Digested blackwater treatment in a partial nitritation-anammox reactor under repeated starvation and reactivation periods
Graphical abstract
Introduction
The increasing water scarcity and resources depletion have triggered efforts on the implementation of sustainable water management approaches (European Commission, 2016; WWAP, 2017). Decentralized wastewater treatment systems become an attractive alternative to be applied in small agglomerations enabling the energy and nutrients recovery, ensuring the local water availability by reusing the treated water and decreasing both investment and operational costs (WWAP, 2017). Source-separation systems allow segregating the different streams for a more intensive treatment depending on their characteristics and their final use promoting the water reuse (Malila et al., 2019; WWAP, 2017). Blackwater (i.e., toilet water) is an organic matter and nutrients concentrated stream contributing to approximately 92% of total nitrogen, 75% of phosphorus and 52% of the organic matter contained in mixed domestic sewage (Gottardo Morandi et al., 2018). Moreover, blackwater composition considerably varies according to its origin, infrastructure, toilet flushing systems and user habits (Gao et al., 2019; Ren et al., 2018), as it is summarized in Table 1 blackwater is more concentrated in residential areas whereas the one deriving from workplaces or touristic installations (museums, parks, etc.) is generally more diluted.
The anaerobic digestion of blackwater allows recovering its energy content as biogas (Gao et al., 2019; Moges et al., 2018). In the case of the anaerobic membrane reactors (AnMBR), the high quality and disinfected nitrogen-rich permeate, after the ammonium oxidation to nitrate, may be used as fertilizer while irrigating. However, the irrigation water requirements (quantity and nutrients concentration) vary along the year and the type of crop growing (European Commission, 2016). Thus, a nitrogen removal system needs to be also considered to obtain a clean effluent suitable for other reuse purposes, or ultimately, to discharge, in order to reduce the potential environmental impact of the anaerobic digestion process.
The combination of the partial nitritation and anammox (PN/AMX) processes represents an adequate and intriguing alternative since it allows the completely autotrophic nitrogen removal from wastewater and promotes the recovery of the wastewater energy content. The anaerobic biodegradability of the blackwater range from 40 to 80%, meaning that residual organic matter is present in the effluent of the anaerobic digester (De Graaff et al., 2010; Gao et al., 2019). Nevertheless, previous studies indicate that it is possible to achieve and maintain stable the PN/AMX processes at moderate nitrogen concentrations and temperature (i.e., the conditions of the anaerobically digested blackwater) when moderate organic matter concentrations are present in wastewater (Hoekstra et al., 2019; Pedrouso et al., 2018). Despite the number of research works about blackwater treatment exponentially rose, scarce information is available about its treatment by anammox based process.
Decentralized systems would have to deal with large fluctuation in both flow and composition of wastewaters (European Commission, 2016). This high variability in wastewater production could top at the extreme case of a single office building where wastewater flow rate would be zero during nights, weekends and holidays. Thus, the biological systems are frequently exposed to famine conditions affecting the process robustness (Wang et al., 2018). Modular treatment trains enable to adapt the treatment requirements depending on the final water purpose. In the periods when treated wastewater is reused for irrigation, nitrogen removal process is not required and the corresponding unit would have no water supply. As anammox bacteria are traditionally considered sensitive to environmental changes, the study of short-term starvation effect over the anammox activity is of great interest. However, limited information is available about the response of simultaneous PN/AMX processes under oxygen and nitrogen absence (Reeve et al., 2016) and the influence of repeated starvation on the biomass from a reactor operating in transient conditions, as can occur in a decentralized system treating blackwater, was not studied.
Hence, the aim of this study is to evaluate the operation of a PN/AMX based processes (ELAN® process), treating anaerobically digested blackwater at room temperature, and to assess the impact of regular stops and reactivation periods on the biological reactor due to the highly variable influent flow rate in a decentralized system. Additionally, the study of the different microbial populations involved (anammox, ammonium and nitrite oxidizers and heterotrophs) was evaluated in terms of their specific activity, to understand the effect of the starvation/reactivation periods over them.
Section snippets
Reactors setup and operation
Three one-stage PN/AMX reactors, with a working volume of 4 L and a volume exchange ratio of 20%, were operated. Reactors were run as sequencing batch reactors (SBR) with a 3-h cycle configuration (See Table S1 in Supporting Material for details of the phase’s distribution in the cycle). The air flow rate was manually adjusted by means of a gas flow meter (P model, Aalborg). Mechanical stirring (with a velocity of 40–50 rpm) was provided in order to guarantee the reactor mixture. The SBRs were
Start-up and performance at the conditions of the digested blackwater
Both SBR-C and SBR-D were inoculated with biomass drawn from a full-scale ELAN® reactor treating the supernatant from a sludge anaerobic digester (Morales et al., 2018), which was acclimated to mesohphilic temperatures (around 30 °C), large nitrogen concentrations (>500 mg TN/L), as well as by lower COD/N ratios (<0.5 g sCOD/TN). However, from the start-up, stable PN/AMX process was achieved (Fig. 1) by adjusting aeration flow rates between 1.5 and 2.0 L/min (resulting in a DO concentration of
Effect of starvation/reactivation over the PN/AMX process
Results demonstrated the feasibility of long-term operation of a PN/AMX system under regular starvation and reactivation periods to treat blackwater at room temperature (14–21 °C). To the knowledge of the authors, no previous study investigated all these factors together, as previous literature was focused only on the anammox activity reactivation after storage and/or at higher temperatures. In the present study, both nitritation and anammox activities were re-established immediately after
Conclusions
Overall, this study demonstrated the technical feasibility of the one-stage PN/AMX process to treat blackwater originated in a decentralized system and operated at room temperatures.
With a synthetic medium simulating blackwater it was proved that the repeated starvation and reactivation periods (nights and weekends) have not adverse effects on the process performance: stable nitrogen removal efficiency (90%) was achieved with no remarkable difference compared to the process performance from a
Declaration of competing interest
None.
Acknowledgements
This work was funded by the Pioneer_STP (PCIN-2015-22 MINECO (AEI)/ID 199 (EU)) project by the WaterWorks2014 Cofunded Call (Water JPI/Horizon 2020) and by MEDRAR (IN852A 2016) project by the Galician Government. The work of G. Tocco was financially supported by the University of Cagliari (Italy) and by European Union within the framework of the Erasmus+ Traineeship Programme (2017-1-IT02-KA103-035458). A. Val del Rio is a Xunta de Galicia fellow (ED418B 2017/075). Authors from the USC belong
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