Entry for April 22, 2008

In the last posting we discussed the use of oxidation in wastewater treatment.  We noted that very high levels of organic in wastewater are likely to make oxidation technologies less economical than other techniques, especially biological.  In this post, we will discuss the types of applications where oxidation may be the preferred solution.

In general oxidation techniques are more compatible with applications that have lower levels of oxidizable materials than higher ones.  Typically this means in the hundreds of ppm (mg/l) versus thousands of ppm.  Second, the contaminants are typically dissolved in water and cannot be filtered.  Thirdly, they are either difficult to absorb on carbon or are biorefractory, i.e. bacteria cannot digest them readily.Other factors that would influence a choice for oxidation include:

  • Limited Floor Space:  An ozone generator for example, can produce a large amount of oxidant in a small space.  Biological treatment systems normally require much more space than an equivalent oxidation system, especially where the compounds are bio refractory. 
  • Carbon Regeneration Cost:  Carbon regeneration costs include the cost of renewing the carbon as well as filling and emptying the carbon beds.
  • Hauling and Offsite Treatment Costs: In some cases, the wastewater can be hauled off site to a treatment facility.  Here the costs include the transport cost for the water as well as the treatment costs. 
  • Purchase and Storage of Chemicals:  Certain oxidation techniques such as ozone, UV and UV/ozone, do not require an purchases of chemicals.  Any oxidants that are used can be generated on site. 
  • Intermittent Generation of Wastewater: Biological systems are efficient when operated continuously.  Oxidation systems can be turned on and off readily.

A few examples of wastewater treatment employing chemical oxidation include: 

  • Color Removal.  Ozone has been shown to be very effective at removing even high levels of color from water in both drinking water applications and textile processing. 
  • Acetone.  Acetone is the oxidation by product of isopropyl alcohol.  It is not readily absorbed on carbon, but can be mineralized (reduced to CO2) using advanced oxidation techniques.
  • CN/Phenol. These compounds are sometimes found together in the foundry industry.  While phenol could be biologically treated with proper technique, CN cannot.  On the other hand both can be oxidized readily by ozone. 

Selecting the proper treatment technique requires understanding the site constraints, the economics for each treatment process, and the final treatment objective.  Oxidation can be an effective option for relatively low levels of difficult to treat organic and inorganic materials.  Spartan Environmental Technologies offers a broad range of oxidation technologies.  Contact us with your wastewater problem to see if oxidation might be a solution.

 

 

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Entry for April 9, 2008

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. 

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