Automated biological sulphate reduction: a review on mathematical models, monitoring and bioprocess control.

In the sulphate-reducing process, bioprocess control can be used to regulate the competition between microbial groups, to optimize the input of the electron donor and/or to maximize or minimize the production of sulphide. As shown in this review, modelling and monitoring are important tools in the development and application of a bioprocess control strategy. Pre-eminent literature on modelling, monitoring and control of sulphate-reducing processes is reviewed. This paper firstly reviews existing mathematical models for sulphate reduction, focusing on models for biofilms, microbial competition, inhibition and bioreactor dynamics. Secondly, a summary of process monitoring strategies is presented. Special attention is given to in situ sensors for sulphate, sulphide and electron donor concentrations as well as for biomass activity and composition. Finally, the state of the art of the bioprocess control strategies in biological sulphate reduction processes is overviewed.

Cr(III) and Cr(VI) removal from aqueous solutions by cheaply available fruit waste and algal biomass.

This study compared the effectiveness of different biosorbents, viz. materials commonly present in natural treatment systems (Scenedesmus quadricauda and reed) and commonly produced fruit wastes (orange and banana peel) to remove Cr(III) and Cr(VI) from a synthetic wastewater simulating tannery wastewater. The Cr(III) removal efficiency followed the order S. quadricauda>orange peel>banana peel>reed, whereas the Cr(VI) removal followed the order banana peel>S. quadricauda>reed>orange peel. The chromium biosorption kinetics were governed by the intraparticle diffusion mechanism. Isotherm data obtained using the different biosorbents were fitted to the Langmuir, Freundlich, and SIPS models, revealing that the experimental data followed most closely the monolayer sorption theory-based Langmuir model than the other models. The maximum Cr(III) sorption capacity, calculated using the Langmuir model, was found to be 12 and 9 mg/g for S. quadricauda and orange peel, respectively, and the maximum Cr(VI) sorption capacity calculated for banana peel was 3 mg/g. The influence of biosorbent size, pH, solid-liquid ratio, and competing ions were examined for Cr(III) biosorption by S. quadricauda and orange peel and for Cr(VI) sorption by banana peel. The solution pH was found to be the most influential parameter affecting the biosorption process: whereas pH 5 was found to be optimum for maximum removal of Cr(III), Cr(VI) was best removed at a pH as low as 3. Interference to chromium sorption by various ions revealed that Cr(III) binding onto orange peel occurs through electrostatic forces, whereas Cr(VI) binding onto banana peel through non-electrostatic forces.

Release and conversion of ammonia in bioreactor landfill simulators.

Bioreactor landfills are an improvement to normal sanitary landfills, because the waste is stabilised faster and the landfill gas is produced in a shorter period of time in a controlled way, thus enabling CH(4) based energy generation. However, it is still difficult to reach, within 30 years, a safe status of the landfill due to high NH(4)(+) levels (up to 3 g/L) in the leachate and NH(4)(+) is extremely important when defining the closure of landfill sites, due to its potential to pollute aquatic environments and the atmosphere. The effect of environmental conditions (temperature, fresh versus old waste) on the release of NH(4)(+) was assessed in experiments with bench (1 L) and pilot scale (800 L) reactors. The NH(4)(+) release was compared to the release of Cl(-) and BOD in the liquid phase. The different release mechanisms (physical, chemical, biological) of NH(4)(+) and Cl(-) release from the solid into the liquid phase are discussed. The NH(4)(+) level in the liquid phase of the pilot scale reactors starts decreasing after 100 days, which contrasts real-scale observations, where the NH(4)(+) level increases or remains constant. Based on the absence of oxygen in the simulators, the detectable levels of hydrazin and the presence of Anammox bacteria, it is likely that Anammox is involved in the conversion of NH(4)(+) into N(2). Nitrogen release was shown to be governed by physical and biological mechanisms and Anammox bacteria are serious candidates for the nitrogen removal process in bioreactor landfills. These results, combined with carbon removal and improved hydraulics, will accelerate the achievement of environmental sustainability in the landfilling of municipal solid waste.

Anammox: an option for ammonium removal in bioreactor landfills.

Experiments carried out in bioreactor landfill simulators demonstrated that more than 40% of the total N was transferred into the liquid and gas phases during the incubation period of 380 days. Ammonium, an end product of protein degradation and important parameter to consider during landfill closure, tends to accumulate up to inhibitory levels in the leachate of landfills especially in landfills with leachate recirculation. Most efforts to remove ammonium from leachate have been focused on ex situ and partial in situ methods such as nitrification, denitrification and chemical precipitation. Besides minimal contributions from other N-removal processes, Anammox (Anaerobic Ammonium Oxidation) bacteria were found to be active within the simulators. Anammox is considered to be an important contributor to remove N from the solid matrix. However, it was unclear how the necessary nitrite for Anammox metabolism was produced. Moreover, little is known about the nature of residual nitrogen in the waste mass and possible mechanisms to remove it. Intrusion of small quantities of O2 is not only beneficial for the degradation process of municipal solid waste (MSW) in bioreactor landfills but also significant for the development of the Anammox bacteria that contributed to the removal of ammonium. Volatilisation and Anammox activity were the main N removal mechanisms in these pilot-scale simulators. The results of these experiments bring new insights on the behaviour, evolution and fate of nitrogen from solid waste and present the first evidence of the existence of Anammox activity in bioreactor landfill simulators.