Optimisation system slashes energy costs

Real time control has cut energy use by 25% at a Wessex Water wastewater treatment plant in Bournemouth.
Michael Häck, manager process optimisation, Hach Lange, explains the system

In May 2010, a new real-time control (RTC) system was installed at a wastewater treatment plant in Bournemouth that has reduced power demand by a remarkable 25%. Wessex Water's Matt Crowhurst is delighted with the results and says: "This represents a substantial reduction in carbon footprint and saves around one sixth of the plant's operational costs."

The new system, known as Wastewater Treatment Optimisation Solutions (WTOS) was developed by Hach Lange and incorporates a selection of RTC modules that continuously monitor the treatment process and provide set points to the local PLC. The Bournemouth plant employs three real time control (RTC) modules for the optimisation of nitrification and a further three to manage sludge retention time (SRT).

In common with other water utilities, Wessex Water constantly seeks new opportunities to improve wastewater treatment plant efficiency and environmental performance. In order to achieve this, it is necessary to maintain the effectiveness of the treatment process, producing a consistent discharge within consent limits, whilst minimising inputs such as energy, labour and raw materials.

This focus on efficiency is in keeping with Wessex Water's long-term aim to be carbon neutral in its operations. To do this the company is working on ways of avoiding energy use and emissions, through leakage control and catchment management, for example. It is also taking opportunities to improve energy efficiency and investing in technology to increase renewable energy generation.

A WTOS system was installed at Wessex Water's Holdenhurst facility in Bournemouth (175,000 PE), which mainly treats domestic wastewater. Aeration for Holdenhurst's fine bubble activated sludge treatment system is provided by four large mains powered variable speed blowers. The site has a good record for maintaining a low ammonia discharge, but has a heavy power/aeration requirement, particularly during storm events.

Prior to the installation of the WTOS, luminescent dissolved oxygen (LDO) probes in the aeration lanes fed dissolved oxygen data to the PLC, which controlled the blowers to maintain dissolved oxygen (DO) at specific levels (approximately 2.5mg/l) depending on the treatment zone. Similarly, under the previous sludge management regime, fixed volumes of sludge were returned based on laboratory mixed liquor suspended solids (MLSS) values and manual settlement tests.

Automated control
There are three main components to WTOS:
  • The RTC modules
  • The process analysers
  • The PROGNOSYS system
Automated control systems necessitate reliable continuous measurement values 24 hours a day, so the Prognosis system has been developed to constantly check the diagnostic signals (health and service status) from Hach Lange instruments in order to achieve the required levels of reliability.

The capital outlay for the addition of WTOS to a treatment plant is relatively small; the most significant extra cost is simply a requirement for extra sensors.

WTOS overlays and compliments existing infrastructure, so it is possible (although very unlikely) to simply turn WTOS off and revert to the former regime. Each RTC is implemented on an industrial PC which communicates with the sc1000 controller network and the local PLC. The WTOS RTC unit delivers set points for the DO concentration and waste activated sludge (WAS) flow rate to the PLC, which applies those set points to the process.

Site specific characteristics such as layout and tank size are also taken in consideration when calculating the set points. All set points can be adjusted either via the SCADA system or the local WTOS controller user Interface.

Nitrification control
The WTOS Nitrification RTC (N-RTC) acts as an experienced process operator, 24 hours per day, seven days a week. At Bournemouth, the ammonia load is measured by Hach Lange's NH4-N probes (ion selective method), which are installed at the beginning and the end of each set of aeration lanes. LDO probes are located in each of the zones within every lane.

WTOS determines the most efficient aeration level and continuously feeds DO set points to the PLC which controls the blowers. This means, when the plant is under RTC control, DO set points are no longer 'fixed', instead they 'float' according to the load.

To enable this, the N-RTC receives information about the actual:
  • NH4-N inflow concentration and flow
  • MLSS concentration
  • Water temperature
A simulation model is integrated within the controller for open loop control to calculate the DO concentrations necessary to achieve the desired ammonia outlet concentration. The algorithms for this 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 feed back control loop and ensures that the DO concentration is increased/ decreased if the ammonia concentration is above/below the desired NH4-N set point. In this way, the N-RTC control module combines the advantages of feed forward and feedback control, which are (1) rapid response and (2) set point accuracy.

Sludge retention time control
Sludge is managed automatically by the sludge retention time module (SRT-RTC), which calculates the volume of WAS in m3/d required to adjust the sludge retention time to a defined value.

Solitax probes were installed to monitor the MLSS and the return activated sludge (RAS). The SRT-RTC calculates the solids retention time and adjusts the WAS flow rate via the PLC which in turn controls the pumps and valves.

Powering down
Matt Crowhurst has measured a 120KW reduction in power demand since the WTOS was installed. This saves approximately 25% of the plant's energy costs which is equivalent to one-sixth of the plant's operational costs.

Locally, staff have noticed that the blowers are working less, especially during low load periods - often only one blower is in operation; they have seen lower final effluent ammonia during storm events and they have been able to see the DO set points changing as influent ammonia levels rise or fall.

Data from the plant influent flow monitor are also fed into the WTOS, which helps the plant to respond faster to heavy rainfall events. The SRT controller helps to reduce labour requirements and produce a more steady process with more stable MLSS.

Continual savings
One of Wessex Water's strategic objectives is to reduce overall energy consumption and aeration is clearly a very significant contributor. Commenting on experience at Holdenhurst, Matt Crowhurst says: "The system is now able to respond to changes in loading very quickly. Following installation, Hach Lange helped us to further optimise the process and it now runs automatically delivering continual savings.

"WTOS obviously presents a major opportunity for a large number of treatment plants; at Holdenhurst we estimate that the payback for the system is about two years, so this would be shorter for larger plants and longer for smaller facilities."

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