Polyphosphate - a key biopolymer in aerobic granular sludge technology

Polyphosphate is the key biopolymer in wastewater treatment (WWT) processes applying enhanced biological phosphorus removal (EBPR) by polyphosphate-accumulating organisms (PAO). Alternating anaerobic (no oxygen and no nitrate) and aerobic phases is used to promote a net phosphorus removal from the wastewater as PAO are capable to take up and store phosphate as intracellular polyphosphate during the aerobic growth phase. In the anaerobic phase, PAO replenish their carbon and energy source in form of polyhydroxyalkanoates (PHA) from volatile fatty acids (VFA) present or formed in the wastewater. The energy and reducing equivalents needed to form PHA come from glycogen and polyphosphate, the polymers that are replenished during the aerobic phase. A few PAO have been already identified, among which Candidatus Accumulibacter phosphatis is a major player in WWT microbial communities enriched with VFA. However, depending on the carbon source the microbial community can strongly fluctuate and other PAO might play a major role in phosphorus removal. We are studying the dynamics of microbial communities in lab-scale aerobic granular sludge sequencing batch reactors subjected to changes of carbon source going from simple VFA to a mixture of VFA, glucose and amino acids. Both metagenomic approaches targeting polyphosphate kinase (ppk) genes and functional analysis of PAO using fluorescence techniques staining the polyphosphate polymer allow us to investigate the key metabolic genes in the synthesis of polyphosphate and expand the knowledge on the diversity of PAO in such engineered systems.

Microbial processes in aerobic granules - can we use them to treat wastewater more efficiently?

Graciela Gonzalez Gil, Christof Holliger, Samuel Lochmatter

Aerobic granular sludge-based reactors represent an attractive alternative to conventional activated sludge systems. Compared to activated sludge, the use of aerobic granular sludge reactors is advantageous because (i) considerably less area is required for construction, (ii) less energy is required to operate the system and (iii) growth yield of granular sludge biomass is lower, hence surplus biomass is reduced. One type of aerobic granular sludge reactors is the sequencing batch bubble column reactor. In sequencing batch bubble column reactors the alternation of anaerobic an aerobic periods is (among others) an important parameter for the formation of aerobic granules. The granules developed in such systems can have high biomass concentration, good settling properties, and high organic matter removal efficiencies. Desirable microbial processes that may develop within the granules are those of phosphorus removal and nitrification. In addition, and depending on bulk oxygen concentrations and granule size, denitrification may also occur in the granules core. Hence, sequencing batch reactors operated with granular sludge have the potential to achieve organic matter and nutrient removal in a single compact system. Development of aerobic granular sludge has been mainly studied using acetate as main energy and carbon source and at reactor temperatures of 20°C. However, some industrial streams may have higher temperatures and often contain, besides acetate, other volatile fatty acids as main components (or contaminants). Reports indicate that, in activated sludge systems, the presence of propionate in the wastewater may favor the proliferation of the microorganisms responsible for phosphate removal. The question remains on whether aerobic granular sludge can be developed with other carbon sources as main components (e.g. propionate) or at higher temperatures and if so, what are the physical and metabolic (or microbiological) properties of such sludge. Therefore we explored these issues in the present study. The development of aerobic granular sludge was investigated in sequencing batch bubble column reactors at temperatures of 20 and 30-35°C as well as with different carbon sources in the feed. Preliminary results show that aerobic granular sludge can be formed at higher temperatures but with different predominant microbial populations carrying out different microbial processes. Phosphate removal was only observed at 20°C whereas nitrification was occurring at higher temperatures.

2008

The effect of different organic substrates on the microbial communities of aerobic granular wastewater treatment sludge

Aline Sondra Adler

Aerobic granular sludge (AGS) is a promising alternative wastewater treatment to the conven- tional activated sludge system, allowing space and energy savings. This process is particularly suited for biological phosphorus removal, avoiding use of coagulant chemicals. Basic under- standing of this process has mainly been obtained in laboratory-scale studies with simple synthetic wastewater containing volatile fatty acids as main carbon source. Yet, the aspect and performance of granular sludge cultivated in such model systems are rather different from those obtained in systems treating real wastewater. In order to make a step toward the comprehension of AGS treating municipal wastewater, two approaches were applied to investigate the impact of the wastewater composition on AGS bacterial communities, settling properties and nutrient removal performance. The first approach was to transform activated sludge performing enhanced biological phosphorus removal into AGS in four parallel lab-scale reactors fed with different wastewater types: simple and complex polymeric synthetic, as well as raw and clarified municipal wastewater. The second approach was to progressively change the composition of the wastewater treated by AGS acclimated to simple synthetic wastewater to complex polymeric wastewater with an intermediary step with complex monomeric wastewater as a control. The whole DNA of biomass corresponding to the three wastewater types was sequenced with PacBio and Illumina technologies. During the two experiments, the bacterial communities, settling properties and nutrient removal performance were monitored. The bacterial communities in AGS treating simple wastewater were drastically different form the ones treating complex wastewater. Several taxa belonging to Actinobacteria and Saccha- ribacteria were largely underrepresented with the simple wastewater. Lower nitrogen removal, lower settling properties and higher proportions of flocs were observed with the polymeric wastewaters compared to the monomeric wastewaters. The lower concentrations of diffusible organic carbon rather than the bacterial community compositions were identified as the cause for these differences. Indeed, genes putatively involved in denitrification and biofilm formation were found in the AGS treating monomeric and polymeric wastewater. Moreover, different denitrification efficiencies and settling properties were observed with AGS having very similar bacterial communities but treating wastewater with a different concentration of organic carbon. The phosphate accumulating organism (PAO) Candidatus (Ca.) Accumulibacter, abundant in most of the AGS samples, was highly diverse and different clade repartitions were found within the AGS treating different wastewater types. The fermentative PAO Tetrasphaera was less diverse and mostly found in the AGS treating complex wastewater. The co-occurrence of two PAO occupying distinct ecological niches likely participated to the quick recovery of the P-removal after the transient but sharp decrease of Ca. Accumulibacter observed during the transition from simple to complex monomeric wastewater and likely due to a bacteriophage attack. The assembly of PacBio sequences associated to a binning based on composition and cov- erage of Illumina sequences produced nearly complete draft genomes attributed to poorly characterized taxa, thus providing information on their potential metabolism and a template for future metatranscriptomic ana

EPFL2019

Multistability and Reversibility of Aerobic Granular Sludge Microbial Communities Upon Changes From Simple to Complex Synthetic Wastewater and Back

Aline Sondra Adler, Christof Holliger

Aerobic granular sludge (AGS) is a promising alternative wastewater treatment to the conventional activated sludge system allowing space and energy saving. Basic understanding of AGS has mainly been obtained using simple wastewater containing acetate and propionate as carbon source. Yet, the aspect and performances of AGS grown in such model systems are different from those obtained in reactor treating real wastewater. The impact of fermentable and hydrolyzable compounds on already formed AGS was assessed separately by changing the composition of the influent from simple wastewater containing volatile fatty acids to complex monomeric wastewater containing amino acids and glucose, and then to complex polymeric wastewater containing also starch and peptone. The reversibility of the observed changes was assessed by changing the composition of the wastewater from complex monomeric back to simple. The introduction of fermentable compounds in the influent left the settling properties and nutrient removal performance unchanged, but had a significant impact on the bacterial community. The proportion of Gammaproteobacteria diminished to the benefit of Actinobacteria and the Saccharibateria phylum. On the other hand, the introduction of polymeric compounds altered the settling properties and denitrification efficiency, but induced smaller changes in the bacterial community. The changes induced by the wastewater transition were only partly reversed. Seven distinct stables states of the bacterial community were detected during the 921 days of experiment, four of them observed with the complex monomeric wastewater. The transitions between these states were not only caused by wastewater changes but also by operation failures and other incidences. However, the nutrient removal performance and settling properties of the AGS were globally maintained due to the functional redundancy of its bacterial community.

FRONTIERS MEDIA SA2020