Product category:
Liquid Waste and Pollution
News Release from: Partech Instruments | Subject: Oil on water detection
Edited by the Processingtalk Editorial
Team on 07 June 2007
Detecting oil film pollution on water
discharges
Highly visible pollution incidents that are caused by floating oil on water can have a hugely damaging effect on both the quality of the watercourse and the environmental reputation of the polluter!
Should the oil then enter a drinking water plant damage can be caused to the processing system or more critically the quality of the water becomes impaired In the power generation business where large quantities of water are used, oil in water can affect the water-cooling system and also be present in used water discharged back into the environment
This article was originally published on Processingtalk on 24 Jun 2004 at 8.00am (UK)
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At Little Barford Power Station in Cambridgeshire, every effort is made to minimise the impact on the environment both in the way that it generates power and its waste emissions.
Operated by npower, part of RWE npower, it is one of a new generation of gas-burning power stations and generates 680MW from two gas turbines, two heat recovery steam generators and one steam turbine linked to another generator.
Cooling water plays a central part in the generating process and to this end water is extracted from the adjacent Great Ouse river.
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A certain amount of the water used in the cooling process is also returned to the river, which means that the Power Station has to ensure that this complies with Environment Agency discharge limits.
Oil on water is one aspect of the discharge water treatment process that demands close attention as water is extracted from the river for drinking purposes downstream of the plant.
Where oil on water is concerned, the EA discharge limit imposed on the Power Station is 10mg/l.
To ensure that this limit is adhered to, the Power Station uses a Partech WaterWatch2920 monitor, which is suspended from a gantry above the water discharge conduit.
In addition to continuous monitoring, the Power Station chemists collect samples on a monthly basis which are analysed by an external company and sent to the EA each quarter.
Cooling water is passed into the condenser where the water temperature is raised, then it goes back into the cooling towers and finally back into the condenser again.
In the cooling towers some of the water in the system is evaporated, so the water extracted from the River Great Ouse is used to top up the cooling system for the steam turbines.
The evaporation process results in a build up of salts and other chemicals in the remaining cooling water, which can lead to scaling of the condenser.
As a result a certain amount of this water has to be purged, so the 900m3 per hour of replacement water brought in from the river makes up the balance.
It is the purged water that is returned to the river.
It is inevitable that plant of this size will produce a quantity of drainage which can include oil, so there is the possibility that this can end up in the cooling water system.
To eliminate contamination of the replacement cooling water, this drainage liquid is first passed through oil interceptors into the waste water sump then on into the cooling system from where it is purged back to the river.
Should the interceptors fail, water containing oil could get through to the cooling water system and eventually to the river.
The EA permit requires the Power Station to have a continuous monitoring installation on the purge and to this end the chemists are monitoring pH, temperature, flow rate, chlorine and as well as oil on water.
Because the Power Station occupies two sites, before the purged water leaves the main side of the plant it is monitored for oil in water.
Should there be an increase in the quantity of oil in water the facility does exist for the link between the two sites to be closed.
However, readings taken by the WaterWatch2920 are relayed to the Power Station main control room and if there is an indication that discharge limits are likely to be exceeded, the purge can be shut down.
The WaterWatch2920 uses a floating infrared sensor to detect visible oil on water and the monitor will react rapidly to both partial and complete oil films.
The measuring system is extremely stable and only needs occasional validation to ensure reliable operation.
In addition to the alarm output, the monitor provides an analogue signal proportional to the probability of oil being present.
The floating sensor has a dual hull design, to channel any passing oil through the centre of the float so that it is presented to the detector.
The infrared sensor emits light that is reflected off the surface.
The presence of oil on the surface increases the amount of light reflected and this is converted into the desired output signals.
The float comes with two fixing cables and a weight to keep it stationary and correctly orientated in the water flow.
A problem commonly associated with detecting oil on water is light reflection and surface ripples.
At Little Barford, there is also the small matter of fish in the culvert where the water is monitored as the movement of fish can throw up false alarms.
However, Partech has addressed these issues and the WaterWatch monitor operates to the satisfaction of the Power Station chemists without throwing up too many undue false alarms and without compromising its sensitivity.
Maintenance is confined to lifting the monitor from the water once a week for cleaning.
When the Power Station applied to the EA for its permits, it was necessary to declare what monitoring instrumentation was in place to stop pollution from oil on water and other potentially harmful chemicals.
Although continuous monitoring of oil on water was not a specific EA requirement, a monthly spot sample being deemed as sufficient, Little Barford Power Station took it upon itself to install this technology.
The Partech WaterWatch2920 is seen as a vital safeguard to ensure that the Power Station meets its own very high levels of environmental protection.
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