C.J. Banks

Trace element requirements for stable food waste digestion at elevated ammonia concentrations.

The work investigated why anaerobic digesters treating food waste and operating at high ammonia concentrations suffer from propionic acid accumulation which may result in process failure. The results showed deficiency of selenium, essential for both propionate oxidation and syntrophic hydrogenotrophic methanogenesis, leads to this while supplementation allows operation at substantially higher organic loading rates (OLR). At high loadings cobalt also becomes limiting, due to its role either in acetate oxidation in a reverse Wood-Ljungdahl or in hydrogenotrophic methanogenesis. Population structure analysis using fluorescent in situ hybridization showed only hydrogenotrophic methanogens. Critical Se and Co concentrations were established as 0.16 and 0.22 mg kg(-1) fresh matter feed at moderate loading. At this dosage the OLR could be raised to 5 g VS l(-1) day(-1) giving specific and volumetric biogas productions of 0.75 m(3) kg(-1) VS(added) and 3.75 STP m(3) m(-3) day(-1), representing a significant increase in process performance and operational stability.

Anaerobic digestion of source-segregated domestic food waste: performance assessment by mass and energy balance.

An anaerobic digester receiving food waste collected mainly from domestic kitchens was monitored over a period of 426 days. During this time information was gathered on the waste input material, the biogas production, and the digestate characteristics. A mass balance accounted for over 90% of the material entering the plant leaving as gaseous or digestate products. A comprehensive energy balance for the same period showed that for each tonne of input material the potential recoverable energy was 405 kWh. Biogas production in the digester was stable at 642 m3 tonne(-1) VS added with a methane content of around 62%. The nitrogen in the food waste input was on average 8.9 kg tonne(-1). This led to a high ammonia concentration in the digester which may have been responsible for the accumulation of volatile fatty acids that was also observed.

Thiol-functionalised mesoporous silica-coated magnetite nanoparticles for high efficiency removal and recovery of Hg from water.

The preparation and testing of thiol-functionalised silica-coated magnetite nanoparticles (TF-SCMNPs) is described. The characteristics of these particles are assessed at different stages in the production process using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), and a magnetometer. The particles were found to be almost spherical with a uniform mesoporous structure with a pore size of ∼2.1nm. The particles were strongly responsive to an external magnetic field making separation from solution possible in less than 1min. The adsorption characteristics of the particles were quantified in a series of isotherm experiments using Hg(II) solution concentrations between 40 and 1000μg l(-1) at adsorbent concentrations of 4 and 8mg l(-1). The adsorption capacity was higher than for other commonly used adsorbents with 90% of Hg(II) removed during the first 5min and equilibrium in less than 15min. Both the Langmuir and Freundlich isotherm models were applied to the isotherm data and the maximum adsorption capacity was achieved when the ratio of adsorbent to adsorbate was low. Both temperature and pH had an effect on adsorption but when the TF-SCMNPs were used for removal of Hg(II) from tap water and bottled water, which contained other ions, there appeared to be no interference. Hg(II) could be successfully desorbed using thiourea in a 3M HCl solution; this did not result in the destruction of the nanoparticles and they could subsequently be reused without loss of their activity in repetitive adsorption tests.

Potential errors in the quantitative evaluation of biogas production in anaerobic digestion processes

Errors that are commonly made in the quantification of biogas from anaerobic digestion experiments were investigated. For liquid displacement gasometers where a barrier solution separates the biogas and the atmosphere, inaccuracy due to gas diffusion was examined experimentally. Acidified saturated saline solution was the most suitable barrier solution, as biogas characteristics changed least with time. Using acidified or tap water caused considerable biogas losses and should therefore be avoided where biogas is stored before measurement. Errors associated with volume calculation from three common liquid displacement gasometer types were investigated theoretically. Corrections that must be made to obtain gas volumes at standard temperature and pressure when using this equipment are discussed. Regarding experimental errors, gasometer designs where displaced liquid is weighed to determine the volume are the most versatile since errors depend mainly upon balance sensitivity. Using liquid heights to calculate volume requires appropriate sizing of the gasometer relative to the volume of gas measured. The calibration of a low flow gas meter was investigated and an approximately linear variation with flow rate was found; hence in situ calibration is advised for this type of instrument. Correction for atmospheric conditions should be performed in real time to reduce errors.

Influence of different aerobic pretreatments on the kinetics of anaerobic digestion of olive mill wastewater

A kinetic study was carried out on the anaerobic digestion of Olive Mill Wastewater (OMW) and OMW that was previously fermented with Geotrichum candidum, Azotobacter chroococcum and Aspergillus terreus. The bioreactor used was batch fed and contained sepiolite as support for the mediating bacteria. Experimental data observed for methane production (G) against time (t) are described by the following equation: G = GM[1 ? exp( ? Axt/S0), over the COD range studied (3.9–14.5 g/l), where: GM is the maximum methane volume obtained at the end of digestion time; S0 is the initial substrate concentration; X is the microorganism concentration and A is the kinetic constant of the process, which was calculated using a nonlinear regression. This kinetic parameter was found to be influenced by the pretreatment carried out, and was 4.6, 4.1 and 2.3 times higher for Aspergillus, Azotobacter and Geotrichum-pretreated OMWs than that obtained in the anaerobic digestion of untreated OMW. The kinetic constant increased when the phenolic compound content and biotoxicity of the pretreated-OMWs decreased. Finally, the yield coefficient, Tp, was 260 (untreated OMW), 300 (Geotrichum-pretreated OMW), 315 (Azotobacter-pretreated OMW) and 350 (Aspergillus-pretreated OMW) ml CH4 STP/g COD.

Anaerobic treatment of palm oil mill effluent in a two stage up-flow anaerobic sludge blanket (UASB) system

A high rate anaerobic treatment of palm oil mill effluent (POME) was achieved in a two-stage up-flow anaerobic sludge blanket (UASB) reactor. The acidogenic reactor acclimated rapidly to the wastewater and was tolerant to a suspended solids (SS) concentration of 5.4 g l?1 in the influent wastewater. Loading was gradually increased over a period of 100 days resulting in a satisfactory hydrolysis and acidification giving a maximum rate of acid production of 4.1 g l?1 d?1 acetic acid at a loading rate of 16.6 g l?1 d?1 COD at a hydraulic retention time of 0.9 days. An increase in alkalinity throughout the acclimatization maintained the effluent from the reactor at around pH 5.8. The methanogenic reactor was initially fed on dilutions of the effluent from the first stage reactor after pH adjustment. The loading was gradually increased, and then stepwise, to 60 g l?1 d?1 COD at which point COD removal efficiency had declined significantly and an accumulation of long-chain volatile fatty acids was observed. It was concluded that the reactor could work efficiently up to loadings of 30 g 1?1 d?1 COD, whilst producing a good methane yield and a COD reduction of greater than 90%. Effluent recirculation alleviated the need for alkali additions to the feed of the methanogenic reactor and a direct coupling of the two reactors was achieved towards the end of the experimental run of 175 days. Both reactors showed granule formation with distinct morphological characteristics; these were observed to be formed after 80 days in the acidogenic reactor and after 110 days in the methanogenic reactor.

Modeling biogas production from organic fraction of MSW co-digested with MSWI ashes in anaerobic bioreactors

This study aims at investigating the effects of MSW incinerator fly ash (FA) and bottom ash (BA) on the anaerobic co-digestion of OFMSW with FA or BA. It also simulates the biogas production from various dosed and control bioreactors. Results showed that suitable ashes addition (FA/MSW 10 and 20 g L(-1) and BA/MSW 100 g L(-1)) could improve the MSW anaerobic digestion and enhance the biogas production rates. FA/MSW 20 g L(-1) bioreactor had the higher biogas production and rate implying the potential option for MSW anaerobic co-digestion. Modeling studies showed that exponential plot simulated better for FA/MSW 10 g L(-1) and control bioreactors while Gaussian plot was applicable for FA/MSW 20 g L(-1) one. Linear and exponential plot of descending limb both simulated better for BA/MSW 100 g L(-1) bioreactor. Modified Gompertz plot showed higher correlation of biogas accumulation than exponential rise to maximum plot for all bioreactors.

Evaluation of carbon dioxide mass transfer in raceway reactors for microalgae culture using flue gases

Mass transfer of CO2 from flue gas was quantified in a 100m(2) raceway. The carbonation sump was operated with and without a baffle at different liquid/gas ratios, with the latter having the greatest influence on CO2 recovery from the flue gas. A rate of mass transfer sufficient to meet the demands of an actively growing algal culture was best achieved by maintaining pH at ∼8. Full optimisation of the process required both pH control and selection of the best liquid/gas flow ratio. A carbon transfer rate of 10gCmin(-1) supporting an algal productivity of 17gm(-2)day(-1) was achieved with only 4% direct loss of CO2 in the sump. 66% of the carbon was incorporated into biomass, while 6% was lost by outgassing and the remainder as dissolved carbon in the liquid phase. Use of a sump baffle required additional power without significantly improving carbon mass transfer.

Impact of the main phenolic compounds of olive mill wastewater (OMW) on the kinetics of acetoclastic methanogenesis

Bench-scale tests were conducted to assess the impact of the most representative phenolic compounds present in olive mill wastewater (OMW), two cinnamic acid derivatives (p-coumaric and caffeic acids) and two benzoic acid derivatives (p-hydroxybenzoic and protocatechuic acids), on the kinetics of acetoclastic methanogenesis. Phenolic compounds were added to cultures transferred from an acetate-enriched seed culture reactor. A control without phenolic compound was included as a basis for comparison. Unacclimated cultures were used to minimize the biodegradation of the toxic organic chemicals during the test. A finite-difference, non-linear, least-squares algorithm was used to estimate kinetic parameters by obtaining a best fit of the experimental data to the classical Monod growth and substrate utilization model. Resulting kinetic coefficients revealed substantial changes in both the maximum rate of acetate conversion, k, and the half-velocity coefficient, Ks, when both cinnamic and benzoic acid derivatives were used. The relative effect of the phenolic compound was manifested in a decrease in the value of k or an increase in the value of Ks as the phenolic compound concentration increased. Therefore, mixed inhibition was occurring. In addition, the toxic effects were clearly related to the molecular structure of the phenolic compound in each pair of toxicants studied, the inhibitory impact being greater for the ortho-diphenols (caffeic and protocatechuic acids) in relation to their corresponding monophenolic compounds (p-coumaric and p-hydroxybenzoic acids, respectively).

Anaerobic digestion of palm oil mill effluent using an up-flow anaerobic sludge blanket reactor

Abstract Anaerobic treatment of palm oil mill effluent (POME) was studied using a 16-litre laboratory scale up-flow anaerobic sludge blanket reactor (UASB) run over a range of influent concentrations from 5.1 to 42.5 g Chemical Oxygen Demand (COD) per litre at a constant hydraulic retention time of four days. Methane production, volatile fatty acid conversion, net sludge growth and Chemical Oxygen Demand reduction were monitored. Over 96% Chemical Oxygen Demand was removed at loadings up to 10.6 g COD l −1 day −1 . At the highest influent concentration reactor instability was observed. Up to this point the results indicated that the UASB could treat POME more effectively than other systems reviewed.