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H2S CONTROL
cost effective control of hydrogen sulfide in municipal sludge
In many municipal treatment facilities, the processing of wastewater
sludge (or the current politically correct term: biosolids)
poses operational and safety challenges due to hydrogen sulfide
(H2S) generation. In addition to being a hazard to operations
personnel (H2S is a poisonous gas), H2S odors generated during
sludge processing can cause community complaints and make plant
working conditions unpleasant. Furthermore, damage to equipment
and concrete structures caused by H2S initiated corrosion can
be substantial. Typical areas where H2S problems occur include
gravity thickeners, mix tanks, and dewatering presses.
Hydrogen sulfide is generated in sludge via the same mechanism
as in collection systems - conversion of sulfates to sulfides
by bacteria. This biochemical generation of H2S is brought about
by anaerobic or oxygen limited conditions. There are numerous chemical and non-chemical treatment technologies
available which can control H2S in municipal sludge processing
operations. This article will discuss the use of hydrogen peroxide
for sludge treatment and give real world examples from field experiments
and full scale plant use.
In municipal wastewater, the reaction time required for complete
oxidation of H2S by H2O2 typically ranges from 10-30 minutes depending
on temperature, pH, and dissolved metal concentrations. In municipal
sludge the reaction rate is much faster. Usually, 5 minutes or
less are required for complete removal of H2S. The faster reaction
rate in sludge is due to relatively high concentrations of solids,
metals, and other catalytic materials. This rapid oxidation of
H2S permits H2O2 to be added only a few minutes ahead of the sludge
presses and react completely without interfering with dewatering
polymers. In sludges a H2O2 to sulfide (liquid) ratio between
5:1 and 10:1 is usually required for complete oxidation.
Although the H2O2 reacts quickly with H2S in sludge, its effect
on H2S is not short lived. One of the reaction products of H2O2
decomposition is oxygen (O2). The additional O2 provided by H2O2
will help maintain aerobic conditions in the sludge, and suppress
H2S regeneration for hours. This extended effect allows H2O2
to be added further upstream to destroy H2S (e.g., in sludge mix
tanks), and still maintain H2S control at the dewatering presses.
Other Oxidants such as potassium permanganate (KMnO4) are also
used for H2S control in municipal sludge. Potassium permanganate
must be added immediately before dewatering because it reacts
instantaneously with H2S, and does not effectively suppress H2S
regeneration. Therefore, it cannot be added to mix tanks or thickeners
and maintain H2S control in subsequent stages of the sludge processing
operation.
An example of hydrogen peroxide's ability to suppress H2S regeneration
was observed while conducting experiments at a municipal treatment
facility in the Northeast. Facility personnel wished to oxidize
H2S in their sludge mix tanks to reduce potential exposure to
operators, and to reduce odors and equipment corrosion. Furthermore,
it was hoped that chemical treatment in the mix tanks would maintain
control of H2S until the sludge reached the belt filter presses.
The residence time in the mix tanks ranged from 1-2 hours depending
on flow rates of primary and secondary sludge. From the mix tanks,
sludge is pumped directly to several large belt filter presses.
Experiments compared H2O2 to potassium permanganate (KMnO4) to
determine the most effective method to control H2S at both the
mix tanks and the filter presses using a single chemical addition
point.
Data was collected at average sludge mix tank holding
times (~90 minutes). Baseline H2S (air) measurements taken from
about 6 inches above the surface of the sludge showed H2S concentrations
exceeding 850 ppm. Subsequent to these initial measurements, H2O2
or KMnO4 was metered into the sludge at concentrations ranging
from 200-500 mg/L (100% oxidant basis). After addition of the
prescribed concentration of oxidant (with mixing) the sludge was
held for 90 minutes without further chemical treatment. After
90 minutes, Measurements of H2S (air) were again taken from just
above the surface of the sludge. Results showed that H2O2 treatment
reduced H2S levels by 87-99% across the range of dose rates.
The KMnO4 treatments were not as effective, with H2S reductions
of 38-68% over the same range of oxidant concentrations. These
results are shown graphically here.
This study demonstrated that H2O2 can control H2S for extended time periods. In addition, H2O2 was clearly superior to KMnO4 when comparable concentrations of each oxidant were used. This information, combined with the lower cost of H2O2 ($0.685/lb H2O2 100% basis vs. $1.21/lb KMnO4 100% basis) made H2O2 a more cost effective choice for this municipality.
The City Of Orlando, FL
The City of Orlando processes sludge at their WATER-CONSERVE 1 Plant. Sludge is pumped to 2 belt filter presses from digesters located adjacent to the sludge processing building. Typical sludge feed rates to each of the presses ranges from about 85-100 GPM (170-200 GPM total). In order to control odors and protect operations personnel from hydrogen sulfide exposure, H2O2 is injected into the sludge lines feeding the presses. The reaction time in the line prior to reaching the belt filter press is only about 3-5 minutes.
Recently, air and filtrate sampling was conducted during sludge processing to examine the level of H2S reduction achieved with H2O2 treatment. With a sludge feed rate of 175 GPM and the H2O2 off, H2S levels above the presses ranged from 18-42 ppm in the air. Filtrate samples collected during this time showed a hydrogen sulfide concentration of 5 mg/L in the liquid. When H2O2 treatment was initiated at a concentration of about 35 ppm (0.6 GPH 50% H2O2) the H2S levels in the air above the presses were quickly reduced to non-detectable levels (STX70 model H2S monitor - Industrial Scientific Inc.). Sulfide levels in the filtrate were likewise reduced to non-detectable levels. These reductions in H2S levels were observed within 5 minutes of starting H2O2 treatment, allowing quick response to unexpected increases in H2S concentrations in the sludge.
The hydrogen peroxide used for this application is stored in a US Peroxide designed H2O2 dosing system which contains the double-contained polyethylene storage tank, pumps, control panel and ChemWatch remote tank monitoring system (see Services Offered by US Peroxide or ChemWatch Monitoring Program information).
Summary
The unique chemistry of H2O2 makes it a versatile tool for destruction of H2S in sludge processing operations. It can be added to sludge lines just prior to dewatering presses to quickly eliminate H2S, or used in mix tanks and thickeners to control H2S generation for longer periods of time. The following advantages of H2O2 should be considered when evaluating sludge treatment options:
 
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