Bubbles Hold the Key to Stopping Grease Problems and Hydrogen Sulfide Buildup in a Wastewater Wet Well

A pulsed hydraulic mixing system resolves capacity and hydrogen sulfide issues in a wet well and reduces a California agency’s operating costs.

Bubbles Hold the Key to Stopping Grease Problems and Hydrogen Sulfide Buildup in a Wastewater Wet Well

Mechanics Nic Faro and Luis Martinez core-drilled four holes through the control room’s concrete wall for the piping connected to the compressor’s control panel.

The 4-foot-thick floating grease blanket in Castroville Pump Station was a continuous challenge for Monterey One Water mechanics in Marina, California.

The cost to remove the grease was prohibitive, and the blanket’s depth reduced valuable capacity. Hydrogen sulfide at 35 ppm degraded the wet well coating and eroded concrete and metal.

“Mechanical seals on the sewage pumps made it difficult to pump down and remove the grease,” says Bret Boatman, field maintenance supervisor. “Even if we could pump down without damaging the pumps, we’d have to do it multiple times per week.”

Castroville’s FOG program and the vendor’s grease removal products didn’t solve the problem. Then Boatman found Kris Drewry of Pulsed Hydraulics through a Technology Affinity Group conference.

“He looked at the pump station and said the PHI-350 Large Bubble Mixer would help us,” Boatman says. “We were skeptical but willing to try anything.” Within 24 hours of startup, the wet well was grease-free, and atmospheric H2S had dropped to 7.7 ppm.

One for the books

In the 1980s, coastal wastewater treatment plants in the area were decommissioned and wastewater was directed to seven new pump stations. The 5-mile 16-inch Castroville interceptor carries about 4 percent of total flow to the 29.6 mgd (design) Regional Treatment Plant operated by Monterrey One Water. The facility treats 18.5 mgd.

The Castroville Pump Station, 42 feet 6 inches in diameter, has a 4-foot-wide wet well with a 2-foot-thick inner wall forming a “doughnut hole” for the pump room and control room above. “We’ve never seen anything like it,” Boatman says. “Apparently, the design addresses the proximity to the Tembladero Slough and soil conditions.”

The station receives 750,000 gpd. Influent enters the wet well at the 12 o’clock position, allowing heavy grease to begin building at the 6 o’clock position. By the time the grease layer reaches the pump suction bells at the 3 and 9 o’clock positions, it has become a thick blanket.

Pulsed Hydraulics engineers designed a system with four sets of two 8-inch bubble-forming plates spaced 16 feet apart. One pair was on a 18-inch-high stand. Mechanics Nic Faro and Luis Martinez were concerned that grit would accumulate beneath the stand, and its elevation prevented the formation of uniform mixing bubbles throughout the system.

Their solution positioned the eight plates level with the floor and equidistant from the supply air tee, ensuring that each plate released uniform bubbles. “We ran the design past Pulsed Hydraulics engineers and they loved it,” Boatman says.

Irresistible offer

A contractor to install the system would have cost an estimate $60,000. Monterrey One Water engineers asked Boatman if he and his mechanical crew could do it. “This is our station and we’re proud of it,” he says. “We jumped at the chance.”

Faro and Martinez spent four days assembling the piping. They laid out the manifolds on the grating according to the plans, adjusted the layout to fit the workspace, and cut and threaded the pipes, taking care to center the tee connecting the individual plate manifolds. All the fittings, anchors, clamps, 1-inch stainless steel braided Teflon tubing for around the walls above the waterline, and the 1-inch piping were 316 grade stainless steel to withstand H2S.

Once that was completed, mechanics bypassed the station’s flow with a 10-inch high-pressure diesel pump, enabling a contractor to remove the 4-foot-thick grease blanket over four days at a cost of $50,000. The contractor returned on the fifth day to vacuum the remaining sewage and expose the wet well floor.

Then the mechanics went down the ladder. “We followed all protocol involving confined spaces, fall protection and personnel safety,” Boatman says. Fresh air entered through the overhead grating.

The crew quickly mounted the bubble-forming plates to the floor using three 1/4-inch wedge anchors per unit. They secured the piping with Unistrut clamps (Atkore International) and brought the risers attached to the manifolds above the grating. Within four hours, the wet well was back in service.

Bubble, bubble, no more trouble

Before Faro and Martinez installed and wired the air compressor and control panel, they core-drilled four holes each through the control room’s concrete wall and floor for the associated piping. In the two weeks it took them to connect all the components, a 3- to 12-inch-thick grease blanket covered three-quarters of the wet well.

On May 31, Ray Sprague, field service manager at Pulsed Hydraulics, initiated startup. He hit the power button and, instead of the compressor whirring to life, an alarm screen showed reverse rotation. “Great. We know the safety system works,” he told the mechanics.

After Martinez changed the wiring, the compressor fired up and maintained 125 psi. Sprague programmed the control panel to open the four solenoid valves for 0.5 seconds every 15 seconds, releasing 60 psi bubbles from the plates that rose up the wet well walls at 4 feet per second.

“They break the surface like exploding depth charges,” Boatman says. “Kinetic energy moves liquid and solids tangentially to the walls and down the sides to the bottom to complete the mix cycle and break apart the blanket.” After 20 minutes of mixing, the mechanics saw water appear as a 5-inch-thick section of grease began to separate. They ran the unit overnight, returning 12 hours later to find only a 3-foot-diameter area of thin grease particles floating above the idle pump suction bell.

High fives

“The Pulsed Hydraulics mixer exceeded our expectations,” Boatman says. “We were super impressed with the result, and the project was fun, creative, and provided job satisfaction.”

The next step was to optimize the system. The mechanics set the time between air bursts to 20 seconds and the run time to three hours in the morning, two hours in the afternoon, three hours in the evening, and two hours in the early morning. “It was too much of a good thing,” Boatman says. “Excessive air rolled the grease into small balls.”

They reduced the run time to 15 minutes twice in the morning and afternoon, but that allowed a slight grease layer to form. More adjustments followed. “Currently, we run for 30 minutes at 0200, 0700, and 1300 hours and for three hours through our peak flow time beginning at 1900,” Boatman says. “Hydrogen sulfide measurements average 2 ppm. Our nightmarish challenge is over.”


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