Specific information about chemicals used for sulfide control

Please select a chemical on the left and type of information on the right

**Unfortunately, currently, there is only information available for Oxygen – provided below.

Oxygen	Mechanism of action
Nitrate	Actions
Ferric Chloride	Source
Magnesium Hydroxide	Dosing arrangement and typical dosing rates
Free Nitrous Acid	Cost considerations
Caustic Biomaterials	Impacts on WWTP
	Online control
	Major limitations
	Case studies

Oxygen : Mechanism of Action

Oxygen prevents the accumulation of H₂S by the following two mechanisms.

Oxidation of hydrogen sulfide

The oxidation of sulfide occurs both chemically and biologically. Chemical sulfide oxidation mainly occurs in bulk phase, while the biological sulfide oxidation takes place both in bulk water and in the aerobic portion of the sewer biofilm.

Maintaining aerobic condition in bulk liquid as well as a portion of biofilm

The growth of sulfate reducing bacteria (SRB) is greatly affected by the presence of oxygen
Due to its low concentration in the bulk, penetration of oxygen in biofilm is generally limited, thereby giving rise to aerobic layer in the outer part and anaerobic layer in the inner part of the sewer biofilm ( see figure). Sulfide production can thus take place in anaerobic biofilm layers despite the presence of oxygen in the bulk phase

Sulfide produced in the anaerobic layer of the biofilm is oxidized as it diffuses out through the aerobic biofilm layer

Results of oxygen dosing

Dosing of oxygen results into the followings:

Oxidation of hydrogen sulfide to intermediate products and the oxidation of the intermediate products to sulfate.
Creation of an aerobic layer on outer surface of the biofilm preventing sulfide generation in the layer. Sulfide generated in the inner anaerobic layer will diffuse out to the aerobic layer, where it will be oxidized thereby preventing its accumulation in the bulk water despite continuous production in inner anaerobic layers.
The presence of oxygen supports the growth of heterotrophic organisms in the anaerobic layer of the biofilm, thereby oxidizing the organic matter present in the wastewater. The oxidation of both sulfide and organic matter occurs simultaneously.

Sources of Oxygen

Followings are the sources of oxygen in sewers:

Oxygen: Dosing arrangement and typical dosing rates

When oxygen is dosed to sewer, it is utilized for both sulfide and COD oxidation. Oxygen dosing requirement can be estimated as follows:

It is assumed that the end product of sulfide oxidation is sulfate and the oxygen uptake rate of sewer biofilm (OUR) is constant throughout the sewer length.

With the injection of pure oxygen, maximum level of DO that could be achieved is only 30-40 mg/L (depending upon the pressure) as not all the oxygen supplied gets dissolved in the wastewater. When the oxygen is dosed at the wet-well of a rising main, then the DO will be depleted in few hundred meters of the pipe, and there will be no protection against the sulfide produced in the remaining anaerobic portion of the sewer pipe. In order to avoid this, the point of injection (POI) should be “reasonably” close to the points where sulfide control is required (Point of Control – POC). An injection site should be selected such that:

1. there would be an adequate hydraulic retention time (HRT) between POI and the POC to enable the full oxidation of any sulfide present at the POI before it reaches the POC, and that
2. aerobic conditions are maintained between the two points with minimum requirement of oxygen. There should be no high points between POI and POC, as otherwise air pockets rich in oxygen and hydrogen sulfide would format such locations causing corrosion of pipes.Typical oxygen dosing rate: 15.9-91.5 kg O₂/ML of wastewater (exact dosing rate depends upon the pressure, type of injection and oxygen transfer efficiency).

Further information can be obtained as follows:

Dosing arrangements
Final Report – ARC Linkage Project on Sewer Biotransformation

Typical dosing rate
Paper: Chemical dosing for sulfide control in Australia: An industry survey by Ganigue et al. (2011)

Cost of oxygen injection

Oxygen injection requires storage facility for the oxygen, and proper dosing facilities depending upon the type of the dosing used. The dosing equipments can be hired.
Estimated cost of oxygen injection is $12.8 – $74.0/ML (the cost depends upon the oxygen dosing rate, pressure at the injection point, and oxygen transfer efficiency).

Further information can be obtained as follows:
Paper: Chemical dosing for sulfide control in Australia: An industry survey by Ganigue et al. (2011)

Impacts on WWTP

The injected oxygen promotes heterotrophic activity in the wastewater and the biofilm, thereby oxidising a significant amount of organic matter in the wastewater, which results in reduced levels of volatile fatty acids (VFA) in the feed to a WWTP. This will significantly affect the biological nutrient removal in the WWTP, the extent of such impact depending upon the rate of oxygen dosing and the dosing location.

The addition of a readily available carbon source (for example, methanol) may be required to improve the nutrient removal performance to the same level as in the case without oxygen injection.

Further information can be obtained as follows:
Paper: Integrated modelling of sewer system and wastewater treatment plant for investigating the impacts of chemical dosing in sewers by Sharma et al. (2012)

Online dosing control of oxygen

A link to a Fact Sheet with the following details will be provided

Methodology
Key requirements
Benefits
Implementation

Oxygen: Major limitations

The oxygen transfer to sewage is limited due to low solubility of oxygen. As a result, it would be almost impossible to keep the entire sewer pipe aerobic, especially in the case of long pipes.
The oxygen injection is effective in controlling sulfide production during periods when oxygen is present. In addition to preventing sulfide from formation, any sulfide present would be oxidized. However, oxygen injection has no long-lasting inhibitory or effect on the SRB activities.
Oxygen injection enhances SRB activities in the downstream sections of sewers. This is due to the re-generation of sulfate (from sulfide) at sites where oxygen is injected so that more downstream sewer biofilms are exposed to sulfate, which enhances the growth of SRB.
The available oxygen would be quickly consumed in sewer and during long quiescent periods, the anaerobic conditions favourable for sulfide production would be quickly established in the system, thereby negating any positive effect achieved by oxygen injection.
High VFA consumption is another main concern as this would create a limitation of carbon source for downstream nutrient removal plant.
Its effectiveness and efficiency are highly sensitive to the injection locations. The selection of the injection sites and the determination of the dosage profiles could be a difficult task, particularly for large, complex networks.

Oxygen: Case studies

Please select a case study below: