A Midwest municipality needed to find a way to eliminate
hydrogen sulfide from the atmosphere in the sludge dewatering area as well as
on the plant grounds. The successful elimination of noxious and toxic gas would
not only make a safer, healthier environment for the workers involved but would
also result in a reduction in the corrosion of metallic components in the
dewatering area, while greatly reducing odor complaints from neighbors.
At this municipality, the primary and secondary sludge
enters two sludge thickeners. The supernatant from these thickeners is sent to
a secondary clarifier before it leaves the plant. The solids are pumped from
the bottom of the thickeners through a 6-in. pipe to a 24,000-gallon mixed wet
well. From the wet well the solids are pumped to one of three belt filter
presses which generally run 24-hours-a-day, seven-days-a-week at 50 to 60 gpm.
The filtrate and wash water from the press operation are returned to the
secondary clarifier, which accepts the supernatant from the thickeners.
Due to the long retention time, the sludge solids are in a
septic condition. This plant had been utilizing potassium permanganate for the
control of odors in the sludge pressing operation with mixed results. The feed
system has been problematic in that it regularly clogs and shuts down, and
requires maintenance on the piping as well as the actual feed system. The plant
was interested in alternative technologies that would provide superior odor
control using a less maintenance intensive feed system.
A team of experts from Vulcan Performance Chemicals of
Birmingham, Ala., surveyed the plant to establish the most effective feed
location and to set demonstration parameters. They determined that a new
product, VX-456, would meet their objectives. This product is a mixed oxidant
containing multiple components that both immediately oxidize reduced sulfur
species, and also prevent their regeneration. The team recommended the
following equipment and testing be conducted during the evaluation:
• A
positive displacement-metering pump capable of delivering approximately 120 gpd
of chemical with manual control to deliver the VX-456. The feed point selected
was the transfer line from the bottom of the sludge thickeners to the mixed wet
well ahead of the sludge pumps for the belt filter presses. The chemical feed
was arranged so that carrier water could be used to facilitate mixing.
• Oxidation-Reduction
Potential (ORP) for control of the oxidant feed system. ORP is a measure of the
oxidizing or reducing nature of an aqueous solution. This method has been used
with great success to measure and control oxidant feed systems in water and
wastewater treatment plants. A liquid having a positive ORP can be said to be
capable of oxidizing and a liquid with a negative ORP can be said to be
reducing in nature. In this application, the ORP of the belt press filtrate
indicated the presence of excess oxidant as well as the possible presence of
hydrogen sulfide.
• H2S
monitoring to be performed through grab sampling. Atmospheric samples were
gathered at the point where the liquid from the top wash box (gravity section
of the press) dropped into the collection basin. This same area was used to test for
mercaptan in the atmosphere. In addition to the monitoring at the base of the
press, atmospheric monitoring was performed at an outside wet well which served
as a surge basin for the pressing operation.
• Testing
on the filtrate to measure H2S and ORP and to determine if the filtrate drain line could be used
as a control location for chemical addition. The Vulcan team conducted a
demonstration to prove that the application of its new technology would be
effective in eliminating hydrogen sulfide from the atmosphere in the sludge
dewatering area as well as on the plant grounds. This demonstration was also
designed to compare Vulcan’s VX-456 to potassium permanganate and to
determine the approximate daily feed rate necessary to control odors.
The VX-456 was introduced to the sludge transfer line prior
to the belt press. It was fed initially at a rate of 64 gpd. This rate was
subsequently determined to be in excess of that needed to alleviate the odors
when testing revealed that the ORP of the filtrate was greater than +100 mV,
and that there was no detectable sulfide. A positive ORP reading indicated that
the treatment was working and the release of hydrogen sulfide was under control
in the wet well and also at the press.
After the first two days of treatment, the required dosage
of VX-456 was reduced to approximately 47.5 gpd. This dosage held more or less
firm until the trial concluded at the end of the week. Data collected during
the demonstration indicated that VX-456 is capable of controlling not only
hydrogen sulfide odors but mercaptan odors as well. ORP was demonstrated to be
an effective measurement tool to determine when the treatment program was
effective.
Vulcan’s VX-456 offers a cost effective solution to
plant odor problems. Based on available pricing information and historical
usage data for permanganate, Vulcan’s VX-456 approach costs approximately
20 percent less, according to the company. In addition, savings can be realized
in operations and maintenance on the chemical feed system.
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