Simultaneous removal of ammonium from landfill leachate and hydrogen sulfide from biogas using a novel two-stage oxic-anoxic system
Graphical abstract
Introduction
Biogas generated by anaerobic digestion of organic matter is a renewable energy source that can either be used to generate electricity and heat via a combined heat and power (CHP) gas engine, or can be upgraded for use as a fuel for vehicles and injection into gas grids (Baena-Moreno et al., 2019). Biogas is primarily composed of methane (CH4) (40–75%), carbon dioxide (CO2) (25–55%) and other compounds in minor quantities (Ullah Khan et al., 2017). Among these trace compounds, H2S stands out as the most undesirable as its mere presence hinders the revalorization of biogas. H2S removal is necessary due to serious problems like corrosion of pipelines or equipment and SOx emissions generated by its combustion (Paolini et al., 2018). Biological desulfurization has received a great deal of attention in the past decade due to its lower operational cost and minimum secondary pollution generation when compared to physico-chemical technologies (caustic chemical scrubbing, activated carbon adsorption, etc.) (Cano et al., 2018). The anoxic biodesulfurization process has numerous advantages over aerobic biodesulfurization and these include the existence of a simultaneous process for denitrification, reduction of explosion risks, no biogas dilution, lower mass transfer limitations and more accurate supply of the electron acceptor which results in better control over the final oxidation products (Fernández et al., 2014; Montebello et al., 2012). In this process hydrogen sulfide (H2S) can be oxidized to elemental sulfur (Eqs. (1), (2)) or sulfate (Eqs. (3), (4)) in the presence of nitrite or nitrate by autotrophic denitrification (Brito et al., 2018).
The anoxic desulfurization of biogas has commonly been carried out in biotrickling filters (BTFs) and this approach is robust and provides high removal efficiencies (Brito et al., 2019; Cano et al., 2019). BTFs have mainly been operated at high N/S molar ratios to produce sulfate as the main oxidation product to avoid the problems caused by the accumulation of elemental sulfur on the packing bed, which forces periodical maintenance to be carried out (Almenglo et al., 2016a; Qiu and Deshusses, 2017). Despite the fact that sulfur accumulation can be slowed by working at high N/S molar ratios (Almenglo et al., 2016b), sulfate generation is undesirable because it can be reduced again to H2S under anaerobic conditions (Celis-García et al., 2008). As a consequence, elemental sulfur stands out as the preferable oxidation product because it can be easily recovered and reused as a renewable feedstock for the fertilizer and chemical industries (Bouranis et al., 2019; González-Sánchez and Revah, 2006). In the present work, the use of a gas-lift bioreactor (González-Cortés et al., 2020) for anoxic desulfurization is proposed. In this way, the absence of a support to which elemental sulfur could adhere means that it could be easily recovered from the suspended biomass bioreactor (SBB). The optimal dosage of nitrate/nitrite should be supplied to the bioreactor in order to avoid extra operation costs, to obtain a high elemental sulfur selectivity and to maintain an outlet H2S concentration below the target limit (Cano et al., 2018; Li et al., 2016). Bearing the above statement in mind, some feedback and feedforward control strategies have already been implemented in anoxic biodesulfurization processes (Brito et al., 2017, Brito et al., 2019). Despite the operational advantages, anoxic desulfurization is associated with higher operational costs due to the high requirements for nitrite/nitrate (NO2−/NO3−) in cases where they are obtained from a chemical supplier (Cano et al., 2018; de Arespacochaga et al., 2014). The use of biological NO2−/NO3− from a nitrification reactor has the potential to solve this cost problem (Zeng et al., 2019). In this way, two pollutants (NH4+ and H2S) could be removed in the same process to obtain desulfurized biogas and an ammonium- and nitrate-free effluent that is rich in recoverable elemental sulfur. Anoxic desulfurization requires high NO2−/NO3− concentrations and, as a result, the use of effluents that are rich in ammonium is essential. These types of effluents can frequently be found in biogas plants. For example, the digestion slurry present in sewage treatment plants (STP) or landfill leachates from municipal solid-waste plants have proven to be potentially nitrified (Capodici et al., 2019; Zeng et al., 2019). The revalorization of landfill biogas has attracted more attention due to its high production rates when compared to STPs. Landfill leachates, which originate from the percolation of rain water through the landfill solid waste (Show et al., 2019), stand out as one of the most concentrated in ammonium. Moreover, the nitrification of landfill leachates has been widely reported (Capodici et al., 2019; Kim et al., 2006; Vilar et al., 2010).
Despite the fact that the microbial communities of the biofilm present in anoxic BTFs have been widely reported in the literature (Almenglo et al., 2016a; Brito et al., 2018; Valle et al., 2018), their composition has never been reported in SBBs. In contrast to BTFs, the characteristics of these bioreactors, in which biomass is suspended, are believed to favor the predominance of very few species and it may be of interest to characterize these further (Davey and O'toole, 2000).
The main objective of the present work was to study the feasibility of a novel two-stage oxic-anoxic (O/A) system. The first stage involved the use of a nitrification bioreactor to treat landfill leachate as an ammonium-rich source while the second was completed by a gas-lift bioreactor under anoxic conditions fed with biogas substitute. The nitrification bioreactor operation was monitored but the main focus of the present work was on the performance of an anoxic H2S-removing bioreactor under these demanding conditions. Studies have not been carried out previously on the operation of an integrated system like the one described here and, as a consequence, there are many unknowns that should be highlighted: (i) the performance of the anoxic desulfurization system when it is fed with nitrite/nitrate produced biologically using landfill leachate; (ii) the microbial resilience of the community in the gas-lift bioreactor when fed with a stream charged with high loads of microbes and (iii) the effect of the toxic compounds present in the landfill leachate on the stability of the integrated system, which has never been investigated before. Moreover, a proportional-integral (PI) feedback control strategy was implemented to ensure the stability of the integrated system. In order to ascertain the effect of the leachate, during the first stage the integration was carried out by feeding synthetic medium used as control supplemented with commercial ammonium to the nitrification bioreactor while, in the second stage, synthetic medium was replaced by real landfill leachate.
Section snippets
Experimental setup
Nitrification of the ammonium-rich effluents was carried out in a 3 L continuous stirred tank bioreactor (CSTBR) (Applikon Biotechnology BV, The Netherlands) with biomass recirculation using a settler (1.2 L). Dissolved oxygen (DO) was kept below 1.5 mg O2 L−1 and pH was controlled by the addition of NaHCO3 (50 g L−1) and H3PO4 (2 N).
Anoxic desulfurization was performed in a 3 L inner loop jacketed gas-lift bioreactor fed with biogas substitute (mixture of H2S and N2) and controlled by two
Effect of GRT on the two-stage O/A system at mid-term operation
The anoxic desulfurization performance is mainly dependent on the variation of the main operational parameters such as HRT, GRT, N/S molar ratio and IL (Cano et al., 2019; Dolejs et al., 2015; Mahmood et al., 2007). Firstly, an appropriate set of gain parameters had to be obtained in order to implement satisfactory and robust PI control. This was achieved using the same method to estimate the different gain parameters as Brito et al. (2019).
The same conditions were tested using synthetic medium
Conclusions
The integrated system demonstrated robustness and resilience when operated with two different ammonium sources, namely synthetic medium and real intermediate landfill leachate. The use of the system demonstrated the possibility of using a nitrified effluent obtained from intermediate landfill leachate. This fact reduces the operating costs of the anoxic desulfurization when compared to the use of commercial nitrate/nitrate. This novel technology can solve the problem caused by the presence of H2
CRediT authorship contribution statement
J.J. González-Cortés: Investigation, Formal analysis, Writing - original draft. F. Almenglo: Conceptualization, Methodology, Supervision. M. Ramírez: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition, Writing - review & editing. D. Cantero: Conceptualization, Project administration, Funding acquisition, Writing - review & editing.
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.
Acknowledgments
The Spanish Government (Ministry of Economy and Competitiveness) and the Vice-rectorate for Research of the University of Cadiz provided financial support through the project CTM2016-79089-R “Enhancement of landfill gas by an integrated biological system (Biointegra3)” and UCA/REC01VI/2017 (Universidad de Cádiz) respectively.
References (60)
- et al.
Techno-economic assessment of biogas plant upgrading by adsorption of hydrogen sulfide on treated sewage–sludge
Energy Convers. Manag.
(2016) - et al.
Effect of gas-liquid flow pattern and microbial diversity analysis of a pilot-scale biotrickling filter for anoxic biogas desulfurization
Chemosphere
(2016) - et al.
Operational conditions for start-up and nitrate-feeding in an anoxic biotrickling filtration process at pilot scale
Chem. Eng. J.
(2016) - et al.
Progressive change from nitrate to nitrite as the electron acceptor for the oxidation of H2S under feedback control in an anoxic biotrickling filter
Biochem. Eng. J.
(2018) - et al.
Life cycle assessment of different physical-chemical and biological technologies for biogas desulfurization in sewage treatment plants
J. Clean. Prod.
(2018) - et al.
Influence of trickling liquid velocity, low molar ratio of nitrogen/sulfur and gas-liquid flow pattern in anoxic biotrickling filters for biogas desulfurization
Biochem. Eng. J.
(2019) - et al.
Achievement of partial nitrification under different carbon-to-nitrogen ratio and ammonia loading rate for the co-treatment of landfill leachate with municipal wastewater
Biochem. Eng. J.
(2019) - et al.
Landfill leachate treatment in Brazil – an overview
J. Environ. Manag.
(2019) - et al.
Biogas deep clean-up based on adsorption technologies for solid oxide fuel cell applications
Chem. Eng. J.
(2014) - et al.
Biogas biodesulfurization in an anoxic biotrickling filter packed with open-pore polyurethane foam
J. Hazard. Mater.
(2014)