Geert Lissens

Electrochemical degradation of surfactants by intermediates of water discharge at carbon-based electrodes

The electrochemical oxidation of anionic (sodium dodecylbenzenesulfonate) and cationic (hexadecyltrimethyl ammonium chloride) aqueous dilute surfactant solutions at a BDD (boron-doped diamond) electrode has been studied by batch electrolysis experiments and potentiodynamic measurements. In the potential region of water decomposition (E>2.3 V vs. SHE), surfactants could be deactivated and oxidised with total organic carbon (TOC) removals up to 82% by the action of intermediates of water discharge (e.g. hydroxyl radicals). Of the investigated process parameters, the initial electrolyte pH had the highest impact on surfactant oxidation. An initial pH of 10 significantly enhanced the electrochemical oxidation of both surfactants. The process was not diffusion-controlled and instantaneous current efficiencies (ICE) for TOC removal were in all cases low, varying from 5 to 12% on average. The surfactant deactivation and oxidation potential of the BDD electrode was compared with other carbon-based electrodes. Applying an equal electrode surface, the BDD electrode showed much higher surfactant removals compared to plane graphite. Graphite granules and carbon felt suffered from abrasion, leading to additional carbon loading of the surfactant solutions. Based on the current electrolysis configuration, the specific energy requirement with the BDD electrode for the electrochemical oxidation of surfactants was estimated at 10–20 kW h m−3 effective wastewater.

Advanced Anaerobic Bioconversion of Lignocellulosic Waste for Bioregenerative Life Support following Thermal Water Treatment and Biodegradation by Fibrobacter Succinogenes

The feasibility of nearly-complete conversion of lignocellulosic waste (70% food crops, 20% faecal matter and 10% green algae) into biogas was investigated in the context of a life support project. The treatment comprised a series of processes, i.e., a mesophilic laboratory scale CSTR (continuously stirred tank reactor), an upflow biofilm reactor, a fiber liquefaction reactor employing the rumen bacterium Fibrobacter succinogenes and a hydrothermolysis system in near-critical water. By the one-stage CSTR, a biogas yield of 75% with a specific biogas production of 0.37 l biogas g−1 VSS (volatile suspended solids) added at a RT (hydraulic retention time) of 20–25 d was obtained. Biogas yields could not be increased considerably at higher RT, indicating the depletion of readily available substrate after 25 d. The solids present in the CSTR-effluent were subsequently treated in two ways. Hydrothermal treatment (T ∼ 310–350 °C, p ∼ 240 bar) resulted in effective carbon liquefaction (50–60% without and 83% with carbon dioxide saturation) and complete sanitation of the residue. Application of the cellulolytic Fibrobacter succinogenes converted remaining cellulose contained in the CSTR-effluent into acetate and propionate mainly. Subsequent anaerobic digestion of the hydrothermolysis and the Fibrobacter hydrolysates allowed conversion of 48–60% and 30%, respectively. Thus, the total process yielded biogas corresponding with conversions up to 90% of the original organic matter. It appears that particularly mesophilic digestionin conjunction with hydrothermolysis at near-critical conditions offers interesting features for (nearly) complete and hygienic carbon and energy recovery from human waste in a bioregenerative life support context.

Wet Oxidation Treatment of Organic Household Waste Enriched with Wheat Straw for Simultaneous Saccharification and Fermentation into Ethanol

Organic municipal solid waste enriched with wheat straw was subjected to wet-oxidation as a pre-treatment for subsequent enzymatic conversion and fermentation into bio-ethanol. The effect of temperature (185-195 °C), oxygen pressure (3-12 bar) and sodium carbonate (0-2 g l-1) addition on enzymatic cellulose and hemicellulose convertibility was studied at a constant wet oxidation retention time of 10 minutes. An enzyme convertibility assay at high enzyme loading (25 filter paper unit (FPU) g-1 dry solids (DS) added) showed that up to 78% of the cellulose and up to 68% of the hemicellulose in the treated waste could be converted into respectively hexose and pentose sugars compared to 46% for cellulose and 36% for hemicellulose in the raw waste. For all wet oxidation conditions tested, total carbohydrate recoveries were high (> 89%) and 44-66% of the original lignin could be converted into non-toxic carboxylic acids mainly (2.2-4.5 % on DS basis). Simultaneous saccharification and fermentation (SSF) of the treated waste at 10% DS by Saccharomyces cerevisae yielded average ethanol concentrations of 16.5 to 22 g l-1 for enzyme loadings of 5 and 25 FPU g-1 DS, respectively. The cellulose to ethanol conversion efficiency during SSF was 50, 62, 65 and 70% for a total enzyme loading of 5, 10, 15 and 25 FPU g-1 DS, respectively. Hence, this study shows that wet oxidation is a suitable pre-treatment for the conversion of organic waste carbohydrates into ethanol and that compatible conversion yields (60-65%) can be achieved at moderate enzyme loadings.