In In this his posting we will discuss the when oxidation might be an appropriate technique for wastewater treatment versus other techniques such as stripping, filtration, absorption or biological treatment. The most important consideration is whether the species of concern can be oxidized. Oxidation is mainly applied to organic compounds although certain inorganic compounds can also be oxidized such as hydrogen sulfide.
There are several types of applications for oxidation:
- To meet discharge requirements for a specific compound of interest where it simply has to be converted into another compound. As an example, phenol is often mentioned in discharge permits and must be reduced to a certain value, say less than 1 ppm. In some cases, the specific compound formed is not considered in the permit.
- Lowering the overall organic loading by some measure such as BOD, COD or TOC. This is probably the most common application for oxidation reactions.
- Reducing the toxicity of the compounds in the wastewater. Here the water is tested to measure the toxicity after treatment either through analytical tests or by exposing sensitive organisms to the water and seeing if they are harmed.
- To make bio refractory compounds bio degradable. In these situations the increase in the BOD reading or respirometry may be used to show that the compound has been converted to a compound or compounds that are biodegradable. This can be a pretreatment step prior to a biological treatment process.
Oxidation processes like biological treatment typically treat the full wastewater stream allowing it to be discharged. Processes such as absorption, filtration and stripping require further treatment for at least a portion of the waste stream such as disposal or regeneration. In the case of stripping, a water pollutant is transferred to an air pollutant.
If oxidation is an option, the treatment objective is understood and a method of measuring the end point is developed, it is necessary to determine the amount of oxidant required to reach this end point. The amount of oxidant is directly related to the cost of the treatment. The oxidant must be purchased (sodium hypochlorite or hydrogen peroxide) or capital must be invested to produce it on-site (e.g. ozone made via an ozone generator). Economics are often the most important factor in selecting a treatment process since multiple options are usually available.
The amount of oxidant required is proportional to the amount of organic material that must be removed. The actual amount of oxidant required should be arrived at through pilot studies, but for ozone, as an example, 2-5 mg of ozone are required to oxidize 1 mg of COD. The range reflects the treatment objective, simple conversion to a less toxic form, or complete mineralization to CO2. As we have noted in previous postings, simple oxidation processes may not be sufficient to remove some compounds from water. Advanced oxidation processes which combine oxidants or pair them with UV radiation may be required.
Let’s consider the example of a wastewater stream that contains 2,000 ppm of COD with the goal of reducing this to less than 600 ppm. The average wastewater flow is 200 gpm. This means that we need to remove 1,400 ppm of COD. Since COD is the chemical oxygen demand, it represents the amount of oxygen required to form CO2 from the organic in the water. As a first approximation it is the minimal amount of oxidant required, oxygen in fact is not a very strong oxidant. In the case shown here the amount of oxidant required would be nearly 3,400 lb/day. As noted above, some multiple of this number would be required. If one were using ozone or some other on-site treatment system the capital costs would be at least $1,000,000 dollars with significant operating costs.
If the wastewater can be treated at an existing publicly owned water treatment plant (POTW) the costs will probably be substantially less. The next choice would probably be an on-site biological system.
If these options were not available, for example the wastewater is not biodegradable, and then oxidation might be an option. In generally, oxidation will not be an economical choice where required COD removal exceed 500 lbs/day for biodegradable materials.In subsequent postings we will talk about some other areas where oxidation might be the optimal choice for treatment.