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Ordering Information:
ORDER NUMBER: 90836
DATE AVAILABLE: Spring 2001
Printed Report |
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PRINCIPAL INVESTIGATORS: Vernon L. Snoeyink, Lutgarde M. Raskin,
and Sun Liang
OBJECTIVES:
The two main objectives of the project were to investigate the biological reduction
of bromate and perchlorate in biologically active carbon (BAC) filters and to
investigate the abiotic reduction of perchlorate in advanced oxidation processes
involving granular activated carbon (GAC). Specific project objectives were
as follows:
- show that perchlorate can be removed in BAC adsorbers under the same conditions that have proven effective for bromate removal, or show how these conditions must be changed to obtain biological removal of perchlorate
- investigate advanced oxidation processes to determine whether these reactions can produce efficient removal of perchlorate, and if so, develop the process at the bench-scale
- optimize the bromate and perchlorate removal processes that show the most encouraging results and monitor the product water for potability
- test the most promising of the bromate and perchlorate removal processes at the pilot-scale
BACKGROUND:
Bromate and perchlorate are inorganic compounds sometimes found in
drinking water. It has been determined that the ingestion of aqueous bromate
caused renal cell tumors in rats. Perchlorate is a health concern due to its
ability to disrupt the thyroid gland's use of iodine in the generation of metabolic
hormones. Thus, the removal of bromate and perchlorate from drinking water or
their reduction to innocuous end-products is an important issue.
HIGHLIGHTS:
Biological bromate reduction can be achieved in BAC filters. The mass of bromate
removed in the filter can be affected by the concentrations of dissolved oxygen
(DO), nitrate, pH, the type of influent water, and the influent bromate concentration.
Bromate-reducing microorganisms seem fairly common since eight bromate-reducers
were isolated from the BAC filters.
Filter history can also greatly affect the subsequent mass of bromate removed in a BAC filter. Additionally, BAC filters demonstrated efficient biological reduction of low microgram-per-liter concentrations of perchlorate under anaerobic conditions with the addition of an external electron donor mixture.
The study also showed that abiotic removal of perchlorate by GAC is inefficient. Results indicated that biological perchlorate reduction can be hindered by the presence of nitrate, but can be enhanced by the occurrence of nitrate reduction.
APPROACH:
The project included bench-scale work conducted at the University of Illinois
at Urbana-Champaign and the Metropolitan Water District of Southern California
(MWDSC). In order to observe its effect on bromate and perchlorate removal,
generally one water quality or operational parameter was changed at a time.
Additionally, pilot-scale work for removal of bromate and perchlorate using
BAC filtration was completed at the La Verne pilot plant at MWDSC. These experiments
were carried out using filter parameters that were tested at the bench-scale.
RESULTS/FINDINGS:
Increasing the influent DO and nitrate concentrations to the BAC filters decreased
bromate removal, but an increased sulfate concentration did not have much effect
on bromate removal.
Depending on the empty bed contact time, an increased influent bromate concentration could cause an increase in the mass concentration of bromate removed. Additionally, bromate removal increased as the influent pH to the BAC filters decreased to near-neutral values.
Filter history was important to bromate removal. The negative effect of high DO concentrations on the bromate-reducing bacteria was not immediately reversible after the DO concentration was reduced. Backwashing the BAC filters did not adversely affect bromate reduction.
Perchlorate was ion exchanged-not reduced-by GAC, and also was not reduced in the presence of ozone or ozone/hydrogen peroxide.
BAC filters demonstrated efficient biological reduction of low microgram-per-liter concentrations of perchlorate under anaerobic conditions with the addition of an external electron donor mixture. Additionally, nitrate reduction enhanced the perchlorate-reducing ability of a BAC filter. Without nitrate reduction, the filter's ability to reduce perchlorate deteriorated and could be lost entirely. However, if the effluent nitrate concentrations from the filter were greater than 0.05 mg/L, perchlorate reduction was significantly inhibited.
IMPACT:
To utilize BAC filtration for bromate and perchlorate reduction, a utility must
determine if reduction of the influent DO concentration to the filter is feasible.
Not only must it be economically feasible, but the utility should also determine
that no unwanted effects are caused by DO reduction. This could be accomplished
through bench-scale testing with the water and filter medium used by the utility.
Depending on the alkalinity of the water, the utility should consider a combination of reduction of bromate formation during ozonation and an increase in biological bromate removal in the BAC filter by pH control.
Utilities that treat water contaminated with both nitrate and perchlorate should
give particular consideration to whether or not BAC filtration is a feasible
treatment option. Research has shown that BAC filtration removes nitrate and
perchlorate very efficiently. Additionally, nitrate reduction can enhance perchlorate
reduction kinetics, making BAC filtration particularly attractive for combined
nitrate-perchlorate remediation.
PARTICIPATING UTILITY:
The Metropolitan Water District of Southern California (La Verne)