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Turbidity tester sees the light


Micro RPX 10 programmable logic controller

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 sewagetreatment works' outfall has helped Crouzet win subsequentorders for complete plant control systems.

Southern Water isaddressing the need for higher quality water supply and wastewater treatment by automating many of its plants and processes,and is tapping into the control systems expertise of Crouzet forassistance with many projects.

Over the last three years SouthernWater has installed over 80 automatic turbidity testers at sitesaround Kent, Surrey and Sussex.

At the heart of each of these isa Crouzet Micro RPX 10 programmable logic controller (PLC), thatis used to monitor alarm conditions and control regularself-cleaning routines The principle is to monitor thecleanliness of water discharged from the treatment process intostreams and rivers by counting the number of particulates insuspension.

An infrared sensor is permanently mounted in theoutfall, well below the waterline.

The RPX is programmed toensure the infrared head is kept clean, so that it is able togive true readings of the water's turbidity, run on a timebase from the RPX's internal clock.

The PLC activates awiper to clean the sensing surface of the underwater components,and at regular intervals a small local compressor is fired up toblow a high pressure bubble of air over the sensing head.

Thisaction, together with sensors built into the wiper, is designedto ensure that nothing, large or small, compromises the reading.The RPX is Crouzet's flagship micro PLC.

It is available invarious sizes and a number of different versions including arugged unit for harsh environments.

Programme capacity is up to2800 steps, depending on the PLC chosen, and programming isaccomplished in ladder logic or Graphcet using Crouzet's C3Windows PC package or a handheld programmer.

Its real time clockoption is complemented by 64 dedicated timers, 32 counters and 16cascadable shift registers.

Battery backup will support thememory for 8000hrs, and data is even retained for four hourswithout a battery With the success of the turbidity tester, itwas natural that Crouzet's input be sought by Southern Wateron other programmes.

A project to upgrade quality performance onsmall, remote rural sewage stations was enhanced bysimultaneously reducing running costs.

The flow through a plantis not consistent throughout the day; there are 'rushhours' before and after work, and the night-time hours arenaturally very quiet.

The idea was to use balancing tanks at theinlet to even out the flow into the treatment plant over the 24hour period, so that the bacteria breaking down the sewage werenever overworked or under-utilised.

Crouzet this time suggesteduse of it RPX 20 PLC, using the clock in real time mode tooperate divert valves to and from the tanks.

For added security,the RPX 20's analogue input is used to measure inflow to theplant.

It operates valves to divert to the balancing tanks whenthe flow is too great, and pumps out of the tanks when the plantis being fed at below optimal rates.

As with all sewage treatmentworks, it was necessary to account for possible emergencies, soCrouzet wrote software to raise alarms and allow overrides of thebase programme.

Looking further, it was realised that each planthas a holding tank for 'untreatables'--heavy metals andother such highly undesirable substances.

The contents of theseare tankered away by road once a fortnight, usually very early inthe morning before the plant gets busy.

This however meant anengineer being on site in time to open the valves and start upthe pumps.

Crouzet wrote a real time programme for the RPX 20that implemented the start-up procedure at 6am on a certain dayevery other week, just in time for the arrival of the tanker.

Thefirst of these control systems was trialed at Handcross SewageTreatment Works near Gatwick Airport during the spring of 1999.Once it was proved out Southern Water quickly made the decisionto adopt the Crouzet arrangement as a template for other plants.To date two other plants are planned for upgrade, with more tofollow.

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Analysis & Quality Control