Water Companies and the Environmental Agency (EA) are by law responsible for the quantity and quality of effluent discharged into the seas or rivers (watercourses). As a consequence, they must control the amount and type of effluent entering the treatment work to a finite capacity. Should this capacity be exceeded pollution would be transferred into the watercourses. Industry, with its diversity of trade effluent, is the most difficult to control and where high volumes of polluting effluent are generated, companies may be required to monitor and control their discharge.

There are many other benefits depending on the nature of your effluent. These include the ability to monitor the amount of water used in your process by subtracting the outgoing flow figure from your incoming figure, better management of your waste, the reduction of losses in your manufacturing process, and trade effluent monitoring is a requisite for ISO14001 accreditation.

Open-channel Flow Meters: The most common type of open-channel flow meters are the ultrasonic airborne type or the Doppler area velocity type.

The ultrasonic type uses an ultrasonic sensor located above the channel to measure the level in the hydraulic structure. This is converted to a flow figure through the use of a set of mathematical formulae known as the British Standard 3680 equations.

Doppler area velocity type systems use a combination sensor submerged in the channel, which will measure the height of the liquid in the channel and the velocity of the liquid flow. Since the

dimensions of the channel are known the flow rate can be determined by computing the area/velocity.

Suspended Solids and Turbidity Monitoring: Solids in suspension will eventually form sludge, either in the watercourse or at the treatment works. As a result, suspended solids are measured and charged as part of your bill. Suspended solid meters usually use a sensor immersed in the effluent that shines modulated light through the effluent to measure the number of solid particles present. Where the numbers of solids (measured in mg/litre) are low (usually less than 300mg/litre), turbidity meters are used to measure the load. Turbidity meters are normally only used where the effluent discharge volume is very large, and although the solids per unit volume are low, the cumulative effect will still be high.

All factories that discharge trade effluent must obtain consent from the Water Companies or the EA to do so. This consent lays down what can and cannot be discharged. Usually, it will define the volume and acidity of the effluent that can be discharged. Should the consent figures be exceeded companies can face heavy fines and in the extreme, closure.

Because you could reduce your trade effluent bill! All effluent discharge attracts charges and the more pollutant your effluent, the higher your bills. Where no equipment is present, water companies take spot samples to calculate bills. Since effluent quality and quantity can vary throughout the working day a spot check does not normally represent the true average discharge. The result is that you may be paying more than the actual figure you should! So, it is in your interest to monitor and treat your effluent where possible. By doing so, companies may benefit from reducing their bills through better management of their effluent discharge.

Closed pipe flow meters: A closed pipe flow meter measures the flow whilst the liquid is contained within a pipe. The two most common types are the Induction type instrument often called the ‘Mag’ flow instrument or the ultrasonic clamp-on instrument. Of these, the Mag Flow is by far the most common and is the most accurate. The Mag flow meter consists of a pipe-shaped sensor that is coupled to the pipe and uses magnetic field changes within the flowing liquid to calculate the flow.

The clamp-on type uses two or four ultrasonic sensors clamped onto the outside of the pipe to measure the flow rate by either the Doppler or a time of flight principle.

Chemical and Biological Oxygen demands to form part of the consent to discharge and can lead to the highest charges on your bill.

On-line automatic monitors are available but it is more normal to take samples and analyse these with a laboratory testing kit. This will help you to validate your charges. As with all analyses the more you know about your effluent the more you can do in your manufacturing process to reduce its pollutant nature and consequently reduce your trade effluent bills.

No! Only those that discharge trade effluent of a polluting nature. Where it can be demonstrated that the effluent is of a clean quality and presents no pollution, online monitoring of the effluent is not required.

Like any other equipment operating in a hostile environment, equipment of this type should be maintained and calibrated by qualified companies (ISO9001 registered) on a regular basis to make certain it is measuring accurately.

Wastewater Samplers: Samplers are used to extract samples of the trade effluent and store them in a bottle for collection and analysis by the regulatory authorities for COD, BOD, pH, solids etc. Samples are usually taken on a flow proportional basis over 24 hours so that they accurately represent the flow regime present. This is known as a composite sample as it gives a 24-hour composition of the liquid that has been discharged. Samplers can consist of 1, 2 and up to 24 bottles. Two bottles are most common, as there is always a bottle available for filling when the 24-hour period is complete. In some instances where the effluent is biodegradable, refrigerated samplers are required. In certain circumstances, the sampler will work on a timed interval sampling programme rather than a flow proportional programme.

Treatment Plant: Where the effluent is particularly polluting and where effluent charges are extremely high it is often cost-effective to put in place a dedicated treatment plant. Usually, a treatment plant retains the effluent for a period of time whilst it is treated by chemicals to reduce COD, BOD, solids etc before being discharged as a clean high-quality effluent.

Charges are made up of a number of factors depending on the nature of the effluent. These include flow volume, solid content and COD and/or BOD. Of these COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand) attract the highest charges as effluents of this nature strip oxygen from the water which eventually ends up, via the treatment works, in the watercourse.

Open Channel Flow: The most commonly used technique of measuring the rate of flow is to use a hydraulic structure (restriction) inserted into an open channel which acts to restrict the flow of liquid in the channel. The shape and dimensions of the hydraulic structure will restrict the flow in a known manner such that the level of liquid behind the structure will rise and fall according to the flow rate.

Hence the measurement of level in the structure can be used to derive the flow. These hydraulic structures are known as primary devices.

pH Meters, pH Dosing and pH Mixing: A large number of trade effluents are either acid (pH less than 7) or alkaline (pH greater than 7). Most water company consents to restrict the acceptable pH figure to be between 6 and 10. Usually, a pH meter is installed in the final effluent to measure this pH figure. Where it is likely that the pH figure will go out of consent a pH dosing system is used to pump either caustic or acid (usually hydrochloric or sulphuric) into the effluent to bring it within the consent limits. Quite often if no

natural turbulence of the effluent is present it is necessary to employ a mixing motor and propeller to ensure that the additive mixes efficiently with the effluent.