L et’s face it. There isn’t a can of Lysol® big enoughto handle the odors that biosolids workers dealwith every day. Yet odor is such an expected byprod-uct of the material produced that many treatmentplant employees simply learn to accept it.

For the City of Tacoma, Wash., odor has alwaysbeen the enemy. Perhaps it’s our industrial historyas a paper mill town, but at our treatment plant wehave always been convinced that people don’t wanta product that smells bad. As a result, we havespent years looking for a better way to produceClass A biosolids that people can be proud of.

Through experimentation, we have found thatdropping the heat levels during the anaerobic phaseof the dual digestion process greatly reduces odorsat the treatment plant and in the biosolids productitself — which can move a facility one step closer toproducing something that its customers will want touse in their back yards.

How It All StartedIn the wastewater treatment industry, we gen-

erally focus on eliminating odors that offend a treat-

ment plant’s neighbors. But in Tacoma, we weren’toffending our neighbors; we were offending ourown workers. In June 2002, the odors were so badthat we placed hydrogen sulfide meters above thebelt-filter presses, because the smell was becominga health issue for the five operators working in the5000-ft2 (465-m2) press room. Although odors neverreached a level where any employees had to vacatethe premises, we were getting readings of 8 ppm —too close to the danger zone of 10 ppm. As opera-tions division manager, I regularly visit the belt-fil-ter press room to check in with staff members. Afterleaving the building one day that June, I realized theodor was still on my clothes. It occurred to me thatuntil we corrected our odor problems, the strongsmell left on my person would prevent me fromattending other city meetings on those days.

In addition, our liquid product for commercialfarmers was especially offensive. We were schedulingaround warm weather and holidays to prevent los-ing customers because of the smell. At that point, weweren’t charging for the liquid, but there were timeswhen we couldn’t even give it away.

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To solve our odor problems, we initially exploreddesigning and installing equipment to vacuum thefoul air out of the building. However, this optionwas quite expensive and, although it would takecare of our health concerns, it wasn’t going to reducethe odor of the biosolids product itself.

Third Time’s the CharmIn the past, the wastewater treatment industry

had explored using temperature phasing for stabi-lization and for vector reduction, but not for odor con-trol. And throughout its history, the biosolids indus-try has focused on finding ways to dispose of solidwaste. Creating products — and considering aesthet-ic issues that come into play when creating products,such as odor — is a relatively new pursuit. The Cityof Tacoma has been an industry leader in developingproducts out of biosolids, beginning its Class Abiosolids product program with the introduction ofTAGRO Mix in 1989. Tacoma uses three levels of tem-perature phasing during the anaerobic step of the dualdigestion process to virtually eliminate the odorsfrom our biosolids. This leads to products that peo-ple want to use in their yards and indoor planters.

In temperature phasing, we gradually lower thetemperature rather than leave it at the same level forthe whole solids processing period. Typically, dur-ing the aerobic step of dual digestion, we keep thesolids at 160°F (71°C) for 24 hours. Then, in theanaerobic step, we process the material at 130°F(54°C) for 21 days.

The thermophyllic temperature of 130°F pro-vides an environment hos-pitable to methane-generatingbacteria and encouragesincreased biological activity,raising the metabolic rate ofthe organisms present. Theresult is relatively high rates ofmethane production. Althoughthe methane produces enoughenergy to run our boilers, italso produces sulfur com-pounds that can be relativelysmelly.

Below the thermophyllicrange is the mesophyllic range,which begins at 100°F (38°C).Here, different organisms areat work, and they produce lessmethane and fewer sulfur com-pounds. At 90°F (32°C), meso-phyllic organisms dominateand are at a relatively lowmetabolic rate, so there’s vir-tually no methane or sulfidegas production.

At these three temperature levels, different pop-ulations of organisms do their jobs then die off, anda different population that doesn’t generate the samesmelly gases takes over. It’s almost like ecological suc-cession in a 21-day period.

A History of InnovationThis is not the first time Tacoma has experimented

with getting rid of odors. Before 1993, we used a sin-gle-phased thermophyllic process to anaerobicallydigest the material at 130°F (54°C) for 21 days. Then,from 1993 to 2002, my predecessor, David Hufford,used a two-phase anaerobic digestion process in aneffort to improve product odor for customers. Hedecreased temperatures from 130°F to 115°F (46°C);the digesters would cool to these temperatures if hemade no adjustments to maintain or lower them.

Although processing at these temperatures madeour product more palatable for our customers dur-ing this time, it was still difficult for our workers tohandle the odor — which prevented us from devel-oping an indoor product. Building on Hufford’s foun-dation, we tried dropping the temperature from130°F to 100°F, and the odors decreased further.When we dropped from 130°F to 100°F and thendown to 90°F, we were able to virtually eliminate theodors (see the figure, below).

My hat goes off to operations supervisor TonyTrotter, filter-press senior operator Dan Frank, andfilter-press operator Dan Mathewson for their inge-nuity in making the three-temperature process workat Tacoma.

Figure 1. Tacoma’s Modified Dual Digestion Process

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The Sweet Smell of SuccessConversion to temperature phasing has

made Tacoma more innovative with itsbiosolids products. Eliminating the odorallowed us to successfully produce a pottingsoil for indoor and outdoor use, which we intro-duced late last summer. In addition to the pot-ting soil, we developed a new odor-free mulch,and we now charge our commercial customers$30 a load to deliver our odorless liquid prod-uct. Customer response has been overwhelm-ing. In April 2004, we broke all previous records,selling 3500 yd3 (2700 m3) of all-biosolids prod-ucts in 1 month — a 57% increase over sales vol-ume in April 2003. We literally can’t keep upwith product demand. Currently, we have awaiting list of 100 customers for our most pop-ular product, TAGRO Mulch. And the liquidcustomers who used to complain about oursmelliest product have praised us instead forthe absence of odor.

We believe our employees don’t want to makea smelly product, let alone try to sell a smellyproduct to businesses or to local residents to usein their back yards and indoor planters. The

three-phase temperature process has allowedus to address our workers’ health concerns andto develop high-quality products that people canbe proud of.

Odor control isn’t just good manners. It’s goodbusiness.

Daniel Thompson, Ph.D., is the operationsdivision manager for the City of Tacoma (Wash.)Wastewater Utility. For more information, visitwww.tagro.com.

Tacoma produces three main biosolids products marketedunder the name of TAGRO (short for Tacoma Grow): TAGROMulch, TAGRO Mix, and TAGRO Potting Soil.

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