title: Studies of modified pumice and titanomagnetite filtration media

authors: Yang, Zailu

abstract: There is a continuing interest in filtration technologies capable of more efficient filtration, particularly of small microbial particles, but which place less reliance on the use of water treatment chemicals. In order to develop new options for granular filtration systems, two indigenous New Zealand media, titanomagnetite (TM) and pumice were investigated. TM was of interest because its particle size is finer than most filtration media and its magnetic properties allow the possibility of magnetic conditioning of the bed. A modified pumice, marketed as Silicon Sponge (SS), is already finding application in filtration systems where its low effective density and porous structure make it ideally suited for use as the upper layer of dual media filters. 

Particle size and surface charge are key parameters influencing the effectiveness of a given filtration medium. Finer medium particle size improves filtration efficiency but at the expense of lower hydraulic conductivity. Magnetic conditioning of fine TM filtration beds was used to improve hydraulic conductivity. Hydraulic conductivity was increased by 140% when a vertically aligned magnetic field 0.018 T was applied. However filtration efficiency was decreased by about 9% in the conditioned (expanded) bed. Adjustment of pH to enhance electrostatic interactions more than compensated for this decrease. 

The particulate matter of most waters requiring filtration is negatively charged. Surface treatment to make a filtration medium more electropositive is known to improve the medium’s filtration effectiveness. The isoelectric point (IEP) of natural TM, even after rigorous chemical cleaning, was found to be 3.64 ± 0.06 This value was confirmed by the preparation of a synthetic TM and the measurement of its IEP. Attempts to raise the IEP of natural TM by treatment with aluminium and iron polycation solutions were unsuccessful. 

Greater success was achieved in the modification of the properties of SS by treatment with aluminium and iron polycation solutions and mixtures of these solutions. IEPs ranging from the 4.10 ± 0.06 of acid rinsed SS to the 8.52 ± 0.06 of aluminium treated SS were achieved. In addition, it was found possible to impart magnetic properties to fine SS by the use of treatment solutions containing a mixture of ferric and ferrous ions. The presence of aluminium in the treatment solutions allowed higher IEPs to be obtained but the magnetic effect was reduced. In the absence of aluminium, the ferric/ferrous mixture gave a product with a magnetic effect that was 7% of that observed for pure magnetite. 

Filtration studies using 1.7 μm negatively charged kaolin as a model negative colloid showed that significant increases in filtration efficiency were achieved by medium pre-treatments with the aluminium and iron solutions. For example, turbidity removal for 710-1000 μm SS was increased from 22% to 94% by aluminium pre-treatment. 

The information on the performance of magnetically conditioned beds and the ability to enhance filtration efficiency by surface modification using iron and aluminium has potential applications in the design of novel systems for water and wastewater treatment. A small-scale system based upon the use of a permanent magnet has been proposed and work in this area is continuing.

text: Studies of modified pumice and titanomagnetite filtration media There is a continuing interest in filtration technologies capable of more efficient filtration, particularly of small microbial particles, but which place less reliance on the use of water treatment chemicals. In order to develop new options for granular filtration systems, two indigenous New Zealand media, titanomagnetite (TM) and pumice were investigated. TM was of interest because its particle size is finer than most filtration media and its magnetic properties allow the possibility of magnetic conditioning of the bed. A modified pumice, marketed as Silicon Sponge (SS), is already finding application in filtration systems where its low effective density and porous structure make it ideally suited for use as the upper layer of dual media filters. 

Particle size and surface charge are key parameters influencing the effectiveness of a given filtration medium. Finer medium particle size improves filtration efficiency but at the expense of lower hydraulic conductivity. Magnetic conditioning of fine TM filtration beds was used to improve hydraulic conductivity. Hydraulic conductivity was increased by 140% when a vertically aligned magnetic field 0.018 T was applied. However filtration efficiency was decreased by about 9% in the conditioned (expanded) bed. Adjustment of pH to enhance electrostatic interactions more than compensated for this decrease. 

The particulate matter of most waters requiring filtration is negatively charged. Surface treatment to make a filtration medium more electropositive is known to improve the medium’s filtration effectiveness. The isoelectric point (IEP) of natural TM, even after rigorous chemical cleaning, was found to be 3.64 ± 0.06 This value was confirmed by the preparation of a synthetic TM and the measurement of its IEP. Attempts to raise the IEP of natural TM by treatment with aluminium and iron polycation solutions were unsuccessful. 

Greater success was achieved in the modification of the properties of SS by treatment with aluminium and iron polycation solutions and mixtures of these solutions. IEPs ranging from the 4.10 ± 0.06 of acid rinsed SS to the 8.52 ± 0.06 of aluminium treated SS were achieved. In addition, it was found possible to impart magnetic properties to fine SS by the use of treatment solutions containing a mixture of ferric and ferrous ions. The presence of aluminium in the treatment solutions allowed higher IEPs to be obtained but the magnetic effect was reduced. In the absence of aluminium, the ferric/ferrous mixture gave a product with a magnetic effect that was 7% of that observed for pure magnetite. 

Filtration studies using 1.7 μm negatively charged kaolin as a model negative colloid showed that significant increases in filtration efficiency were achieved by medium pre-treatments with the aluminium and iron solutions. For example, turbidity removal for 710-1000 μm SS was increased from 22% to 94% by aluminium pre-treatment. 

The information on the performance of magnetically conditioned beds and the ability to enhance filtration efficiency by surface modification using iron and aluminium has potential applications in the design of novel systems for water and wastewater treatment. A small-scale system based upon the use of a permanent magnet has been proposed and work in this area is continuing.

year: 2001

title: Investigation of variations in the saturated hydraulic conductivity of sand and the influencing factors in the laboratory

authors: Nikghalb Ashouri, Saeed

abstract: The most critical hydraulic property of soil is the saturated hydraulic conductivity (Ks) of its matrix. Accurate determination of Ks is vital for understanding and simulating flow and transport phenomena. Hydrologists require accurate estimations of soil moisture content and retention to better discern rainfall distribution between runoff and infiltration; agronomists similarly rely on this data to inform crop yield models and formulate appropriate irrigation timetables. A diverse range of methodologies currently exists to determine the Ks, with the constant head test in the laboratory being a common approach. Traditionally, it is presumed that the Ks values obtained from both field and laboratory assessments remain constant. In addition, Ks is considered as a constant in various applications, such as solute transport modelling and surface water as well as groundwater simulations.  

Given that soils can undergo saturation and desaturation they can be subjected to gradual and/or rapid changes in the hydraulic head, and thus the Ks of the soil will be affected. Consequently, any changes in Ks will affect the hydraulic properties of the soil. Therefore, to fully understand the hydraulic behaviour of a soil requires investigating the changes in Ks during short and long timeframes.  

A primary objective of this study was to systematically investigate the changes that could occur in Ks during short-term and long-term constant head tests. Two soil columns were prepared and subjected to continuous constant head tests for a duration exceeding 50 days The hydraulic heads were increased gradually in a similar way as for a constant head test, with the only difference being that the hydraulic heads were increased every 5 to 7 days. The outcomes reveal that Ks exhibits negligible variations over short intervals, showing minor daily fluctuations. However, the long-term variations highlight significant shifts in Ks, with a maximum decrease in value of approximately 90% from the onset of the experiments, followed by intermittent brief periods of slight increase. These findings challenge the notion of Ks as a constant value over time. Moreover, a comparative analysis of soil properties before and after the tests indicates notable modifications in the sample characteristics. Specifically, the particle size distribution and particle density differ pre and post-experiment, with an observable migration of fine, low-density particles towards the upper section of the sample closer to the outlet, and aligned with the flow direction. This phenomenon results in an increased particle density at the lower part of the sample adjacent to the water inlet. The migration of particles is evidence of the constant head test impact on the distribution of particles. The change in the distribution of the particles pre and post experiment, caused by the standard procedure of the tests, shows that the Ks value determined in the lab through constant head tests is influenced by the tests, and might not reflect the actual Ks of the sample.  

In addition, a series of short-term experiments were carried out on 10 samples that were prepared with different particle size mixtures. These mixtures featured varying proportions of fine and coarse materials to study the range of changes in the Ks in different mixtures. The hydraulic heads of the tests on these samples were systematically adjusted in ascending increments, to investigate the range of changes in Ks under low and high hydraulic heads during short-term tests. This test series revealed a distinctive 3-phase pattern exhibited by the samples: a low and relatively consistent Ks was initially observed prior to a transition through a phase change as the hydraulic head escalates, leading to a stable constant Ks under high hydraulic gradients. These observations underscore the significance of the changes of Ks during constant head tests, and show a need to consider that Ks  has a range of values over time rather than being a constant value.  

Determining a suitable value for Ks in the lab and field is time consuming, it can be costly, and test results can be impacted by the test conditions. For instance, in the lab, ensuring the full saturation of the samples before the tests needs determination of solid particle density, which requires a pycnometer. Moreover, due to the spatial variations in Ks, the number of samples needed to represent the typical Ks in a study area can be costly to retrieve. Similarly, to investigate the spatial variation of Ks, the field tests (slug tests, pumping tests, or tracer tests) add extra cost and time to projects. Additionally, taking soil samples or performing Ks field tests in remote areas or harsh weather conditions is an arduous task. Therefore many researchers have endeavoured to establish predictive relationships to estimate Ks accurately without relying on resource-intensive laboratory or field tests. Indirect estimation methods are generally based on easily obtainable data such as soil texture, particle size distribution, specific surface area, or porosity. These relationships are developed into pedotransfer functions and aim to provide estimations of Ks. However, the reliability of these functions remains uncertain due to discrepancies observed when comparing the results obtained using different pedotransfer functions with actual field or laboratory measurements.  

To address this issue, the Ks derived from Darcy's equation for the short-term tests were compared with those computed using 7 of the most commonly used pedotransfer functions. The objective of the comparison was to evaluate the effectiveness of these equations in predicting Ks. The analysis revealed that the majority of pedotransfer functions tend to underestimate the Ks values of the soil samples. Subsequently, a novel method was employed to adjust the parameters of one of the equations, which is suitable for non-plastic sandy soils as used in the earlier experiments, by linking the equation parameters to the coefficient of uniformity and void ratio of the samples. This modification led to a more precise estimations of Ks for the range of sandy samples used in the research.  

Given the inaccuracies related to the constant head tests and the impact on Ks determination (such as the disturbance in connection of drainable pores due to varying hydraulic heads, inaccuracies in reading the flow rate and sample head gradient, improper sample saturation, effect of air entrapment in the sample), it was necessary to look for improvements in determining the Ks in the soil.  

Tracer tests on soil samples in the laboratory and field have been used by several researchers to estimate the soil hydraulic properties. These tests offer essential insights into site hydrogeology, aiding in the delineation of crucial factors such as natural groundwater velocity and hydraulic connectivity between neighbouring aquifers and aquitards. When coupled with groundwater flow and transport models, tracer tests can enhance the precision of defining Ks and its spatial distribution accurately.  

In order to acquire soil hydraulic properties from tracer tests, a tracing element is injected into the samples, and the changes of concentration in the outflow is plotted over time. The resulting graph is called a break through curve (BTC). In order to acquire soil hydraulic properties from BTCs, advection-dispersion equations (ADE) and the models that are built upon ADE equations are widely used. The ADE links soil hydraulic properties (Ks and dispersivity coefficient) to the BTC. Trial and error is required to achieve the best fit between predicted and measured BTC which can be challenging and may require several trials to process multiple sets of BTCs.  

In this study, the use of a statistical distribution, specifically the Inverse Gaussian distribution (IG), was examined for fitting BTCs obtained from 94 tracer tests utilizing sodium chloride as the tracer compound. The results of fitting the BTCs using the IG distribution revealed that solute transport in porous media could be effectively articulated using this distribution, particularly given its suitability for positively skewed data. The majority of the tracer tests fitted with the IG distribution exhibited model efficiencies exceeding 0.9 (based on Nash–Sutcliffe model efficiency coefficient), indicating a strong fit of the BTCs. A comparison of the IG method with alternative methodologies from existing research further underscored the superior fit achieved by the IG distribution in modelling BTCs.  

The test series outlined in this thesis represent a significant advancement in the comprehension of saturated hydraulic conductivity within porous media. The noticeable long-term changes of  Ks in the laboratory show this parameter cannot be considered as a constant, and the assumption of linearity between the hydraulic head and flow in the samples is only valid in the short-term tests. Additionally, Ks can have a range of low and high values, during rapid changes of the hydraulic head. The implications of these findings offer potential revisions to the existing approaches for integrating saturated hydraulic conductivity within equations of surface and groundwater models. Furthermore, the study highlights the viability of employing statistical methods to analyse the BTCs from tracer tests. The use of statistical methods with the tracer tests can save time and cost in the post-processing of the tracer test results.

text: Investigation of variations in the saturated hydraulic conductivity of sand and the influencing factors in the laboratory The most critical hydraulic property of soil is the saturated hydraulic conductivity (Ks) of its matrix. Accurate determination of Ks is vital for understanding and simulating flow and transport phenomena. Hydrologists require accurate estimations of soil moisture content and retention to better discern rainfall distribution between runoff and infiltration; agronomists similarly rely on this data to inform crop yield models and formulate appropriate irrigation timetables. A diverse range of methodologies currently exists to determine the Ks, with the constant head test in the laboratory being a common approach. Traditionally, it is presumed that the Ks values obtained from both field and laboratory assessments remain constant. In addition, Ks is considered as a constant in various applications, such as solute transport modelling and surface water as well as groundwater simulations.  

Given that soils can undergo saturation and desaturation they can be subjected to gradual and/or rapid changes in the hydraulic head, and thus the Ks of the soil will be affected. Consequently, any changes in Ks will affect the hydraulic properties of the soil. Therefore, to fully understand the hydraulic behaviour of a soil requires investigating the changes in Ks during short and long timeframes.  

A primary objective of this study was to systematically investigate the changes that could occur in Ks during short-term and long-term constant head tests. Two soil columns were prepared and subjected to continuous constant head tests for a duration exceeding 50 days The hydraulic heads were increased gradually in a similar way as for a constant head test, with the only difference being that the hydraulic heads were increased every 5 to 7 days. The outcomes reveal that Ks exhibits negligible variations over short intervals, showing minor daily fluctuations. However, the long-term variations highlight significant shifts in Ks, with a maximum decrease in value of approximately 90% from the onset of the experiments, followed by intermittent brief periods of slight increase. These findings challenge the notion of Ks as a constant value over time. Moreover, a comparative analysis of soil properties before and after the tests indicates notable modifications in the sample characteristics. Specifically, the particle size distribution and particle density differ pre and post-experiment, with an observable migration of fine, low-density particles towards the upper section of the sample closer to the outlet, and aligned with the flow direction. This phenomenon results in an increased particle density at the lower part of the sample adjacent to the water inlet. The migration of particles is evidence of the constant head test impact on the distribution of particles. The change in the distribution of the particles pre and post experiment, caused by the standard procedure of the tests, shows that the Ks value determined in the lab through constant head tests is influenced by the tests, and might not reflect the actual Ks of the sample.  

In addition, a series of short-term experiments were carried out on 10 samples that were prepared with different particle size mixtures. These mixtures featured varying proportions of fine and coarse materials to study the range of changes in the Ks in different mixtures. The hydraulic heads of the tests on these samples were systematically adjusted in ascending increments, to investigate the range of changes in Ks under low and high hydraulic heads during short-term tests. This test series revealed a distinctive 3-phase pattern exhibited by the samples: a low and relatively consistent Ks was initially observed prior to a transition through a phase change as the hydraulic head escalates, leading to a stable constant Ks under high hydraulic gradients. These observations underscore the significance of the changes of Ks during constant head tests, and show a need to consider that Ks  has a range of values over time rather than being a constant value.  

Determining a suitable value for Ks in the lab and field is time consuming, it can be costly, and test results can be impacted by the test conditions. For instance, in the lab, ensuring the full saturation of the samples before the tests needs determination of solid particle density, which requires a pycnometer. Moreover, due to the spatial variations in Ks, the number of samples needed to represent the typical Ks in a study area can be costly to retrieve. Similarly, to investigate the spatial variation of Ks, the field tests (slug tests, pumping tests, or tracer tests) add extra cost and time to projects. Additionally, taking soil samples or performing Ks field tests in remote areas or harsh weather conditions is an arduous task. Therefore many researchers have endeavoured to establish predictive relationships to estimate Ks accurately without relying on resource-intensive laboratory or field tests. Indirect estimation methods are generally based on easily obtainable data such as soil texture, particle size distribution, specific surface area, or porosity. These relationships are developed into pedotransfer functions and aim to provide estimations of Ks. However, the reliability of these functions remains uncertain due to discrepancies observed when comparing the results obtained using different pedotransfer functions with actual field or laboratory measurements.  

To address this issue, the Ks derived from Darcy's equation for the short-term tests were compared with those computed using 7 of the most commonly used pedotransfer functions. The objective of the comparison was to evaluate the effectiveness of these equations in predicting Ks. The analysis revealed that the majority of pedotransfer functions tend to underestimate the Ks values of the soil samples. Subsequently, a novel method was employed to adjust the parameters of one of the equations, which is suitable for non-plastic sandy soils as used in the earlier experiments, by linking the equation parameters to the coefficient of uniformity and void ratio of the samples. This modification led to a more precise estimations of Ks for the range of sandy samples used in the research.  

Given the inaccuracies related to the constant head tests and the impact on Ks determination (such as the disturbance in connection of drainable pores due to varying hydraulic heads, inaccuracies in reading the flow rate and sample head gradient, improper sample saturation, effect of air entrapment in the sample), it was necessary to look for improvements in determining the Ks in the soil.  

Tracer tests on soil samples in the laboratory and field have been used by several researchers to estimate the soil hydraulic properties. These tests offer essential insights into site hydrogeology, aiding in the delineation of crucial factors such as natural groundwater velocity and hydraulic connectivity between neighbouring aquifers and aquitards. When coupled with groundwater flow and transport models, tracer tests can enhance the precision of defining Ks and its spatial distribution accurately.  

In order to acquire soil hydraulic properties from tracer tests, a tracing element is injected into the samples, and the changes of concentration in the outflow is plotted over time. The resulting graph is called a break through curve (BTC). In order to acquire soil hydraulic properties from BTCs, advection-dispersion equations (ADE) and the models that are built upon ADE equations are widely used. The ADE links soil hydraulic properties (Ks and dispersivity coefficient) to the BTC. Trial and error is required to achieve the best fit between predicted and measured BTC which can be challenging and may require several trials to process multiple sets of BTCs.  

In this study, the use of a statistical distribution, specifically the Inverse Gaussian distribution (IG), was examined for fitting BTCs obtained from 94 tracer tests utilizing sodium chloride as the tracer compound. The results of fitting the BTCs using the IG distribution revealed that solute transport in porous media could be effectively articulated using this distribution, particularly given its suitability for positively skewed data. The majority of the tracer tests fitted with the IG distribution exhibited model efficiencies exceeding 0.9 (based on Nash–Sutcliffe model efficiency coefficient), indicating a strong fit of the BTCs. A comparison of the IG method with alternative methodologies from existing research further underscored the superior fit achieved by the IG distribution in modelling BTCs.  

The test series outlined in this thesis represent a significant advancement in the comprehension of saturated hydraulic conductivity within porous media. The noticeable long-term changes of  Ks in the laboratory show this parameter cannot be considered as a constant, and the assumption of linearity between the hydraulic head and flow in the samples is only valid in the short-term tests. Additionally, Ks can have a range of low and high values, during rapid changes of the hydraulic head. The implications of these findings offer potential revisions to the existing approaches for integrating saturated hydraulic conductivity within equations of surface and groundwater models. Furthermore, the study highlights the viability of employing statistical methods to analyse the BTCs from tracer tests. The use of statistical methods with the tracer tests can save time and cost in the post-processing of the tracer test results.

year: 2024

title: The life story model of identity: A bridge between two spaces

authors: Houkamau, Carla Anne

subjects: Maori

abstract: Two questions which continue to absorb Western social psychological researchers are: What is the ‘self’? And ‘What is identity in relation to self?’ At the NMPPS this year this Dr. Love’s keynote threw these matters into relief prompting us as indigenous psychologists to reflect deeply on the nature of ‘selves’. Many questions arise from her address. Do we have ensembled selves? Are we self contained? Are we a bit of both depending on the situation? How do we work within psychology while honouring Māori perspectives?

text: The life story model of identity: A bridge between two spaces Two questions which continue to absorb Western social psychological researchers are: What is the ‘self’? And ‘What is identity in relation to self?’ At the NMPPS this year this Dr. Love’s keynote threw these matters into relief prompting us as indigenous psychologists to reflect deeply on the nature of ‘selves’. Many questions arise from her address. Do we have ensembled selves? Are we self contained? Are we a bit of both depending on the situation? How do we work within psychology while honouring Māori perspectives?

year: 2008