By Kevin Bossy and Don Burgess
High manganese is a common seasonal occurrence for the Municipal District of Opportunity #17 in northern Alberta, which draws its water from the South Wabasca Lake. For years, operators at the Desmarais Water Treatment Plant had been able to successfully control manganese in finished water, using potassium permanganate and a blower-powered conventional aeration system.
But, during the summer of 2014, manganese levels rose significantly and chemical treatment along with microfiltration membrane treatment was unable to reduce them to the desired limits. As a result, finished water during the summer months would be distributed with an average manganese concentration of 0.2 mg/L. This then reacted with the chlorine disinfectant and caused the water to change to a muddy brown colour. Though the water was still safe to drink, residents began to notice unpleasant taste, odour and colour issues with the tap water, prompting many concerns and complaints to the water utility.
The municipal water utility investigated several options to try and reduce manganese levels in the finished water, including optimization of potassium permanganate dosing and clarification process. However, the problem persisted and it seemed that an affordable solution could not be found. Moreover, new federal guidelines were coming into effect in April 2019 that would limit the maximum allowable manganese concentration in drinking water to 0.12 mg/L.
DWG Process Supply, a manufacturer’s representative in Western Canada for Bishop Water Technologies, evaluated the situation and suggested that operators try OctoAir™-10, a new micro bubble diffuser manufactured by Canadianpond.ca Products Ltd. It produces abundant micro bubbles that provide mixing and high oxygen transfer to the raw water. The resulting high level of dissolved oxygen causes oxidation and precipitation of dissolved manganese. OctoAir-10 has a 120V, 3/4 HP compressor that uses a fraction of the electricity of conventional aerators.
In July 2018, plant operators installed the OctoAir-10 diffuser into the storage pond that supplies raw water to the treatment plant. They just had to connect an air line from the compressor to the new diffuser and lower it into the desired location. There was no impact to the plant operation and the installation was completed in about four hours.
“We noticed a big drop in manganese within two or three days of starting the new diffuser,” said Earl Gullion, utilities manager for MD Opportunity #17. “We experience the highest manganese concentration in the summer, when it can reach 0.8mg/L in raw water. But, once we started the OctoAir-10 diffuser, manganese in raw water dropped significantly and consistently remains in a range of about 0.4 to 0.27 mg/L.”
This drop in levels enabled operators to optimize potassium permanganate dosage and membrane filtration to reduce manganese concentration in finished water from an average of 0.2 mg/L to only 0.01 mg/L. This exceeds federal guidelines and eliminates taste, odour and colour problems for residents.
“The OctoAir diffuser requires no maintenance,” Gullion said. “We haven’t touched it since installation and it continues to function well and helps provide a consistent reduction of manganese in our drinking water.”
The plant also experienced numerous other benefits that have improved water quality and helped reduce operating costs. There has been a 55% reduction in potassium permanganate use as the dose dropped from 1.8 mg/L to just 0.82 mg/L. The cleaning interval for the membrane system was extended by two to three times. Turbidity in finished water has dropped from about 0.03 NTU to 0.001 NTU.
Finally, operational savings enabled the community to recover the cost of the OctoAir diffuser in less than a year.
Kevin Bossy is with Bishop Water Technologies. Email: firstname.lastname@example.org. Don Burgess is with DWG Process Supply. This article appears in ES&E Magazine’s December 2019 issue.