Coagulants and flocculants:
What, why, when, and issues to look out for
Batch dosing means applying a measured amount of flocculent to dirty water in a single dose. It is also known as ‘shock dosing’.
Batch dosing is usually a one-off, reactive measure to treat impounded runoff that has not been treated to the correct standard. You add liquid reagent to the surface of the impounded runoff, to increase the rate of settlement and achieve the required standard of discharge.
It can also be a contingency measure in devices that have been treated by either a rainfall-activated system or a floc sock system.
Batch dosing is mainly used in the following situations:
- Impounded runoff has not been treated to the correct standard
- Sediment-laden water has been collected by pumping
- Impounded sediment-laden water needs pumping
- During dewatering.
Issues to look out for
- Batch dosing is only suitable for collected runoff.
- It doesn’t work during rainfall.
- If you are planning to batch dose runoff, size your sediment retention tools to retain all the runoff from the catchment. Doing only part of the runoff is ineffective. Either sediment retention tools will be overtopped with dirty, untreated water discharging into receiving waterways, or by the time you have stopped the outflow the dirtiest water will have already been discharged.
- Calculate the volume of impounded water to be treated.
- Bench test to determine the appropriate volume of liquid reagent to apply to the impounded runoff.
- Give the pond or bund enough time for sediment to settle out.
- Keep the device’s outflow closed until the treatment has achieved an appropriate level of clarity or water quality.
- Periodically check water quality to determine progress. If the flocculent you are using could affect the water’s pH, regularly monitor the pH to ensure that it is within 6.5–8.0 and can be discharged. If it is too low, you may need to buffer the pond with gypsum.
- Time the batch dosing according to weather forecasts. If bad weather is predicted, there may not be time to achieve effective sediment settling before the next inflows arrive.
Construction, operation and maintenance
When carrying out batch dosing
- Only implement this treatment under the supervision of a suitably experienced and qualified professional.
- Raise the T-bar and block the outlet from the device before you dose.
- Determine the volume of the impounded runoff. From the bench testing, apply the volume of reagent needed.
- Add the reagent by spraying it on the surface of the device or by carefully using a bucket or other small container to evenly disperse the reactant over the surface.
- Mix the impounded water with the reagent in one of these ways:
- Mixing with a pole, paddle or oar
- Mixing by circulating the impounded flows through a pump
- Mixing by dragging semi-submerged floats through the surface area of the impounded water.
- Once settlement has happened (usually after 2–3 hours), check the pH and clarity within the device. If the pH is between 5.5 and 8.5 and the clarity is greater than 100 mm, open the decant and drain the device.
- If the clarity is less than 100 mm, add a further dose of reagent at 50% of the original rate and allow a further settlement period of 2–3 hours before re-testing.
- If the pH levels fall outside the acceptable range of 6.5–8.0, seek advice from a suitably experienced and qualified professional to determine appropriate actions. These could include taking the impounded water offsite or adding alternate substances, such as gypsum, sodium carbonate or sodium bi-sulphate, to alter the pH. Subject to the weather conditions and the level by which the range has been exceeded, you may be able to use the impounded water for dust control.