Pumping concentrated brown grease at 180 degrees F into a heated 10,000-gallon storage tank at the Easterly Wastewater Treatment Plant in Cleveland, Ohio, occasionally tricked the ultrasonic level sensor when steam affected the return signal to the transducer. Overflows looked like liquid peanut butter oozing down the sides of the tank and pooling on the floor.

Water in the grease seeped behind the external electric heating jacket on the tank, shorting out elements, corroding the metal, and causing leaks. As the pads lost heating capacity, the oil cooled enough to cake on the tank walls and conical bottom.

Mixing was done by recirculating the grease through pumps and grinders, but suspended water settled out when levels were low. "The liquid also has a considerable amount of chopped plastics," says maintenance supervisor Morris Taylor. "If they aren't kept in suspension, the bits coagulate and block suction ports."

During an upgrade of the tank, plant engineers chose the PHi-300 mixing system from Pulsed Hydraulics. Combining it with other components solved the problems.

Labor intensive

Cleaning an average spill took three operators eight hours. "Cold grease has the consistency of wax and is difficult to remove," says Taylor. "The guys would shovel up as much as they could, then transport it in a wheelbarrow to a grease sump. To wash the area, they used powerful degreasers and a gas-fired steam cleaner [Jenny Products]." Dirty water collected in the sump.

Operators pumped the sump to one of two flotation tanks, which also received grease from the plant's primary settling tanks and chlorine contact tanks. Air injected into the flotation tanks from a 50 hp compressor with water separator made the grease rise to the surface with the denser water below.

Arms in the flotation tanks skimmed the grease and plastic floatables to an inline Muffin Monster grinder (JWC Environmental) and a macerator (NOV Monoflo Pipeliner Grinders) in series. "From there, we pumped to our 5,000-gallon day tanks, labeled A and B, or to C tank, the storage tank," says Taylor. "Grease rendered in this plant goes to the day tanks for heating to 180 degrees."

Just before operators incinerate a batch, they subcant or draw off as much water as possible. Then a gun injects the oil into a Dorr Oliver fluidized sand bed furnace. The sand, heated to 1,300 degrees F, is suspended 12 inches above the floor by a pneumatic force. The resulting turbulent mixing, much bubbling fluid, incinerates the oil.

The Southerly and Westerly treatment plants each ship 5,000 gallons of concentrated, hot brown grease to Easterly monthly. Their product goes into C tank, a conical vessel 14 feet 8 inches in diameter and 17 feet 6 inches high. Grease is transferred to A and B tanks as needed. Easterly incinerates 15,000 gallons of grease per month.

Through the roof

The plant's Engineering Department hired Bay Mechanical and Electrical Corp. to remove the flat roof on the processing building, extract the old tank, set the new one, and reinstate the roof. The upgrade took a month; plumbing the pre-engineered 10,000-gallon Tenco Hydro Thermix heated batch process tank took two weeks.

"Besides connecting grease lines at the bottom of the tank, they also plumbed the overflow line running to the sump," says Taylor. Bay also plumbed the Chromalox 50 kW electric heater boiler supplying water to the tank's heating jacket.

The mixing system from Pulsed Hydraulics has a Hydro-Pulse bubble-forming plate inverted in the center of the tank inches above the bottom. Bay tapped into the plant's air supply line and attached it to the unit at an easily accessible location.

A computer controls the bubble-pulse intensity, duration and intervals. Operators program it using an Allen-Bradley PanelView Plus 700 terminal (Rockwell Automation). Every 30 seconds, a valve releases a 100 psi burst of air beneath the plate.

The resulting massive bubble powers liquid and sediment off the bottom of the tank and forces them upward. When the bubble breaks the surface, liquid and solids move tangentially to the walls and down the sides to the bottom, completing the mix cycle. After 10 minutes, the kinetic energy decreases, then achieves steady-state mixing for 30 minutes.

To prevent overflows, the tank has SITRANS radar level measurement transmitters (Siemens Water Technologies) that use a low-energy electromagnetic pulse guided along a probe. When the pulse reaches the surface of the grease, the energy is reflected up the probe to the circuitry that calculates the fluid level from the time difference between the pulse sent and the pulse reflected.

Spill-proof

The tank has automatic mixing and temperature controls. Sensors turn them on and off based on the liquid level in the tank. "The hot-water heating system, pulsed-air mixer, and radar level sensor work like a dream," says Taylor. "With no spills to clean up, the upgrade is maintenance free."

On previous plant improvements, operators had to follow behind vendors to debug and modify products after installation,
says Taylor. "This was the first time we didn't have to tweak anything. That speaks volumes."