Successful lagoon operation in northern communities

By Ken Johnson

Lagoons are the most common type of sewage treatment in Canada, and are often the treatment process of choice for small and medium sized communities. This is due to their very low operating costs and proven capability to achieve high quality effluent. In high latitudes where the costs and operation challenges of mechanical systems are magnified several times, lagoons are particularly effective.

Located at 60°34’ north latitude, and 135°4’ west longitude in the Yukon, the City of Whitehorse used a four cell primary sewage lagoon system for many years.

In the late 1980s, regulatory demands for improved effluent quality prompted the City to investigate options for achieving a secondary quality or better effluent. A number of studies considering mechanical and lagoon systems were completed. In the end, an area north of the City, near the Livingstone Trail, was chosen as the site for a large lagoon system.

In 1994, Stanley Engineering, now called Stantec, was retained to complete engineering work on the Livingstone Trail Environmental Control Facility (LTECF) to serve the 18,000 residents of Whitehorse. The LTECF includes the following major detention and retention components:

• Two 115,000 m³ primary lagoons, with a combined detention time of 20 days.
• Four 293,000 m³ secondary lagoons, with a combined detention time of 100 days.
• One 5,813,000 m³ long-term storage pond, with a one-year retention time.

Its primary cells can fill to a depth of 6.2 metres and the secondary cells to a depth of 2.3 metres. The long-term storage area, a wetland three kilometres long and two kilometres wide, can fill to a depth of six metres. Flow between lagoons is controlled by a variety of control structures.

These facilities were constructed over a period of two years, and included clearing and extending the Marwell forcemain from the old Whitehorse primary lagoons to the LTECF. Other facilities associated with the collection system were also upgraded.

Completion of the work in September 1996, allowed Whitehorse to end direct discharge of primary treated effluent into the Yukon River. The total capital cost of the LTECF in 1996 was approximately $20 million, or $1,100 per resident.

The initial design of the facility included a discharge structure from the long-term storage pond for seasonal discharge into the Yukon River. However, with such a high quality effluent expected, the City considered an innovative opportunity to avoid direct discharge to the Yukon River. Adjacent to the long-term storage pond is Pothole Lake, which lies 16 metres below the level of the surrounding lakes. It is less than 10 cm above the level of the Yukon River and is hydraulically connected to it. Materials between the lake and the river are sands and gravels.

The City applied to the Yukon Territory Water Board to obtain an additional Class A water licence for a trial discharge of up to two million cubic metres of fully treated effluent into Pothole Lake. The discharge would gradually seep into the groundwater, along with other water from the lake, and very slowly make its way to the river. The trial discharge into Pothole Lake was a success. From the fall of 1998 to 2009, treated effluent was discharged from the long-term storage pond into the lake.

Time, wind and sunlight do most of the work at Whitehorse’s sewage lagoon. In a typical July, the City receives approximately 256 hours of bright sunshine and has an average daily temperature of 14°C. Average annual precipitation is 269 mm. The LTECF is designed to hold sewage for at least 360 days at optimum capacity. During that time, wind stirs the holding cells and oxygenates the system, helping microbes and natural chemical processes break down the sewage. The only addition to the system are biological enzymes that enhance biodegradation.

By mid-May of each year the secondary lagoons are generally “ice-free” and algal blooms quickly develop. This significantly increases pH levels (maximum 11.3) and dissolved oxygen concentrations (maximum >20 mg/l). Elevated pH levels promote the volatilization of ammonia, reducing levels to below detection (0.005 mg/l) within five weeks of becoming ice-free.

The water licence states fecal coliform levels in effluent from the system may not exceed 2000 counts per 100 millilitres. Tests of the system have found fecal coliform counts ranging from less than three per 100 ml to a high of 240 per 100 ml. The old system would discharge over 100,000 counts per 100 ml.

In 2003, approximately 3,770,000 m³ of sewage was received at the LTECF. Discharge of treated effluent from the LTS into Pothole Lake commenced on August 1, 2003 and ended on October 31, 2003, a total of 92 discharge days. A total of 3,374,660 m³ of treated effluent was discharged into Pothole Lake.

The utility of the Pothole Lake discharge diminished with time, and in 2008 the City tendered a design/build project for the construction of a direct discharge system into the Yukon River from the long-term storage pond. One of the problems was that the water licence dictated a short time period when the City could discharge effluent into the lake. This period was only 90 days from August to October. Even though approximately 3,500,000 m³ of treated sewage was released into Pothole Lake each year, this wasn’t enough to completely empty the long-term storage pond. In addition, the exfiltration capacity of Pothole Lake was diminishing.

This situation made direct effluent discharge into the Yukon River an essential facility capability for Whitehorse. The successful design/build for the direct effluent discharge was $5.6 million and the work was completed in 2009.

The Livingstone Trail Environmental Control Facility remains a showcase project to this day, demonstrating the opportunity for a sewage lagoon system to produce a very high quality sewage effluent at high latitudes in Canada.

Ken Johnson is with Stantec. This article appeared in ES&E’s September/October 2014 issue.