Cambridge, Ohio retrofitted a portion of its vacuum assisted drying beds into a covered composting facility, processing about 300 dry tons/year of biosolids and wood chips.
BioCycle October 2013, Vol. 54, No. 10, p. 37
Hauling and disposal fees were between $50,000 and $100,000/year, and Lou Thornton, Cambridge’s environmental compliance and safety manager, was looking for a more sustainable alternative. He began exploring the option of composting biosolids with green waste, and toured a variety of facilities to see what would work best for Cambridge. “We had stopped using the vacuum assisted drying beds that were in a greenhouse structure, so that area was available for composting,” says Jamiel. “Lou decided that given that space, windrow composting would work out best.”
The city had been struggling with removing solids throughout the treatment facility and reviewed various treatment options. It experimented with a dewatering tube system that resulted in 8 to 10 percent solids, and ultimately opted to install a rotary fan press that produces 19 to 22 percent solids. “We were paying to haul a lot of liquid, and the higher solids content is much more conducive to composting,” he notes. The rotary fan press has been operating since 2010.
In 2011, after evaluating various windrow turning equipment, Thornton decided to purchase an R24 Brown Bear aerator that it attaches to a CAT skid loader. It experimented with using a mixture of leaves and wood chips as amendment, but the leaves clumped together, in part because of the wetness of the solids. Thornton and Jamiel switched to a mixture of all wood chips along with finished compost once it became available. The wood chips are dropped off at the plant by several tree service companies. “Basically, we put down a row of new wood chips, then add a layer of finished compost, and then put the biosolids on top,” explains Jamiel. “The materials are mixed together with the Brown Bear. Once the pile reaches 131°F, the windrows are turned every three days for 15 days to meet pathogen reduction [PFRP) requirements. If the temperature doesn’t rise back to 131°F after a turning during that time period, we adjust the recipe.”
Windrows are 3-feet high, 5-feet wide and 70-feet long. After the pile meets PFRP, it is moved to another bed for curing. Last year, the city stopped composting in November because it was too cold to achieve PFRP in the windrow, and then resumed in the spring of 2013. “We expect we will do the same thing this year,” he says. The city is still developing markets for the compost, and uses some of it for municipal projects. Finished compost is stored at a separate facility.
Less than 10 percent of the 300 dry tons of biosolids generated annually is composted. The remainder is collected by quasar energy group and taken to the company’s anaerobic digestion facility in Zanesville, about 25 miles away. The City of Cambridge hopes to compost more of the biosolids, especially as it develops markets for the end product. “We have seen a huge savings just as a result of the higher percentage of solids in the material,” explains Jamiel. “Having the ability to compost on-site opens up more cost-saving opportunities.”