Chlorine Replacement Applications with Hydrogen Peroxide
For over 50 years, pre-chlorinating the influent has been the standard treatment program to keep hydrogen sulfide (H2S) from entering wastewater treatment plants. Due to regulations and safety concerns, however, an increasing number of POTW's have revisited their approach and found a different answer - hydrogen peroxide (H2O2). Documented results at numerous municipal treatment facilities have proven that H2O2 can replace chlorine with little to no cost increases and notable benefits to downstream plant operations.
Reasons for Increasing Use of Hydrogen Peroxide for Sulfide Control
Reasons for the shift from chlorine to H2O2 include:
- Efficient reaction of H2O2 with H2S
- Lower cost of H2O2 compared to sodium hypochlorite
- Industry trend to eliminate gas chlorine and its associated risks
Efficient reaction of Hydrogen Peroxide with Hydrogen Sulfide
At the pH of municipal wastewaters (typically around 7), the oxidative reaction of H2O2 with sulfide is as follows, yielding elemental sulfur and water.
Therefore, 1 mg/L H2O2 (100%) is theoretically required to oxidize 1 mg/L H2S (a 1:1 dose ratio). In practice, slightly higher dose ratios (e.g. 1.2-1.5 : 1) are typically required for effective headworks H2S control. The reaction efficiency depends on many factors, the most important of which are available reaction time and duration of control. The optimal range is typically between 5-20 minutes and 1-2 hours. Operating outside of this range is most likely the reason why dose ratios reported in some of literature are 4-8 times the theoretical requirement. However, as the examples below show (see Figure 1), practical ratios much closer to theoretical are attainable if one can operate nearer to the optimal range.
Figure 1. Effective H2O2 : Sulfide dose ratios for various headworks applications
|Location||Effective||Available||Aqueous Sulfide Level|
|Dose Ratio||Reaction||Before (mg/L)||After (mg/L)||Removal %|
San Jose, CA
5 - 10 min
2 - 4
0.3 - 0.5
80 - 90
MWRA (Boston, MA)
8 - 10 min
3 - 5
0.2 - 0.3
90 - 95
One concern expressed with changing to H2O2 is what becomes of the excess chemical. Unlike chlorine or hypochlorite, it does not react with ammonia or the many organics present in the wastewater. In fact, it naturally decomposes to dissolved oxygen (D.O.) and water. This added D.O. has practical value in helping to control sulfide generation in downstream primary clarifiers, an aspect not afforded by chlorine or hypochlorite. The decomposition reaction of H2O2 is often referred to as the "preventative" mechanism and is represented as:
Lower cost of hydrogen peroxide compared to sodium hypochlorite
A typical first response when searching for alternatives to gas chlorine might be to change to sodium hypochlorite (NaOCl), which is not a PSM-listed material. This is not a perfect substitution, however, as NaOCl costs about five times chlorine on an active basis. As a result, other non-chlorine H2S control agents must be considered. In this case, H2O2 becomes the best option as the standard strengths of H2O2 used at wastewater treatment facilities (35% and 50%) are exempt from the PSM ruling. Furthermore, it has been proven that H2O2 is considerably cheaper than NaOCl, with the actual costs being closer to chlorine (see Figure 2) for headworks H2S control.
Figure 2. Comparative costs for headworks odor control chemicals - (typical costs to treat 200 lbs/day of liquid-phase H2S, e.g., 5 mg/L in 5 mgd)
|Oxidizer||Practical Weight||Typical||Requirement||Effective Cost|
|Ratio (as 100%)||Unit Cost||(per day)||($ per day)|
Chlorine (1-ton cylinders)
6 - 8 : 1
0.6 - 0.8 Tons
$150 - $180
Sodium Hypochlorite (12.5%, 1.1 lbs/gal)
6 - 8 : 1
960 - 1,280 Gal
$380 - $510
Hydrogen Peroxide(50%, 5.0 lbs/gal)
1.2 - 1.5 : 1
48 - 60 Gal
$165 - $210
Note: The costs indicated above for hydrogen peroxide include supply of storage and handling equipment, maintenance and technical support.
Industry trend to eliminate gas chlorine and its associated risks.
Various regulatory agencies have been making concerted efforts to increase safety. For example, OSHA's Process Safety Management (PSM) standard, 29 CFR 1910.119, requires that facilities undergo comprehensive preparedness for catastrophic releases of certain toxic materials. This involves an analysis of process hazards, standard operating procedures, employee training, incident investigations, emergency planning and response, and periodic compliance audits. As a toxic compressed gas, chlorine is subject to these regulations if stored in quantities > 1,500 lbs. This includes the standard industry packaging unit - one-ton cylinders. Continuing the use of chlorine gas becomes less attractive, especially when the costs for POTWs to comply with just this one law run in the hundreds of thousands of dollars. As a result, treatment facilities turn to non PSM-listed materials as alternatives in an effort to keep costs down.
The trend over the past 15+ years at major municipalities is to replace chlorine with hydrogen peroxide for odor control at treatment plants. Hydrogen peroxide is a proven cost effective method for controlling sulfide problems at headworks and primary treatment operations within the plant. It is more cost effective than sodium hypochlorite and has the added benefit of eliminating the safety, health and regulatory concerns associated with gas chlorine.
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