The District of Chetwynd Water Treatment Plant in British Columbia, Canada, treats nearly 2,500 cubic meters of water per day using gas chlorination as its primary form of drinking water disinfection. The plant was built in 1997, and recent concerns about the safety of chlorine gas, including potential eye, skin and respiratory irritation and the risk of death with high exposure, have led the district to switch to hypochlorite generation as its primary means of disinfection.
Al Tricker, chief operator at the facility, recognized that the 12-15% concentration solution strength of potable grade bulk sodium hypochlorite is considered a hazardous material. He recommended the use of an on-site generating system, which produces a hypochlorite solution concentration of 0.8%. The use of on-site sodium hypochlorite generation offers significant advantages over the use of gaseous chlorine for disinfection. The disinfectant is produced and stored in liquid form, so the danger of gas leaks from high-pressure chlorine cylinders is not present. As a result, on-site sodium hypochlorite generating system users do not need to develop and maintain a Risk Management Plan. Utilities using on-site systems also do not need to provide HAZMAT training or provide for the availability of self-contained breathing apparatuses.
Generating sodium hypochlorite onsite is a simple process that uses three common consumables: salt, water and electricity. The system operates by feeding softened water into a brine dissolver. The salt dissolves to form a brine solution, which is further diluted to the desired salt solution. The salt solution is then passed through the electrolytic cell(s), which apply a low voltage DC current to the brine to produce the sodium hypochlorite. The sodium hypochlorite is then safely stored in a 3,700 liter, five-day tank. When it reaches the low-level set point, the system automatically restarts to replenish its supply.
The Chetwynd plant operates at an average flow rate of 1.5 million liters per day, with seasonal fluctuations ranging to more than 3.5 million liters per day during the local sawmills' busy season. With a municipal population of just over 3,000, the plant required a 12 lb./day on-site generating system. In July 2005, Tricker oversaw the installation of a skid-mounted ClorTec  12 lb./day on-site generation system.
Before entering the ClorTec system  in Chetwynd, water goes through two sand filtration beds and an ultraviolet disinfection system. When raw water enters the plant, chemicals are added before the water moves into an upflow filter, through which flocculated particles are transported into the granular media filter bed. The filter bed consists of anthracite on top of silica sand supported by a layer of gravel. Upon media saturation, a backwash cycle is activated per square inch. To ensure more thorough cleaning of the media during the backwash cycle, raw water is used to flush the media with the assistance of an air scour. The wastewater generated from this backwash cycle is then directed over a backwash trough and run to waste. This backwash process continues until the filter bed is cleaned, at which point water flows to the top of the filter section and is drawn through the filter media into an underdrain system.
An ultraviolet disinfection process is used to inactivate waterborne pathogens and viruses. UV does not affect the chemistry of the water, nor does it remove any beneficial minerals. However, UV lacks a residual disinfection effect, resulting in dependence upon the sodium hypochlorite generation system to provide residual protection.
The ClorTec system  produces a week-long supply of equivalent chlorine disinfection, which is held in the storage tank, ready for distribution to the community. The district maintains a full stock of all spare parts associated with the on-site generating system in the event the duty system is not operational.
Labor and maintenance for the on-site system has been routine. Typical annual manufacturer recommendations for on-site system maintenance include acid washing of the electrolytic cells, cleaning the salt tanks, cleaning/changing filters, and cleaning the chlorine product tanks. The District of Chetwynd has extremely hard water, which leads to an increase of deposit build up on the interior of the electrolytic cell and requires a more frequent cell cleaning schedule. In order to cut down on the amount of chemical used at the plant, Tricker opted to use vinegar to clean the electrolytic cells instead of acid washing. Cells are cleaned approximately every three months.
After more than one year of operation, Tricker claims that the on-site sodium hypochlorite generating system has met and surpassed the district’s expectations.
Nadia Abbott is the marketing manager at Severn Trent Services. She can be reached at (215) 997-3733 or at firstname.lastname@example.org .