Waste water going to sewer has to be measured to the satisfaction of the sewerage undertaker. This can be as simple as a ’v-notch’ weir where the height of water flowing through the notch is proportional to the flow or more complex systems involving electronic or ultrasonic technology. Measurement is essential because most consents contain limits on peak flow rates as well as average flows over longer periods.
Many companies find that one of the first steps towards minimising water discharge is to install accurate localised metering on the water they use so as to understand exactly where in their sites water is being used the most. This helps them target their waste minimisation efforts.
Sampling sites
Sampling sites for both sewer and surface water discharge should be selected to provide representative samples. For example, if effluent temperature is critical, then sampling will have to be close to the point at which the effluent meets the receiving water. Flows containing suspended solids may lose some of this material as the flow slows down, and this must be borne in mind. It is, therefore, desirable to site sampling points as close to the point of discharge as possible.
Especially in the case of discharges to surface water, it is advisable for the discharger to sample upstream as well as downstream of their discharge point. Any pollutant which has not originated in their discharge can then be identified – to the benefit of the discharger and the investigating authorities.
Sampling points need to be sited to give safe access for samplers.
Sampling strategies
Spot samples provide a snapshot in time. The timing of such samples must give representative readings as far as possible.
Mechanical devices such as carousel samplers can be set so that samples are taken at regular intervals. These can be analysed individually to give a full picture of the discharge or pooled to give an average sample over the whole period covered by the carousel.
Chemical and electrochemical sampling techniques can be used to give real-time analysis of critical parameters such as pH. Thermometer readings can also be used in this way.