WSP SLUDGE MEASUREMENT TECHNIQUES
Wet sludge volume, sludge solids concentrations and inert fraction of dry solids are the main considerations when quantifying or surveying the accumulated sludge in a WSP. All survey levels should be referenced to a clear fixed datum nearby. Ideally this survey datum should be identifiable against the WSP as-built drawings.
MEASURING WET SLUDGE VOLUME
A systematic methodology should be used when determining the quantity of sludge in WSP. The WSP should be divided up using clear and repeatable transect lines across both the length and the width of the pond. For repeatability, the location of these transects could be marked using stakes or permanent markers on either the waveband or fence posts. Graduated ropes pulled taught across the pond or GPS can then be followed to position the measurement and sampling boat.
The number of locations at which sludge depth should be measured to allow a reasonable estimate of total sludge quantity to be made will be influenced by the size of the pond. Larger ponds will require more measurements to be taken. As an example, for a WSP 100m by 100m, transects at 20m intervals in both directions would give a total of 16 locations for sludge depth measurement. This would allow a reasonable estimation of sludge volume to be determined.
Calm weather is essential and a rope line pulled tight across the pond can be used to hold the survey vessel stationary while measurements and samples are taken. The depth probe used to measure total depth should have a blunt end so that it does not damage a geomembrane pond liner or penetrate into a clay pond liner. The water level on the day of the survey must be recorded in relation to a fixed height if it is used as the datum for all measurements. This will then need to be compared to the water level in following surveys and any difference subtracted or added accordingly.
Many techniques have been used to measure to the sludge-water interface (top of sludge layer) with varying degrees of accuracy.
A simple and effective method for measuring both the sludge depth and total pond depth is through use of a graduated “sludge judge”. A sludge judge allows a “core” of WSP liquid and sludge to be brought to the surface for visual determination of the depth of water above the sludge layer. The total pond depth can be determined by pushing the sludge judge to the base of the pond. There are a number of different infrared sludge blanket detectors available that are relatively inexpensive, simple to use and accurate.
The accuracy of using sonar depth sounding to measure to the top of the sludge layer can depend entirely on the nature of the sludge, turbidity, type of sonar device and depth of the sludge below the transducer. Sonar has the advantage of being able to produce a highly detailed survey but that detail is of little use if inaccurate. A sonar device should be calibrated in each pond at multiple points against a sludge judge or infrared sludge blanket detector.
Sludge measurement accuracy and repeatability of +/- 25mm is possible and should be targeted. All data should be made available to the operator in hard and electronic format for reference as part of the plant history records. With the data gathered, 3D computer software can then be used to calculate volumes and create plans and cross sections.
MEASURING SLUDGE SOLIDS CONCENTRATIONS
Solids concentrations are an important consideration when calculating dry mass, dewaterability, space and costs needed for disposal.
Undisturbed in-situ sludge samples need to be taken to be analysed for dry solids content. The depth at which the sludge sample is taken also needs to be considered. A comprehensive sludge survey should include a representative number of sample locations and ideally samples taken at up to three different depths at each location (depending on the depth of the sludge layer). The reason for this is that sludge accumulations deeper than about 1 metre can sometimes show compaction with higher solids concentrations nearer the base than in the upper sludges. This stratification in solids concentration needs to be considered when calculating dry solids volumes. An example of 3 differing layers in the sludge column is shown below.
Any stratification in solids concentration will show up in the sample analysis at which point it can be determined whether there is a significant difference in total dry solids calculated by either applying an overall average dry solids percentage to the wet volume of sludge or splitting that volume into two or three layers and applying the corresponding average dry solids percentages to each layer. If there is a significant difference, it would be more accurate to separate out the layers and apply the appropriate average dry solids percentage to each layer. It is also important when comparing the solids concentrations to the depths at which the samples were taken to note that the thickness of each layer could vary throughout the pond especially where the base profile is inconsistent.
In summary, to provide a more accurate approximation of the dry mass present, it is important to consider variation in sludge solids concentration relative to depth. However, it is recognised that specialist sampling equipment is required to be able to effectively collect discrete samples from different sludge depths.
If such specialist sampling is not possible, an alternative is to homogenise a sludge core collected through the depth of the pond. The resulting TSS concentration, multiplied by the depth of sludge, provides a reasonable estimation of the mass of dry solids present. Examples of sludge core collection include use of a ‘sludge judge’ and use of an open tube with a valve on top.
When using a sludge judge to collect a core sample, care should be taken as the sludge may not enter the tube at the same rate as the tube is lowered like water does. This means that the tube will collect sludge at the sludge surface but may begin to push sludge out of the way instead of letting it enter the tube as it proceeds through the sludge column. This may mean that you do not capture the whole sludge column.
When using an open tube that has a valve on top, the valve is open when lowering the tube and closed when the tube has reached the base. The tube is then raised up with the entire sludge column inside, including any layer of grit or soft clay from the base. Depending on the weight of the water on top and the depth of the sludge, the grit and clay can often drop out before you have a chance to close off the bottom of the tube when it is just below the pond water surface.