A solar drying system helps a Mississippi treatment plant save on biosolids management and progress toward Class A status
When the sun shines on Natchez, Miss., it does more than just brighten the day. It saves the Natchez Wastewater Treatment Facility tens of thousands of dollars in biosolids handling costs. Natchez is the site of a new “greenhouse” solar biosolids drying system. Operational for about one year, it is only the second such drying system in Mississippi.
“We used to have to haul our biosolids to a site near the Natchez airport for liquid injection,” explains plant manager Michael Stewart. “It was very expensive, because our solids content was only around 10 percent.”
Now most of that water is evaporated in the Thermo-System active solar drying system from Parkson Corp., and he feels the cake will be popular with farmers and gardeners when it comes time to begin distribution of the solids. “We’re not doing this to make money, but to save money,” Stewart says. “Even if we have to still haul some biosolids off site it will be a lot less expensive than it was. We don’t intend to ever need to haul wet sludge off site again.”
The Natchez facility serves about 7,400 customers in a historic city on the east bank of the Mississippi River. About 2.5 mgd (more during rainy periods) flows into the plant through a manual bar screen. Three 25 hp Fairbanks Morse submersible pumps move the flow through a Parshall flume with a metering device and then to an aerated grit chamber.
A Parkson automatic bar screen removes remaining debris before the flow passes to two aeration basins, each with four Ovivo Process surface aerators. Treated wastewater is clarified, chlorinated, and dechlorinated with sulfur dioxide before flowing by gravity to the Mississippi. An alarm system notifies operators of high levels in the influent wet well or of any leaks in the chlorination system. Natchez has an operations and maintenance staff of six, who work 10-hour shifts.
The biosolids process has changed significantly. In the past, aerobic digesters stabilized the biosolids, which then were pumped to a pair of 3-acre, 8-foot-deep lagoons on the plant site. About every 10 years, the lagoons were cleaned, and the largely liquid biosolids were trucked to fields near the airport, where a private contractor injected the material into the ground as fertilizer. The treatment facility produces about 575 dry tons of biosolids per year.
Even though the lagoon cleanings were infrequent, they were expensive. “It cost us more than $300,000 a year ago when we emptied just one-half of one of the lagoons,” Stewart says. “Ninety percent of what we removed and hauled away was water. And that still left 17,000 dry tons of solids in the two lagoons combined.”
Like many small cities, Natchez has been losing industry and population, especially during the recession. Reducing operating costs is critical. That’s why superintendent and city engineer David Gardner, the engineering firm of Williford, Gearhart and Knight Engineering, and Stewart’s team got together with the drying specialists at Parkson to investigate a new way to manage biosolids.
Gardner had read an article about Parkson’s solar sludge drying system, which draws 95 percent of its drying energy from the sun and is in use at more than 150 sites around the world. Parkson says that due to low operating costs, the system can achieve significant savings compared to alternatives like hauling and wet biosolids.
The solar drying system, built by a contractor, cost $5 million. The American Recovery and Reinvestment Act (federal stimulus program) provided $4.3 million, and Natchez borrowed the remainder with a 20-year, low-interest loan.
At Natchez, biosolids are pumped from the existing lagoons into a 65,000-gallon tank using a FLUMP dredge manufactured by SRS Crisafulli. From there, the material is pumped to a belt press (supplied by BDP Industries) and conditioned with a Sedifloc 680CL polymer from 3F Chimica Americas. The biosolids are delivered to the press at 2 to 2.5 percent solids and leave the press at about 22 percent solids. Filtrate from the press is directed to the head of the treatment plant.
“The press does a real good job,” says Stewart. He credits proper polymer type and belt tension for the high performance. “We tested five different polymers, working with suppliers, before we settled on the Sedifloc 680,” he says. “The ratio between the solids feed rate and polymer feed is critical, and we worked with polymer manufacturers’ experts to achieve this.”
The dewatered cake falls onto a conveyor belt that moves it into a hauling trailer. Manufactured by Henderson Products Inc., the trailer takes the cake to one of two 8,500-square-foot, 24-foot-high “greenhouse” structures, which are the heart of the solar drying system. The cake is spread onto the concrete floor until it reaches a depth of about six inches.
Each greenhouse is equipped with a “mole” — a computer-operated machine that moves randomly about the surface, tilling and leveling the cake throughout the greenhouse. The mole looks like a small Volkswagen car, about four feet long by four feet high. “It covers the entire area, corner to corner, and turns and aerates the cake pile as it goes,” he says.
The mole starts up automatically, according to the moisture content of the solids. The wetter the material, the more it runs, and as the solids dry, it runs less. “That’s one of the great features of the system,” says Stewart. “Like a smartphone, we have a smart mole.” Panels in the greenhouse roof are made of polycarbonate to withstand wind, and function just like the roof panels in a flower or plant greenhouse, letting in sunlight and concentrating heat. Exhaust fans come on automatically to control heat and humidity in each greenhouse.
The drying process is closely monitored by PLCs in monitoring sheds near each greenhouse. Terrence Logan, Natchez laboratory technician, takes samples twice each week, analyzes them in the onsite laboratory, and then inputs the data to the solar drying system PLC. The PLC instructs the operator to “start new batch” if the biosolids have reached the desired level of dryness (at least 75 percent solids) and meet Class A standards.
“We can get as much as 90 percent dry solids in as little as 10 days in the summer,” says Stewart. “In the winter, it may take a week longer.”
Heading to market
Construction on the new system commenced in March 2010 and wrapped up in December. Except for the interruption caused by this spring’s floods it has run continuously since. Sometime soon, Stewart and his team will start planning to market the material to area farmers and others as an agricultural or horticultural amendment.
Stewart has received quite a bit of interest and also points out that the dried material might also serve as a fuel, since it has the same heating value as brown coal. He’s optimistic that Natchez has found a cost-effective way to manage biosolids. “The solar drying system uses the cheapest energy source available — the sun,” he says. “If it’s approved as a Class A biosolids by the Mississippi Department of Environmental Quality, it will be a marketable product.”
He notes that the solar drying system keeps human interface with biosolids to a minimum and that the system is safe to operate. “The process is doing exactly what we envisioned,” he says. “We’re excited about it.”
He expects to see more solar drying systems installed in his state and around the country: “It’s going to be more and more difficult for treatment plants to manage biosolids by land spreading because of stricter regulations or lack of space. We think this is the way to go.”