A city water plant in southeastern Missouri, which has treated millions of gallons of groundwater and Mississippi river water, had built up twelve inches of calcium deposits in the sand and anthracite filter media due to excess lime softening in its groundwater clarifier. The staff at Alliance Water Resources investigated the filters’ calcium penetration by obtaining samples from different depths. Finding calcium deposits in all levels of the media, we knew the standard procedure called for total filter media replacement.
The estimated cost to replace the media for fourteen filters was $525,000. To minimize this cost, we developed an innovative method for rehabilitating the media by treating it with acid, neutralizing the calcium and restoring the filter media to its original performance. We performed a trial run on one filter in order to determine the best dosage of acid, optimize treatment, determine cost and look for any problems associated with the media cleaning process.
Worker safety was a prime concern because using acid would result in acid vapors being released into the air. We engineered safety controls by building a cover over the filter as a means to safely exhaust the acid vapors. Next, we determined what protective equipment would be required for each employee, and then wrote a step-by-step procedure that workers followed to safely conduct the cleaning process. Discharge of the spent acid would be monitored and tested before its proper disposal.
The procedure was to mount a plastic, pneumatic diaphragm pump to a 300-gallon hydrochloric acid tote. The acid was injected into the filter through a clear plastic hose. An air scour blower ran the entire time as acid was being pumped into the filter, aiding in distribution of the acid. Two acid totes (600 gallons total) were pumped into the filter. The reaction of the acid with calcium caused foaming, which was limited by a water spray on the surface. After the acid reaction with the calcium was completed, the filter was backwashed. The cleaning process took five days and 3,000 gallons of acid to complete each filter. There was a mark on the filter wall that indicated the height of the media when it was originally placed, so when the calcium was completely removed, the top of newly cleaned media was level with the mark. Samples were taken from the media bottom to ensure all calcium was removed.
The acidizing process brought the filter media back to new condition. The cost of materials and acid was $42,000, or $3,000 per filter. The process saved the client $483,000 due to Alliance’s ability to restore the filter media.
Each year the top layer of anthracite is checked for calcium coating in the filters. To date, there has been no reaction when acid is applied to the media sample, which means the filter media remains in good condition.