CASE
STUDY:
Hanford, California
Arsenic, Color and Hydrogen Sulfide Removal from Ground Water
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State of
the Art Treatment for Arsenic, Color, and Hydrogen Sulfide.
New Arsenic Regulations
How meaningful to the average
water customer are µg/l or parts per billion? Not very. How about
the word “arsenic”? That
will ring some bells as the mystery novel poison-of-choice. But with
new, lower EPA arsenic standard of 10 ppb that went into effect Jan.
16, cities with arsenic problems are required to inform customers in
writing of non-compliance, which is sure to cause an uproar with customers.
Hanford, CA, is faced with just such a problem. In addition to the long-standing
hydrogen sulfide and some color difficulties in its 15 active groundwater
wells, many also have been bordering on non-compliance with the old
EPA arsenic standards of 50 ppb. Under the new standard, 14 of Hanford’s
wells are out of compliance.
Hanford is a medium sized city in central California serving approximately
40,000 residential, commercial and industrial customers. In anticipation
of the new arsenic standard, a pilot study at one of Hanford’s
wells was conducted in November 1998 using a treatment system manufactured
by Filtronics, Inc., a water-treatment system manufacturer located in
Anaheim.
Mike Wegley, Engineer; Terry
Carr, Utility Superintendent; David Weisser, Utility Supervisor and
Richard Hobbs, Utility worker, all with the City of Hanford, examine
the city’s new Filtronics treatment unit.
Color and Hydrogen Sulfide
Problems
Drilled at considerable expense
in 1990, the 1700-foot-deep well was operated 18 months before it began
producing colored water in the range of 30 to 60 color units (it looks
like weak tea). The water has a noticeable hydrogen sulfide odor and
arsenic levels ranging from 25 to 92 µg/l. As a result, the well
was never put on-line.
Pilot Test
When the portable pilot
plant began treating water from the well, water was produced with non-detectable
color, zero hydrogen sulfide and arsenic levels from non-detectable
to 3.4 ppb near the end of the test run.
The city of Hanford was really impressed with the pilot study, and asked
Boyle Engineering to design not just a treatment system, but a Filtronics
treatment system,” said Gunter Redlin, Principal Sanitary Engineer
at Boyle Engineering Corp.
Based on the success of the
test, a full-scale wellhead treatment plant was designed and constructed
in 2000 at an approximate cost of $600,000. Built to a 20-year municipal
standard, the 1000 gpm system uses a pressure flocculator vessel (with
a 10-minute retention time), in which the coagulant and other treatment
chemicals react with the contaminants in the water. So treated, the
water then enters two steel pressure filter vessels (ASME Code approved
and epoxy lined using an ANSI approved coating) in parallel at a loading
rate of 10 gpm per square-foot of filter area.
Two filters were used to reduce the backwash flow requirement (one filter
is backwashed at a time), to reduce demands on Hanford’s distribution
system. The non-hazardous sludge produced during backwash is discharged
to the sewer and processed with other wastewaters normally.
The latter point is significant.
“Take ion exchange, an arsenic-removal process that works similar
to how home water softeners do, the filter media is regenerated with
concentrated salt. But now you have a waste regenerate that in addition
to being high in arsenic is also high in salt. The brine waste is very
difficult and expensive to dispose of,” Redlin Said.
Backwash
The backwash
to filtration ratio is 0.2 percent for the Filtronics treatment system,
so in addition to being non-hazardous, there is a comparatively small
volume of wastewater and sludge of which to dispose versus other possible
treatment schemes. The system is fully automated and routine maintenance
is limited to checking control valves and monitoring chemical-feed equipment.
Conclusion: One Process
for Multiple Contaminants
Since the Filtronics system
works on multiple contaminants using a single special filtration media,
it has proven to be cost effective. For a city like Hanford with several
contaminants, multiple treatment schemes aren’t necessary, keeping
equipment and facility costs down.
With the positive results of this first plant, Hanford has been able
to use a previously unusable water source in which it had invested considerable
capital. The city is now embarking on a wellhead treatment program that
may one day encompass its entire well water supply.
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