Some of these cables were 132kV lines.

The location was also on a hillside and had old coal mine workings underneath. By examining the layout of the works geologically, spatially and hydraulically, the design team, working closely with the construction team, was able to come up with an orientation for the structure that improved the layout, and resulted in construction savings. In order to deal with the geological difficulties, Halcrow's leading expert in mining geology and geological hazards was involved in the design stage of the project to agree a strategy for dealing with the risk of constructing over a mined area.

Two of the biggest potential problems identified were migrating voids, common over abandoned mines and uncharted mineshafts. The proposed solution was to pressure grout the ground on a grid pattern, adapting in pattern density to suit the varying ground conditions.

During excavations for the ASP, the construction team witnessed evidence of migrating voids. They also uncovered a coal seam going through a corner of the construction site. These events reinforced and confirmed the geotechnical team's predictions and reaffirmed usage of widespread grouting as well as usage of sulphate resisting mixes for all main concrete.

Hydraulics modelling
Another significant challenge was analysing the hydraulics of the works, both existing and proposed. Knostrop WwTW is large and over the years a lot of development in various places has been undertaken.

Piecing together the current workings has required considerable detective work, going through almost 100 years of engineering drawings. Once the necessary information was gathered, the next challenge was delivering the required operational flexibility in diverting varying amounts of flows to different parts of the treatment process.

This has demanded examinations of a multitude of different flow scenarios, which resulted in varying flow rate maxima for individual pipes and individual process structures. The specification required physical hydraulic modelling of key parts of the proposed works to ensure accurate flow splitting so that treatment capacity would not be exceeded in various streams.

A physical model of the proposed main distribution chamber was built at 1/8 scale.

Once constructed, this chamber will bring flows from the high- and low-level inlet works together, and will distribute the combined flows to the three nitrifying ASPs.

It was important to get the flows from the two sources to mix fully in order to achieve homogeneity of the loads across all ASPs.

To this end, the model incorporated two separate flow sources, one saline water and the other fresh water, representing flows from high- and low-level inlet works. This allowed the mixing efficiency of the structure to be modelled.

In addition, the anoxic zones of the new ASP lanes were modelled using computational fluid dynamics (CFD). This was to achieve a baffle and mixer layout that minimises short-circuiting. The design of the aeration systems for the new ASPs has seen the introduction of new aeration technology in the form of high-efficiency fine-bubble strip diffusers.

A common header air distribution system has enabled the size and number of blowers for each of the systems to be optimised, resulting in significant savings in project costs. Because of accurate flow-splitting requirements, most of the flow splitting and distribution is achieved by use of weirs.

While resolving the issue of accurate flow splitting, weirs could cause other problems downstream. Two such problems are the break-up of floc caused by large drops and subsequent aeration, especially upstream of anoxic processes. In order to avoid these problems, every critical pipe intake has been examined and designed with these issues in mind.

Delivery
The Laing O'Rourke - Halcrow team was appointed as design and construct contractors in August 2006. The Knostrop Delivery Team (KDT) comprises mainly of Laing O'Rourke and Halcrow as well as the client Yorkshire Water and their framework consultants Arup, Turner & Townsend and Castleton.

An office has been established on site with space for contractors, sub-contractors, designers, the client and NEC project management staff, all in open-plan accommodation.

From the designer's perspective, working so closely with the construction team improved buildability and enhanced safer working practices in the design solutions.
For instance, it has been possible to optimise the design and construction programme for the new 13,000m2 footprint ASP structure, so that all the steel for the 7m- to 9m-high walls could be pre-fabricated and erected using two mobile tower cranes, reducing the need to work at height.
Bulk excavation100,000m3
New ASPEight lanes each 65m by 25m by 6m operating depth
Final settlement tanks11nr 40m diameter
Detritors2nr 14m diameter
SAS tanks2nr 26m diameter by 7m high
Outfall1.2km 2m diameter

The need to construct a number of major structures on a fairly constricted site, to tight deadlines, presents a significant challenge for the project team. Compliance with the current discharge consent cannot be compromised, even when major elements of the existing processes are taken out of service for modifications.

To achieve this, a phased approach has been adopted to ensure that there will be sufficient biological treatment capacity available at all times. Construction has been separated into two phases.

Phase one requires the new activated sludge plant and three new final settlement tanks to be constructed and commissioned by the end of 2007. The following phase is to be completed by July 2009.

A review of the commissioning plan prepared by the process team, demonstrates the complexity involved, as it breaks down the two phases into some further 22
sub-phases.

In addition to these, the high-voltage network is being modified and extended together with the installation of a new 5km fibre optic ring.

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