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Enhancing wastewater lagoon nitrification in cold climates

Cold Climate Focus

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BioCord Reactors
A bubble curtain surrounding the BioCord reactors keeps water above them ice free during winter months.

By Christine Gan and Kevin Bossy

Regulatory bodies continue to emphasize the importance of more stringent discharge guidelines, so many northern and cold climate communities face challenges in biological treatment. In particular, ammonia removal during cold weather conditions can be difficult to achieve, due to the reduced metabolic rate of the already slow-growing nitrifiers in cooler waters.

To make things even more complicated, many smaller communities that experience year-round or seasonal freezing temperatures utilize wastewater stabilization pond or lagoon facilities for treatment. These facilities are especially susceptible to the effects of cold weather on biological treatment due to their size and exposure to the natural elements.

With the increasing pressure to maintain high effluent quality, along with growing populations and increased wastewater volumes, wastewater stabilization pond or lagoon facilities may consider costly upgrades, requiring new infrastructure or mechanical plants. However, there are more cost- and energy-efficient solutions to enhance biological treatment and better meet target effluent ammonia concentrations, even in cold weather conditions.

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Customized solutions for cold weather nitrification

Recognizing the need for “easy to install and operate” solutions for cold weather nitrification, Bishop Water Technologies developed and tested a semi-passive fixed film treatment system with the ability to enhance biological wastewater treatment and nitrification at low temperatures.

In relatively warm weather conditions and under typical loads, an aerated lagoon may contain enough proliferating bacteria to achieve even stringent discharge limits. However, as water temperatures begin to drop, the growth rate and metabolism of these bacteria decline sharply. Nitrifiers in particular are especially susceptible to lowered water temperatures, with nitrification rates becoming increasingly inhibited as temperatures drop below 15°C. As these bacteria stop proliferating and are washed out with the hydraulic flow, no amount of aeration will be able to return the cell to the treatment level seen previously during summer months.

To enhance nitrification in the winter, attached growth systems can be installed into the lagoon at a time when microorganisms are in an ideal environment. As temperatures drop, the concentrated biofilm layers remain active and attached to the media surface, resulting in continuous treatment without washout as water temperatures are lowered.

The treatment system developed by Bishop Water utilizes modular and customizable BioCord™ reactors, which provide optimal growth and proliferation conditions for contaminant-reducing microorganisms. Each modular unit is comprised of a unique media substrate that provides a high surface area for the development of a stable, robust biofilm.

As the microorganisms form in multiple layers on the charged polymeric fibres, the development of microclimates within the biofilm insulates inner-layer microorganisms from the cold water. Thus, nitrification continues as water temperatures drop. In addition, the pressurized aeration system integrated underneath each reactor to deliver oxygen to the proliferating biofilm, also has the benefit of providing extra warmth to wastewater surrounding the reactor.

The movement of fine bubbles, also used in a bubble curtain surrounding the reactors, keeps the water above the reactors open and the system operational even at sub-zero ambient temperatures. Thus, unlike other biofilm technologies, the BioCord units can be installed directly within a lagoon or basin and require a minimal footprint.

To test the system’s ability to act as a solution for enhanced contaminant removal in cold climates, Bishop Water conducted two pilot-scale studies to assess the reactors’ nitrification rates at cold temperatures.

Fleming College’s cold chamber study

An environmental cold chamber study was performed in 2015 in collaboration with Fleming College’s Centre for Alternative Wastewater Treatment. During this study, raw septic wastewater was fed into four 1000-L experimental tanks operating in parallel, each containing one BioCord reactor. An additional 1000-L tank containing diffused aeration and a mixer was employed as a control. Starting at 20°C, the ambient temperature in the cold chamber was gradually lowered until the chamber reached 1°C. Wastewater temperatures and contaminant concentrations in the influent and effluent were monitored over time.

Chart – Ammonia concentrations
Total ammonia concentrations in BioCord effluent at water temperatures below 6°C

Results showed that the tanks containing BioCord technology showed much higher reductions in ammonia than the control and were able to achieve effluent ammonia concentrations as low as 0.75 mg/L at wastewater temperatures below 5°C.

Cold weather pilot

In late 2018, a six-week cold weather pilot project was conducted in a city in central western Canada. Two BioCord reactors were set up in series to receive lagoon wastewater as temperature and contaminant reductions were monitored over time. At water temperatures below 6°C, the system was able to consistently reduce influent ammonia concentrations by an average of 94%.

A lagoon receiving the same influent wastewater, but not containing a BioCord reactor system, was only able to produce ammonia reductions of less than 3%.

Christine Gan and Kevin Bossy are with Bishop Water Technologies. This article appears in ES&E Magazine’s February 2019 issue.

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