Process optimisation by Real-Time Control at Taunton STW


Wessex Water is investing around £20m at Taunton (Ham) sewage treatment works to improve the facilities for wastewater and sludge treatment in a project that is due for completion by the end of March 2015. The upgrade to the works under the DWF Improvements Scheme will increase the site’s treatment capacity whilst also improving the efficiency and quality of the treatment process, lowering energy costs and reducing the site’s carbon footprint.

Prior to the implementation of the DWF Scheme, the STW was comprised of an inlet pumping station and balance tank, coarse and fine screens, grit removal (detritor), primary settlement tanks, a conventional ASP & biological filter beds, final & humus tanks and final effluent lagoons. The construction work involves the creation of a new four-lane ASP to replace the existing 16 biological filters. To facilitate this, one of the lagoons and four of the filters are being taken out of service to create space for the new works, and this has allowed all development to remain within the existing site boundaries enabling most works to be constructed under permitted development rights.

Process optimisation of the new ASP stage will be achieved through implementation of HACH LANGE’s Real-Time Control (RTC) system, which monitors influent ammonium concentration and dissolved oxygen concentrations along the aeration lanes, providing more efficient control of the fine bubble diffused aeration. The measurement of other quality parameters in the process train provides feedback to the RTC. A reduction of up to 15% energy usage is anticipated as a result.

Balfour Beatty Regional Civil Engineering Ltd has provided the civil works and Nomenca Ltd is responsible for the supply, installation, commissioning, and performance testing of the mechanical and electrical components of the new works. Contracts Manager Trevor Farrow says, “Nomenca’s reputation is built on a track record of successfully delivered projects, and the relationships that we develop with both clients and suppliers are key to this success. We have already worked with HACH LANGE’s instrumentation on a wide variety of projects, so we are confident that this project will be a further success.”

As Project Manager for Wessex Water, Garry Orford says: “The drivers for this works upgrade include an increased treatment capacity requirement and a tightening of the consent, taking in to account longer-term requirements that may be implemented in AMP6. We have already implemented HACH LANGE’s RTC process optimisation systems at our Holdenhurst plant - 175,000 PE - near Bournemouth, and this has delivered energy savings of around 25% so we are confident that we can repeat this success at Taunton - 85,000 PE.”

Following completion of the new works, the site will meet the following consent conditions:

  • Dry Weather Flow (DWF) 30,595 m3/d
  • Sanitary parameters BOD:SS:AmmN 15:30:3 mg/l

In addition to the upgrade of the sewage treatment facilities, a third anaerobic digester (AD) is also being built at the Taunton works. “This will increase our capacity to generate renewable energy and further reduce our electricity bill,” says Garry Orford. “The power generation of the AD plants is fairly stable, but the energy demand of the treatment plant varies according to the load, so there will be occasions where we can sell energy back to the grid, and others where we will continue to have a power requirement. It is essential therefore that we use this power as efficiently as possible.”

Real-Time Control in industrial processes is commonplace. However, wastewater monitoring represents a greater challenge because of its physical and chemical variability. Historically, wastewater monitoring technology was prone to drift (especially galvanic dissolved oxygen monitors) and required a high level of maintenance, so RTC was not feasible. However, the latest sensors offer much higher levels of reliability than was possible in the past, with substantially lower levels of maintenance and recalibration. This has been a major factor in enabling the development of RTC in wastewater treatment. In addition, many of the latest sensors provide a ‘health status’ output in addition to the readings. As a result, if any problems arise they can be quickly remedied, and control systems can ignore data from sensors that are not performing to their target specification.

Monitoring technology
The capital outlay for the addition of RTC to a treatment plant is relatively small; the most significant extra cost is the requirement for extra sensors plus the RTC unit. The Taunton build includes the installation of the latest sensors for dissolved oxygen, ammonium and turbidity, controlled by an sc1000 network, providing reliable data on the influent, and from within the treatment process.

The LDO sc dissolved oxygen sensor employs an optical luminescence method for calibration-free and drift-free measurements. Once the construction work is complete there will be four new lanes, each with three zones, so a total of 12 LDO probes will monitor dissolved oxygen. In addition, two SOLITAX ts line dip probes will measure Mixed Liquor Suspended Solids (MLSS) content in the aeration lanes and the solids content of the Returned Activated Sludge. The RTC at Taunton will also control sludge retention time, which enhances plant efficiency. The suspended solids probes employ a patented dual scattered light method with a built-in wiper, to provide colour-independent measurement of solids without a requirement for calibration. Ammonium measurements will be undertaken at both the entrance and exit of the aeration lanes with two AMTAX sc instruments; high-precision analysers that continuously collect samples via an air-bubble cleaned filter probe. The ammonium analysers will be mounted directly over the filters to minimise the distance travelled by samples.

Real-Time Control
The HACH LANGE RTC is implemented on an industrial PC which communicates with an sc controller network and the local PLC. The RTC system determines the most efficient aeration level and continuously feeds DO set points to the PLC which controls the blowers. This means that under RTC, DO set points are no longer 'fixed', instead they 'float' according to the load. The RTC modules continuously deliver set points to the PLC, which applies them to the process. This ensures that response to changing conditions is immediate. The algorithms employed by the N-RTC (Nitrification Real Time Controller) are mainly based on the Activated Sludge Models of the International Water Association.

The N-RTC also constantly reads the NH4-N concentration at the outlet of the aeration lane. This value provides a feedback control loop and ensures that the DO concentration is fine tuned to achieve the desired ammonium set point at the end of the ASP. In this way, the N-RTC control module combines the advantages of feed forward and feedback control, which are (1) rapid response, (2) set point accuracy and (3) robust compliance.

Aeration to achieve the biological oxidation of ammoniacal compounds to nitrate is the most energy intensive process at activated sludge plants because blower power consumption can represent over 50% of total costs at some plants. However, in addition to the advantages of the process optimisation system, four new Sulzer high speed HST-20 turbo-compressors are being installed by Nomenca, following trials on similar units by Wessex Water. These machines employ a control system that manages both the number of blowers to run, and the speed of the blowers, which will further improve energy efficiency.

Summarising, Garry Orford says: “Wessex Water has an ambitious long-term objective of carbon neutrality, and these improvement works projects provide us with useful opportunities to make a significant contribution to that goal.”

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