Through continual upgrades and innovations, Gold Bar, opened in 1956, remains at the forefront of wastewater treatment technology. The treatment processes, technologies and equipment that are described below provide insight into one of the most innovative wastewater treatment operations in North America. Its tradition of wastewater treatment excellence is evident today with the plant’s Wastewater Research and Training Centre, part of the Edmonton Waste Management Centre of Excellence.
Infrastructure
One of Canada’s largest Class IV wastewater treatment plants
Includes enhanced primary treatment (for wet weather flows) and UV disinfection
ISO 14001-certified (pending)
Features a membrane-based wastewater recycling facility which supplies local industry with reclaimed water
Has four wet ‘scrubbers’ for odour control
Recovers approximately $1 million in annual fuel savings by using biogas produced on site
Produces treated effluent that consistently surpasses regulatory standards
It treats more than 100,000 million litres annually – enough to fill an Olympic-size swimming pool more than 110 times a day.
Plant Capacity
Current Design Treatment Capacity: 310 million litres per day
Peak Primary Treatment Capacity: 910 million litres per day
Peak Secondary/Tertiary Treatment Capacity: 420 million litres per day
Headworks (grit removal & screening) Capacity: 1600 million litres per day
Eight anaerobic digesters for sludge treatment. Combined Digester Capacity: 67.5 million litres
Biogas Produced (2011): 10.7 million m3
Average Daily Flow: 255 million litres per day
Highest Peak Daily Flow (2007): 747.1 million litres (May 5/07)
Highest Annual Flow: 102,710 ML (in 2011). Despite increasing dry weather flows, pronounced yearly fluctuations are due to effects of annual precipitation entering combined sewer system (~15% of city’s area).
Biosolids to Farmland: Since the program’s inception in 1978, close to 500,000 dry tonnes of biosolids have been distributed over 16,000 hectares of farmland
Biosolids to Compost: Since the composter’s start-up in 2002, more than 123,000 dry tonnes of biosolids have been used to make compost products for sale
Treatment Duration – 17-18 hours from start to finish for full tertiary treatment
Treatment Process
Pre-treatment
As incoming raw wastewater flows through Gold Bar’s aerated grit tanks, heavy material (sand, rocks) drops to the bottom while large, lighter material (plastics, paper, rags, wood, etc.) is trapped by the large bar screens at the ends of the tanks. Removing this material is necessary to prevent damage to equipment, and allow for the following treatment processes to focus on removing the organic matter and pollutants.
Once the large inorganic solid matter and grit is removed from the raw wastewater, it is trucked to the City of Edmonton Waste Management Centre.
Enhanced Primary Treatment (for wet weather flows)
The wastewater collection system comprises two types of sewers – sanitary and combined sewers. Combined sewers, which are typical of older areas of the city, convey sanitary and storm water in a single pipe. In newer areas a sanitary and storm water system do this work separately.
During heavy wet weather periods the combined sewers (~ 15% of total system) often capture more runoff than they can handle, resulting in wastewater overflows to the river. Without such overflows there would be no way of preventing untreated wastewater in those areas from backing-up into household basements or from spilling out of manholes and flooding roadways.
With recent upgrades at the Gold Bar Plant, however, the amount of untreated overflow entering the river is greatly diminished. These overflows are instead directed to Gold Bar's Enhanced Primary Treatment clarifiers, after the pre-treatment step, which allows the plant to take in and treat more peak seasonal flows (e.g. summer storms and spring snowmelt).
Primary Treatment
During normal, dry weather periods, the wastewater effluent flows directly from the pre-treatment stage directly to the regular primary treatment clarifiers where approximately 50% of the wastewater’s impurities are removed. In the deep clarifier tanks, the heavier solids (sludge) settles to the bottom while the lighter solids (scum) rises to the top. Both sludge and scum are then scraped or skimmed off and piped away for solids handling, while the primary effluent (as it is referred to folloing this stage) travels on to the secondary treatment phase.
From this point forward in the treatment process, the solids and liquids are dealt with separately for the most part.
Solids Handling
Fermenters
Primary sludge’s first stop is the fermenters – four unheated, anaerobic (no oxygen) vessels. After 3-7 days, the thickened sludge is sent first to a blend tank, to be mixed with secondary sludge (see Final Clarification) just prior to its longer stay in the digesters where it undergoes further treatment. Liquid from the fermenters, which has become enriched by what are known as 'volatile fatty acids' or VFAs, is used in the secondary treatment process and is an essential ingredient in what's known as 'Biological Nutrient Removal'.
Digesters
Anaerobic digesters, which are maintained at 37°C, serve in breaking down and stabilizing the massive amounts of organic sludge removed during the treatment processes. The digesters do in weeks what in nature would take many months.
Some of the odorous biogas (65-70% methane) produced by the sludge digesting or breaking down is captured and used to heat the Gold Bar plant buildings and digesters. Digested sludge (~ 2-4% solids) is pumped some 11 KM to offsite storage lagoons at Clover Bar for further thickening, processing and distribution.
Biosolids
As the solids and liquid gradually separate, the liquid is pumped back to the Gold Bar plant for treatment. The thickened sludge, referred to at this stage as ‘biosolids’, is handled by the City of Edmonton’s Waste Management Branch, who either apply the biosolids on agricultural fields or use it as an input for their co-composter, also located at their Clover Bar site.
Biosolids contain significant concentrations of nitrogen and phosphorus; they have been used around the world for decades as a fertilizer and a soil amendment.
Secondary Treatment
The remaining organic matter in the effluent are dissolved solids that will not settle out by gravity alone and therefore cannot simply be scraped out by rakes or machines. So Gold Bar harnesses the wastewater's biological properties, namely the microorganisms (bacteria, fungi, protozoa) and organic matter, to further clean the effluent.
Microorganisms feed on the dissolved organic matter and other pollutants, breaking them down. The microorganisms multiply in large bioreactor tanks and their appetite for waste materials is further stimulated by mixing in large volumes of air.
Biological Nutrient Removal
Modifications to the secondary bioreactors allow the microorganisms to also remove the nutrients phosphorus and ammonia which can be harmful if released back to the river in high concentrations. Once these nutrients have been taken care of, the secondary effluent is sent for final clarification.
Final Clarification
Upon leaving the bioreactor, effluent enters the secondary clarifiers which function much like the primary clarifiers. In this case, the well-fed microorganisms bind together as a 'floc', settle to the bottom and from there are removed by mechanical rakes. By this stage almost all the impurities have been removed.
Most of this settled floc is channeled back into the bioreactors, while a small portion is removed, thickened and pumped to the digesters for solids treatment.
UV Disinfection
Before being returned to the North Saskatchewan River after almost 18 hours of treatment, the clear wastewater effluent is disinfected using high intensity ultra-violet light. In a few short minutes, this chemical-free process destroys any of the harmful bacteria that remain and renders the treated wastewater safe for contact during recreational activities (e.g. boating, wading and fishing).
Following disinfection, the wastewater, having been treated in accordance with regulatory standards, can now be safely returned to the North Saskatchewan River via the plant's main outfall.
Membrane Filtration
About 5% of the final treated effluent bypasses UV disinfection and goes straight to membrane filtration, producing high-grade process water for industry. Membrane filters are a series of porous synthetic strands containing millions of microscopic pore openings that allow water to pass through but act as a barrier to even the smallest bacteria. This filtering process is referred to as ‘effluent polishing’, and produces the same disinfection quality as if it were treated by UV.
Expansions and Upgrades
Plant Expansions
Since its beginnings, the plant has undergone two major processing capacity (primary and secondary treatment) increases; they occurred a decade apart, with the last in 1981. Solids handling capacity increases occurred right up to 2011, with the addition of 2 new digesters. The plant site has room on the eastern perimeter, if needed and if the same configuration is followed, for three more bioreactors/clarifiers or up to 126 million litres per day.
Treatment Upgraded
Improved effluent quality - tertiary treatment upgrade began in 1995; completed in December 2001, three years ahead of mandated deadline from province.
Tertiary Treatment Description
Disinfection of final effluent and nutrient (phosphorus and ammonia) removal. Both disinfection and nutrient removal are non-chemical processes; they are accomplished by high-intensity UV light and Biological Nutrient Removal (BNR) respectively. Facilities to support the BNR upgrade component include primary sludge fermenters; sludge blend tanks; Waste Activate Sludge (WAS) thickeners; two new bioreactors/clarifiers; and a new 3,500 horse power blower.
