Quick Payback

A California plant saves substantial money on power, chemicals and hauling costs with a new biosolids process that reduces volume
Quick Payback
Steel framing goes up around the solids separation module of the Cannibal process.

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The Big Bear Area Regional Wastewater Agency in Big Bear City, Calif., generated 5,800 tons of Class B biosolids per year. The 300 trips to the landfill were on winding mountain roads that dropped 7,000 feet in elevation.

Treatment plant workers dewatered digested biosolids with a belt filter press, producing 14 percent solids cake, dried further in two drying beds. However, long winters and sometimes rainy summers affected results. From 2000 to 2010, landfill costs increased from $32 to $100 per ton.

“We were paying a lot to haul water,” says plant superintendent Joe Hanford. “Reducing our biosolids volume became a major concern.” Agency staff studied alternatives such as natural-gas-fired dryers to produce Class A biosolids, but they involved costly expansions.

Then they found the Cannibal solids reduction process from Siemens Water Technologies. In January 2008, Big Bear became the third plant in the state and the eighth nationwide to use the technology. In 2010, the agency produced 2,100 tons of biosolids and hauled 100 loads. In a little less than four years, the process has saved nearly $400,000.

 

Two-part system

The 4.9 mgd (design) activated sludge extended aeration treatment plant averages 2.4 mgd. After primary treatment, flows go to three oxidation ditches.

The Cannibal system has a solids separation module and bioreactor with two interchange tanks. Return activated sludge flows through the module, in which ultrafine mesh screens and hydrocyclones remove inert debris. A 7 hp motor drives the drum, and another powers the auger that compacts and dewaters the material. Screenings come out at about 30 percent solids and are landfilled.

The screened sludge is pumped to either the bioreactor or to the oxidation ditches. The bioreactor, next to the building housing the separation module, creates a low oxidation reduction potential (ORP) environment. It promotes no new aerobic bacterial growth and actually destroys some microorganisms to feed the facultative bacteria, thereby reducing the solids that would normally be wasted.

Probes for ORP and pH monitor the environment. “ORP is a measurement of the liquid’s ability to oxidize organic material,” says Hanford. “It has a much stronger value than just measuring dissolved oxygen.”

 

Thorough mixing

Each interchange tank has a floating mixer with a 30 hp motor that keeps the material in suspension. The mixers run six to eight hours per day as 4,000 pounds of screened sludge is added to the tanks. It takes 12 days for the biomass to break down. Suspended solids sensors and flow sensors monitor the mass flow rate and mixed liquor suspended solids concentrations throughout the process.

When the mixers shut off, effluent is decanted during the night and is routed by a 10 hp submersible pump to the oxidation ditches where the facultative bacteria die. From the oxidation ditches, mixed liquor returns to the separation module.

“Cycling the solids between aerobic and anaerobic bacteria destroys the material,” says Hanford. “By lowering our digested sludge yield, we reduced the cost of running the aeration blowers by 10 percent and saved $4,000 per month.”

The SmartCannibal system controls the plant and minimizes power usage. “The program isn’t tricky to use, but it does take some thought to calculate how much to decant and how often to aerate the liquid,” says Hanford. “The numbers are based on previous plant operations, so it isn’t too difficult.”

 

Trickle down benefits

The agency hired W.M. Lyles Co. to install the sidestream process. It took nine months. “We were down a couple of hours or sometimes overnight,” says Hanford. “It really didn’t affect the rest of our work that much.”

The bioreactor will eventually need cleaning. At that time, workers will drain it to four feet and pump the sludge to the oxidation ditches. So far, the separation module has helped to reduce some maintenance tasks. “It removes the inert debris that used to plug our downstream pumps occasionally,” says Hanford.

The biological process also substantially reduced the hours on the belt press and lowered polymer usage from seven 55-gallon drums per year to five. “Not using mechanical means to reduce our biosolids production is huge for us,” says Hanford. “It saves a significant number of staff hours and reduces wear and tear on the equipment and vehicles.” The agency calculated that the system would pay for itself in the first 10 years.



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