M. Figueroa

Relationship between microbial activity and microbial community structure in six full-scale anaerobic digesters.

High activity levels and balanced anaerobic microbial communities are necessary to attain proper anaerobic digestion performance. Therefore, this work was focused on the kinetic performance and the microbial community structure of six full-scale anaerobic digesters and one lab-scale co-digester. Hydrolytic (0.6-3.5 g COD g(-1) VSS d(-1)) and methanogenic (0.01-0.84 g COD g(-1) VSS d(-1)) activities depended on the type of biomass, whereas no significant differences were observed among the acidogenic activities (1.5-2.2 g COD g(-1) VSS d(-1)). In most cases, the higher the hydrolytic and the methanogenic activity, the higher the Bacteroidetes and Archaea percentages, respectively, in the biomasses. Hydrogenotrophic methanogenic activity was always higher than acetoclastic methanogenic activity, and the highest values were achieved in those biomasses with lower percentages of Methanosaeta. In sum, the combination of molecular tools with activity tests seems to be essential for a better characterization of anaerobic biomasses.

Treatment of anaerobic sludge digester effluents by the CANON process in an air pulsing SBR

The CANON (Completely Autotrophic Nitrogen removal Over Nitrite) process was successfully developed in an air pulsing reactor type SBR fed with the supernatant from an anaerobic sludge digester and operated at moderately low temperatures (18-24 degrees C). The SBR was started up as a nitrifying reactor, lowering progressively the dissolved oxygen concentration until reaching partial nitrification. Afterwards, an inoculation with sludge containing Anammox biomass was carried out. Nitrogen volumetric removal rates of 0.25 g NL(-1)d(-1) due to Anammox activity were measured 35 d after inoculation even though the inoculum constituted only 8% (w/w) of the biomass present in the reactor and it was poorly enriched in Anammox bacteria. The maximal nitrogen removal rate was of 0.45 g NL(-1)d(-1). By working at a dissolved oxygen concentration of 0.5 mg L(-1) in the bulk liquid, nitrogen removal percentages up to 85% were achieved. The reactor presented good biomass retention capacity allowing the accumulation of 4.5 g VSS L(-1). The biomass was composed by ammonia oxidizing bacteria (AOB) forming fluffy structures and granules with an average diameter of 1.6mm. These granules were composed by Anammox bacteria located in internal anoxic layers surrounded by an external aerobic layer where AOB were placed.

Applications of Anammox based processes to treat anaerobic digester supernatant at room temperature

The supernatant of an anaerobic digester was treated at 20 degrees C in two systems. The first one is a two units configuration, conformed by two sequencing batch reactors (SBR), carrying out partial nitrification and Anammox processes, respectively. Partial nitrification was achieved by granular biomass with a mean diameter of 3 mm, operating at a dissolved oxygen concentration of 2.7 mg/L. The combined system allowed the removal of nitrogen loading rates around 0.08 g N/(Ld). Afterwards, Anammox biomass was spontaneously developed in the inner core of the nitrifying granules of the SBR and therefore, partial nitrification and Anammox process were carried out in a single unit. Once the stable CANON process was established, a mean nitrogen removal rate of 0.8 g N/(Ld) was registered. The settling velocities of the granules ranged from 70 to 150 m/h with sludge volumetric index values lower than 50 mL/g VSS during the whole operation.

Treatment of saline wastewater in SBR aerobic granular reactors

Fish canning effluents characterized by their salt content, up to 30 g NaCl/L, were treated, previously diluted to desired concentration, in a SBR where aerobic granular sludge was produced. The formation of mature aerobic granules occurred after 75 days of operation with 3.4 mm of diameter, SVI of 30 mL/g VSS and density around 60 g VSS/L-granule. Treated organic loading rates were up to 1.72 kg COD/(m3.d) with fully organic matter depletion. Ammonia nitrogen was removed via nitrification-denitrification up to 40% when nitrogen loading rates were of 0.18 kg N/(m3.d). The presence of salt in the treated effluent did not cause a detrimental effect on the operation of the reactor once the aerobic granules were formed.

Aerobic granular SBR systems applied to the treatment of industrial effluents

Four lab scale sequencing batch reactors (SBRs) were operated to remove organic matter and nitrogen from four different industrial wastewaters. The biomass grew in the reactors in the form of aerobic granules characterized by good settling properties. The high biomass concentrations achieved inside the reactors allowed reducing the solids concentration in the effluent down to 0.2 g VSS L(-1). The organic loading rates (OLR) applied to reactors ranged between 0.7 and 5.0 g CODL(-1)d(-1) with removal efficiencies of 60-95%. The nitrogen loading rates (NLR) applied varied between 0.15 and 0.65 g NH(4)(+)-NL(-1)d(-1) with variable removal efficiencies in the four systems (between 15% and 76%).

Influence of gas flow-induced shear stress on the operation of the Anammox process in a SBR

The start up and performance of the Anammox process were tested in sequencing batch reactors with two different configurations: a bubble column (SBR-B) and a gas-lift reactor (SBR-G). Different off-gas upflow velocities were tested (3.53-12.3 cm min(-1)) in order to expose the biomass to different shear conditions and to study their effects on both efficiency and physical properties of the Anammox granular biomass. For the SBR-B the minimum gas upflow velocity needed to achieve biomass suspension inside the reactor was 12.3 cm min(-1). Such velocity made impossible the stable operation of the process. The fluidization of biomass for the SBR-G was reached at a gas upflow velocity of 3.52 cm min(-1). This system maintained an efficiency of nitrite removal around 98% at values up to 5.29 cm min(-1) but when the gas upflow velocity was increased from 5.29 to 9.70 cm min(-1) a significant decrease of the specific Anammox activity of the biomass from 0.35 to 0.05 g Ng(-1) VSS d(-1) was measured. The system lost 85% of its nitrogen removal efficiency which was not restored in spite of returning the gas upflow velocity to its initial value.

Influence of transitional states on the microbial ecology of anaerobic digesters treating solid wastes.

A better understanding of the microbial ecology of anaerobic processes during transitional states is important to achieve a long-term efficient reactor operation. Five wastes (pig manure, biodiesel residues, ethanol stillage, molasses residues, and fish canning waste) were treated in five anaerobic reactors under the same operational conditions. The influence of the type of substrate and the effect of modifying feeding composition on the microbial community structure was evaluated. The highest biomethanation efficiency was observed in reactors fed with fish canning waste, which also presented the highest active archaeal population and the most diverse microbial communities. Only two Bacteria populations could be directly related to a particular substrate: Ilyobacter with biodiesel residues and Trichococcus with molasses residues. Results showed that the time to achieve steady-state performance after these transitional states was not dependent on the substrate treated. But reactors needed more time to handle the stress conditions derived from the start-up compared to the adaptation to a new feeding. Cluster analyses showed that the type of substrate had a clear influence on the microbiology of the reactors, and that segregation was related to the reactors performance. Finally, we conclude that the previous inoculum history treating solid waste and higher values of active Archaea population are important factors to face a successful change in substrate not entailing stability failure.

Post-treatment of effluents from anaerobic digesters by the Anammox process.

The application of the Anammox process was studied under two different approaches for the post-treatment of anaerobic digester supernatants: two independent units, the combined SHARON-Anammox system, performed in a chemostate and a SBR, respectively, and, a single unit system composed by an air pulsing SBR to carry out the CANON process. The technology based on the combination of the SHARON-Anammox process was used to treat the effluent of an anaerobic digester from a fish canning industry. The presence of organic matter in the influent caused fluctuations in the efficiency of the SHARON unit and an optimal nitrite to ammonium ratio was not achieved in this system to feed the Anammox reactor. Nevertheless an overall percentage of nitrogen removal of 40-80% was obtained when the Anammox reactor operated at nitrite limited conditions. In those periods when the effluent from the SHARON unit contained a NO2(-)-N/NH4(+)-N molar ratio higher than 1.3 the Anammox process lost its stability due to nitrite accumulation. The effluent from an anaerobic digester placed at a WWTP was treated by a CANON system operated at room temperature (20-24 degrees C). This system was developed from a nitrifying air pulsing reactor working at limiting dissolved oxygen conditions which was inoculated with Anammox biomass. A quick start-up of the system was observed and the reactor reached a nitrogen removal rate of 0.25 g N/(L d) 40 days after inoculation. The maximum nitrogen removal rate reached 0.5 g N/(L d). These results indicate the feasibility of the treatment of effluents from psychrophilic anaerobic digesters using the Anammox process.

Aerobic sludge granulation: state-of-the-art

Recently, the research focused on the development and operation of aerobic granular biomass has increased in interest, due to the advantages of this option as compared to the conventional activated sludge systems traditionally used for the biological treatment of wastewater. The aerobic granular sludge is produced in Sequential Batch Reactors (SBR) where a large amount of biomass is accumulated by means of the improvement of its settling properties. These compact systems allow the reduction of the required space for implantation and favour the treatment of large loads of contaminants and the performance of the nitrification and denitrification processes simultaneously.

Population dynamics of nitrite oxidizers in nitrifying granules

The competition between Nitrospira and Nitrobacter species was analyzed in this work under conditions of excess of nitrite. A population of nitrite oxidizing bacteria (NOB) was developed from nitrifying biomass grown as granules with a mean diameter of 0.8 mm, whose feed was switched from ammonium to nitrite. The initial population distribution of the granules was: 60% Nitrosomonas and 30% Nitrospira and it evolved to 45% Nitrobacter and 40% Nitrospira measured 177 days after the change in the feeding. The disappearance of Nitrosomonas allowed the development of an important population of Nitrobacter demonstrating that these organisms, characterized by being r strategists NOB, are poor competitors when oxygen is the limiting substrate. Interestingly, the physical structure of the granules was not altered by the change of its microbial composition during the 220 days of operation.