AWWA ACE59883

For seventy years, the City of Ottawa used quicklime (calcium oxide) to providecorrosion protection in the treated water. A treated water pH target of 8.5 had been usedto minimize corrosive effects, yet still resulted in water that is mildly corrosive. In orderto improve corrosion protection and to eliminate operational problems associated withquicklime, a comprehensive review was initiated in 1999 to develop a new corrosioncontrol strategy.A number of chemical alternatives were evaluated through bench-scale and pilot tests todetermine impacts on turbidity, pH response, chlorine demand, chloramine stability,corrosion indexes, taste, and chemical dose.Several criteria were used to compare various corrosion control strategies: water qualityimpacts (health & aesthetic); effects on industrial customers; regulatory compliance;operating and capital cost; safety and environmental risk; and, process control impacts.From the analysis, a new corrosion control strategy was established at pH=9.2 with aminimum alkalinity target of 35 mg/L (CaCO₃).During the implementation phase, an old area of the City was unexpectedly found toexperience high levels of lead (0.010 - 0.015 mg/L for flowing samples). The discoverycame as a result of customer testing, and was confirmed by City water quality staff. Anintensive investigation was initiated in 300 area homes, due to health concerns of leadexposure from drinking water.The cause of the problem was found to be pH depression induced by nitrification withinthe distribution piping. The pH of treated water was increased from 8.5 to 9.2 in order tosuppress lead dissolution. This operational measure was taken 1 year ahead of schedule,but was immediately successful in lowering lead values to the 0.006 - 0.008 mg/L range.Several sentinel sites were subsequently monitored over the next year to observe seasonaleffects of water temperature and nitrification on lead exposure. A number ofrecommendations are made for handling similar water quality events dealing with lead indrinking water.

Product Details

Edition:

Vol. - No.

Published:

06/17/2004

Number of Pages:

8

File Size:

1 file , 400 KB