Environ. Sci. Technol. 1999, 33, 4092-4095
Composition and Biodegradability of Products of Wet Air Oxidation of Polyester R I C H A R D M . D I N S D A L E , * ,† MATS ALMEMARK,‡ FREDA R. HAWKES,§ AND DENNIS L. HAWKES† School of Design and Advanced Technology and School of Applied Sciences, University of Glamorgan, Pontypridd, Mid Glamorgan CF37 1DL, U.K., and IVL Swedish Environmental Research Institute, Environmental Technology and Aquatic Effects, Ha¨lsingegatan 43, P.O. Box 21060, 10031 Stockholm, Sweden
Knowledge of the chemical composition of wet air oxidation (WAO) effluents is essential in determining the effectiveness of the WAO system and in selecting further treatment. WAO with excess O2 of cured green architectural polyester (33.77 g L-1 total solids) at 300 °C for 80 min gave 97% destruction of volatile solids, with 92% of the remaining total organic carbon (TOC) in the soluble form. Volatile fatty acids (C1-C5 with acetic acid as the major component) and other short organic acids (maleic and glyoxylic) and benzoic acid were also identified, in total comprising 98.5% of the soluble TOC. The polyester sample oxidized under O2 limitation (107.01 g L-1 total solids) for 4 h at 290 °C showed 84% volatile solids destruction, with 67% of the remaining TOC in the soluble form, and gave a more complex range of organic compounds. Soluble effluent from both WAO processes showed good biodegradability both aerobically and anaerobically.
Introduction Increasing interest is being shown in integrating chemical and biological systems to treat polluting organic compounds. Chemical treatments used to break down toxic and recalcitrant compounds to molecules suitable for biotreatment (1, 2) include wet air oxidation (WAO), ozonation, photooxidation, and Fenton’s reaction and are generally termed advanced oxidation processes. In the process of WAO, effluents are oxidized under air or oxygen with no additional reagents. With wastes of 20 000 mg L-1 COD or above, the process is self-sustaining energetically as compared to incineration where wastes should be 300 000-400 000 mg L-1 COD (1). Mishra et al. (1) report that in 1995 there were up to 200 full-scale WAO processes operational worldwide, the majority built to dispose of sewage sludge, but several applications are known from the chemical industry (3). Other applications include recovery of fillers from fiber sludge (4). In WAO, the waste components are oxidized in the liquid phase at elevated temperatures (125-320 °C) and pressures * Corresponding author phone: +44 1443 482227; fax: + 44 1443 482231; e-mail:
[email protected]. † School of Design and Advanced Technology, University of Glamorgan. ‡ IVL Swedish Environmental Research Institute. § School of Applied Sciences, University of Glamorgan. 4092
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 22, 1999
(0.5-20 MPa) with oxygen. Carbon is oxidized to CO2; nitrogen is oxidized to NH3, NO3-, or elemental nitrogen; halogens are oxidized to inorganic halides; and sulfur is oxidized to sulfates. The degree of oxidation is a function of temperature, oxygen partial pressure, residence time, and composition of the components. The percentage of oxidation can be up to 99.9%, and WAO effluents can have COD values of 17 000 mg of O2 L-1(5). Biological treatment is often used for further stabilization of WAO effluents (1, 2). In particular, short organic acids such as acetic acid can accumulate in wet air oxidation effluents. These tend to be resistant to further oxidation but would be ideal for biological treatment (1). Most wet air oxidation experiments have utilized watersoluble components or solids such as waste-activated sludge that have some degree of biodegradability before wet air oxidation. The application of wet air oxidation technology would be expanded if it was shown that a solid waste with limited biodegradability, for example, plastic waste, was successfully broken down. Plastics recycling is difficult when different types of plastics are mixed with each other and/or with silicone-based filler and heavy metal stabilizers, fire retardants, and colorings are present. Polyesters have been widely used in the building industry due to their long life and good chemical capabilities (6). However, this robustness which is essential for their utilization makes the treatment of these products at the end of their life more difficult. Another problematical waste is the primary screening residues from wastewater treatment plants. These contain significant levels of organic material including plastic and cellulose components, which make the waste difficult to biodegrade but also make up a complex matrix that is difficult to recycle using conventional technologies. Other disposal routes for these wastes such as landfill and incineration are becoming less favorable as their costs increase due to new legislation and increasing environmental concerns. Wet air oxidation has been used to break down polyethylene or polystyrene (7) and a mixture of polyethylene, rubber, poly(vinyl chloride) (PVC), and cellulosics (5); WAO and alkali hydrolysis were used to oxidize PVC alone (8). However, little or no information was presented on the breakdown components or biodegradability. To our knowledge, there are few other studies published on the WAO of plastics from which data on end products can be obtained. Knowledge of the physicochemical characteristics and chemical composition of wet air oxidation effluents is essential in determining the effectiveness of the WAO system and selecting further treatment or disposal. This paper examines what WAO conditions broke down plastic waste (green architectural polyester) and the physical, chemical, and biodegradability characteristics of the WAO effluent.
Materials and Methods Polymer Sample. Cured green architectural polyester (PPG Industries (UK) Ltd, Birmingham, U.K.) was chosen as representative of a polyester type used as a coating 10 years previously, with products now requiring disposal. The sample was broken up with a domestic blender to a particle size less than 1.4 mm. Wet Air Oxidation. A 2-L batch reactor made of inconel steel alloy (8% Fe, 76% Ni, and 15.5% Cr as major constituents) (Parr Instrument Company, Moline, IL) was used to perform the wet air oxidation experiments. The reactor was equipped with a stirrer and with stainless steel Swagelok tubes and valves, which allowed the addition of oxygen and the withdrawal of samples during a run. Heating was provided by an external bomb heater. 10.1021/es9905086 CCC: $18.00
1999 American Chemical Society Published on Web 10/07/1999
TABLE 1. Physicochemical Characteristics of the WAO Effluenta
a
analysis
experiment A
experiment B
total solids (g L-1) % reduction in TS volatile solids (g L-1) % reduction in TS suspended solids (g L-1) volatile suspended solids (g L-1) total COD (mg of COD L-1) soluble COD (mg of COD L-1) total TOC (mg of C L-1) soluble TOC (mg of C L-1) soluble TOC/initial polyester (mg g-1)
37.94 (3), SD ) 0.61 65 12.81 (3), SD ) 0.31 84 24.81 (3), SD ) 0.55 4.55 (3), SD ) 0.51 75613 (3), SD ) 2623 47616 (1) 36711 (4), SD ) 1218 24774 (4), SD ) 994 231
8.49 (3), SD ) 0.77 75 0.77 (3), SD ) 0.14 97 8.71 (3), SD ) 0.41 0.87 (3), SD ) 0.30 3713 (9), SD ) 93 3513 (9), SD ) 117 2229 (4), SD ) 70 2051 (4), SD ) 48 61
SD ) standard deviation. Parenthetical values ) number of analyses.
In experiment A, 100.1 g of green polyester was added to 935 g of water purified by deionization and reversed osmosis. The reactor was then run at 290 ( 10 °C for 4 h. Sufficient oxygen was added initially to meet 19% of the theoretical oxygen demand with a further 13% of the theoretical oxygen demand added during the course of the experiment. Assuming that the residual gas pressure was all oxygen, then the oxygen consumption measured manometrically was 1332% of the theoretical oxygen demand. In experiment B, 10.4 g of green polyester was added to 308 g of purified water and sufficient oxygen to be 122% in excess of the theoretical demand. The reactor was kept at 300 ( 6 °C for 80 min. The oxygen consumption measured manometrically was 50% of the theoretical demand. The effluent samples from WAO experiments A and B in acid-washed containers were received within 24 h in the laboratory for analysis and stored frozen/ refrigerated until analyzed. Analyses. Total solids (TS) and volatile solids (VS) were determined by drying to constant weight at 105 °C and subsequent incineration at 500 °C, leaving ash (fixed solids). Totals suspended solids (TSS) and volatile suspended solids (VSS) were determined by standard methods (9). Fumaric acid (the trans form of maleic acid) and maleic, benzoic, glyoxylic, oxaloacetic, and oxalic acids were determined by HPLC using a Dionex 500 ion chromatograph (Dionex Ltd, Camberley, Surrey) fitted with a 4 mm Ionpac AS4ASC with suppresser and 1.8 mM Na2CO3 eluant. Gas chromatography was used to determine volatile fatty acid (VFA) concentration (10). Acetic acid and formic acid were determined by ion chromatography using a Dionex 500 ion chromatograph fitted with a 4 mm Ionpac ICE-AS6 with 5 mM tetrabutylammonium hydroxide regenerant and 0.4 mM heptafluorobutyric acid eluant. Total organic carbon (TOC) was determined using a Rosemount Dohrmann DC190 (Sartec Ltd., Borough Green, Kent) by high-temperature combustion and subsequent analysis of carbon dioxide with an infrared (IR) detector. COD was determined by the mercury-free method (11). Soluble TOC and COD were performed on the supernatant of a sample centrifuged at 1500g for 10 min in a bench centrifuge. GC/MS analysis was performed on a Hewlett-Packard 5890 Series II gas chromatograph with EPC (electronic pressure control) together with a Hewlett-Packard 5971A mass selective detector in EI mode, turned on after 0.5 min. The Wiley mass spectra database was used to determine compounds present. A SPME Microfibre 7 µm poly(dimethylsiloxane) was suspended in the liquid sample and heated with a hair-dryer for 2 min before being withdrawn and placed in the splitless GC injection port at 250 °C. The GC acquisition was started immediately, and the fiber was left in the hot injection port for the first 5 min of the run to ensure that all compounds were desorbed from the fiber. The temperature ramp employed was 40 °C for 3 min to 340 °C for 5 min at 16 °C
min-1 with 0.5 psi constant flow of helium gas. A 25 m × 0.22 mm BPX5 capillary column with 4 mm splitless borosilicate liner was used. To improve the identification of polar compounds, an ether extract was injected onto a free fatty acid phase (FFAP) capillary column of 25 m length, 0.2 mm i.d., 0.33 µm film thickness. X-ray microanalysis was performed with a Stereoscan 240 electron microscope (Cambridge Systems, Cambridge) fitted with a Link Systems X-ray analyzer (Link Systems, High Wycombe, U.K.). The solids from the WAO experiments were washed three times in deionized water by centrifugation at 1500g for 15 min and coated with carbon. The metal content of polyester fixed solids, WAO soluble fraction, and WAO fixed solids fraction after washing was determined (after wet ashing with sulfuric acid and nitric acid for the solids) by analysis using a Perkin-Elmer 400 ICPAES instrument. The residual material was assumed to be silicone as the X-ray microanalysis did not pick up any other elements. Biodegradability Testing. The potential for aerobic biodegradability was determined by performing a 5-day biochemical oxygen demand (BOD5) assay using Oxitop pressure transducer instrumentation (WTW, Germany), with buffers and nutrients described by HMSO (12) and using a EPA-recommended seed inoculum, Hach Polyseed (Camlab, Cambridge). Total BOD5 was determined on a well mixed suspension, and soluble BOD5 was determined on a sample centrifuged at 1500g for 10 min in a bench centrifuge. Anaerobic biodegradability was tested by batch biodegradability tests (13).
Results and Discussion Analysis of WAO Products. The effluent from experiment A was a suspension of black particles in a green-yellowish liquid phase and had a strong solvent odor, while the effluent from experiment B was a suspension of green particles in a virtually clear liquid phase with a slight solvent odor. The pH of the effluent was determined immediately after WAO, being 3.9 for experiment A and 4.7 for experiment B. On arrival at the laboratory for analysis, the pH had become pH 4.18 for experiment A and pH 4.90 for experiment B. The initial concentration of polyester suspended in water for WAO in experiment A was 107.01 g L-1 TS (79.19 g L-1 VS), while that for experiment B was 33.77 g L-1 TS (24.99 g L-1 VS). Table 1 shows a volatile solids destruction of 84% and 97% for experiments A and B, respectively. This compares favorably with results of Clark (5), who achieved up to 97% COD destruction of a mixture of polymers (cellulosics, silicone rubber, Tygon, and Nalgene) at 293 °C for 1 h. Up to 98% TOC and 100% COD destruction of pure polystyrene and polyethylene were achieved at higher temperatures of 320 °C and a longer duration of 4 h (7). VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. VFA Analysis of WAO Effluents by Gas Chromatographya acid (mg L-1) acetic propionic i-butyric n-butyric i-valeric n-valeric
experiment A
experiment B
acid
experiment A (mg L-1)
experiment B (mg L-1)
8087 (3), SD ) 103 557 (3), SD ) 24 197 (3), SD ) 9 138 (3), SD ) 8 13 (3), SD ) 1 29 (3), SD ) 5
3460 (3), SD ) 36 9.5 (3), SD ) 0.2 2.2 (3), SD ) 0.7
