Tech talk 2: Coagulants and Flocculants Part 2

In our first technical talk we discussed what are coagulants and flocculants, how they operate to settle out suspended sediment, and finally how their varied chemistry may impact basin performance (through different mixing/distribution requirements and floc stability). In this technical talk we will look at how to select the most effective product and determining the optimum dose rate to apply.
Regardless of the type of sediment basin, the key considerations in selecting a coagulant or flocculant will typically include:

  1. Effectiveness of product to rapidly settle suspended sediment
  2. Dose rate required to achieve optimum settling
  3. Dose range to achieve acceptable settling
  4. Product cost per volume of water
  5. Intended application of product (automated dosing, batch treatment, passive treatment system) 

To address points 1 to 4 above, we need to undertake a jar test. Conducting a jar test is quick, simple, does not require specialised equipment and will give you hands on experience in how various products will perform for your given site soils. A jar testing procedure is included within the finalised Appendix B (2018) document as well as the accompanying factsheet. It is suggested that a range of products be trialled as part of your jar test as some may be more effective than others.
As Type A and B basins require rapid settlement, a coagulant or flocculant should only be considered if the jar test achieves a clarity of at least 100mm within 15 minutes. With a little patience it is likely that you will be able to achieve a far quicker settlement time within your jar tests by trialling different products and progressively increasing dose rates until an optimum rate is determined. Just as important as finding the optimum dose rate of the chosen product is determining the effective (or operational) dose range (i.e. what is the lowest and highest dose rates which can be applied and still achieve settlement within design parameters). A product which provides a large effective dose range will be far more desirable from a operational perspective as it gives some flexibility in accuracy of application, method of application and changing site conditions. Another consideration when undertaking jar tests and selecting a suitable coagulant or flocculant is whether a variety of soils (due to varying soil properties within the natural profile, or imported fill)  are likely to enter a basin during its operational life. Jar testing should aim to find a product which is effective for all soil conditions at each basin, ideally which can be settled within the operational dose range.
Just as the effectiveness of coagulants and flocculants will vary so does the unit cost ($/L or $/kg of raw product). Comparing products based solely on unit cost will be misleading, as it does not consider the varying effectiveness (i.e. quantity or dose rate required) and indirect costs related to handling, transport and labour. It is suggested that products be compared in terms of $/ML (i.e. cost of product to treat 1 ML of stored water) by multiplying your optimum dose rate by unit rate.
Within our October newsletter edition, we will continue the technical talk theme and look at options available to dose coagulants and/or flocculants into your basin.

Meanwhile, if you would like to find out more about designing and operating Type A, B and D sediment basins, Topo has specialised courses coming up in Brisbane and Darwin, which will cover how to size basin varieties, how to adapt to site conditions and practical considerations to maximise the use and benefit of each basin type.

Kyle Robson