Deptford WTW: Run-to-Waste & Process Control (2023)
Thames Water’s Deptford Water Treatment Works (WTW) is one of five supplying potable water to Oxleas Wood Reservoir, in South East London. From 1701, until it became too polluted, water was abstracted from the River Ravensbourne and distributed across Deptford, Lee, Lewisham, Greenwich, and Rotherhithe. From 1861, groundwater was abstracted via wells located in the still-operational James Engine House, and the works currently produces approximately 22 million litres of water per day (MLD). The site is typically unmanned and is operated remotely from Thames Water’s Operations and Maintenance Centre (OMC) at Kemble Court, Reading.
MWH Treatment are carrying out site-wide works to upgrade and improve plant across the entire water treatment process.
The effectiveness of the current Calgon based system to reduce scaling caused by the sodium hypochlorite dosing plant has declined and become less reliable over recent years. A new water softening system, including a bulk storage salt saturator, is to be installed to remove scaling risk.
There is no current operational RTW facility at Deptford. RTW from contact tanks is currently directed toward an existing sewer connection, which is insufficiently sized to accommodate tank volumes. RTW from the existing transfer pumps is also inadequate and causes flooding. An existing redundant DN450 pipeline has been hydraulically reviewed and inspected by CCTV for integrity and suitability and is being re-purposed to allow RTW to the River Ravensbourne.
The existing chemical dosing skids have passed their design life and are a resilience and potential health and safety risk. The bunds have suffered corrosion and the spray shields require improvement. The skids do not have dedicated controllers and generally do not comply with current Thames Water asset standards. These skids are being completely replaced with asset standard compliant equipment, containing dedicated dosing control programmable logic controllers (PLC) and local human machine interfaces (HMI).
Disinfection process control
The existing dosing control system is monitored by a single process controller and there is no controller on individual dosing skids. Loss of the single controller could result in loss of production across the entire process stream. This controller is to be replaced by dedicated controllers for each dosing skid.
There is a lack of real time visibility of process operation, due to the slow communication link between Deptford and the OMC in Reading, with a lag of up to two minutes recorded. The signal time lag between the start signal of the borehole pumps and the start of the transfer pumps carries the risk of over pressurising the contact tank and tripping the pumps. Re-start currently occurs under local control.
There is also a lack of a process HMI on site, meaning there is no local visibility of how the overall treatment process is performing. During local restart, the OMC has visibility of site but no control until the site is returned to remote mode.
To avoid the risk of contact tank pressurisation, the works can only be started with the smaller Well Pump No. 1, switching to one of the larger well pumps only when transfer pumps have been started. Failure of Well Pump No. 1 results in a sub-optimal restart sequence with the potential to over pressurise the contact tank.
Well Pump No. 1 is to be removed, and Well Pumps No. 2 and 3 are to be replaced. Variable speed drives (VSD) for these pumps are being included in the solution to allow a safe ramp up of pressure. The existing control system is to be replaced to enable real time full remote control of the site via the OMC. Some of the existing BBL control units will be replaced with a new Rockwell Automation CompactLogix PLC and HMI.
A Bluesky 3D Building Model is acquired. This is an accurate interactive 3D representation of the site and is used as the basis for the VPI.
The MWH Treatment project team utilise the Bluesky model as a basis to develop a storyboard which is a visual representation of the project scope of works. Areas of site are highlighted by taking sections of the Bluesky model and overlaying text and imagery to demonstrate works to be completed.
From the storyboard, the Bluesky model and, if available, a 3D model created using BIM Glue, the MWH Treatment visualisation team can create an accurate depiction of the project scope. This can be used across the project lifecycle to:
- Clearly demonstrate the scope of works and the proposed solution.
- Highlight any potential issues with the proposed solution including access risks, health and safety risks and site-specific issues that may arise during construction and commissioning.
- Provide a digital construction and commissioning plan prior to commencement of works.
- Demonstrate the optimal sequence of works.
A scope of works and sequencing VPI were developed for Deptford WTW. The visual representations were useful in conveying proposed solutions to Thames Water, and in identifying opportunities to reduce cost and outage durations and shorten the project programme.
Works requiring outages at Deptford include the following:
- Post-contact tank run-to-waste tie-in (1 day).
- Post-well run-to-waste tie-in (1 day).
- Modify the pumphouse MCC for process distribution board feeder (1 day).
- Install new PLC hardware communications for BBL units (2 weeks).
- Well Pumps No. 2 and 3 replacements (2 weeks).
- Replacement of sodium bisulphite and sodium hypochlorite dosing systems (3 weeks).
Completing these works in stages would require a total outage duration of nearly 8 weeks.
An outage management workshop was completed to identify the optimal outage sequence. The initial proposed changeover sequence involved upgrading the control system in stages.
This would allow for multiple shorter outages, instead of one lengthy offline duration. However, it was expected that this method of changeover would result in complications for operation and commissioning of site by having BBL control units DEP 1-6 I/O integrated to the new system, and DEP 7 I/O to the existing system.
By diverting site I/O from DEP1-7 to the new ControlLogix PLC within one outage, the complications involved in construction and commissioning are reduced. However, the outage duration will be longer (4 weeks) and will put more of a strain on the reservoirs supplying the surrounding area.
Deptford is one of five water treatment sites supplying Oxleas Wood Zone Reservoir. Prior to commencing outages, it must be confirmed that the other four (Darenth, Wilmington, Bexley and Dartford WTWs) are available and running. It must also be confirmed that a minimum of three large pumps at Honor Oak Pumping Station are available to support the reservoir from the loss of output from Deptford. The Eltham, Northumberland Heath and Castlewood Reservoirs must also be at reasonable levels to provide additional support for the area.
Combining outages required to complete works on the BBL units, well pumps and chemical dosing systems would reduce the total outage duration from 7 weeks to 4 weeks. The VPI was used as a digital run-through of the changeover sequence for each outage required on site, acting as a rehearsal for the site and commissioning teams prior to construction and installation.
- Principal designer & contractor: MWH Treatment
- GPR surveys: Select Surveys
- Asbestos survey: RSK Environmental
- Civil works: Derryard Construction Ltd
- Pump supply & installation: G Stow Plc
- Chemical dosing package: WES Ltd
- Mechanical installation: Alpha Plus Ltd
- Electrical installation: Bridges Ltd
- MCC & systems integration: Total Automation & Power
- Disinfection: Veriflo Ltd
- Ground investigation: Structural Soils Ltd
- 3D scanning & model: Engenda Group
- Harmonic survey: Power Quality Management Ltd
- Documentation: Cognica
- Pipe pigging: JMC Northern Ltd
- CCTV: MTS Cleansing Services Ltd
- Static mixers: Statiflo International Ltd
- Pressure transmitter: ABB Ltd
- Pressure switch: IFM Electronic
- VSDs: Total Automation & Power
- Chemical lances: Prochem Services
- Valves: AVK UK Ltd
- Actuators: Auma Actuators
- Float switch: Xylem Water Solutions
- RTU: Rockwell Automation
- ICA long lead: Routeco Ltd
- Bluesky 3D Model: Bluesky International Ltd
- Ventilation: Air Technology Systems
By completing an outage management workshop an optimal changeover sequence was identified. By combining process control, chemical dosing and well pump works the total outage duration was reduced by three weeks, resulting in an earlier delivery of project benefits.
Combining these outages also allows for the outages to be completed prior to the peak demand period.
Optimising the changeover sequence also reduced complications involved in construction and commissioning of upgraded process control and removed requirement for temporary measures for operation and off-site visibility.
The initial project solution specified the extension of the existing chemical dosing and pump building to provide space to house sodium hypochlorite and sodium bisulphite dosing skids in separate rooms. Utilising the Visual Project Initiation, it was possible to provide an alternative layout whereby both dosing skids were located together in the existing chemical dosing room.
The layout modelled the existing room and dosing skids at Deptford WTW, whereby any space constraints were immediately clear. Following this exercise, the design was deemed a viable option, eliminating the requirement for a building extension, and thus reducing project costs, environmental impact and project programme.
Visual Project Initiation (VPI)
The VPI has been proven to be an important tool in conveying ideas between the project delivery and Thames Water Operations teams. During early stages of the project, the VPI was utilised to develop and confirm the scope of works, and thus finalise the project brief. It has been relied on during internal design workshops to optimise equipment design and layout, pipe configuration, and highlight design risks.
The VPI was also used as a guide in completing the Operation and Maintenance (OPMAN) review, acting as a digital twin of the site to identify areas of improvement for access and health and safety measures.
Within the context of changeover optimisation, the VPI was integral in communicating the potential outages to Thames Water, comparing options, and ultimately providing the basis for outage planning within the wider Thames Water business.
The major benefits from using visualisation technology have allowed MWH Treatment and Thames Water to review the entire site and treatment process. It has been possible to identify different design, construction, and commissioning options very quickly and efficiently.
The model emphasises risks associated with the design and allows design changes, where necessary, to be implemented very efficiently. The use of this digital media has allowed for better explanation and communication with key stakeholders and has offered a strong basis for collaboration between design, construction, commissioning, and operations teams.
The benefits of this technology can be utilised in future projects of this type, and are being deployed across MWH Treatment, providing a cost saving, reducing project programmes, highlighting health and safety risks, and reducing environmental impacts of projects.