BioCycle September 2004, Vol. 45, No. 9, p. 50
Biosolids composting facility in Utah uses combination of tools – including odor modeling and field measurement equipment – to help guide operational changes and improve neighbor relations.
THERE is an old Irish limerick that goes something like this:
There once were two cats
Each thought there was one cat
So they fought and they fit
And they scratched and they bit,
‘Till except for their tails
And the tips of their nails,
Instead of two cats there weren’t any!
Often, wastewater treatment plants (and their associated odors) and facility neighbors are quite similar to the two cats from Kilkenny. People living close by desire a quality of life that excludes odors they don’t like. Officials of the governing body overseeing the wastewater treatment plant (WWTP) often believe they were there first and neighbors who move in should tolerate odors from time to time because they knew the plant was there when they built or purchased their homes. Without some compromise, the theme of the two old cats would arise and there probably wouldn’t be any left happy. The way this tale played out in our case has a happier ending, however. Both cats are well and alive because those involved chose change and compromise over conflict.
The Central Davis Sewer District (CDSD) in Kaysville, Utah – not far from Salt Lake City – has a 9.9 MGD wastewater treatment plant (WWTP), with actual flows closer to 5.5 MGD. The plant uses oxidation ditch aerobic treatment and trickling filters with anaerobic digestion (mesophilic). When it first started, sludge from these treatment trains was air dried then delivered to area farms for application. In the late 1980s, CDSD acquired 200 acres of farmland around the treatment plant for its land application program. By the beginning of the 1990s, all biosolids were applied to the district-owned farmland in accordance with federal and state requirements. Sod and alfalfa hay were among the crops raised.
In 1995, new homes were constructed in the area near the WWTP. Nearby residents began complaining about odors from the air drying process. About a year later, an existing food processor violated its pretreatment permit and sent excessive amounts of wastewater to the treatment plant. This increased the organic load to the plant, generating significantly higher quantities of biosolids. The increased load, plus the drying beds and surface land application – all with their related odors – caught the attention of the plant’s new neighbors.
In 1996, a group of residents met with the CDSD governing board and requested changes to reduce or eliminate the odors. After a review period, two goals emerged: 1) Odors needed to be reduced; and 2) Beneficial reuse of biosolids should continue. After a review of alternatives, the decision was made to switch to windrow composting for sludge stabilization. Use of drying beds was discontinued. Aerobic solids were removed from the oxidation ditch, dewatered in a belt press to about 15 percent solids, mixed with wood chips and formed into windrows (in the area where the drying beds had been). Scrap wood from a pallet manufacturer was (and is still) used as the bulking agent, supplemented during certain times of the year with ground tree trimmings from community clean-ups. Anaerobically digested solids continued to be land applied after thickening.
Peace returned to the neighborhood and everyone was happy and for the next few years, the cats played happily in their own areas. Complaints went down from 150 in 1996 to no complaints between 1997 and 2000. The governing board created a plan for continuous improvement for odor reduction and control. A formal complaint response system was put in place.
By late 2001, however, more and closer homes were constructed and new noses were in place. A significant increase in aerobic biosolids production – going from about 390 dry tons/year to about 550 dry tons/year – and a winter with many inversions stirred the cats to again begin hissing. During an inversion, the air is very still so the plumes (steam containing odor-causing compounds) coming from the compost piles stay close to the ground and there is little to no mixing with ambient air, i.e., little or no dilution and dispersion. There were 30 complaints in late 2001 and about 80 complaints in the first couple months of 2002. It was obvious that change and compromise were needed again. The governing board convened a meeting with neighbors to determine the extent of the problem and to discuss potential solutions. A survey mailed to the 200-plus closest homes revealed that about 25 percent were concerned and dismayed by the extent of the odors. There was desire by some neighbors to see all aerobic solids hauled to the landfill for disposal. This action plan may have solved the odor problem, but there was the reality of goal two – beneficial reuse.
To better understand the feelings of the entire District population, a public opinion poll was commissioned and completed. While it was determined there was sympathy for odor problems, less than half the population wanted to pay much to correct the situation. However, it also was determined that almost 75 percent of those surveyed wanted to continue to recycle the biosolids and were willing to pay more to maintain this program.
With an understanding that some additional expenditure was reasonable to maintain recycling of the solids, CDSD put funds into the composting operation and odor management equipment and consulting services. On the composting side, a Scat elevating face windrow turner was purchased to improve pile aeration. On the odor side, later in 2002, CDSD purchased a field olfactometer from St. Croix Sensory (see sidebar). Known as the Nasal Ranger, the portable, hand-held unit measures ambient odor in dilutions-to-threshold (D/T). The district uses the device as part of its response when an odor complaint is received, as well as when doing odor monitoring.
ODOR MIGRATION STUDY
As part of its odor management strategy, the District also decided to hire CH2M Hill for technical assistance with developing an action plan and preparing an off-site odor migration study. In addition to using the Nasal Ranger to quantify odor strength, a Jerome 621-X H2S analyzer was procured to detect and measure hydrogen sulfide, a primary odor compound associated with wastewater treatment. Using both instruments to measure off-site migration helped quantify more accurately the extent of the problem.
Dispersion modeling was used to estimate the baseline off-site odor impacts from the plant and to quantify the reduction in impacts using different odor control strategies. Data from field sampling conducted in the summer of 2003, physical parameters of the odor sources, local topography and meteorological data from the Salt Lake City airport were input into the air dispersion model. The results were used to rank odor sources and prioritize recommended improvements.
The District has over 1,500 feet in most directions from the composting site before reaching the boundary of its property and the site of an odor receptor. Air modeling of potential plumes indicated the most likely D/T that would be experienced and the duration that the odors would be at that D/T. The baseline odor assessment included all the treatment process units typically on-line in the summer. Three baseline models were run using different 5-minute odor threshold values: 4 D/T, 20 D/T and 50 D/T.
The highest off-site odor impacts from the baseline model were tabulated by source group (process area) to assess their contribution to off-site impacts. Table 1, prepared by CH2M Hill, presents the maximum 5-minute odor impacts and maximum annual average odor impacts expressed as D/T for each major process area. The ratio of the maximum annual odor to the maximum 5-minute odor, peak-to-mean ratio, is a parameter indicating the frequency of the maximum odor occurrence, as shown in the table. The major odor sources (having an odor impact above 4 D/T) at the plant were the compost process, solids building, oxidation ditches, headworks and primary clarifier before the trickling filters.
IMPLEMENTING PROCESS CHANGES
While the noses were out “sniffing the air,” staff and the consultant identified incremental changes that could reduce the potential for off-site impacts. Now the goal was to implement changes that would contain odors to an acceptable level and duration. First the definition of acceptable was needed. After public input on the subject was solicited and received, a goal was set to limit any offsite odor to less than two percent of the time or 175 hours/year. The D/T for off-site incidents was to remain below 20 with no emissions over 50 D/T at the property boundary.
With the measurement tools and the limits, all that was left was making the incremental changes needed to insure continued compliance. Changes would be implemented based on cost and potential improvement. Low-cost changes with a high potential for improvement would be implemented first. Two changes were identified. First, better mixing was needed during pile construction to improve aeration and reduce odor generation. CDSD purchased a Rotomix truck mounted mixer for the initial feedstock blending. Dewatered sludge is loaded directly from the conveyor into the mixer; wood chips are added and then the truck drives out to the pad to put the material directly into a pile.
The second change identified was to leave new piles dormant for 15 to 20 days before active mixing with the Scat was to begin. During this time, the release of odorous gases would be minimized. In addition, piles are capped with large wood chips – overs from the initial three-quarter inch grind. As an odor control “back-up,” the district purchased used air blowers and some piping if it needs to switch to negative aeration of piles and biofiltration of process air. Finally, CDSD found that lengthening the residence time of sludge in the oxidation ditch increased stability of the material, making it less odorous prior to dewatering.
The active composting phase, when piles are monitored and turned, is about 12 weeks. There is a three to nine month curing process so when the compost is screened, it is about seven to eight months old. Finished, screened compost is sold for $7/cubic yard.
ODOR MONITORING AND COMPLAINT RESPONSE
Parallel to the operating changes, the District also implemented a more aggressive odor complaint response program. Aggressive investigation of complaints and response has addressed the concern that the District did not take the odor issue seriously enough. In addition, CDSD does weekly monitoring of off-site areas (identified in the CH2M Hill study as projected migration areas), once in the morning and once in the evening. Staff also goes to one or two places outside of those areas to obtain background measurements (e.g. to see what the H2S levels are as in some cases, the odors are coming from the Great Salt Lake). The Jerome meter is used for the monitoring; the Nasal Ranger is used if an odor is detected – both during monitoring and in response to a complaint.
Since implementing the changes and the aggressive monitoring and response program, the District has received only a handful of complaints from a couple of people in the most affected area. In situations when people smell something, the D/T has been in the range of 2 to 4 and has never been measured over 7. So far, the District had met its goal and the neighbors are satisfied.
During the 2003 – 2004 winter, the area suffered significant periods of atmospheric inversion. Off-site consequences were kept to a minimum and the cats stayed home without even a hiss at each other!
Leland Myers is District Manager with the Central Davis Sewer District in Kaysville, Utah.
TAKING SUBJECTIVITY OUT OF ODOR DETECTION
“SMELL is the oldest and most sophisticated sensory system that mammals have for detecting information about their environment at a distance,” wrote Pamela Dalton of the Monell Chemical Senses Center in a BioCycle article last year (see “How People Sense, Perceive And React To Odor,” November 2003). Despite its sophistication, there is still a fair amount of subjectivity when it comes to classifying odors and their intensity, especially when those odors are objectionable. In the organics recycling world, malodors can spark significant public outcries. As the years have gone by, a great deal has been learned about how to manage odors in ways that earn the public’s support – or at least tolerance. A key part of that public relationship is making subjective situations more objective and “science-based.” Increasingly, equipment is being developed in the “odor world” to bring objectivity to sensitive situations.
For example, about two years ago, St. Croix Sensory, an odor analytical laboratory in Lake Elmo, Minnesota, introduced an odor measurement tool called the Nasal Ranger. It is a hand-held, field olfactometer that takes real time measurements of the level and strength of ambient odors (vs. measurements taken directly from the odor source). “We had developed a laboratory olfactometer designed around new standards that were being developed in Europe in the mid-1990s,” says Michael McGinley of St. Croix Sensory. “It was for our own use, but we began getting requests from universities that wanted to buy one for their own research. We then began getting requests, especially from universities and state agencies working on odor issues related to agricultural operations, for a unit that could be used to get real time measurements, for example, at homes, or in areas downwind from an operation. The principle of field olfactometry is actually included in at least ten state regulations on odors.”
A field olfactometer instrument, known as the “Scentometer,” had been developed in the late 1950s through project grants from the U.S. Public Health Services. The Nasal Ranger uses the same principles of field olfactometry, but differs in one significant way – it has a flow sensor device designed to standardize the “sniffing rate,” thus reducing the variability between users. “Essentially, we’ve standardized the amount of mass concentration of odorants being put into the nose, presenting consistent volumes,” adds McGinley. An LED display on top of the unit has three symbols that light up – a minus sign means sniff harder, plus means sniff less, and the target symbol, which indicates the user is sniffing at the correct rate, calibrated at 16 to 20 liters of air/minute. The field olfactometer creates a calibrated series of discrete dilutions by mixing the odorous ambient air with odor-free (carbon) filtered air. The dilution to threshold (D/T) ratio – which measures the number of dilutions needed to make the odorous ambient air nondetectable – is determined by dividing the volume of carbon-filtered air by the volume of odorous air.
Confined animal feeding operations (CAFOs) are a prime market for the field olfactometer (which costs $1,475), notes McGinley, with monitoring done by regulators, university researchers, and/or the facility owners themselves. Units also are used by landfills to monitor odors, as well as biosolids land application and storage sites and composting facilities. In almost every case, an odor problem exists and there is some community pressure being exerted. “It usually starts with complaints by citizens, and then it’s a matter of whether regulators get involved or if the facility clues in quickly enough to keep the situation from escalating,” he says. “With situations involving citizens that have been impacted by the odors, we’ve found that this tool provides all parties with a starting point to work from – actual measurements that can be used versus trying to describe what something smells like at that moment or trying to remember months later. And if a facility makes changes, citizens, regulators and others can tell whether the situation is better, the same or worse.” – N.G.