Nitrous oxide emissions during microalgae-based wastewater treatment: current state of the art and implication for greenhouse gases budgeting

Nitrous oxide (N₂O) is a greenhouse gas (GHG) and a potent ozone depleting atmospheric pollutant (Ravishankara et al. 2009) that is biologically produced via numerous pathways (Ciais et al. 2013; Plouviez et al. 2019b). The Intergovernmental Panel on Climate Change (IPCC) considers that wastewater treatment generates significant N₂O emissions via two routes: (1) directly via nitrification or nitrification–denitrification during centralised aerobic wastewater treatment; and (2) indirectly via the discharge of nitrogen (N) triggering bacterial nitrification and denitrification in aquatic environments (Box 1, based on Bartram et al. 2019). The ability of microalgae to synthesise N₂O however challenges this ‘bacterial centric’ approach to computing N₂O emissions. Hahn & Junge (1977) early hypothesised that N₂O was the by-product generated during the reductive assimilation of nitrate (NO₃⁻) by phytoplankton, and Weathers and co-authors (Weathers 1984; Weathers & Niedzielski 1986) later demonstrated the ability of axenic eukaryotic microalgae and cyanobacteria to synthesise N₂O in the presence of nitrite. More recently, Guieysse et al. (2013) showed axenic Chlorella vulgaris could synthesise N₂O via nitrate reduction under oxia and several routes for N₂O synthesis in Chlamydomonas reinhardtii were established by Plouviez et al. (2017b) and Burlacot et al. (2020).

Box 1

N₂O emissions from wastewater treatment and discharge

The Intergovernmental Panel on Climate Change (IPCC) currently considers that only bacterial nitrification and denitrification causes N₂O emission during wastewater treatment (Bartram et al. 2019). These emissions can occur directly during centralised aerobic wastewater systems, and indirectly in aquatic environments receiving nitrogen (N)-laden effluents (Bartram et al. 2019).

The IPCC proposes a tiered methodology approach to estimate N₂O emissions during centralised wastewater treatment and discharge. Countries with limited data should follow the Tier 1 method and therefore use default values for the emission factor and activity parameters. The Tier 2 method is similar to the Tier 1 method but allows for country-specific emission factors and activity data. Finally, countries with good data and advanced methodologies (e.g. plant specific emissions factors), can follow a country-specific Tier 3 method.

Direct N₂O emissions are computed based on the amount of nitrogen found in wastewater and the degree of utilisation of treatment/discharge pathways or systems:

where is the amount of N₂O emitted from wastewater treatment plants (kg N₂O·yr⁻¹); is the amount of total nitrogen in wastewater (kg N·yr⁻¹); is the fraction of population in income group i; is the degree of utilisation of treatment/discharge pathway or system j, for each income group fraction i; i is the income group (rural, urban high income and urban low income); j is each treatment/discharge pathway or system; is the emission factor for treatment/discharge pathway or system j (default = 0.016 kg N₂O-N·kg N⁻¹); and is the conversion factor of kg N-N₂O into kg N₂O. The value of can be computed as the sum of the total annual amount of nitrogen in wastewater for each treatment pathway:

where is the total annual amount of nitrogen in wastewater for treatment pathway j (kg N·yr⁻¹); is the human population who are served by the treatment pathway j (person·yr⁻¹); is the annual per capita protein consumption (kg protein·person⁻¹·yr⁻¹, estimated as a fraction of protein consumed and the annual per capita protein supply (kg protein·person⁻¹yr⁻¹)); is the fraction of nitrogen in protein (default = 0.16 kg N·kg protein⁻¹); is the additional nitrogen from household products added to the wastewater (default value of 1.1); is the factor for nitrogen in non-consumed protein disposed in sewer system (kg ·kg N⁻¹, value of 1 if food waste is disposed with solid waste) and is the factor for industrial and commercial co-discharged protein into the sewer system (default value of 1.25 kg N·kg N⁻¹ for centralised treatment and 0 for decentralised treatment or untreated wastewater discharge).

Indirect emissions from effluent discharges are computed based on the fraction of the N emitted as N₂O as:

where is the total indirect N₂O emissions (kg N-N₂O·yr⁻¹); is the nitrogen in the effluent discharged to aquatic environments (kg N·yr⁻¹); and is the emission factor for N₂O emissions from discharged wastewater (kg N-N₂O·kg N⁻¹). While the default value for is set at 0.005 kg N-N₂O·kg N⁻¹ for a Tier 1 method, it is set at 0.019 kg N-N₂O·kg N⁻¹ for a Tier 3 method (i.e. for wastewater discharged in nutrient-impacted (eutrophic) or hypoxic aquatic environments). The value of is computed as:

where is the degree of utilisation of treatment system j (from all income groups); j is each wastewater treatment type used; and ⁠. is the fraction of total wastewater N removed during wastewater treatment per treatment type j (including transfer to sludge and nitrification–denitrification with concomitant N loss to the atmosphere).

The occurrence of studies reporting N₂O emissions from microalgae-based systems has also intensified and N₂O emissions have now been reported from a range of cultivation systems representative of the microalgal industry (see Plouviez et al. 2019b for examples). In light of this knowledge and the current popularity of microalgae-based wastewater treatment as a potential sustainable biorefinery platform (Javed et al. 2019; Pancha et al. 2019), this review summarises recent advances on N₂O emissions during microalgae-based wastewater treatment and discusses the implications of these findings. This work seeks to inform researchers and policymakers of a potential new source of N₂O emissions that must be better understood and, if deemed to be significant, must be addressed in GHG budgeting, climate change policy, and the assessment of the environmental credentials of microalgae-based wastewater treatment technologies.

High rate algal ponds (HRAPs) are broadly heralded as sustainable processes that enable combining wastewater treatment with economic biomass cultivation for resource recovery (Craggs et al. 2012; Ángeles et al. 2019). Challenging this view, two studies have investigated N₂O emissions during microalgae-based wastewater treatment: Alcántara et al. (2015) first recorded average N₂O emission of 23 μg N-N₂O·m⁻²·d⁻¹ during synthetic wastewater treatment in bench scale HRAPs. Based on their average value, these authors concluded that at large scale the N₂O emissions potentially generated from HRAPs treating domestic wastewater would have a low environmental impact. However, the authors also acknowledged that monitoring under full-scale conditions would still be needed to confirm their findings. Plouviez et al. (2019a) more recently quantified N₂O emissions from an outdoor pilot HRAP fed primary domestic wastewater. Over a year of monitoring, these authors recorded highly variable N₂O emissions of 70–18,300 μg N-N₂O·m⁻²·d⁻¹. Based on the 25–75% data range emission recorded (2,250–9,700 μg N-N₂O·m⁻²·d⁻¹), these emissions represented 0.13–0.57% of the influent N load when the pond was operated at a hydraulic retention time of 7.5 days (see Plouviez et al. (2019a) for further details). In comparison, the IPCC documents N₂O emissions representing 0.016–4.5% of the influent N load and the IPCC recommends a default value of 1.6% for ‘centralised aerobic wastewater treatment’ (Bartram et al. 2019). While a meaningful assessment is currently not possible given the scarcity and variability of the data available, the current data show HRAPs have the potential to generate significant amounts of N₂O despite harbouring a very different ecology than advanced centralised systems. This challenges the IPCC assumption that direct N₂O emissions only take place during centralised aerobic treatment systems (aerobic shallow ponds are considered as ‘unlikely source of CH₄ and N₂O’) and raises the question if other microalgae-based treatment system also release N₂O because microalgae can abound in the waste stabilisation ponds used by countless farms all over the world (Shilton & Walmsley 2005). Glaz et al. (2016) indeed reported median emissions of 0.04 and 0.53 μg N-N₂O·m⁻²·d⁻¹ from ponds located in Australia and Quebec, while Hernandez-Paniagua et al. (2014) reported emissions of 8–605 μg N-N₂O·m⁻²·d⁻¹ representing 0.0001–0.01% of the influent N input load (1,715 g TN·d⁻¹) from ponds located in Mexico. Although these emissions appears to be lower than the emissions reported during advanced wastewater treatment, caution is needed given the very limited number of studies available and the inherent challenges associated with detecting microalgae N₂O synthesis (see further discussion below).

As explained in Box 1, the IPCC estimates indirect N₂O emissions from wastewater discharge based on the assumptions that a fraction of the N discharged is emitted as N₂O via bacterial nitrification and/or denitrification. This assumption must be challenged in view of the ability of microalgae to synthesise N₂O in ecosystems receiving wastewater effluents. Interestingly, as part of its Tier 3 methodology, the IPCC proposes to use an EF of 0.019 kg N-N₂O·kg N⁻¹ for ‘nutrient-impacted’ aquatic environments (against 0.005 kg N-N₂O·kg N⁻¹ in the Tier 1 methodology). This is critical because N₂O emissions from eutrophic ecosystems have been documented for years (see the review of Plouviez et al. 2019b) and Delsontro et al. (2018) found that N₂O emissions from lakes and impoundments are expected to increase as a function of lake size and chlorophyll a (a proxy for the presence of microalgae). These authors therefore predicted global N₂O estimates of 190–450 kt N-N₂O·yr⁻¹ for lakes, which is equivalent to 32–75% of the current IPCC estimate of 600 kt N-N₂O·yr⁻¹ for N₂O emitted from all rivers, estuaries and coastal zones (Ciais et al. 2013). Focusing on eutrophic lakes, Plouviez et al. (2019b) conservatively estimated that eutrophic lakes and reservoirs could generate 110 kt N-N₂O·yr⁻¹ (or 18% of the current IPCC estimate for all rivers, estuaries and coastal zones). In contrast, Webb et al. (2019) showed that 67% of 101 constructed agricultural reservoirs monitored (<0.01 km²) acted as N₂O sinks despite of their highly eutrophic status (99 ± 289 μg⋅L⁻¹ chlorophyll a). While we cannot explain these contradictory findings based on the limited amount of relevant data in the field, the magnitude of the potential emissions from eutrophic ecosystems is critical to properly assess indirect N₂O emissions from wastewater discharges because nitrate pollution triggers microalgae growth and microalgae can synthesise N₂O when fed nitrate (Plouviez et al. 2017a). It is therefore possible that N discharge from wastewater effluents is fuelling both algae growth and N₂O synthesis. There is also increasing evidence that the combination of N and phosphorus (P) is critical to trigger microalgae growth in eutrophic ecosystems (Conley et al. 2009; Xu et al. 2010; Paerl et al. 2016; Smith et al. 2016). This means that, in comparison with the current methodology used by the IPCC, a new methodology based on both N and P loads on receiving waters may be needed to accurately compute indirect N₂O emissions from wastewater effluent in GHG inventories.

The large variability in the emissions factors reported from centralised aerobic wastewater treatment and microalgae-based systems (Table 1) generates a large uncertainty in the estimation of direct emissions from wastewater treatment. This means that both centralised and microalgae-based systems may generate amounts of N₂O that are significantly different than what is currently estimated, and that further research must seek to improve the accuracy of predictions (Vasilaki et al. 2019). Wastewater discharge also contributes to fuel both microalgae growth and N₂O synthesis by microalgae so further research must therefore seek to reduce the considerable uncertainty associated with indirect emissions factors (Table 1) and to define specific EFs for ecosystems affected by different degree of eutrophication. To our opinion, further research must finally also establish how P discharge contribute to indirect emissions as this has the potential to dramatically change the way indirect N₂O emission are quantified and addressed.

Microalgal N₂O synthesis is influenced by factors such as the cell biology and the type and concentration of the nitrogen source microalgae receive (Plouviez et al. 2019b), meaning extensive monitoring (i.e. long-term with wide spatial coverage and high sampling frequency) of several microalgae-rich environments may be required to improve the accuracy of N₂O emissions from these systems. In addition, microalgal N₂O synthesis is influenced by light supply (Plouviez et al. 2019b), meaning that sampling methodologies may need to be adapted to capture this ‘microalgal activity’. For example, the rate of microalgal N₂O synthesis can vary greatly over short time scales (minute) due to, in particular, changes in solar radiation (Plouviez et al. 2017a). Frequent grab samples should therefore be preferred to, for example, the use of static floating chambers in order to capture temporal variability and prevent issues such as shading of the microalgae or N₂O re-dissolution when N₂O production is intermittent. Based on the recent identification of key genes involved during microalgal N₂O synthesis (Plouviez et al. 2017b; Burlacot et al. 2020), the use of genomics could also help to generate new and comprehensive insights into the occurrence and ecological implications of microalgal N₂O synthesis.

While aerobic shallow ponds are currently considered as ‘unlikely source of N₂O’ (Bartram et al. 2019), the magnitude of N₂O emissions during real wastewater treatment in HRAPs (0.13–0.57% of the influent N load) is within the range reported for direct emissions during centralised aerobic wastewater treatment (0.016–4.5%; Bartram et al. 2019) and indirect emissions from ‘nutrient-impacted’ aquatic environments (0.41–9.1%; Bartram et al. 2019). Monitoring using methodologies specifically designed to capture microalgal N₂O synthesis must now be conducted. This work will be critical to better understand the mechanism of microalgal N₂O synthesis and assess its significance. If significant, this mechanism should then be addressed in GHG budgeting, climate change policy, and the assessment of the environmental credentials of microalgae-based wastewater treatment technologies.

This research was supported by Massey University, New Zealand.