How to cope with NOB activity and pig manure inhibition in a partial nitritation-anammox process?
Introduction
The increasing pig production in EU and the current trend to concentrate it in small and localized areas to reduce the production costs have led to increased livestock wastes [1]. The abusive use of manure as fertilizer causes a rise in nutrient concentration in soils (mainly N and P), groundwater and surface water [2]. This growing situation forced pig farms to confront with an increasing number of environmental regulations, concerning the application of manure as direct fertilizer on agricultural land. Thus, sustainable solutions for pig manure treatment regarding nitrogen removal need to be implemented, considering that swine wastewaters are among the most problematic livestock waste streams [3], [4].
One of the most attractive treatment alternatives to land application is the previous anaerobic digestion (AD) for the removal of organic matter and the production of biogas as a valuable product, whereas biological removal is the most widely adopted approach regarding the removal of nitrogenous compounds [5]. With respect to the latter one and far from conventional systems like nitrification-denitrification, new schemes have been investigated based on the anammox (anaerobic ammonium oxidation) process, and the autotrophic nitrogen removal has mainly drawn the attention of researchers [1].
In particular, partial nitritation coupled to the anammox process (PN-AMX) is regarded as one of the best suitable options for the removal of nitrogen following anaerobic digestion [6]. In this process, ammonium oxidizing bacteria (AOB) transform the ammonium present in the wastewater to nitrite during partial nitritation, and in a second step the autotrophic anammox bacteria use this nitrite as electron acceptor to oxidize the ammonium to nitrogen gas, while nitrate formation is prevented. The interest for this process focuses on the lower operational costs in comparison with conventional processes (e.g. nitrification-denitrification): less energetic and aeration requirements, negligible sludge production and no necessity of external carbon (organic matter) addition [1], [6].
Regarding the application of the PN-AMX process to manure effluents after anaerobic digestion, some compounds of the digested pig manure may inhibit the bacterial activities, like for example salts [7], and certain concentrations of biodegradable organic matter [3]. Furthermore, the presence of an excessive concentration of biodegradable organic matter may provoke the competition for the nitrite between heterotrophic bacteria and autotrophic anammox bacteria, rather than the competition with AOB for the oxygen [8]. However, it is the presence of nitrite oxidizing bacteria (NOB) activity one of the most reported problems at full scale [9], that need to be controlled to avoid the complete oxidation of ammonium to nitrate, and also to avoid the competition with AOB for oxygen and with anammox bacteria for nitrite [10]. Microaerobic conditions, for example, can favor the partial nitritation to nitrite instead of the total nitrification to nitrate [8], as well as sufficiently high concentrations of free ammonia (FA) in the system [11].
More than 100 PN-AMX installations have been implemented at full-scale over the last decade, the majority of them applied the one-stage configuration for the treatment of the supernatant from sludge anaerobic digester in municipal WWTPs [9], [12]. However, the full-scale application of the PN-AMX process for the treatment of manure using the aforementioned alternative is still scarce [13] and more research is necessary, principally to evaluate the presence of NOB activity in the available inoculum and the possible inhibitory effects of manure compounds over the anammox activity.
For this reason, the present research work presents the first results about the operation of a PN-AMX system for the treatment of pig manure with significant NOB activity in the inoculum. Furthermore, the loss of anammox activity due to the conductivity and the organic matter content of the pre-digested manure is discussed. The gathered knowledge will be useful to address potential issues during the start-up of the process for its application at full-scale.
Section snippets
Experimental set-up
Two laboratory sequencing batch reactors (SBRs) with working volumes of 0.65 and 1.4 L were used for three short experiments (SE) and a continuous operation period, respectively (Table 1). The aeration system for each of the reactors consisted of a diaphragm pump (Laboport N86, KNF) for the air supply and a diffuser located at the bottom of the reactor, providing good mixture inside the reactor. The dissolved oxygen (DO) concentration was periodically measured with a DO probe (Hach Lange, model
Short experiments to evaluate the start-up of the PN-AMX process for swine slurry treatment
In the first short experiment (SE-1, Fig. 1a/b) the inoculum used was collected from a PN-AMX pilot plant treating the supernatant of an anaerobic sludge digester in a municipal WWTP. The inoculated biomass had a SAAPBS of 0.071 g N/(g VSS·d) and the concentration of solids was of 8.81 g VSS/L (Table 2). This means a NRRmax of approximately 0.63 g N/(L·d), which was lower than the applied NLR in the first 9 operational days (between 0.7 and 0.8 g N/(L·d), Fig. 1a). Despite the later decrease in
Salinity and conductivity
The pre-treated swine slurry used as feeding in this research work contained different ions, being Na+, K+ and Cl− the ones with the highest concentrations (Table S3, Appendix A). The salinity as NaCl was in the range of 0.2–1.0 g NaCl/L for the short experiments (diluted) and 0.99 ± 0.05 g NaCl/L for the continuous operation (without dilution) (Table 1 and Fig. S5, Appendix A). According to previous reports [19], [22], [23], [24], [25], this NaCl concentration is low to induce inhibition over
Conclusions
The batch activity experiments performed indicated that the pig manure can at least produce an inhibition up to 44.4% in the maximum specific anammox activity. The conductivity of the pig manure (between 6 and 8 mS/cm) could be the main reason, while the presence of organic matter (sCOD/N = 0.18–1.14 g/g) seemed not relevant. The presence of NOB activity in the available inoculum hindered the fast start-up of the PN-AMX process for pig manure treatment, despite the presence of enough anammox
Declarations of interest
None.
Acknowledgements
This work was performed in collaboration with Abengoa Water, and funded by the Spanish Government (AEI) through the projects AgroEnergy (IDI-20140388), GRANDSEA (CTM2014-55397-JIN) and TREASURE (CTQ2017-83225-C2-1-R), co-funded by FEDER (EU), as well as through the FPU Grant Program (FPU-014/05476). The authors wish to express their gratitude to Nuria Fernández and Andrea Fra for their contributions on the FISH and sequencing analysis. The authors from the USC belong to CRETUS Strategic
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