Technologies and Costs for Removal of Arsenic From Drinking Water

een developed provide important information to be considered for AA processes; these are discussedhere.AA beds may be operated in series or parallel. Series operation increases removal and helpsprevent leakage, but limits throughput (leakage simply refers to elevated levels of arsenic in theeffluent). Parallel operation on the other hand increases throughput, but does not improve effluentquality (AWWA, 1990). When operated in series, a “merry-go-round” configuration is often used.This configuration uses three beds: two in production and one in regeneration mode at a given time.When exchange capacity of the first bed in series is exhausted, the first bed is removed from serviceto be regenerated. The second bed in series then becomes the first and a fresh regenerated bed isbrought on-line to become the second. This allows the maximum exchange capacity of beds to be usedand prevents leakage since a fresh bed is always last in line. This also helps minimize regenerationfrequency. Systems operating activated alumina without regeneration will also benefit from seriesoperation. Such an approach will provide greater utilization of the media before it is disposed.Degradation of AA media must also be considered. Alumina tends to dissolve over successivecycles due to the strong base/strong acid cycling during regeneration. As a result of this, alumina bedsmay become “cemented” if close care is not given (EPA, 1994). Backwashing the AA media may helpprevent cementation. Another important consideration is operator involvement. Strong acid andstrong base are handled on a frequent basis and can present a safety hazard. An operator must becapable of handling these chemicals and must have a good understanding of pre-treatment, posttreatment,and regeneration practices if the process is to be operated efficiently. This presents aproblem particularly for small systems. For these reasons, it is recommended that systems utilizedisposable activated alumina rather operate the process with regeneration.Secondary EffectsAA processes may produce changes to the effluent water quality (EPA, 1994). When pretreatmentis used to reduce the pH to low levels (less than 6.0) to optimize the process, the effluentpH will be less than typically desired in the distribution system. For this reason, post-treatmentcorrosion control to raise the pH would be necessary for those systems. Some systems, especiallysmall systems, may choose to operate the process at the natural water pH. The two full-scale plantsstudied in the ORD project (Wang, 2000) were operated at natural pH values of 8 and 8.3. While thismay not yield the optimal run length, it may be sufficient for smaller systems.2-19