WET AIR OXIDATION FOR INDUSTRIAL WASTEWATER AND SLUDGE TREATMENT: FIRST RESULTS OF A NEW RESEARCH PROGRAM IN QUEBEC

Jean-François Vermette, Centre de transfert technologique en écologie industrielle [email protected] Sophie Girard, Centre de transfert technologique en écologie industrielle Patrick Desjardins, Cégep de Sorel-Tracy

Key words: Wet air oxidation, supercritical fluids, wastewater, sludge, green process

Introduction Sub-critical technologies, in particular wet air oxidation (WAO), have been developing across Europe in recent decades. This process is now used at the industrial scale for the effective treatment of wet organic material, mainly sludge from wastewater treatment plants (WWTP), and is done with energy self-sufficiency [1, 2]. In WAO, hot pressurised water (200-350 ˚C, 30-200 bars) is enriched with or air to initiate exothermic oxidation of organic material, mostly into CO2 and H2O. Since the operating temperature is much lower than typical , no gaseous emissions of NOX, dioxins or furans are generated. Water is being kept in a liquid state due to high pressure, avoiding energy losses by vaporisation. These characteristics allow WAO to be a competitive and environmental friendly process for a variety of wet organic wastes and chemical effluents. However, WAO is still non-existent in the province of Quebec (Canada) and unknown to industries.

In 2014, Centre de transfert technologique en écologie industrielle (CTTÉI), an applied research center in , acquired a WAO laboratory unit and launched the first research program in this field in the province of Quebec. This program, financed by NSERC (Natural Sciences and Engineering Research Council of Canada), Produits Chimiques Magnus and the City of Sorel-Tracy, aims to study the potential of this technology on various industrial wastes in Quebec and facilitate its transfer to industries. CTTÉI is also supported by European partners Innovation Fluides Supercritiques (IFS) and Aix-Marseille University.

After the first year of work, numerous WAO laboratory experiments were conducted on aerated lagoon sludge, waste latex paint, surfactants, lubricants and solvents from the chemical industry. An overview of these results as well as the perspectives for WAO deployment in Quebec will be presented.

Methodology CTTEI’s WAO laboratory equipment was custom-built by TOP Industrie (150 ml batch reactor, operating range up to 300 bars/350 °C, passivated inox 316 Ti alloy) (Fig. 1).In a standard oxidation procedure, the reactor is first filled with sludge or wastewater. Deionised water is also added to obtain an initial (COD) of 5000 to 10 000 mg/L. The reactor is then inserted with N2(g) and heated to desired temperature (200 – 300 °C). When the temperature is reached, the required amount of air is added (20% to 100% excess of the stoichiometric amount for complete oxidation). The reaction is monitored over time by taking liquid samples (≈ 1 mL) for COD analysis (Hach 435 High Range method, absorbance measurement at 320 nm on DR1900 spectrophotometer). Some oxidation essays were made in triplicate to assess the reproducibility of the method. - - - 2- 3- 2- 3- Analysis for anions (Cl , F , Br , SO4 , PO4 , NO , NO and acetate) were conducted by ion chromatography (IC) on selected samples, with a Dionex ICS-1100 and a IonPac AS9-HC column. Some samples taken before and after WAO treatment were also sent to external laboratories for DBO5, NTK, NH4 and Phosphorus analysis. 2+ In some cases, the effect of both homogeneous catalyst (Cu as CuSO4) and heterogeneous catalyst (MnO2) were investigated.

Figure 1 : CTTÉI’s wet air oxidation laboratory unit

Results – Aerated lagoon sludge A literature review enlightened the fact that municipal sludge treatment by WAO is the subject of several studies, mostly focusing on activated sludge processes [3,4]. However, no study was found on WAO treatment of sludge from aerated lagoons, which is a process used by many cities in Quebec for the treatment of municipal wastewater. The sludge generated is difficult to recycle due to its high water content (90 – 95 %), its water retention properties (costly dewatering), very low calorific value and various contaminants (ex.: sulphur, trace metals, chlorides and emerging organic pollutants like PAHs, hormones and pharmaceuticals) [5]. The City of Sorel-Tracy will soon have to dispose of ≈ 30 000 m3/y of such sludge by conventional technologies: dewatering followed by landfilling or incineration. These solutions are very costly, both economically and environmentally, and WAO could be a viable alternative.

An experimental design was conducted on Sorel-Tracy lagoon sludge to investigate the effects of pressure and temperature on WAO treatment efficiency, with other variables fixed (oxygen stoichiometry, agitation speed, sludge concentration, treatment duration). A summary of the results obtained, expressed as COD reduction efficiency, is presented in Fig. 2.

Figure 2 : Treatment of lagoon sludge by WAO

210°C - 100 bars

5000 210°C - 175 bars 240°C - 100 bars 270°C - 100 bars 270°C - 175 bars 4000 Initial diluted sludges: 300°C - 100 bars 5200 mg/L 300°C - 175 bars

3000 (mg/L)

2000 COD samples COD of sludge 210°C

240°C

1000 270°C

300°C

0 0 10 20 30 40 50 60 70 80 90

Reaction time (min)

The results show that temperature has a major effect on COD removal by wet air oxidation, while pressure has no significant effect in the range studied (100 – 175 bars). The solids remaining in the reactor after each WAO run were also analysed and showed a 98 to 99 % reduction in terms of total organic compounds, indicating a near complete dissolution and destruction of all organic matter. The biodegradability index (COD/BOD5 ratio) of the treated liquid vs the initial sludge is also greatly enhanced, dropping from 52,1 to 1,58. This indicates that the treated liquid, which is rich in ammonia and acetic acid, could be returned to the lagoons for efficient biological treatment. IC results from experiments at 300°C showed a final concentration in acetic acid of about 670 mg/L, which accounts for ≈ 66% of the residual COD. The results also showed that phosphorus, a key parameter in wastewater treatment, is mostly recovered in the solid phase after WAO treatment (14 000 mg/kg in the solid after treatment), and thus would be removed from the lagoons and could be recycled.

Results – Mix of chemicals Produits Chimiques Magnus (Magnus) is a manufacturer of equipment and chemicals for water treatment and industrial fluids since 1946, located in Boucherville, Quebec. Their industrial activities generate ≈ 10 000 L/week of wastewater that is too concentrated in various chemicals to be disposed of through the municipal sewage system. This effluent contains ≈ 95% water contaminated with various organic solvents, surfactants, oils and refrigerants. Several experiments were conducted to assess the technical potential and limitations of WAO process on their wastewater.

Figure 3 presents a series of experiments conducted on an aqueous mix of 6 common products used by Magnus and their industrial clients (surfactants, lubricants and organic solvents).The initial concentration of the mix was 10 000 mg/L of COD. The molar mass of the 6 compounds was ranging from 76 to 600 g/mol, mostly composed of hydrocarbons with oxygenated functions, with some amounts of nitrogen, sulphur, phosphorus and chlorinated compounds. The WAO experiments were conducted at 300°C and 150 bars, with and without addition of catalysts. Figure 3 : Treatment of a mix of chemicals by WAO, with and without catalysts

10000 Initial sample : ≈ 10 000 mg/L

8000

6000

(mg/L) Without catalyser 4000

COD COD of samples With CuSO4 With MnO

2000 1 550

2 1 280 810 0 0 10 20 30 40 50 60 Reaction time (min)

The results indicate an interesting potential for treatment of a broad range of chemical products by WAO, since the experiments were conducted on a complex mixture as could be found in the industry. At 300°C – 150 bars and without any catalyst, the performance of the process reached about 85% in terms of COD reduction, after 60 min. IC analysis showed that acetic acid accounted for 63% of the remaining COD. The effect of adding a small amount of catalyst was found to be beneficial, particularly in the case of CuSO4 where a performance > 91% was observed.

Conclusion and perspectives WAO shows promising performances for the treatment of municipal lagoon sludge and a mix of common chemical products. The results were transferred to CTTÉI’s industrial partners so they could assess the technical and economic potential of WAO. This waste treatment process could benefit other industrial fields such as pulp and paper mills, the pharmaceutical industry, the oil industry, etc. Further laboratory experiments, as well as economic analysis and energy efficiency studies will be necessary on each individual case in order to validate the relevance of this process as compared to conventional technologies. It is expected that in many cases, WAO will be less expensive than usual disposal routes (ex.: incineration, dewatering + landfilling), as well as generating a useful source of energy (hot water or steam) with lesser environmental impacts.

References [1] Debellefontaine, H., J. N. Foussard (2000). Wet air oxidation for the treatment of industrial wastes. Chemical aspects, reactor design and industrial applications in Europe. 20:15-25. [2] Levec J. (2007). Catalytic wet-air oxidation processes: A review. Catalysis Today 124:172–184. [3] Hii K. et al. (2014). A review of wet air oxidation and thermal hydrolysis technologies in sludge treatment. Bioresource Technology 155: 289–299. [4] Baroutian S. et al. (2013). Relative influence of process variables during non-catalytic wet oxidation of municipal sludge. Bioresource Technology 148, p.605–610. [5] Conseil canadien des ministres de l’environnement (2010). Contaminants d’intérêt émergent dans les biosolides : concentrations et impacts des procédés de traitement. Project # CCME 447-2009. Revue de littérature – Rapport final, 11 p.