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

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.

Polyphosphate - a key biopolymer in aerobic granular sludge technology

Aline Sondra Adler, Arnaud Michel Gelb, Christof Holliger

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.

2016

Aerobic granular sludge formation and performance at 30 and 35°C in sequencing batch bubble-column reactors

Sirous Ebrahimi, Sébastien Gabus, Graciela Gonzalez Gil, Christof Holliger, Maryam Hosseini

Aerobic granular sludge-based reactors represent an attractive alternative to conventional activated sludge systems due to their small footprint and their low energy consumption and excess sludge production. The granules developed in such systems have high biomass concentration, good settling properties, high COD removal efficiencies and eventually high phosphorus removal capacity. In addition, and depending on bulk oxygen concentrations and granule size, both nitrification and denitrification can occur in the granules. Therefore sequencing batch reactors operated with granular sludge have the potential to achieve organic matter and nutrient removal in a single compact system. So far, aerobic granular sludge development has been studied at 20°C as well as at relatively low temperatures (e.g. 5 and 10°C). Compact wastewater treatment systems are especially interesting for industry. Since they often produce wastewaters with temperatures above 20°C, we investigated whether aerobic granular sludge could be developed at higher temperatures and if so, what the physical and metabolic properties of the sludge were. The formation and performance of aerobic granular sludge was studied in sequencing batch bubble-column reactors. Two start-up strategies were examined. Reactor 1 and 2 were inoculated with activated sludge from a municipal wastewater treatment plant and operated at 20°C and 30°C, respectively. After failure of Reactor 2, it was inoculated with aerobic granular sludge of Reactor 1 and operated with a stepwise temperature increase from 20 to 35°C. Granular sludge with good physical and metabolic properties was obtained in Reactor 1. Virtually all acetate was consumed during the anaerobic period with concomitant high phosphorus release suggesting the predominance of phosphate accumulating organisms (PAO). After the aerobic phase, an average of 63% of phosphate was removed. Nitrogen removal was not observed in this system, not even nitrification. In Reactor 2, a mixture of smooth granules and irregular structures was obtained. Complete COD removal was achieved, however, the sludge had low density (

2008

DENITRIFYING PAO AND GAO IN AEROBIC GRANULAR BIOFILM CULTIVATED WITH ACETATE AND PROPIONATE

Graciela Gonzalez Gil, Christof Holliger, Samuel Lochmatter, David Gregory Weissbrodt

Phosphate accumulating organisms (PAO) and glycogen accumulating organisms (GAO) play an important role in aerobic granular biofilm systems used for wastewater treatment. Both are slow growing organisms which can take up and store carbon under anaerobic conditions. The growth of PAO over GAO is preferred because of their capacity to remove phosphate. As some PAOs and GAOs can utilize nitrite or nitrate as electron acceptor in absence of oxygen, they act also as denitrifiers (i.e. DPAO/DGAO). The presence of active DPAOs in a treatment system is desirable because the same carbon source is used for dephosphatation and denitrification. The aim of this study was to investigate and to compare the denitrification potential of aerobic granular biofilms having already a good phosphate removal activity, and that were cultivated with either propionate or acetate as model contaminats. The results show that both biofilms had considerably higher specific uptake rates for nitrite than for nitrate (3.26 times for acetate and 4.79 times for propionate). These results agree with a study (Martin et al., 2006) which showed, that some DPAO have no nitrate reductase genes and can therefore only reduce nitrite but not nitrate. Therefore, DGAO were likely responsible for nitrate and DPAO together with DGAO for nitrite removal. This assumption is supported by the observation that the nitrite uptake rate decreased in the absence of phosphate, whereas the nitrate uptake rate was not influenced by phosphate presence or absence. Furthermore we observed a 27% higher specific nitrite uptake rate in the propionate reactor than in the acetate reactor. This supports the hypothesis that granules fed with propionate may favor the development of PAO with nitrite reducing capacity. We conclude that (i) to enhance the denitrification and create a link to the P-removal, nitrite oxidation to nitrate has to be inhibited, and that (ii) propionate leads to more robust DPAO populations than acetate. In addition to the biochemical results presented in this abstract, microbial community analysis of granules will be done to complete the results.