This paper describes a preliminary evaluation of the performance of carbonaceous materials prepared from sewage sludges (SBCMs) in a hybrid water treatment process based on adsorption and catalytic wet air oxidation; phenol was used as the model pollutant. Three different sewage sludges were treated by either carbonisation or steam activation, and the physico-chemical properties of the resultant carbonaceous materials (e.g. hardness, BET surface area, ash and elemental content, surface chemistry) were evaluated and compared with a commercial reference activated carbon (PICA F22). The adsorption capacity for phenol of the SBCMs was greater than suggested by their BET surface area, but less than F22; a steam activated, dewatered raw sludge (SA_DRAW) had the greatest adsorption capacity of the SBCMs in the investigated range of concentrations (<0.05 mol L(-1)). In batch oxidation tests, the SBCMs demonstrated catalytic behaviour arising from their substrate adsorptivity and metal content. Recycling of SA_DRAW in successive oxidations led to significant structural attrition and a hardened SA_DRAW was evaluated, but found to be unsatisfactory during the oxidation step. In a combined adsorption-oxidation sequence, both the PICA carbon and a selected SBCM showed deterioration in phenol adsorption after oxidative regeneration, but a steady state performance was reached after 2 or 3 cycles.
A new sequential process -AD-OX -for (post)treatment of water polluted by poorly biodegradable organic compounds has been investigated. It is based on hybridizing classical adsorption on a fixed bed of activated carbon (AC) followed by batch wet catalytic oxidation at higher temperature and pressure on the same bed of AC, which is then regenerated in situ. The basic idea is to take advantage of both operations: 1-efficient water purification at room temperature by adsorption, 2-effective concentrated pollutant degradation by batch air oxidation achieving simultaneously AC regeneration.A small fixed bed reactor, 10 mm diameter, 0.18 m long, filled with granular activated carbon and equipped with convenient 3 way valves, may achieve successively the two main steps. Several AD-OX cycles have been performed with a phenol solution to quantify the regeneration of the activated carbon adsorption capacity. Results show that activated carbon is fast damaged during the first cycles, due to oxidative coupling, but then a quasi steady state is obtained proving that significant oxidative regeneration has been achieved.A dynamic model of the adsorption step has been first developed, including intraparticle diffusion, liquidsolid external mass transfer and axial dispersion of the liquid phase. It has been applied to simulate the performance of the regenerated activated carbon.Using oxidation kinetics over this aged carbon and its adsorption isotherm, separately determined in autoclave at reaction temperature, the oxidation step after several cycles has been simulated including the heating period, where desorption and oxidation simultaneously occur. The proposed model conveniently predicts the complex phenol concentration-time profile and gives insight to the hydrodynamic behavior of the recycle reactor and the role of mass transfer resistances.
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