Evaluating the suitability of synthetic organic polymers to replace iron salts in the purification of humic and sediment-rich runoff

Abstract

Peat extraction runoff water requires chemical treatment to remove organic matter and phosphorus. In Finland, ferric sulphate (FS) is normally used as coagulant agent, but significant variations in runoff water quality and the lack of optimisation of process parameters has led to increased acidity, metal and sulphate concentrations in the purified water. The use of synthetic organic polymers as an alternative to the commonly applied metal salt coagulant is suggested to better cope with typical variations in runoff water quality. This study evaluated the suitability of two synthetic organic polymers (polyDADMAC and polyAmine) for the purification of humic and sediment-rich diffuse runoff by comparing their performance to the normally applied iron-based coagulant. FS was found to require up to fourfold higher dosages but achieved higher overall purification levels than the organic polymers. In particular, removal of organic matter was substantially higher when FS was used. Of the two synthetic organic products, polyDADMAC achieved slightly better purification rates and required lower effective dosages than polyAmine. Low water temperature (2°C) had a detrimental effect on the performance of all coagulants, especially regarding removal of suspended solids. Decreasing the initial water pH (6.5–4.5) resulted in a substantial decrease in the coagulant dosages required to achieve acceptable purification levels. Although FS presented higher overall removal efficiency, the synthetic organic polymers performed satisfactorily and can potentially replace metal salts as primary coagulants in the treatment of humic and sediment-rich water.

Keywords:

• Coagulation
• Ferric sulphate
• Humic substances
• Organic polymers
• Settling
• Temperature

Acknowledgements

This research was funded by Vapo Oy and also supported by Maa-ja Vesitekniikan tuki r.y and Thule Institute. The authors would like to recognise the support received from the personnel of the Water Resources and Environmental Engineering Research Group, University of Oulu.