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Making groundwater potable a chemist's dream, pipeline solution

Posted December 6, 2011

Groundwater in Tottenham is a lot like the prescription drugs advertised on television. On the one hand they provide relief, but the side affects can trigger a whole rash of other problems.

In this case, the Tottenham Municipal Water Supply Disinfection and Treatability Study completed by Genivar on November 29, and presented to New Tecumseth councillors last night, shows the raw water supply contains elevated levels of ammonia and methane which are treated with chlorine as part of the process to make it potable. But Chlorine disinfection is elevating the level of Trihalomethanes (THMs). THMs are a group of chemical compounds formed when chemicals in the natural water react with chlorine which is used for disinfection of the water supply.

"In order to achieve a free-chlorine residual (...) chlorine is dosed on average around 11 mg/L. This dose is very high compared to typical drinking water treatment systems where the range is generally between 1-3 mg/L," according to the study.

That means treating water from the two wells supplying Tottenham requires about tens times more chlorine than "average" to meet Ontario Drinking Water Quality Standards (ODWQS) which reduced the allowable maximum concentration limits of THMs in treated drinking water from 300 ug/L to 100 ug/L.

Below is a quick trip through the water treatment process in the former village:

Wells #4 and #5 are located on Walkem Drive and were constructed in 1970 and 1977 respectively. The existing Permit to Take Water (PTTW) allows pumping of either of Wells 4 and 5 at rates up to 1,633 m3/day. The wells are also starting to pump more sand way into the system.

Water from the Walkem Drive and Coventry Park Pump houses is sent to the Mill Street Reservoir where it is treated with chlorine (sodium hypochlorite) for both primary and secondary disinfection. After chlorine dosing, water enters the Mill Street Reservoir, which is a chlorine contact-ehamber (CCC) that provides residence time to meet primary disinfection requirements.

A second chlorine application point is located downstream of the CCC to ensure an adequate chlorine residual is maintained in the distribution system. Treated water is pumped to the distribution system, which also contains an elevated water tower.

As well, the Tottenham drinking water supply is presently fluoridated at the Walkem Drive pumphouse. It can also be dosed at the Coventry Park pumphouse though it currently acts only as a back up to the Walkem Drive dosing. (...) Consideration of future water supply options will continue to take fluoridation into account.

"Results of the investigation demonstrated that there are two major contributors to the high THM levels in the Tottenham treated water supply: (1) elevated bromide in raw water and (2) high chlorine dosing needed to overcome the elevated ammonia and methane levels. Bromide in the raw water leads to the formation of brominated THM, which account for the majority of the total THMs in treated water," according to the study. "Moreover, THMs form within a relatively short time frame. Consequently, it will not be possible to decrease the chlorine dose to levels that adequately reduce THM formation. Nor will reducing the travel times in the distribution system realize an acceptable decrease in THM formation."

The Ministry of Environment is for all intents, forcing the Town's hand to remedy the problem with solutions that range in estimated cost from a high of $25.7 million to replace the well system completely with piped water from Alliston, via Georgian Bay at Collingwood, to a low of developing a new well supply, plus UV/Chloramination treatment.

"The THM formation study demonstrates that the existing chlorination system cannot be optimized to reduce THM formation, while maintaining adequate secondary disinfection in the distribution system."

The preferred solution includes blending the supply augmented with the pipeline from Alliston, with construction of a new well. One of the immediate actions to determine whether it's feasible is analysis of whether the water types can be mixed "to guarantee that THMs do not exceed the permissible level."

"The Town's long-term water supply plan for Tottenham includes supply augmentation of local groundwater supplies with surface water from a new pipeline from Allison to Tottenham. This pipeline is also intended to improve supply security to Beeton. The new pipeline will increase the system capacity to meet future water demands. The capacity of the existing Tottenham groundwater system (Wells 4, 5, 6A, and 7) is 6,000 m3/d (as per the PTTW). The Town's Water Distribution and Storage Master Plan (2007) projects a maximum day demand of 11,413 m3/d for the Community ofTottenham in 2031. An additional supply of approximately 5,500 m3/d is therefore needed to meet future supply needs."

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