Technologies and Costs for Removal of Arsenic From Drinking Water

Since the available arsenic capacity of the “old” polishing column is lower than the maximumcapacity, the steady-state run length must be lower than 18,500 bed volumes.G. The run length was determined by an iterative approach. The integral equation was used toestimate the arsenic adsorbed onto the first column for the lower run lengths. The availablearsenic capacity of the second column was also estimated using the arsenic leakage determinedfor the new run length. When the arsenic adsorbed on column 1 was equivalent to theavailable arsenic capacity of column 2, the steady-state run length was determined. Thefollowing will illustrate this process:At 15,400 bed volumes:Column 1 - arsenic adsorbed = 4651 gramsColumn 1 - arsenic passed through = 978 gramsColumn 2 - arsenic leakage = 346 gramsColumn 2 - arsenic adsorbed = 632 gramsColumn 2 - available arsenic capacity = 5288 - 632 = 4656 gramsThe available arsenic capacity of column 2 is approximately equivalent to the amount ofarsenic that can be adsorbed onto it when it becomes the roughing column (column 1) basedon the maximum run length. This is the steady-state run length to meet a standard of 2 Fg/L atthe effluent to the second column. After 15,400 bed volumes, the media in the roughing columnis taken off-line and replaced. While this is occurring, the polishing column becomes the newroughing column and the redundant column becomes the new polishing column.H. The Phase 3 Report (12) recommended that the run length be increased by up to 50% toaccount for improved performance when hydrochloric acid is used instead of sulfuric acid toadjust the pH. Based on the Phase 3 Report, low and high estimates of run length were madebased on the use of hydrochloric acid for pH adjustment. For the low option, a 20% increasewas assumed and a 50% increase was assumed for the high option. If hydrochloric acid isused for pH adjustment, the run length would range from 18,480 bed volumes to 23,100 bedvolumes.D-19