As the cold winter months approach, many wastewater treatment plant operators start to worry about how the frigid temperatures will impact their treatment processes. Extreme cold can slow down or even halt important biological processes like nitrification, putting facilities at risk of permit violations. However, with careful planning and the right technologies, effective wastewater treatment can be maintained even when the mercury plummets.
How Cold Temperatures Impact Wastewater Treatment
The biggest challenge cold temperatures present for wastewater treatment is reduced biological activity. Bacteria and other microorganisms are less active in cold water, slowing down critical treatment processes:
- Nitrification: The oxidation of ammonia to nitrate, performed by nitrifying bacteria, is highly temperature-dependent. Nitrification rates can drop 50% or more at 50°F compared to 68°F. Below 41°F nitrification often ceases entirely.
- Organics removal: Rates of biochemical oxygen demand (BOD) removal also decline in cold temperatures as bacterial metabolism slows. Older treatment facilities relying on non-heated, outdoor treatment units are most vulnerable.
- Settling/separation: Thickening and settling of solids is hindered as water viscosity increases and biomass goes dormant. Poor solids separation can cause violations of permit limits for BOD, TSS, and ammonia.
- Disinfection: At low temperatures, chemical disinfection processes slow while pathogens remain active longer. It becomes harder to achieve adequate disinfection without increasing chemical dosage.
Maintaining Compliance
Permit noncompliance due to cold weather is unacceptable, so treatment plant operators need strategies to continue effective treatment through winter. The EPA recognizes temporary cold weather issues, but facilities still must take proactive measures to maintain their wastewater treatment processes. Critical strategies include:
- Enhanced monitoring: Increase testing during cold snaps to catch permit excursions early. Target key parameters like ammonia, BOD, TSS, fecal coliforms.
- Adjusting treatment processes: Increase hydraulic residence time to allow slower biological processes to catch up. Boost aeration to maintain dissolved oxygen. Upgrade disinfection systems to handle higher chemical demand.
- Using cold-active technologies: Supplement existing processes with specialized cold-weather technologies like rotating biological contactors (RBCs), sequencing batch reactors (SBRs), and breakpoint chlorination.
- Heating systems: Ensuring raw influent, clarifiers, aeration basins, anaerobic digesters, and disinfection systems stay warm enough for biological activity to continue. Heating is costly but sometimes necessary.
Innovative Cold-Weather Wastewater Solutions
In addition to traditional upgraded wastewater infrastructure, some innovative new technologies are specially designed to continue removing ammonia, BOD, and solids even in frigid conditions without costly heating systems:
Nitrifying Bioreactors
Fixed-film bioreactor products from suppliers like Biowater Technology utilize cold-active ammonia oxidizing bacteria (AOB) immobilized on structural media. Influent flows over these robust biofilm reactors, allowing the AOB community to convert ammonia to nitrate through nitrification even at 39°F or below. These compact bioreactors create treatment zones for reliable nitrification outside the main wastewater infrastructure, avoiding bottlenecks.
Advanced Aeration Systems
Fine bubble aeration technologies distribute oxygen efficiently for continued BOD reduction and nitrification. For example, Environmental Dynamics International’s Sludge Reduction System couples patented turbo-accelerator transfer devices with mixers, subsurface aerators, and ultrasonic units. The powerful aeration prevents biomass settling and lowers treatment basins’ oxygen demand despite freezing air temperatures.
Rotating Biological Contactors
Attached growth processes like rotating biological contactors (RBCs) are less impacted by cold temperatures than suspended growth systems. As disks slowly rotate through wastewater, cold-resistant biofilm on the RBC surfaces removes BOD, TSS, ammonia, and other contaminants. RBCs take up minimal space plus easily integrate into existing infrastructure.
Sequential Batch Reactors
SBRs condense the activated sludge process into timed cycles in one tank, creating excellent operational control for winter conditions. SBRs provide aeration, mixing, settling, decanting, and idling phases in sequence to optimize flocs’ formation and enhance treatment. They adaptle easily to variable winter flows and slow microbial activity compared to continuous-flow systems.
The Bad News: Cold Weather Is Coming
The Good News: We Have Solutions!
With forward-looking preparation and the right cold-optimized technologies, wastewater treatment facilities can stay in compliance through the winter. Proven technologies like advanced aeration devices, nitrifying bioreactors, SBRs, and rotating biological contactors overcome problems like reduced nitrification, BOD removal, and solids separation. Staying abreast of the latest winter wastewater innovations provides plant operators the tools needed to successfully navigate even the most challenging cold snaps. With the proper cold weather treatment strategy, operators can relax rather than worry each time the thermometer plunges.
