Grease and related odors from a trickling filter wet well were so bad at the Silicon Valley Clean Water treatment plant that the staff couldn’t eat lunch at the picnic tables on the deck outside the office.

But 3 1/2 years ago, the plant — located in Redwood City, California — installed a MILLI hydroxyl radical misting system (Vapex) to dissolve and eliminate the grease layer. The odors have disappeared, and so has corrosion of plant infrastructure in that area from hydrogen sulfide.

“I was familiar with the technology,” says Monte Hamamoto, chief operations officer. “It had always been appealing to me, so I felt it was worth a try.” The Vapex team came on site and pilot-tested the technology for six months. Then, based on favorable results, plant staff purchased the unit.  

Resource recovery

Silicon Valley Clean Water is a joint powers authority providing wastewater treatment for some 220,000 customers in Redwood City, San Carlos and Belmont, plus the West Bay Sanitary District based in Menlo Park. Wastewater from all four communities is conveyed through pump stations and 9.8 miles of sewer force main to the treatment plant.

Normal flow through the plant is about 14 mgd, although the volume can increase to 80 mgd from inflow and infiltration throughout the communities’ collections systems. The activated sludge plant includes tertiary filtration and chlorination with sodium hypochlorite. Discharge is mostly to San Francisco Bay, although a portion is pumped to Redwood City for reuse.

“We send about 1.5 mgd for recycling in the summer,” Hamamoto says. “We could produce as much as 12 mgd for recycling.”

Biosolids are anaerobically digested and dewatered to a 20% solids cake. In summer, the cake is dewatered further on drying beds and then land-applied. Biogas fuels a cogeneration unit with two engines that produce 1.2 MW of electricity to fulfill 70% of the plant power needs. Hot water from the process heats the digesters and buildings.

Attacking grease

Vapex has installed more than 350 odor control systems since it introduced the hydroxyl radical misting technology 15 years ago. The hydroxyl radical mist, made from ozone, water and air, breaks up grease while eliminating hydrogen sulfide.

A six-nozzle MILLI unit was piloted-tested on the trickling filter wet well for about six months. When the technology worked, the authority purchased the unit. A grease layer of as much as 18 inches used to build up in the influent channel. The concrete walls and aluminum hatch covers were showing signs of corrosion.

“We cleaned the channel out to give the Vapex unit a fresh start,” Hamamoto says. Since the unit became operational, the grease layer has never exceeded 1 inch. It is simply washed away with the wastewater and does not congeal in downstream processes.

A second smaller unit was later purchased to eliminate grease and odors at a receiving station where grease is trucked in from restaurants. Hamamoto says the Vapex organization has been easy to deal with, providing sound training in operations and maintenance. Nozzle cleaning is the only regular maintenance required.

The units use no chemicals and operate on electricity and water supplied by the treatment plant. The units use plant effluent that has passed through a simple spin-down cyclone filter to remove particles that might plug the misting nozzles. The absence of chemicals is a boon to operators: “We used to use permanganate to try to scrub these odors. It was ineffective, and the operators hated it. It was not fun to deal with.”

Always innovating

On its website, the authority champions innovation, citing breakthrough developments in energy, water, solids and new technology. “We’re in Stanford University’s backyard,” Hamamoto says. “We partner with them, and we’re constantly trying new things.”

In that vein, the plant is constructing new storage and flow equalization structures. The team also plans to add new drying technology to its biosolids processing train this year. A new pyrolysis unit will turn the dried solids into a carbon biochar that can be used as an agricultural soil amendment.

The odor control technology is further evidence of looking to the future. “The units have done a heck of a job for us, reducing grease, odors and corrosion,” Hamamoto says. “Plus, we can use the picnic deck again.”