M. Beccari

Interaction between acidogenesis and methanogenesis in the anaerobic treatment of olive oil mill effluents

The treatment of olive oil mill effluents (OME), even when performed with the most appropriate technology (i.e. anaerobic digestion) meets great difficulties in reaching the depuration efficiencies required by national regulations all over the Mediterranean area. This paper was aimed at gaining better insight into the degradation of the main compounds contained in the OME; in particular, the interaction between the two successive stages occurring in the anaerobic digestion, the acidogensis and the methanogenesis, was investigated. First of all, the best operating conditions were identified. Most of the lipids were degraded both in acidogenesis and methanogenesis tests. On the other hand, polyphenol-like substances were not degraded at all in acidogenic conditions, whereas they were partially removed in methanogenic conditions. A little methanogenic activity, established in acidogenic conditions because of the partial degradation of the chemical inhibitor, seems to be the key factor determining lipids degradation even in acidogenesis tests. Synthetic solutions were also tested. Oleic acid was degraded both in acidogenesis and methanogenesis tests, provided that an easily biodegradable substrate (glucose) was added to the solution. The presence of glucose was also required for the degradation of p-hydroxybenzoic acid in methanogenic conditions. This acid was not degraded at all in acidogenic conditions, thus confirming the results obtained in the tests on OME. Even if slightly lower, bioconversion yields of OME in acidogenesis are remarkably less sensitive to the effect of pH and substrate conditions than in methanogenesis. This result suggests that a two-phase anaerobic digestion might be adopted as a suitable process for optimizing OME degradation.

Integrated treatment of olive oil mill effluents: Effect of chemical and physical pretreatment on anaerobic treatability

Difficulties met in the anaerobic treatment of olive oil mill effluents (OME) suggest the use of a chemico-physical pretreatment for the removal of biorecalcitrant and/or inhibiting substances (essentially lipids and polyphenols) as selectively as possible before anaerobic digestion. Laboratory scale experiments were carried out in order to identify pretreatment type and conditions capable of optimizing OME anaerobic digestion in terms both of kinetics and methane yield. Ultrafiltration, even if it allowed very high removals of lipids and polyphenols, was affected by poor selectivity (indeed, large amounts of biodegradable COD were also removed). Centrifugation turned out to be preferable to sedimentation owing to smaller volumes of separated phase. Results of great significance were obtained by adding Ca(OH)2 (up to pH 6.5) and 15 g/l of bentonite, and then feeding the mixture to the biological treatment without providing an intermediate phase separation. Indeed, the biodegradable matter adsorbed on the surface of bentonite was gradually released during the biotreatability test, thus allowing the same methane yield (referred to the total COD contained in untreated OME) both in scarcely diluted (1: 1.5) pretreated OME and in very diluted (1: 12) untreated OME. Application of a continuous process combining pretreatment (with Ca(OH)2 and bentonite) and anaerobic digestion without intermediate phase separation is suggested.

Effects of dissolved oxygen and diffusion resistances on nitrification kinetics

The effects of the dissolved oxygen concentration on biological nitrification in suspended biomass processes are investigated. A model of nitrification, in which the kinetics is expressed by considering both the intrinsic rate of ammonia oxidation and the diffusion rate of substrates inside the biological floc, is presented. The model is calibrated and validated utilizing experimental data of tests performed in conditions of oxygen limiting kinetics. Excellent agreement is found between experimental results and model predictions. Finally, the model is applied to evaluate the effectiveness factor, η, i.e. the ratio between the actual substrate removal rate and the intrinsic rate, as a function of the bioflocs diameter at different levels of dissolved oxygen concentration. The results obtained show that in nitrification processes, the effects related to oxygen internal diffusion resistances cannot be neglected in evaluating the overall kinetics. A marked decrease in the η value is found at a biofloc diameter greater than 100 μm, particularly when the dissolved oxygen concentration is ⩽ 2 mg l−1.

Sequencing batch reactor: Influence of periodic operation on performance of activated sludges in biological wastewater treatment : Novel reactor engineering for the new millennium

This paper presents the main results obtained during several years of studies with periodically fed biological reactors (sequencing batch reactors, SBRs) from the perspective of wastewater treatment. Substrate (acetate) removal mechanisms under transient conditions were studied both in the SBR and in batch tests, as were their dependence on the applied operating conditions in the SBR (organic load rate and/or sludge age, feed length, aerobic or anoxic conditions). The most general evidence was the relevant role of storage (usually representing about 70% of the overall observed yield) and the negligible role of growth (usually less than 10%) during acetate removal for all tested conditions. However, when acetate was present for a long enough time, physiological adaptation could occur, and the growth contribution could become more important. The applied operating conditions affected the biomass behavior. In particular, with an increase in the applied organic load rate, the observed yield in the SBR and the acetate removal rate in the batch tests decreased, whereas with an increase in the feed length (other conditions being the same), the relevance of the storage response decreased. The role of dynamic conditions in selecting a floc-forming or filamentous biomass was also investigated. Even though a floc-forming biomass usually developed, filament growth was also sometimes observed. Thus, the usual assumption that filaments are less able than floc-formers to store the substrate should not be considered as an absolute rule. Finally, an empirical kinetic model, including growth and storage both in parallel and in sequence, was defined and applied to describe and interpret the experimental results.

Removal of molecular weight fractions of COD and phenolic compounds in an integrated treatment of olive oil mill effluents

Previous works (Beccari et al. 1999b; Beccari et al. 2001a; Beccari et al. 2001b)on the anaerobic treatment of olive oil mill effluents (OME) have shown: (a) apre-treatment based on the addition of Ca(OH)2 and bentonite was able toremove lipids (i.e. the most inhibiting substances present in OME) almostquantitatively; (b) the mixture OME – Ca(OH)2 – bentonite, fed to amethanogenic reactor without providing an intermediate phase separation,gave way to high biogas production even at very low dilution ratios; (c) theeffluent from the methanogenic reactor still contained significant concentrationsof residual phenolic compounds (i.e. the most biorecalcitrant substances present inOME). Consequently, this paper was aimed at evaluating the fate of the phenolicfractions with different molecular weights during the sequence of operations(adsorption on bentonite, methanogenic digestion, activated sludge post-treatment).The results show that a very high percentage (above 80%) of the phenolic fractionbelow 500 D is removed by the methanogenic process whereas the phenolic fractionsabove 1,000 D are significantly adsorbed on bentonite; the 8-day activated sludgepost-treatment allows an additional removal of about 40% of total filtered phenoliccompounds. The complete sequence of treatments was able to remove more than the96% of the phenolic fraction below 500 D (i.e. the most toxic fraction towards plantgermination). Preliminary respirometric tests show low level of inhibition exerted bythe effluent from the methanogenic reactor on aerobic activated sludges taken fromfull-scale municipal wastewater plants.

The storage of acetate under anoxic conditions

Till now the role of storage in activated sludge processes under transient conditions has been deeply investigated under anaerobic (EBPR processes) or aerobic (bulking control) environments. Little attention has been given to the role of storage in processes including anoxic environments. Hence, the aim of the present work was to investigate the anoxic storage along with other substrate removal mechanisms under transient conditions. Several mixed culture were ad hoc selected under anoxic environment and periodic feeding (acetate as carbon source) at different organic load rate (OLR) and feed length; then their transient response to substrate spike was investigated by batch tests under both anoxic and aerobic conditions. The relative role of different mechanisms in the substrate removal was established on the basis of COD balance assuming that the acetate COD removed from the liquid phase could be oxidised for energy needs or recovered into solids as poly-3-hydroxybutyric acid (PHB) (storage), other internal precursors or intermediates (accumulation) and active biomass (growth, as estimated by ammonium uptake). In all tested conditions, growth response was very little while PHB storage was prevailing. In some operating conditions, indirect evidence of accumulation (in forms still to be identified) was also found. The transient response was not affected by the presence of free amino acids, at least for the unacclimated mixed culture under observation. Transient response under aerobic condition was quite similar to the anoxic one.

Effect of the length of the cycle on biodegradable polymer production and microbial community selection in a sequencing batch reactor.

The effect of the length of the cycle on the enrichment and selection of mixed cultures in sequencing batch reactors (SBRs) has been studied, with the aim of biodegradable polymers (namely, polyhydroxyalkanoates (PHAs)) production from organic wastes. At a fixed feed concentration (20 gCOD/L) and organic loading rate (20 gCOD/L/day), the SBR was operated at different lengths of the cycle, in the range 1–8 h. Process performance was measured by considering the rates and yields of polymer storage and of the competing phenomenon of growth. The selected biomass was enriched with microorganisms that were able to store PHAs at high rates and yields only when the length of the cycle was 2 or 4 h, even though in these conditions the process was unstable. On the other hand, when the length of the cycle was 1 or 8 h, the dynamic response of the selected microorganisms was dominated by growth. The best process performance was characterized by storage rates in the range 500–600 mgCOD/gCOD/h and storage yields of 0.45–0.55 COD/COD. The corresponding productivity of the process was in the range 0.25–0.30 gPHA/L/h, the highest values obtained until now for mixed cultures. The microbial composition of the selected biomasses was analyzed through denaturing gradient gel electrophoresis (DGGE) and reverse‐transcriptase denaturing gradient gel electrophoresis (RT‐DGGE). The instability of the runs characterized by high storage rate was associated with a higher microbial heterogeneity compared to the runs with a stable growth response.

A critical analysis of nitrification alternatives

Abstract A kinetic model has been applied for comparing the one-sludge system with the two-sludge system in the oxidation of nitrogenous substrates. It has been verified that some organic substances, members of different classes of compounds usually present in sewage, do not exert appreciable inhibiting effect on the activity of the nitrifying bacteria. The growth yield coefficient ( Y ) and the maximum growth rate (μ M ) for these bacteria were determined using new methods since correct application of the kinetic model requires the most accurate estimate possible for these parameters. The comparison between the two processes was made in the case of two ‘limiting’ concentrations for the nitrification reactor in the two-sludge system. The important role played by the settling characteristics of the sludge has been pointed out.

Role of Lipids and Phenolic Compounds in the Anaerobic Treatment of Olive Oil Mill Effluents

The inhibitory effects of olive oil mill effluents (OME) on methanogenesis were studied by using p-hydroxybenzoic acid (HBA) and oleic acid (OA) as model compounds for phenolic compounds and lipids respectively. The addition of HBA (0.3 g l−1) to diluted OME (5.7 g COD l−1) had a negligible effect on methanogenesis in terms of kinetics and final methane yield. Strong inhibition was observed only at high HBA concentrations (6 g l−1). The OA inhibited methane production much more strongly. Indeed, 0.35 g l−1 of OA were able to double the lag phase for methanogenesis from diluted OME. With both compounds, the presence of an easily biodegradable co-substrate (glucose) did not increase the rate of substrate degradation nor methane formation. OME acidogenesis was significantly less sensitive than methanogenesis to the inhibitory effects exerted by OA and HBA, even at relatively high concentrations of the model compounds.

Inhibitory effects on nitrification by typical compounds in coke plant wastewaters

Abstract This paper describes a theoretical and experimental approach to evaluating nitrification inhibition (types and degrees) by some substances to be found in coke plant discharges.