Music City Gold

It turns out Nashville residents are putting more of themselves into their local parks and gardens than they may realize. Since 2009, Metro Water Services has been turning the nutrients from human waste into fertilizer at its Central Biosolids Facility. The fertilizer — which MWS refers to as “Music City Gold” — is used at many of the city’s public parks, including Centennial Park, as well as in 15 community gardens and the landscape at Vanderbilt University and at the home of the Nashville Sounds, First Horizon Park.

Categorized by the Environmental Protection Agency as a Class A EQ Biosolid, the organically rich fertilizer is safe and legal to use on farms, in vegetable gardens and in public parks. Class A EQ Biosolid is a designation for dewatered and heated sewage sludge that meets EPA guidelines for land application with no restrictions.

During the wastewater treatment process, both water and solids are reclaimed from the sewage system. The water is cleaned through a complex series of steps before being returned to the Cumberland River. After the water is separated and cleaned, the remaining solids and microorganisms are pumped to the biosolids facility. In a heated tank, microorganisms digest the organic nutrients from the other solids and release methane gas. Next, the microorganisms are heated and dried to destroy any remaining pathogens and remove moisture. MWS’ dryers are powered by reusing the methane gas released during the digestion process. The dried biosolid pellets are Music City Gold.

Biosolids have cut down on MWS’ operating expenses, conserved landfill space and eliminated greenhouse gases. In 2020, Metro Water produced more than 19,000 tons of fertilizer at its biosolids facility, according to MWS communications director Sonia Allman.

“While 19,000 tons of fertilizer may seem like a large number, it’s only about 20 percent of the 103,000 tons of sludge that MWS hauled to landfills before our biosolids facility was constructed,” Allman tells the Scene. “[The Central Biosolids Facility] has significantly reduced transportation expenses and landfill fees.”

Last year, those savings equated to more than $4 million, Allman says.

Before the Central Biosolids Facility was constructed in 2009, about 20 truckloads of the biomatter were taken to landfills each day. Allman says roughly 200,000 cubic yards of landfill space — the size of 61 olympic pools — was conserved and 120,000 metric tons of greenhouse gas emissions from landfill decomposition were eliminated in 2020.

Metro Water Services uses microbes and high temperatures to repurpose the nutrients in the sewage into fertilizer. First, Whites Creek and Central Wastewater treatment plants pump their extra microorganisms and other materials through pipes to the biosolids facility. When it arrives at the facility, the muddy-looking water goes into a tank for processing. Air bubbles are added to float the solid material to the top of the tank, and food-grade polymers are added to help the solids stick together. Rakes pull the solid material out of the water. The water that is removed is sent back across the street to the wastewater treatment facility, where it is cleaned and sent to the Cumberland River. Meanwhile, the solids are pumped into digesters.

Allman compares the facility’s digesters to stomachs, with microbes breaking down the material. To keep the microbes active, the digesters are kept at a constant 95 degrees Fahrenheit. Like a stomach, the digesters create gas, and that gas is 60 percent methane. MWS stores the methane gas and uses it to run the equipment in the plant. The boilers are powered using methane gas, and the hot water is used to keep the materials inside at the necessary temperature. 

After 15 days, the water and microbes that have eaten the nutrient-rich materials are removed from the digester. Polymers are added to the mixture before it is placed into a centrifuge to separate the water. Again, water is cleaned and returned to the river. According to Allman, the material coming out of the centrifuge resembles a dark, wet cake and has very little odor. The “wet cake” is mixed with fertilizer pellets recycled from a previous day’s work. 

“Similar to baking a cake, consistency is key,” Allman explains. “You have to make sure you add the right amount of flour to your batter, before you bake it.”

Once the cake and fertilizer are combined, the mixture is transferred to a dryer where it is heated to more than 1,000 degrees Fahrenheit. The dryer is powered by the methane gas released from the microbes in the digester. Inside the dryer, moisture evaporates and the wet cake sticks to the small pellets. Then, the pellets are sifted and separated by size. Most of the pellets are the right size to fit the spreading equipment farmers use. However, pellets that are too large are crushed and mixed with pellets  that are too small. These fertilizer pellets are recycled and will be mixed with the wet cake that goes into the dryer to make more pellets.

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