For seventy years, the City of Ottawa used quicklime (calcium oxide) to provide
corrosion protection in the treated water. A treated water pH target of 8.5 had been used
to minimize corrosive effects, yet still resulted in water that is mildly corrosive. In order
to improve corrosion protection and to eliminate operational problems associated with
quicklime, a comprehensive review was initiated in 1999 to develop a new corrosion
control strategy.
A number of chemical alternatives were evaluated through bench-scale and pilot tests to
determine 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 quality
impacts (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 a
minimum alkalinity target of 35 mg/L (CaCO<sub>3</sub>).
During the implementation phase, an old area of the City was unexpectedly found to
experience high levels of lead (0.010 - 0.015 mg/L for flowing samples). The discovery
came as a result of customer testing, and was confirmed by City water quality staff. An
intensive investigation was initiated in 300 area homes, due to health concerns of lead
exposure from drinking water.
The cause of the problem was found to be pH depression induced by nitrification within
the distribution piping. The pH of treated water was increased from 8.5 to 9.2 in order to
suppress 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 seasonal
effects of water temperature and nitrification on lead exposure. A number of
recommendations are made for handling similar water quality events dealing with lead in
drinking water.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 400 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 8 |
| Published : | 06/17/2004 |
