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

Aerobic granular sludge reactor technology - Aerobic granulation with different substrates

Aerobic granular sludge is a promising alternative to traditional activated sludge basins in wastewater treatment plants (WWTP). Granular sludge has much better settling characteristics than activated sludge. Furthermore it can remove carbon, nitrogen and phosphorus within the same reactor. Aerobic granules can be formed in bubble column reactors (BCR) in sequencing batch mode (SB) by alternating anaerobic and aerobic phases. Our investigation consisted in running two reactors one with acetate as C-source, like in most studies, and one with propionate, to see if it is also possible to form granules with propionate. Propionate is assumed to favor the growth of phosphate accumulating organisms (PAO) over glycogen accumulating organisms (GAO). This is an important point for the removal performances, because in contrary to GAO, PAO can take up phosphate and may also denitrify (DPAO). A first objective was reached as we obtained stable, good settling granules with propionate. Concerning the reactor performances, we got 100 % of substrate removal, 83 % and 55 % of phosphate removal in the propionate and acetate reactor respectively, but only moderate ammonium removal. Based on these results we concluded that granule formation, C-removal and P-removal with acetate and also propionate as substrate is possible, but that further investigation is needed to cultivate nitrifying bacteria in the granules

Optimization of operation conditions for the startup of aerobic granular sludge reactors biologically removing carbon, nitrogen, and phosphorous

The transformation of conventional flocculent sludge to aerobic granular sludge (AGS) biologically removing carbon, nitrogen and phosphorus (COD, N, P) is still a main challenge in startup of AGS sequencing batch reactors (AGS-SBRs). On the one hand a rapid granulation is desired, on the other hand good biological nutrient removal capacities have to be maintained. So far, several operation parameters have been studied separately, which makes it difficult to compare their impacts. We investigated seven operation parameters in parallel by applying a Plackett-Burman experimental design approach with the aim to propose an optimized startup strategy. Five out of the seven tested parameters had a significant impact on the startup duration. The conditions identified to allow a rapid startup of AGS-SBRs with good nutrient removal performances were (i) alternation of high and low dissolved oxygen phases during aeration, (ii) a settling strategy avoiding too high biomass washout during the first weeks of reactor operation, (iii) adaptation of the contaminant load in the early stage of the startup in order to ensure that all soluble COD was consumed before the beginning of the aeration phase, (iv) a temperature of 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 using this optimized startup strategy produced again AGS with good nutrient removal performances within four weeks and the system was stable during the additional operation period of more than 50 days

MICROBIAL AND BIOMOLECULAR HETEROGENEITY OF AEROBIC GRANULES WITHIN A SINGLE BIOREACTOR ?

Aerobic granulation is considered as a promising alternative to conventional activated sludge in biological wastewater treatment. Instead of forming flocs the bacterial communities aggregate to dense granular biofilms. In the present study, granules were cultivated in bubble column reactors operated in sequencing batch mode. The operating cycle consisted of 4 phases: i) an anaerobic feeding phase, ii) a long aeration phase where the sludge was mixed, iii) a short settling phase and finally iv) a withdrawal phase where the treated water was taken out of the reactor. Feeding with synthetic wastewater was done from the bottom of the reactor in plug-flow mode without a mixing of the settled sludge. We made two observations which let us conclude that all the granules were not equally provided with substrate: 1) We measured the carbon consumption rate and calculated that only the lower 15-25% of the sludge bed got carbon in each cycle. If all granules would have had the same settling properties, this heterogeneity would have been cancelled over time. But 2) we observed a clear gradient in granule size within the sludge bed. Larger granules settled faster than smaller ones. Sludge from the bottom of the reactor was composed up to 80% by granules with diameter larger than 3.5 mm, whereas over the whole reactor the threshold diameter for 80% of the sludge volume was only at 2.4 mm. The heterogeneity in carbon accessibility is expected to influence the composition of bacterial communities and the wastewater treatment performances. For example, dephosphatation and denitrification are directly depending on the presence of bacteria containing internal carbon reserve in form of polyhydroxyalkanoates. We are presently investigating possible differences between “feast” and “famine” granules

Nitrogen Removal over Nitrite by Aeration Control in Aerobic Granular Sludge Sequencing Batch Reactors

This study investigated the potential of aeration control for the achievement of N-removal over nitrite with aerobic granular sludge in sequencing batch reactors. N-removal over nitrite requires less COD, which is particularly interesting if COD is the limiting parameter for nutrient removal. The nutrient removal performances for COD, N and P have been analyzed as well as the concentration of nitrite-oxidizing bacteria in the granular sludge. Aeration phase length control combined with intermittent aeration or alternate high-low DO, has proven to be an efficient way to reduce the nitrite-oxidizing bacteria population and hence achieve N-removal over nitrite. N-removal efficiencies of up to 95% were achieved for an influent wastewater with COD:N:P ratios of 20:2.5:1. The total N-removal rate was 0.18 kgN·m−3·d−1. With N-removal over nitrate the N-removal was only 74%. At 20 °C, the nitrite-oxidizing bacteria concentration decreased by over 95% in 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 concentration decreased too but less, and nitrite oxidation could not be completely suppressed. However, the combination of aeration phase length control and high-low DO was also at 15 °C successful to maintain the nitrite pathway despite the fact that the maximum growth rate of nitrite-oxidizing bacteria at temperatures below 20 °C is in general higher than the one of ammonium-oxidizing bacteria

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

Optimized aeration strategies for nitrogen and phosphorus removal with aerobic granular sludge

Biological wastewater treatment by aerobic granular sludge biofilms offers the possibility to combine carbon (COD), nitrogen (N) and phosphorus (P) removal in a single reactor. Since denitrification can be affected by suboptimal dissolved oxygen concentrations (DO) and limited availability of COD, different aeration strategies and COD loads were tested to improve N- and P-removal in granular sludge systems. Aeration strategies promoting alternating nitrification and denitrification (AND) were studied to improve reactor efficiencies in comparison with more classical simultaneous nitrification–denitrification (SND) strategies. With nutrient loading rates of 1.6 gCOD L−1 d−1, 0.2 gN L−1 d−1, and 0.08 gP L−1 d−1, and SND aeration strategies, N-removal was limited to 62.3 ± 3.4%. Higher COD loads markedly improved N-removal showing that denitrification was limited by COD. AND strategies were more efficient than SND strategies. Alternating high and low DO phases during the aeration phase increased N-removal to 71.2 ± 5.6% with a COD loading rate of 1.6 gCOD L−1 d−1. Periods of low DO were presumably favorable to denitrifying P-removal saving COD necessary for heterotrophic N-removal. Intermittent aeration with anoxic periods without mixing between the aeration pulses was even more favorable to N-removal, resulting in 78.3 ± 2.9% N-removal with the lowest COD loading rate tested. P-removal was under all tested conditions between 88 and 98%, and was negatively correlated with the concentration of nitrite and nitrate in the effluent (r = −0.74, p < 0.01). With low COD loading rates, important emissions of undesired N2O gas were observed and a total of 7–9% of N left the reactor as N2O. However, N2O emissions significantly decreased with higher COD loads under AND conditions

Role of ammonia-oxidizing bacteria in micropollutant removal from wastewater with aerobic granular sludge

Nitrifying wastewater treatment plants (WWTPs) are more efficient than non-nitrifying WWTPs to remove several micropollutants such as pharmaceuticals and pesticides. This may be related to the activity of nitrifying organisms, such as ammonia-oxidizing bacteria (AOBs), which could possibly co-metabolically oxidize micropollutants with their ammonia monooxygenase (AMO). The role of AOBs in micropollutant removal was investigated with aerobic granular sludge (AGS), a promising technology for municipal WWTPs. Two identical laboratory-scale AGS sequencing batch reactors (AGS-SBRs) were operated with or without nitrification (inhibition of AMOs) to assess their potential for micropollutant removal. Of the 36 micropollutants studied at 1 mu g l(-1) in synthetic wastewater, nine were over 80% removed, but 17 were eliminated by less than 20%. Five substances (bisphenol A, naproxen, irgarol, terbutryn and iohexol) were removed better in the reactor with nitrification, probably due to co-oxidation catalysed by AMOs. However, for the removal of all other micropollutants, AOBs did not seem to play a significant role. Many compounds were better removed in aerobic condition, suggesting that aerobic heterotrophic organisms were involved in the degradation. As the AGS-SBRs did not favour the growth of such organisms, their potential for micropollutant removal appeared to be lower than that of conventional nitrifying WWTPs

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

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

Significance of Rhodocyclaceae for the formation of aerobic granular sludge biofilms and nutrient removal from wastewater

The competition of two Rhodocyclaceae-affiliated bacterial populations was assessed during the formation and maturation of aerobic granular sludge in two different types of anaerobic-aerobic sequencing batch reactors. Zoogloea spp. (57%) were predominant in the bacterial community of aerobic granules cultivated in a 2.5-L bubble-column SBR under wash-out conditions selecting for a fast-settling biomass. Zoogloea spp. were favored over “Candidatus Accumulibacter phosphatis” relatives by high transient biomass specific organic loads leaking into the aeration phases. Accumulibacter proliferation over Zoogloea spp. and enhanced dephosphatation were observed after 80 days, as soon as the granular sludge concentration (>7 gVSS•L-1) and the sludge blanket (>30 cm) were sufficiently high to ensure full anaerobic carbon uptake. Formation of up to 2-mm granular aggregates was also observed in a 2.0-L conventional stirred-tank SBR operated at biomass steady-state for Accumulibacter enrichment from activated sludge. Granules formed in this reactor showed a low abundance of Zoogloea spp. (<5% during the first 20 days). The bacterial community was predominated by Accumulibacter (50-60%) and exhibited enhanced orthophosphate cycling profiles. Ensuring full anaerobic carbon uptake by progressively adapting the food-to-microorganism ratio and the anaerobic contact time is proposed to be the key for cultivating dephosphatating aerobic granules