Located in the heart of central Texas is the growing City of Killeen. Lured by cheap land and a strong economy, people moving in from other states could drive population up 36% over the next two decades to nearly 200,000.

The Bell County Water Control Improvement District No. 1 treats sewage generated by this growing population via two plants: a 21 mgd central plant and a 6 mgd south plant (maximum capacities). Both were meant to treat primarily residential sewage, but as the city’s southern corridor grew so have the number of restaurants.

Wastewater treatment plants are designed around three metrics: flow, load, and effluent quality. At 3 mgd, flow to the south plant was well under maximum treatment capacity in 2013. However, declining dissolved oxygen (DO) levels in the fill-and-draw sequencing batch reactor activated sludge system made operators suspect that loads into the plant were above design levels. They were correct: The plant had been receiving higher-than-anticipated loads since it began operating in 2007.

Further investigation indicated high levels of fat, oil, and grease (FOG) in the wastewater. FOG is the byproduct of feeding humans: bacon grease, meat fat, oil, shortening, butter, margarine, sauces, dairy products, etc. Lipids can be washed down the drain with hot water, but they solidify and stick to the inside of sewer pipes when they cool. Given enough time, they completely clog the pipe. If they get to the treatment plant, they create issues like lower DO levels.

FOG is common in residential wastewater, but the high concentrations coming into the south plant indicated more restaurants were discharging into the city’s sewer collection system. The materials tend to collect anywhere water is quiet enough to allow them to separate from the water and float to the surface. After observing a large buildup in the district’s influent lift station, plant operators began routine cleaning and maintenance. Despite their efforts, FOG continued to collect in other locations around the treatment plant.

Restaurants keep grease from entering the collection system via a trap that allows dishwasher water to cool so fat, oil, and grease floats to the surface. They’re supposed to regularly remove this material. The district doesn’t own the city’s collection system, so it couldn’t require them to do so.

That left another option: modifying the plant to cost-effectively manage the higher loads. District managers hired the Waco, Texas, office of planning, engineering, and program management firm Lockwood, Andrews & Newnam Inc. (LAN) to explore the issue and make recommendations. Together, they devised an unconventional but effective solution to a problem common to wastewater treatment plants nationwide.

Successful solution = hybrid option

The plant’s arrangement and subsurface conditions required minimal excavation, a compact design, and capabilities for future expansion. In the process of evaluating separators, primary treatment, and chemical treatment, the project team realized the best solution was a combination of the two.

Option 1: Oil-water separator like an American Petroleum Institute (API) settler. This offered simple design and easy operation and maintenance, but was cost-prohibitive due to size and space requirements.

Because a settler relies on gravity, adequate flow requires a non-pumped location with enough height difference between the settler’s inlet and the next structure. The recommended flow path for the plant’s 6 mgd average flow was 120 feet by 10 feet by 20 feet. This limited the team to two locations: 20 feet below the ground before the lift station, which would require drilling into rock; and 20 feet above ground between the pump discharge and headworks screen, which would require building an elevated structure.

The economics of construction in either location ruled out an oil-water separator.

Option 2: Some form of primary treatment. Primary treatment, such as a clarifier, usually removes at least 30% of organic loading (primarily solids to the bottom, but also FOG to the surface). Like settling, it uses gravity to remove FOG but also provides enough vertical distance for grease to separate and be removed by a surface skimmer.

Therefore, primary treatment presented the same construction obstacles even though it required a 30-foot-diameter structure. In addition, primary treatment equipment required a pump to move settled solids to digesters or into the biological process. Long-term operation and maintenance costs, combined with the construction issues, eliminated this option.

Option 3: Combine screening, settling, and skimming at the plant. The project team decided a hybrid solution that combined some parts of an API settler and some parts of physical screening would be most cost-effective. Wastewater moves from an elevated, open-channel screen at the lift station through a below-grade pipe and into a pipe chase. Velocity through the chase keeps material from settling so all wastewater constituents reach the headworks. The open channel allows some cooling of the wastewater through evaporation, enabling FOG to agglomerate and form particles that are separated from the influent via screens.

Combining screening and settling via a grease trap is anticipated to remove 75% of grease and reduce organic loading on the plant’s secondary treatment process by up to 15%.

Upgrading headworks screening

To separate grease from other organic constituents in influent, the headworks’ ¼-inch perforated plate screens were replaced with 1/8th-inch Flexrake FPFS-M, bar screens made by Duperon Corp. of Saginaw, Mich. The bar screen’s opening is 50% smaller than the plate screen’s, but eliminates negative impact on headworks hydraulics. FOG is discharged into a new Duperon washer compactor with better spray and cleaning characteristics to reduce buildup in the compactor and provide cleaner screenings for disposal.

A 40-foot, custom-designed grease trap now follows the plant’s existing vortex grit removal. It uses the entire basin to develop a length of travel for influent and is divided into two sections to provide sufficient hydraulics for the existing 6 mgd design and an additional 6 mgd in the future. Wastewater flows into the trap slightly below normal operating water level, reducing turbulence and creating the quiet environment necessary for grease to separate. It enters from the top, flows down at a low velocity and under a baffle, then rises upward in a higher-velocity segment to exit over a weir at the end of the trap. The slow downward velocity creates a settling zone similar to a primary clarifier with an underflow baffle similar to a grease trap. Grease floating on the settling zone’s surface is removed by a tipping skimmer made by Envirodyne Systems Inc. of Camp Hill, Pa.

Upgrading the headworks with new coarse and fine screens and creating a skimming area cost almost $1.7 million.

The problem of settling solids and the need for additional pumping were addressed in two ways.

First, because the headworks is fed by the lift station, fairly consistent flow velocities are maintained during pumping periods. This allows the upzone following the baffle to create sufficient velocity to carry solids over the weir and to the secondary treatment process.

Second, the tank’s floor slopes to a bank of diffusers along the dividing wall. When large air bubbles enter wastewater at depth, they create a rapid rise that promotes mixing in the tank and creates an effect whereby flowing water on the basin’s floor helps collect solids. Aeration can be operated intermittently on a timed cycle or manually in the basin to reduce solids accumulation along the floor.

Hydraulically, the trap will pass the 18 mgd peak flow to the treatment plant, but with a corresponding decrease in the ability to separate grease. Tripling downflow and upflow velocity through the trap effectively eliminates the zones necessary for grease removal. To minimize this effect and the washout of collected FOG into the plant, the trap is a side-stream structure. Valving was installed so operators can split flow to both the grease trap and secondary processes or bypass the trap without affecting downstream hydraulics.

Phased construction plan

This solution was rolled into a $7.5 million improvement project that began in February 2016. Close coordination with operating staff and night-time construction allowed Atlanta-based contractor Archer Western Construction to install bypass valves and piping with minimal interruptions. Fine screens were installed first and completed in February 2017. The grease trap, mixing equipment, baffles, and piping were finished in June 2017 and are being tested.