Best of Two Worlds

The San Antonio Water System applies belt filter presses and sand drying beds to prepare biosolids for its contracted composting process

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The managers of San Antonio’s water recycling operations believe in working with nature. Today, the city’s three recycling plants use the sun’s warmth as a major part of their strategy for drying biosolids.

Drying beds covering some 25 acres work in concert with belt filter presses to produce dewatered material for contracted composting operations. All told, the San Antonio Water System’s three water recycling plants produce 37,000 dry tons of biosolids per year.


The staff of the SAWS centralized biosolids processing facility at the Dos Rios Water Recycling Center (WRC) focuses on optimizing both drying processes. That includes deploying natural predators for vector control on and around the drying beds.


An experienced staff keeps the biosolids facility running smoothly. It’s all part of a management program that is increasing beneficial use of both biosolids and the methane gas from the anaerobic digesters.


Combining flows

The Dos Rios WRC (design flow 125 mgd) receives combined biosolids from the Medio Creek WRC (16 mgd) and the Leon Creek WRC (46 mgd). Dos Rios and Leon Creek are conventional activated sludge facilities, while Medio Creek uses extended aeration.


Tad Eaton, WRC manager, previously worked at the utility’s compost facility. When management decided to contract for composting operations, Eaton moved upstream and began working to improve operations of the drying beds and other biosolids processes.


Solids arriving at Dos Rios are blended, thickened and digested before being dewatered. “The weather in San Antonio is conducive to drying beds, but it’s also unpredictable,” says Robert Yrle, director of treatment operations for San Antonio Water System (SAWS), the umbrella organization for wastewater and biosolids management. “We handle 400 wet tons of solids a day. Because of the high volume we have to balance both methods.”


Dos Rios handles two types of solids. Combined waste activated sludge (WAS) from all three facilities is first treated using a diffused air floatation thickener (Eimco Water Technologies). First-stage activated sludge from


Dos Rios goes through one of two Ashbrook Simon-Hartley gravity belt thickeners. (Within the year, the diffused air floatation thickeners will be taken out of service and replaced with two more gravity belt thickeners.)


Using the methane

After thickening, the two streams are combined at 4.7 percent solids. From the blend tank, they are run through strain presses (Parkson Corporation) where solids are augered through a fine mesh screen, removing foreign materials like hair and grass, which are detrimental to the digestion process and associated equipment. The material then goes to one of eight 2.2-million-gallon anaerobic digesters.

A portion of the digester gas (methane) is used to fire boilers that heat the digesters. During fall and winter, about 20 percent of total methane production goes to the boiler, and in summer, 10 to 15 percent. Excess methane is flared. A project is now underway to transport methane to a retail distribution pipeline.


Solids remain in the digesters for 25 days and would stay longer if Eaton had his way. “Longer means more gas produced and more volatiles destroyed,” he says. From the digesters, the material is pumped to one of two holding tanks. Depending on the weather, solids are sent to the drying beds or belt filter presses.


Applying experience

“The plant was designed to use drying beds exclusively, but there were operational issues, so 20 years ago the belt filter press was added,” Yrle says. “Through the years, the two processes worked in tandem. Then about 10 or 12 years ago, we decided to focus on improving the efficiency of both. With our expertise and our employees’ experience, we use both to our advantage.”


The facility operates a dozen 2.2-meter Winklepress filter presses (Ashbrook Simon-Hartley) that produce material at approximately 20 percent solids. Solids not destined for the belt filter press are distributed to one of the 132 drying beds next to the biosolids processing area. While those beds encompass 25 acres, they are less of an imposition than one might think, considering that the Dos Rios WRC spans 700 acres.


“There is minimal cost in labor and electricity,” says Eaton. “And the end result is a product that’s 80 to 85 percent solids. We can’t get that off the belt filter press or a centrifuge.”


Solids from the storage tank are pumped to the beds through an underground manifold piping network. Each bed has its own controls and must be manually opened and closed. The beds are 84 by 100 feet and are poured to a depth of 10 to 18 inches, depending on the time of year (shallower in fall and winter, deeper in spring and summer). There are 12 rows of 11 beds.


Filtrate from the beds passes through an underdrain system with a 12-inch top layer of fine aggregate mix and a 12-inch bottom layer of coarse aggregate. Perforated 6-inch pipes collect the filtrate and allow it to drain through 8-inch collection lines to pump stations, which return the filtrate to the Dos Rios plant headworks.


The biosolids are not turned in the drying beds. When the material reaches about 80 percent solids, an operator uses a Bobcat compact track loader to remove the dried biosolids from the beds. “The time spent in the bed varies, but there are some extremes,” says Eaton. “I’ve seen them pulling off dry solids in seven days when we’ve had temperatures in the 100s. The other extreme was the last six months when we had our highest rainfall in San Antonio for years. We poured in October and didn’t clean out until February. On average, it’s about 15 to 20 days.”


Composting operations

Biosolids from both the filter presses and the drying beds are stockpiled for no longer than three days. Material is removed during work hours Monday through Friday and as needed on Saturday. Composting contractors haul the material.


San Antonio used to operate its own composting facility at the Leon Creek WRC, but in 2001 the agency contracted with Texas Disposal Systems Garden-Ville to operate the facility. Since 2008, SAWS has also contracted with New Earth Soils and Compost, which operates its own composting facility 22 miles away. Both companies sell various mixes of commercial compost.


“Basically, we made some really good compost, but we didn’t market it very well,” says Eaton. “Both Garden-Ville and New Earth are really good at marketing. And because of that, they’re better at selling, they get better prices, and their operations are more cost-effective.”


Long-term contracts stabilize the process. San Antonio is required to provide material at 18 percent solids from the belt filter press and 50 to 95 percent solids from the drying beds. The expectation is that 70 percent of the solids will come from the belt filter press.


At the composting facilities, the biosolids are mixed with local wood chips and formed into windrows, where they are turned and allowed to cure. Wood chips come from the City of San Antonio, nearby military bases, City Public Service Energy, the local electric utility, and various other commercial landscaping and tree trimming sources.


Even with cost-effective composting, a portion of solids always needs to be landfilled. “Our goal for biosolids was for 80 percent to go to beneficial use,” says Eaton. “Last year, 98 percent was beneficially used, and 2 percent did go to the landfill. And it always pays to have multiple avenues of disposal.”


Eaton sees composting as a budding business. “We feel there will be even more players in the composting business because the business is growing and retail markets are expanding,” he says. “In 2009, between the two compost contracts, 300,000 cubic yards of compost was produced and 500,000 cubic yards of carbon material was used.”


Managing vectors

In operating sand drying beds, it helps that the facility is on 700 rural acres. As people move closer, there could be more challenges, but Yrle and Eaton feel up to the task. “We’ll always have a challenge with public perception,” Yrle says. “It’s the nature of the business. If it happens, we will overcome it and find a solution. We’ve worked together for 20 years, and we’ve resolved many issues to the satisfaction of the public.”


One issue with potential to flare public opinion is insects around the drying beds. To head off problems, Eaton started a program at Dos Rios similar to one he applied with success when he ran the SAWS composting facility. “We purchased and installed eighty 12-compartment purple martin houses surrounding the solids drying beds,” he says. “This effort is for fly control, but only adult flies.” The team also attacks the flies by deploying tiny parasitic wasps that feed on the fly pupae.


Another problem is drain flies, commonly found on trickling filters. “About five years ago, they just showed up. They caught us off guard,” says Eaton. To manage these pests, workers inject Bt (a microbial insecticide) and methoprene (a growth regulator) into the solids as they are being pumped to the drying beds.


Pests were one reason the drying beds weren’t fully utilized until Eaton investigated various control strategies. “A guy would fog with pesticide, putting toxic chemicals on the drying beds,” says Eaton. “Employees would have to go inside, which kept them from working. It wasn’t friendly or green.”


Yrle observes, “Now that Tad has developed this program, vectors issues are minimized. Because we can control odor and vectors, we’re able to use the drying beds to our advantage.” But Eaton is always looking to improve operations.


“If solids were able to stay longer in the digester, there would be less odor and even less vectors,” Eaton says. And as improvements are made at the facility, Eaton may just get his wish.


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