Location, location, location!
Selecting the correct position for samplers is vital if companies are to secure the most accurate and representative sample. Care, attention and expertise are integral to obtaining a representative sample that will satisfy a range of environmental and legislative requirements. David Claridge explains.
Water companies installed large numbers using either outsourced contractors or in-house staff that generally knew the considerations required in selecting the correct sampling location.
Recently, the new drive to self-monitor discharges, born 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.
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 using 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 flow rate is constant
- Quantity-proportion sampling: This method again takes set size samples but in this case, at varying time intervals based on the discharge flow rate. This form of sampling is dependent 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 flow rate 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
The appropriate sample bottle configuration would need to be determined. Here, time-proportional samples could be collected as a composite sample, while the event samples may be required as individual discrete samples.
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°C and 4°C (without freezing) for the range of ambient and sample inlet temperatures likely to be
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 several 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 watercourse, 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.
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, while 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 that 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, while 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:
- Be accessible for maintenance reasons
- 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
- Avoid movement in the fluid that could cause the surface or sediment layers to be included in the sample
- Avoid channel walls, pipework or equipment to remove the possibility of dislodged accumulated material being drawn into the sample
- 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.
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.
While 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.
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 that will satisfy a range of environmental,
legislative and financial requirements.
David Claridge is water industry manager at Endress+Hauser. T: 0161 286 5000.