Lime-Softening Plant Takes Advantage of New Batch Lime Slaking System to Ease Maintenance
Management at a 22 mgd lime-softening plant in Florida City, FL, reports that replacement of its continuous-feed lime slaking system ...
Management at a 22 mgd lime-softening plant in Florida City, FL, reports that replacement of its continuous-feed lime slaking system with a new batch-process type has ended heavy maintenance burdens previously endured with the slaking operation, while providing more consistent concentration of calcium hydroxide(hydrated lime) slurry output.
The Tekkem Slaker System was manufactured by RDP Technologies of Norristown, PA, under license from PR Stephansen, AS of Norway.
Originally built in 1943 as a 6 mgd plant by the U.S. Navy to serve the needs of its submarine base boilers, the facility was turned over to the Florida Keys Aqueduct Authority in 1976. The Authority, which had previously been buying its water from the Navy, then built a new plant, including two slaker systems, in 1989.
By 1999, frequent slaker system breakdowns and ongoing maintenance needs had become enough of a problem to lead management to seek alternative means for its slaking needs. Upon learning of the Tekkem alternative technology, they asked their engineering firm to investigate.
"We were aware of the technology, and had reports of success," said Paul Wobma, senior water process engineer at CH2M Hill in Deerfield Beach, FL.
The firm learned that RDP Technologies was manufacturing and selling the system, and a plant visit was arranged at an RDP installation in California.
"We were real impressed, and gave it the green light," said W. Michael Tam, superintendent of the water treatment plant at the Florida Keys Aqueduct Authority. "The only alternative was to use the same type of process that we had been using, and we weren't confident that simply changing manufacturers would help much. With the new process, we would be making batches that were consistent from one to the next. With the previous continuous-type system, we couldn't be sure about the lime concentration leaving the slaker."
The plant's feed water comes from the Biscayne Aquifer, with the calcium carbonate content attributed to the presence of Miami Oolite honeycombed limestone there. While the 300 ppm concentration is well within the 500 ppm allowed limit for drinking water, management wants it reduced down to 100 ppm to protect its distribution piping against buildup, while providing a better product for its consumers, who also use the water for laundry and other needs.
A slurry pump in the new feed loop helps minimize buildup maintenance by maintaining high-velocity flow that provides scouring of piping.
The 22 mgd design plant operates at an average of 17 mgd, with a permit being sought for a revised maximum of 24 mgd. The new slaker technology was installed as two redundant systems. In addition to the slakers, RDP also provided slurry tanks, the main feed loops, and the feed lines to the treatment units, including a flowmeter and pinch valve. The treatment units, with special solid contact basins, consist of two larger ones @16 mgd max, and a smaller one @5 mgd max.
Designed for 2000 lbs/hr maximum lime output per slaking system, average output is about 25,000 lbs/day. Each system runs for about 30 days at a time, with the redundant system taking over while routine, simple maintenance is performed. The unit is first filled with water that is allowed to stand for a couple of days before draining. Then, in a 30-minute operation, high pressure spraying is performed to remove any lime before the system returns to standby status.
A high pH of about 10.7 is maintained in the treatment units, with CO2 buffering used to reduce it to about 9.4 in the finished water. Alkalinity is kept at about 30 ppm.
The RDP Tekkem Slaker uses a direct weight control operation, coupled with temperature control, to allow for accuracy in control of the slaking reaction, as well as subsequent dilution. The system has received numerous patents.
The slaker operates at temperatures in excess of 180 degrees F, and provides up to 10 minutes slaking time, with the system automatically adjusting parameters to maintain the high slaking temperatures for each batch. Slaker operation is designed to insure that all lime fed to the system is slaked at optimum slaking temperatures.
The system is described as almost maintenance-free. The slaker is completely sealed during slaking operations, which contains dust and steam. All control sensors are located outside the tank, away from the lime and water, to help eliminate maintenance and sensor errors.
"Maintenance doesn't even know that building exists anymore," Tam said. "And we also have a very stable lime slurry output now, averaging 10-12% lime. We've been running the new system since July 2002, and it's like night and day. We have had no maintenance needs at all with the slaker slurry tanks, or any breakdown of any other component. We're also slaking all of the lime now, and the operators don't have to keep adjusting pH, which is staying consistent.
"In addition, with the new feed loop, the high velocity of the 35-50 gpm flow provides scouring of the piping, so lime can't settle and build up like it used to. And the slaker 'learns' from batch to batch. If it made a batch that wasn't strong enough, say around 10%, it corrects in the next batch by making 13-14% to compensate."
RDP Technologies specializes in providing complete lime slaking systems. For further information, visit the company's website at www.rdptech.com.