Sludge to solid: Double the green
NATCHEZ — The City of Natchez may soon be able to bag and tag a recycled product that was once flushed down residents’ toilets.
The Natchez Wastewater Treatment Plant’s new solar-powered greenhouse recently produced its first batch of biosolids made from decontaminated and dried out sewage sludge.
The batch was completed last week, and the greenhouse’s computer system is currently manipulating weather conditions to speed up the evaporation process for the second batch.
The result is a dirt-like biosolid that, when confirmed grade-A by the Mississippi Department of Environmental Quality, can be used and even sold as organic fertilizer.
“Farmers can apply it to pastures as fertilizer; it’s like a compost,” City Engineer David Gardner said.
Gardner said the system takes household sewage from sinks and toilets and treats it through a natural process.
The water is separated from the sludge and made clean enough to pump into the Mississippi River through a gravity flow piping system.
Reducing the sludge to a solid and treating it to be usable does three things: it eliminates the liability of storing the toxic sludge in a DEQ-approved dump site, saves the city an average expense of $200,000 a year to transport a sludge product to a dump site near the airport and creates a marketable compost when dried.
The waste water treatment plant produces 576 dry tons of the sludge every year. Gardner said it would take 25 years to empty the two lagoons, converting the yearly waste along with gradually emptying the 17,000 dry tons already in the lagoon.
The Department of Environmental Quality classifies the sludge as a toxic hazard and only permits the sludge to remain in the lagoons for 10 years.
Rather than continue to collect the sludge in lagoons or pile up the toxic waste near the airport, incurring the cost of transporting a 90-percent water product, the new system can begin the process of recycling the waste.
Savings and income
Gardner said the new system will save the city an average of $200,000 each year. The city recently spent $300,000 hauling away a few years worth of sludge, he said.
“We were hauling 90 percent water,” Gardner said.
At this stage, Gardner said the main financial benefit of creating grade-A biosolids is the savings.
“The main thing is savings, not income,” Gardner said.
With gasoline prices rising, Gardner said the average annual savings of $200,000 would probably increase since the savings were calculated using the price of gas per gallon at $2.50.
Gardner said once the product is tested and confirmed as grade-A by DEQ, the city can market the dry product to farmers, give it away or bag it for sale at retail stores.
Tests on the final product will be sent to the DEQ after a minimum of three batches is produced in order to give composite results, Gardner said.
“(The dry product) is very rich and organic,” Gardner said.
Ernie Jacobs with Parkson, the green house manufacturing company, directed a tour of the greenhouse last Friday for city officials. Jacobs showed off the first batch of environmentally friendly dried sludge.
Jacobs said the same product made in Carmel, Ind., is selling at Home Depot.
“They’re calling it Carmel Green,” Parkson said.
“(The finished product) is the same stuff you see in those little tubes for $7 or $8 at Home Depot,” he said.
Jacobs said the dried biosolids can also be used as a fuel source, having the same efficiency as brown coal.
Gardner said coal-to-liquid fueld producer Rentech Industries has expressed interest in using the product.
How it works
When the process began, the sludge collected from the city’s sewage system was 20 percent solid and dispersed on the floor in a 6-inch layer.
The Natchez Wastewater Treatment Plant produces 576 dry tons of the sludge every year. Gardner said it would take 25 years to empty the two lagoons, converting the yearly waste along with gradually emptying the 17,000 dry tons already in the lagoon.
The process to create the class-A biosolid starts with a newly installed fine mechanical screen filter system. The filter system is used to sort rags, jugs and other large debris that will be automatically pumped into a Dumpster.
From there, the sludge goes into a contaminating chamber where the waste is introduced to naturally occurring bacteria that feed on the waste. Gardner said it becomes a feeding frenzy once oxygen is introduced to the mix from air raiders.
Stalked ciliates, nematodes and rotifers are among the microorganism that help clean the water and waste, plant Manager Michael Stewart said.
Once the bacteria are full, they go into clarifier that separates the sludge from the liquids.
The water is then chlorinated to kill the bacteria and de-chlorinated with sulfur dioxide, before it is sent to the Mississippi River using a gravity piping system.
The sludge is approximately 20-percent solid at that point when it pumps into a 64-gallon tank.
Polymer is then added to the sludge for thickness pumped onto a belt filter press, giving it a brownie consistence that is easy to transport.
Trucks then load the brownie product in the greenhouses six inches deep.
The greenhouse, a solar drying unit, harnesses the energy of the sun to reduce the brownies volume until it is at least 75 percent solid.
“We’re using the cheapest source of energy which is free to us — the sun,” Jacobs said.
“It came out very well. We put (the sludge) in as 20 percent dry sludge, and in four weeks now its 90 percent dry solids, which is excellent,” Stewart said.
Sludge must be 75 percent solid and meet other specifications to be considered grade-A.
“You look at it in magazines and see pictures, and you’re always anticipating your sludge will look like (the sludge) in the pictures; and it does,” Stewart said.
Jacobs said the greenhouse computer system monitors sunlight, temperature, humidity and wind.
When the beds are loaded with six inches of sludge, a technician pushes a button that reads, “Start new batch,” the process is completely automatic until it is complete.
“(The system) keeps human interfaces to a minimum,” Jacobs said.
Jacobs the solar drying system was originally invented to dry fruit and wood.
The computer will monitor and adjust settings every two seconds to optimize the environment. It even controls a plow called a mole, which resembled a new-age go-cart and tills the product in a random motion.
Other than till randomly, the mole has only two settings, one to wiggle itself out of a corner and another to till near the walls where moisture is more prone to collect.
“Other than that he runs around like a kid in a sandbox,” Jacobs said.
Jacobs said Natchez’s climate has favorable drying conditions and a batch could take as little as 10 days to dry out during hot months.
The greenhouse structure is constructed with poly carbonate, which can withstand high winds.
Parkson has designed greenhouse systems for 17 or 18 cities in the United States and approximately 150 globally, Jacobs said.
Jacobs said Parkson has had zero reports of explosions or injuries associated with its solar drying system.
“Parkson has been doing this (process) over in Europe for years, but it’s just now catching on in the United States; it’s using a lot of German technology,” Gardner said.
The upgrades including the greenhouse and filter system are partially being built using $4.3 million in American Recovery and Reinvestment Act funds, and Natchez Water Works will take out a 20-year loan at 1.57 percent interest to make up the difference.
The original estimate for the work was $5.8 million.
“This will be a good use of taxpayer money and will keep our rates competitive,” Gardner said.
Construction began on the plant upgrades, especially on the air raiders and containment tank in early 2010, Gardner said.
Greenhouse construction started last fall and was completed approximately three months ago.
The plant employs six people, including Stewart. Stewart said technicians are constantly testing the waste water products the plant produces at every step of the process to ensure its quality.
Big for Natchez
Before the tour, Stewart was patted on the back and complimented on the wastewater sludge biosolid.
Stewart said he is excited for the City of Natchez to have one of two similar systems in the state.
The other wastewater treatment plant was built in Clinton roughly the same time the one in Natchez was built.
“This is big,” Stewart said.