title: Carbon dosing of denitrifying bioreactors to remove nitrate from agricultural drainage

authors: Moghaddam, Reza

subjects: Woodchip bioreactors

abstract: Several studies have demonstrated that denitrifying woodchips bioreactors effectively remove nitrate from a broad spectrum of wastewater. Even though bioreactors have a comparatively small spatial footprint and are straightforward to construct, the low long-term nitrate removal efficiency they provide compared to other mitigation options, such as constructed wetlands, may limit their utilization by catchment managers and landowners. This thesis investigated the use of external carbon dosing in bioreactors to improve nitrate removal and quantified some potential adverse effects of carbon dosing on bioreactors and downstream aquatic environments. Through observations ranging from field to mesocosm experiments, this thesis sought to (1) determine whether and to what extent external carbon dosing could improve nitrate removal in bioreactors and (2) assess the potential adverse effects of carbon dosing, such as reduced hydraulic performance and pollution swapping. 
The first component of the study evaluated the carbon dosing of a pilot-scale (58 m³) bioreactor treating drainage waters from a 0.65 ha paddock on a dairy farm. The effects of carbon dosing on the bioreactor's nitrate removal rates were assessed following a constant dosage of 8% (v/v) methanol solution at 10 mL min⁻¹ to the bioreactor's inlet distributor. Sulfate reduction and losses of added methanol from the bioreactor’s outlet were quantified as the potential adverse effects of external carbon dosing. Methanol dosing increased seasonal nitrate removal rates to 8 g N m⁻³ day⁻¹ in 2020 and 5 g N m⁻³ day⁻¹ in 2021 (with a halved dosing rate), compared to 1 g N m⁻³ day⁻¹ in the undosed bioreactor in 2018. Even under nitrate-limiting conditions, the added methanol was effectively removed in the bioreactor, with a mean removal rate of 106 g CH₃OH-C m⁻³ day⁻¹
. Methanol concentrations decreased by order of magnitude along the bioreactor (lengthwise) and never exceeded 50 mg CH₃OH-C L⁻¹ at the outflow. The added carbon caused sulfate reduction with a mean removal rate of 8.5 g SO4²⁻-S m⁻³ day⁻¹ in 2020 (higher dosing rate) and 0.5 g SO4²⁻-S m⁻³ day⁻¹ in 2021 (halved dosing rate). Carbon dosage at a continuous rate was shown to be a viable method for increasing nitrate removal while minimizing capital and operational costs. The simplicity of this approach is a significant advantage, and it could be used by a variety of industries, including farmers.
While field observations showed that carbon dosage could improve nitrate removal, the very variable flows and nitrate concentrations made determining nitrate, sulfate, and methanol removal rates difficult. Therefore, the second component of the thesis was to assess nitrate, sulfate and added carbon removal rates more accurately using mesocosms. An empirical model was also developed to calculate the amount of methanol required to remove specific loads of nitrate. Mesocosm bioreactors were designed to share some characteristics with the field bioreactor (for example, added carbon component and dosage ratio) but diverged in that they were hydrologically regulated to eliminate the impact of transient operating circumstances on bioreactor performance. Carbon dosing increased nitrate removal rates from 7 g N m⁻³ day⁻¹ in the control treatment bioreactors (without methanol addition) to 27 g N m⁻³ day⁻¹ in the methanol-dosed treatment bioreactors. Added methanol was removed in either the absence or presence of nitrate, with mean methanol removal rates of 23 g CH₃OH-C m⁻³ day⁻¹ in nitrate prevailing conditions and 17 g CH₃OH-C m⁻³ day⁻¹ in nitrate limiting conditions. The empirical model suggested that a methanol to nitrate ratio of 0.7 resulted in complete nitrate removal. 
In the third component of this thesis research, observations from both mesocosm and full-scale bioreactors were used to assess the hydraulic characteristics of bioreactors under various carbon dosage regimes. Field measurements revealed a decrease in hydraulic conductivity from 4601 m day⁻¹ in 2018 (season without carbon dosing) to 1600 m day⁻¹ in 2021 (second year of dosing). Based on tracer tests of the mesocosm bioreactors, I investigated the effects of added carbon on the internal hydraulic performance of bioreactors. The results of the tracer tests revealed that carbon dosing had no significant effect on the hydraulic parameters of the mesocosm bioreactors (p-value > 0.05). 
The findings of this thesis have implications for improving the performance of existing bioreactors or for developing new, externally dosed bioreactors with smaller spatial footprints. The constant dosage strategy, demonstrated in the full-scale bioreactor, was effective in increasing nitrate removal rates while also being safe regarding added carbon losses to the receiving environments. The ease of implementation of the constant dosage strategy could increase take-up from landowners and managers. The thesis also provides an understanding of whether and to what extent carbon dosing affects the hydrology of bioreactors, which can be used to develop new bioreactor designs with less backflow and bypass.

text: Carbon dosing of denitrifying bioreactors to remove nitrate from agricultural drainage Several studies have demonstrated that denitrifying woodchips bioreactors effectively remove nitrate from a broad spectrum of wastewater. Even though bioreactors have a comparatively small spatial footprint and are straightforward to construct, the low long-term nitrate removal efficiency they provide compared to other mitigation options, such as constructed wetlands, may limit their utilization by catchment managers and landowners. This thesis investigated the use of external carbon dosing in bioreactors to improve nitrate removal and quantified some potential adverse effects of carbon dosing on bioreactors and downstream aquatic environments. Through observations ranging from field to mesocosm experiments, this thesis sought to (1) determine whether and to what extent external carbon dosing could improve nitrate removal in bioreactors and (2) assess the potential adverse effects of carbon dosing, such as reduced hydraulic performance and pollution swapping. 
The first component of the study evaluated the carbon dosing of a pilot-scale (58 m³) bioreactor treating drainage waters from a 0.65 ha paddock on a dairy farm. The effects of carbon dosing on the bioreactor's nitrate removal rates were assessed following a constant dosage of 8% (v/v) methanol solution at 10 mL min⁻¹ to the bioreactor's inlet distributor. Sulfate reduction and losses of added methanol from the bioreactor’s outlet were quantified as the potential adverse effects of external carbon dosing. Methanol dosing increased seasonal nitrate removal rates to 8 g N m⁻³ day⁻¹ in 2020 and 5 g N m⁻³ day⁻¹ in 2021 (with a halved dosing rate), compared to 1 g N m⁻³ day⁻¹ in the undosed bioreactor in 2018. Even under nitrate-limiting conditions, the added methanol was effectively removed in the bioreactor, with a mean removal rate of 106 g CH₃OH-C m⁻³ day⁻¹
. Methanol concentrations decreased by order of magnitude along the bioreactor (lengthwise) and never exceeded 50 mg CH₃OH-C L⁻¹ at the outflow. The added carbon caused sulfate reduction with a mean removal rate of 8.5 g SO4²⁻-S m⁻³ day⁻¹ in 2020 (higher dosing rate) and 0.5 g SO4²⁻-S m⁻³ day⁻¹ in 2021 (halved dosing rate). Carbon dosage at a continuous rate was shown to be a viable method for increasing nitrate removal while minimizing capital and operational costs. The simplicity of this approach is a significant advantage, and it could be used by a variety of industries, including farmers.
While field observations showed that carbon dosage could improve nitrate removal, the very variable flows and nitrate concentrations made determining nitrate, sulfate, and methanol removal rates difficult. Therefore, the second component of the thesis was to assess nitrate, sulfate and added carbon removal rates more accurately using mesocosms. An empirical model was also developed to calculate the amount of methanol required to remove specific loads of nitrate. Mesocosm bioreactors were designed to share some characteristics with the field bioreactor (for example, added carbon component and dosage ratio) but diverged in that they were hydrologically regulated to eliminate the impact of transient operating circumstances on bioreactor performance. Carbon dosing increased nitrate removal rates from 7 g N m⁻³ day⁻¹ in the control treatment bioreactors (without methanol addition) to 27 g N m⁻³ day⁻¹ in the methanol-dosed treatment bioreactors. Added methanol was removed in either the absence or presence of nitrate, with mean methanol removal rates of 23 g CH₃OH-C m⁻³ day⁻¹ in nitrate prevailing conditions and 17 g CH₃OH-C m⁻³ day⁻¹ in nitrate limiting conditions. The empirical model suggested that a methanol to nitrate ratio of 0.7 resulted in complete nitrate removal. 
In the third component of this thesis research, observations from both mesocosm and full-scale bioreactors were used to assess the hydraulic characteristics of bioreactors under various carbon dosage regimes. Field measurements revealed a decrease in hydraulic conductivity from 4601 m day⁻¹ in 2018 (season without carbon dosing) to 1600 m day⁻¹ in 2021 (second year of dosing). Based on tracer tests of the mesocosm bioreactors, I investigated the effects of added carbon on the internal hydraulic performance of bioreactors. The results of the tracer tests revealed that carbon dosing had no significant effect on the hydraulic parameters of the mesocosm bioreactors (p-value > 0.05). 
The findings of this thesis have implications for improving the performance of existing bioreactors or for developing new, externally dosed bioreactors with smaller spatial footprints. The constant dosage strategy, demonstrated in the full-scale bioreactor, was effective in increasing nitrate removal rates while also being safe regarding added carbon losses to the receiving environments. The ease of implementation of the constant dosage strategy could increase take-up from landowners and managers. The thesis also provides an understanding of whether and to what extent carbon dosing affects the hydrology of bioreactors, which can be used to develop new bioreactor designs with less backflow and bypass.

year: 2022

title: Food miles or carbon emissions? Exploring labelling preference for food transport footprint with a stated choice study

authors: Caputo, Vincenzina

abstract: The ecological footprint of food transport can be communicated using carbon dioxide emissions (CO₂ label) or by providing information about both the length of time and the mileage travelled (food miles label). We use stated choice data to estimate conventional unobserved taste heterogeneity models and extend them to a specification that also addresses attribute nonattendance. The implied posterior distributions of the marginal willingness to pay values are compared graphically and are used in validation regressions. We find strong bimodality of taste distribution as the emerging feature, with different groups of subjects having low and high valuations for these labels. The best fitting model shows that CO₂ and food miles valuations are much correlated. CO₂ valuations can be high even for those respondents expressing low valuations for food miles. However, the reverse is not true. Taken together, the results suggest that consumers tend to value the CO₂ label at least as much and sometimes more than the food miles label.

text: Food miles or carbon emissions? Exploring labelling preference for food transport footprint with a stated choice study The ecological footprint of food transport can be communicated using carbon dioxide emissions (CO₂ label) or by providing information about both the length of time and the mileage travelled (food miles label). We use stated choice data to estimate conventional unobserved taste heterogeneity models and extend them to a specification that also addresses attribute nonattendance. The implied posterior distributions of the marginal willingness to pay values are compared graphically and are used in validation regressions. We find strong bimodality of taste distribution as the emerging feature, with different groups of subjects having low and high valuations for these labels. The best fitting model shows that CO₂ and food miles valuations are much correlated. CO₂ valuations can be high even for those respondents expressing low valuations for food miles. However, the reverse is not true. Taken together, the results suggest that consumers tend to value the CO₂ label at least as much and sometimes more than the food miles label.

year: 2013

title: The Proceedings of the National Māori Graduates of Psychology Symposium 2002: Making a difference

authors: Nikora, Linda Waimarie

subjects: Maori

abstract: This is the full proceedings of the National Māori Graduates of Psychology Symposium 2002. The proceeding include the following themes: Kia matāra - negotiating the challenges in Māori development, kia mau – recruitment and retention, Tuhia mai, whiua atu – research and methodology, tinia mai – interventions and treatment, taitaia i te ahi manuka – pride upon the skin.

text: The Proceedings of the National Māori Graduates of Psychology Symposium 2002: Making a difference This is the full proceedings of the National Māori Graduates of Psychology Symposium 2002. The proceeding include the following themes: Kia matāra - negotiating the challenges in Māori development, kia mau – recruitment and retention, Tuhia mai, whiua atu – research and methodology, tinia mai – interventions and treatment, taitaia i te ahi manuka – pride upon the skin.

year: 2003