Thumbnail Image

Characterization of Manganese Oxide Coated Filter Media

Abstract Several methods for removal of soluble maugauese from drinking water exist. One of these options involves adsorption of the maugauese onto Mn oxide coated filter media. Dissolved maugauese (Mn(H)) is adsorbed to sites on the Mn oxide coating aud then oxidized by free chlorine, thus regenerating the media sites for further Mn adsorption. This method has been successfully used by numerous water treatment plauts, decreasing Mn levels to below treatment goals. A disadvautage to this method is the use of chlorine prior to filtration, which cau lead to increased levels of disinfection by-products as compared to post-filter chlorination. Nine plauts that use oxide coated filter media for Mn removal participated in this study. The objective of this research was to characterize the filter media at these plauts. The media was characterized with respect to size, oxide coating level, surface area aud Mn uptake capacity. Laboratory aud full scale studies were conducted to assess the impact of chauges in pre-filter chlorination on the Mn adsorption process. The authracite aud saud media from nine drinking water treatment plauts had a wide rauge of oxide coating levels, rauging from less thau 0.01 to more thau 120 mg Mn per gram of media. All dual media filters show a depth profile with more oxidized Mn on the surface of the media in the upper authracite layers thau in the lower saud layers. Filters with homogeneous media showed no such depth variable profile. The effective size of authracite media aualyzed rauged from 0.67 mm to 1.0 mm. The effective size of saud media rauged from 0040 to 0.73 mm. Samples were analyzed for Mn uptake in a recirculating laboratory batch system before and after regeneration with a 20 mglL solution of free-chlorine for 15-24 hours. Media in the laboratory regenerated state had approximately 130% of the Mn uptake exhibited by media in the as-is (un-regenerated) condition for cases where media had been exposed to continuous pre-filter chlorination at the full scale plants. The media from the nine plants exhibited a wide range ofMn uptake, ranging from 0.01 to 0.7 mg Mn per gram of media after regeneration (20 mglL free chlorine). In general, it was found that the Mn uptake by the media was directly proportional to the amonnt of Mn oxide coating on the media. However, at coating levels above 20 mg Mn per gram of media, an increase in coating level was not fonnd to always yield a proportional increase in Mn uptake. The surface area of oxide coated filter media was also characterized. Media with Mn coating levels ranging from less than 0.5 to over 120 mg Mn per gram of media had surface areas ofl.2 to 92 m2/gram of media. Results show that the surface area of media is directly proportional to the amonnt ofMn oxide coating on the media for traditional media filters. However, samples taken from Mn removal filters that have a particle filtration step upstream in the treatment process had relatively low surface area despite high levels ofMn oxide coating. Laboratory and full scale studies were conducted to determine the impact of restarting pre-filter chlorination after a period of months when the filter did not receive pre-filter chlorine. Media from the Stamford water treatment plant, which had its Mn uptake capacity exhausted, was shown to resume Mn removal soon after receiving a chlorine dose of2.0 mglL. At the full-scale, after a period of 5 months without chlorine, the oxide coated media produced satisfactory Mn effluent concentrations upon resumption of prefilter chlorination. These results indicate that the characteristics of oxide coated filter media and the process of Mn adsorption are interdependent. A sound understanding of the filter media at a particular water treatment plant could facilitate an optimization of the Mn adsorption process. Further research is recommended to better understand the contribution of particles encapsulated in Mn oxide coating to the surface area of the filter media.