title: Rapid online detection of nitrate and quantification of nitrate removal via capacitive deionisation (CDI)

authors: Jose, Jojo

subjects: Nitrate detection

abstract: Abstract
The increasing nitrate contamination in water poses significant environmental and public health challenges. With the rapid development of Capacitive Deionization (CDI) as a low-cost point-of-use (POU) solution to the problem, there was a rapid need for an online in-situ method for nitrate detection to support emerging POU technologies. 

Due to these circumstances, it was necessary to investigate a new method for detecting nitrates, optimised for CDI’s requirements. Spectrophotometric based methods in small batch configuration (cuvette style) became our focus at the onset. A review of all the viable techniques reduced the design to the following common steps that needed to be converted into a viable miniature scale process: i) sample the liquid continuously, ii) pretreatment, iii) nitrate to nitrite reduction, iv) dye addition, v) colour development, vi) spectrophotometric measurement, and vii) dispense the waste.

With the aim of trialling a range of methods that had the potential of functioning as a continuous flow cell system within this design process, reverse engineering attempts were made using commercially available reagent sets from Palintest, and Hach Nitraver5 showing poor colour development. Thus, they were eliminated from further testing 

Subsequent experiments employed the zinc reduction method, which offered a cost-effective means of nitrate concentration through its reduction to nitrite, followed by colorimetric analysis. Even though the zinc method demonstrated promising results, challenges such as sensitivity to reaction time, over-reduction of nitrite to ammonia, and variability in reproducibility limited its effectiveness. Additionally, side reactions and maintenance issues, including zinc oxidation and clogging, hinder its suitability for our continuous process.

Vanadium Chloride reduction method was finally chosen for its efficiency, rapid reaction time, and robustness under varying conditions. This method, combined with UV-VIS spectrophotometric analysis and MATLAB-based data processing, enabled real-time nitrate monitoring with over 93.05% accuracy. Key parameters such as residence time, flow rate, and heat transfer were optimised to enhance system performance. A reaction temperature of 85°C enabled complete reduction of vanadium species within 2 minutes, ensuring safe operation, consistent repeatability, and high precision. To address axial dispersion in flow-through systems, a novel sample-and-hold configuration was implemented, utilising seven parallel reactors with 17-second intervals to achieve discrete temporal resolution and accurate measurement. The system achieved a high-frequency sampling rate of 210 samples per hour (105 samples per CDI cycle), allowing near real-time tracking of nitrate fluctuations during semi-batch CDI operation.

A Stella model was developed to simulate nitrate removal via CDI, providing high resolution insight into nitrate adsorption and desorption dynamics. The model predicted transient nitrate responses that did not far exceed the system’s temporal resolution, which guided the implementation of a discrete sampling interval of 17 seconds. The integration of experimental validation with system simulation underscores the effectiveness of this combined approach in achieving accurate, real-time nitrate monitoring. The outcomes of this research contribute to the advancement of automated water quality analysis systems and offer a scalable, field-deployable solution for mitigating nitrate pollution in drinking water supplies.

text: Rapid online detection of nitrate and quantification of nitrate removal via capacitive deionisation (CDI) Abstract
The increasing nitrate contamination in water poses significant environmental and public health challenges. With the rapid development of Capacitive Deionization (CDI) as a low-cost point-of-use (POU) solution to the problem, there was a rapid need for an online in-situ method for nitrate detection to support emerging POU technologies. 

Due to these circumstances, it was necessary to investigate a new method for detecting nitrates, optimised for CDI’s requirements. Spectrophotometric based methods in small batch configuration (cuvette style) became our focus at the onset. A review of all the viable techniques reduced the design to the following common steps that needed to be converted into a viable miniature scale process: i) sample the liquid continuously, ii) pretreatment, iii) nitrate to nitrite reduction, iv) dye addition, v) colour development, vi) spectrophotometric measurement, and vii) dispense the waste.

With the aim of trialling a range of methods that had the potential of functioning as a continuous flow cell system within this design process, reverse engineering attempts were made using commercially available reagent sets from Palintest, and Hach Nitraver5 showing poor colour development. Thus, they were eliminated from further testing 

Subsequent experiments employed the zinc reduction method, which offered a cost-effective means of nitrate concentration through its reduction to nitrite, followed by colorimetric analysis. Even though the zinc method demonstrated promising results, challenges such as sensitivity to reaction time, over-reduction of nitrite to ammonia, and variability in reproducibility limited its effectiveness. Additionally, side reactions and maintenance issues, including zinc oxidation and clogging, hinder its suitability for our continuous process.

Vanadium Chloride reduction method was finally chosen for its efficiency, rapid reaction time, and robustness under varying conditions. This method, combined with UV-VIS spectrophotometric analysis and MATLAB-based data processing, enabled real-time nitrate monitoring with over 93.05% accuracy. Key parameters such as residence time, flow rate, and heat transfer were optimised to enhance system performance. A reaction temperature of 85°C enabled complete reduction of vanadium species within 2 minutes, ensuring safe operation, consistent repeatability, and high precision. To address axial dispersion in flow-through systems, a novel sample-and-hold configuration was implemented, utilising seven parallel reactors with 17-second intervals to achieve discrete temporal resolution and accurate measurement. The system achieved a high-frequency sampling rate of 210 samples per hour (105 samples per CDI cycle), allowing near real-time tracking of nitrate fluctuations during semi-batch CDI operation.

A Stella model was developed to simulate nitrate removal via CDI, providing high resolution insight into nitrate adsorption and desorption dynamics. The model predicted transient nitrate responses that did not far exceed the system’s temporal resolution, which guided the implementation of a discrete sampling interval of 17 seconds. The integration of experimental validation with system simulation underscores the effectiveness of this combined approach in achieving accurate, real-time nitrate monitoring. The outcomes of this research contribute to the advancement of automated water quality analysis systems and offer a scalable, field-deployable solution for mitigating nitrate pollution in drinking water supplies.

year: 2025-06-10

title: Beginning teachers learning to teach mathematics through problem-solving

authors: Bailey, Judy

abstract: New Zealand curriculum documents have referred to mathematics as a problem-solving endeavour for more than 25 years. Although an intended curriculum focus, problem-solving seems to be an aspect of mathematics that many beginning teachers are not familiar with.

text: Beginning teachers learning to teach mathematics through problem-solving New Zealand curriculum documents have referred to mathematics as a problem-solving endeavour for more than 25 years. Although an intended curriculum focus, problem-solving seems to be an aspect of mathematics that many beginning teachers are not familiar with.

year: 2018

title: Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2015

authors: Hicks, Brendan J.

abstract: The aim of the survey was to provide on-going monitoring of the fish communities and abundance by boat electrofishing in the Ohau Channel, especially fish species that are taonga to Maori (eels, goldfish, and koura). In the current study, we present the findings from the ninth year of sampling (2015) and a summary of previous surveys.
We used the University of Waikato's 4.5 m-long, aluminium-hulled electrofishing boat to catch a total of total of 1,198 fish and koura (18.9 kg) at 13 sites on 2 December 2015, which comprised 2,671 lineal m and 10,684 m² in area. Koura (freshwater crayfish) and 6 fish species were present, with common bully the most abundant species (up to 45.5 fish 100 m⁻² at the site 6, edge habitat). Goldfish (up to 4.85 100 m⁻²) was the next most abundant species, with most goldfish at sites 7 and 12-13 in and around the side channel. Rainbow trout densities were up to 0.28 fish 100 m⁻². Mean bully density (11.41 fish 100 m⁻²) was much higher than for smelt (0.29 fish 100 m⁻²). Koura had a patchy distribution; only 3 individuals were caught at one site.
Comparing catches over the 9 years of sampling, the mean abundance of common bullies in 2015 was consistent with densities in most post-wall years (after 2007), but lower than in 2007 before wall closure (ANOVA P = 0.001). The cause of fluctuating bully abundance is not known, and was not accounted for by changes in water clarity expressed as black disc distance (BDD), water temperature, or water conductivity. Poor water clarity can reduce the efficiency of electrofishing, but high BDD did not correspond with high common bully densities. In 2015, smelt abundance had recovered somewhat from the low catch in 2014.
Goldfish biomass increased initially (2009-2010) because of targeted fishing in the excavated side branch (site 11), which has dense macrophytes and offers good habitat for goldfish. The continued rise in density from 2012 on, however, suggests a real increase in goldfish numbers. In 2012 and 2013 shortfin eels were caught, but no eels were caught in 2014 and only a single longfin eel was caught in 2015.
Analysis of fish densities before and after wall closure is hampered by the single data point before closure. However, we now have 9 years of post-wall data, and comparison of means suggest that the number of bullies has been lower since 2007. An obvious cause could be interruption of bully migration into the Ohau Channel from Lake Rotoiti by the wall. This suggests that the bully population in the Ohau Channel before wall construction and closure was a mixture of fish from lakes Rotorua and Rotoiti, and that recruitment from Rotoiti is now restricted. This hypothesis is testable with otolith microchemistry.

text: Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2015 The aim of the survey was to provide on-going monitoring of the fish communities and abundance by boat electrofishing in the Ohau Channel, especially fish species that are taonga to Maori (eels, goldfish, and koura). In the current study, we present the findings from the ninth year of sampling (2015) and a summary of previous surveys.
We used the University of Waikato's 4.5 m-long, aluminium-hulled electrofishing boat to catch a total of total of 1,198 fish and koura (18.9 kg) at 13 sites on 2 December 2015, which comprised 2,671 lineal m and 10,684 m² in area. Koura (freshwater crayfish) and 6 fish species were present, with common bully the most abundant species (up to 45.5 fish 100 m⁻² at the site 6, edge habitat). Goldfish (up to 4.85 100 m⁻²) was the next most abundant species, with most goldfish at sites 7 and 12-13 in and around the side channel. Rainbow trout densities were up to 0.28 fish 100 m⁻². Mean bully density (11.41 fish 100 m⁻²) was much higher than for smelt (0.29 fish 100 m⁻²). Koura had a patchy distribution; only 3 individuals were caught at one site.
Comparing catches over the 9 years of sampling, the mean abundance of common bullies in 2015 was consistent with densities in most post-wall years (after 2007), but lower than in 2007 before wall closure (ANOVA P = 0.001). The cause of fluctuating bully abundance is not known, and was not accounted for by changes in water clarity expressed as black disc distance (BDD), water temperature, or water conductivity. Poor water clarity can reduce the efficiency of electrofishing, but high BDD did not correspond with high common bully densities. In 2015, smelt abundance had recovered somewhat from the low catch in 2014.
Goldfish biomass increased initially (2009-2010) because of targeted fishing in the excavated side branch (site 11), which has dense macrophytes and offers good habitat for goldfish. The continued rise in density from 2012 on, however, suggests a real increase in goldfish numbers. In 2012 and 2013 shortfin eels were caught, but no eels were caught in 2014 and only a single longfin eel was caught in 2015.
Analysis of fish densities before and after wall closure is hampered by the single data point before closure. However, we now have 9 years of post-wall data, and comparison of means suggest that the number of bullies has been lower since 2007. An obvious cause could be interruption of bully migration into the Ohau Channel from Lake Rotoiti by the wall. This suggests that the bully population in the Ohau Channel before wall construction and closure was a mixture of fish from lakes Rotorua and Rotoiti, and that recruitment from Rotoiti is now restricted. This hypothesis is testable with otolith microchemistry.

year: 2016