Jan Degrève

Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion

In this work, the influence of a low temperature (70-90 degrees C) thermal treatment on anaerobic digestion is studied. Not only the increase in biogas production is investigated, but attention is also paid to the solubilisation of the main organic (proteins, carbohydrates and volatile fatty acids) and inorganic (heavy metals, S and P) sludge constituents during thermal treatment and the breakdown of the organic components during the subsequent anaerobic digestion. Taking into account the effects of the treatment on the sludge composition is of prime importance to evaluate its influence on the subsequent anaerobic digestion and biogas production using predictive models. It was seen that organic and inorganic compounds are efficiently solubilised during thermal treatment. In general, a higher temperature and a longer treatment time are beneficial for the release. The efficiency of the subsequent anaerobic digestion slightly decreased for sludge pre-treated at 70 degrees C. At higher pre-treatment temperatures, the biogas production increased significantly, up to a factor 11 for the 60 min treatment at 90 degrees C.

Peracetic acid oxidation as an alternative pre-treatment for the anaerobic digestion of waste activated sludge

Anaerobic digestion is generally considered to be an economic and environmentally friendly technology for treating waste activated sludge, but has some limitations, such as the time it takes for the sludge to be digested and also the ineffectiveness of degrading the solids. Various pre-treatment technologies have been suggested to overcome these limitations and to improve the biogas production rate by enhancing the hydrolysis of organic matter. This paper studies the use of peracetic acid for disintegrating sludge as a pre-treatment of anaerobic digestion. It has been proved that this treatment effectively leads to a solubilisation of organic material. A maximum increase in biogas production by 21% is achieved. High dosages of PAA lead to a decrease in biogas production. This is due to the inhibition of the anaerobic micro-organisms by the high VFA-concentrations. The evolution of the various VFAs during digestion is studied and the observed trends support this hypothesis.

Mechanisms of solute rejection in solvent resistant nanofiltration: the effect of solvent on solute rejection.

The separation performance of solvent resistant nanofiltration (SRNF) membranes was studied in a systematic way to elucidate the complex mechanisms involved in rejection of solutes. Rejection of three dyes (Sudan II, Sudan Black, Sudan 408) from common organic solvents (methanol, ethanol, acetone, methyl ethyl ketone, toluene and n-hexane) through a polyimide based SRNF membrane, STARMEM™122, was studied. It was found that the rejection of the STARMEM™122 membrane was lower than that indicated by the manufacturer. The experimental observations for Sudan II were not promising for the rejection study as they were lower than expected. Sudan Black and Sudan 408, which are larger solutes than Sudan II, provided more interesting insights. The effects of the solvent on the membrane and solute were studied separately. A higher permeation rate of ketones and alcohols was observed, while permeabilities of non-polar solvents were low which shows that this membrane shows higher affinity toward semi-polar solvents (alcohols, ketones). The effect of the solvent on the solutes rejection, based on the results for Sudan Black and Sudan 408, was studied for solvents in the same chemical groups, since the membrane showed a similar separation performance for solvents with similar functional groups (e.g. alcohols). The effect of solvent on solute molecular size was investigated by using simulation with Molecular Dynamics. It was shown that the effective size of a molecule is dependent on the solvent due to solvation and hydration of the solute by the solvent. The size of the solute in the solvent belonging to a similar family was studied separately. It was clear that the rejection was influenced by molecular size of the solute in the same group of solvents. A surprising negative rejection of solutes was achieved for n-hexane. Although solutes in n-hexane have higher volume compared to those in other solvents, the affinity between the solute and membrane increases the solute permeation in the presence of n-hexane. The affinity of solvent and solute for the membrane was investigated by means of solubility parameters for solvents within the same chemical family. In two different systems including two different solvents and one solute (Sudan Black and methanol, Sudan Black and ethanol), lower rejection (in this case for Sudan Black and methanol) was achieved when the solutes have higher affinity toward the solvent. Finally, it was found that in a system comprising the solvent, solute and membrane, interactions between solvent and membrane have much more effect on separation than solvent-solute interactions.

Influence of microwave pre-treatment on sludge solubilization and pilot scale semi-continuous anaerobic digestion

Anaerobic digestion is widely applied for the recovery of energy from waste activated sludge. Pre-treatment methods are of high interest to increase the biodegradability of the sludge and to enhance the digestion efficiency. This paper studies the application of a microwave pre-treatment. An experimental set-up of two pilot scale semi-continuous digesters was used. During a long term experiment, one of the reactors was fed with untreated sludge, while microwave pre-treated sludge (336 kJ/kg sludge) was introduced in the second one. A solid retention time of 20 days was kept during the experiments. (Organic) dry solids, carbohydrates, proteins and volatile fatty acids were monitored during digestion. It was seen that the microwave pre-treatment resulted in an effective solubilization of the organic matter in the sludge. The changes to the sludge composition resulted in an increase in biogas production by 50%, while the methane concentration in both reactors remained stable.

Comparing the influence of low power ultrasonic and microwave pre-treatments on the solubilisation and semi-continuous anaerobic digestion of waste activated sludge.

Anaerobic digestion is a well-known technique for the recovery of energy from waste sludge. Pre-treatment methods are useful tools to improve the biodegradability of the sludge and to enhance the digestion efficiency. In this study, an ultrasound (US) and a microwave (MW) pre-treatment were compared in a long-term digestion experiment, using 3 small pilot scale semi-continuous digesters (SRT=20 days). A specific energy of 96 kJ/kg sludge was applied, hence enabling to compare the effectiveness of both pre-treatment methods towards sludge solubilisation and biogas production enhancement. Total and volatile solids (TS and VS), COD, carbohydrates and proteins were monitored throughout the digestion experiment. It was seen that US was most effective in COD solubilisation. The average biogas increment was 20% for the microwave pre-treatment and 27% for the ultrasonic pre-treatment. However, this additional biogas production did not outweigh the energy consumed by the pre-treatment, leading to a negative energy balance.

Performance of solvent resistant nanofiltration membranes for purification of residual solvent in the pharmaceutical industry: experiments and simulation

This study explores the possibility of developing a sustainable extraction method for use in pharmaceutical production, based on purification with membrane processes. Two types of commercial polymeric organic solvent nanofiltration membranes (StarMem122 and DuraMem150) were selected and tested for their abilities to recover the solvent from a pharmaceutical/solvent mixture (5, 10, 50 mg L−1). Five different pharmaceutical compounds have been selected in this work, namely: Imatinib mesylate, Riluzole, Donepezil HCl, Atenolol and Alprazolam. Solvents tested in the experiment were those used in the manufacturing process, i.e., methanol, ethanol, iso-propanol and ethyl acetate. An acceptable performance (rejection over 90%) was obtained for DuraMem150 in all tested pharmaceutical and solvent mixtures except for iso-propanol. No flux was observed for iso-propanol over the DuraMem150 due to its high viscosity. No separation was observed by using StarMem122 for Imatinib mesylate in iso-propanol (over 80%). Commercially available solvent resistant nanofiltration (SRNF) membranes (StarMem™122 and DuraMem™150) show promising performances as alternative tools to traditional separation units such as distillation columns for the recovery of solvents. Furthermore, to evaluate the potential of SRNF as a substitution for traditional solvent recovery, a model was developed for nanofiltration membrane units and implemented in a common process simulation software (Aspen Plus). These models were based on the pore flow mechanism and describe a single membrane module. A membrane module is not available in Aspen Plus and in its Model Library. In this study, this shortcoming was overcome through implementation of the NF membrane module within the Aspen Custom Modeler link to Aspen Plus. The model has been tested for two model solutes (Disperse orange 3 and Disperse red 19) since the pharmaceutical physical properties are not included in the Aspen Properties Database. The results presented here confirm the value of the Aspen Custom Modeler as a simulation tool for the use of NF as a novel and sustainable tool in pharmaceutical manufacturing.

Simulation of a hybrid pervaporation-distillation process

Abstract This study explores the possibility of simulating a hybrid pervaporation membrane process with the help of Aspen Plus™ (Aspen Tech) flowsheeting. Because Aspen Plus does not contain membrane modules in its Model Library, the pervaporation membrane is simulated within Excel Visual Basic for Applications (VBA). Excel VBA is then linked with Aspen Plus to perform the hybrid simulation. In this way, the user can control the simulation even during the calculations. Case studies, in which industrially relevant hybrid distillation–pervaporation processes are simulated, are used to test the program. First, the dehydration and recycling of ethanol in an industrial plant is looked at, to explore whether an economic improvement can be established with a hybrid process. Secondly, the same is done for the purification of acetic acid in an industrial plant. The results presented here indicate the value of this software as a design tool.

Transport of Binary Mixtures in Pervaporation through a Microporous Silica Membrane: Shortcomings of Fickian Models

Abstract This study explores the applicability of the adsorption‐diffusion mechanism to describe the transport of binary “methanol‐water” and “ethanol‐water” mixtures in pervaporation through a commercial microporous silica membrane. Two different adsorption‐diffusion models are considered: one based on Ficks diffusion equation and another based on the Maxwell‐Stefan formulation. Basic models (Fick) assume concentration independent parameters; more complex models (Maxwell‐Stefan) incorporate flux coupling and other non‐idealities. The influence of feed temperature (40°C–90°C) on permeation flux was analysed in terms of activation energy for flux, permeability and diffusion, and heat of adsorption and vaporization. Also the occurrence of coupling effects was studied by determining the effect of feed composition (entire composition range) on permeation flux, permeability and selectivity. Adsorption‐diffusion models based on Ficks diffusion equation can be used to describe coupling effects if they are modified with concentration dependent diffusion and/or sorption coefficients. They are incapable of describing drag effects by water on alcohols. These drag effects should be modeled through models based on the Maxwell‐Stefan theory.

Removal of PCDD/F from Incinerator Flue Gases by Entrained-Phase Adsorption

Abstract The emission abatement of polychlorinated dioxins and furans (PCDD/F) issued from municipal solid waste incineration is growing in importance because of more stringent emission standards and general concern about their toxic characteristics. These substances cannot be separated by conventional gas cleanup processes but are successfully removed through adsorption onto carbonaceous materials. The simplest technique is the entrained-phase injection of pulverized adsorbents in the flue gas, followed by fabric filter separation. The various related techniques are briefly reviewed here. Operating conditions and results obtained from Flemish MSWIs are given. The results illustrate the excellent overall removal efficiency. Furans are adsorbed to a slightly higher extent than dioxins. PCDD/F removal by carbonaceous adsorbents is thereafter modeled from first principles for the contribution of both entrained-phase (η1) and cake filtration (η2) to the overall efficiency (ηT). Application of the model equations and comparison of measured and predicted overall efficiencies for the Flemish municipal solid waste incinerators (MSWIs) demonstrate that the approach is meaningful and that the dominant parameters are the operating temperature, the dosage and activity of adsorbent, and the fraction of adsorbent in the filter cake. The model equations enable the MSWI operators to predict the adsorption efficiencies for any combination of operating parameters and to assess the sensitivity of the process to varying operating conditions.

Pervaporation of Binary and Ternary Mixtures of Water with Methanol and/or Ethanol

Abstract This article explores water, methanol, and ethanol transport through hydrophilic membranes, by measuring the flux and separation factor in pervaporation as a function of the feed composition for binary water/methanol and water/ethanol mixtures. Additionally, the influence of adding a third component to the feed is examined. Because the solution‐diffusion model is considered to be the basic reference model for the description of transport through pervaporation membranes, it was used for understanding differences in transport characteristics. It was found that the feed composition has a large influence on flux and selectivity of all components. The results show that for a given feed concentration, the permeability is different for binary and ternary mixtures. Permeability thus depends also on the other feed components and their concentration, which contradicts the assumptions of the solution‐diffusion model. The deviations from the ideal are explained qualitatively, and may be attributed to the swelling behavior of the membrane, and to differences in size and polarity of the components. Because of the low polarity (and the larger size) of ethanol, ethanol permeation is only possible in the presence of enough water. Furthermore, it was observed for both alcohols that the water–methanol or water–ethanol separation factor in the ternary mixtures was (much) higher than in the binary mixtures, indicating that the presence of a second alcohol in the feed decreases the permeation of both alcohols.