Wastewater Pond Sludge Level Monitoring
Periodically checking the sludge levels in the WSP can be performed to assess sludge
accumulation, and to assist with planning and budgeting for sludge removal. Ideally sludge depth profiles should be undertaken at least every 5 years. The evaluation of sludge accumulation is complicated, in that sludge build up within WSP varies both over the pond area and with depth.
Sludge levels are often found to be higher near the inlet, outlet and in the corners of the WSP. Older, more compacted sludge is found on the base of the pond, while newer and less dense sludge is found at the sludge-water interface. When the depth of sludge is significant (e.g. approaching 0.9 m from the water surface), it can potentially impact treatment performance, and sludge depth profiling should be undertaken more frequently.
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.
References: WATER NEW ZEALAND Good Practice Guide for WASTE STABILISATION PONDS: DESIGN AND OPERATION