ULTRASOUND IN ENVIRONMENTAL PROTECTION AND
WASTE CONTROL
Research into the use of ultrasound in environmental protection has received a considerable amount of attention with the majority of investigations focusing on the harnessing of cavitational effects for the destruction of biological or chemical pollutants in water and the processing of sewage. The field is much broader than this however and a summary of topics is given in the Table.
Control of air-borne contamination agglomeration of smokes and aerosols
defoaming of liquids
Washing of soils Removal of organic and inorganic contamination
Water treatment
biocidal action
• direct mechanical action e.g. cell rupture and the break-up of bacterial clumps
• indirect mechanical action e.g. increased cell permeability to bactericide
• stabilization and dewatering of sludge
Removal of chemical contamination
• direct oxidation of chemical and pesticide residues
• in combination with other techniques e.g. ozonation, uv light
Removal of surface contamination and biofilms
n chemistry, the study of sonochemistry is concerned with understanding the effect of sonic waves and wave properties on chemical systems. The chemical effects of ultrasound do not come from a direct interaction with molecular species. Studies have shown that no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry[1] or sonoluminescence[2]. Instead, sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. This is demonstrated in phenomena such as ultrasound, sonication, sonoluminescence, and sonic cavitation.
The influence of sonic waves traveling through liquids was first reported by Robert Williams Wood (1868-1955) and Alfred Lee Loomis (1887-1975) in 1927, but the article was left mostly unnoticed.[3] Sonochemistry experienced a renaissance in the 1980s with the advent of inexpensive and reliable generators of high-intensity ultrasound.
Upon irradiation with high intensity sound or ultrasound, acoustic cavitation usually occurs. Cavitation – the formation, growth, and implosive collapse of bubbles irradiated with sound— is the impetus for sonochemistry and sonoluminescence.[4] Bubble collapse in liquids produces enormous amounts of energy from the conversion of kinetic energy of the liquid motion into heating the contents of the bubble. The compression of the bubbles during cavitation is more rapid than thermal transport, which generates a short-lived localized hot-spot. Experimental results have shown that these bubbles have temperatures around 5000 K, pressures of roughly 1000 atm, and heating and cooling rates above 1010 K/s.[5][6] These cavitations can create extreme physical and chemical conditions in otherwise cold liquids.
With liquids containing solids, similar phenomena may occur with exposure to ultrasound. Once cavitation occurs near an extended solid surface, cavity collapse is nonsphereical and drives high-speed jets of liquid to the surface[7]. These jets and associated shock waves can damage the now highly heated surface. Liquid-powder suspensions produce high velocity interparticle collisions. These collisions can change the surface morphology, composition, and reactivity.[8]
Three classes of sonochemical reactions exist: homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or solid-liquid systmes, and, overlapping with the aforementioned, sonocatalysis.[9][10][11] Sonoluminescence is typically regarded as a special case of homogeneous sonochemistry.[12][13] The chemical enhancement of reactions by ultrasound has been explored and has beneficial applications in mixed phase synthesis, materials chemistry, and biomedical uses. Because cavitation can only occur in liquids, chemical reactions are not seen in the ultrasonic irradiation of solids or solid-gas systems.
For example, in chemical kinetics, it has been observed that ultrasound can greatly enhance chemical reactivity in a number of systems by as much as a million-fold [14]; effectively acting as a catalyst by exciting the atomic and molecular modes of the system (such as the vibrational, rotational, and translational modes). In addition, in reactions that use solids, ultrasound breaks up the solid pieces from the energy released from the bubbles created by cavitation collapsing through them. This gives the solid reactant a larger surface area for the reaction to proceed over, increasing the observed rate of reaction.
While the application of ultrasound often generates mixtures of products, a paper published in 2007 in the journal Nature described the use of ultrasound to selectively effect a certain cyclobutane ring-opening reaction.[15]
Sonochemistry can be performed by using a bath (usually used for ultrasonic cleaning) or with a high power probe.
Air Cleaning
>
The inhalation of airborne particles is now recognized as a serious public
Using this type of device airborne ultrasound has been used for both the precipitation of airborne powders and defoaming.
Land Remediation
For contaminated soil wastes the currently available options for management and disposal are principally:
• Permanent storage in a secure landfill. This will result in a permanent retained liability by the waste generator.
• Incineration in a permitted waste incinerator. This is costly and entails the risk of atmospheric emissions.
• Soil washing to produce bulk soil with low-level contamination. However the washing process itself will produce a volume of solvent that must be treated before disposal.
For many years ultrasound has been considered as a technology to promote the process of soil washing and if subsequent disposal of the washings was considered at all this was perhaps to be a separate treatment. An integrated system has been developed in Canada (by Sonic Environmental Solutions Inc.) for large scale continuous processing using acoustic frequencies in the audible range that incorporates the clean-up of the washings and recycling of the solvent. The equipment itself affords vibrational amplitudes considerably larger than those available using ultrasound and it has proved to be particularly efficient for the removal and destruction of PCB contaminants in soils. The equipment generates vibrational energy through the use of resonant bending modes in a large cylindrical steel bar. The bar is driven into a cloverleaf type of motion by firing six powerful magnets (three at each end of the bar) in sequence. The bar is supported by air springs so that the ends and the centre are then caused to rotate at a resonance frequency depending on its size.
One such unit, operating at a power of 75kW, drives a bar that is 4.1 metre long and 34 cm in diameter at its resonance frequency of 100 Hz. The bar weighs 3 tonnes and produces an amplitude of vibration at each end of 6 mm. For the washing of soils a mixing chamber is rigidly mounted on each end of the bar and these are used in three process areas: PCB extraction, PCB destruction and solvent recovery. The use of this generator for pilot testing has proved that processing can be achieved at a commercial scale of around 3 to 4 tonnes of soil/hour.
Water Remediation
Removal of biological contamination
Some species of bacteria produce colonies and spores, which agglomerate in spherical clusters (e.g. Bacillus subtilis). The use of a biocide can destroy microorganisms on the surface of such clusters but often leaves the innermost bacteria intact. Flocs of fine particles e.g. clay can entrap bacteria which can also protect them against disinfection [Mir, 1997]. Due to these problems alternative methods of purifying water are being investigated and amongst these the application of ultrasound is proving to be of considerable interest. Ultrasound is able to inactivate bacteria, make them more susceptible to biocides and/or deagglomerate bacterial clusters or flocs depending upon the power and frequency applied through a number of physical, mechanical and chemical effects arising from acoustic cavitation.
Removal of Chemical contamination
The mechanical effects of cavitational collapse together with the production of radical species combine to provide the essential elements for water decontamination. The primary radicals produced during the sonication of water are OH. and H. and the fate of these is quite complex (Scheme 18). The HO. radical is extremely reactive and is capable of oxidising most chemical compounds dissolved in the water. This oxidation is mainly responsible for the degradation of organic pollutants in sonicated aqueous media. The efficient generation of HO. is therefore an important goal in waste treatment.
1. Degradation of dye effluent, J.P.Lorimer, T.J.Mason, M.Plattes, S.S.Phull, and D.J.Walton, Pure and Applied Chemistry, 73, 1957-1968 (2001).
2. Potential uses of ultrasound in the biological decontamination of water, T.J.Mason, E.Joyce, S.S.Phull, and J.P.Lorimer, Ultrasonics Sonochemistry 10, pp 319-324 (2003).
3. Ultrasound in Advanced Oxidation Processes, T.J.Mason and C.Petrier, Chapter 8 in Advanced Oxidation Processes for Water and Wastewater Treatment, pp 185-208, ed S Parsons, IWA Publishing (2004).
4. Application of UV radiation or electrochemistry in conjunction with power ultrasound for the disinfection of water Eadaoin M. Joyce, Timothy J. Mason and John P. Lorimer , Int. J. Environment and Pollution 27, 222-230 (2006)
5. Oxygen-induced concurrent ultrasonic degradation of volatile and non-volatile aromatic compounds Christian Pétrier, Evelyne Combet and T.J.Mason, Ultrasonics Sonochemistry 14, (2007) in press.
Examples of projects
Water purification:
Advanced oxidation methods involving sonochemistry
“Degradation of water pollutants using ultrasound”
Biological decontamination
“The effect of ultrasound in combination with uv radiation and/or electrolysis for the biological decontamination of potable water”
“The effect of sonication at different frequencies on microbial disinfection usinghypochlorite”
“Controlling algae in reservoirs with ultrasound”
“Assessment of hydrodynamic cavitation methods compared with sonochemistry for the decontamination of water”.
Soil remediation
“Sonic and ultrasonic removal of chemical contaminants from soil in the laboratory and on a large scale”
Airborne pollution
“Ultrasound for the removal of dust, suppression of foam”
Surface Cleaning
“Membrane fouling and integrity in the municipal sector: a multi-faceted approach to their amelioration”
“Surface decontamination in the food industry”
MICROBIOLOGY
The effect of ultrasound on biological systems and biotechnological processes depends strongly on frequency, intensity and sonication time.
Low intensity effects (i.e. under conditions which occur below the cavitation threshold) are the result of microstreaming and acoustic streaming. At these intensities, where no cavitation damage will occur, the beneficial effects are:
• activation of enzymes in enzymatic reactions
• improvements in microbial reactions (e.g. fermentation)
• improvement of the bioavailability of contaminants in environmental remediation using microorganisms
Higher intensity effects are the result of cavitational damage and may be summarised as follows:
• destruction of cell walls and release of cell components into the surrounding solution (damage to cell components e.g. DNA, proteins is limited if sonication time is short)
• extraction of organic substances from plants
• emulsification of food (see Food section)
• damage of cell walls and cell components at very high intensity
• killing of microorganisms (see Environmental Remediation)
• improvement of the conventional bacterial decontamination (disinfection) of water
• destruction of biological tissue e.g. tumours or kidney stones (see Therapeutic Ultrasound)
1. The use of ultrasound in microbiology - Sonomicrobiology. S.S.Phull and T.J.Mason, Advances in Sonochemistry, Vol 5, ed. T.J.Mason, JAI Press, 175-208 (1999)
2. Potential uses of ultrasound in the biological decontamination of water, Mason, T.J., Joyce, E., Phull, S.S. and Lorimer, J.P., Ultrasonics Sonochemistry 10, pp 319-324 (2003).
3. The effect of sonication on microbial disinfection using hypochlorite, H. Duckhouse, T.J. Mason, S.S. Phull, and J.P. Lorimer, Ultrasonics Sonochemistry 11, 173-176 (2004).
4. A review of research into the uses of low level ultrasound in cancer therapy, Tinghe Yu, Zhibiao Wang and T.J.Mason, Ultrasonics Sonochemistry 11, 95-103 (2004).
Examples of Projects
“The effect of ultrasound and ultraviolet radiation on bacterial suspensions”
“The effect of ultrasound and ultraviolet radiation on gram positive and gram negative bacteria”
“The influence of ultrasound on the uptake of chemotherapeutic agents into cells”
Wastewater treatment is an issue in many countries both developed and underdeveloped. The need to treat efficiently these wastewaters to reduce the levels of contamination on them is always present. Traditionally, wastewaters have been treated using biological processes and physico-chemical processes such as denitrification, phosphorous removal, reverse osmosis, micro and ultra-filtration, chemical precipitation, carbon adsorption, electro-dialysis, and ion-exchange.
Sonochemistry, the use of acoustic waves, can improve wastewater treatment. A variety of methods to treat polluted water using sonochemistry have been used. More specifically, the combination of electrolysis and sonochemistry, that is Sonoelectrochemistry, has proved to be very successful in treating both drinking or polluted water. Traditionally, chlorine is produced by electrolysis of hydrochloric acid on effluents. This process produces around 1% of chlorine, the active disinfectant. But, if ultrasound is coupled with electrolysis then efficiency of chlorine production increases dramatically. This way more wastewater can be treated with same amount of hydrochloric acid electrolyzed.
Removal of phenols from toxic waste also is improved significantly when acoustic waves are used coupled with traditional phenol-removal methods. Normally phenol in removed from industrial effluents by a process electrochemical oxidation. But if a sonochemistry device is coupled with electrical oxidation then efficiency increases by as much as 160% when compared with the electrochemical oxidation alone. Also, sonoelectrochemical destruction of aromatic compounds in water samples has been obtained using very low acoustic waves frequencies.
Another interesting technology that could be used to increase the efficiency of wastewaters treatments is the use of a process called sonoelectrical coagulation. Traditionally, electrocoagulation has been used for destruction of a variety of pollutants (mainly inorganic and organics). Electrocoagulation method of treating wastewaters in which effluents are electrolyzed with special electrodes that release coagulant precursors (aluminum iron ions) into the treated sample. The electrocoagulation process has a variety of issues that prevents to get the full “cleaning” effect. Coupling electrocoagulation with sonochemistry gets rid of many of those obstacles increasing its efficiency.
Recently, it has been demonstrated that heavy metals can be efficiently be recovered from photographic waste effluents using sonoelectrochemical methods. For example, traditional electrochemical removal of mercury from photographic waste effluents could take up to 7 hours to reduce Hg levels to 1 ppm. By using sonochemistry (in addition to electrochemistry)
the processing time can be reduced to 2.2 hours
Sonochemistry in a new technology that can be utilized to treat a variety of industrial effluents including chemical dyes, chemical wastes, mercury contaminated waters, fluorinated waters, mine effluents, etc. Efficiencies of scale can be realized if the technology is incorporated in current wastewater treatment processes.
nvironmental Protection
Research into the use of ultrasound in environmental protection has received a considerable amount of attention with the majority of investigations focusing on the harnessing of cavitational effects for the destruction of biological or chemical pollutants in water and the processing of sewage.
Air cleaning
The inhalation of airborne particles is now recognized as a serious public health concern. Fine particles originate in the emissions associated with carbon-fired power plants, cement factories, chemicals industries, and diesel-powered vehicles have increasingly become the focus of stricter government regulations.
The ideal solution to the problem is to stop these emissions at source but current filters and electrostatic precipitators have problems in coping with the smallest particles.
It has been shown that airborne acoustic energy in the ultrasonic frequency range can be used to precipitate suspended particles (aerosol or smoke).
Water remediation
In the removal of biological contamination ultrasound is able to inactivate bacteria, make them more susceptible to biocides and/or deagglomerate bacterial clusters or flocs depending upon the power and frequency applied through a number of physical, mechanical and chemical effects arising from acoustic cavitation.
On the other hand in the removal of chemical contamination the production of oxidative radicals using ultrasound and the subsequent oxidation and chemical breakdown of many organic pollutants is an important factor in the decontamination of waste water.
Land remediation
Ultrasound has been employed in soil washing to produce bulk soil with low-level contamination and ultrasonic companies have developed equipment that can treat both the contaminated soil and the effluent water waste prior to disposal.
Examples of projects
Water purification
• Degradation of chemical water pollutants using ultrasound
Biological decontamination
• The effect of ultrasound in the biological decontamination of potable water
• Controlling algae in reservoirs with ultrasound
Airborne pollution
• Ultrasound for the removal of dust
• Ultrasonic suppression of foam
Surface Cleaning
• Use of ultrasound to prevent membrane fouling and integrity in the municipal sector
• Surface decontamination in the food industry
Relevant publications
• Ultrasound in Advanced Oxidation Processes, T.J.Mason and C.Petrier, Chapter 8 in Advanced Oxidation Processes for Water and Wastewater Treatment, pp 185-208, ed S Parsons, IWA Publishing (2004).
• Application of UV radiation or electrochemistry in conjunction with power ultrasound for the disinfection of water Eadaoin M. Joyce, Timothy J. Mason and John P. Lorimer, Int. J. Environment and Pollution, 27, 222-230 (2006)
• Oxygen-induced concurrent ultrasonic degradation of volatile and non-volatile aromatic compounds Christian Pétrier, Evelyne Combet and T.J.Mason, Ultrasonics Sonochemistry, 14, (2007) in press.
Sonochemistry
Sonochemistry is the use of high intensity acoustic fields to enhance chemical reactions. High-frequency acoustic pressure variations literally tear water apart in the process known as cavitation and chemists have demonstrated that this greatly enhances many chemical reactions.
When cavitating bubbles collapse in a sonochemistry reactor, high local temperatures and pressures arise producing excited free radicals. Other beneficial effects arise, including the transport of products and increase in active surface area.
Unfortunately, this non-linear process is not readily scalable from the test-tube to industrial size. This Holy Grail remains elusive.
The Landfill Directive states that liquid industrial waste can no longer be sent to landfill. The adoption of the Water Framework Directive has put emphasis on the protection of water resources. If the UK is to meet its obligations under these directives a range of novel effluent processing technologies will need to be introduced.
Sonochemistry is central to the Green Chemistry Movement and has been demonstrated to enhance the degradation of effluent and to oxidise toxic waste prior to disposal - offering a cost-effective solution to these key legislative issues.
DETERMINATION OF THE ULTRASONIC EFFECTIVENESS IN ADVANCED WASTEWATER TREATMENT
*S. Nasseri, F. Vaezi, A. H. Mahvi, R. Nabizadeh, S. Haddadi
Department of Environment Health Engineering, School of Public Health and Center for Environmental Research,Medical Sciences/University of Tehran, Tehran, Iran
*Corresponding author-Email: siminasseri@hotmail.com , Tel: +98 21 8895 4914, Fax: +98 21 8895 0188
Received 19 October 2005; revised 5 February 2006; accepted 20 March 2006
Code Number: se06017
ABSTRACT
Ultrasonic technology may be used for water and wastewater treatment as an advanced oxidation process. Application of this technology, leads to the decomposition of many organic compounds during cavitation process. In this study, the efficiency of ultrasonic in advanced treatment of municipal wastewater has been investigated by use of an ultrasonic bath. COD and BOD5 tests were used as the indicators of organic matter concentrations and three detention times for treatment were appointed at 10, 30 and 60 minutes. Two frequencies of 35 and 130 kHz for sonication were used. Results indicated that sonication can reduce 25% to 30% of COD in less than 60 minutes. Also, it was indicated that suspended COD was mainly converted to soluble COD during sonication. The rate of hydrogen peroxide production and thus the efficiency of treatment were higher at 130 kHz, but this efficiency was not much increased by prolonging sonication time. In other words, maximum efficiency was achieved at the initial time of sonication. Furthermore, no considerable change in nutrients concentration was detected and pH variations of samples were negligible (<0.3). In contrast, significant temperature change occured which was about (18-20)oC increase in 60 minutes. However, this temperature change had no considerable effect on treatment efficiency.
Key words: Ultrasonic, organic matter, capitation, secondary effluent treatment
INTRODUCTION
Biological treatment is the most commonly applied method for treatment of wastewaters. However, biological treatment can be inhibited by bacteriotoxic or persistent pollutants present in wastewater (Lifka et al., 2003). As a result, this technology may be incapable of reducing the levels of contaminants below which they are not considered as a potential threat to public health. Therefore, new technologies that offer significant improved levels of treatment or constituent reduction need to be tested and evaluated (Metcalf and Eddy, 2003). Advanced oxidation processes (AOPs) are used to oxidize organic constituents found in wastewater that are difficult to degrade biologically. AOPs typically involve the generation and use of hydroxyl free radical as a strong oxidant to destroy compounds (Metcalf and Eddy, 2003). Ultrasonic technology (as an AOP) has been used for water and wastewater treatment (Naffrechoux et al., 2000). Ultrasound (US) was defined as the sound of a frequency that is beyond human hearing above 16 kHz. The ultrasound energy which has been used in sonochemistry is in the distinct ranges of 16-1000 kHz i.e. power ultrasound (Zheng, 2004). Ultrasonic irradiation of aqueous solutions can result in the growth and collapse of gas bubbles (cavitation) so producing high transient temperatures and pressures, which leads to the formation of free radicals (oOH , oOOH) via thermal dissociation of water and oxygen. These radicals penetrate into water and oxidize dissolved organic compounds. Hydrogen peroxide (H2O2) is formed as a consequence of oOH and oOOH radicals recombination in the outside of the cavitation bubble (Langenhore, 1998; Jang et al., 2002; Visscher et al., 2004). Three regions, gas phase, inerfecial region surrounding the cavitation bubble, and the bulk solution are present during cavitation (Laughrey et al., 2001). High-volatile compounds diffuse more easily into the cavitation bubbles and hence are degraded mainly through pyrolytic reactions. The aquasonolytic degradation of low volatile pollutants by oOH radicals takes place in the surrounding water (Lifka et al., 2003). The concentration of HOo at a bubble interface can be as high as 4×10-3 M, witch is 108-109 times higher than that in the other advanced oxidation processes (Crittenden et al., 2004). Furthermore, there are no additives introduced into the ultrasonic system and no by products generated by ultrasonic technology. Therefore, there are no anticipated environmental concerns associated with this technology (Buchholz et al., 1998). In contrast to many other processes which are negatively affected when suspended solids of effluent increase, US efficiency may even improve by increase of turbidity or suspended solids (Manson and Lorimer, 2002). Although the technology has been shown to be feasible on a small scale, the commercialization of sonolysis is still a challenge, due to the high energy requirement of the process (Crittenden et al., 2004).
Many studies have been performed on sonolytic degradation of different compounds and related factors which affect the rate of decomposition. Francony and Petrier showed that the rates of reactions involving hydroxyl radicals (H2O2 formation and phenol degradation) have a maximum value at 200 kHz compared with lower and higher frequencies (20, 500 and 800 kHz) (Francony and Petrier, 1997). Goel and co-workers recognized that decomposition rates of non-volatiles were lower than volatiles (Goel et al., 2004). Study of the effect of temperature revealed that the destruction rate of 1,2-DCA (dichloroethane) is almost independent of temperature (in the range of 15-30oC) (Kruger et al., 1999). Treatment of raw sewage by sonuv (combined sonication and UV irradiation) in 90 min was not effective to mineralize the organic matter. A significant reduction of COD was observed after 4h of sonuv treatment (Naffrechoux et al., 2000). Ultrasonic can decompose other organic substrates such as chlorinated hydrocarbons, pesticides, phenol, explosives such as TNT, and esters, and transform them into short-chain organic acids, CO2 and inorganic ions as the final products. The time for complete degradation ranges from minutes to hours (Haffmann et al., 1996). The main purpose of this study was to determine the efficiency of sonication process in treatment of secondary effluent municipal wastewater.
MATERIALS AND METHODS
Sampling
Secondary effluent samples were collected from two sewage treatment plants, Ghods and Shoosh sites, in Tehran. Activated sludge biological treatment is used in both treatment plants. Sampling was performed between 8-10 am in the spring of 2005. In order to determine the effect of ultrasonic on wastewater constituents, individual samples were taken for organics and nutrient treatment. Samples had been taken after secondary clarification (before chlorination) and sent for analysis in less than one hour.
Treatment
The wastewater samples were treated in an ultrasonic bath with the characteristics shown in Table 1. 500mL beakers were used for this purpose and each sample was treated in three sonication times of 10, 30 and 60 minutes. Thereupon, the treatability tests had been performed in batch system.
Analyses
Samples were analyzed before treatment for determination of: total COD (TCOD), suspended COD, soluble COD (SCOD), total BOD5 (TBOD5), suspended BOD5, soluble BOD5 (SBOD5), pH, temperature, total suspended solids(TSS), total phosphorous (TP) and total Kjeldal nitrogen (TKN). After sonication, samples were analyzed for determining COD (TCOD, suspended COD and SCOD), BOD5 (TBOD5, suspended BOD5 and SBOD5), pH and temperature. All the analyses were performed according to the procedures described in the Standard Methods (APHA, 2005). Temperature and pH were measured by a thermometer and a pH meter, respectively. Producible H2O2 was analyzed by a Hatch Model Kit.
Interferences and their removal
In this study, some increase in COD was detected for all samples after sonication. Reduction of COD was possible in the initial sonication times (during 10 and 30 min), however, at longer times (60 min), it raised often to 1.3 times of the initial COD. This effect which was not recorded in the previous studies may be attributed to the radicals and H2O2 formation (H2O2 was detected after sonication). Two methods can be recommended for removal of this interference. By one way, COD formation by radicals and H2O2 can be measured and then the result is substracted from total COD. But, by the second way it is necessary to remove interferences (radicals and H2O2). Regarding the volatile characteristic of radicals and H2O2, removal of these chemicals is possible after 20-30 min maintaining in ambient air, and this way was considered as a simple method to omit this interference. Thus, a same period of 30 min was used and COD of all samples had been determined after this period. OH radicals may also interfere with BOD5 determination by increasing dissolved oxygen of samples. A same method was again used for removal of this interference.
RESULTS
Effect of ultrasound on organic matter
Results showed that US reduces BOD5 of secondary effluent (Fig. 1). But sanitation time had no considerable effect on the efficiency of this treatment (p>0.05). Suspended BOD5 was removed completely (near 100%). However, soluble BOD5 was increased in some cases, may be because of suspended BOD conversion to soluble forms. As COD concentrations were much more than BOD5 concentration, the effect of ultrasound on organic matter oxidation has been studied by use of COD results. In this study, the efficiency of total COD removal was determined to be 17-28% (Tables 3 and 4). The effects of US on soluble and suspended COD have also been determined. As shown, removal of suspended COD is better accomplished than SCOD. Two reasons may be mentioned: suspended COD may be really better affected by US and/or this form may be converted to SCOD by US treatment. To find the reason, an extra experiment has been performed after preparation of a new special sample by first removing the SCOD and then adding TSS which had the inorganic nature in concentration equal to the original samples (namely about 10 mg/L). Results which can be seen in Fig.1 indicated that there is no considerable difference (such as the high difference between total and soluble COD, shown in the previous Figure) between these two types of COD, and treatment efficiencies are relatively similar. This means that suspended COD has converted to SCOD during sonication. Similar to suspended COD, the removal of suspended BOD is better accomplished than SBOD.
Effect of sonication time on organic matter
Tables 3 and 4 show the effect of sonication time on the efficiency of organics removal. Much of the COD decomposition was accomplished in the initial sonication time and the efficiency of this decomposition was not much increased by increasing time. For example, this efficiency was 20% in 10 min (Table 3: COD=49.2 mg/L and f=130 kHz) and was only increased 3.5% and 5% after 30 and 60 minutes, respectively. But, the effect of time was significant (p<0.01).
Effect of ultrasound frequency
As shown in Fig.2, better decomposition of secondary effluent organics has been performed at 130 kHz compared with the lower frequency (p<0.05). The efficiency of treatment in 60 minutes sonication at the frequency of 35 kHz was about 24%, but it raised to about 28% at 130 kHz frequency. H2O2 formation at 130 kHz frequency was about 2.5 times higher than that at the frequency of 35 kHz (Table 2). It should be noted that oOH radicals formation and thus H2O2 formation in distilled water is less than that of the effluent, but due to absence of organics in distilled water much of these radicals remain and so H2O2 measurement in distilled water may better demonstrate the radical formation. The effect of US frequency on suspended COD can be seen in Fig.3. In contrast to TCOD, the removal efficiency of suspended COD was better at the frequency of 35 kHz, may be because of formation of finer bubbles and therefore more intensive collapse of these bubbles at lower frequencies.
Effect of ultrasound on nutrients
Nitrogen and phosphorous are among the most important pollutants in secondary effluent which should be removed by wastewater treatment. In this research, the effect of ultrasonic on these pollutants has been determined by TKN and TP analyses. The concentrations of TKN in the initial effluent samples were as low as 3.6-6.5 mg/L. It was revealed that 45-60 min sonication had no detectable effect on these low concentrations of TKN. Also, it should be noted that these two frequencies had no significant effect on total phosphorous concentration (initial concentrations were always < 4.2-5 mg/L). It is noteworthy that the concentrations of both nutrients in the initial effluents were low .
Effect of ultrasound on pH
The ultrasound had no considerable effect on pH of samples, and the little change occurred was insignificant (p>0.05).
Effect of ultrasound on temperature
In an ultrasonic reactor, the temperature increases with sonication if it is not controlled. In this research, temperature increase in 60 min was about 18-20 oC and it is due to cavitation. The increase in temperature in 35 kHz frequency was about 2-3 oC more than in 130 kHz frequency, but this difference was not significant (p>0.05). Besides, by preserving a constant temperature during sonication (through use of an ice bath) it was detected that temperature increase of samples during ultrasound had no considerable effect on COD removal by itself. In general, increase of temperature can increase or sometimes decrease the degradation rate.
DISCUSSION
Treatment of secondary effluent by ultrasonic can reduce about 30% of the remained organics in these effluents. This treatment efficiency is probably the result of organics characteristics. Most of the organics in secondary effluent are low-volatile. Besides, it is predictable that most of the remained matter in effluent have hydrophilic characteristics. Therefore, it is probable that the main mechanism of organics removal is treatment by oOH radicals in bulk solution. Pollutants which decompose in this region are less degradable by ultrasound than pollutants which decompose in gas phase. Besides, secondary effluent contains different organic compounds with specific characteristics. Thus, each have different behavior in treatment by ultrasonic. Moreover, these different compounds may interfere with the decomposition process of eachother and deteriorate or enhance the ultrasonic treatment. Inorganic matter can affect the decomposition of organics too. Sometimes, treatment by US covnerts complex organics to much smaller compounds and it is obvious that much sonication times are needed for complete demineralization. Often, relative conversion of organics suffices for meeting much of the requirements. As these simple compounds have organic nature, the effect of treatment can not be detected by routine tests of COD and BOD5 and in other words, by these tests it is difficult to show the effect of ultrasound on organics decomposition. For example, in sono-oxidation of humic acids (Chemat et al., 2001), complete degradation of these compounds occured in 60 minutes whereas, reduction of TOC was only 40%. Suspended COD has converted to SCOD during sonication. Previous works on SCOD of wastewater sludge confirm our result about conversion of suspended COD to SCOD.
For example, one of the previous studies showed considerable increase of SCOD of sludge after sonication such that the SCOD was reported to increase from 620 mg/L to 2100 mg/L after 2.5 minutes and to 4200 mg/L after 10 minutes (Gronroos and Hyllonen, 2005). The mechanical shear forces caused by ultrasonic may be the dominant factor for the disintegration enhancement (Mao et al., 2004). In a few studies (Pandit et al., 2001), the low improvement of efficiency versus time has been attributed to the degasification effect of ultrasound. Degasification of solution leads to increase in cavitation threshold and thereby to reduction in efficiency. Besides, most of the decomposable organics by ultrasound are removed in initial sonication time and the remained fraction of organics may be less removable. Suspended solids are also effective in the process of cavitation and their reduction may lead to increase cavitations threshold. Ultrasonic can reduce TSS, but in this study the TSS of effluent samples were low (less than 8 mg/L) and this concentration reached to less than 2 mg/L after 60 minutes.
It is expected and also reported that the rate of degradation of organic compounds increases with the increase in frequency of sonication, although, the effect of frequency is somewhat system specific (Goel et al., 2004). The optimal frequency for aquasonolysis of high-volatile pollutants ranges between 300 and 800 kHz. The generation of oOH radicals and the degradation of low-volatile pollutants by oxidation is optimal at frequencies of approximately 200 kHz (Lifka et al., 2003). As decomposition by oOH radicals is expected to be the main mechanism for sonolysis of the organics present in the secondary effluent, it can be accepted that meeting the better efficiency at 130 kHz frequency is due to better formation of oOH radicals in bulk solution. Results of H2O2 measuring in distilled water in these two frequencies (Table.3) can be considered as the conformation for this claim.
Sonochemical treatment of various organics generated low molecular weight carboxylic acids. Simultaneously, water decomposes to oH and oOH radicals. oOH radicals react with organics, and remained hydrogen may produce acidic compounds hence pH drop may result. But, these effects are not considerable in real samples, because water has bufferic characteristics. However, more work is needed to confirm these results. In a previous study (Kruger et al., 1999), sonication of 1,2 DCA solution in deionized water has resulted in pH drop, but, in groundwater the pH has raised from 6.2 to 7. Presence of carbonate system in natural waters was reported to be the reason of this phenomenon.
Finally, it is concluded that treatment of secondary effluent organics by ultrasonic seems not very efficient but it should be noted that the efficiency of many other advanced treatment processes is not much higher. On the other hand, not much higher efficiency is always needed at this stage. Finally, if we consider the disinfection capability of this method, we can expect much better position for this technology. According to a research accomplished in our country, the efficiency of this method was determined to be as much as more than two logs for total coliforms disinfection (Dehghani, 2005) and this is an outstanding advantage for accepting US in the process of secondary effluent treatment.
ACKNOWLEDGEMENTS
The authors express their thanks to Mrs. A. Ghasri, Mrs. A. Kheiri and Miss. Sh. Hosseipour, colleagues of Water and Wastewater Chemical Laboratory in the Department of Environmental Health Engineering, for their assistances throughout the experiments analysis.
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© 2006 Tehran University of Medical Sciences Publications
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