September 21, 2005 | General


BioCycle September 2005, Vol. 46, No. 9, p. 41
Small wastewater treatment plant in northern California installs solar drying beds to manage 340 dry tons/year of biosolids.

THE Town of Discovery Bay, California, a hamlet about 40 miles west of San Francisco, is facing growing population pressures. New home development is expected to double the population. Biosolids from the town’s 2.2 mgd wastewater treatment plant, which currently receives about 1.7 mgd of wastewater, had been stored in a lagoon and eventually removed and disposed by a contractor. With the anticipated rise in population, Virgil Koehne, Discovery Bay’s general manager, began looking for alternative management options for the approximately 340 dry tons/year of biosolids generated. “I was very aware that our options for disposing of the Class B biosolids we had been producing were becoming increasingly limited,” says Koehne. “The State of California issues fewer and fewer Class B permits to farmland owners each year, and it does not allow Class B biosolids to be burned.”
During his search for alternatives, Koehne saw an advertisement for a solar sludge drying technology. The Thermo-System™ Solar Sludge Dryer from Parkson Corporation utilizes a drying bed housed inside a greenhouse-like structure (referred to as a drying chamber). The building is equipped with ceiling fans, exhaust fans and air flaps, all of which are controlled by a microprocessor. There is also an electronic “mole” – a small, automated vehicle – that moves randomly around the drying bed, tilling the biosolids to expose as many surfaces as possible and to aerate the mass. The microprocessor draws information from sensors inside the chamber, and from a weather station that monitors exterior humidity and temperature, solar radiation, and wind speed. It calculates this data and manipulates ceiling fans, exhaust fans, and air flaps accordingly. Essentially, solar and wind power are the primary energy sources used to dry the solids.
Koehne asked Herwit Engineers, the town’s engineering adviser, to gather more information on this system. A single chamber unit was operating at a wastewater treatment plant in Oregon. He decided to send an engineering team to Germany, where the solar drying technology originated. According to Roland Mueller, Parkson’s project manager, the Thermo-System was originally developed in the early 1990s by researchers at the University of Hohenheim as a solar drying technique for agricultural production. Its applicability to wastewater management was recognized shortly thereafter.
“The engineers came back saying they thought that this was something that could work – and save us money in the process,” Koehne remembers. “I toured the site operating in Oregon, then decided to take a proposal to use this technology to the Discovery Bay Town Council. The council approved construction of two 42-ft by 205-ft side-by-side drying chambers.”
A belt press was installed along with the drying chambers. The facility became operational in the fall of 2004. In addition to handling daily output from the treatment plant, the backlog of biosolids in the storage lagoon is being processed – about 10 million gallons or roughly 1,600 dry tons of material. The design capacity of the solar dryer is about 2,000 dry lbs/day. “We are currently running at about 1,600 dry lbs/day of sludge produced and are using the excess capacity to process sludge out of the lagoons,” says Gregory Harris of Herwit Engineers. “After several years of doing this, we should be able to completely process the backlog of sludge in the lagoons ahead of the increasing solids loading from the continued housing construction.”
Dewatered solids (about 15 percent dry solids) are loaded into one of the two chambers. Once a chamber is full, the batch takes about two weeks in the summer and up to two months in the winter to meet the EPA Part 503 Class A pathogen and vector attraction reduction requirements. No supplemental heat is provided. The average percent dry solids coming out of the chamber is 75 percent. According to Koehne, the system in Discovery Bay produces upwards of 85 to 90 percent solids content, while reducing vector attraction and pathogens sufficiently to meet EPA standards for Class A biosolids. Operators test for fecal coliform, Helminth ova and enteric viruses.
Harris calculates that it requires about 30 kW-hours/dry ton to operate the solar drying facility, versus an estimated 600+ kW-hours/dry ton to operate a conventional heat dryer. “Total operating and maintenance costs for the dryer are about $42/dry ton, based on 300 dry tons/year processed,” he says. “O&M costs for the belt press are $62.70/dry ton.” Capital costs for the belt press, drying chambers and associated equipment and a truck loader were $2.1 million.
Parkson Corporation is working with the U.S. EPA based on the pathogen testing to get the Discovery Bay facility certified as a Class A production facility. “We are taking the composite samples and running the pathogen tests as required in order to submit the application for Class A certification,” says Harris.
Current end uses for the dried biosolids are fuel and land reclamation. “We learned about two cement companies in the area that were interested in our product,” says Koehne. “It turns out that Class A biosolids, when burned, produce one-half the BTU value of the brown coal used in cement kilns. Here was a golden opportunity for all parties involved. Discovery Bay would have a convenient outlet for its processed biosolids, and the cement companies would have a ready supply of highly efficient fuel for nothing but the cost of transporting it.”
In addition, Koehne found a depleted mining site not far from its plant. Originally a source for sand used in the manufacture of wine bottles, the mining company was looking to reclaim the site. Class A biosolids from Discovery Bay have proved to be an ideal solution, providing both fill material and clean, fertilized soil. – N.G.
AN analysis of agitated air drying on paved beds was conducted by Diane Garvey of Garvey Resources, Inc. in Lansdale, Pennsylvania ( The report notes the solar drying plant, marketed as the Thermo-System (see accompanying article), is “based on an improved greenhouse construction mounted on paved flooring with underdrains to allow the drainage and filtration of the free water of the liquid sludge…..In Europe, the solar optimized paved beds were shown to reduce fecal coliform and salmonella to below detectable limits.”
The report explains advantages and disadvantages of paved air drying beds with agitation. Among the advantages of solar optimized paved beds (compared to conventional beds) are low maintenance and cost-effectiveness for small to medium-sized WWTPs; increased efficiency in reducing fecal coliform in summer; and a high volume reduction (50 percent in autumn/winter and 93 percent in summer) that leads to reduced costs for biosolids management. Among the disadvantages are large land requirements; reduced performance in autumn and winter; and need to demonstrate pathogen reduction by monitoring fecal coliform, Helminth ova and enteric virus indicator organisms for each batch or over each monitoring period. (The last cited disadvantage would be addressed by receiving certification as a Class A pathogen and vector attraction reduction process by the USEPA Pathogen Equivalency Committee.)

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