Over the last three years Southern Water has installed over 80 automatic turbidity testers at sites around Kent, Surrey and Sussex. At the heart of each of these is a Crouzet Micro RPX 10 PLC.
Success in developing a quality monitoring system for sewage treatment works' outfall has helped Crouzet win subsequent orders for complete plant control systems.
Southern Water is addressing the need for higher quality water supply and waste water treatment by automating many of its plants and processes, and is tapping into the control systems expertise of Crouzet for assistance with many projects.
Over the last three years Southern Water has installed over 80 automatic turbidity testers at sites around Kent, Surrey and Sussex.
At the heart of each of these is a Crouzet Micro RPX 10 programmable logic controller (PLC), that is used to monitor alarm conditions and control regular self-cleaning routines The principle is to monitor the cleanliness of water discharged from the treatment process into streams and rivers by counting the number of particulates in suspension.
An infrared sensor is permanently mounted in the outfall, well below the waterline.
The RPX is programmed to ensure the infrared head is kept clean, so that it is able to give true readings of the water's turbidity, run on a time base from the RPX's internal clock.
The PLC activates a wiper to clean the sensing surface of the underwater components, and at regular intervals a small local compressor is fired up to blow a high pressure bubble of air over the sensing head.
This action, together with sensors built into the wiper, is designed to ensure that nothing, large or small, compromises the reading. The RPX is Crouzet's flagship micro PLC.
It is available in various sizes and a number of different versions including a rugged unit for harsh environments.
Programme capacity is up to 2800 steps, depending on the PLC chosen, and programming is accomplished in ladder logic or Graphcet using Crouzet's C3 Windows PC package or a handheld programmer.
Its real time clock option is complemented by 64 dedicated timers, 32 counters and 16 cascadable shift registers.
Battery backup will support the memory for 8000hrs, and data is even retained for four hours without a battery With the success of the turbidity tester, it was natural that Crouzet's input be sought by Southern Water on other programmes.
A project to upgrade quality performance on small, remote rural sewage stations was enhanced by simultaneously reducing running costs.
The flow through a plant is not consistent throughout the day; there are 'rush hours' before and after work, and the night-time hours are naturally very quiet.
The idea was to use balancing tanks at the inlet to even out the flow into the treatment plant over the 24 hour period, so that the bacteria breaking down the sewage were never overworked or under-utilised.
Crouzet this time suggested use of it RPX 20 PLC, using the clock in real time mode to operate divert valves to and from the tanks.
For added security, the RPX 20's analogue input is used to measure inflow to the plant.
It operates valves to divert to the balancing tanks when the flow is too great, and pumps out of the tanks when the plant is being fed at below optimal rates.
As with all sewage treatment works, it was necessary to account for possible emergencies, so Crouzet wrote software to raise alarms and allow overrides of the base programme.
Looking further, it was realised that each plant has a holding tank for 'untreatables'--heavy metals and other such highly undesirable substances.
The contents of these are tankered away by road once a fortnight, usually very early in the morning before the plant gets busy.
This however meant an engineer being on site in time to open the valves and start up the pumps.
Crouzet wrote a real time programme for the RPX 20 that implemented the start-up procedure at 6am on a certain day every other week, just in time for the arrival of the tanker.
The first of these control systems was trialed at Handcross Sewage Treatment Works near Gatwick Airport during the spring of 1999. Once it was proved out Southern Water quickly made the decision to adopt the Crouzet arrangement as a template for other plants. To date two other plants are planned for upgrade, with more to follow.
