The South Bend Water Works in Indiana supplies drinking water to 120,000 people in St. Joseph County. This includes 98,000 residents of the city of South Bend. An essential part of the company’s overall treatment process is its Pinhook filtration plant, which has gravity filters that have relied on manganese greensand to remove iron and manganese from its groundwater source since 1998.
“If you’re treating groundwater anywhere in the U.S., you’re going to have a problem with iron and manganese,” said Dave Tungate, water quality specialist for the South Bend Water Works.
During a routine filter maintenance overhaul in the spring of 2004, operators at South Bend found that the manganese greensand levels in each of the plant’s eight gravity filters were too low. The company decided to replace the greensand in the eighth filter with Calgon Carbon’s CalMedia GSR Plus to compare its performance to manganese greensand, and also to a silica sand filtration unit being operated at another facility.
“Manganese greensand is hard to come by, and in our case, we were told that the lead time to replenish our filters would be as much as 18 months,” Tungate said. Unlike manganese greensand, the CalMedia GSR Plus fialtration media was readily available.
The manganese dioxide coating on the CalMedia GSR Plus granules enhances the oxidation reactions that cause dissolved iron and dissolved manganese to form solid, insoluble precipitates. The manganese dioxide coating also acts as a buffer to reduce any excess potassium permanganate used for regeneration in the water, ensuring that this powerful oxidation agent and its signature purple color does not taint the treated water or enter downstream service lines.
The oxidation capacity of the CalMedia GSR Plus bed can be continuously maintained by adding to the water ahead of the filtration unit a constant feed of both chlorine (fed as hypochlorite solution or in gaseous form) and potassium permanganate.
Chlorine is relatively inexpensive compared to potassium permanganate, and does most of the work converting Fe2+ to Fe3+. Therefore, when both oxidizing agents are used simultaneously, smaller amounts of the more costly potassium permanganate can be used. When compared to manganese greensand, the granules that comprise the CalMedia GSR Plus media are larger, less dense and more lightweight.
Another important distinction between the two competing filtration materials is that filtration media require no air-scouring step during the backwash cycle, which is not the case with the greensand filtration system. The elimination of this step not only simplifies and streamlines the backwash cycle, which saves money and effort, but it also reduces particle attrition. In greensand beds, the particle breakage that results from air scouring often leads to clogged pores and reduced bed permeability, which reduces throughput over time and shortens the life of the filtration media.
The plant operators found that the iron and manganese removal capabilities of the CalMedia bed were comparable to the greensand vessels, even at faster throughput rates and lower retention times.
“We found that the CalMedia bed can actually filter more water than a comparably sized greensand bed and still meet our iron and manganese removal standards,” Tungate said. Meanwhile, the elimination of the air-scouring step during the backwashing of the CalMedia GSR Plus not only reduces granule attrition, but it also considerably shortens and simplifies this part of routine filter maintenance.
The filtration vessel loaded with CalMedia GSR Plus has no anthracite cap that is susceptible to blowout, and has higher porosity and permeability. Therefore, it is possible to use higher backwash rates (up to 5,000 gpm).
According to Tungate, this, coupled with the elimination of the air scour step, reduces the entire backwash cycle to 22 minutes from start to finish, as compared to 45 minutes for conventional greensand.