CTMP Revision 0 The dosing point should be at a location where the chemical will fall into the sediment laden flow during periods of low flow. The end of the dosing tube should be only a few centimetres above the diversion channel to ensure that the chemical falls into the sediment laden runoff and is not blown away during periods of strong wind. 4.1.4 Alternative Rainfall Activated System Alternative systems such as ‘Auto-Floc’ are available that are rainfall activated by a rainfall sensor located on the devices. Advice on the suitability of those systems is required on a case by case basis. 4.2 Flow Activated System Various flow activated systems such as Erosion Control Dose Boxes or Green1 Technologies Electronic Dosing Device (EDD) or Inline Dosing Device (IDD) and are available depending on the circumstances of each site. These include pumping based systems set to the pumping rate, and systems that employ sensors to read the flow moving through the inlet channel to a sediment control device. Again, these systems should be considered on a case by case basis. 4.3 Batch Dose Treatment Batch dosing is largely undertaken as a reactive measure to treat impounded runoff that has not been treated to the correct standard. Batch dosing is achieved by adding liquid reagent to the surface of impounded runoff to increase the rate of settlement to achieve the required standard of discharge. Batch dosing may be undertaken as a contingency measure in devices that have been treated by a rainfall activated system. Batch dosing can be utilised during dewatering / pumping processes. The criterion to establish the need for batch dosing is the clarity of the sediment laden runoff. Clarity will be measured by either of the following two techniques: • Black disc o A 50-80mm diameter disc is attached to a 1m long stick with a centimetre scale starting as the disc is lowered vertically into the water to be tested until it disappears, and then is raised until it just reappears. The depth of reappearance is recorded as the clarity of the water. • Clarity Tube o A clarity tube containing a magnetic black disc will be filled with water from the device. The tube will be laid horizontal and the disc will be moved down the tube until it disappears, when viewed from the end of the tube and the distance is recorded. The disc is then moved back until it reappears, and the distance is recorded. o Readings should be taken in diffuse sunlight or shade. If it is impossible to avoid bright sunlight, work with the tube perpendicular to the sun’s plane. o Readings will not be taken in very low light conditions, i.e. before sunrise or after sunset (insufficient for colour perception). Water with a clarity of 100mm or greater is considered to be acceptable for discharge. Water with a clarity of less than 60mm should be batch dosed. If the sediment laden runoff has clarity between 60100mm after rainfall has ceased, it should be left for 48 hours to settle. If the clarity has not reached 100mm after 48 hours, or if sediment laden runoff has to be discharged within 48 hours because the pond is full, the sediment laden runoff should be batch treated. The batch dose rate will be based on the recommendations of the CART or specific bench testing for that area and calculated against the volume of the device to be treated. The batch dose rates will be provided in Appendix D of this CTMP.
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