Entry for December 17, 2007

A number of clients that call us regarding ozone water treatment systems often don’t know how much ozone they will need.  They know the amount of water they treat and the treatment objective, but they don’t know how much ozone will be required to meet that treatment objective.

This is not much of a surprise since it is difficult to predict the required ozone dosage.  In some applications well informed estimates can be made based on theoretical requires to oxidize a specific species, such as Fe or Mn.  Even in these applications, other elements in the water might also exert a demand on the ozone making the estimates based on the Fe or Mn concentrations low.

A thorough water analysis can often provide most of the information for making a good estimate for the size of the ozone system.   In a smaller system with well understood compounds, it may be possible to use the water analysis with a margin of safety to eliminate the need for pilot testing.  It is certainly possible to use such information to provide a rough estimate of the cost of an ozone system so a client can determine if ozone is likely to be a viable solution for their application.

In many cases, the amount of ozone required for treating certain compounds and the full range of contaminants that might consume the ozone are not known.  So while an educated guess of the system size and cost might be possible, the actual design and a firm cost estimate would not be.  In order to achieve this type of information, laboratory and pilot studies are needed.

The testing can be done on site or off site.  Testing off site assumes that the nature of the materials in the water do no change with time and can be preserved in shipping.  This can be a problem if BOD reduction is a goal of the ozonation process.  BOD levels can decrease with time, the presence of naturally occurring bacteria and oxygen.  If samples can be kept cold and tested within 24 hours, it might be possible to do the testing off site; otherwise doing the testing at the ultimate place of use would make more sense.

The laboratory or pilot testing equipment should be such that the results can be scaled to full size.  Fortunately, typical ozone water treatment equipment is scalable.  The vendors of the various components offer different sized pieces of equipment with technical data that allows for this scale up.  The testing facility has to be able to estimate the amount of ozone produced, the amount of ozone transferred to the water and how much of the ozone has been consumed.  In the case of the ozone production, either the testing facility must have monitors to measure the concentration of the ozone gas, or have ozone generators that have been well characterized using such devices with accurate and reproducible production curves.

Normally there is a specific objective for the ozonation process, for example color removal.  Colorimeters can measure when the ozonation process has achieved its objective.  If we know how much ozone we applied, from production curves or ozone concentration monitors/flow meters, we can then design a larger system to achieve the color removal.  Color removal, COD reduction, Fe/Mn/H2S removal, destruction of specific organic compounds, e.g. phenol, are typical applications for ozone.  A curve can be plotted to show the treatment objective versus the applied dose of ozone, allowing a full scale system to be designed.  In disinfection applications, it is important to know the ozone residual over a period of time (CT) in order to estimate the degree of pathogen inactivation.  In these circumstances one needs a dissolved ozone monitor or wet chemical method for measuring ozone.

If you want to know how efficiently the applied dose is being used, which would be useful for optimizing the design, it is necessary to be able to measure the ozone in the off gas from the process.  It is extremely rare or 100% of the ozone to dissolve into the water.  So measuring the ozone in the off gas will allow you to know the ozone transfer efficiency and the effectiveness of the ozone dissolving equipment.  For larger applications this efficiency can be very important.

Spartan Environmental Technologies can provide on-site or off-site testing for a number of applications.  This information will allow for detailed designs and firm price estimates.   If you are just trying to get a feel for whether ozone might be an economical process for your application, we can also provide rough estimates based on a water analysis. 

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Entry for December 10, 2007

In this posting we will discuss the use of ozone for residential wastewater disinfection.  While not practiced extensively in the US versus chlorination and UV, ozone is used more often for wastewater treatment in Europe.  The advantages of ozone include: it is more effective than chlorine in destroying viruses and bacteria.  The ozonation process utilizes a short contact time (approximately 10 to 30 minutes).   There are no harmful residuals that need to be removed after ozonation because ozone decomposes rapidly and, after ozonation, there is no regrowth of microorganisms, except for those protected by the particulates in the wastewater stream.  Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling.  Ozonation elevates the dissolved oxygen (DO) concentration of the effluent (the increase in DO can eliminate the need for reaeration and also raise the level of DO in the receiving stream).

The key process control parameters are dose, mixing, and contact time. An ozone disinfection system strives for the maximum solubility of ozone in wastewater, as disinfection depends on the transfer of ozone to the wastewater. The amount of ozone that will dissolve in wastewater at a constant temperature is a function of the partial pressure of the gaseous ozone above the water or in the gas feed stream. It is critical that all ozone disinfection systems be pilot tested and calibrated prior to installation to ensure that they will meet discharge permit requirements for their particular sites.

Ozone generation uses electrical power. Attention should be given to the system to ensure that power is optimized for controlled disinfection performance.  The operator must on a regular basis monitor the appropriate subunits to ensure that they are not overheated.  Like oxygen, ozone has limited solubility and decomposes more rapidly in water than in air. This factor, along with ozone’s reactivity, requires that the ozone contactor be well covered and that the ozone diffuses into the wastewater as effectively as possible.

Ozone in gaseous form is explosive once it reaches a concentration of 240 g/m3. Since most ozonation systems never exceed a gaseous ozone concentration of 50 to 200 g/m3, this is not a problem. However, ozone in gaseous form will remain hazardous for a significant amount of time; thus, caution is needed when operating the ozone gas systems.

It is important that the ozone generator, distribution, contacting, off-gas, and ozone destructor inlet piping be purged before opening the various systems or subsystems.

Key O&M parameters include:  Clean feed gas with a dew point of -60° C (-76° F), or lower, must be delivered to the ozone generator. If the supply gas is moist, the reaction of the ozone and the moisture will yield a very corrosive condensate on the inside of the ozonator.  The output of the generator could be lowered by the formation of nitrogen oxides (such as nitric acid). Maintain the required flow of generator coolant (air, water, or other liquid).  Lubricate the compressor or blower in accordance with the manufacturer’s specifications. Ensure that all compressor sealing gaskets are in good condition. Operate the ozone generator within its design parameters. Regularly inspect and clean the ozonator, air supply, and dielectric assemblies, and monitor the temperature of the ozone generator. Monitor the ozone gas feed and distribution system to ensure that the necessary volume comes into sufficient contact with the wastewater.  Maintain ambient levels of ozone below the limits of applicable safety regulations.

The cost of ozone disinfection systems is dependent on the manufacturer, the site, the capacity of the plant, and the characteristics of the wastewater to be disinfected. Normally, costs for wastewater ozonation are based on at least a secondary level of treatment (i.e., average BOD and SS of 30 mg/L or less).  Costs are influenced by many factors that are process- and site specific.  Because the concentration of ozone generated from either air or oxygen is so low, the transfer efficiency to the liquid phase is a critical economic consideration. For this reason, the contact chambers used are usually very deep and covered.  The annual operating costs for ozone disinfection include power consumption, chemicals and supplies, miscellaneous equipment repairs, and staffing requirements.

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