Sampling is straightforward. But where, why and how?
15 Jan 2000
Self-regulation, growing public concern over pollution, and ever increasing commercial pressures on effluent treatment, are making the subject of sampling more important than ever, writes David Claridge, water industry manager for Endress+Hauser.
Sampling not only proves that discharge from treatment facilities meets regulatory requirements, but can also identify process problems within a plant, determine effects of incoming pollutants, and test new technology. Sample results can also be used to calculate the discharge costs from individual sites, or indeed the cost of non-compliance.
As a result, extra thought is required in the choice of equipment, the type of sampling required, positioning of the sample point, and the preservation of that sample until it is transported and analysed.
The aim of the sampling programme dictates the choice of sampling method. There are four main types of sampling method:
(1) Event sampling: Samples are taken during an event such as produced by a pumped discharge or storm event. Sampling can be initiated by level or flow measurements.
(2) Time proportion sampling: This form of sampling takes discrete set size samples at set time intervals. Consequently, this method is independent of the flow quantity, and the sample can only be guaranteed if the flow rate is constant.
(3) Quantity proportion sampling: This method again takes set size samples, but in this case at varying time intervals based on the flow rate of the main body of fluid. This form of sampling is dependent of flow quantity.
(4) Flow proportional sampling: This final method is based on taking samples at fixed time intervals. However, in this case the size of the sample varies in accordance with the flowrate of the main body of fluid at that time. This method more accurately mirrors the actual process inflows/discharges and can also assist in evaluating total loading into the receiving water course more accurately than previous methods.
It is useful to use a sampler that is capable of running a number of control regimes at any time. Once collected, samples are deposited into collection bottles. But measures must be taken to preserve the sample. Some samples have inherent instability which causes variations with time, i.e. high organic levels. Sample preservation is achieved by cooling the collecting area to between 0 and 4 degrees C. Combined with darkness, this can maintain stability for up to 24 hours.
The choice of sampling equipment entails many factors, most of which are determined by either site conditions or the sampling regime required. However, the choice is important if the original aim of obtaining representative, useful and pristine samples, is to be achieved. PE
