Flocculent Treatment Management Plan
3.
Use of flocculants and coagulants
The use of flocculants and coagulants (commonly referred to as flocculation) can decrease the settlement time within sediment retention devices and therefore increase the sediment removal efficiency of these devices. In order to maximise the efficiency of sediment retention devices utilised on site, and therefore minimise the adverse environmental effects associated with the discharge of sediment the use of flocculants and coagulants is to be investigated for all sediment retention devices on site. Typically, flocculation for a sediment retention device will be provided unless: • •
Bench testing of soils in the contributing catchment confirms there is no benefit from flocculation, or Where the duration of the earthworks is very short.
Flocculation will be undertaken in accordance with Section F2.0 Coagulant and Flocculant Treatment of Auckland Council Guideline Document 2016/005 “Erosion and Sediment Control Guide for Land Disturbing Activities in the Auckland Region” (GD05). The details of the specific erosion and sediment control measures to be utilised, including the design of the flocculation management practices is detailed within the SSESCP prepared for each stage of work. The initial design of the flocculation management practices will be based on previous information of soil types until the soil testing (Section 3.1) can be undertaken. The implementation and as-builts of any flocculation device will be in accordance with the inspection and as-built procedures contained within the ESCP.
3.1 Soil testing As part of the initial earthworks associated with areas where flocculation is to be considered, soil samples will be taken and will be bench tested to confirm flocculation options for those soils, including flocculants or coagulants to be used, optimum dosage rates and dosage methodology. This bench testing will also identify any soils where the existing pH is outside of the baseline range of 5.0 – 9.0. This assessment of the additive to be used will also determine the most efficient method of dosing (refer Section 4). In accordance with the monitoring and maintenance detailed in Section 8, as the soil types in each area changes due to deeper cuts and/or altering fill sources, the optimum flocculation option may also change. Any changes to the flocculation for a device will be recorded as part of the monitoring records for that device.
3.2 Test procedure Testing is undertaken by mixing a representative sample of soil from the site with water to produce a weak slurry, representative of sediment laden runoff from the site. Page 15
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