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
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.
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
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
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
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.
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