BioCycle July 2004, Vol. 45, No. 7, p. 24
Lenoir City, Tennessee develops simple method for achieving Class A biosolids product that avoids a “hefty annual landfill charge.”
THE COMMUNITY of Lenoir City, Tennessee (pop. 6,719) is nestled in a valley bordered by the Great Smoky Mountains on the east and the Cumberland Plateau on the west. The once mighty Tennessee and Little Tennessee Rivers (now harnessed by TVA dams) converge with their banks bordering the city.
The wastewater treatment plant serving Lenoir City began operation in 1969. It is a 2 mgd designed trickling filter plant with two stage anaerobic digesters. Effluent discharge is into the Tennessee River. Until 2001, 12 sand drying beds were used to dry the anaerobically digested biosolids and for many years the dried solids were given away. As regulatory agencies took turns to enact more stringent regulations, Lenoir City abandoned the biosolids giveaway program and the solids were disposed of at the county landfill. Although the solids were later classified as class “B”, land application was hindered by site restrictions, application rates, and use restrictions.
With the escalating cost of landfill disposal (current cost: over $30,000 per year – a large sum for a small town!), it became obvious that implementing a method to create a class “A” biosolids product needed to be seriously considered. A class “A” product would meet EPA and state regulations and the product would be marketable, or simply given away for unrestricted use.
At that time, the Lenoir City Utilities Board (LCUB) was preparing for an expansion to 4 mgd and was experimenting with a screw press in conjunction with an alkaline stabilization procedure to produce a class “A” biosolids product.
SAVING DOLLARS WITH LOW-TECH COMPOSTING
Staff from the LCUB attended a presentation on biosolids composting, given by Dr. Richard Buggeln, University of Tennessee Center for Industrial Services, and soon composting seemed like a reasonable cost-saving way of handling biosolids. Working with Buggeln, LCUB staff developed a simple, “low-tech” composting procedure that made use of the sludge drying beds for carrying out most of the composting operation. A sufficient quantity of coarse green wood chips, generated on a steady basis by the LCUB electric division, was dumped into an empty concrete drying bed, followed by the introduction of approximately 12,000 gal liquid biosolids (ca. nine percent solids content), so that the final ratio of chips to solids was at least 3:1. Water was allowed to drain from the bed for a day or two before the chips and biosolids were thoroughly mixed with a Bobcat outfitted with a front-mounted screw-auger attachment.
The mixture was transferred to an adjacent drying bed where it was piled in windrow fashion (7-8 inches high) on a 12 inch layer of clean wood chips, and then covered with a layer of clean wood chips (ca. 8-12 inches thick) acting as a biofilter and thermal blanket. The pile was never turned during the two to three weeks of the EPA’s well-known “time-temperature” phase of the composting process. In addition, temperature records showed that the EPA’s minimum PFRP temperature (130°F) could be achieved, not only in the core of the windrow but out at the interface between the chips/biosolid mixture and the biofilter. Salmonella tests at this interface have always achieved the EPA standard.
In order to keep the process moving, windrows that have passed the PFRP temperature regime are moved out of the drying beds to make room for the next batch of biosolids. The composted biosolids and accompanying biofilter layer are piled for another 45-60 days for curing. At this point, wood chips are in excess, but in the future, we may decide to recover the biofilter layer for use as bulking agent. The cured compost product has a coarse texture due to the wood chips but a local contractor is taking the unscreened product as it becomes available, at no charge. If the demand for the product exceeds the current supply, charging a fee will be considered. At present, Lenoir City is pleased to be saving money by avoiding the previous, hefty annual landfill charge. Further, we hope that this low-tech composting procedure that we have successfully implemented in Lenoir City will be a role model for other small municipalities with populations under 10,000.
M.L. Blackburn is Pretreatment Coordinator for the Lenoir City, Tennessee Utilities Board. The Tennessee Department of Environment and Conservation Division of Community Assistance also provided project funding.
July 22, 2004 | General
BIOSOLIDS COMPOSTING FOR CITIES WITH UNDER 10,000 POPULATION
BioCycle July 2004, Vol. 45, No. 7, p. 24