Dawson Wastewater Treatment Project, Canada

The Dawson wastewater treatment project will be Dawson City's second treatment facility. The city is under court order to build a secondary treatment facility by the end of 2011. An initiative of the Yukon government, the proposed plant will be designed and constructed by Corix Water Systems at an estimated cost of C$25m.

On 7 July 2009, the project broke ground when Dawson City partnered with the Yukon government by signing a Memorandum of Understanding to build the project. A test hole was drilled at the project site between 10 and 14 August 2009.

The plant is scheduled for operations in July 2010. While the design and permitting will be undertaken between July 2009 and July 2010, the plant construction will begin from April 2010 and extend up to July 2011.

Plant components

The Dawson facility will use an aerobic activated sludge process called Vertreat (vertical treatment), which will consist of two 1.4m diameter shafts installed 100m deep in the ground. Two steel cylinders for collecting sludge will be grouted to the ground. Air will be supplied through 150hp air compressors. The plant will also consist of a hold tank, head tank, dissolved air floatation clarifier and effluent biofilters.

The reactor will consist of three separate treatment zones. The oxidation zone occupying the upper part of the reactor will include a central concentric draft tube for mixed liquor circulation. Immediately below will be the mixing zone. The bottom of the reactor will act as the saturation zone. The reactor will be installed at a depth of 75-110m using conventional drilling or excavation techniques.

Treatment process

Compared with traditional wastewater treatment plants, the Vertreat technology saves 50% more energy, is compact, robust and requires low maintenance.

Degritted and screened influent stream is mixed with return activated sludge in the hold tank, in this process. An airlift effect caused by the injection of air into the mixing zone draws the influent from the hold tank into the bioreactors. It remains in contact with air and biomass for several minutes before it reaches the head tank.

Due to high reaction rates, a large portion of organic compounds are bio-oxidised in the oxidation zone. The recirculating liquor travels up the riser and flows into the head tank where entrained used gas bubbles release into the atmosphere. To remove any volatile organic compounds the vent stream is treated in biofilters.

A relatively lesser amount of mixed liquid flows from the mixed zone to the saturation zone. Characterised by high dissolved concentration and longer resistance time, this zone creates a high degree of residual biochemical oxygen demand (BOD) oxidation. The high dissolved gas concentration also leads to solid separation in the floatation clarifier.

To ensure that the bottom of the reactor does not accumulate grit or solids, the treated effluent travels from the bioreactor to dissolved air flotation clarifiers. Rapid depressurisation separates biomass from the effluent and also results in well-aerated floc of low density. At the bottom of the clarifiers, biomass thickens to nearly 4% solids.

Treated and high quality effluent flows to the nitrification biofilters. Part of the sludge is recycled back to the hold tank and waste sludge pumps by gravity. Equipped with centrifugal air blowers, the nitrification biofilters nitrify the effluent. Containing acclimatised biomass on porous media, it also reduces residual TSS and BOD to extremely low levels by polishing any organic material that takes more time to biodegrade.

After filtration, the effluent flows to a backwash tank where it is directed either to the filter as backwash water or discharged to the battery limit. Using an intermittent backwash pump, the biofilters are periodically backwashed. The spent backwash is recycled to the hold tank.

Contractors

Sub contractors to the project include Allnorth Consultants, Keith Plumbing and Heating, EBA Consulting, StreamLine Architects, Construction Drilling, Ben's Electric and Dorward Engineering. The Vertreat technology to the plant will be provided by Noram Engineering.

An AECOM team is acting as the owner's representative on behalf of the Yukon government to supervise the plant's design, construction and commissioning.