Since 1995, the Florida city of Vero Beach used Jones and Attwood twin rotary drum thickeners (Ovivo USA) and two aerobic digesters to store 2 percent liquid biosolids. Ranchers and farmers applied 6.5 million gallons annually.

By 2010, watershed restrictions had reduced the agricultural sites available. City officials hired a contractor with a portable centrifuge to dewater and landfill the material. “We’d run a 400,000-gallon batch through the rotary drum thickener whenever the contractor arrived, which was monthly in summer and every three weeks during the three-month winter tourist season,” says Rob Bolton, director of the city’s Water and Sewer Department.

The two aerobic digesters gave operators 60 days of storage and the city a $90,000 annual electric bill for a 150 hp blower. To circumvent the digesters and save money, the contractor dewatered return activated sludge from the bottom of clarifier No. 2. “The odor was horrendous,” Bolton says. “We’re right on the Indian River Lagoon next to homes and a busy intersection. Odor control is of major importance.”

To address rising biosolids handling costs, the city purchased a CleanB treatment system (BCR) for conditioning and odor control, and a CS18-4HC 2Ph FH solid bowl centrifuge skid (Centrisys/CNP) for dewatering. “Our goal is to process digested sludge as we transition to direct processing of RAS, thereby eliminating the digesters,” Bolton says. “The tanks will be cleaned and kept as a backup to the centrifuges or repurposed for nitrogen removal.”

Chemistry works

The city’s 4.5 mgd (design) activated sludge treatment plant averages 2.5 mgd. After four smaller tile filtration beds (Hendrick Screen) replaced the original sand drying beds, their foundation became the site for the new biosolids facility. BCR built a 102- by 45- by 28-foot-high building with open end walls for the processor and centrifuge, which went online in early 2017.

“We’re still in startup mode with the processor, feeling it out and finding the sweet spots on everything,” Bolton says. “It seems to be working very well.” Upon activation, a Moyno pump (NOV) on a variable-frequency drive feeds digested sludge or RAS to the processor, with a static mixer before and after the chlorine dioxide generator. Feed lines with pulsating positive displacement pumps inject a 1:1 ratio of 50 percent sulfuric acid and 15 percent sodium chlorite into the generator.

As chlorine dioxide mixes with the material in the 10-inch-diameter serpentine piping, the chemical disinfects and deodorizes by oxidizing sulfides, sulfates, and mercaptans. The chemical addition has a minimum detention time of 10 minutes.

Chlorine dioxide also changes the chemical properties of the material, enabling the polymer to produce drier solids. “When we ran digested sludge through the rotary drum thickeners and processed it in the centrifuge, we had 16 to 17 percent solids,” Bolton says. “The minute we turned on the chlorine dioxide, solids jumped to 19 to 20 percent.”

Feed rates are based on the centrifuge’s design points of 80 to 120 gpm, with a range of 50 to 160 gpm. “Our goal is to process RAS at 120 gpm, which equals 6.5 hours given a retention time of 13 to 14 minutes in the coils,” Bolton says. “One cycle fills up half the tractor-trailer, and we’re processing 24 to 25 wet tons per load two to three times per week. To avoid a regrowth of odors, we don’t want the biosolids sitting any longer than two days.”

Sweet spots

Currently, the system feeds 0.6 gallons of sulfuric acid and sodium chlorite and 0.35 gallons of Praestol K 275 FLX polymer (Solenis) per 1,000 gallons of RAS at 1.28 to 1.6 percent solids. For April, finished biosolids cake averaged 21.5 percent solids.

“My operators want to see how dry they can get the biosolids and how much they can load on the truck,” Bolton says. “They want visual results; I want monetary results. Dialing down the polymer usage is one way to do that.”

The thicker the sludge, as from the digesters, the slower the flow rate. Nevertheless, operators are keen to see what the system can do. “They were up to 140 gpm with flows from the clarifier, and there were no odors,” Bolton says. “The chlorine dioxide knocked them out.” The chemical supplier tops off the 2,500-gallon tanks of liquid acid and chlorite every 90 days.

An automatic, self-adjusting drive system discharges solids from the centrifuge through a 9-inch tubular conveyor to the nearby tractor-trailer. Operators standing on a platform move the conveyor arm to load the truck evenly. “Once we set up the camera in the bay, the men can check the trailer from the control room,” Bolton says. “Everyone wants to play with the new toy and are happy to make the rounds.”

Refining operations

Compared to the electric, tankage, aeration system, and maintenance involved with the aerobic digesters, the all-in-one CleanB system is simpler. “We program how many thousands of gallons to waste that day, push the button, and walk away,” Bolton says. “The machine automatically flushes the coils and shuts down when done.”

Bolton’s biggest predicament is the plant’s constant wasting mode to the rotary drum thickeners. It interferes with shutting down the system on weekends or at least on Saturdays due to staffing. “We’re moving toward batch processing but are still determining how much to waste per day and weekly,” he says. “Once we figure that out, we can program the CleanB for a flow rate, a time, or a batch.”

Other components also remain in flux, making it difficult to evaluate cost-effectiveness. For example, a dedicated in-line meter records monthly electric costs remotely using fiber optic data transmission with Allen-Bradley operating software. It communicates with the plant iFIX SCADA system (GE Digital).

Bolton wants to test other polymers, and the city has bid out the chemical and hauling contracts. “One local bidder said he has land application sites north of us,” Bolton says. “That would cut 30 percent off our total hauling costs. With the numbers still not finalized, we’re estimating a total savings of $200,000 annually and a return on investment in five or six years.”