Alternating anoxic feast/aerobic famine condition for improving granular sludge formation in sequencing batch airlift reactor at reduced aeration rate

In this study the influence of a pre-anoxic feast period on granular sludge formation in a sequencing batch airlift reactor is evaluated. Whereas a purely aerobic SBR was operated as a reference (reactor R2), another reactor (R1) was run with a reduced aeration rate and an alternating anoxic-aerobic cycle reinforced by nitrate feeding. The presence of pre-anoxic phase clearly improved the densification of aggregates and allowed granular sludge formation at reduced air flow rate (superficial air velocity (SAV) = 0.63 cm s⁻¹). A low sludge volume index (SVI₃₀ = 45 mL g⁻¹) and a high MLSS concentration (9–10 g L⁻¹) were obtained in the anoxic/aerobic system compared to more conventional results for the aerobic reactor. A granular sludge was observed in the anoxic/aerobic system whilst only flocs were observed in the aerobic reference even when operated at a high aeration rate (SAV = 2.83 cm s⁻¹). Nitrification was maintained efficiently in the anoxic/aerobic system even when organic loading rate (OLR) was increased up to 2.8 kg COD m⁻³ d⁻¹. In the contrary nitrification was unstable in the aerobic system and dropped at high OLR due to competition between autotrophic and heterotrophic growth. The presence of a pre-anoxic period positively affected granulation process via different mechanisms: enhancing heterotrophic growth/storage deeper in the internal anoxic layer of granule, reducing the competition between autotrophic and heterotrophic growth. These processes help to develop dense granular sludge at a moderate aeration rate. This tends to confirm that oxygen transfer is the most limiting factor for granulation at reduced aeration. Hence the use of an alternative electron acceptor (nitrate or nitrite) should be encouraged during feast period for reducing energy demand of the granular sludge process.     

A comparative study of the removal of trivalent chromium from aqueous solutions by bentonite and expanded perlite

Local bentonite and expanded perlite (Morocco) have been characterised and used for the removal of trivalent chromium from aqueous solutions. The kinetic study had showed that the uptake of Cr(III) by bentonite is very rapid compared to expanded perlite. To calculate the sorption capacities of the two sorbents, at different pH, the experimental data points have been fitted to the Freundlich and Langmuir models, respectively, for bentonite and expanded perlite. For both sorbents the sorption capacity increases with increasing the pH of the suspensions. The removal efficiency has been calculated for both sorbents resulting that bentonite (96% of Cr(III) was removed) is more effective in removing trivalent chromium from aqueous solution than expanded perlite (40% of Cr(III) was removed). In the absence of Cr(III) ions, both bentonite and expanded perlite samples yield negative zeta potential in the pH range of 2-11. The changes of expanded perlite charge, from negative to positive, observed after contact with trivalent chromium(III) solutions was related to Cr(III) sorption on the surface of the solid. Thus, it was concluded that surface complexation plays an important role in the sorption of Cr(III) species on expanded perlite. In the case of bentonite, cation-exchange is the predominate mechanism for sorption of trivalent chromium ions, wherefore no net changes of zeta potential was observed after Cr(III) sorption. X-ray photoelectron spectroscopy measurements, at different pH values, were also made to corroborate the zeta potential results.     

Efficient algorithms for singleton arc consistency

In this paper, we propose two original and efficient approaches for enforcing singleton arc consistency. In the first one, the data structures used to enforce arc consistency are shared between all subproblems where a domain is reduced to a singleton. This new algorithm is not optimal but it requires far less space and is often more efficient in practice than the optimal algorithm SAC-Opt. In the second approach, we perform several runs of a greedy search (where at each step, arc consistency is maintained), possibly detecting the singleton arc consistency of several values in one run. It is an original illustration of applying inference (i.e., establishing singleton arc consistency) by search. Using a greedy search allows benefiting from the incrementality of arc consistency, learning relevant information from conflicts and, potentially finding solution(s) during the inference process. We present extensive experiments that show the benefit of our two approaches.     

Development of a molecular method for the rapid discrimination of red seaweeds used for agar production

In Tunisia, agar is produced from the species Gracilaria verrucosa. Other species, such as Gracilariopsis sp. which have a very similar morphology to Gracilaria, can be harvested in a mixture with Gracilaria with the result that the quantity and quality of agar extracted is different than initially expected. In this study, we tested the use of ITS sequences from 5.8s rDNA for species discrimination between G. verrucosa and Gracilariopsis harvested from two production sites (Bizerte and Tunis Lagoons). The amplification of genomic ITS sequences and sequencing shows that the samples collected from Tunis can be identified by PCR fragment of 1124 bp whereas the samples from Bizerte are characterized by the presence of two fragments of 1124 and 983 bp, respectively. A BLAST investigation in Genbank shows that the sequence similarities between the fragment of 1124 pb and G. verrucosa was 85% and between the fragment of 983 pb and Gracilariopsis sp. Plymoutn was 94%. Gracilariopsis sp. possesses a PCR fragment with a length of 983 bp, which discriminates it from G. verrucosa when these two species are collected in a mixture as in the lagoon of Bizerte. The RFLP method after EcoRI application provides a characteristic pattern for G. verrucosa, which is composed of two main fragments with respective sizes of 823 and 301 bp whereas amplified DNA of Gracilariopsis was uncut under these conditions.     

Methode rapide de dosage selectif des chlorophylles a et b utilisation de la separation par chromatographie liquide haute pressionA selective and rapid method of chlorophylls a and b separative determination by high-pressure liquid chromatography

The usual extraction methodology of the phytoplankton chlorophylls by acetone at 90% and their usual detection by spectrophotometry of the global extract at different wave lengths seem to be insufficient to many authors. With intent to improve this method (extraction, detection), we have verified first that methanol extraction was more effective than 90% acetone extraction, and we have set up a method of detection as simple and rapid as the usual one but which allows measuring the chlorophylls quantitatively and specifically, once they are separated. The chlorophylls a and b were separated by high-pressure liquid chromatography in reversed phase with a spherisorb ODS C₁₈ column eluted with methanol-water ($\text{97}\text{3}$) at a flow-rate of 1 ml min^?1; after being eluted they were detected by spectrofluorometry (excitation 427 nm and detection 470 nm) (Figs 1 and 2). This selective, sensitive rapid (20 min) determination can be automated by using an automatic injector and the recording on an integrator. The standardization of chlorophylls a and b was carried out with fluoranthen (internal standard) which does not want preparing again at every set of determinations (Fig. 3). This is a great advantage when we consider the fast degradation of the standards. This method allows using absolute methanol which is an extracting solvent more efficient than acetone 90% in the specific case of Scenedesmus subspicatus, whose pigments are known not to be easily extracted, and, in a broader sense, in the case of natural phytoplanktonic populations. To add BaCO₃ as a basic material during extraction is not recommended. This methodology may be used for biomass estimates by routine analysis as well as for the delicate measuring of pigments. Chromatographic profiles of Scenedesmus subspicatus extract and Seine river extract are presented in Figs 4 and 5.     

Chemical composition and some biological activities of the volatile oils of a chemotype of Lippia chevalieri Moldenke

The volatile oils of a chemotype Lippia chevalieri collected in Burkina-Faso were obtained by hydrodistillation and analysed by GC and GC/MS. Most of the constituents identified in leaves and flowers oils were sesquiterpenoids. The dominant components in the oil from leaves were β-caryophyllene (27%), elemol (22%) and caryophyllene oxide (9%) whereas those found in the oil from flowers were β-caryophyllene (30%), germacrene d (15%) and elemol (12%). The latter compound and 1,8-cineole may be considered as metabolic markers of flowers differentiation taking into account the local environmental changes. The antibacterial and antifungal activities of both oils were tested against six microorganisms. While a strong inhibitory effect was shown regarding the growth of Staphylococcus aureus and Enterococcus hirae, a moderated effect was observed for Candida albicans and Saccharomyces cerevisiae. Elemol, 1,8-cineole, camphor and para-cymene can be considered as the principal antimicrobial components of these oils. No antioxidant capacity comparable to that of α-tocopherol was demonstrated from either oil.     

Domain filtering consistencies for non-binary constraints

In non-binary constraint satisfaction problems, the study of local consistencies that only prune values from domains has so far been largely limited to generalized arc consistency or weaker local consistency properties. This is in contrast with binary constraints where numerous such domain filtering consistencies have been proposed. In this paper we present a detailed theoretical, algorithmic and empirical study of domain filtering consistencies for non-binary problems. We study three domain filtering consistencies that are inspired by corresponding variable based domain filtering consistencies for binary problems. These consistencies are stronger than generalized arc consistency, but weaker than pairwise consistency, which is a strong consistency that removes tuples from constraint relations. Among other theoretical results, and contrary to expectations, we prove that these new consistencies do not reduce to the variable based definitions of their counterparts on binary constraints. We propose a number of algorithms to achieve the three consistencies. One of these algorithms has a time complexity comparable to that for generalized arc consistency despite performing more pruning. Experiments demonstrate that our new consistencies are promising as they can be more efficient than generalized arc consistency on certain non-binary problems.