Feeding ferric chloride to digested biosolids paid big dividends in drier material, lower hauling costs and reduced odor complaints in Fort Worth, Texas.
For some two decades, Renda Environmental has applied more than 40,000 dry tons of Class A exceptional quality lime-stabilized and dewatered biosolids on agricultural sites for the city of Fort Worth, Texas.
“We had some odor complaints throughout the years, but nothing serious until they spiked in spring 2013,” says Ben Davis, environmental program manager for the dewatering facility. “That’s when operators noticed a change in the characteristics of the anaerobically digested biosolids in response to a significant drop in iron concentration. The material became wetter and visually unattractive.”
Davis, who was familiar with Kemira Chemicals, a global chemistry company, called applications manager Tafadzwa “Tee” Mariga, who analyzed the material and then ran a trial mixing it with ferric sulfate. Results were encouraging, but not the solution. To reduce odors further, Mariga switched to feeding PIX-111, a ferric chloride based on trivalent iron (Fe3+).
Although a six-month trial was necessary to customize the flow-based equipment, operators saw the biosolids characteristics change within hours of feeding the primary coagulant. “The material went from sloppy to granular and was more aesthetically pleasing,” says Davis. “Previously, we were fortunate if the belt presses achieved 14 percent solids. With the addition of ferric chloride, 16 to 18 percent solids became the norm.” More good news lay ahead.
The 166 mgd (design) Village Creek Water Reclamation Facility averages 100 mgd from 900,000 city residents and 22 communities. Pumps in a wet well at the plant send digested biosolids through a mile-long, 12-inch pipe to a 500,000-gallon storage tank at the dewatering facility.
Five 2-meter belt filter presses (Andritz Separation) keep up with demand. The facility’s dump trucks make numerous weekly trips to application sites 40 to 70 miles away. Mechanical spreaders disperse the material. Renda Environmental staffs the entire operation.
“Because of the distance between the treatment plant and dewatering facility, the trial’s biggest factor was selecting the appropriate injection point for the chemical,” says Davis. “The treatment plant had an ideal location.”
Kemira technicians fabricated a dosing point to increase surface area for maximum mixing of the ferric chloride and sludge. Proprietary KemConnect software monitors the process and adjusts doses based on flow meter readings. It also monitors the tank level, creates daily reports on the process, creates alarms and alerts, and initiates chemical delivery to ensure product availability.
“Tee had calculated the startup feed rate,” says Davis. “Operators then increased it incrementally based on orthophosphate levels in the filtrate. The city checks it five days a week. Initially, we had readings of 120 ppm to 130 ppm. When they dropped to the single digits, we knew we had the correct dosage. At the time, it was 2 gallons of chemical per 1,000 gallons of digested biosolids.”
The 90 percent reduction in orthophosphate ended the formation of struvite. Previously, operators had noticed struvite accumulating in the piping and grinder pumps, blinding the belts and perforated rollers on the presses. They worked overtime breaking down the equipment, chipping off the mineral and restoring the presses’ efficiency. Today, the facility has no struvite issues.
While drier cake reduced the number of weekly truckloads, it’s difficult to determine monetary savings. Crews dredge the settling ponds occasionally, and that volume spikes the hauling numbers. “If we didn’t have ferric chloride to turn to, we would be in big trouble,” says Davis. “Application costs would have gone up exponentially because we would have had more truckloads hauling greater distances to more isolated application sites.”
Down with polymer
Because ferric chloride acts as a coagulant, as soon as the first batch of biosolids hit the belt presses, operators could reduce polymer usage by about 30 percent. “According to a study paid for by the city, polymer is an odor-contributing agent,” says Davis. “Complaints did die off tremendously after we began adding ferric chloride, but we still had changes in product chemistry that lasted a day or two and could have generated legitimate odor complaints.”
In a March 2016 trial study, Renda Environmental fed chlorine dioxide (Water Solutions, a division of Azure Water Services) to its biosolids storage tank. The oxidizer, dosed at 100 ppm, was implemented full time in December 2016. Davis says, “This additional treatment step has greatly diminished the likelihood of odor events.”