Public comment about wastewater treatment operations is rarely positive. Usually, when the phone rings or the newspaper writes a story about the local wastewater treatment plant it has to do with the need to increase sewer rates, improve outdated facilities, biosolids application, or the need for odor control. One Northwest Iowa Wastewater Treatment Plant has been the feature of positive press and frequent "pats on the back" by local citizens in recent months.
Le Mars, IA, a growing community of about 10,000, has had a long-standing and ever-worsening odor problem. Much of the odors were a result of the heavy dairy waste loading at the wastewater plant. With influent CBOD loadings that average over 5,000 lbs/d and sometimes spike to as high as 60,000 lbs/d, odor has been the talk of the town.
The wastewater treatment plant was constructed in 1958. On average, every seven years since then the city has conducted major upgrades to accommodate the increased loadings. The plant consists of preliminary treatment, flow equalization, primary treatment, a deep roughing filter with plastic media, trickling filters, and activated sludge. Sludge is treated anaerobically then thickened to 3-5 percent and land-applied year around.
Mark Simms, Wastewater Superintendent, wanted to find a solution to the odor problem and turn the public perception around.
"It just didn't seem right to be called the Ice Cream Capitol of the World and have a lingering, often very strong odor hanging over the community. Citizens expected better and so we set out to find the answer," Simms said.
First, he established a volunteer citizen task force made up of neighbors, city officials, and interested people to assist the city in evaluating various products. Those trials have included chemical addition, process control improvements, the most recent plant upgrade (1997), and "odor sleeves". The primary job of these people was to monitor and record odor events (or the lack of).
While the task force was helpful, over time the people seemed to lose interest. West Central Environmental Consultants of Morris, MN, was hired to conduct a detailed "odor study" and develop specific recommendations for a long-term solution. WCEC analyzed a number of air and water samples and soon issued a detailed report that identified the sources of the odors and levels of hydrogen sulfide at each source.
While it was found that odor was being released from nearly every plant process, the roughing filter, primary clarifiers, and trickling filters were identified as those with the highest intensity. WCEC went on to make a public report to the City Council recommending that a biofilter be installed to handle the roughing filter odors, and that biocovers be installed over the two primary clarifiers. Estimated equipment and construction cost was $95,000. The City Council approved the recommendation and authorized the expenditure. However, with another major plant upgrade also in the works, the City Administrator wanted first to see what impact the improvements would have, without adding the biofilter and biocovers.
It had now been some three years and there had been many local news stories published about the plant upgrade. Still there were odors. The city's design engineers, McClure Engineering Company of Fort Dodge, IA, was asked to review the WCEC recommendation and do further analysis. While the plant upgrade may have helped slightly, it was not near enough. McClure suggested a number of operational modifications such as increased trickling filter recirculation, increased sludge withdrawal from the clarifiers, and frequent flooding/flushing of the roughing filter. The operational changes were made and again there was improvement. But it wasn't enough. It looked like the biofilter (or an air scrubber) was the only option.
In the spring of 1998, Mike Trotter of McClure Engineering suggested the city try one or two more things before proceeding with the project. He had obtained information on a couple of products and felt that the city should evaluate them. The products included odor sleeves and granules from the Odor Control Company of Phoenix, AZ, which use essential oils technology. Simms agreed to purchase a small quantity to see what happened.
The product comes in 25-pound pails and looks something like kitty litter with a strong fragrance. The product is placed into a 3-foot-long cloth sleeve and simply hung between the odor sources and the public. The sleeves are placed every 8-10 feet along the perimeter of the plant. The granules must be replaced once every three months. It seemed to help, so Simms ordered enough to do about 800 feet of his fence. The sleeves also were hung inside the roughing filter (which is covered by a dome) and in the screenings dumpster.
Citizen input again was solicited by issuing press releases in the local paper. It is interesting to note that since the sleeves have been hung, the 3-4 complaints received came during a time when the roughing filter was being flooded or the primary digester was down for cleaning.
The local paper did yet another story, this time on the "odor sleeves". Again, the phone began to ring; however the calls and public comments were all positive! One man, who has lived next to the WWTP for many years, stated that he had sort of forgotten about the smell this summer. It wasn't until he read the news article that he realized there had been no odor all summer.
Following this, liquid products from the same company were ordered and are now being "misted" into the screenings, roughing filter, and the atmosphere. The liquid products are diluted 150:1 or more. Simms and many others have noticed a significant reduction in odors. Within minutes of starting the misters, the odors were gone. This is not to say that there are never any odors, but they now are manageable.
Annual cost for the products from Odor Control Company is probably less than the cost to operate and maintain the recommended air scrubber or biofilter.
"How do we know it is working?" Simms said. "Because people tell us it is. I often get comments from a variety of people and they are all positive. The only negative feedback has come when we either were flooding the roughing filter, or cleaning our digester and we quickly were able to take care of that by adjusting the liquid products we mist into the air."
