Pushing the Limits of 

Cold Temperature Nitrification

By Wes Ripple, NHDES

Concerns about cold temperature nitrification usually arise when water temperature in the biological treatment system drop 5 degrees Celsius or below. At this temperature the nitrifying bacteria responsible for oxidizing ammonia tend to go dormant. If this happens, it usually means effluent violations for those plants with ammonia limits.

If you operate an activated sludge plant in New Hampshire this is usually not a huge problem since most aeration basins here probably don't get much colder than 7 to 8 degrees Celsius during the dead of winter. A common process control technique for dealing with cold temperatures is to increase your solids inventory. This helps to insure you will maximize the number of nitrifiers in the system. Hopefully, having enough bacteria on hand will help to overcome their sluggishness when the temperature nose dives.

For the aerated lagoon operator, however, complying with ammonia limits is more of a challenge. Temperatures of 0 degrees Celsius are not uncommon, especially when the ice is 2 feet thick. Anything left alive in that environment, including the nitrifiers, are going to be moving pretty slowly. Most lagoons do seem to nitrify fairly well during the summer months, often achieving ammonia concentrations of well below 1.0 mg/l. They do this eighth a very low mixed liquor suspended solids concentration ( if you can call it that ) of 150 mg/l, on a good day! Unfortunately, lagoon operators do not have the capabilities to increase the system's solids inventory. So, during the winter months, these systems are at a severe disadvantage, with ammonia violations running rampant from December through early summer.

Significant research has been devoted to understanding lagoon nitrification and finding ways to manipulate lagoons so that they can nitrify year round at 0 degrees Celsius. That may be asking a lot, but the stakes are high. The conventional approach to achieving ammonia limits is to convert the lagoon to activated sludge, which is guaranteed to nitrify all the time. But the price tag will be in the millions of dollars, hardly a cheap solution. And with every year that passes, the possibilities of additional lagoons having ammonia limits added to their permit increases.

One research project under way at the Exeter, New Hampshire aerated lagoon system seeks to increase the nitrifier population so that the lagoons have at least a fighting chance. Exeter is a worst case scenario, with summer ammonia limits of 1.0 mg/l and winter limits of 2.0 mg/l. Since nitrifying bacteria are not free swimming organisms, they must be provided with a home upon which to grow. The home can be anything that has a lot of surface area, is nontoxic, and is not biodegradable. In this case we are doing a s side by side comparison of two different types of synthetic media. One type is a commercially marketed Japanese product designed specifically for use in wastewater treatment and is referred to as looped cord media going under the trade name Biomatrix. Picture a nylon rope with many of the starnds pulled out to form thousands of mini loops along the length of the cord. The nitrifiers grow on the surfaces of the loops. 3,800 feet of this material along with 4 support frames was purchased from Biomatrix Technologies Inc. of Lincoln, Rhode Island.

The second media was the thought of Scott Butler, Chief Operator of the Exeter lagoons and responsible for the day to day operations of the pilot plant. here we used 3M scrubbing pad material manufactured by the 3M Company of St. Paul, Minnesota. This is the same material used on the bottom of floor scrubbing machines and on abrasive kitchen sponges. The material was purchased in a roll 3 feet in width and 132 feet long. Its was cut into 3 foot squares and hung on 4 homemade support frames. The material provided substantial surface area for nitrifier attachment.

The media and frames are immersed in a 4,000 gallon rectangular storage tank. The tank is divided down its length by a center dividing wall, effectively creating two separate tanks out of one. each tank holds one of the types of media for the side by side comparison study,. Both tanks are separated into 5 individual compartments through the use of cross baffling. each of the first four compartments contains one frame of media. the final compartments do not contain any media and are basically used for clarification of the sloughed solids. Aeration is provided using conventional lagoon aeration tubing and a blower.

They pilot plant operates in a plug flow mode and receives its influent from the discharge pipe of the final lagoon via submersible pump. Flow rates for winter operation are designed to be low, averaging 0.75 gpm. This provides a detention time of approximately 1.5 days per side ( the longer the better ). The influent and effluent from both tanks are monitored for typical nitrification on a regular basis.

The pilot plant went online on October 12, 1999. As of this writing, thirteen weeks of certified lab results for ammonia have been received, covering the period from October 28, 1999 through January 20, 2000.

Exeter Pilot Plant Ammonia Results
click on graph for larger size

The results show that both media have consistently produced an effluent containing less ammonia than found in the lagoon. With the exception of week 9, the 3M media regularly outperformed the Biomatrix media. During December, weeks 6 through 10, lagoon effluent ammonia concentration steadily increased as lagoon temperatures dropped. The pilot plant results of the same period held fairly steady. Pilot plant temperatures averaged 5.2 degrees Celsius. In terms of percent removal rates for ammonia, Biomatrix averaged 81% and 3M averaged 87% removal for weeks 6 through 10. If week 9, which is inconsistent with the results for the rest of the month, if thrown out as a fluke or lab error, removal rates for Biomatrix increase to 84% and 3m increases to a whopping 97%!

As January arrived, lagoon temperatures plummeted and lagoon ammonias continued to increase. For the first 2 weeks of January, which corresponds to week 11 and 12 of the project, the pilot plant did remarked well. Pilot plant temperatures averaged around 2.5 degrees Celsius for these two weeks. The medias averaged 90% removal for Biomatrix and 99% for 3M. Actual ammonia concentrations as measured on January 12 (week #12) were 13.6 mg/l for the lagoon, 1.89 mg/l for Biomatrix, and 0.28 mg/l for 3M.

The reality set in. The relatively mild winter that we had been experiencing turned bitterly cold. Air temperature settled in at well below ) degrees Fahrenheit for over a week straight. Lagoon and pilot plant temperatures plunged to ) degrees Celsius. Unlucky week #13 presented significant problems. In addition to the low temperatures, the pilot plant discharge and feed pipes froze solid. Scot Butler went to considerable lengths to keep the pilot unit functioning during this time. Hay bales were brought in for added insulation. Needless to say, ammonias sky rocketed but remained below that of the lagoon. Biomatrix showed a 16% reduction while 3M reduced it by 32%. We are not sure if all of the ammonia increase can be attributed to temperature alone. During the freezing period pilot plant flows were intentionally doubled in order to privet additional freezing of the discharge piping. This also had the effect of reducing the pilot plant detention times by half. Nitrification was never totally lost and continues to show ammonia reduction well into February.

This pilot program will continue through October 2001. So far we have learned several important facts: 1.) Lagoon effluents can be improved through the addition of synthetic media. 2.) The media doesn't necessarily need to be made specifically for wastewater treatment. 3.) Nitrification can proceed to significantly low numbers (0.28 mg/l NH3-N) at temperatures down to 2 degrees Celsius. 4.) An undetermined degree of nitrification will continue at temperatures as low as 0 degrees Celsius. We are currently evaluating the use of recycle pumps in the pilot plant in order to pass the water through the media several times prior to its discharge.

Many thanks go to Victoria Abbey, Exeter Water and Sewer Superintendent, and Keith Noyes, Exeter Public Works Director for allowing this project to proceed. Special thanks to Scott Butler for his diligent efforts at keeping the pilot running, and Ernie Braham, assistant operator for his invaluable help. A big thank you to the entire Water and Sewer staff for helping to assemble the project (it was a bigger job than imaged.) Most of all a very special thank you to Chuck Conway and the New England Interstate Water Pollution Control Commission for providing the grant money to make it all possible.