Cleaning out Walkerton digesters required
extraordinary planning

A biosolids haulage truck waiting to be loaded.

Digester cleaning crews from Azurix North America/Terratec were called in to assist the Town of Walkerton, Ontario, during the recent drinking water contamination crisis. Crews cleaned out potentially contaminated sludge from the primary and secondary digesters and sludge storage at the town's wastewater treatment plant. Sludge was transported to the Greenway WWTP in London and new seed sludge was delivered to re-start the digesters.

Although the specialized nature of the work was well within the normal range of residual management services, the heightened sensitivity surrounding Walkerton required careful planning.

There was a close working relationship and constant dialogue between the Ministry of the Environment (MOE), town officials and Azurix. All procedures had to be approved by the command centre that had been set up.

MOE approval was received to conduct the work under an existing C of A. The sludge was tested for the particular strain of E.coli to ensure worker safety. None was detected. All equipment such as tankers and hoses that had come in contact with the sludge had to be cleaned.

Proper on-site handling procedures were developed so that staff would not be exposed unnecessarily. For example, drivers' duties were restricted to driving only in order to prevent carry-over of material onto clothing or into the cab. Designated operators who conducted the pumping functions were washed down on-site at the end of each day. To enter the digesters, clean-out crews were equipped with normal safety equipment, including self-contained breathing apparatus with air lines, gas detectors, safety harnesses, boots and gloves, and explosion-proof lights.

As emergency spill contingency plans were put in place, a communications strategy was developed to respond to the media and public. The MOE issued a news release explaining the project.

After a thorough cleaning of the primary digester, followed by disinfection with chlorine, new seed sludge was delivered from the Hanover plant and pumped into the primary digester. To prevent any possible contamination of the seed stock, clean tankers were utilized rather than those previously used for transporting sludge.

The secondary digester was then cleaned, again using a vacuum truck with a 3,300 cubic feet per minute pump and six-inch suction. The holding tank was pumped down using the sludge truck loading pump at the plant to within two feet of the bottom, after which a vacuum truck was used to clean out the heavy solids from the bottom. Water was used for dilution and to clean the floor and walls.

The work had to be conducted efficiently so that the plant could be put back into operation quickly, and also because the holding tank was reaching capacity. Crews worked 12 to 14 hours a day, but only during daylight hours to avoid disturbing the neighbours.

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