Wastewater Odor Control Applications with Hydrogen Peroxide
Hydrogen Peroxide as a Replacement for Wastewater Odor Control using Sodium Hypochlorite
Hydrogen
peroxide may be used in both mist scrubbers and packed tower scrubbers
as a replacement for sodium hypochlorite (bleach). Like bleach, the
process involves two concurrent mechanisms: 1) absorption of the odors
(H2S) into the alkaline scrubbing solution; and 2) oxidation of the
absorbed sulfide in solution.
| Step 1: | H2S + NaOH → NaSH + H2O |
|---|
| Step 2: | 4H2O2 + H2S → H2SO4 + 4H2O |
|---|
Typical
dose ratios are 5 parts H2O2 per part H2S or, when used in place of
bleach, one gallon 50% H2O2 for every 10 gallons of 15% sodium
hypochlorite (NaOCl). This generally translates into a break-even cost
scenario. Sufficient caustic soda (NaOH) is added to maintain a pH of
10.0 - 10.5 in the scrubbing solution.
There is also in practice a
process which uses H2O2 in series with bleach to scrub composting
odors. This process relies on a series of three packed tower scrubbers:
the first is a pH neutral water wash (to remove ammonia and amine
odors); the second uses a conventional caustic/bleach solution in which
the bleach is purposely overdosed (to oxidize the complex organic sulfur
odors); and the third uses a caustic/H2O2 solution (to remove the
unreacted chlorine vapors carried over from the second stage).
| H2O2 + HOCl → HCl + H2O + O2 |
Typical
dose ratios are 0.5 parts H2O2 per part hypochlorite (OCl-), with
sufficient caustic soda (NaOH) added to maintain a pH of 8.5 in the
scrubbing solution.
Practical Considerations
With the
elimination of bleach, the benefits realized in using H2O2 include no
formation of chlorinated VOC's, no chlorine odors due to overdosing,
substantially reduced corrosion of process equipment, and reduced
scaling of spray nozzles and packing material. However, because of the
different properties of H2O2 versus NaOCl, the substitution is not
straightforward. For example:
- Since H2O2 is more
concentrated and leaves no salt residues, there is less need for
blowdown in packed tower scrubbers. Typically, blowdown rates may be
reduced 5-10 fold over those using bleach.
- Since the reaction
between H2O2 and sulfide is slower than that between NaOCl and sulfide, a
higher working concentration of oxidant is needed in the scrubber
solution. Typically, this is 50 - 200 mg/L H2O2.
- Because of the
requirement for a high residual concentration of oxidant, conventional
control systems which rely on ORP sensing may not be suitable for
controlling H2O2 feed. ProMinent Controls has developed an online H2O2
controller which has shown great value in this application.
- Some
scrubbing systems which currently use bleach may be operating at a pH
< 9 which volatilizes chlorine, thereby allowing direct oxidation of
H2S to occur in the gas phase. Since H2O2 will not perform in this
manner, changing the operating characteristics of the scrubber may
highlight a overstressed design.
- H2O2 is less reactive toward
organic odors than bleach and, where this is a concern (e.g., with
thiols or mercaptans), the H2O2 must be activated through catalysis.
This entails special considerations and the user is encouraged to
contact USP Technologies for further guidance.
Evaluation Process
The
evaluation process for using H2O2 in odor scrubbers begins with an
initial paper assessment, but inevitably must involve a field pilot
test. The following information will greatly assist in completing an
initial paper assessment:
- Schematic of unit processes showing the type and number of scrubbers;
- Gas flow rates, including influent / effluent H2S concentrations;
- Comments regarding the relevance of non-H2S (organic) odors;
- Design detail on the scrubbers showing unit volume, packing type/density (if applicable), and gas/liquid contact parameters;
- Recirculation and blowdown rates for the scrubbing solution;
- Current operating and control parameters (e.g., pH and ORP); and
- Current chemical use-rates, costs, and any comments on effectiveness (for both bleach and caustic).
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