Abstract
Granular activated carbon (GAC) has been widely introduced to advanced drinking water purification plants to remove organic matter and ammonium. Backwashing, which is the routine practice for GAC maintenance, is an important operational factor influencing the performance of GAC and its microbial biomass. In this study, the effects of backwashing on the ammonium removal potential of GAC were evaluated. In addition, abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) on GAC were analyzed. GAC samples before and after backwashing were collected from a full-scale drinking water purification plant. Samplings were conducted before and after implementation of prechlorination of raw water. The results showed that the ammonium removal potential of the GAC increased by 12% after backwashing before prechlorination (p < 0.01). After implementing the prechlorination, the ammonium removal potential of the GAC decreased by 12% even after backwashing (p < 0.01). The AOA was predominant on the GAC in the two samplings. Regardless of prechlorination, the amounts of the AOA and the AOB remained at the same level before and after backwashing. Analysis of the backwashing water indicated that the amounts of the AOA and AOB washed out from the GAC were negligible (0.08%–0.26%) compared with their original amounts on the GAC. These results revealed the marginal role of backwashing on the biomass of ammonia oxidizers on GAC. However, the results also revealed that backwashing could have a negative impact on the ammonium removal potential of GAC during prechlorination.
Highlights
Ozonation followed by granular activated carbon (GAC) has been widely installed in drinking water purification plants (DWPPs) to improve water quality [1,2]
GAC, which had been in use for 19 months, was collected before and after backwashing from a full-scale DWPP in Tokyo in October 2012
The first significant decrease in the ammonium concentration after the primary sand filtration (p < 0.01) in Figure 1a should be attributed to the biological activities of the ammonia-oxidizing microorganisms
Summary
Ozonation followed by granular activated carbon (GAC) has been widely installed in drinking water purification plants (DWPPs) to improve water quality [1,2]. GAC has a large specific surface area [3]. While physical adsorption is a major function of GAC, microorganisms proliferating on the surface of GAC play an important role in water purification [4]. The oxidation of ammonium is one of its key biological functions. Waterborne ammonium reacts with chlorine to form monochloramine, which in some countries serves as a long-lasting disinfectant, ensuring pipeline water security [5].
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