Optimization of Reactor Startup and Nitrogen Removal of Aerobic Granular Sludge Systems

Aerobic granular sludge (AGS) in sequencing batch reactors (SBR) has recently made its proofs as full-scale technology for the biological treatment of urban wastewater. In contrast to conventional flocculent activated sludge, microorganisms aggregate to dense granular biofilm due to shear stress from aeration. Hence, AGS has much better settling characteristics and allows the construction of more compact wastewater treatment plants (WWTP) without secondary clarifiers. Due to diffusion limitations of oxygen through the biofilm, aerobic and anaerobic/anoxic zones coexist in AGS. Hence, AGS-SBR has the potential to treat organic matter (COD), nitrogen (N), and phosphorous (P) simultaneously in one single reactor. The focus of the present PhD thesis was on the startup of AGS-SBR and the optimization of biological nutrient removal (BNR). For the investigation of the optimal startup conditions, a study testing seven parameters in parallel was conducted. The main conditions identified for a rapid startup of AGS-SBRs with good nutrient removal performances were (i) the alternation of high and low dissolved oxygen (DO) phases during aeration, (ii) a settling strategy avoiding too high biomass washout, (iii) the adaptation of the pollution load in the early stage of the startup in order to ensure that all soluble COD was consumed before the aeration phase, (iv) higher temperature (20°C), and (v) a neutral pH. Under such conditions it took less than 30 days to produce granular sludge with high removal performances for COD, N and P. A control run of the best startup strategy led to very similar results, proving the reproducibility of the experimental approach. This control run was operated for 80 days without any problems concerning the stability of the granular sludge or the nutrient removal performances. Concerning the bacterial community composition during the startup phase, a general shift of the predominant populations from Intrasporangiaceae and Sphingobacteriales to Dechloromonas and Zoogloea was observed. This shift was mainly due to general conditions with lab-scale AGS-SBR, rather than the specific operation parameters tested in the different experimental runs. However, it has been observed that polyphosphate-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO) related populations were favored by the adaptation of the pollution load in the early stage of the startup in order to ensure that all soluble COD was consumed before the aeration phase. Besides the pollution load, the temperature and the pH had a significant impact on the global bacterial community structure. The predominant PAO were presumably Dechloromonas. In order to optimize BNR by AGS-SBR, different aeration strategies were tested. It has been concluded that the N-removal efficiency of COD-limited systems can be considerably enhanced with improved aeration strategies. Strategies promoting alternating nitrification and denitrification (AND) were significantly more efficient than simultaneous nitrification and denitrification (SND) strategies. The introduction of low DO phases or even anoxic phases in an early stage of the total aeration period probably enhanced denitrifying P-removal and led to COD savings. Intermittent aeration, which is a realistic AND strategy for full scale applications, led to the highest N-removal efficiency. The short mixing times implemented with this strategy were not problematic for the stability of the granules. Finally, to further optimize N-removal in COD-limited systems, the potential of aeration control for the achievement of N-removal over nitrite was investigated. It was shown that aeration phase length control combined with intermittent aeration or alternating high-low DO, is an efficient way to achieve N-removal over nitrite. N-removal efficiencies of up to 95% were achieved with this way of reactor operation. At 20°C, N-removal over nitrite was achieved within 20 – 60 days and it was possible to switch from N-removal over nitrite to N-removal over nitrate and back again. At 15°C, the nitrite-oxidizing bacteria population could be reduced, but nitrite oxidation could not be completely repressed. However, the combination of aeration phase length control and high-low DO was successful to maintain the nitrite pathway at 15°C where the maximum growth rate of nitrite-oxidizing bacteria is clearly higher than the one of ammonium-oxidizing bacteria. In conclusion, this thesis showed the potential of the optimization of operation conditions for the startup of AGS-SBR and BNR. An efficient strategy for the startup with flocculent inoculum sludge has been developed, maintaining high BNR. Moreover, it has been shown that N-removal under COD-limited conditions can be improved by aeration control, either by enhancing denitrifying P-removal or achieving N-removal over nitrite.

Co-culture of microalgae and bacteria for wastewater treatment

Thierry Mounir

To address the problem of insucient wastewater treatment and eutrophication of lakes and rivers, the symbiotic association of microalgae and bacteria has shown great potential and numerous advantages. First, co-cultures can exhibit ecient treatment of domestic wastewater, with the combination of high nutrients (nitrogen and phosphorus) removal abilities of the microalgae and ecient degradation of organic matter by bacteria. Second, the symbiosis between microalgae and bacteria exchanging CO2 and O2 makes aeration not necessary, thus signicantly reducing the operating costs of the wastewater treatment plant. Third, recovered biomass of algae could be used as highpro t products like biofuels and food source. Fourth, the xation of the atmospheric CO2 by the microalgae through photosynthesis would contribute to greenhouse gas reduction in the atmosphere. Finally, the decayed microalgal biomass could serve as organic carbon source for denitrication by bacteria. However, this technology is not well-known yet, and further research on the in uence of the operating parameters have to be conducted. Furthermore, its main drawback is the harvesting of the microalgae. Due to their small size, microalgae demonstrate a low settling rate of 3.6 x 10􀀀3 m/h. A solution is to enhance aggregation of the biomass through occulation, followed by granulation to reach fully-developed microalgal-bacterial consortia. These granules exhibit high treatment performances and high settleability. The rst objective of this project was to identify the optimal microalgae:bacteria ratio for COD and nutrients removal. The second objective was to form fully-developed granules, starting from a co-culture, and assess the in uence of dierent agitation speeds on the granulation. The photobioreactor with a microalgae:bacteria ratio of 75:25 expressed the most promising results in COD removal eciency (95% after 8 days), nitrogen removal eciency (94% after 9 days), phosphorus removal eciency (93% after 7 days), and biomass productivity (144 mg/L/d). In this photobioreactor, nitrogen was removed by a combination of assimilation into microalgal biomass and nitrication/denitrication by the bacteria. The COD was removed by degradation by bacteria and mixotrophic microalgae that used the organic matter as carbon source. Finally, phosphorus was removed by assimilation into microalgal biomass. Although obvious aggregation between microalgae and bacteria could be observed, no fully-developed granules were formed after 89 batches of 24 hours. Because of low COD and nitrogen removals, no starvation period was experienced by the co-cultures. Therefore, extracellular polymeric substances were not released in sucient amounts to enhance the granulation. Furthermore, excessive stirring (200 rpm) may have lead to microalgal cells damage, thus promoting bacterial growth over microalgal growth. On the other hand, slower agitation speeds (80 and 120 rpm) seemed to enhance microalgal growth. Overall, the agitation speed appeared to have an indirect impact on the treatment performances and the settleability of the co-cultures, by signicantly in uencing the microalgal and bacterial growths.

2020

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

Bacterial Resource Management for Nutrient Removal in Aerobic Granular Sludge Wastewater Treatment Systems

David Gregory Weissbrodt

The approach of wastewater treatment has shifted towards a holistic view in order to achieve sustainability in addition to environmental protection. The aerobic granular sludge (AGS) technology, which relies on the use of fast-settling granular biofilms called granules, is progressively becoming a new standard for high-rate biological nutrient removal (BNR) and secondary clarification in single sequencing batch reactors (SBR). The AGS intensive process has been related with definite savings in land area, construction, and operation costs. In an economical analysis, theoretical savings of CHF 0.45 per m3 (1 Swiss franc CHF ≈ 0.83 €) were computed for a Swiss wastewater treatment plant (WWTP) of 200’000 capita removing all nutrients biologically. Besides scale-up, fundamental research was required to understand and tailor the structure of underlying bacterial communities that form granules and remove nutrients. Mechanisms of bacterial selection were investigated in a systems approach to propose strategic axes for optimal management of the bacterial resource for efficient process performances. This research led to the following advances. After having designed a flexible reactor infrastructure for AGS research, a mathematical modeling methodology was developed to understand the hydraulic and biological processes involved during plug-flow transport of wastewater across the settled AGS bed during the feeding phase. BNR relies on the preferential selection of polyphosphate-accumulating organisms (PAO) by proper uncoupled supply of electron donors and electron acceptors. A bed height to diameter ratio of 6 was deduced to be optimal for full bed loading and efficient anaerobic acetate uptake under reference simulation conditions (20°C, pH 7.0). A bioinformatics methodology called PyroTRF-ID was developed for the identification of bacterial relatives involved in BNR processes by combination of terminal-restriction fragment length polymorphism (T-RFLP) and pyrosequencing data. This procedure provided high resolution of bacterial community structures and dynamics of AGS systems. Electrical conductivity and polyphosphatase assays were proposed for rapid and low-cost assessment of the fractions of active PAO and of the dephosphatating potential of activated sludge and granular sludge. Positive linear correlations were obtained between the fraction of PAO, the biomass specific rate of conductivity evolution measured in anaerobic metabolic batch tests, and the polyphosphate-hydrolyzing enzymatic activity of cell extracts. Wash-out conditions used to stimulate granule formation were shown to exert a selection pressure not only on physical properties of early-stage granules, but also on the underlying bacterial community composition. Operation with constant volumetric organic loading rates (OLR) during wash-out resulted in the formation of slow-settling fluffy granules dominated by filamentous Burkholderiales affiliates formed under low aeration (< 2 cm s-1) and high mesophilic temperature (30°C), and of fast-settling dense granules dominated by Zoogloea spp. under high aeration (4 cm s-1) and with an inoculum originating from a BNR-WWTP. These exopolysaccharide producers were considered as model organisms for granulation. However, their proliferation over PAO and nitrifiers correlated with poor BNR. PAO estab-lished at mature stage as soon as sufficient AGS was present to fully remove volatile fatty acids (VFA) during the anaerobic feeding phase. Without purge of excess sludge, glycogen-accumulating organisms (GAO) competed with PAO at a later stage of reactor operation. Granulation was shown to occur in stirred-tank SBRs operated under steady-state condi¬tions to cultivate PAO and GAO enrichments. The Accumulibacter- and Competibacter-dominated communities displayed granulation potential without involvement of Zoogloea spp. Fast-settling nuclei formed despite initial operation with a settling time as high as 60 min, and evolved towards granules after decrease of settling time to 10 min. Dynamic control of the OLR, anaerobic contact time, and sludge retention time (SRT) selected for active PAO in early-stage granules. Confocal laser scanning microscopy (CLSM) revealed that granula¬tion mechanisms depend on the predominant organisms involved. Fast-growing Zoogloea spp. formed smooth biofilm continuum matrices. The slower-growing PAO and GAO formed het¬erogeneous granular structures by proliferation in compact colonies around flocs. Fluctuations in operation variables were shown by multivariate statistics to impact on process performance and bacterial community structures in anaerobic-aerobic AGS-SBRs. The size of granules impacted on nitrification and dephosphatation by affecting oxygen mass transfer. Efficient BNR was obtained with at least 500 mgCOD L-1of acetate, 30 gCOD gP-1, and 10 gCOD gN-1. Although clades of PAO can denitrify, nitrogen removal was thus not restricted only to PAO. A broad denitrifying community was identified. The bacterial community continuum comprised two major opponent clusters, composed of members of the core microbiome of full-scale BNR-WWTPs. The first mainly comprised Accumulibacter, Nitrospira, Xanthomonadaceae, and Aminobacter affiliates selected by conditions of efficient BNR. The second mainly comprised Competibacter, Sphingobacteriales, Cytophaga, and Tetrasphaera affiliates correlating with periods of low BNR. Bacterial selection in AGS systems was mainly impacted by pH, according to multifactorial experiments. Accumulibacter and BNR were favored with alkaline pH, low mesophilic temperatures, and in the presence of propionate. Competibacter proliferated with acidic pH, temperature close to 30°C, and only acetate. Tetrasphaera spp., potential major PAO of certain full-scale plants, withstood Competibacter-selective conditions. Enhanced BNR was obtained under non-limiting conditions with food-to-microorganism ratios above 25 mgCODs gCODx-1. With fixed 2-h starvation, dephosphatation was only complete with full aeration. Since alternating aerobic-anoxic starvation conditions led to partial conversions, the length of starvation phases should be controlled in function of the redox conditions applied. Overall, optimal bacterial resource management in AGS systems requires full control of system behavior by taking advantage of the flexibility of the SBR technology. A methodology is proposed to this end, comprising milestones for wastewater characterization, SBR design, “anaerobic selector” design, control of the starvation phase length, shaving of fluctuations in operation variables, purge of excess sludge, proper selection for a stable, active and cooperative bacterial community, as well as efficient BNR. As key research perspective, investigations with real wastewater should validate the knowledge gained on bacterial behaviors under low-complexity conditions. Significant differences in predominant bacterial relatives were detected between lab-scale and full-scale BNR sludge. Metagenomics will provide a broader view on key organisms and functions of BNR and AGS microbiomes.

EPFL2013