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Leachate Treatment

Hydrogen Peroxide Dosing

Process Type: Chemical
Description:

Hydrogen peroxide (H2O2) is a strong oxidising agent generally supplied as a 35% w/v solution.

Use of hydrogen peroxide has found many applications to oxidise contaminants in industrial wastewaters. In the presence of a catalyst, such as iron, hydrogen peroxide (H 2 O 2 ) generates hydroxyl radicals (*OH), which can react with reduced compounds and specific organics.

For leachate treatment, peroxide treatment systems have ranged from very simple drip feed dosing into open leachate lagoons, through pumped dosing into the inlet of large recirculation pumps, to fully engineered dosing systems into mixed reactors.

Dosing of hydrogen peroxide has sometimes also been incorporated within simple methane stripping systems for leachate (see earlier), in order to meet discharge consents for entry of leachate into the public sewer.

Hydrogen peroxide and potassium permanganate have also been used successfully to treat odorous leachates for short periods by turning the leachates aerobic and reducing the potential to cause odour.

In leachate treatment, hydrogen peroxide oxidation has been applied principally to oxidise sulphide, although experience from other industries has shown that many other contaminants which might be found in leachates can also potentially be treated (e.g phenols, sulphite, cyanide, formaldehyde, etc).


For oxidation of sulphide, reactions depend on pH-value as below:


(a) acidic or neutral pH

H2O2 + H2S? 2H2O + S

Reaction time 15-45 minutes (much quicker if catalysed by Fe
2+)


(b) basic pH

4H2O2 + S2-?SO4 2- + 4H2O

Reaction time 15 minutes

Sulphide levels have been successfully managed at between 10 20 UK landfill sites, either to control odours, or to comply with limits for discharges of leachates into sewers. Under the optimum pH-value conditions of neutral or slightly acid, the reaction of peroxide and sulphide is relatively specific, and chemical requirements of about 25 percent greater than those predicted in equation (a) have generally proved to be appropriate, with a reaction time of about 30 minutes. Laboratory trials may be valuable in optimising chemical dosing rates.

Hydrogen Peroxide treatment

1. Hydrogen peroxide has several key advantages over alternative chemical oxidising agents for leachate treatment applications. It does not produce toxic chlorinated by-products, nor any increase in AOX, as does chlorine and hypochlorite, nor does it increase salinity.

2. Hydrogen peroxide can provide a temporary buffer against septicity, in the form of dissolved oxygen, because it readily decomposes to water and oxygen within the environment.

Chemical precipitation of metals

1. Lime or other chemicals used as part of the process must be selected on the basis of a high purity to avoid introducing other potential contaminants into the process, or reducing the reactivity of the reagents.

2. Equipment used to prepare and dose slurries is critical to operation of the process and must be subject to a preventative maintenance programme.

3. Certain chemicals e.g. lime are susceptible to the effects of moisture and must be stored in dry conditions.

4. Treated water will be of much higher pH-value than the feed water, and may require addition of acids to reduce pH-values to suitable levels for discharge or subsequent treatment.

5. The precipitated metals will be settled out of the water stream, and will be contained within the waste sludges generated by the process, which will also exhibit high pH values. Handling and disposal of waste sludges must be appropriate to the nature and hazard of the metals present. These sludges may be designated as a hazardous waste.

Precipitation /coagulation/flocculation

The optimum pH-value and coagulant dosing should be established by detailed laboratory testing prior to operation.

Advantages:
Disadvantages:

Chemical coagulants, flocculants, and pH-control chemicals may be hazardous and require appropriate precautions in use and storage. Cationic and anionic flocculants can be very toxic to fish and their storage and use should ensure appropriate containment and dosing.

The main risk to the process is lack of appropriate control, resulting in failure to meet treatment objectives. A high degree of process control should be maintained at all times.

Where best used:

Where the leachate is smelly.

Costs comments:

Not costs available.

Sustainability comments:

Principle concerns over Hydrogen Peroxide relate to storage and handling issues and ensuring that in the event of a spillage adequate controls are in place (spill kits, bunding and training) to protect sensitive environmental receptors. Hydrogen Peroxide is a strong oxidising agent and as such must not be allowed to
come into contact with incompatible materials.

Energy usage comments:

Low: if all very low-key.

Chemical usage/by-product production:

Significant quantities are sent landfill in most countries.


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