§§ Something water related
Timothy Lebrecht, Air Products, USA, and Neil Hannay, Air Products,
UK, offer several ways to treat H2S in wastewater and discuss the benefits
and drawbacks of each.
S
ulfur species are typically treated within a process and
managed through reaction engineering. But what happens
when small concentrations of sulfur species make their way
into wastewater? For many substances, the result is the
creation of hydrogen sulfide (H2S), a dangerous, odorous substance.
This article looks at various ways to handle sulfur species,
particularly H2S, and offers specific treatment methods, along with
the benefits and drawbacks of each method. Although sulfur
treatment can be a significant challenge, a cost effective solution is
available through the creative use of oxygen based chemistry.
What is H2S?
H2S is a flammable, colourless gas that smells like rotten eggs. It
occurs both naturally and from manmade processes. H2S can be
released from volcanoes, sulfur springs, undersea vents, swamps,
stagnant bodies of water and, most commonly, in areas with crude
petroleum and natural gas production and refining. Other industries
that have to manage the creation and treatment of sulfur are water
treatment, landfill gas processing, manure handling, and pulp and
paper. In all of these areas, sulfur species continue to be difficult to
isolate and manage.
In wastewater treatment, sulfur and H2S concentrations tend to
be relatively low, yet high enough to cause issues with safety,
corrosion and odour complaints. This article will review some
techniques for treating sulfur species in wastewater and help
identify the most effective way to manage this odorous gas when it
occurs in the process.
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2015
Table 1. Methods of sulfur control
Typical
requirement,
lb/lb H2S
Further
treatment
required
Process
application
area
Gas stream H2S
removal
Effluent flow
pH and stripping
control can
be difficult to
maintain at
equilibrium, plus
H2S is highly
soluble.
HS
Neutralisation
for discharge
In lagoon
pH too high for
biological activity.
Prevention,
removal and
control
SO
SO4
Maintenance
of high redox/
ORP
Effluent flow
and in lagoon
Must be
maintained to
prevent reduction
of S and SO4
H2O2
Permanganate
Chlorine
Chlorine
dioxide
Hypochlorite
Ozone
Removal and
control
S
SO
SO4
Removal of
solid and/or
prevention of
reconversion
to H2S
Effluent flow
and in lagoon
Expensive;
undesirable
organic reactions;
toxic chemical
handling
considerations.
Stripping
Air
CO2
Gas
stripping for
downstream
gas phase
treatment
H2S in carrier
gas
Gas collection
and scrubbing
Effluent flow
Low pH
maintenance
required for
effective full
sulfide removal;
CO2 carrier gas
is therefore
preferable to air.
Precipitation
Ferrous sulfate
Reaction
to solid
precipitate
Fe2S3 solid
Solids removal
and disposal
Effluent flow
Bactericidal
Acid/alkali
Chlorine
Permanganate
Kill all
bacteria
to remove
biological
reduction of
sulfur species
n/a
Ongoing
requirement
to ensure no
biological
activity
Effluent flow
Method of
control
Methods
Action
Main sulfur
compounds
produced
pH control
acid
Acid dose for
stripping pH<5
Removal
H2S dissolved
by pushing
equilibrium to
H2S (sol)
pH control
alkali
Base dosing for
maintaining in
solution pH>9
Control by
pushing
equilibrium
to HS
Redox/ORP
control
Oxygen (as
air or pure
oxygen)
Nitrate
Oxidation
Issues with H2S
H2S is a chemical that comes with severe dangers. It is a strong acid
when dissolved, extremely flammable and highly toxic. Since this
article focuses on wastewater, the flammability hazard is not part
of the discussion. However, the toxicity and odour of the
substance cause this material to be one of the most challenging to
handle. For example, elevated H2S levels can cause headaches and
nausea at just 5x the odour detection threshold, assuming a
detection threshold of 8 ppb (toxicity issue threshold would then
be 40 ppb). H2S has been lethal to humans at acute concentrations
generally exceeding 500 ppm.
H2S removal versus control
In general, companies are very aware of processes that can
generate or accumulate sulfur and H2S. The dangers associated with
H2S, as well as the extreme odour, have companies doing what
they can to make sure they remove as much of the material as
possible within the process. Treating in process is the best way to
address H2S.
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Comments
4 - 15
Applicable for
corrosion and
odour control in
pipelines.
An example from the refining industry is the Claus reaction,
which transforms H2S to elemental sulfur. Sour water can be
encountered and will need to be treated, but the vast majority of
H2S is handled outside of the wastewater process. Treatment
through specific reactions is most commonly done when volumes
are large and there can be another use for the sulfur. The challenges
become greater when H2S exists in small quantities. Developing a
way to cost effectively manage the substance is challenging, but
possible.
In general, sulfur is an element that is necessary to sustain life.
When sulfur is within an aerobic digestion wastewater system, it is
readily converted to an odourless sulfate. When sulfurs are present
in an anaerobic process, such as anaerobic digestion, however, H2S,
mercaptans or thiols can be formed. The odours associated with
sulfides can range from the smell of garlic to rotten eggs and worse.
The wastewater team at a given facility needs a strategy to
actively treat H2S in the wastewater stream, rather than wait for it
to become an issue that can create problems at the facility or in
the community. There are several good ways to treat H2S, each
with its own advantages and disadvantages. These strategies may
Table 2. Comparison of redox control methods
Method
of redox
control
Efficiency
losses w/
ea 1 mg/l
DO rise
above 0
Typical
energy
required
for
dissolution
(power)
Typical
method of
application
Typical
dose
rates
quantity
required
Mechanism
Side
reactions
Effect on
ecosystem
Benefits
Issues
Pure
oxygen
2%
0.0 - 0.3
kW/kg
Continuous
dissolved
oxygen;
automatically
controlled
injection
8 - 10 g
O2/m3
5 - 10 lb/
lb H2S
Maintains
high dissolved
oxygen
preventing
reduction of
sulfur species.
Local
oxidation of
sulfur species
by biological
and chemical
processes to
H2SO4, SO4
and S.
Biological
BOD removal.
Promotes
bacterial
activity.
Improves
biodiversity
and aerobic
treatment.
High
dissolved
oxygen
for final
discharge.
High
efficiency.
Low
agitation of
basin.
Small
footprint.
No
chemical
handling.
Fully
automated.
Low power.
Good
dissolution
required
to ensure
economics.
Air
10%
0.8 kW/kg
Continuous
surface
aeration
50 - 100 g
O2/m3
(150 - 300 g
air/m3)
Maintains
dissolved
oxygen levels
> 0.5 mg/l
preventing
reduction of
sulfur species.
Biological
BOD removal.
Stripping of
dissolved
H2S due to
nitrogen
waste gas.
Improves
biodiversity
and aerobic
treatment.
Low
technology
investment.
Dissolution
rates drop
in warm
weather,
when
biological
activity
increases.
Waste
nitrogen
gas can
cause over
mixing and
exacerbate
the release
of H2S from
the system.
Air will
strip H2S
at effluent
inlet due to
high volume
waste
nitrogen.
H2O2
N/A
N/A
Batch
delivery and
application
N/A
reaction
not DO
rise 1 - 5
lb/lb
@ 100%
H2O2
Local
chemical
reaction to S.
Stochiometric
1:1 but in
process nearer
5:1.
Chemical
oxidation
of organic
compounds
and biological
components.
Decomposes
to oxygen and
water.
Effectively
disinfects
local
injection
region
entirely
stopping
water
treatment.
Emergency
control of
H2S levels.
Biological
destruction
stimulates
increased
activity and
anaerobic
processes
once
oxidation
potential
lost.
Hazardous
chemical
handling.
Usually
very local
injection
required.
vary by treatment type and include equalisation ponds, anaerobic
ponds and deep storage industrial ponds.
In general, the goal for H2S control in an equalisation pond is
to keep organic material in solution and move it along quickly so
that anaerobic conditions do not develop. Anaerobic ponds are
the largest challenge, as sulfide gas bubbles (most commonly H2S)
can rise from the pond. Strategies to eliminate this include pH
control, stripping with scrubbing, and oxidation reduction
potential (ORP) control/oxidation. Deep industrial ponds can
encounter the same issues as anaerobic ponds, but on a more
seasonal basis. In general, the concepts of pH control, stripping
and scrubbing, and ORP control/oxidation are the best means to
control sulfur in wastewater.
The level of acidity in the treatment basin can be a key way
to make sure H2S does not leave the basin. When wastewater has
a pH level of >9, nearly all H2S will stay in solution as HS and
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Reprinted from February 2015
Figure 1. Air Products' Halia® mixer aeration
system, consisting of oxygen supply, either as
liquid oxygen or onsite generation, combined with
OxyMix® technology jointly developed by
Aqua-Aerobic Systems, Inc. and Air Products.
Figure 2. OxyMix® technology installed in a
wastewater treatment tank.
will not exit the treatment area. This may sound like a simple
approach, however, in biological wastewater, the micro
organisms are the key to treatment. These organisms cannot
live in such a high pH environment. The challenge is that for
these organisms to be healthy, pH must be much closer to
neutral. Doing so keeps the bacteria healthy, but it does not
keep the H2S in solution.
Another means of controlling H2S is stripping it from the
stream with air or CO2 and then scrubbing the H2S. This tends to
be a costly approach due to the large volume of water that
would need to be stripped of H2S, as well as the operating cost
of a scrubber. In most instances, this is not the chosen path of
treatment.
The most common approach in water treatment is ORP
control/oxidation. Oxidation involves the reaction of H2S with
oxygen (O2) to form sulfur (S), sulfate (SO4), sulfuric acid (H2SO4)
or other soluble sulfur compounds. In this way, the challenge of
H2S is changed to an alternate chemical that is more simply
treated or controlled. The issue with oxidation is that it requires
the oxygen molecule in the wastewater at a concentration high
enough to react with the H2S without slipping into anaerobic
conditions. Ponds or treatment basins try to maintain dissolved
oxygen (DO) content at an acceptable level, but due to process
variation and seasonality, DO levels will vary.
A summary of the different types of control for sulfur species
odour is shown in Table 1. As described above, there are multiple
Reprinted from February 2015
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paths to consider, but the most common path, especially when
anaerobic issues are the root cause, is oxidation.
When employing oxidation or ORP control techniques,
process demand variance or seasonality require a dynamic way to
control the input of oxygen. These variations can be controlled
proactively or reactively. A proactive approach is to install the
necessary wastewater equipment to make sure that the dissolved
oxygen level is relatively consistent. A reactive approach is to
wait for the pond or treatment basin to be overcome with an
impurity and then treat on an as needed basis. The reactive
approach can be lower cost if H2S does not start to leave the
treatment system. However, if the levels of the gas leave the
water and create a toxic or odorous environment, the price can
be quite high both in cost and company image.
Oxidation and ORP control
The most common way of controlling sulfur species in a
lagoon or treatment pond is through oxidation or redox/ORP
control. Achieving the necessary level of dissolved oxygen in
the pond or treatment basin requires air, pure oxygen or
hydrogen peroxide. Each of these substances has pros and
cons, which are compared in Table 2.
Air typically has the lowest operating cost; however, due
to the way oxygen is added to the treatment pond, air can
actually cause more H2S to leave the pond than it stops
through reaction with the oxygen. Typically, air is not a
reliable means for control due to the possibility of stripping
and the seasonal variability of O2 retention with temperature.
Although oxygen in air is readily available, the challenge
is getting it into the water solution. Large air based mixers
and aerators can require large horsepower motors and
significant capital to get enough oxygen into the water
solution. One issue this may create is an increase in overall
VOCs into the atmosphere by stripping the treatment pond
of impurities and pushing them into the air. Additionally, over
mixing the pond can exacerbate the release of H2S because
of the high volume of waste nitrogen coupled with increased
sediment disturbance in lagoon type basins. Also, dissolution
efficiency rates tend to drop in warm weather when
biological activity increases, requiring even more energy. For
H2S, the air based approach needs to be handled carefully, as
the solution may cause more issues than the initial problem
or vary by season.
Pure oxygen requires unique equipment to ensure
appropriate mixing and distribution in the treatment ponds.
Good dissolution is required to ensure the economics make
sense for oxygen. The safety of oxygen also needs to be
carefully understood, and special consideration of the
handling and care for the equipment is necessary. Oxygen
based equipment can add the product without stripping the
VOCs. Several methods involve adding the O2 under water in
ways that increase its ability to mix with the wastewater
based on a sensor’s measurement of dissolved oxygen.
Lastly, hydrogen peroxide (H2O2) can offer a solution.
Typically, H2O2 is used as a reactive approach to water
treatment. Ongoing supply can be challenging to distribute
appropriately, which lends itself to be more of a reactive
means of control. Since distribution and control are
challenges, the cost in terms of product, labour and yield are
significant issues. Due to the substance’s high degree of
Table 3. Choosing the right oxygen supply mode
reactivity, H2O2 can react with
impurities other than H2S,
Supply features
Liquid oxygen
Onsite generation
Pipeline*
thereby reducing its ability to
Flow range (tpd)
0 - 50
50 - 150+
100+
effectively treat H2S. One of
Commitment
Low
High
Medium
these side reactions is biological
Time
to
implement
(months)
1
2
10
18
6-8
destruction of organisms in the
Location limitations
Yes
No
Yes
basin, which can stimulate a
second activity in anaerobic
Application best fit
processes once oxidation
Flow
Low
Medium/high
High
potential is lost. The safety of
Use pattern
Variable
Steady
Variable/steady
using H2O2, which requires
*Gas piped in from remote air seperation plant
hazardous chemical handling,
also increases the cost of this
flexible source of oxygen. Evaluating the optimal mode of supply
option.
requires the review of a host of factors, including:
As illustrated in Table 2, oxidation through the use of pure
nn Size of the oxygen requirement (average and peak demand).
oxygen creates the safest and lowest cost option.
nn Expected use pattern (continuous, seasonal, erratic).
nn Presence of other nearby oxygen consuming applications, such
Oxygen injection
as ozone.
Pure oxygen can be injected into wastewater in a number of
nn Power availability and cost.
ways. For years, this approach has been used in activated sludge
nn Proximity of delivered oxygen source.
systems to boost the dissolved oxygen content when other
means have been exhausted. Oxygen tends to be a way to boost
Table 3 provides rough guidance about the best mode of
a treatment pond’s performance without increasing its size.
supply in the context of these parameters. From the early stages of
These same methods are also good for the treatment of H2S. In
the project, an industrial gas company like Air Products will work
the treatment of H2S, oxygen tends to be better because it does
closely with a wastewater treatment plant to jointly determine the
not lift impurities in the air through stripping, and most of the
best mode of oxygen supply.
O2 is injected into the water without significant agitation of the
pond.
Some typical ways of adding oxygen are: diffusion into a
Conclusion
pipeline, diffusion into a grid in the treatment pond, a floating mixer
Sulfur species are problematic for many industries. However, there
aerator unit or a floating diffuser based system. Each of these
is hope for a way to control this substance even when treatment
systems varies in yield and complexity. The simplest way to add the
conditions are variable. Equipment designed to use pure oxygen
pure O2 is through basic diffusion. This can be done by adding O2
with an accompanying DO probe and a programmable logic
controller can maintain the necessary dissolved oxygen content
through a lance into a line of water that feeds the treatment basin.
required to transition H2S and other sulfur species into treatable
Another simple way is to have a static diffusion grid in the treatment
pond bubbling oxygen through the depth of the pond. Each way has
sulfates. Other types of treatment can prove to be more costly or
definite limits on efficiency. Typically, only 10 - 15% of the oxygen
create more problems than they solve. For example, temporary
put into the pipe or diffusion grid is captured by the water, and the
fixes, such as hydrogen peroxide, do not solve the issue. Use of
rest escapes into the atmosphere.
H2O2 is costly, creates safety challenges, and is not a permanent
There are several other approaches that use a more complex
solution. The more practical solution is pure oxygen, which
method for adding oxygen. Diffusion based forced water/oxygen
provides a continuous supply to a treatment pond without
systems can be operated. Industrial gas companies like Air Products
exposure to a hazardous chemical for workers and without danger
offer this type of equipment. Oxygen efficiency can reach up to 90%
of hurting or causing an issue with the planned biological treatment
under specific operating parameters. Air Products offers such items
of the wastewater.
as the Halia® Mixer Aerator and the Halia® Venturi Aerator units for
both deep and shallow treatment pond conditions. These units,
References
1. Hydrogen Sulfide; MSDS No. 300000000081; Air Products: Allentown,
pictured below, can add up to 10 000 lb/d of oxygen per unit to a
Pa., February 8, 2014.
treatment zone.
2. MOUSSAVI et al., The Removal of H2S from Process Air by Diffusion
Oxygen supply
The supply of oxygen is also an area where great expertise is needed.
Oxygen can be provided from a liquid oxygen tank or an onsite
generator.
The most common mode of supply for delivered oxygen is via on
road liquid oxygen tankers from a central manufacturing facility. The
oxygen is stored as a liquid at the site in an insulated tank and
vaporised at the time of use. This is the most flexible mode of supply.
Oxygen can also be generated onsite using cryogenic or
adsorption technologies. At locations in the vicinity of an oxygen
pipeline, supply via pipeline could be the most cost effective and
into Activated Sludge, Env Tech, Vol 28, pp. 987 - 993, 2007.
3. HJORTH et al., Redox Potential as a Means to Control the Treatment of
Slurry to Lower H2S Emissions, Sensors 2012, 12,
pp. 5349 - 5362.
4. Septicity in Sewers: Causes, Consequences and Containment, Boon
1995, Water Sci tech, Vol 31, No 7, pp. 237 - 253.
5. Metcalf and Eddy Wastewater Engineering 2003 Fourth Edition.
6. EPA: Process Design Manual for Sulfide Control in Sanitary Sewerage
Systems 1974.
7. Economical, Efficient and Effective Mixing: Three Approaches to
Controlling Odor in Wastewater Treatment Ponds – White Paper
Medora Corp.
8. NIELSEN at al., Aerobic and Anaerobic Transformations of Sulphide in
a Sewer, WEFTEC 2006.
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Reprinted from February 2015