wastewater
Golder Associates / Urban Systems
higher concentration would protect the environment opened the door to less expensive treatment alternatives. Golder Associates and Urban Systems were retained to complete a detailed concept study. The study concluded that a lagoon could be modified by partitioning it with floating membranes and adding equipment to create the BNR process conditions. The system would achieve similar tertiary nutrient removal levels as a more expensive mechanical BNR process. The resulting lagoon-based tertiary treatment process that was designed and constructed is a simple but state-of-the-art, sustainable solution that will save the city millions over the 20-year life of the facility. It also uses existing operator skills and reduces greenhouse gas emissions.
bic environmental conditions. These mechanical plant configurations can be quite complex: the first BNR process in North America consisted of 21 individual bioreactor tanks. While simpler configurations have since been developed, BNR processes are still generally considered to be among the most complex and expensive domestic wastewater treatment systems to build and operate. In contrast, lagoon wastewater treatment processes are generally considered to be among the simplest and lowest cost treatment processes. Their primary disadvantages are the large amount of land that they typically require and the inability to significantly alter or control the treatment conditions. They are also generally considered to be, at best, a passive means of achieving secondary wastewater treatment.
ing recirculation pumps, aeration equipment, and mixers, along with a concrete wall and two floating bafflewalls to partition the lagoon into aerobic, anoxic and anaerobic zones, as shown in the photo below. The concrete wall that extends into the lagoon between the anaerobic and aerobic zones provides structural support for the mechanical equipment and electrical power required for the tethered mixers and recirculation pumps, as well as serving to anchor the two floating baffle-partitions (shown in red) that separate the three zones. Two new clarifiers were constructed to retain and recycle bacteria back to the front of the modified aerated lagoon. The treated effluent is disinfected using ultraviolet light, eliminating the need for chlorine. This approach addresses the Canadian EnviMechanical BNR vs. lagoons ronmental Protection Act requireAs illustrated in the diagram on p. 24, Modifying the lagoons ment that effluent is free of toxic biological phosphorus and nitrogen Ignoring this technology bias, the chlorine residuals. removal is achieved in a mechanical project team developed a design conThe design modified an existing tertiary treatment plant by recirculatcept that involved modifying one of anaerobic lagoon for the purpose of ing wastewater and bacteria between the existing secondary treatment lapre-treating the raw wastewater while structural tanks that are designed to goons to achieve tertiary treatment. generating volatile fatty acids that are maintain anaerobic, anoxic and aeroThe modifications included installessential to biological nutrient removal. The anaerobic lagoon was covered to isolate the water from oxygen in the atmosphere, as well as to capture methane. Although the city is currently flaring the methane while decreasing greenhouse gas emissions by an estimated 8,900 tonnes per year (carbon dioxide equivalent), the intent is to consider energy recovery options in the future. Several of the existing aerated lagoons were also retained to Above: BNR-modifed lagoon with mixers, recirculation pumps, aeration diffusers and concrete wall in place. The red sustainably treat 5,500 lines illustrate the location of the floating baffle partitions. They are anchored to the concrete wall and separate the anaerobic, anoxic and aerobic zones.
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