SubscribeDavid Claridge, Water Industry Manager at Endress+Hauser Ltd will discuss the importance of selecting the correct sampler location in securing the most accurate and representative sample.
The use of samplers, both automatic and portable, has been widely adopted to assist in many facets of the Environmental and Water industries, from self monitoring of discharges, to complete catchment surveys. The initial drive came from the Urban Waste Water Treatment Directive, which required samples to be taken at wastewater treatment works to confirm that discharge consents were being met. Large numbers were installed by water companies using either outsourced contractors or inhouse staff who generally knew the considerations required in selecting the correct sampling location.
Recently the new drive to self-monitor discharges, borne out of the Water Resources Consent to Discharge system, has required both large and small industrial dischargers to install sampling systems. In many cases, the personnel charged with this requirement are not conversant with sampling and the potential problems that can be encountered in ensuring the sample is representative.
This is often due to lack of experience or knowledge. In the future, the ongoing impact of Integrated Pollution Prevention and Control (IPPC) and the Framework Directive will ensure a requirement for sampling in one form or another. It is important to provide information and guidelines to enable personnel to determine the best location and installation details for self-monitoring their individual application. The aim of this article is to provide these guidelines.
When initially considering the installation of a sampler, the purpose of the sample should be considered in addition to the nature of the effluent being sampled. For example, it may be to ensure the discharge from an industrial plant does not contain above a certain level of a chemical that may be discharged by the plant. The production process also needs to be considered - will the discharge be constant? Is the production a batch process discharging varying volumes and concentrations throughout the day? In such cases, the sampling has to be designed to ensure capture of a representative sample by utilising the correct method. There are four main types of sampling method:
- Event sampling
Samples are taken during an event produced by a pumped discharge, batch production or a storm event. Sampling can be initiated by level or flow measurements. - Time proportional 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 as representative if the flowrate is constant. - Quantity proportion sampling
This method again takes set size samples but in this case, at varying time intervals based on the flowrate of the discharge. This form of sampling is dependant on flow quantity, with information provided by a level or flow measurement. - Flow proportional sampling
This final method is based on taking samples at fixed time intervals. However, 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 flow and can also assist in evaluating total loadings with greater accurately than other methods.
It is possible and, in many cases preferable, to use a sampler that is capable of running a number of control regimes at any one time. For example, a steady flow may require a time proportional sampling regime, but at certain times may experience an event such as a pumped discharge, requiring additional event samples to be collected. The appropriate sample bottle configuration would need to be determined. Here, time proportional samples could be collected as a composite sample, whilst the event samples may be required as individual discrete samples.
Although not a consideration of this article, the temperature at which the sample is held once collected is also important, particularly when considering samples containing organic matter. The sample should be maintained at a temperature of between 0 and 4oC (without freezing) for the range of ambient and sample inlet temperatures likely to be experienced.
Once the type of sampling required has been determined, the most appropriate location has to be selected. Often this is dictated by the logistics of access for the sample-hose into the pipe or channel as well as power, space and access (for removal of the samples) considerations. Ideally, the sample should be taken at the point of discharge to a water course or public sewer, allowances for surcharge of the receiving system should also be made. It is also important to consider individual production processes that may require samples of the waste stream.
There are a number of considerations relating to sample quality that are determined by selecting the correct sampling location. The following points highlight a number of the important factors:
- Location of most representative sample
In all cases the sample has to be fully representative of that discharge, which means primarily well-mixed, but ideally, without being aerated. For example, greases and oils float and although may not be sampled, could affect the receiving water course, or treatment equipment and processes downstream. Similarly solids, which could be the main contributor to pollution, will settle to the lower levels of the flow and may not be collected in the sample if mixing is not sufficient. - Depth of flow
Consideration also needs to be given to the depth of the fluid being sampled. Is the level constant or variable with flow? Ideally a sample should be taken at the most representative point which could be at mid-depth, for example. If this is the case, the variance in depth needs to be considered and allowances made. A float system may be a solution to ensure the sample-hose is maintained at a set depth below the surface. - Sample hose protection
Protection of the sample hose will be required to protect against damage or blockages from large particles, rags, fats and greases, whilst ensuring a representative sample can still be withdrawn. This may entail a form of protection plate or filter, but could induce a maintenance requirement. Any maintenance regime required would then have to be established and enforced. - Flow and Sample lift velocity
For a more representative sample collection, the intake velocity at the sample hose intake should ideally match the actual flow velocity. Any forced intake, by a high intake velocity in relation to the actual flow velocity for example, could induce scouring. Similarly, the purge velocity prior to sample collection could agitate and scour sediments which would then be included with the subsequent sample - rendering the sample unrepresentative.
Additionally, the sample lift velocity has to be sufficient in order to lift particles that may be inherent within the sample. The required lift velocity will vary depending on the nature of the particles present. For example, organic particles have densities similar to water and will not settle easily, whilst sand particles have a greater density and will require higher velocities to ensure they are collected. The lift and intake velocities, which in turn can be dependent on the nature of the sample, could alter the type of sampler and sample pump required. - Positioning of sample intake
In addition to the points above regarding sampling positioning, consideration must be given to the positioning of the intake hose for physical reasons. The position must:
i) be accessible for maintenance reasons
ii) be located to reduce the possibility of damage from debris, rags and blockages. The hose must also be protected from personnel and machinery throughout its entire length, from the sample point to the sampler
iii) avoid movement in the fluid which could cause the surface or sediment layers to be included in the sample
iv) avoid channel walls, pipework or equipment to remove the possibility of dislodged accumulated material being drawn into the sample
v) avoid pumped inflows or other flows which would put undue strain on the hose. This could reposition it or cause unrepresentative sampling to occur by possibly jetting into the sample inlet. - Securing of sample intake
In order to mitigate the possibility of the hose moving from its selected position, it must be securely fastened. However, the bore of the hose must not be restricted and it must be fastened so that it can be easily removed for maintenance purposes.
Whilst not exhaustive, all the above points must be incorporated into the design of a sampling system - from the initial need and requirement, through the nature of the flow being sampled, to the practical and physical implementation and running of the system.
This article shows that selecting the correct sampling location and method should not be taken lightly. There are many intricacies to consider in order to self-monitor discharges that require knowledge and understanding of their importance. Care, attention and expertise are integral to obtain a representative sample which will satisfy a range of environmental, legislative and financial requirements.