Performance of a two-stage partial nitritation-anammox system treating the supernatant of a sludge anaerobic digester pretreated by a thermal hydrolysis process
Graphical abstract
Introduction
Efficient management of water and energy resources is crucial for human sustainable development given their importance for the global economy and life. Wastewater treatment plants (WWTPs) are central to water-energy interactions where the human footprint on the natural water environment is reduced by applying energy to remove pollutants [1]. For this reason, recent efforts to improve WWTPs have focused mainly on promoting their energy efficiency. Some improvements involved implementing control systems to adjust the treatment capacity to current demand, while others involved modifications of the WWTP operation, such as changes in their configurations or even the installation of new units [2]. Maximizing the recovery of the energy contained in the chemical bonds of organic matter is one of the key paths for increasing WWTP energy efficiency. When organic matter is aerobically degraded, the chemical energy simply dissipates as heat while the aeration needed to promote degradation also consumes energy. In contrast, energy consumption is greatly reduced by anaerobic degradation of organic matter into methane, the latter serving as a source of energy. For these reasons, proposals were made to accumulate a large part of the organic matter in the sludge by improving the efficiency of primary decanters or the use of high-load aerobic systems [3].
Higher mass flow to primary and/or secondary sludge would increase the biogas production in the WWTPs but would also increase the organic load towards the anaerobic digesters, potentially exceeding their design capacity. Several operating strategies have been studied to improve the overall rate of anaerobic digestion (AD) [4], and therefore, the treatment capacity of anaerobic digesters. The implementation of thermal hydrolysis units prior to anaerobic digestion has been the most successful strategy applied at the WWTP sludge line [5]. However, the promotion of sludge hydrolysis also increases ammonium concentration in the AD dewatering liquor that is returned to the mainstream. This increase entails both the need, in the biological reactor, for more organic matter to carry out the denitrification process (less methane production) and for more dissolved oxygen (DO) for the nitrification process. Ammonium from the AD dewatering liquor would have to be removed in the sludge line to avoid these drawbacks. Since AD dewatering liquors have a low chemical oxygen demand to nitrogen ratio (COD/N) and their temperature is around 30 °C, the partial nitritation/anammox (PN/A) process is the most suitable nitrogen removal process to treat this stream. PN/A process has been successfully applied in the sludge line of >100 WWTPs with a single-stage configuration, where both PN and anammox processes occur under limited DO conditions (0.3–0.6 mg O2/L) in the same unit [6]. The joint implementation of sludge thermal pre-hydrolysis (THP) and PN/A systems for the AD dewatering liquor treatment seems to be a winning combination to increase the WWTP energy efficiency [7]. However, recent research has shown that incorporating a THP process can cause stability problems during the operation of single-stage PN/A systems [8]. Zhang et al. [9] attributed these problems to a direct inhibitory effect of the THP/AD dewatering liquor, which caused a decrease in the microbial growth rate and an indirect inhibitory effect related to substrate diffusional limitations due to the presence of particulate and colloidal fractions of COD. Nevertheless, the causes of such operational stability losses are still not clear [10].
Single-stage PN/A systems have a relatively low operational flexibility since they must be operated at low DO concentrations to maintain a balance between the activities of ammonia oxidizing bacteria (AOB, aerobic) and anammox bacteria (anoxic). Moreover, the solids retention time (SRT) needs to be fixed in a narrow range that avoid the proliferation of both nitrite oxidizing (NOB) and denitrifying bacteria but allowing the development of AOB and anammox bacteria. Generally, sludge anaerobic digesters operate at high hydraulic residence time (HRT) to buffer large fluctuations in the effluent quality. The implementation of THP units, however, allows a shortening of the HRT of the sludge anaerobic digesters and causes fluctuations in their effluents. This fact, coupled with the increase in the concentration of solids and organic matter due to the THP process itself, has caused operational problems in the single-stage PN/A systems [8]. In this sense, the application of a two-stage PN/A system could be an alternative to treat THP/AD dewatering liquors where ammonium oxidation and anammox processes are carried out in separate units and, therefore, the operating conditions for each process can be optimized independently.
In this work, the long-term operational stability of a two-stage PN/A system treating THP/AD dewatering liquor was studied. In addition, batch activity tests were carried out to understand the inhibitory effect of the THP/AD liquor over AOB activity and the benefits of performing an aerobic pre-treatment. Finally, an economic analysis was performed to compare the viability of one-stage and two-stage processes.
Section snippets
Partial nitritation reactor
A sequencing batch reactor (SBR) system, with a working volume of 3 L (total height of 50 cm and diameter of 10 cm), was used to carry out the partial nitritation process. The volumetric exchange ratio was fixed at 50%. The length of the operational cycles was adjusted according to the desired HRT and ranged from 3 to 12 h. The cycle consisted of the following series of sequential process phases: simultaneous feeding and reaction under aerobic conditions (120–660 min), aerobic reaction without
Start-up and stabilization
The partial nitritation reactor was initially operated at an HRT of 6 h and was fed with diluted THP/AD dewatering liquor (Fig. 1a). The dilution applied to the THP/AD dewatering liquor was gradually decreased as the reactor maintained an ammonium oxidation efficiency of around 50%, which is the maximum efficiency expected according to the alkalinity present in the system (3.5 mg CaCO3/mg NH4+-N). During the application of this operating strategy, there were several episodes of operational
Conclusions
The PN reactor start-up strategy based on feeding the undiluted THP/AD effluent and gradually decreasing the HRT allowed to obtain a stable reactor performance. This shows that it is not necessary to apply dilution water to this type of effluents as is usually done at full scale, which would reduce treatment costs.
Contrary to single nitrogen removal systems, the PN reactor can be operated under non limiting conditions of both solids retention times and DO levels since the proliferation of NOB
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by the Chilean Government through the Projects ANID/FONDECYT/1200850 and CRHIAM Centre grant number ANID/FONDAP/15130015, by the Spanish Government through TREASURE [CTQ2017-83225-C2-1-R] and by the European Commission LIFE ZERO WASTE WATER [LIFE19ENV/ES/000631] projects. The authors from Universidade de Santiago de Compostela belong to CRETUS Strategic Partnership [ED431E 2018/01] and the Galician Competitive Research Group [GRC ED431C 2017/29]. All the Spanish
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