As if the many variables of wastewater treatment aren’t enough to keep a treatment plant operator busy from day to day, winter weather makes the task even more challenging. The onset of cold temperatures brings a whole new set of variables that have to be monitored.

What’s worse, cold temperatures slow down the activity of the system’s microorganisms, which do the important work of breaking down the wastewater contaminants. This means a larger colony of bacteria is needed to do the same work that a smaller colony was able to do in warmer temperatures.

In order to adequately handle the amount of MLSS in the system, at least two options are available: adjust the return activated sludge or waste activated sludge parameters, or use bioaugmentation to boost the level of microbial activity. The benefits and important considerations of bioaugmentation will be the topic of this article.

Effects of cold on WWTPs

Winter can drastically change the conditions at a wastewater treatment plant. For example, freezing temperatures force plants that have activated sludge or lagoon systems to implement various cold weather measures such as deicing, supplying hot air, adjusting aeration, and sometimes even covering aeration tanks to keep the wastewater flowing and processes working.

Lagoons are especially at risk in cold weather, with the potential to freeze at the surface in extremely low temperatures, but even the performance of indoor aeration beds can suffer from colder temperatures during winter.

In spite of common precautions taken, wastewater is still at a disadvantage because of reduced, if not freezing, temperatures. For every temperature drop of around 10 degrees F, microorganisms lose about one log of potential growth, a 90 percent reduction.

The optimal temperatures for the growth of bacteria that help degrade wastewater contaminants and reduce BOD, TSS, sludge and odor is a range of 77 to 113 degrees F. Bacteria continue to grow sufficiently at temperatures of 59 to 113 degrees F.

However, when the water temperature reaches the lower ranges of these bounds (below 59 degrees F), the metabolism and growth of helpful microorganisms slows down extremely, which in turn slows the contaminant breakdown process. To make matters worse, filamentous microorganisms can unfortunately experience more growth during the winter, causing other potential problems such as a decrease in DO or an upset in pH and food to microorganism ratio.

The following factors influence bacterial growth and should receive especially close monitoring during winter seasons:

Countering winter upsets with bioaugmentation

One of the best ways to keep a healthy system functioning and to prevent or treat plant upsets in the winter is through bioaugmentation. By adding extra microorganisms to the system, treatment plant operators can create a colony of biological workers that will be able to keep up with the load of food (contaminants) that needs to be digested in the system.

Bacteria that can grow and function at temperatures as low as 41 degrees F should be chosen according to the type of wastewater contaminants they digest best. Specialized blends of bacteria have been developed to target the major contaminants found in the following types of treatment systems and/or waste:

• Lagoons
• Anaerobic digesters
• Dairy waste
• Refinery and petrochemical waste
• Chemical waste
• Fruit and vegetable processing waste
• Pulp and paper waste
• High grease waste
• Surfactant-rich waste
• Fish farm waste
• Manure

Maximizing performance

To achieve the best performance possible, the microorganisms should be activated overnight in a tub of water at room temperature. This awakens the “bugs” so they’re active and ready to start “eating” contaminants and growing when they are added to the lagoon or secondary digester. This pre-activation is especially important when the temperature drops below 50 degrees F.

DO should continue to be monitored, because even though the amount of DO in wastewater tends to increase with colder temperatures, the use of more microorganisms will require more oxygen to support growth, offsetting the DO gain. Optimal DO concentration (see table) is 2 mg/L. If necessary, this can be achieved by performing additional aeration.

Nutrients are another important factor for achieving the best waste treatment performance of microorganisms. Bacteria need nitrogen and phosphorus in a BOD:N:P ratio of 100:5:1. More nutrients can be added if these are deficient. On the other hand, effluent water also needs to be analyzed to make sure nutrient levels do not exceed required levels.

Cold-weather bioaugmentation in action

A practical example of how bioaugmentation can be used to enhance secondary treatment in cold weather is seen in the use of specialized bacteria for the winter startup of a pulp and paper mill and its accompanying wastewater system (aerated lagoons).¹ After being shut down for more than a year, the plant had a new owner that wanted to start up the plant during January and February, two of the coldest winter months.

Kraft stock production meant that wastewater output was approximately 60,000 m³ (78,477 yd³) a day at a BOD5 level of approximately 350 mg/L (2.9x10^-3 pounds per gallon). To make matters more difficult, the temperature in the first lagoon was 32 degrees F at the inlet and between 41 and 53.6 degrees F at the outlet. The heating for the lagoon would come from plant processes, which meant no warming could occur prior to startup. This also left only a very short time to build up biomass in the treatment system.

Basic steps were taken to get the biologically driven lagoon system started. Nitrogen and phosphorous were added in the typical manner, and a truckload of sludge was brought in to provide a dose of biologically rich medium to facilitate waste treatment processes in the lagoon.

Because of the quick startup required, additional microorganisms were added in powder form to augment the biodegradation activity of the microorganisms naturally occurring in the sludge. The extra bacteria were specially formulated to target pulp and paper waste at low temperatures.

For the first six days, a shock dose of 132 pounds per day of microorganisms was added. For the next seven days, this was reduced to 66 pounds per day, then to 33 pounds per day the third week. Extremely cold temperatures prompted an increase of the dose back up to 66 pounds per day the fourth week. After this, a maintenance dose of 17.6 pounds per day was applied until the end of the winter for a total addition of 2,425 pounds of bacteria.

The startup was successfully completed, and all discharge levels were met, with bioaugmentation playing an important role in facilitating the quick startup. The same type of method can be used in similar situations to offset the slower activity of microorganisms and the increase of filamentous microorganisms during winter.

Acknowledgements: Special thanks to Diana Di Marco, Technical Sales Director at Bionetix International, for sharing various winter wastewater treatment insights.

References: 1. Bionetix International. “The Use of BCP57CT in the Winter Startup of Wastewater Treatment in a Pulp and Paper Mill.” Retrieved 25 Jan 2018 from http://www.bionetix-international.com/resources/casestudies/cs_bcp57ct.pdf

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About the authors: Tonya Decterov holds degrees in soil chemistry, environmental engineering, and waste management and is a technical sales representative at Bionetix International, a provider of natural biological wastewater treatments. She can be reached at tdecterov@bionetix.ca.

Julie Holmquist is content writer at Cortec Corporation, which specializes in providing more environmentally friendly corrosion solutions and is the parent company to Bionetix International. Julie can be reached at jholmquist@cortecvci.com.