• Energy Research

Ammonium recovery using a two chamber microbial fuel cell (MFC) was investigated at high ammonium concentration. Increasing the ammonium concentration (from 0.07 to 4 g ammonium-nitrogen/L) by addition of ammonium chloride did not affect the performance of the MFC. The obtained current densities by DC-voltammetry were higher than 6A/m(2) for both operated MFCs. Also continuous operation at lower external resistance (250 Ω) showed an increased current density (0.9A/m(2)). Effective ammonium recovery can be achieved by migrational ion flux through the cation exchange membrane to the cathode chamber, driven by the electron production from degradation of organic substrate. The charge transport was proportional to the concentration of ions. Nonetheless, a concentration gradient will influence the charge transport. Furthermore, a charge exchange process can influence the charge transport and therefore the recovery of specific ions.

Abstract After dredged sediments have settled in a temporary upland disposal site, ripening starts, which turns waterlogged sediment into aerated soil. Aerobic biological mineralization of organic matter (OM) and chemical oxidation of reduced sulfur compounds are the major biochemical ripening processes. Quantitative data describing these processes are scarce. Therefore, aerobic oxidation and mineralization of five previously anaerobic dredged sediments were studied during a 160-d laboratory incubation experiment at 30°C. A double exponential decay model could adequately describe sulfur oxidation and OM mineralization kinetics. During the first 7 d of incubation, 23 to 80% of the total sulfur was oxidized, after which no further sulfur oxidation was observed. Oxygen used for sulfur oxidation amounted up to 95% of the total oxygen uptake in the first 7 d and up to 45% of the oxygen uptake during the entire incubation period. Mineralization rates of the rapidly mineralizable OM fractions that degraded during the first 14 to 28 d of incubation were 102 to 103 times higher than the mineralization rates of the slowly mineralizable OM during the remaining period. First-order mineralization rates of the slowly mineralizable OM were 0.22 × 10−3 to 0.54 × 10−3 d−1 and can be compared with those of terrestrial soils. Yields of biomass on substrate ranged from 0.08 to 0.45 g Cbiomass/g COM and appeared to be higher for rapidly mineralizing OM than for slowly mineralizing OM. The results of this study can be used to optimize conditions during temporary disposal of sediments, to estimate the potential decrease in OM, and for future studies on the possible link between OM mineralization and degradation of hydrophobic organic contaminants.

Microbial extracellular polymeric substances (EPS) were produced in two membrane bioreactors, each separately treating fresh and saline synthetic wastewater (consisting of glycerol and ethanol), with the purpose of applying them as sustainable bioflocculants. The reactors were operated under nitrogen-rich (COD/N ratios of 5 and 20) and limited (COD/N ratios of 60 and 100) conditions. Under both conditions, high COD removal efficiencies of 87–96% were achieved. However, nitrogen limitation enhanced EPS production, particularly the polysaccharide fraction. The maximum EPS recovery (g EPS−COD/g CODinfluent) from the fresh wastewater was 54% and 36% recovery was obtained from the saline (30 g NaCl/L) wastewater. The biopolymers had molecular weights up to 2.1 MDa and anionic charge densities of 2.3–4.7 meq/g at pH 7. Using kaolin clay suspensions, high flocculation efficiencies of 85–92% turbidity removal were achieved at EPS dosages below 0.5 mg/g clay. Interestingly, EPS produced under saline conditions proved to be better flocculants in a saline environment than the corresponding freshwater EPS in the same environment. The results demonstrate the potential of glycerol/ethanol-rich wastewater, namely biodiesel/ethanol industrial wastewater, as suitable substrates to produce EPS as effective bioflocculants.

A model based on three equations was developed and validated for the design of the inlet and outlet of the Chinese dome digester (CDD) to prevent biogas emission or leakage. The model was implemented in MATLAB software and validated with results from a pilot study. Biogas and temperature data from the pilot experiment were used to validate the model and the results fitted well with the experimental data at low gas volume (G) and the slurry displacement in the expansion chamber, inlet pipe head (h) and head inside the digester (hG). The relationship between the gas and h was not linear at high gas volume (>20 mols). The model can be used to predict the optimal size of the CDD for daily biogas storage based on different reactor and expansion chamber sizes, in order to mitigate the emission of surplus biogas from the inlet and outlet pipes.

Fine particles often create problems in flotation applications. In this article a new laboratory flotation system for the selective separation of small particles was designed and tested. The device contains an active counter current sedimentation that should prevent entrainment of the fine hydrophilic particles. The cell was used to selectively float fine particles in the size range 2-25 mum. To create small bubbles dissolved air was used. The study is linked to the problems that fine particles cause by remediation of soils and sediments. Therefore, small silica and small-oxidized carbon black (MT-OX) particles were used as model system. Three different frothers, sodium dodecylsulfate (SDS), Aerofroth, and Montanol were applied to obtain a stable froth. The results showed that the equipment works excellent to separate the fine MT-OX particles from the small silica particles. Especially with Aerofroth as frother, the Grade of the flotation experiments was extremely high (98.1%). The MT-OX Recovery was best with SDS (74.6%). The new flotation design provides a promising method for the remediation of contaminated sediments and soils. Next to that it offers an interesting option to separate fine particles and powders in other industrial applications.

The Residence Time Distribution (RTD) technique was applied to evaluate mixing of liquid and solid phases in laboratory scale Chinese dome digesters mixed via hydraulic variation. To achieve this purpose, six laboratory scales digesters with different mixing modes and two total solids (TS) concentrations using appropriate tracers were studied over a theoretical hydraulic retention time (HRT) of 30 days. The three different mixing modes were impeller, unmixed and hydraulic mixing, each at influent concentration of ca. 7.5 and 15% TS concentrations. The mode of mixing strongly affected the effective or actual residence time (ta) and directly influenced the percentage of dead zones. The Chinese dome digesters had more dead volumes than the impeller mixed reactors and less than the unmixed reactors. This implied that mixing was more efficient in the impeller mixed reactors followed by the hydraulic mixed reactors and then the unmixed reactors, irrespective of the TS concentration. There was a clear relation between the RTD results and anaerobic digestion performance viz. methane production. There is need to optimize the hydraulic variation in the Chinese dome digester to reduce dead zones while also optimizing the effective residence time (ta).

This study examines the effect of mixing on the performance of anaerobic digestion of cow manure in Chinese dome digesters (CDDs) at ambient temperatures (27–32 °C) in comparison with impeller mixed digesters (STRs) and unmixed digesters (UMDs) at the laboratory scale. The CDD is a type of household digester used in rural and pre-urban areas of developing countries for cooking. They are mixed by hydraulic variation during gas production and gas use. Six digesters (two of each type) were operated at two different influent total solids (TS) concentration, at a hydraulic retention time (HRT) of 30 days for 319 days. The STRs were mixed at 55 rpm, 10 min/hour; the unmixed digesters were not mixed, and the Chinese dome digesters were mixed once a day releasing the stored biogas under pressure. The reactors exhibited different specific biogas production and treatment efficiencies at steady state conditions. The STR 1 exhibited the highest methane (CH4) production and treatment efficiency (volatile solid (VS) reduction), followed by STR 2. The CDDs performed better (10% more methane) than the UMDs, but less (approx. 8%) compared to STRs. The mixing regime via hydraulic variation in the CDD was limited despite a higher volumetric biogas rate and therefore requires optimization.