Scavenging The Way To Biosolids Composting In Coalville

BioCycle October 2004, Vol. 45, No. 10, p. 45

Facility manager gets good results by converting an old agricultural grain elevator into a screen to separate chips for composting.

Dan Emerson

WITH BUDGETS unusually tight, local governments often have to use old-fashioned ingenuity in coming up with ways to provide needed public services without spending more money. Coalville, Utah (population 1,470) is a case in point.

A few years ago, the city began using treated sewage sludge to make compost. Dennis Gunn, Coalville’s wastewater treatment plant operator, needed a device to screen woodchips and other unwanted debris in the materials. There wasn’t room in the budget to buy a modern trommel screen.

“We didn’t have a lot of money to spend on equipment. Sometimes in a small community you have to get a little creative,” says Gunn. He had read an October, 1999 BioCycle magazine article about a farmer who designed an in-vessel composting system using a second-hand concrete mixer. Gunn then conferred with City Public Works Director Doug Moore, who is also a farmer. Moore offered a solution that is anything but high-tech: a discarded, Case grain elevator, circa 1910. Gunn picked up the dust covered agriculture antique, made a few alterations, and wound up with an efficient reliable device for screening material used to make compost. Gunn says the city had to invest less than $2,000 in the gasoline-powered device, originally used to load grain into silos and also stack bales of hay.

This community of 1,400, located northeast of Salt Lake City on Interstate 80, treats its municipal wastewater using an aerobic stabilization process. Sludge is dewatered prior to composting. Using the treated sewage sludge to make compost has largely replaced the city’s former method of placing it on drying beds and then trucking the dried material to the local landfill (although a portion of the material is still handled that way). The thickened biosolids are then mixed with woodchips and sawdust that the city buys from a lumber mill in Evanston, Wyoming, about 45 miles away. (The mill delivers the material to Coalville). “It’s a great range of material, from split two by fours to sawdust,” says Gunn.

The converted grain elevator used for screening conveys the material through a 32-foot long trough that is 14 inches wide. The unscreened material is poured across a two-inch gravel screen with a shaker to remove the large materials. Then the sized material falls through a hopper onto the screen. The chain-driven conveyor belt has steel paddles that move the material across the 3/8th-inch screen. Fines are collected below in the chain return trough and discharged on the opposite end from the two-inch clean chips.

The elevator is normally set at a 12.5 degree slope, although the angle can be adjusted “depending on how big the chip sizes are,” Gunn says. He uses an eight-horsepower Honda motor to power the device.

While municipal wastewater treatment plants must regularly screen and test for metals such as arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, selenium, nickel and zinc, “we don’t have a history of problems with them, because we’re strictly handling domestic waste,” Gunn explains. “The ones that are in there are far below the EPA standards.”

At the beginning of the composting process, Gunn builds windrows 100 feet long and 5 feet high. Adjusting the size of each pile is an effective method to ensure that the desired temperature, moisture and oxygen conditions are maintained throughout the process, he notes. The large windrows “start out a little on the wet side, so they have enough moisture in them to continue the process. As they have a higher need for oxygen, we make them long and narrow so they get plenty of air.” As the piles mature and dry, they become lighter in weight – as a result, he adds, “they don’t compact as much so they can still get the air flow.”

The material is composted for 15 days at temperatures above 55°C. During the summer months, the active composting phase begins within three days, with internal temperatures running from the high 60s to mid-70s (degrees centigrade) until the curing stage.

The city, which began composting its treatment plant sludge in the late 1990s, will produce about 800 cubic yards of compost this year, Gunn reports. The city uses a certain amount of the finished product in landscaping at the local cemetery and other public areas. Sales to local residents and commercial growers have been increasing each season, as word spreads regarding the material’s effectiveness as a growing medium. (Sales have far surpassed the less than $2,000 invested in the grain elevator). The city sells the material for $15 a yard, producing four different grades of compost, depending on user needs. With an insert in the screen, also made of recycled “junk,” “we can produce fines the consistency of dirt,” Gunn adds.

Local demand for the sludge-based compost is still “a little weak” among growers who still have some misgivings about using sewage. But “everyone who uses it absolutely loves it,” Gunn says. “It’s amazing what benefits the biosolids have for compost. The plant growth is exponentially faster and the colors are so much deeper and richer.”As one example of the material’s fertility, after the Coalville public work’s director’s son built a new home, he used the material as a growing medium for his lawn seed. “A couple of weeks later, it looked like he had sodded his yard; the grass was one to two inches high. Around here (in Utah’s arid climate) that’s pretty fast growth,” comments Gunn.

In his quest for inexpensive ways to run the composting system, Gunn didn’t stop with the recycled grain elevator. He recently scavenged another discarded piece of equipment to improve the design of his screening device. He removed the sand hopper from the back of a municipal snow plow. He adapted the chain conveyor from the sand-spreader and is using it to feed solid waste material to the screen. “The chain drive and the screen will be operated hydraulically using the salvaged equipment from the snow plow truck, which will give us the ability to control the feed rate and speed of the screen depending on compost conditions, to optimize performance and quality,” he concludes. “It’s more efficient than manually sprinkling material on the screen.”

Where The Screening Idea Came From
The article that got Coalville, Utah’s David Gunn thinking about making the screen for his biosolids composter was in the October 1999 issue of BioCycle. Titled an “Innovative In-Vessel Composting System,” the article described how a commercial chicken farmer in Tasmania, Australia spent $3,000 on a secondhand concrete mixer that included a car engine and gears. Written by Martin Line – an agricultural microbiologist at the University of Tasmania – the report included the following details on how the homemade small-scale in-vessel system works to process chicken mortalities:

Chicken mortalities are loaded into the mixing chamber with poultry litter in a ratio of about 2:3 by volume. A total of approximately 1,200 birds are composted over a 21-day cycle, with the chamber being tipped up during the early stages of composting. Approximately six meters of waste are handled in 21 days. The machine is left open, with daily turning for ten minutes.

The end product shows some degree of fist-size balling, but is otherwise benign; it has been either spread over pasture or given to a local market gardener, who composts it further in open heaps prior to use. Moisture content has not been a problem, with the addition of chicken litter sawdust combining with evaporation from the chamber to keep free liquid to a minimum.

Running costs amount to $475/year in fuel, with servicing every six months (oil, points and plugs) adding $100/year. Total labor costs (the farmer’s time) are estimated to be about $900/year. If the machine is run at maximum capacity on a continuous cycle, it could deal with approximately 52 tons/year. Estimated operations and maintenance total about $1,475/year or $28/ton, which compares favorably with the static in-vessel units reported for composting food residuals from institutions in the U.S.

The clear advantages of using a redundant concrete mixer as an in-vessel composting unit are those of low cost outlay, provision of an built-iin turning and self-emptying system, and simplicity of operation. It should provide an attractive option for any small- to medium-scale poultry farm operator, with scope for automation if desired.

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