Wastewater Coagulant Dosing Control

Control of coagulant dosing chemicals in wastewater treatment is an important issue, it links directly to both the site operating cost and to the effectiveness of the treatment process. Partech (UK) have successfully worked with one of the UK's water companies to apply their TurbiTech Self Cleaning sensors in the monitoring of Turbidity in Crude and Settled Sewage as a control parameter for the addition of coagulant chemicals.

Unreacted coagulants are highly toxic to aquatic life primarily due to the low pH levels they cause, it is therefore important that overdosing does not occur, whilst maintaining sufficient dosing levels to ensure effective treatment. The Environment Agency will require a full risk assessment and normally impose discharge permits which include limits on metal residues. Reliable and responsive dosing control is therefore required to optimise the use of the selected coagulant chemical whether this is Aluminium or Iron based.

This document looks at the use of Turbidity as the control parameter, consideration of Phosphorus measurement may also be needed, either as a permanent inlet or effluent measurement or as part of the design process when considering the viability of relying on a diurnal P load profile.

The principal dosing regime on the majority of sites utilising coagulants for nutrient removal and for chemically assisted dosing is time or flow based depending on the size of the plant. However simply relying on flow or on historic data does not allow the control system to respond to changes in load, this is particularly important in areas where large population change occurs due to tourism or where industrial load impacts on the treatment system. This is where Turbidity has proven to be a highly effective, responsive measurement that is easy to apply and understand without high capital or operating costs.