Why the recent upgrade of the Saline Wastewater Treatment Plant (WWTP) did not include plans for odor mitigation is a not clear. Brian Rubel of Tetra Tech, the company that designed the upgrade and supervised its construction, said he did not even hear about an odor problem until well after the rebuild had begun.
Nevertheless, a recent analysis by Webster Environmental Services has helped to identify the cause and point to solutions. As it turns out, the largest source of odor appears to be a poorly functioning odor scrubber that was installed 20 years ago which has reached the end of its useful life.
On Monday, Saline City Council members heard a preliminary report from Jim Ross of Webster on the studies Webster did in late November of 2016.
The company analyzed both the water and the air at all stages of the waste treatment process. They measured odors by chemical analysis, instruments and an odor panel.
An odor panel is a group of human sniffers. They smell WWTP air samples at various dilutions to determine how much dilution makes the sample odorless. This might be a good task for Mike Rowe on “Dirty Jobs.”
The offensive odors come primarily from small (low molecular weight) sulfur-containing compounds. One of the most common of these is hydrogen sulfide which has a characteristic rotten-egg odor.
Webster analyzed for and detected various sulfurous compounds, but they used hydrogen sulfide as a surrogate for all of them. They deployed instruments called OdaLogs that can continuously monitor this chemical. Placing these instruments at various places in the WWTP, they collected hydrogen sulfide data for a period of one week, showing the rise and fall over time.
In addition to seeking the sources of the odiferous substances, they also monitored the performance of two scrubbers that were designed to pull air away from the sources and employ a chemical process to remove the substances from the air. A Duall scrubber, installed in 1996, and an ERA Tech scrubber, installed in 2000, were both found to have serious problems.
They found that the Duall scrubber was removing under 20 percent of the hydrogen sulfide effluent. The ERA Tech scrubber removed 85 percent, but also had numerous chemical leaks creating a hazard to workers. Furthermore, the company that made it no longer exists.
A properly functioning scrubber should remove more than 99 percent of the offensive molecules it processes. There should also be sufficient airflows to maintain negative pressure in all of the vessels where smells are generated. Saline’s scrubbers failed on both counts.
A pie chart generated by Webster estimated that 62 percent of the noisome vapors that so annoy the neighbors were blowing out of the Duall scrubber. Actually, they determined that 78 percent came from the Duall, but they guessed an additional 20 percent contribution would come from the septage receiving facility which was not receiving septage during the study.
Surprisingly, Wastewater Treatment Supervisor Bob Scull suggested that the Duall might even be making the problem worse, not because it’s contributing to the odor, but because, after removing very little, it blows the aroma up a stack, increasing dispersion.
Though it may come as a surprise to some of the near neighbors of the WWTP, Ross characterized the odors in Saline as low to moderate.
“We’ve seen a lot worse,” Ross said. “It’s bad for you because you have residents on your fence line.”
The recommended solution to the odor problem is to remove and replace the odor scrubbers. The new scrubbers would have greater airflow and would connect to all of the waste treatment equipment that generates odors. They would also add a small scrubber to the southside lift station, an offsite source of some odors.
Rather than a chemical scrubber, like the city now uses, Webster recommends using either an activated carbon adsorption system or an organic biofilter. The two would be similar in cost with the biofilter being slightly more expensive, but the biofilter would also have a lower yearly operating expense.
Unfortunately, that initial cost will not be cheap. Whichever system is chosen, demolishing the old scrubbers, installing the new ones, making all the ductwork connections and adding a small scrubber to the lift station will cost around two million dollars.
In addition, it will not be quick. To further refine the data, Webster plans to do a second study in July when higher temperatures accelerate the production of sulfur compounds.
The final report should be completed in August, but design work would not be completed until March of 2018. Construction would take a year, so that relief does not come until April 2019.
City Council is planning a field trip to decide on what type of scrubber system to build. Kalamazoo has a carbon adsorption system and Grand Haven has an organic biofilter. Both could be visited in a day.
More importantly, they are planning for funding.
“The low interest loan program that the city has used in the past has one day a year that you can submit an application,” said Rubel. “If you miss that date you have to wait a whole additional year. It’s July 1, and with a 30-day public notice period, that deadline is really right upon us.”
Council seemed to have unanimous consensus that they should begin preparing this application for a state loan very soon.
The map above shows where odor complaint calls originated.