Sulfate radical-based advanced oxidation processes (SR-AOPs) employing heterogeneous catalysts to generate sulfate radical (SO4 −) from peroxymonosulfate (PMS) and persulfate (PS) have been extensively employed for organic contaminant removal in water. This article aims to provide a state–of–the–art review on the recent development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on PMS activation. The hybrid heterogeneous catalyst/PMS systems integrated with other advanced oxidation technologies is also discussed. Several strategies for the identification of principal reactive radicals in SO4 −–oxidation systems are evaluated, namely (i) use of chemical probe or spin trapping agent coupled with analytical tools, and (ii) competitive kinetic approach using selective radical scavengers. The main challenges and mitigation strategies pertinent to the SR-AOPs are identified, which include (i) possible formation of oxyanions and disinfection byproducts, and (ii) dealing with sulfate produced and residual PMS. Potential future applications and research direction of SR-AOPs are proposed. These include (i) novel reactor design for heterogeneous catalytic system based on batch or continuous flow (e.g. completely mixed or plug flow) reactor configuration with catalyst recovery, and (ii) catalytic ceramic membrane incorporating SR-AOPs.

Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects

In seeking greater sustainability in water resources management, wastewater is now being considered more as a resource than as a waste-a resource for water, for plant nutrients, and for energy. Energy, the primary focus of this article, can be obtained from wastewater's organic as well as from its thermal content. Also, using wastewater's nitrogen and P nutrients for plant fertilization, rather than wasting them, helps offset the high energy cost of producing synthetic fertilizers. Microbial fuel cells offer potential for direct biological conversion of wastewater's organic materials into electricity, although significant improvements are needed for this process to be competitive with anaerobic biological conversion of wastewater organics into biogas, a renewable fuel used in electricity generation. Newer membrane processes coupled with complete anaerobic treatment of wastewater offer the potential for wastewater treatment to become a net generator of energy, rather than the large energy consumer that it is today.

Domestic Wastewater Treatment as a Net Energy Producer–Can This be Achieved?

Optimisation of the anaerobic digestion of agricultural resources.

Intensive poultry production generates over 100,000 t of litter annually in West Virginia and 9×106 t nationwide. Current available technological alternatives based on thermophilic anaerobic digestion for residuals treatment are diverse. A modification of the typical continuous stirred tank reactor is a promising process being relatively stable and owing to its capability to manage considerable amounts of residuals at low operational cost. A 40-m3 pilot plant digester was used for performance evaluation considering energy input and methane production. Results suggest some changes to the pilot plant configuration are necessary to reduce power consumption although maximizing biodigester performance.

Macroscopic mass and energy balance of a pilot plant anaerobic bioreactor operated under thermophilic conditions.

Hydrogen production for energy: An overview

From start-up to maximum loading: An approach for methane production in upflow anaerobic sludge blanket reactor fed with the liquid fraction of fruit and vegetable waste.

The heterogeneous character of fixed-film reactors may create highly specialized zones with a stratified distribution of microbial groups and varying capabilities to withstand high organic loads in anaerobic digestion (AD) systems. The microbial distribution and methane-producing potential of biomass from different regions (feeding zone and structured bed) of two second-stage thermophilic (55 ºC) fixed-film reactors were assessed. Three levels of food-to-microorganism (F/M) ratio (0.4, 1.0 and 3.0 g-COD g−1VS) using fermented (two-stage AD) and fresh (single-stage AD) sugarcane molasses were tested in batch reactors, simulating low to high organic loads. Specific methane production rates increased as the F/M increased when using fermented molasses, maintaining efficient methanogenesis at substrate availability levels threefold higher than single-stage schemes (3.0 vs. 1.0 g-COD g−1VS). Success in methane production derived from the homogenous establishment (similar in both feeding zone and bed) of syntrophic associations between acetogens (Pelotomaculum, Syntrophothermus, Syntrophomonas and Thermodesulfovibrio), acetate oxidizers (Thermoacetogenium, Mesotoga and Pseudothermotoga) and hydrogenotrophic methogens (Methanothermobacter and Methanoculleus) replacing acetoclastic methanogens (Methanosaeta). Phase separation under thermophilic conditions was demonstrated to boost methane production from sugar-rich substrates, because the process depends on microbial groups (hydrogenotrophs) that grow faster and are less susceptible to low pH values compared to acetotrophs. 

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Methanogenic consortia from thermophilic molasses-fed structured-bed reactors: microbial characterization and responses to varying food-to-microorganism ratios

Improving the Continuous Multiple Tube Reactor: an Innovative Bioreactor Configuration with Great Potential for Dark Fermentation Processes

AbstractAnaerobic digestion (AD) is a complex biological process widely used to decompose various types of organic matter, as well as to produce some metabolites and biogas. Diverse microorganism groups cooperate in many intricate metabolic routes so that organic matter can be degraded. However, any imbalance on these routes can lead to process instability or even failure. Therefore, a proper monitoring system, as well as a good understanding of the process, are key steps to improve performance and stability. Several mathematical models have been developed to represent AD. Despite this, process monitoring is mostly conducted by analytical methods, whose equipment is either expensive or the analyses are time consuming, which may be a hindrance to low-budget developments. The objective of this study was to develop a low-cost electrochemical sensor to monitor components in wastewater treatment plants. Hundreds of synthetically supplemented sugarcane vinasse and synthetic domestic sewage samples were characterized. The obtained signals were used to calibrate principal component regression, partial least square and artificial neural network estimation models. The predictable variables were chemical oxygen demand, volatile fatty acids, sodium bicarbonate, beef extract, and lipids, and their R2 ranged from 0.84 to 0.99, depending on the component.

Development of a Low-Cost Electrochemical Sensor for Monitoring Components in Wastewater Treatment Processes.

This work evaluates four different physicochemical pretreatments (acidic, thermal, acidic-thermal and thermal-acidic) on an anaerobic inoculum used for alcohol production from acetate and butyrate. All experiments were conducted in single batches using acetate and butyrate as substrates at 30°C and with a pressurized headspace of pure H2 at 2.15 atm (218.2 MPa). Thermal and acidic-thermal pretreatments lead to higher production of both ethanol and butanol. Mathematical modelling shows that the highest attainable concentrations of ethanol and butanol produced were 122 mg L-1 and 97 mg L-1 for the thermal pretreatment (after 17.5 days) and 87 mg L-1 and 143 mg L-1 for the acidic-thermal pretreatment (after 18.9 days). Acetate was produced in all assays. Thermodynamic data indicated that a high H2 partial pressure favoured solventogenic metabolic pathways. Finally, sequencing data showed that both thermal and acidic-thermal pretreatments selected mainly the bacterial genera Pseudomonas, Brevundimonas and Clostridium. The acidic-thermal pretreatment selected a bacterial community more adapted to the conversion of acetate and butyrate into ethanol and butanol, respectively. Thermal-acidic pretreatment was unstable, showing significant variability between replicates. Acidic pretreatment showed the lowest alcohol production.

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Marcelo Zaiat

Papers

From start-up to maximum loading: An approach for methane production in upflow anaerobic sludge blanket reactor fed with the liquid fraction of fruit and vegetable waste.

Methanogenic consortia from thermophilic molasses-fed structured-bed reactors: microbial characterization and responses to varying food-to-microorganism ratios

Improving the Continuous Multiple Tube Reactor: an Innovative Bioreactor Configuration with Great Potential for Dark Fermentation Processes

Development of a Low-Cost Electrochemical Sensor for Monitoring Components in Wastewater Treatment Processes.

Microbial Communities Performing Hydrogen Solventogenic Metabolism of Volatile Fatty Acids