Intensification of lignocellulosic bioethanol production process using continuous double-staged immersed membrane bioreactors.

Processing complexities associated with different lignocellulosic bioethanol production stages have hindered reaching full commercial capacity. Therefore, in this study efforts were made to remediate some issues associated with hydrolysis and fermentation, by the integration of immersed membrane bioreactors (iMBRs) into lignocellulosic bioethanol production process. In this regards, double-staged continuous saccharification-filtration and co-fermentation-filtration of wheat straw slurry was conducted using iMBRs at filtration fluxes up to 51.0 l.m-2.h-1 (LMH). The results showed a stable long-term (264 h) continuous hydrolysis-filtration and fermentation-filtration with effective separation of lignin-rich solids (up to 70% lignin) from hydrolyzed sugars, and separation of yeast cells from bioethanol stream at an exceptional filtration performance at 21.9 LMH. Moreover, the effect of factors such as filtration flux, medium quality and backwashing on fouling and cake-layer formation was studied. The results confirmed the process intensification potentials of iMBRs in tackling commonly faced technical obstacles in lignocellulosic bioethanol production.

Selective carboxylate production by controlling hydrogen, carbon dioxide and substrate concentrations in mixed culture fermentation.

This research demonstrated the selective production of n-butyrate from mixed culture by applying 2 bar carbon dioxide into the headspace of batch fermenters or by increasing the initial substrate concentration. The effect of increasing initial substrate concentration was investigated at 8, 13.5 and 23 g COD/L with potato processing waste stream. Within 1 week of incubation, n-butyrate fraction selectively increased up to 83% by applying 2 bar hydrogen or 78% by applying carbon dioxide into the headspace whereas it was only 59% in the control reactor. Although the fraction of n-butyrate was elevated, the concentration remained lower than in the control. Both the highest concentration and fraction of n-butyrate were observed under the highest initial substrate concentration without headspace addition. The concentration was 10 g COD/L with 73% fraction. The operational conditions obtained from batch experiments for selective n-butyrate production were validated in a continuous process.

Effect of hydrogen and carbon dioxide on carboxylic acids patterns in mixed culture fermentation.

This study investigated the carboxylate spectrum from mixed culture fermentation of three organic waste streams after supplying 2 bar hydrogen and carbon dioxide or a mixture of these two gases to the headspace. Under any modified headspace, propionate production was ceased and butyrate, caproate and the total carboxylate concentrations were higher than in the reactors with N(2) headspace (control). Production of one major compound was achieved under hydrogen and carbon dioxide mixed headspace after 4 weeks of incubation. Both the highest acetate concentration (17.4 g COD/l) and the highest fraction (87%) were observed in reactors with mixed hydrogen and carbon dioxide headspace independent of the substrate used. In the control reactor, acetate made up maximum 67% of the total products. For other products, the highest concentration and fraction were seldom observed together. Selective butyrate production reaching a 75% fraction was found under the carbon dioxide headspace on the carbohydrate rich waste.

Modelling the degradation of micropollutants in wastewater: parameter estimation and application to pilot (laboratory-scale) MBR data in the case of 2,6-NDSA and BTSA.

The paper summarises the definition of an extended biokinetic model dedicated to micropollutant degradation in wastewater treatment and the parameter estimation methodology for this model. Additionally it describes results on experimental parameter estimation for two target micropollutants, naphthalene disulfonate (2,6-NDSA) and benzothiazole sulfonic acid (BTSA). Subsequently the parameterised model is applied to real operational data from two laboratory-scale (MBR) installations. The work presents the full chain of theoretical model development, model analysis and practical application to case study data for micropollutants.

Recalcitrant COD degradation by an integrated system of ozonation and membrane bioreactor.

In order to treat wastewater to a low residual COD-concentration such as 125 mg/L, classical biological treatment is not sufficient for many types of industry. This research focused on the integrated treatment of the wastewater of the paper industry, with a membrane bioreactor (MBR) and an oxidation step. The optimal configuration was examined. Screening tests with different types of oxidation showed that ozonation after biological treatment could reduce the COD with 40% with an ozone dose of 0.4-0.8g O3/g COD. BOD/COD ratio could be increased up to 0.19. Neither combination of ozone with UV and/or hydrogen peroxide nor the process H2O2/UV or (photo-)Fenton reagents gave any improvement in COD reduction or BOD increase, unless the doses were very high. Based on these results, an integrated system MBR-ozonation was designed, with recirculation of MBR effluent over ozonation. This test showed that reduction of COD up to 125 mg/L immediately behind the MBR required a lot of ozone. A technically feasible solution was to discharge the water after an extra ozonation step, which resulted in a high total ozone dosage. The alternative, the consecutive treatment activated sludge-ozonation-activated sludge, did not give a better COD-removal with the same ozone dose as the integrated concept. The economic evaluation proves that the integrated chemical and biological treatment is expensive for the paper industry if a low discharge limit of COD has to be complied with.

Closing the water loop in a maltery: reuse tests at pilot-scale.

This paper reports on the potential for water reuse in the malting sector. Core unit of a treatment train to close the water loop was a membrane bioreactor (MBR). We compared three different commercial submerged membranes for their fouling potential in this application and related this to the presence of extracellular polymeric substances (EPS). In a second step, we subjected MBR permeate to reverse osmosis and several (advanced) oxidation processes to evaluate the water quality achieved. Finally we performed a set of water reuse tests with waters obtained through different scenarios. The optimal scenario was then tested in a closed water loop over several malting cycles at pilot scale and the effect on water and malt quality was investigated.

Removal of persistent polar pollutants through improved treatment of wastewater effluents (P-THREE).

The EU-project P-THREE started with the establishment of analytical methods for persistent polar pollutants (P3) and quality assurance, followed by screening of P3 in influents and effluents of known wastewater (WW) treatment plants (TP), receiving waters and tap water produced thereof in several European countries. A final selection of analytes for further studies has been performed. Model MBR reactors have been constructed and an optimisation on synthetic wastewater spiked with P3 (linear alkylbenzene sulfonates (LAS), naphthalene sulfonates) has been performed. An initial dynamic modelling of treatment processes has also started. Advanced oxidation process (AOP) treatment has been done in groundwater with isoproturon and nonylphenol ethoxylates (NPEO). The analysis of individual P3 and potential degradation products was also performed. An integrated systeme analysis of the WW treatment processes has also been initiated.

Modelling the degradation of low concentration pollutants in membrane bioreactors.

This paper deals with the development of a description of membrane bioreactor systems, focussing on wastewater treatment and removal of specific low concentration persistent organic pollutants (e.g. persistent polar pollutants--P3 compounds). In the present work we offer a basic model, adapted to Membrane Bioreactor (MBR) conditions, which reflects the biodegradation of recalcitrant (organic) substances. We put a particular focus on organic aromatic pollutants and demonstrate a model parameter estimation approach, with experiments carried out for linear alkylbenzene sulphonates (LAS).

Comparison of linear alkylbenzene sulfonates removal in conventional activated sludge systems and membrane bioreactors.

The potential of a membrane bioreactor (MBR) and a conventional activated sludge (CAS) system to remove polar micropollutants was evaluated using linear alkylbenzene sulfonates (LAS) as model components. Removal efficiencies over 97% were achieved in both reactor systems. The appearance of biological breakdown metabolites and the respirometric response of the sludges to LAS addition indicated that LAS removal was due to biodegradation, rather than sorption phenomena. The effect of operational variables, such as hydraulic retention time, LAS composition and hydrophobicity of the membrane used in the MBR, was negligible in the range tested. A stepwise increase in LAS influent concentration resulted in higher residual effluent concentrations but did not change the procentual removal efficiency. Because an increase in LAS and SPC effluent concentration occurred to a larger extent in the CAS than in the MBR under similar operating conditions, MBRs may turn out to be be more robust with respect to biological degradation of micropollutants than CAS.

Long-range (1)H-(15)N heteronuclear shift correlation at natural abundance: a tool to study benzothiazole biodegradation by two rhodococcus strains.

The biodegradation of benzothiazole and 2-hydroxybenzothiazole by two strains of Rhodococcus was monitored by reversed phase high-pressure liquid chromatography and by (1)H nuclear magnetic resonance (NMR). Both xenobiotics were biotransformed into a hydroxylated derivative of 2-hydroxybenzothiazole by these two strains. The chemical structure of this metabolite was determined by a new NMR methodology: long-range (1)H-(15)N heteronuclear shift correlation without any previous (15)N enrichment of the compound. This powerful NMR tool allowed us to assign the metabolite structure to 2,6-dihydroxybenzothiazole.

Microbial transformations of 2-substituted benzothiazoles.

The occurrence of benzothiazoles in the environment seems to be restricted to aquatic compartments and is mainly associated with the manufacture and use of the rubber additive 2-mercaptobenzothiazole (MBT) and its derivatives. Although data on benzothiazole biotransformations in natural environments at ppb and ppt levels are scarce, the unsubstituted benzothiazole (BT) and 2-hydroxybenzothiazole (OBT) are generally considered to be biodegradable, whereas 2-methylthiobenzothiazole is recalcitrant. The fungicide 2-thiocyanomethylthiobenzothiazole is assumed to be hydrolysed to MBT, which is then further methylated. At higher concentration levels, similar conclusions can generally be drawn. In addition, BT, MBT, 2-aminobenzothiazole and benzothiazole-2-sulphonate can be biodegraded, although side- and end-products may form. For BT and MBT, threshold concentration were reported above which inhibitory effects on biological treatment processes occur. Due to the limited availability of axenic bacterial cultures capable of benzothiazole mineralization, only the initial steps of the degradation pathways have been elucidated so far.

Reductive dehalogenation of trichloroacetic acid by Trichlorobacter thiogenes gen. nov., sp. nov.

A bacterium able to grow via reductive dechlorination of trichloroacetate was isolated from anaerobic soil enrichments. The isolate, designated strain K1, is a member of the delta proteobacteria and is related to other known sulfur and ferric iron reducers. In anaerobic mineral media supplemented with acetate and trichloroacetate, its doubling time was 6 h. Alternative electron donor and acceptors were acetoin and sulfur or fumarate, respectively. Trichloroacetate dehalogenation activity was constitutively present, and the dechlorination product was dichloroacetate and chloride. Trichloroacetate conversion seemed to be coupled to a novel sulfur-sulfide redox cycle, which shuttled electrons from acetate oxidation to trichloroacetate reduction. In view of its unique physiological characteristics, the name Trichlorobacter thiogenes is suggested for strain K1.

Initial transformations in the biodegradation of benzothiazoles by Rhodococcus isolates.

Benzothiazole-2-sulfonate (BTSO3) is one of the side products occurring in 2-mercaptobenzothiazole (MBT) production wastewater. We are the first to isolate an axenic culture capable of BTSO3 degradation. The isolate was identified as a Rhodococcus erythropolis strain and also degraded 2-hydroxybenzothiazole (OBT) and benzothiazole (BT), but not MBT, which was found to inhibit the biodegradation of OBT, BT, and BTSO3. In anaerobic resting cell assays, BTSO3 was transformed into OBT in stoichiometric amounts. Under aerobic conditions, OBT was observed as an intermediate in BT breakdown and an unknown compound transiently accumulated in several assays. This product was identified as a dihydroxybenzothiazole. Benzothiazole degradation pathways seem to converge into OBT, which is then transformed further into the dihydroxy derivative.

2-Mercaptobenzothiazole degradation in laboratory fed-batch systems.

Rubber additives manufacture yields waste-waters with recalcitrant and/or toxic benzothiazole compounds. Biodegradation of such compounds was investigated in fed-batch systems. 2-Mercaptobenzothiazole (MBT) was best degraded by a mixture of MBT-history and non-MBT-history sludge. Concentrations up to 200 mg.l-1 were removed. From 100 mg.l-1 onwards, high percentages of the recalcitrant disulphide were accumulated in the sludge. MBT slowed down the biodegradation of benzothiazole-2-sulphonate. MBT and benzothiazole did not mutually influence their degradation. Under some experimental conditions high levels of unidentified so-called polar compounds were formed.

Toxicity of 2-mercaptobenzothiazole towards bacterial growth and respiration.

Active sludge systems containing benzothiazoles may be intoxicated by 2-mercaptobenzothiazole (MBT). This toxicity towards several bacteria is now confirmed and is situated at around 100 mg MBT l-1. Octanol-water partition coefficients indicated that MBT might interact with membrane-bound systems. This was confirmed through experiments showing that bacterial cell respiration was inhibited using lactate or succinate as substrates. Using these substrates and also NADH, it was found that their oxidation was also inhibited using isolated membrane fragments of Escherichia coli and Paracoccus denitrificans. Methylene blue reduction was also found to be inhibited. The oxidation of ascorbate was not inhibited in P. denitrificans. From these results it is suggested that MBT might interact with the respiratory chain at the level of flavoproteins or quinones and Fe-S clusters.

Parameters affecting the degradation of benzothiazoles and benzimidazoles in activated sludge systems.

It was found that benzothiazole, 2-oxybenzothiazole and 2-benzothiazolesulphonate were degraded in activated sludge systems. 2-Mercaptobenzothiazole (MBT) was more resistant, although the first step in MBT degradation seemed to be transformation to the sulphonate form. At higher MBT concentrations, it was transformed into a disulphide, which accumulated in the sludge. MBT was also found to be mainly responsible for the toxicity of rubber chemical waste-water towards activated sludges. It inhibited the degradation of the other heterocycles. Only at concentrations of around 20 ppm was MBT degraded. Mercaptobenzimidazole ranked second in resistance to degradation.