The Hornsby Bend Biosolids Management Plant has come a long way since its creation in the 1950s. Back then, its lagoons stored biosolids from the city of Austin’s wastewater treatment plant. After treatment, all the separated water was discharged to the Colorado River under a discharge permit.

“That’s just how things were done here back then,” observes Ken Lockard, facility superintendent.

Today, Hornsby Bend is a zero-discharge facility that produces Class B biosolids for land application, uses the city’s yard wastes to create Class A biosolids compost for wholesale and retail trade, and generates biogas to produce electricity for sale to the power grid and heat for plant processes. The city has a history of beneficially using all biosolids produced as either Class A or B, keeping it out of the landfill.

On the radar screen for the future is the possible addition of residential food waste and fats, oils and grease (FOG) to the plant’s eight anaerobic digesters, producing still more biogas and potentially more renewable energy. All that is in line with the city’s larger green energy and waste reduction master plans.

The facility’s present operations and future outlook have been strengthened by federal stimulus-funded projects that improved the composting facilities, significantly enhanced digester performance and added a biogas-fueled cogeneration system. Jody Slagle, biosolids reuse manager, observes, “We are beginning to transform our thinking from just handling biosolids to taking a broader role as a site for resource recovery.”

Reuse pioneer

Austin, a heart-of-Texas city of 900,000 and the state capital, has been a national leader in beneficial use of biosolids. Land application of Class B material began in the early 1980s. Production of compost, brand-named Dillo Dirt (after the armadillo, the city’s unofficial mascot), began on a pilot basis in 1987 and went commercial two years later.

Hornsby Bend receives a 70-30 mixture of primary and waste activated sludges — about 1.5 mgd in total — via pipeline from the city’s two secondary wastewater treatment plants:

Walnut Creek (75 mgd design, 52 mgd average, about 8 miles away)

South Austin Regional (75 mgd design, 42 mgd average, about 3 miles away)

Biosolids are screened, thickened and anaerobically digested, then dewatered on belt presses for land application or to mix with ground wood and grass clippings for composting. Net annual biosolids production is about 20,000 dry tons.

The Hornsby Bend site is also home to the Austin Water Utility Center for Environmental Research, a joint project with the University of Texas and Texas A&M University devoted to ecology and sustainability studies. In addition, the site’s 200 acres of ponds are a magnet for migrating waterfowl and a top area destination for birdwatchers.

Long evolution

The city acquired the Hornsby Bend property over several decades starting in the 1950s at a bend in the Colorado River. Today the facility totals about 1,200 acres, about 800 acres actively used. The rest of the land is left as a natural buffer from the river and is mostly open to the public. In the mid-1960s, a conventional activated sludge wastewater treatment plant with its own discharge permit was built on site to serve nearby Bergstrom Air Force Base and the surrounding community of Del Valle.

The modern version of Hornsby Bend began taking shape in the 1980s. Discharge from the biosolids lagoons to the river stopped, the digesters were built, and five 5-acre open-air concrete evaporation basins were added to dry biosolids for land application. Today, about two-thirds of the biosolids are land-applied, 70 percent of that offsite by a contractor and the rest on 454 acres at Hornsby Bend, where a contract farmer grows hay and pays the city a share of the sale proceeds (about $60,000 last year).

The Dillo Dirt program, which takes care of the remaining one-third of biosolids, expanded greatly in the mid-1990s. The Air Force base closed, and the city relocated its airport to that site. That meant the city’s landfill, which was next to the base, had to close by order of the Federal Aviation Administration because it attracted birds that could jeopardize aircraft.

Faced with paying tipping fees to a private landfill, the city needed to reduce waste volume and did so by diverting yard waste and tree trimmings — 15 percent of the waste stream — to Hornsby Bend for composting. The concrete composting pad was doubled in size to 14 acres and grinding equipment added.

Then, as now, the city used the windrow composting method. “It’s a simple, fairly forgiving process,” Slagle says. Team members use Wildcat (Vermeer) and Scarab windrow turners to mix the material.

After windrow processing, the material is cured, screened and sold wholesale for $13 per cubic yard to about 50 garden centers, nurseries and landscapers, who in turn sell it to consumers. “Our approach has been to keep our end of this operation as simple as possible,” says Lockard. “Rather than dealing with mom and pop customers and trying to bag the material, we keep the focus on producing a quality compost product.” Dillo Dirt generates about $250,000 in annual revenue.

To the next level

The past few years have seen dramatic steps forward at Hornsby. The city took advantage of the American Reinvestment and Recovery Act of 2008 (federal stimulus) to get a $31.8 million zero-interest loan for wide-ranging improvements.

A key component was a new 15-acre concrete compost pad. “We had already laid it out the way we wanted it, with slopes and retention basins to hold a 100-year flood event,” says Slagle. “Our consultant had estimated a cost of $12 million, but we were able to build it for a little over $7 million.” That’s because the economy was down, materials were cheap and contractors were hungry for work.

All water that runs off the old and new pads is captured and treated on site. “We haven’t discharged at Hornsby Bend for over 25 years,” says Lockard. “We have an irrigation system installed back in the 1980s with four large central pivots that handle the water from our ponds. All our process water and part of our stormwater goes through our Bergstrom Treatment plant, which we now call the Side Stream Treatment Plant.

“Water then goes through our pond system and eventually is used to irrigate about 150 acres on site. With the Class B land application and on-site irrigation, we keep our farmers happy. Even during drought conditions, we are the ‘green spot’ in the county.”

The old 14-acre concrete pad is now used for compost curing and screening and for loading customers’ trucks. Also on that pad is a newly remodeled structure containing three new belt presses (Alfa Laval Ashbrook Simon-Hartley) that dewater digested biosolids to 18 to 20 percent solids. Space has been allocated for a fourth press.

Digestion re-imagined

The bulk of the federal stimulus funds went to a major upgrade of the plant’s eight 2-million-gallon digesters, split into two complexes of four units each. The aims were to increase energy efficiency, expand capacity and increase biogas generation. “The digesters were operating at about half their capacity,” recalls Fred Ramirez, facilities engineer. “There was so much buildup at the bottoms that we were only able to use the top half. They had floating covers that were corroding. They were not being well-heated or well-mixed.”

Contractor Parker Ag Services cleaned out some 25 years of sediment. Two sludge holding tanks were structurally relined and the digester interiors recoated. Gravity belt thickeners (Alfa Laval Ashbrook Simon-Hartley) were updated with new controls and an automated polymer feed system ahead of the digesters.

Old mixing systems in each digester that drew 125 hp were replaced with 20 hp LM (Linear Motion) mixers (Ovivo). Fixed digester covers replaced the floating covers and a dual-membrane gas holder (Evoqua Water Technologies) were added to increase biogas storage capacity.

Process ingenuity

Process changes have also enhanced performance. Two old digesters that receive sludge from the pipeline were reconfigured long ago as a flow equalization basin and blending tank, and were recently fitted with LM. “Our operators noticed an almost immediate improvement in sludge quality,” says Ramirez. “We are getting a much more homogeneous material than before.”

Ferric chloride for odor control is fed into the material at the screens upstream from the equalization basin, a concession to housing developments already built and being planned near Hornsby Bend.

The operation of the digesters themselves has also changed: Three units in each complex now serve as primary digesters and the fourth (secondary) unit receives material from the other three before it is sent to the belt presses. “We continuously feed the primary digesters on a two-hour cycle,” says Lockard. “That is, we feed one tank for two hours, then move to second for two hours, and so on.

“The primary tanks are full at all times. There is an overflow mechanism so that when we’re feeding, material flows to the secondary digester, which acts as a gas holder and is our main gas production tank.” Total retention time is 23 to 25 days.

The improvement in performance has been notable. “The primary/secondary treatment configuration enables the acid phase and gas-producing phase of digestion to be better separated. That means we get more gas, and better solids destruction,” Lockard says.

“Before, we were running all eight digesters and only getting volatile solids reduction percentages in the low 40s. Recently, because of construction issues on the site, we have been running only one complex of four digesters, yet we’re handling the same or slightly more flow. Volatile solids reduction percentage is now in the upper 50s to low 60s, depending on the season — a little higher in summer. Gas production has increased from 500,000 to more than 700,000 standard cubic feet per day. So all signs indicate that everything is working the way we intended.”

Fuel in abundance

The increase in biogas is part of the impetus behind a second $5.3 million stimulus project funded by the U.S. Department of Energy, Austin Energy and the Austin Water Utility to install an 848 kWh engine-generator unit (GE Energy). Other parts of the project include upgrades to gas conditioning, hot water piping, and electrical and instrumentation systems and controls.

Cogeneration at Hornsby Bend goes back to the late 1980s with installation of two 400 kWh diesel engine-generators converted to burn digester gas. Fuel impurities severely limited those engines’ performance, most notably siloxanes, which are compounds of silicon found in cosmetics and personal-care products that can form hard, ceramic-like deposits on internal engine components during the heat of combustion.

The new system is specially designed to burn biogas and includes a fuel conditioning system that removes siloxanes, moisture and acid-forming sulfur compounds before they reach the engine. As a result, the system operates continuously at full load. Heat recovered from the engine exhaust and fluids provides heat for the digesters; biogas-fueled boilers provide supplemental heat in winter. About half the facility’s biogas serves as fuel, and the balance is flared for now.

The electricity is fed directly to the local utility grid. In return the city receives a credit against Hornsby Bend’s daily consumption (typical demand is about 500 kWh) and monetary payments per kWh for the balance.

Sustainable future

The facility, equipment and process enhancements have positioned Hornsby Bend for more resource recovery in years to come. An on-going master planning process helps the team look several years down the road at potential projects.

One of these aligns with the city’s desire to remove food and other organic materials from the waste stream going to landfill. “We’re looking at the possibility of building a FOG receiving station on site,” says Lockard. “FOG is certainly amenable to anaerobic digestion. It would add very little complexity to the operation of our digesters but would further improve volatile solids reduction and produce significantly more gas.” Greater gas production, in turn, could make it feasible either to generate more power or fuel a biosolids dryer.

“The point is that these possibilities exist because of the stimulus project and the digester improvements,” Lockard says. “We couldn’t have even considered these things without that. We’ve set ourselves up in a good place to bring about a new era of resource reuse and sustainability for the city of Austin.”

More Information

Alfa Laval Ashbrook Simon-Hartley - 800/362-9041 - www.as-h.com

Alpine Technology - 512/328-5829 - www.alpinetechnology.com

Evoqua Water Technologies, LLC - 866/926-8420 - www.evoqua.com

GE Energy - 773/414-3459 - www.ge.com/energy

Ovivo USA, LLC - 512/834-6000 - www.ovivowater.com

Scarab Manufacturing and Leasing, L.P. - 806/883-7621 - www.scarabmfg.com

Vermeer - 888/837-6337 - www.vermeer.co