At Gold Bar, solid, organic, and chemical waste is separated from wastewater so that it can be safely re-introduced into the ecosystem. Wastewater enters the plant from two large sewer channels, then undergoes several treatment phases before it is ready to exit the plant and enter the North Saskatchewan River via the plant's main outfall pipes.
Because of the nature of the city sewer system, with its combined (sanitary and storm water) sewers, the Gold Bar Plant is permitted to bypass secondary/tertiary treatment during periods of wet weather flow as set out in its Approval To Operate. This means that some portion of the flow will receive only primary treatment, while the remainder will receive full tertiary treatment. (In 2009, secondary bypass flows were 1,395 ML, or 1.5% of the total treated flow of 92,887 ML.)
Gold Bar treats an average of 260-270 million litres per a day with a peak primary treatment design capacity of 910 million litres per day, and peak secondary/tertiary treatment capacity of 420 million litres per day. Note: 310 million litres per day is the plant's overall design capacity.
Process Schematic
See a schematic of the following treatment process.
Pre-treatment
Large solid matter and grit is screened and removed from the raw wastewater. As it passes through the aerated grit tanks, large rakes and screens separate the larger solids material (plastics, paper, rags, metal, grit, etc.) from the influent stream. This is necessary to prevent damage to equipment, and allow for the following treatment procedures to focus on removing the organic matter and pollutants.
Primary Treatment
Once all the grit and screened solids have been removed and eventually trucked to the landfill, the wastewater enters the primary clarifiers. It still contains a large amount of sludge and scum (partially-dissolved waste). In the deep clarifier tanks, the heavier sludge settles to the bottom while the scum rises to the top. Both sludge and scum are then scraped and skimmed off and piped away for separate treatment, while the primary effluent travels on to the next treatment phase. By the end of this process, approximately 50% of pollutants are already removed.
Enhanced Primary Treatment
During wet weather periods the combined sewers 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 from backing-up into household basements. With recent changes at the Gold Bar Plant, however, the amount of untreated overflow entering the river is being greatly diminished. These overflows are directed to Enhanced Primary Treatment Clarifiers, which allow the plant to take in and treat more peak seasonal flows.
For a detailed description of the EPT process, see How it Works.
Secondary Treatment
The remaining approximate 50% of pollutant loading is more difficult to remove. Most of these solids will not settle out by gravity alone and therefore cannot simply be scraped out by rakes or machines.
However, rather than use chemicals to aid treatment, Gold Bar employs biological reactions to further clean the effluent. In the bioreactors, microorganisms are introduced into the primary effluent in ever greater number. As they continue to grow and multiply, the microorganisms feed on the dissolved organic matter and other pollutants, breaking them down.
Tertiary Treatment
Modifications to the secondary bioreactors allow the microorganisms to perform the further task of removing even more impurities, namely the nutrients phosphorus and ammonia. Once these nutrients and the dissolved solids have been take care of, the clear final effluent is ready for disinfection.
Final Clarification
The effluent is then sent to the secondary clarifiers. As in the primary clarifiers, gravity and mechanical rakes separate matter from the effluent. In this case, the microorganisms bind together as a 'floc', settle to the bottom and are separated by mechanical rakes. Most of this floc is channeled back into the bioreactors, while a small portion is removed and broken down with the rest of the solid waste. All solid waste is treated separately in fermenters or digesters. Again, no chemicals are used.
Membrane Filtration
A small portion of the treated wastewater (~5%) is diverted for 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 but act as a barrier to even the smallest bacteria. Filtering the final effluent is referred to as ‘effluent polishing’.
UV Disinfection
Before being returned to the North Saskatchewan River after almost 18 hours of treatment, the clear wastewater effluent is disinfected by high intensity ultra-violet light. In a few short minutes, this chemical-free process renders the treated wastewater safe for contact through different recreational activities (e.g. boating, wading, fishing).
The water is now treated and meets regulatory standards to be returned to the North Saskatchewan River via the plant's main outfall.