Biosolids Composter Satisfies Customers and Neighbors

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Courtesy of BioCycle Magazine

A ticket to Penn State football games isn’t the only hard thing to come by in State College, Pennsylvania. So is the biosolids compost produced by State College’s University Area Joint Authority (UAJA) wastewater treatment plant. “For the last four years, we’ve been sold out before we’ve made the compost,” says Stephen Welch, who manages the UAJA composting plant. All biosolids generated — about 43 wet tons/day — are composted. About 15,000 cubic yards (cy) of compost were produced in 1999. “Typically, we’ll have 1,100 to 1,200 cy available on the first of a month, and most of that is gone by the end of that week,” he adds.

The biosolids composting facility started operating in March, 1992. The $6 million plant uses the agitated bay technology marketed by U.S. Filter (formerly known as the IPS system). There are 12 bays and three agitator/mixers housed in a 40,000 sq. ft. building. Based on a five-day work week, the composting process takes about 26 days; finished compost cures for a minimum of 30 days in another building, where the product also is stored.

Raw primary and secondary solids are blended and dewatered in a belt filter press to about 20 percent solids. Biosolids are mixed with amendment in a stationary SSI mixer. The mix ratio per batch is 7,800 lbs of biosolids, 1,700 lbs of green sawdust, 1,700 lbs of kiln dried sawdust and 1,700 lbs of recycled compost. “We are looking for a mix that is 40 percent solids, has a C:N ratio of 20 to 30:1, and good porosity,” explains Welch. “Grab samples are taken of one mix a day to see if we are making 40 percent. We like the blend of the two sawdusts. If the presses are not having a good day, we go with more kiln-dried sawdust in the mix because at about 98 percent solids, it acts as a good buffer for moisture. The green sawdust is about 50 percent dry solids.”

Early on in the operation, Welch found that the composting process took off so fast that all the moisture evaporated, which stopped biological activity. UAJA solved the situation by installing an overhead watering system.

There are five aeration zones per bay. The air in the building goes through seven changes an hour. The interior of the composting building has a two inch coating of polystyrene foam as a base layer. That is covered with a 20 mil film of butyal rubber. There also is a final 20 mil layer of epoxy. This interior insulates the building and helps to minimize corrosion. It has not been necessary to replace any of the interior structures.

UPPER HAND ON ODOR CONTROL

UAJA installed a 120 foot by 200 foot biofilter to treat air from the composting building. The media was made from compost, hardwood chips and bark mulch in a 1:1:1 ratio. “The biofilter failed within two years,” says Welch. “The literature says they are rated for five to seven year longevity. What we found is that the air being treated deteriorated the media. It accelerated breakdown, and produced fines, which washed down and clogged the filter. Untreated air escaped around the edges.”

In November, 1994, the authority was issued a notice of violation by the Pennsylvania Department of Environmental Protection (DEP) for fugitive air emissions, said to be originating from the composting building. UAJA already had authorized its consultant to perform an odor emission study (begun in June, 1994) to assess and prioritize potential odor emission sources and recommend alternative control strategies. DEP was notified of the ongoing study and was supportive.

The consultant’s scope of work included conducting an odor panel to identify sources of odor emissions from the composting facility. The panel consisted of a newspaper reporter, a student, a retired professor and three neighbors. The panel’s work also helped to determine that the number of dilutions to threshold (D/T) needed to get to acceptable levels of emissions was 45 to 50 D/T. The consultants built that level into its recommend odor control strategy.
 
 The “Proactive Odor Management Study” report was finalized in February, 1995, and included a number of specific action steps. The study was forwarded to the DEP. Steps taken include the following: Removal of heat exchanger coils from the exhaust duct work to improve air flow; Remove existing geotextile fabric and media in the biofilter and replace it with new geotextile materials and an improved media; Install an in-duct air flow humidification system ahead of the biofilter, as well as install a surface irrigation system; Modify compost building ventilation duct work to improve air flow within the building; Modify the building’s heating system to provide a warmer air flow to the biofilter during the winter months (thus improving performance).

“The biofilter we installed in 1994 is still in service today,” says Welch. “Odor complaints related to the composting facility were practically eliminated, and we have not had any further DEP violation notices.” The base of the biofilter has 36 inch diameter fiberglass manifolds with 60 eight inch perforated laterals. The manifolds are bedded in eight to nine inches of washed river stone. Next, there is a layer of quarter inch mesh geotextile fabric, with a second layer of 40 mesh fabric on top to help prevent clogging. “During media replacement, we go down and take the top layer of fabric out and we can leave the bottom layer, which is protected from fines building up,” he adds.

One significant change between the original and new biofilter was switching the type of chips in the media. The authority started out with hardwood chips. A study was done to compare hard versus softwood chips. It was found that ammonia accelerated decomposition of the hardwood chips, notes Welch, therefore a switch was made to softwood. “We gambled with white pine and hemlock chips and bark. By volume, it is two parts softwood chips, one part bark mulch and one part leaf compost. We put in 48 inches and it compacts to three feet.”

The biofilter is monitored for pH and moisture. Every three months, staff core down to the support gravel to see where the fines are. (Accumulation on the support gravel is an indication that the media needs to be changed.) “We also can spot short circuiting by looking at the biofilter before the sun rises and watching the pattern of the mist,” he says. “There is a cloud if we are getting good air distribution.” The surface of the biofilter is rototilled twice a year in the fall and spring to prevent the roots of any plants on the surface from growing down to the distribution channels.

MARKETING SUCCESS

UAJA’s end product, ComposT, meets EPA’s exceptional quality criteria. All compost goes through VAR verification and sampling for fecal coliform before distribution. The compost is unscreened. The authority has three separate beneficial use “orders” from the Pennsylvania DEP. They are for marketing and distribution — including turfgrass and ornamental horticulture applications — agricultural utilization and reclamation of drastically disturbed land. End users include landscapers, nurseries and greenhouses, as well as colleges and schools that apply compost on athletic fields. The material also is used to reclaim drastically disturbed lands, limestone quarries, etc.

ComposT is sold in bulk for $3.50/cy plus transportation. For amounts under 1,000 cy, the cost is $5/cy. Material typically is moved out in 70 cy walking floor trailers. All loads have to be tarped. To avoid storing large amounts of compost produced during the late fall and winter months, UAJA encourages large bulk customers to stockpile compost at their sites. “We do, however, have a fair amount of storage capacity if we need to use it,” says Welch. “That storage was built into the design because we knew it would take several years to develop end markets.”

UAJA has sponsored several compost utilization research projects, including a disease suppression study with seedling beds in a state tree nursery, and pathogen analysis in conjunction with vegetables grown in biosolids compost. The authority received an EPA Beneficial Use of Biosolids Award in 1997 for its composting facility.

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