An excellent biosolids beneficial use program in Texas is about to get even better.

The Trinity River Authority’s Central Regional Wastewater System, which now applies some 69,000 dry tons per year of dewatered Class A biosolids to nearly 90,000 acres of cropland, is planning major advances in the solids side of its process in the next four years.

The net result likely will be a lower volume of a higher-quality material, saving on hauling costs and opening possibilities for new uses, such as for lawn and landscape fertilizer. The planning is still underway and many details remain to be decided. What’s certain is that the authority will continue improving its solids processes with an eye on changing regulations and the use of technology to enhance sophistication.

“Much of what I tell you about our solids process today may no longer be true four or 4 1/2 years from now,” says Bill Tatum, project manager for the Central Regional system. “We started a solids master plan in 2009 and carefully evaluated six alternatives. The option with the highest opportunity for return on investment included thermal hydrolysis and anaerobic digestion — technologies that create new opportunities for our treated solids stream. We still have to determine exactly what those opportunities may include.”

Texas-sized territory

The Trinity River Authority (TRA) of Texas is a conservation and reclamation district that provides water and wastewater treatment along with recreation and reservoir facilities for municipalities in the Trinity River basin. The basin, spanning some 18,000 square miles, cuts a generally north-south swath across east Texas with Dallas-Fort Worth at its approximate geographic center.

The authority operates five wastewater treatment plants and six water-supply systems. The Central Regional Wastewater System, by far the largest with 162 mgd design capacity and 130 mgd average flow, serves 21 cities including Arlington, Irving, Grand Prairie and part of western Dallas, about 1.2 million people in all.

The Central Regional system has beneficially used biosolids since 1996. The biosolids from the other four wastewater treatment plants (design capacities 3 to 24 mgd) are landfilled. “Every time we do a master plan for those facilities, we look at the business case,” says Julie Hunt, P.E., assistant manager of operations for the TRA’s Northern Region. “Right now, the business cases for those four plants indicate that it’s more economical for our customer cities if we take the biosolids to the landfill.”

Tertiary process

The TRA’s Central Regional system discharges excellent-quality effluent to the Trinity River with a tertiary treatment process. Influent passes through coarse and then fine bar screens, removing debris down to about 6 mm. Two pump stations, each rated 350 mgd, deliver screened wastewater to parallel north and south treatment trains that begin with grit removal, followed by settling in eight primary clarifiers.

Primary effluent enters 12 aeration basins, each holding 3 million gallons, with fine-bubble diffusers. The outflow from each basin enters two secondary clarifiers with traveling bridges that siphon activated sludge for wasting or return to aeration. (In a year or so, the TRA plans to add anaerobic selector technology to the aeration process for phosphorus removal.)

Flow from the final clarifiers passes to tertiary filtration, where 30 sand filters are being progressively replaced by AquaDiamond cloth media filters (Aqua-Aerobic Systems). “We are retrofitting the cloth filters in the existing sand filter basins,” says Tatum. “The sand filters are rated at 7.5 mgd, whereas we can get 25 mgd off the Aqua-Aerobic filters. A master plan 10 years ago called for adding 10 more sand filters, but in 2005 we switched gears and looked at the cloth filtration technology. Instead of adding more sand filters, we will ultimately have 16 cloth media filters in those 30 sand filter basins. At present, we have 10.”

Final effluent from the filters typically contains about 1.0 mg/L BOD and 1.0 mg/L TSS. The effluent is disinfected with chlorine for a 20-minute contact time and dechlorinated with sulfur dioxide before discharge to the river.

Processing solids

On the solids side, primary sludge is gravity-fed into four gravity thickeners. For waste activated sludge, a dissolved air flotation process is being phased out in favor of gravity belt thickeners (Alfa Laval Ashbrook Simon-Hartley). Four 2-meter units are in place, and two 3-meter units are being installed.

The thickened primary and waste activated sludges contain 4 to 4.5 percent solids. “Those are mechanically mixed in two sludge-blending tanks of about 125,000 gallons each,” says Tatum. “The blended sludge is drawn out by pumps for dewatering.”

There are two separate dewatering processes. About one-fourth of the material is conditioned with ferric chloride and lime and dewatered on three 2-meter Passavant Chamber filter presses (Bilfinger Water Technologies). “We have a dose rate of about 25 percent lime and 5 to 6 percent ferric chloride,” says Tatum. “We wind up with about 43 percent solids cake. That material drops out of the filter presses directly into trucks we own. We haul it to an area where it is stored for five to seven days, after which it is agronomically applied as Class A biosolids to hay ground in the Dallas-Fort Worth metroplex area.”

The balance of the biosolids is dosed with polymer and dewatered to about 26 percent solids on four 2-meter belt presses (one Andritz, three Alfa Laval Ashbrook Simon-Hartley). In the next two to three years, TRA proposes to add two more belt presses to replace the filter press dewatering process.

“When the material comes out of the belt presses, we convey that to a lime stabilization process [RDP Technologies],” says Tatum. “We add pulverized lime at a dose rate of about 30 percent and mix the material with thermal blenders. Then we hold it for 30 minutes in a pasteurization vessel to achieve the temperature required to qualify it as Class A.”

Material coming out of the pasteurization vessel is placed in piles, then stored in bins for 24 hours and monitored to verify the required pH for Class A status (pH 12 for two hours, pH 11.5 for 22 hours).

To the land

Contractor Renda Environmental removes material from both solids process trains and applies it to farmland, mostly within 35 to 50 miles of the treatment plant. The firm typically hauls about 36 truckloads per day, each with a 25-ton payload. The program operates year-round. Farmers pay $10 per acre at the time they sign up. TRA’s 2014 land application budget was $4.7 million.

“We save the farmers a lot of money on fertilization,” says Tatum. “They get a much better crop yield at a dramatically reduced cost. It’s a very good program from an economical and agronomic standpoint. If you look at what farmers are charged throughout the U.S., I don’t think you’ll find many other programs as economical for them as ours.”

Renda handles all noticing for application sites (permitting is not required for Class A material) and maintains complete application records. Every August, the TRA submits a yearly summary to the Texas Commission on Environmental Quality. “It’s a verification and certification of all the Class A material we produced,” says Tatum. “We show where everything went. We have extensive records of our land applications, from cradle to grave.”

Better things ahead

TRA is now exploring thermal hydrolysis and digestion of biosolids at the Central Regional system, with the aim to reduce the volume of solids handled.

Tatum notes that thermal hydrolysis began in Europe about 20 years ago and has been steadily refined. During the process, primary and waste activated sludges at about 4 percent solids are blended in a holding tank under anaerobic conditions to achieve release of about one-third of the biological phosphorus.

Material emerging from that tank is pre-thickened with centrifuges or belt presses to 13 to 15 percent solids and transferred to a stainless-steel pulper/preheating vessel, from which it is fed at a metered rate into the thermal hydrolysis reactor.

“One of the systems we are considering has four reactors, each one holding 6 cubic meters of material,” says Tatum. “The reactors are filled, topped off with steam and brought up to about 330 degrees F at 160 psi for 20 minutes. Each reactor then discharges into a flash tank.”

When the material emerges from the bioreactor, the microorganisms’ cell walls are ruptured (lysis). The resulting material is held in the flash tank for an interval, then cooled to about 110 degrees F and fed to anaerobic digesters. A portion of digester material is continuously recirculated to the hydrolysis process.

“In the end, you can get 60 to 65 percent volatile solids reduction, which is a major reduction,” says Tatum. “You get methane gas, and you get an odorless Class A biosolids on which biological regrowth will not occur.”

TRA expects to burn some of the digester methane (biogas) as fuel to heat the thermal hydrolysis process. “We still haven’t determined what we will do with the remaining gas,” Tatum says. “Combined heat and power may not be practical because our utility electric rates are very low, at about 4.8 cents per kilowatt-hour. We may look at using the methane to dry a portion of the biosolids. Taking the material from about 30 percent to more than 90 percent solids would reduce hauling costs significantly.”

It’s the kind of innovation that helps a biosolids program make the leap from good to great.  

More Information

Alfa Laval Ashbrook Simon-Hartley - 866/253-2528 - www.alfalaval.us

Andritz Separation, Inc. - 800/433-5161 - www.andritz.com/separation

Aqua-Aerobic Systems, Inc. - 877/271-9694 - www.aqua-aerobic.com

Bilfinger Water Technologies, Inc. - 800/833-9473 - www.water.bilfinger.com

RDP Technologies, Inc. - 610/650-9900 - www.rdptech.com