Entry for November 26, 2007

Previously we had discussed the removal of color from drinking water.  We will now discuss the removal of color from textile wastewaters.

The textile industry is familiar with ozone and comfortable with its use.  Ozone has been used for successfully for removal of color from textile waste water streams.  Ozone has also been used in the dyeing process itself in a step called stripping.  In the stripping process some of the color is removed from the textile and the fabric is dyed again.  This process is used to even out the color on the textile.  Depending on the effect desired this can be done more than once. 

In wastewater treatment, ozone is often used in conjunction with biological treatment systems such as activated sludge.  Organic dyes are mostly refractory due to their large molecular size and they can be poorly removed by adsorption on activated sludge.  In some cases ozone has been used before the biological process, but mainly after biological treatment.  If the wastewater is hardly biodegradable or toxic to activated sludge pretreatment is an option. 

Ozone can be used prior to a biological process[1] since it has a tendency to convert organic molecules into smaller more biodegradable species.  This can enhance the efficiency of the biological process.  In addition, ozone treatment of wastewater increases the oxygen content of the water (unconverted oxygen and ozone that decomposes back to oxygen that was mixed with the water) which results in improvement in aerobic processes.   While this benefit is well known in the literature it is difficult to practically apply since the amount of improvement is difficult to predict and pilot studies involving ozone and biological processes are difficult to carry out.  In textile wastewater processes, a 20-30% improvement in the action of the biological system has been observed.

The effect of ozone on improving biodegradability and reducing toxicity is worth noting in terms of the effect of the treated water on the receiving stream.  Where the treated water is tested for toxicity, the impact of the treatment process on this parameter must be considered.  Destroying one organic molecule, but creating more toxic ones in a treatment process has been observed, for example the ozonation of MTBE without any additional agents or treatment processes can result in a more toxic wastewater.  Another consideration is the presence of surfactants and the need to remove these compounds from the water.  In some locales surfactant concentrations are tightly controlled and must be kept under 1 ppm.  This creates an additional demand for oxidant.  Some textile wastewaters contain both color and surfactants.

Ozone is effective in removing the color from all dyes used in textile processing.  The amount of ozone can vary depending on a number of factors: how much color was removed in the biological process, the type of dye used, where ozone is applied in the process, etc.  Knowing the proper amount of ozone required to meet the color removal objective for the receiving water body is critical to the economics of the ozone system.  In general it is not easy to predict the amount of ozone required, so in virtually all cases where specific previous experience is not available, pilot testing is employed.

Tosik[2] has shown that about 1 mg ozone/mg dye is required to achieve 95% color removal, although this ratio varies by dye type.  The ratio increases to about 1.5 for 100% removal.  Reaction times were on the order of 10 minutes.  In the textile industry a typical dosage might be 15 mg/l post biological treatment, but the levels could easily reach 25 mg/l.  It is important to note that the ozone dose only needs to make the dye compound uncolored and not necessarily completely mineralize the material.

Example of Ozone Color Removal

We recently looked at a situation where a textile plant in the US wanted to reuse their wastewater.  Color levels reached 150 as measured by the ADMI method.  We did not reach the pilot plant stage for this project, but we did a cost analysis on the basis of dosing the water with 25 mg/l of ozone.  The flow ranged from 1 to 1.5 MGD.  The required amount of ozone would be ont eh order of 300 lbs/day.

For the example discussed above the, following system costs have been estimated at $467,000 installed.

[1] Removal of Dissolved Organic and color from dying Wastewater by Pre-Ozonation and Subsequent Biological Treatment, Takahashi, Nobuyuki; Kumagai, Tomoyo; Ozone: Science and Engineering, 28: 199-205

[2] Dyes Color Removal by Ozone and Hydrogen Peroxide: Some Aspects and Problems, R. Tosik, Ozone: Science and Engineering 27: 265-272


 

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Entry for November 16, 2007

We are continuing our discussion of ozone applications in the treatment of drinking water.  One of these applications is the enhancement coagulation and flocculation which often results in improved filtration and improved turbidity.   Rakness has surveyed water treatment plants that use ozone and has found that many of them have observed an improvement in coagulation and filtration.  This has been observed as a reduction in chemical use in the coagulation flocculation process.   Data from water treatment plants show that pre-ozonation (ozonation of the raw water) was more effective than pre-chlorination to reduce filter effluent turbidities.  Besides drinking water plants, operators of aquariums have also observed enhanced filtration with the use of ozone.

Several suggestions have been proposed for the cause of the improved coagulation, but no definitive answer has been accepted.  Ozone forms organic compounds with functional groups such as carboxylic acid.  These compounds may form complexes with aluminum oxide.   Carboxylic functional groups can also complex calcium which may improve adsorption of organics to the metal oxide surfaces.  Ozonation of organic compounds may create organic polymers that may enhance coagulation.  Ozone can breakdown Fe and Mn which are complexed with organic compounds.  The release of the oxidized metals may create a source of natural coagulant.

For a large number of WTP improved particulate removal for filtration has been observed after the installation of ozone generators in terms of lower turbidity, lower particle counts in the filtered water and increased filter runs.  In pre-ozonation applications, lower dosages of coagulant have also been observed.  This reduction in chemical use can be in the range of 20 to 50%.

Enhanced filtration can be an important economic and performance benefit observed with the use of ozone generators in the treatment of drinking water and other water treatment applications.

Kerwin L. Rakness, “Ozone in Drinking Water Treatment: Process Design, Operation and Optimization”.  AWWA 2005

Gurol, M.D. and M. Pidatella. 1983. “A Study of Ozone-Induced Coagulation.” Conferenceproceedings, ASCE Environmental Engineering Division Specialty Conference. Allen Medine and Michael Anderson (editors), Boulder, CO.

Reckhow, D.A., J.K. Edzwald, and J.E. Tobiason. 1993. “Ozone as an Aid to Coagulation and Filtration.” AWWARF and AWWA, Denver, CO.

Reckhow, D.A., P.C.Singer, and R.R. Trussell. 1986. Ozone as a coagulant aid. Seminar proceedings, Ozonation, Recent Advances and Research Needs, AWWA Annual Conference,Denver, CO.

Stolarik, G. F., and J.D. Christie. 1997. “A Decade of Ozonation in Los Angeles.” Conference proceedings, IOA Pan American Group Conference, Lake Tahoe, NV.

Tobiason, J.E., J.K. Edzwald, O.D. Schneider, M.B. Fox, and H.J. Dunn. 1992. “Pilot Study of the Effects of Ozone and Peroxone on In-Line Direct Filtration.” JAWWA. 84(12):72-84.

Hiltebrand, D.J., A.F. Hess, P.B. Galant, and C.R. O’Melia. 1986. “Impact of Chlorine Dioxide and Ozone Preoxidation on Conventional Treatment and DirectFiltration Treatment Processes.” Conference proceedings, AWWA Seminar on Ozonation: Recent Advances and Research Needs, Denver, CO.

Prendiville, D.A. 1986. “Ozonation at the 900 cfs Los Angeles Water Purification Plant.” Ozone Sci. Engrg. 8:77.

Farvardin, M.R. and A.G. Collins. 1990. “Mechanism(s) of Ozone Induced Coagulation of Organic Colloids.” Conference proceedings, AWWA Annual Conference,Cincinnati, OH.

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Entry for November 11, 2007

Continuing the postings related to the application of ozone in water treatment, we will cover the use of ozone for disinfection.   In general, disinfection with ozone, and other oxidizing disinfectants, is measured by CT.  This is a product of the disinfection residual by the time of exposure to the residual.  If one imagines a curve of disinfection residual versus time, the area under the curve would represent CT.  For ozone people refer to the law of fours, 0.4 mg/l for 4 minutes will reduce viruses by 4 orders of magnitude, i.e. a 99.99% reduction.

In drinking water treatment the EPA has established regulations for calculating CT and the CT value associated with the inactivation of specific pathogens at a particular temperature.  Spartan has a presentation which discusses these regulations (EPA CT Regulations).   Depending on the particular situation of a water treatment plant, a certain number of disinfection credits are required for different pathogens. 

The EPA is promoting multiple pathogen barriers to accumulate the total number of credits.  Thus one combines the benefits of filtration, ozone and say chlorine to reach the degree of inactivation that is needed.  Besides the obvious benefit of wearing “belts and suspenders” for such an important application, synergies can develop between the methods that actually enhances disinfection.  In addition, ozone provides other benefits, which we have discussed recently.  Thus adding ozone to provide some additional disinfection credits while gaining benefits for color removal or taste and odor is may be a good idea.

Ozone in particular works by damaging the membranes of micro organisms.  For a more detailed discussion of how ozone disinfects follows this link.  Compared to other oxidizing disinfectants, ozone works very quickly.  For example, for giardia lamblia, ozone requires a CT that is 1,000 times less than chlorine.

In order to use ozone effectively, one needs to know two things about the system, the hydraulic detention time (HDT), the amount of time the water is exposed to the ozone, and the ozone residual.  In a multi chamber contact vessel, ozone residual may be measured in several of the chambers, especially at the exit.  The HDT is more difficult to measure since the flow through a contact vessel or tank and have some short circuiting.  Thus one cannot simply take the volume for the tank and divide by the flow rate to know the HDT. 

Dissolved ozone monitors are available for determining the ozone concentration accurately.  The HDT is often measured by tracer studies.  A tracer chemical is added to the inlet of the contact vessel and the tracer measured at the outlet of the tank.  In a multi chamber contact vessel,  the actual HDT verses the apparent HDT typically is 60-70%. 

Ozone is a multi functional water treatment chemical that is an extremely effective disinfectant either alone or in combination with other compounds and methods.  Spartan supplies ozone disinfection systems for water treatment.  

 

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Entry for November 5, 2007

Continuing our series on applications for ozone in water treatment, we will cover the topic of taste and odor control.  Taste and odors found in water can be from a variety of inorganic or organic sources.  Iron and hydrogen sulfide are two common inorganic compounds known to produce odors in water.  Both are easily treated via ozone oxidation.

Organic compounds found in water are often caused by micro organisms and can be  found in nanograms (ng)/l concentrations.  These organically derived tastes and odor tend to be seasonal with most of the issues arising in summer months.  Geosim and MIB (taste and odor thresholds of 6 and 10 ng/l) are important examples of these type of compounds.  There are other types of odor causing compounds such as those from industrial pollution and by-products from the water treatment process itself.  The latter is most notably caused by the reaction of chlorine or chloramines with materials found in the water.   The reaction of chlorine with phenol is a specific example.

Treatment form taste and odor can be carried out by both oxidative and non oxidative methods.  Air stripping to remove dissolved gases, powdered activated carbon and granulated activated carbon are several examples.  The effectiveness of these methods depends on the specific organic compounds causing the tastes and odors.  Various oxidants have been used with varying success for taste and odor control.  Simple aeration may be effective for iron and hydrogen sulfide depending on pH and, in the case of iron, whether the iron is complexed with organic compounds.  Chlorine is effective in some situation and in others may actually creates odors.   Potassium permanganate at high concentrations is effective against some compounds, but not very effective against geosim and MIB.

Ozone used by itself or with peroxide/UV has been found to be effective in removing many taste and odor compounds including geosim and MIB (1,2,3).   In some cases the same levels of ozone used for disinfection are also effective at removing taste and odor. Studies on the use of ozone and ozone with peroxide found that with ozone alone dosages of 4 mg/l were necessary to treat pilot plant effluent spiked with geosim and MIB.  When peroxide was added, the ozone dosage was dropped to 2 mg/l (4)

Peroxide is added with ozone when the concentration of Geosim and MIB are particularly high, on the order of hundreds of ng/l.  Adding ozone alone in the first chamber of an ozone contactor can permit CT credits for disinfection to be earned.  Ozone and peroxide can then added in later chambers of the contactor to create condition favorable for hydroxyl radical formation for taste and odor control.  This allows two treatment objectives to be achieved in the same unit operation. The use of ozone to affect two or more treatment objective is a good reason to employ this powerful oxidant and disinfectant.

The effectiveness of ozone is also enhanced if it is followed by biological filtration. Ozone partially oxidizes the geosim and MIB and the biological filters continue the degradation of these compounds.  The combination can result in 90-100% removal of these compounds (5). The explanation for this effect may be that ozone is very effective at taking non biodegradable compounds and making them biodegradable. This may be what is occurring in systems that employ both ozone and biological filters, normally the biological filters would not be able to treat the geosim or MIB, but after exposure to ozone and the presence of the high dissolved oxygen levels the filters become effective.

Spartan can provide complete ozone water treatment systems using ozone and advanced oxidation processes including ozone/peroxide for treatment of taste and odors in water.  Please contact us for further information by phone, fax or e-mail using the contact information at the bottom of the page.

1. Terashima, K. 1988 Reduction of Musty Odors Substances in Drinking Water-A Pilot Plant Study. Wtr. Sci. Tech. 20:8/9:275.

2. Daniel, P.A. and Meyerhofer, P.F. 1989. Ozonation of Taste and Odor Causing Compounds. 1989. Proc. 9th Ozone World Congress, vol. 1 edited by L.J. Bollyky. IOA, NY. pp. 239-242

3. Duguet, J.P. et al. 1989. Geosim and 2-Methylisoborneol Removal Using Ozone and Ozone/Hydrogen Peroxide Coupling. Proc. 9th Ozone World Congress, edited by L.J. Bollyky. IOA, NY. pp. 709-719

4. Gramith, J.T., Ferguson, D.W., McGuire, M.J., and Tate, C.H., Overview of the Metropolitans Ozone-Peroxide Demonstration Project, by the Metropolitan Water District of Southern California with Headquarters in Los Angeles, CA and James M. Montgomery Engineers of Pasadena, CA, presented at the AWWA Ann. Conference, Cincinnati, OH, June 1990.

5. Nerenberg, R., Rittmann, B.E., Soucie, W.J. 2000. Ozone/Biofiltration for Removing MIB and Geosim. Jour. AWWA, 92:12:85

 

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