Microbial characterization during composting of biowaste

Windrow composting of source-separated biowaste was studied in a pilot plant in Crete, with regard to abiotic factors, gas concentration in the pile and succession of functional microbial groups. The pH, C/N ratio and VS content, as well as the O₂ and CO₂ concentration, correlated well with composting time, indicating typical composting behaviour. Most of the microbial groups examined exhibited their highest counts towards the end of the thermophilic phase, with declining trends thereafter. The population of total mesophilic and thermophilic bacteria increased during the mild thermophilic phase and followed the temperature decline thereafter. Results on these microbial groups and fungi indicate that the timing of the thermophilic stage in the composting process, in addition to the peak temperature and duration of the stage, affects the microbial succession. Escherichia coli were detected for over 2 months of processing, in spite of the high temperatures achieved; only after about 3 months of composting did its population decline below the detection limit.     

Chemical characterization of emissions from a municipal solid waste treatment plant

Gaseous emissions are an important problem in municipal solid waste (MSW) treatment plants. The sources points of emissions considered in the present work are: fresh compost, mature compost, landfill leaks and leachate ponds. Hydrogen sulphide, ammonia and volatile organic compounds (VOCs) were analysed in the emissions from these sources. Hydrogen sulphide and ammonia were important contributors to the total emission volume. Landfill leaks are significant source points of emissions of H₂S; the average concentration of H₂S in biogas from the landfill leaks is around 1700 ppmv. The fresh composting site was also an important contributor of H₂S to the total emission volume; its concentration varied between 3.2 and 1.7 ppmv and a decrease with time was observed. The mature composting site showed a reduction of H₂S concentration (<0.1 ppmv). Leachate pond showed a low concentration of H₂S (in order of ppbv). Regarding NH₃, composting sites and landfill leaks are notable source points of emissions (composting sites varied around 30–600 ppmv; biogas from landfill leaks varied from 160 to 640 ppmv).Regarding VOCs, the main compounds were: limonene, p-cymene, pinene, cyclohexane, reaching concentrations around 0.2–4.3 ppmv.H₂S/NH₃, limonene/p-cymene, limonene/cyclohexane ratios can be useful for analysing and identifying the emission sources.     

Hydrogen production from food wastes and gas post-treatment by CO2 adsorption

The production of H₂ by biological means, although still far from being a commercially viable proposition, offers great promise for the future. Purification of the biogas obtained may lead to the production of highly concentrated H₂ streams appropriate for industrial application. This research work evaluates the dark fermentation of food wastes and assesses the possibility of adsorbing CO₂ from the gas stream by means of a low cost biomass-based adsorbent. The reactor used was a completely stirred tank reactor run at different hydraulic retention times (HRTs) while the concentration of solids of the feeding stream was kept constant. The results obtained demonstrate that the H₂ yields from the fermentation of food wastes were affected by modifications in the hydraulic retention time (HRT) due to incomplete hydrolysis. The decrease in the duration of fermentation had a negative effect on the conversion of the substrate into soluble products. This resulted in a lower amount of soluble substrate being available for metabolisation by H₂ producing microflora leading to a reduction in specific H₂ production.Adsorption of CO₂ from a gas stream generated from the dark fermentation process was successfully carried out. The data obtained demonstrate that the column filled with biomass-derived activated carbon resulted in a high degree of hydrogen purification. Co-adsorption of H₂S onto the activated carbon also took place, there being no evidence of H₂S present in the bio-H₂ exiting the column. Nevertheless, the concentration of H₂S was very low, and this co-adsorption did not affect the CO₂ capture capacity of the activated carbon.     

Degradation of petroleum hydrocarbons from bottom sludge of crude oil storage tanks using in-vessel composting followed by oxidation with hydrogen peroxide and Fenton

In this research the feasibility of aerated in-vessel composting process followed by chemical oxidation with H₂O₂ and Fenton for removal of petroleum hydrocarbons from oily sludge of crude oil storage tanks was investigated. The ratios of the sludge to immature compost were 1:0 (as abiotic control), 1:2, 1:4, 1:6, 1:8 and 1:10 (as dry basis) at a C:N:P ratio of 100:5:1 and 55 % moisture content for a period of 10 weeks. Six concentrations of H₂O₂ and Fenton were added to the compost mixture for a period of 24- and 48-h reaction times. Results showed that petroleum hydrocarbons removal in ratios of 1:2, 1:4, 1:6, 1:8 and 1:10 were 66.6, 73.2, 74.8, 80.2 and 79.9 %, respectively. The results of the abiotic experiments indicated that the main mechanism of hydrocarbon removal in the composting reactors was biological. The application of combined composting and chemical oxidation demonstrated a remarkable (about 88 %) overall removal. The study showed that in-vessel composting combined with chemical oxidation is a viable choice for the remediation of the sludge.     

Emission of volatile sulfur compounds during composting of municipal solid waste (MSW)

Volatile sulfur compounds (VSCs) are the main source for malodor from composting plants. In this study, the VSCs generated from composting of 15–80 mm municipal solid waste (T0), kitchen waste (T1) and kitchen waste mixed dry cornstalks (T2) were measured in 60 L reactors with forced aeration for a period of 30 days. The VSCs detected in all treatments were hydrogen sulfide (H₂S), methyl mercaptan (MM), dimethyl sulfide (DMS), carbon bisulfide (CS₂) and dimethyl disulfide (DMDS). Over 90% of the VSCs emissions occurred during the first 15 days, and reached their peak values at days 4–7. The emission profiles of five VSCs species were significantly correlated with internal materials temperature and outlet O₂ concentration (p < 0.05). Total emissions of the VSCs were 216.1, 379.3 and 126.0 mg kg^?1 (dry matter) for T0, T1 and T2, respectively. Among the five VSCs, H₂S was the most abundant compound with 39.0–43.0% of total VSCs released. Composting of kitchen waste from separate collection posed a negative influence on the VSC and leachate production because of its high moisture content. An addition of dry cornstalks at a mixing ratio of 4:1 (wet weight) could significantly reduce the VSCs emissions and avoid leachate. Compared to pure kitchen waste, VSCs were reduced 66.8%.     

The effect of dust emissions from open storage piles to particle ambient concentration and human exposure

The current study focus on the determination of dust emissions from piles in open storage yards of a municipal solid waste (MSW) composting site and the subsequent atmospheric dust dispersion. The ISC3-ST (Industrial Source Complex Version 3 – Short Term) model was used for the evaluation of the PM₁₀ ambient concentrations associated with the dispersion of MSW compost dust emissions in air. Dust emission rates were calculated using the United States Environmental Protection Agency proposed dust resuspension formulation from open storage piles using local meteorological data. The dispersion modelling results on the spatial distribution of PM₁₀ source depletion showed that the maximum concentrations were observed at a distance 25–75 m downwind of the piles in the prevailing wind direction. Sensitivity calculations were performed also to reveal the effect of the compost pile height, the friction velocity and the receptor height on the ambient PM₁₀ concentration. It was observed that PM₁₀ concentrations (downwind in the prevailing wind direction) increased with increasing the friction velocity, increasing the pile height (for distances greater than 125 m from the source) and decreasing the receptor height (for distances greater than 125 m from the source). Furthermore, the results of ISC3-ST were analysed with the ExDoM (Exposure Dose Model) human exposure model. The ExDoM is a model for calculating the human exposure and the deposition dose, clearance, and finally retention of aerosol particles in the human respiratory tract (RT). PM₁₀ concentration at the composting site was calculated as the sum of the concentration from compost pile dust resuspension and the background concentration. It was found that the exposure to PM₁₀ and deposited lung dose for an adult Caucasian male who is not working at the composting site is less by 20–74% and 29–84%, respectively, compared to those for a worker exposed to PM concentrations at the composting site.     

Removal of Reactive Blue 19 dye from synthetic wastewater using UV/H2O2 and UV/Cl advanced oxidation processes

The textile and dyeing industries are among the largest water-consuming and polluting industries in the world. The most important feature of the textile dyeing industry wastewater is its color, due to the use of colored materials. Most of these dye compounds are resistant to conventional purification methods and their biodegradation is very low through secondary purification processes, resulting in incomplete removal. Therefore, selecting the optimal method to remove these color compounds is essential. In this study, we studied the removal of an organic dye contaminant (Reactive Blue dye 19 [RB19]) using advanced oxidation processes (AOPs). For this purpose, ultraviolet (UV) mercury lamps with a wavelength of 254 nm and a voltage of W16 inside a reactor were used as an energy source. The experiments were performed in a collimated beam reactor inside a dark chamber. Two oxidizers, sodium hypochlorite (NaOCl) and hydrogen peroxide (H₂O₂), were used to remove RB19 from the artificial sewage stream. Removal of RB19 with a concentration of 20 mg/L with variable pH (5, 7, and 9), oxidant concentrations (5, 10, and 20 mg/L), and time (5, 10, 15, and 30 min) were investigated during the processes of photolysis, chemical oxidation (by H₂O₂ and NaOCl), and UV/NaOCl and UV/H₂O₂ AOPs. The photolysis process did not remove the RB19. The highest removal efficiencies of RB19 by chemical oxidation processes with NaOCl and H₂O₂, UV/NaOCl, and UV/H₂O₂ at optimal conditions (pH = 5, [oxidant] = 20 mg/L, RB19 = 20 mg/L, and radiation intensity of 1005 mJ/cm²) were 64.49%, 0.88%, 99.7%, and 13.31%, respectively. These results indicate that the hydroxyl radical was produced, under optimum conditions, more in the acidic medium; thus, the RB19 removal efficiency was higher in the acidic medium. The combination of UV rays with oxidants resulted in the production of more hydroxyl radicals and increased removal efficiency.     

Desulfurization of coke oven gas from the coking of coking coal blended with a sorbent and waste plastic

A new way to implement the simultaneous reutilization of solid waste, the desulfurization of coke oven gas (COG), and even the desulfurization of coke by the co-coking of coking coal (CC) and waste plastic (WP) blended with a sorbent is proposed; the evolution of H₂S and the removal efficiency of H₂S from COG during the co-coking process were investigated in a lab-scale cylindrical reactor. The experimental results indicated that for the coking of CC blended with ZnO, Fe₂O₃, or blast furnace dust (BFD) as a sorbent, the instantaneous concentration of H₂S in COG was lower than 500 mg/m³ (which meets the technical specification requirement of the Chinese Cleaner Production Standard–Coking Industry, HJ/T 126-2003) when the molar ratio between the key component of the sorbent and the volatile S in CC or the CC/WP blend, n _Zn+Fe/n _S, was about 1.2 for ZnO and Fe₂O₃, but not for BFD under the same conditions, suggesting that ZnO and Fe₂O₃ are promising sorbents, but that BFD must be treated chemical or thermally before being used as a sorbent because of the size and complicated nature of the influence of its phase/chemical composition on its desulfurization ability. However, for the co-coking of CC and WP blended with ZnO as a sorbent, n _Zn+Fe/n _S must increase to 1.4 and 1.7 for 100/2 and 100/5 blends of CC/WP, respectively, to ensure a satisfactory efficiency for H₂S removal from COG. Part of this paper was presented at the International Symposium on EcoTopia Science 2005 (ISET05), Aug 8–9, 2005, Chikusa-ku, Nagoya, Japan     

Stabilization of sewage sludge in the presence of nanoscale zero-valent iron (nZVI): abatement of odor and improvement of biogas production

The potential benefits of nanoscale zero-valent iron (nZVI) on sludge stabilization, either the abatement of odor or the improvement of biogas production, were investigated in this study. Two commercial-grade microscale iron powders were also utilized for comparison. Adding 0.10 wt% of nZVI in sludge during anaerobic incubation significantly reduced the concentration of H₂S in biogas by 98.0 % (96.2–98.9 %), probably attributed by reactions between sulfides and the neo-formed hydrous Fe(II)/Fe(III) oxides layer at the surface of ZVI nanoparticles. Meanwhile, the percentage of P in bioavailable fractions decreased from 76.8 to 52.5 %, possibly due to the formation of vivianite [Fe₃(PO₄)₂]. Furthermore, 0.10 wt% of nZVI in anaerobic digestion for 17 days enhanced the concentration of CH₄ in biogas by 5.1–13.2 % and improved the production of biogas and methane by 30.4 and 40.4 %, respectively. The amendment of iron nanoparticles during anaerobic digestion can not only effectively reduce H₂S in biogas, but also potentially boost methane production significantly.     

Attenuation of hydrogen sulfide at construction and demolition debris landfills using alternative cover materials

The attenuation of H₂S emissions by various landfill cover materials was evaluated using both laboratory and field experiments. The results demonstrated that cover materials consisting of selected waste products (compost and yard trash) and soils amended with quicklime and calcium carbonate effectively attenuated H₂S emissions and detectable H₂S emissions were only encountered in a testing plot using a sandy soil cover (average emission rate was 4.67 × 10^?6 mg m^?2 s^?1). H₂S concentration profiles in the cover materials indicated that H₂S was removed as it migrated through the cover materials. At the same depth in the testing area, the H₂S concentration in the sandy soil field plot was always higher than that of other testing plots because the sand (a) demonstrated less ability to remove H₂S and (b) exhibited a higher H₂S concentration at the base of the cover. Laboratory experiments confirmed these observations, with a combination of physical adsorption, chemical reactions, and biological oxidation, accounting for the enhanced removal. In addition to removal, the results suggest that some of the cover materials reduced H₂S generation by creating less favorable conditions for sulfate-reducing bacteria (e.g., high pH and temperature).     

Photocatalytic Degradation of Poly(Vinyl Alcohol) on Pt/TiO<Subscript>2</Subscript> with Fenton Reagent

In this research Fenton reagent (Fe²⁺/H₂O₂) was investigated as oxidants to degrade poly (vinyl alcohol) (PVA). The role of nano-TiO₂ photocatalyst was discussed as an additive in Fenton reagent (Fe²⁺/H₂O₂). Pt/TiO₂ composites were also synthesized by photo-reaction to be used as additive in Fenton reagent. The rapid degradation of PVA was obtained when Pt/TiO₂ composites served as photocatalyst. The different photocatalytic efficiency of Pt/TiO₂- Fenton reagent (Fe²⁺/H₂O₂) was studied compared with TiO₂- Fenton reagent (Fe²⁺/H₂O₂) during the degradation of PVA.     

Photochemical Degradation of Aqueous Polyvinyl Alcohol in a Continuous UV/H<Subscript>2</Subscript>O<Subscript>2</Subscript> Process: Experimental and Statistical Analysis

Polyvinyl alcohol (PVA), being a dominant contributor of total organic carbon (TOC) in textile wastewater, is not easily degradable by conventional methods of wastewater treatment. This study investigates the degradation of aqueous PVA in a continuous UV/H₂O₂ photoreactor since the feeding strategy of hydrogen peroxide proves to have considerable effects on the process performance. Response surface methodology involving the Box–Behnken method is adopted for the experimental design to study the effects of operating parameters on the process performance. Experimental analysis shows that the TOC removal varies from 16.11 to 42.70 % along with a reduction of the PVA molecular weights from 56.7 to 95.3 %. The TOC removal is significantly lower than the molecular weight reduction due to the generation of the intermediate products during oxidation. Operating the UV/H₂O₂ process in a continuous mode facilitates the degradation of highly concentrated polymeric solutions using a relatively small hydrogen peroxide concentration in the feed with a small residence time ranges from 6.13 to 18.4 min.     

Effect of MgO and CaO/SiO2 on the immobilization of chromium in synthetic slags

This work investigated the chemical and mineralogical properties of CaO–SiO₂–Cr₂O₃–CaF₂–MgO slags. Synthetic slags were prepared and the effect of the slag basicity (mass ratio CaO/SiO₂) and MgO contents on the stability of the mineralogical species formed was analyzed. The morphology and composition of the slags were analyzed by X-ray powder diffraction (XRD) and scanning electron microscope-energy dispersive spectroscopy (SEM–EDS), whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that in slags with CaO/SiO₂?=?1, the main Cr-compound was MgCr₂O₄ spinel, which forms octahedron crystals. Small amounts of CaCr₂O₄ and CaCrO₄ were also observed. It was found that with increasing the slag basicity from 1 to 2 the compounds MgCr₂O₄ and CaCr₂O₄ were formed together with the Cr(V)-containing compound complex Ca₅(CrO₄)₃F which forms hexagonal crystals. The results showed that the highest Cr concentration levels in the leaching liquors corresponded to slags with CaO/SiO₂?=?2, probably owing to the formation of CaCrO₄ and Ca₅(CrO₄)₃F, whilst the lowest chromium concentration levels corresponded to MgO-based slags owing to the stable binding of chromium in spinel with MgO. Additionally, potential–pH diagrams for the Ca–Cr–H₂O and Mg–Cr–H₂O systems at 25?°C were calculated.     

Removal of H2S using molten carbonate at high temperature

Gasification is considered to be an effective process for energy conversion from various sources such as coal, biomass, and waste. Cleanup of the hot syngas produced by such a process may improve the thermal efficiency of the overall gasification system. Therefore, the cleanup of hot syngas from biomass gasification using molten carbonate is investigated in bench-scale tests. Molten carbonate acts as an absorbent during desulfurization and dechlorination and as a thermal catalyst for tar cracking. In this study, the performance of molten carbonate for removing H₂S was evaluated. The temperature of the molten carbonate was set within the range from 800 to 1000 °C. It is found that the removal of H₂S is significantly affected by the concentration of CO₂ in the syngas. When only a small percentage of CO₂ is present, desulfurization using molten carbonate is inadequate. However, when carbon elements, such as char and tar, are continuously supplied, H₂S removal can be maintained at a high level.To confirm the performance of the molten carbonate gas-cleaning system, purified biogas was used as a fuel in power generation tests with a molten carbonate fuel cell (MCFC). The fuel cell is a high-performance sensor for detecting gaseous impurities. When purified gas from a gas-cleaning reactor was continuously supplied to the fuel cell, the cell voltage remained stable. Thus, the molten carbonate gas-cleaning reactor was found to afford good gas-cleaning performance.     

Effect of the mixing ratio on the composting of OFMSW digestate: assessment of compost quality

This study presents the results obtained in compostability tests of organic fraction of municipal solid waste (OFMSW) digestate. The final aim was to obtain mature compost without phytotoxic effects. For the evaluation of the composting process, a novel parameter describing the performance of the composting process, the relative heat generation standardized with the initial volatile solid content (RHGVS₀), was defined and evaluated at laboratory-scale. From these laboratory-scale test, the optimum operational conditions were obtained, a mixing ratio (v/v) of 1:1:0 (bulking agent:digestate:co-substrate) and with 15% of mature compost as inoculum. Subsequently, these optimum operational conditions were applied in the active phase of the composting pilot-scale reactor. The active composting stage took 7 days, subsequently a curing phase of 60 days was carried out at ambient conditions. After 30 days of curing, the mature compost showed a specific oxygen uptake rate (SOUR) of 0.14 mg O₂/g VS·h, a germination index (GI) of 99.63% and a low volatile fatty acids (VFA) concentration (41.3 AcH mg/kg_dm), being indicative of the good compost stability and maturity of the compost. The very good quality of the final compost obtained indicated that the RHGVS₀ accurately describes the performance of the composting process.

     

Groundwater treatment and aquatic toxicity reduction at a chemical production site

Contaminated groundwater at a chemical antioxidant and phenolic resin chemical production site was subjected to treatability studies to develop design criteria for surface water discharge. Raw groundwater required pretreatment for total suspended solids (TSS) and color removal prior to treatment by ultraviolet light/hydrogen peroxide (UV/H₂O₂). Because of high capital and operating costs for UV/H₂O₂, biological treatment was evaluated as an alternate. Respirometric analyses showed that completely mixed activated sludge could be applied as a treatment technology to the groundwater. Biotreatment resulted in an approximately 70 percent reduction in soluble chemical oxygen demand (SCOD). Residual SCOD was recalcitrant to further biodegradation. The treated effluent was tested for aquatic toxicity using fathead minnows (Pimephales promelas) and Ceriodaphnia dubia and was found to be toxic. Toxicity reduction of biotreatment effluent was evaluated in bench-scale experiments using activated carbon adsorption, filtration, and UV/H₂O₂. Subsequent toxicity testing showed that filtration alone could reduce the bioeffluent toxicity and that residual SCOD was not the primary source of toxicity.     

Effect of temperature on bacterial emissions in composting of swine manure

Swine manure was subjected to laboratory scale composting in order to quantify bioaerosols, i.e., airborne culturable bacteria and endotoxin, in the exhaust gas, which provided details on the effect of temperature on bacterial emissions. The concentration of airborne bacteria reached 31,250 colony-forming units (CFU)/m³ during the thermophilic stage of composting, and positively correlated with the temperature profile of the compost pile. Initially, the endotoxin concentration was 1820 endotoxin units (EU)/m³, but it decreased exponentially as the composting process proceeded. The temperature can be an excellent indicator of bacterial emissions during the composting process, indicating that the composting process requires a consistently high temperature to ensure sanitization of both compost and bacterial emissions. The cumulative emission data showed that emission factors was 11.2?13.5 CFU/g dry swine manure and that of endotoxin was 0.5?0.9 EU/g dry swine manure. The bacterial diversity in the bioaerosol was analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis, revealing the presence of various gram-negative bacterial consortia.     

Greenhouse gas emissions from home composting of organic household waste

The emission of greenhouse gases (GHGs) is a potential environmental disadvantage of home composting. Because of a lack of reliable GHG emission data, a comprehensive experimental home composting system was set up. The system consisted of six composting units, and a static flux chamber method was used to measure and quantify the GHG emissions for one year composting of organic household waste (OHW). The average OHW input in the six composting units was 2.6–3.5 kg week^?1 and the temperature inside the composting units was in all cases only a few degrees (2–10 °C) higher than the ambient temperature. The emissions of methane (CH₄) and nitrous oxide (N₂O) were quantified as 0.4–4.2 kg CH₄ Mg^?1 input wet waste (ww) and 0.30–0.55 kg N₂O Mg^?1 ww, depending on the mixing frequency. This corresponds to emission factors (EFs) (including only CH₄ and N₂O emissions) of 100–239 kg CO₂-eq. Mg^?1 ww. Composting units exposed to weekly mixing had the highest EFs, whereas the units with no mixing during the entire year had the lowest emissions. In addition to the higher emission from the frequently mixed units, there was also an instant release of CH₄ during mixing which was estimated to 8–12% of the total CH₄ emissions. Experiments with higher loads of OHW (up to 20 kg every fortnight) entailed a higher emission and significantly increased overall EFs (in kg substance per Mg^?1 ww). However, the temperature development did not change significantly. The GHG emissions (in kg CO₂-eq. Mg^?1 ww) from home composting of OHW were found to be in the same order of magnitude as for centralised composting plants.     

Laboratory studies on the remediation of mercury contaminated soils

Mercury, in contrast to other toxic metals, cycles between the atmosphere, land, and water. During this cycle, it undergoes a series of complex chemical and physical transformations. Because of these transformations, it is found in the environment not only as simple inorganic and organic compounds, but also as complex compounds. As a result, it is difficult to remediate mercury contaminated materials. Laboratory studies were conducted with a mercury contaminated complex waste from an industrial site to evaluate the ability of extractants such as H₂O₂, H₂SO₄ and Na₂S₂O₃ to decontaminate the waste. Up to 87 percent of the total mercury present in the waste was extracted. Mercury was recovered as insoluble mercury sulfide by adding Na₂S solution to the combined filtrates from the H₂O₂ + H₂SO₄ and Na₂S₂O₃ treatment steps. The technique described in this article is capable of recovering mercury in a usable form and can be used as a pretreatment to remediate mercury contaminated waste before laud disposal.     

Treatment of municipal landfill leachate by catalytic wet air oxidation: Assessment of the role of operating parameters by factorial design

The wet air oxidation (WAO) of municipal landfill leachate catalyzed by cupric ions and promoted by hydrogen peroxide was investigated. The effect of operating conditions such as WAO treatment time (15-30 min), temperature (160-200 °C), Cu²⁺ concentration (250-750 mg L⁻¹) and H₂O₂ concentration (0-1500 mg L⁻¹) on chemical oxygen demand (COD) removal was investigated by factorial design considering a two-stage, sequential process comprising the heating-up of the reactor and the actual WAO. The leachate, at an initial COD of 4920 mg L⁻¹, was acidified to pH 3 leading to 31% COD decrease presumably due to the coagulation/precipitation of colloidal and other organic matter. During the 45 min long heating-up period of the WAO reactor under an inert atmosphere, COD removal values up to 35% (based on the initial COD value) were recorded as a result of the catalytic decomposition of H₂O₂ to reactive hydroxyl radicals. WAO at 2.5 MPa oxygen partial pressure advanced treatment further; for example, 22 min of oxidation at 200 °C, 250 mg L⁻¹ Cu²⁺ and 0-1500 mg L⁻¹ H₂O₂ resulted in an overall (i.e. including acidification and heating-up) COD reduction of 78%. Amongst the operating variables in question, temperature had the strongest influence on both the heating-up and WAO stages, while H₂O₂ concentration strongly affected the former and reaction time the latter. Nonetheless, the effects of temperature and H₂O₂ concentration were found to depend on the concentration levels of catalyst as suggested by the significance of their 3rd order interaction term.