Environ. Sci. Techno/. 1995, 29, 2016-2022
Removal of CYom and Nitro by Ozomtion and Biotreatnlent HERMANN STOCKINGER, ELMAR HEINZLE,* AND OEMER M. KUT Chemical Engineering Department, ETHZ, CH-8092 Zurich, Switzerland
Synthetic wastewater containing a mixture of 10 different chloro and nitro aromatic pollutants was treated in batch ozonations and in a continuous combined ozonation-biodegradation system. The pH dependency of elimination of individual aromatics and of total organic carbon (TOC) was studied in batch ozonations. Experiments at pH 7 showed higher degradation and elimination rates than those at pH 2 and pH 12. A rapid increase of the dissolved ozone concentration after elimination of the aromatics was observed. This was very useful for on-line control of the combined process. Ozonation products were shown to be highly biodegradable through high oxygen uptake rates of biomass. Total mineralization of recalcitrant chloro and nitro aromatics was observed in the combined ozonation- biotreatment. Most significant improvement of ozone efficiency was obtained by controlling dissolved ozone concentration at optimal levels. A slight further enhancement was observed by recycling the liquid between the bioreactor and the ozone reactor.
Introduction The increasing turnover of materials and the increasing consumption of different products generate wastes during production, use, and disposal. In addition, many materials in the production are toxic, mutagenic, or carcinogenic and therefore have high environmental relevance and must be treated before release into water, soil, or air. The environmentally compatible design of products is the first and most important step to avoid waste formation during the production and application and to allow recovery of materials. Unavoidable waste compounds are often nonbiodegradable or toxic to microorganisms and can have very long half-life periods in the environment. Different treatment processes, especiallyfor wastewater and air, have been developed to eliminate refractory wastes. Refractory in this context describes compounds not degradable by microorganisms in waste treatment plants with usual retention times. Production of dye stuffs, pharmaceuticals, herbicides, and pesticides often release wastewater containing multi* Corresponding author: Telephone: +41-1632-3040; Fax: +411632-1053; e-mail address:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8,1995
substituted aromatic compounds. In this work, the selection of model compounds and their combinations with high environmental relevance is based on the reports of the German Advisory Committee on Existing Chemicals of Environmental Relevance (BUA)( I ) . Although this is a series of German reports, chemical industries of other countries also produce the mentioned substances as intermediates or final products and have similar wastewater to be treated in special plants (2). A list of 60 selected substances was given, mainly organic chloro compounds as well as substituted and condensed aromatics. From this list, five chloronitro aromatics and further nitro aromatics were chosen to study ozonation of compounds. Additionally, chlorobenzene (CB)and nitrobenzene (NB), which are not highlyrefractory,were used as model compounds since published data concerning the elimination allow a critical evaluation of the combined process developed. Ozonation of Wastewater. Ozonation of water, mainly drinking water, has been studied since about 1840. In the last 40 years, ozonation reactions were investigated in pure water and in water containing different compounds, dissociated or non-dissociated, organic or inorganic (35). Ozonation involves a large number of reactions. The most important reactions in the complex system for elimination of pollutants are the ozonolysis with molecular ozone and the free radical reactions (6). Although ozone has a higher oxidation potential than chlorine compounds (7), it reacts only selectively with nucleophilic molecules. Contrary to molecular ozone, the 'OH radical which is the product of ozone decomposition and one of the most reactive agents, reacts non-selectivelywith almost all kinds of substances (8). Ozone forms 'OH radicals through decomposition reactions with -OH ions or dissociated hydrogen peroxide. The formation of hydrogen peroxide during the ozonation of organic compounds was observed (9). Various intermediate radicals provide the establishment of radical chain reactions leading to 'OH (6, 10).All radicals can be scavenged by combination with other radicals or through production of secondary radicals with low reactivity. The most important scavengers in wastewater are carbonate and bicarbonate (5,8). The initiation of the radical chain reactions through -OH ions and its promotion through H+does not allow a simple separation of the complex reaction scheme into pure 'OH radical or molecular ozonolysis regimes by varying the pH value (I1, 12). Reaction rate constants for the ozonolysis of organic compounds (13,141 as well as for free radical reactions are available (8, 15, 16) but were measured at specific conditions. The scope of this work was to investigate the ozonation of different chloro- and nitro-substituted aromatic compounds and to study the combined chemical oxidation by ozone with biological treatment. The crucial question was whether such combinations can improve the efficient use of ozone for the elimination of recalcitrant compounds through increasing the biodegradability in the ozonation (17,18).
Experimental Section Synthetic Wastewater. In all experiments synthetic wastewater was used, containing deionized water with a mixture
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of the bioreactor was controlled at 30 1 "C. A process control system (PCS) (Miinzer & Diehl Electronic GmbH, Overrath, Germany) stored the measuring data and controlled pH by adding phosphoric acid or sodium hydroxide solution. For continuous combined ozonation-biotreatment. the setup shown in Figure 1 was used. The ozone concentration in the liquid bulk was controlled by manipulating the feed flow of wastewater using a PID type controller (19). Increasing ozone concentration caused an increase in the feed flow. Thus, an increased supply of organic compounds caused an increased consumption of ozone. This system was used because the control of the inlet gas concentration could respond only with high time delays in this setup. Wastewater was pumped from the feed tank into the ozonation reactor (r 7-12 h). From there it entered the aeration tankand was pumped into the fluidized bed biofilm reactor. The recirculation ratio [fluidizingflow rate of fluidized bed (FLP) per feed flow rate (FP)I was about 60. The treated wastewater left the reactor system by the effluent ofthe aeration tank. Recycle could be run between fluidized bed and ozone reactor using the pump RP given in Figure 1. For batch ozonations, only the ozone reactor with appropriate measuring units was used. Oxygen Uptake Rate Measurements. Oxygen uptake rate (OUR) of biomass was measured in a respiration cell (40 mL volume, 30 "C) with an oxygen electrode. A total of 25 mL of wastewater with macro and micro substrates beside the carbon source was initially saturated with air. Microorganisms as the consortium of an industrial wastewater treatment plant (Ciba Geigy, Grenzach/Germany), several times washed by deionized water, were added at time 0 to give a final concentration of 890 g ofbiomass m-3 in the respiration cell. Foreachexperiment, new organisms were used except in the adaptation test. Analytical Methods. Aromatic and other extractable compounds from wastewater were analyzed by GC/MS. Beforeinjection, the compounds were extracted into diethyl ether. For all aromatic compounds tested, the extraction efficiencywas better than 99.5%. Quantification was done by internal standard calibrationwith 4-bromophenol. The gas chromatograph used was a Hewlett Packard [type5890A, Avondale, PA1 with a split-splitless injection system. The mass spectrometer connected to the GC was an ion trap detector (electron impact ionization. Model ITD 800) of Finnigan MAT (San Jose, CAI. The overall standard deviation of the concentrations was within f4%. Detection limits were typically less than0.5 mg/L (-3.3pmol/L). The substances were identified bylibrarydatacomparison using the ITD software or MASSLIB (Max-Planck-Institut fiir Kohlenforschung, Miihlheim a. d. Ruhr). Ozonation products, mainly organic acids as well as chloride, nitrite, and nitrate, were detected by ion chromatography. The system consisted of a HPLC (Waters. Milford, MA1 equipped with an ion exchange column (Ion Pac AS1 1,4 mm diameter, Dionex, Sunnyvale,CA), followed by a membrane suppresser (anion self-regenerating suppresser 1, Dionex) and a conductivity detection cell (Dionexl. The standard deviation of the concentrations was within +3.5%, and the detection limits were less than 10 pmol/L. Total organic carbon (TOC) was analyzed as NPOC (nonpurgeable organic carbon) and POC [purgeable organic carbon) using a DC 180 TOC analyzer (Dohrmann, Santa Clara,CAI that oxidizesorganic compoundswith potassium
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FIGURE 1. Experimental setup: feed lank, FI; ozonation bubble column. 0$3C: waste gas reflux cooling unit. C: fluidized bed, FB; mixing tank, MT; feed pump, FP; fluidizing pump, FLP recycle pump, AP; ozone photometer. O R ozone electrode, O+: oxygen electrode. 02E pH electrode and control, pHEC flow control of feed, FC flow indicationof feed, FI: processcontrol system. PCS: personalcomputer. IBM-PC.
of aromatic compounds. The following aromatics were used 0-, m-, andp-dinitrobenzene (oDNB,mDNB, pDNB); 0-,m-,and p-chloronitrobenzene (oCNB, mCNB, pCNB1; 2.4-dichloro-1-nitrobenzene(DCNBI and 1,2-dichlorobenzene (oDCB);chlorobenzene (CB); and nitrobenzene (NB). AUwerepurchasedfromFhka,Buchs, Switzerland (>99%). For continuous experiments, additional macro elements (20 mmol of NH4S04/molof organic C contained in the wastewater, 24 mmol of KHzP04/molof C) as well as micro elements [120pmolofFe/mol ofC, llOpmolofCo/mol of C, 30 pmol of Mn/mol of C, 1.7 pmol of Culmol of C, 4.6 pmol of Zn/mol of C, 3.9 pmol of Ni/mol of C, 2.8 pmol of Al/mol of C (all from Fluka) >98%1were added to support biofilm growth. These elements had no significant influence on the ozonation reactions. Experimental Setupand Procedure. The experimental setup used is shown in Figure 1. The ozonizer (Labor Ozonisator, Sanders, Germany) fed with pure oxygen providedozoneof62-65 mg1L. Ozone concentrations were detected in the gas phase by a W-photometer (Anseros Ozomat GP, E. Sanders,Tiibingen,Germany, detection limit 50.1 mg/Ll and in the liquid phase by an amperometric ozone electrode (Model 26501, sensor 2301, Orbisphere, Geneva, Switzerland, detection limit 50.005 mg/L). The ozone reactor and the fluidized biofdm reactorwere made of glass with 1.85 L volume, 1 m height excluding disengagement section, and 0.04 m internal diameter. The gas stream (40 mL/min) was fed through a porous glass plate of 16-30pmporediameter. The temperatureoftheozone reactor was controlled at 21 1 "C, and the temperature
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FIGURE 2. Batch ozonation at pH 2. (A) Gas and liquid phase ozone concentrations: Fo2 = 40 mumin, Osn = 65 mgA, fin = 1.24 bar, T = 20 "C. (B)Concentrations of (0)CB, (m) NB, and (A) oDCB. (C) Concentrations of (0)oWB, (e) mCNB, and (W pCNB. (D) Concentrations of ( 0 ) oDNB, (0)mDNB, and (m) pDNB. (E) Concentrations of (m) Wm&, (0) CI-e,~c~, (0)NOS--, and (0) N03-e,~cd.(F)Concentrations of (0)acetate, (r)fonnate,(0)oxalate, and@) glyoxylate. (G)Concentrationsof ( M ) T O L and (0)TOC,rcd.
peroxodisulfate and W light with infrared COZdetection. It was recently observed by the Swiss distributor (Schmidlin AG, Neuheim, Switzerland) that not all highly substituted aromatics can be oxidized completely by this method.
Results and Discussion Batch Ozonation at pH 2. At this pH, free radical formation is very low; therefore, radical reactions are expected to be negligible, whereas direct molecular ozonolysis should be much more important. Figure 2 depicts the results of a batch ozonation at pH 2. Ozone concentrations in the gas and liquid phases as well as concentrations of substrates, intermediates, and end products are given in that figure. Ozone in both phases was initially zero. Dissolved ozone concentration began to increase at 460 min and increased even faster at -630 min (Figure 2A). At 650 min, the ozone electrode failed. The first significant increase occurred when faster reacting compounds (CB, NB, CNBs) were eliminated (Figure 2B,C); the second increase coincided with the disappearance of dinitrobenzenes (Figure2D).The oxidation of aromatic rings therefore seems to be provided by ozonolysis of double bonds. The ozone concentration in the outlet gas stream began to increase at -340 min, before the dissolved ozone concentration increased (at -700 min). Chloronitrobenzenes are faster degradable (Figure 2C) than dinitrobenzenes (Figure 2D). As expected, the nitro groups deactivate the aromatic ring more than chloro groups for electrophilic attack of ozone. The initial increase of the DNBs could be due to solubility problems of aromatic compounds in wastewater. CDNB was removed only by 2018 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 8.1995
92%, and mDNB and pDNB were eliminated only by 97% even after 1100min. Figure 2E shows concentration profiles of inorganic ions produced. The curves for Cl-cdcd and N O J - were ~ ~ ~calculated ~ from the decrease of aromatics with the assumptions that the initial degradation step was dechlorination or denitration, and no chloro- or nitrosubstituted intermediates were produced. Chloro substituents are removed as chloride and the nitro group first as nitrite that is finally oxidized to nitrate. The concentration of nitrite remained near or below the detection limit. Higher measured C1- concentration (cl-measd) than calculated (cl-cdcd) could be due to initial adsorption effects of CB or oDCB on the glass walls. Decreasing concentration in the bulk could lead to desorption and higher C1- levels by increasing reaction time. Measured nitrate concentration (NOs-measd) was lower than calculated ( N O J - ~ ~Probably ~~). nitrated intermediates are produced that could not be detected. Figure 2F presents profiles ofthe most important organic ozonation products. Acetate and formate were accumulated just after starting the ozonation. Oxalate and its precursor glyoxylate showed a delayed increase. This points to *e formation of undetected intermediate products. Additionally, malonate, maleate, and succinate were detected at very low concentrations (
