Development of an attached growth reactor for NH₄–N removal at a drinking water supply system in Kathmandu Valley, Nepal

Abstract

Higher concentrations of ammonium (NH₄–N) and iron (Fe) than a standard for drinking are typical characteristics of groundwater in the study area. To remove NH₄–N and Fe, the drinking water supply system in this study consists of a series of treatment units (i.e., aeration and sedimentation, filtration, and chlorination); however, NH₄−N in treated water is higher than a standard for drinking (i.e., <1.5 mg NH₄–N/L). The objective of this study, therefore, is to develop an attached growth system containing a fiber carrier for reducing NH₄–N concentration within a safe level in the treated water. To avoid the need of air supply for nitrification, groundwater was continuously dripped through the reactor. It made the system simple operation and energy efficient. Effects of reactor design (reactor length and carrier area) were studied to achieve a high NH₄−N removal efficiency. In accordance with raw groundwater characteristics in the area, effects of low inorganic carbon (IC) and phosphate (PO₄–P) and high Fe on the removal efficiency were also investigated. The results showed a significant increase in NH₄–N removal efficiency with reactor length and carrier area. A low IC and PO₄–P had no effect on NH₄–N removal, whereas a high Fe decreased the efficiency significantly. The first 550 days operation of a pilot−scale reactor installed in the drinking water supply system showed a gradual increase in the efficiency, reaching to 95−100%, and stability in the performance even with increased flow rate from 210 to 860 L/day. The high efficiency of the present work was indicated because only less than 1 mg of NH₄−N/L was left over in the treated water.

Keywords:

• NH₄−N removal
• groundwater treatment
• drinking water supply system
• attached growth reactor

Acknowledgments

The authors would like to thank GCOE program, University of Yamanashi for the financial support, Center of Research for Environment Energy and Water (CREEW) in Nepal for operating and maintenance on−site reactors. Further, the authors are also thankful to Prof. Kei Nishida for E. Coli analysis, Dr. Vishnu Prasad Pandey for valuable comments and suggestions, and Mr. Hiroyuki Isshiki for assistance during the EDS analysis.