17/7/2018: Mt Crosby Water Treatment Plant – Acrylamide

Non-routine Investigative monitoring returned an ADWG health exceedance for acrylamide of drinking water leaving Cameron’s Hill 1 Reservoir.

Acrylamide monomer is an impurity found in polyacrylamide polymers which is used at Mt Crosby WTPs (and widely across the water industry) and is used as a flocculant aid, a filter aid and in the centrifuges to assist in the sludge dewatering process (which is returned of the head of the treatment works).

Under the operating conditions around the time of sampling the acrylamide monomer concentration in the drinking water was modelled at about 0.0001 mg/L (the ADWG limit is 0.0002 mg/L). The external laboratory analysed the sample in duplicate obtained results within measurement uncertainty at a concentration of approximately 0.0005 mg/L (0.00048, 0.00052) upon which Seqwater were verbally notified as per our notification protocols. However, prior to issuing the Certificate of Analysis (CoA) a second duplicate was taken from the same sample by the lab which returned a result of 0.00012 mg/L. This is what the lab reported on the CoA. A subsequent low-level quantification method returned a result of 0.000056 mg/L (i.e. an order of magnitude lower than those initial results that were verbally reported). Optimisation of the centrifuge operating conditions enabled the polymer dose to be reduced, decreasing the load and concentration of acrylamide monomer in the drinking water and an intensive acrylamide monomer monitoring program for a period of two weeks was undertaken. A predictive ‘Live acrylamide concentration monitoring system’ has been implemented using the SCADA system at Mt Crosby Eastbank WTP. This is a predictive modelling tool that identifies to the operator the predicted acrylamide monomer concentration in the drinking water under the current operating conditions. Enabling early operational intervention to avoid concentration above the ADWG.