Final Report Summary - AQUA-PULSE (Photocatalysis with UV LED Sources for Efficient Water Purification)
Executive Summary:
Aqua-pulse is an EU sponsored project in the 7th Framework Programme of the EU funded under the “Research for the Benefit of Specific Groups” programme, specifically research for the benefit of SME’s.
Water purification requirements feature in a wide range of applications, from residential homes, office and hotel buildings, to high-specification environments such as hospitals, laboratories and industrial production facilities, as well as municipal water supply and wastewater treatment. Consequently, the world demand for water treatment products is estimated at $44.6 billion, with a projected 5.7% annual growth. The European Technology Platform, WssTP (Water Supply and Sanitation Technology Platform), in its Strategic Research Agenda 2010, has identified the need for new water technologies, and greater technology transfer between industry and the research sector.
The AQUA-PULSE project aims to realise a low-power, low-maintenance water purification solution based on high-brightness UV Light Emitting Diodes (LEDs) and a photocatalysis method. Such a system would be effective against viruses, bacteria and organic compounds, and would provide an attractive and innovative alternative to current technology utilising mercury-based UV lamps. It brings together three European SMEs in three different, but complementary, technology areas, and links them with three RTD Performers to develop new knowledge and a new water purification product which will have significant commercial benefits for all of the SME partners.
Photocatalysis has been shown to be a highly effective method for removing selective bacteria and compounds from a variety of systems. By combining this process with UV LED technology, the project hopes to achieve a low cost water purification unit. The catalyst can be tailor to target a specific or range of compounds. It can also be tailored to work with specific wavelengths of light. UV LED’s are now starting to compete with their mercury counterparts in terms of power outputs and have significant advantages over them in terms of footprint, power usage and environmental impact. The photocatalytic material can be deposited on a wide variety of surfaces, thus the potential for an equally wide range of water purification reactor designs exists.
Kinetic and optical design parameters will be explored and a focus towards efficiency will be key in the overall project. A prototype reactor will then be validated through the SME end user and its ability to deliver a suitable solution assessed. There exists also the possibility for various iterations of designs to follow once the prototype has been fabricated. The areas of optical design and implementation, photocatalyst development and assessment and end user applicability and validation each have an SME and RTD partner associated with them, ensuring that the overall project provides specific targets for the related SME partners.
Project Context and Objectives:
While UV photocatalysis systems have been incorporated in some water purification systems, a major barrier to their widespread deployment is the source of the UV radiation. These are almost exclusively mercury-based UV lamps, which are relatively bulky, inefficient, and have health and environmental issues due to the mercury contained in them. Moreover, the bulbs typically need replacing on a yearly basis, leading to high maintenance costs. Such size, power and cost considerations have excluded Photo-Catalytic Oxidation systems from a number of potential applications which require a compact, low-cost solution, for example in private residences. AQUA-PULSE aims to draw on the strengths of the SME partners in high brightness UV LEDs, photocatalytic materials, and water treatment technologies, to produce such a solution.
The expected impacts of the FP7 Research for SMEs call include “strengthening the competitiveness of SME participants” and “new/improved products, processes or services” with a “clear economic impact for the SME participants”. The AQUA-PULSE project clearly conforms to these targets, aiming to produce a new purification product which will present new applications/customers for each of the SMEs involved, whether as suppliers of components, manufacturer or seller of the final unit. The product will represent a disruptive technology with strong market competitiveness, offering a cheaper, more compact alternative to existing mercury-lamp based technologies.
The Project Objectives needed to achieve this aim can be summarised as follows:
• Realise and optimise UV LED emitters with the required brightness (e.g. >1 W/cm2) and wavelength (<380nm, tailored to photocatalyst) (Month 12)
• Optimise the photocatalysis process using appropriate catalyst material (e.g. nanostructured TiO2 immobilised on porous supporting matrix) (Month 15)
• Model, design and construct the UV photocatalysis reaction chamber (Month 16)
• Fabricate the prototype water purification unit (Month 18)
• Validate the performance and effectiveness of the purification unit, with regard to a number of key potential applications (Month 24)
The coordinating SME in this project, Epi-Light, produce a range of Light Emitting Diode (LED) solutions for Life Science applications, which to date have principally focused on imaging and product inspection applications. One of their flagship products are very high power UV LEDs, which are the brightest such LEDs available on the market, and have found uses in curing processes and imaging techniques. A key aspect of the company’s development roadmap is to identify new target markets for their LED products and develop innovative solutions relevant to them. A promising and lucrative such market which they have identified is that of water purification using UV techniques. Here, there is a considerable disruptive opportunity for replacing the currently used bulky UV mercury lamps with compact, lightweight, low power UV LEDs.
While the elimination of bacteria and viruses can be achieved by direct exposure to UV light, UV photolysis of persistent organic pollutants (and emerging contaminants) is not wholly effective, but can be achieved with UV-photocatalysis. Here, a photocatalyst material such as TiO2 is illuminated with UV light, creating reactive oxygen species, which result in the complete mineralisation of chemical pollutants and the inactivation of pathogenic microorganisms. This is a process with which the SME partner AM is very familiar, as they are a producer of photocatalytic materials, principally for multifunctional coatings. These coatings, when applied to surfaces such as walls or roofs, use the UV and visible light from sunlight to clean and disinfect the surfaces via the photocatalysis process. These specialist paint products have been very successful for AM, and the company are now looking to expand their application space. By combining their material with a UV source tailored to the wavelength response of the TiO2, the efficiency of the photocatalysis process can be greatly improved and applied to more intensive purification processes such as water purification.
The third SME partner in the consortium, Unik, manufacture and supply filter and purification systems for water treatment applications. Their products include a UV purification unit which utilises conventional UV lamps for direct germicidal elimination. Adapting this unit to use the much more compact and more power-efficient LED sources, in combination with a photocatalyst to address a wider range of purification aspects, would provide them with a ground-breaking new product with enormous market potential.
In the AQUA-PULSE project, these three SMEs have thus joined forces with the aim of realising a new water purification product that would have significant commercial benefits for each of the companies. However, the achievement of this UV LED photocatalysis water purifier requires a substantial amount of research and development in a number of areas, activities which are beyond the resources of the SMEs themselves. Consequently, they have linked with RTD Performers who have been carefully chosen for their expertise and facilities in each of the necessary research areas.
LED technology is needed in order to achieve the required output power and wavelengths; here, the experience of CIT in the design and characterisation of semiconductor devices will be invaluable. Secondly, the formulation of the photocatalyst material, and the method of deployment, will need to be optimised; Ulster’s expertise in photocatalysis makes them ideally suited to this task, along with chemical engineering to ensure an effective reactor design. Finally, the integration of the new UV photocatalyst unit into a water purification system requires a significant design and testing effort; this will draw on the extensive background of TI in water technology development. Thus, the AQUA-PULSE project brings together a well-balanced and tightly-focused consortium, in which the complementary skills of the RTD Performers will combine to provide valuable R&D results, leading to new products and markets for all the SME partners.
Project Results:
Main Science and Technology Results:
Optical design overview
At CIT, a number of conceptual designs have been modelled and optimized for UV-LED illumination using ZEMAX optical software. These optical designs then were used for kinetics calculations of the mass transport and have been examined in more detail and refined to fit mass transport requirements. The process of the reactor design has undergone thus a number of iterations, and several design concepts have been proposed and evaluated. The main factors for the reactor design proposed in D1.3 (such as illumination irradiance, wavelength, irradiated surface area, mass transport, and cost) have been considered for choosing an optimal design for the reactor prototype.
The model parameters were chosen to fit realistic data. The upper estimation was taken for the water absorption and scattering coefficients, based on the measured data for highly turbid water. The Mie scattering model was incorporated to simulate scattering of light by the particles dissolved in turbid water. The water properties in the model are adjusted for the complete reactor depending on the pollution level and contaminants targeted for treatment. The LED far field distribution was measured and modelled to achieve accurate representation of the source far field (see D2.2-D2.4).
The reactors efficiency from an optical perspective was optimised via modelling. The reactor types considered and their parameters were summarized. For all of the reactor types the key was to maximise the surface area illuminated at the desired level of > 5 mW/cm2 (the comments on the levels of illumination and photo-oxidation efficiency are given in D4.1). Consideration was also given to the type of packing material and whether the material is coated or uncoated. Spherical beads were chosen for packing due to the large surface to volume ratio and availability. The optical properties of the photocatalyst material, i.e. transmittance and reflectance were also considered, as they strongly affect the illuminated area in the reactor. For the Lambertian scattering models the most promising models from optical prospective were a tubular reactor with inner coated wall and disc-type reactor illuminated from top. These configurations were chosen for further investigations both from optical and kinetics aspects. The following sections on optical design show the development process of the optical modelling and ultimately the final optical design that was chosen to be implemented.
Selection/Model of UV LED and reactor modelling:
In order to be integrated within the final product, the LED must satisfy a number of criteria in relation to power output, illumination pattern, etc. The main commercially available UV LEDs were reviewed and the most appropriate was chosen for initial testing and ultimately, inclusion into the final design. The reactor efficiency from an optical perspective is optimised via modelling. The ability to accurately model the LED leads in turn to a more accurate reactor model. A number of direct suppliers for UV LED devices were considered to select the most appropriate devices for initial testing on the Aqua-Pulse project. The four leading suppliers of high brightness UV LEDs: Nichia, LED Engin, Luminus and SemiLeds were considered. A number of selection criteria were employed to determine which supplier might best meet the initial project’s needs. Devices/suppliers were considered under the following:
• Range of devices available
• Selection of output powers available
• Driving current/voltage required
• Optical characteristics suitable to the envisaged application
• Cost per device
For the optical characteristics, the current application of mercury lamps being employed at Ulster in their stirred tank test reactor was used as a reference. The devices would be required to supply several watts of optical power and run at voltages up to 40V. In terms of the spatial distribution of power almost all LEDs had a similar optical “far-field”. Considering all of the above requirements and also the availability of the devices it was decided that the LED Engin range of UV LEDs would be used for initial and interim experimental work. Of particular usefulness with these devices was the availability of the optical model of the device which caould be incorporated in the Zemax modelling program that will be used throughout the project. The LED Engin devices come in three forms suitable for project work.
The LZ1 a single 250mW device:
http://www.ledengin.com/files/products/LZ1/LZ1-00U600.pdf
The LZ4 a four die 1W device:
http://www.ledengin.com/files/products/LZ4/LZ4-00U600.pdf
and the LZC a 3W device:
http://www.ledengin.com/files/products/LZC/LZC-00U600.pdf
The respective cost of these devices are €25, €70 and €230 which make them competitive for use both in initial experimentation and also possible incorporation into a more developed UV purification unit. These prices would also be significantly reduced if ordered in larger quantities.
Initial trials involved using these devices as direct replacements for the UV lamps currently used by Ulster and comparisons were made between the supplied optical model data and the real optical characteristics of the devices. Ultimately increased optical ray models were generated to give improved accuracy to the modelling calculations. Preliminary results have shown that the UV LEDs perform at least equally well for driving the photocatalytic degradation of phenol. Also the modelled and measured data for the LEDs is closely matched giving a solid platform for the development of the technology. It should also be noted that UV LED technology is improving rapidly. While more efficient and more powerful UV LEDs are now available at the end of this project the development of the modelling and the fabrication of the reactor have shown that the distribution rather than the ultimate power output is much more important.
After the verification of the LED model various configurations of LEDs were considered to form different basic reactor types. Three basic designs were chosen for further investigation:
• Single tube reactors,
• multiple tube reactors and
• dish type reactors (flat thin profile).
The symmetry and manufacturing criteria were strong considerations here. Initially single tube reactors were considered. The optimum illumination of such a design was determined from the model data, with the appropriate number and positioning of LEDs being the critical factors. For the reactor to work effectively a sufficient level of light intensity must fall on the photocatalyst. The model allows for variation of the LED positions and an output of the illumination intensity inside the tube to be generated.
Modelling and representation of the photocatalyst
Initially some basic experiments were undertaken to determine the transmittance and reflectance of a typical photocatalyst covered plate. This would be required once a basic optical model was set-up. After producing a clean beam by passing the LED output through a pinhole the light is focussed on the plate in question. The detector is then placed either behind the plate for transmittance measurements or in front for reflectance. Rotating the detector angularly gives estimates of the angular values for the transmittance and reflectance.
Optical designs:
The tubular collector illuminated from top was shown to be extremely efficient in terms of optical illumination due to wave guiding. As the glass tube has higher refractive index than that of the air, the light rays entering the tube at small angles experience total reflection, and hence the majority of light entering the tube propagates inside it. The wave guiding makes illumination from top of tubular reactors highly effective with over 86 % of the source radiation propagating within the tube. Here the efficiency of illumination is the ratio of the LED power in the collector to the total source power. We maximized the flux on the detectors by varying the position of the source.
Figure 1: ZEMAX model (a) and schematic diagram (b) of the tubular cylinder illuminated from top.
By placing detectors at different places along the tube we calculated irradiance and power absorbed by a coating layer along the tube.
The optical design incorporates optimisation of the source position. The tubular reactor illuminated from top has shown to be a promising option for the illumination via LED, with an efficiency of 83-86% for the optimised source position. The optimised length of the coated cylinder filled with turbid water was ~15 cm, resulting in an illuminated area of 138 cm2 via a single LED. This is a lower estimation of irradiated area, as the worst case assumption for the water absorption coefficient was used in the model. The result can be further improved by incorporating multiple tubes of smaller diameter. Illumination from the side was also investigated and required multiple LEDs for uniform intensity distribution.
Reflectors and “packing material”
Due to refraction of the beam the light intensity dropped dramatically at the tubular walls normal to the source, resulting in dead zones along the cylinder. A significant fraction of the radiation missed the cylinder due to conical shape of the LED far field, which resulted in poor efficiency for this configuration. This makes the use of reflectors necessary for this configuration.
Figure 2: Schematic representation of the elliptical reflector (a) and ellipse parameters (b). 3D ZEMAX models of the elliptical reflector (c).
A design with an optimised elliptical reflector has shown to decrease the level of lost light with an increase of the average intensity from 50 mW/cm2 to 120 mW/cm2 and an efficiency increase to 75%. This can also potentially lessen the required number of UV emitters. Dual-elliptical reflectors provided similar benefits with more uniform illumination. However some dead zones were still present with reflectors. The goal of optimising any design is to effectively illuminate the reactor while reducing the area occupied by the dead zones.
The next step was to introduce a “packing material” into the reactor design. This would serve as a support structure for the photocatalyst and also promote increased kinetic activation. A packing material that is relatively cheap and has a large surface area to volume ratio was chosen. The tubular reactor design was assessed with the coated packing material. A significant portion of the incident light is absorbed by the outer layers of packing material making the inner regions of the reactor less effective.
Figure 3: 3D ZEMAX model of the tube filled with beads illuminated from side (a), and of the flat dish packed bead reactor (b).
This result led the research team to focus on a design to evenly distribute the light throughout the reactor. Two possible options were assessed:
1) A flat plate/dish type reactor which reduces the effective thickness and can be illuminated from both sides. For this model the beads placed in the reactor can double as light detectors, effectively mapping the light distribution though the reactor. This type of design is also more suited to the illumination pattern from the LEDs and leads to less dead zones.
2) An alternative arrangement was to consider changing the coating thickness on the packing material to achieve higher transmission and also mixing coated and uncoated beads. This would increase the distribution of light towards the centre of the reactor and allow for a thicker (larger volume) dish type reactor.
For each design the key is to maximise the total surface area. A larger volume realises a larger amount of treated water for a given throughput. Consideration was also given to the type of packing material and whether the material is coated or uncoated. Also of further significance are the properties of the photocatalyst material, i.e. transmittance and reflectance. These properties strongly affect the optimum illuminated area in the reactor. Our modelled designs use transmission values guided by literature and experiments.
For the dish model illuminated from the top, figure 9, the maximum radius allowing for sufficient irradiation by a single LED was 47 mm, which resulted in ~70 cm2 illuminated area with a high efficiency of 81.4%. This result can be improved for the dish packed with coated spherical beads; however, due to high absorption of the photocatalyst, two LEDs are required for the packed bead dish reactor. The sphere packing was considered as the most promising due to the largest surface area to volume ratio. However, other packing materials can provide for better light penetration and possibly more efficient mass transport. Defining the best packing material for the packed bed reactors is driven by the market availability and large surface-to-volume ratio.
Table 1: Summary of the optical parameters obtained from simulation of the different reactor types (a). This summary is used to guide the kinetic modelling of the reactor. Also shown is the active surface area in the tubular reactor filled with 3 mm beads for different formulations of the catalyst reflectance (R) and transmittance (T), (b).
Table 1 gives a very brief selection of the criteria that were considered for the different types of reactors. A surface area illuminated per LED figure of merit was generated for the different designs and this data (along with others) guided the process of finalising a design that is both efficient in terms of its operational capacity and also affordable as a product.
Refined dish reactor model
One of the solutions available to combine large illuminated area with turbulent water kinetics was to use a fluidic dish with specially designed profile (see D4.2) and a dish cap covered with a photocatalyst from the side contacting the water. This model was chosen as a best compromise between the optical data available and the kinetic data being generated in Ulster. The centre of the dish cap gets most of the light, the peak irradiance is 23.9 milliwatts per cm2. The irradiance of the peripheries of the dish cap drops to 3.9 milliwatts per cm2 which still ensures the reaction on the catalyst.
In the presence of reflector the center of the dish cap again gets most of the light, the peak irradiance is 23.9 milliwatts per cm2. The irradiance of the peripheries of the dish cap drops to 4.8 milliwatts per cm2 which is higher than 3.9 milliwatts per cm2 shown without reflector The difference in the results is small which infers that reflection from the dish cap is negligible. This is confirmed by experimental testing (see D2.4).
A further refinement of the dish model to increase quantum efficiency is to install a lens to make illumination more evenly distributed. The optimal lens parameters were considered in D2.3 – D2.4. The best lens was found to be a 2 inch unit with focal length of 50 mm. Optimised distances are 20 mm from the light source to the lens and 49 mm from lens to the dish cap. The 3D model of the dish cap setup is shown on Figure 10.
In the presence of the lens the light is distributed more evenly along the surface with a maximal irradiance of 6.8 milliwatts per cm2 in the central part of the dish cap and of 5.3 milliwatts per cm2 at the edges. These levels of irradiation are desired for the efficient reaction (see D4.2).
Fluidic dish with coated beads illuminated from the top
In order to increase the illuminated catalyst surface area one can incorporate surfaces with a high surface-to-volume ratio as a packing material inside the dish. We considered a fluidic disc with coated beads. The parameters for fluidic dish were: the channel width – 5.2 mm, dish diameter – 100 mm, dish thickness - 10 mm, dish cap thickness - 1 mm. Beads diameters were 5 mm. Distance from the light source to dish top was set to 42 mm (see D2.2).
To improve illumination of the side beads one can add a lens to the model as it was done for the dish top (see Section Dish cap with lens).
Figure 6: Fluidic dish with beads and lens
We have considered a number of illumination designs. The main factors we took into account are the kinetics of the mass transport and ratio between volume and illuminated surface. The full list of factors is presented in the table below
The following reactor types were considered (Table A)
The following modification to fluidic dish design were considered
LED cooling
Considering all the data we found that the most promising designs are the disc type ones:
• Dish illuminated from top
• Dish cap with reflector
• Dish cap with lens
• Fluidic dish with beads illuminated from the top
• Fluidic dish with beads and lens
As they have demonstrated good characteristics from the point of view of kinetics as well as light distribution.
Further modelling of light propagation in ZEMAX proved that adding a reflector to the setup does not bring advantages as the reflection from the dish cap is negligible. Adding a lens on the other hand helps to redistribute light along the dish surface providing a more uniform illumination of the dish and beads.
The considered optical designs were combined with reaction kinetics and CFD modelling (see D4.2) to investigate final reactor efficiency. The incorporation of the rate equation into the CFD model has been undertaken and a design that results in good mass transport has been developed. The result of the work gave a number of pre-prototype systems optimized both from optical and mass transport concerns.
WP3 Photocatalyst Development (M1-M15) WP Leader Ulster
The main objective for this work package is to adapt AM’s photocatalytic formulation for water treatment applications. This objective involves three key tasks, benchmark testing, photocatalytic development and development and analysis of photocatalytic supports.
As the project moved from catalyst assessment to LED based reactor development, and the integration of results and data from WP2 and WP4, recommendations on which photocatalysts should be used were based on performance data and commercial factors (ability to prepare materials at commercial scale and cost). AM prepared variety of photocatalytic nano materials with different structures and morphologies to meet requirements of all theoretical types of reactors delivered in WP2 and WP4. The photocatalysts were prepared in the pure powder form, in suspensions and as films on selected supports. In collaboration with AM it was decided that the catalysts recommended for further trials would be FN3a and 60 wash.
Work Package 3 Photocatalyst Development
Benchmarking existing materials:
One of the main problems in photocatalytic research is the lack of standard test systems to allow the direct comparison of photocatalytic efficiencies of new materials and reactor configurations. To allow comparative studies within and across research groups Evonik (formerly Degussa) P25 is commonly used as a research standard as it is well-characterised and widely available. It has been reported to have good photocatalytic activity because of its mixed phase composition (80% anatase: 20% rutile) which promotes charge pair separation. Employing a photocatalyst in a suspension (or slurry) is efficient due to the large catalytic surface area to volume ratio and efficient mass transfer of the pollutant to the catalyst surface. However, to remove small catalyst particles from the treated water would increase the complexity and cost of industrial scale photocatalytic treatment. To negate post treatment recovery, the catalyst may be immobilised onto a supporting substrate and the catalyst film incorporated into water treatment reactor. The use of borosilicate glass as the catalytic support allows irradiation of the catalyst through the glass thus preventing photon loss due to absorption by the solution (i.e. back-face configuration). Previous studies at Ulster in a stirred tank reactor with good mass transfer have demonstrated this configuration to be suitable for the analysis of the intrinsic kinetics of immobilised photocatalysts.
Samples of photocatalyst were received from AM and sprayed onto borosilicate glass plates at a catalyst loading of 1mg cm-2. These plates were tested for their photocatalytic activity under backface UVA irradiation in the custom built stirred tank reactor using phenol and formic acid as model pollutants. The results were compared with the photocatalytic activity of P25 (Evonik Aeroxide) photocatalyst which was spray coated on borosilicate glass substrate (1 mg cm-2). The rates of degradation of formic acid and phenol were observed with the rates being just slightly lower than that observed for the P25 films. Following annealing of the FN3 films to 450oC, the rate of photocatalytic degradation of formic acid was similar to that obtained with P25. The rate of photocatalytic degradation observed with the FN2 films was much lower and was not markedly improved following annealing to 450oC (table 2).
Table 2: Initial rates for the photocatalytic degradation of formic acid using P25 and FN photocatalysts
Assessment of novel materials
AM developed a range of novel photocatalytic materials and provided seven different materials for characterisation and testing:
1. Nanofibres OR
2. Nanofibres 550C
3. Nanofibres reduced
4. Beads Classified (~45-70 µm)
5. 60 Wash
6. Pk20
SEM analysis (courtesy of AM) showed that samples comprised of predominantly anatase phase, which has been suggested as the primary phase responsible for photocatalytic activity. BET analysis demonstrated that the novel materials spanned a range of surface areas, with Pk20 showing the highest specific surface area (88 m2 g-1).
At Ulster, novel reduced graphene oxide/TiO2 composite films were prepared. Reduction of the GO to RGO in the composite mixture was confirmed by XPS and Raman analysis. The novel titania powder samples received from AM were assessed for their photocatalytic efficiency towards degradation of phenol, formic acid and E. coli as batch suspensions in the STR. The most efficient material for the photocatalytic degradation of phenol was the hollow spheres, which have a slightly higher efficiency than P25 (factor 1.15) and 60 Wash. However, P25 had the highest initial rate of degradation for formic acid and E. coli inactivation (followed by Pk20 and 60 Wash in both cases) (figure 4). No direct correlation between the rate of pollutant degradation and the BET specific surface area or the crystallinity of the novel photocatalytic materials was observed (table 2). The activity of the novel materials towards the disinfection of E. coli did not correlate with their activity towards the degradation of the chemical pollutants.
Figure 7: E-coli inactivation for a selection of photocatalysts received from AM and P25.
The efficiency of the TiO2-RGO composite material, prepared as immobilised films, was analysed using phenol as a model pollutant. The initial rate of degradation with composite catalyst (loadings of 0.5 and 1 mg cm-2) was found to be comparable to that of immobilised P25 (optimal catalyst loading for STR, 1.1 mg cm-2).
Table 3: Initial rates for pollutant degradation and BET specific surface area for the various catalysts in order of high to low surface area
1 The initial rate for sample ‘60 Wash’ has been normalised with respect to STR 1 due to slight differences in the dimensions between STR1 and STR 2.
2 The rates highlighted in green represent the highest rate obtained for the corresponding pollutant.
Experiments examining the disinfection activity of FN2a and FN3a films demonstrated that the rate of kill was similar to that observed for P25 (catalyst loadings 0.5 mg cm-2) As the project moved from catalyst assessment to LED based reactor development, and the integration of results and data from WP2 and WP4, recommendations on which photocatalysts should be used were based on performance data and commercial factors (ability to prepare materials at commercial scale and cost). In collaboration with AM it was decided that the catalysts recommended for further trials would be FN3a and 60 wash.
Development and testing of LED photocatalytic reactors:
Novel LED based photoreactors incorporating immobilised photocatalysts were successfully developed and tested (See WP 2&4). Stable and homogenous photocatalyst coatings were produced by spray coating of P25 and FN3a onto borosilicate sheets and via rotary evaporation onto glass spheres. Both these methods are inexpensive processes for the production of coatings on non-toxic and widely available substrates; in addition both processes are easily scalable.
The prototype recirculation reactor was developed in WP4. The TiO2 coated plate was sandwiched between the Perspex guides with the TiO2 coating facing down towards the flow channels. The coating was irradiated from above using a UV-A LED (1W) at a fixed distance of 4.2 cm from the sample plate. 800 mL of pollutant solution was added to the 1 L reservoir and pumped through the LED photo-reactor with a flow rate of 1.4 L min-1 using a submergible aquarium water pump (figure 7). An aquarium air pump provided oxygen to the reservoir at a flow rate of 1 L min-1. The pollutant solution under treatment was covered and stirred continuously throughout the experiment. The LED source was allowed to stabilise for 30 min prior to each experiment. TiO2 coated beads were added into the concentric groves within the Perspex guide depending on the nature of the experiment. An uncoated borosilicate glass plate was used in UVA LED only control experiments. The Mk1a reactor contained a TiO2 coated plate without beads.
Figure 8: Mk1a LED photo-reactor and Mk2 reactor.
Using a coated borosilicate window in the Mark 1a LED photor-eactor, significant organic pollutant degradation was observed, however disinfection of E. coli was not observed. The inclusion of TiO2 coated beads (creating a 3D porous supporting media), a blank borosilicate window and the reduction in flow rate (the Mark 1b LED photo-reactor) significantly enhanced the rate of disinfection with complete inactivation observed in 270 min.
To further enhance the inactivation kinetics a Peltier cooling system was introduced. This reduced the LED running temperature to 40oC, effectively doubling the UVA output, and resulted in the complete inactivation of E. coli within 210 min. Catalyst adhesion analysis determined that initially a small quality of titania was released from the coated beads under worst case scenario testing. A washing step is therefore recommended to remove any catalyst dislodged during reactor assembly - this could easily be included within the reactor leak testing step.
The optimal reactor conditions for the disinfection of E. coli were determined to be:
a) LED photo-reactor incorporating TiO2 coated beads and a blank borosilicate window.
b) Reactor should be used in batch recirculation, with a flow rate of 0.11 L min-1.
c) Air sparging (1 L min-1) must be included, via a standard aquarium air pump.
d) Peltier cooled LED fixed at 4.2 cm from reactor surface (UVA output of 9.8 mW cm-2).
The Mark 1b photo-reactor was examined to determine photocatalyst longevity during extended testing (1 week) using both chemical and microbial pollutants in 20 L of simulated water. The system demonstrated the expected performance for chemical (phenol) removal and disinfection of E. coli. Catalyst stripping analysis was undertaken using flow rates three times that of the standard operating conditions. Upon reactor construction incorporating TiO2 coated plates/beads a small amount of TiO2 was mechanically dislodged from the system (< 6 mg) and entered the water exiting the reactor. TiO2 did not continue to be removed from the glass supports during extended use. The water leakage test, carried out following prototype construction, should include a catalyst recovery.
Risk assessment:
Due the concern with respect to the risk of nanomaterial contamination of the environment, a report describing the EU legislation in place to regulate the use of nanotechnology and nanomaterials within products – and more specifically Titania - was compiled. If produced in volumes greater than 1 tonne, nanoparticle (NP) Titania will fall under the REACH regulations, and manufacturers have a legal duty to comply with REACH in line with the published deadlines. The classification, labelling and packaging (CLP) regulations are also relevant with respect to provision of information to those using products containing nanomaterials.
A review of the scientific literature was also undertaken with respect to nanoparticle interaction with the aquatic environment and toxicity of nanoparticle Titania. The available scientific literature highlights the complexity of the interaction of NPs with the inherent chemistry of natural waters and demonstrates that this can influence both toxicity measurement and bioavailability of nanoparticles. Whilst a range of aquatic based toxicity assays have been conducted, there is great variation in the concluded levels of nano TiO2 toxicity - with many studies not observing any toxic effects in whole organisms such as fish.
Although further research is required to design full environmental risk assessment strategies, use of the median L(E)C50 value across a range of species via EU-Directive 93/67/EEC suggests that the nano TiO2 can be classified as “harmful”. In a worst case scenario, if the weight of TiO2 in the current Aqua-Pulse product was discharged into 1 L of water, the concentration would still be within the levels associated with the “harmful” category described above.
In conclusion, the use of nanoparticle Titania in Aqua-Pulse products does not appear to pose a significant toxicity risk to the aquatic environment. However, as the field of nano-ecotoxicology develops appropriate environmental risk assessment and life cycle analysis should be conducted to ensure compliance with best practice.
A risk assessment (based on the University of Ulster template) for the use of the Aqua-Pulse prototype system was conducted prior to shipping of the prototype devices to partners.
Market assessment:
To aid the SME’s in the development of a product for the aquarium market assessments of the priority pollutants and products currently available within this sector were compiled.
The main chemical and microbial priority pollutants present in fresh water aquaria were summarised. The inclusion of a well-developed biological filter can manage the levels of nitrogen within the aquarium water ecosystem and weekly small water changes (10 - 15%) are usually sufficient to ensure the chemical stability of aquarium water. Disease causing organisms pose a more of a threat to the survival of fish – a table of priority organisms and disease symptoms was compiled. The latter area offers significant potential for the development of a product based on the Aqua-Pulse technology. To this end, a report summarising aquarium treatment devices available to remove chemical and microbial pollutants from freshwater aquaria in a global market place was developed. Mechanical, biological, chemical and sterilisation systems are widely used for aquarium water treatment as individual units or more typically through a combination of one or more of these processes in a single device. These technologies were briefly described, along with a range of available products from the main brands within this growing market. The variety of devices available, and the range of prices products sell for was also included. All available products require regular maintenance with the requirement to purchase replacement consumables parts. Whilst UVC disinfection can be included in large volume external and canister based filtration systems, combined products are not widely available for smaller volume tanks. There is therefore a market opportunity for new compact disinfection technologies which could inactivate fish pathogens. The product could be included into internal and external filter assemblies used within small to medium volume aquariums.
WP4 Photoreactor development (M7-M17) WP Leader Ulster
The main objectives for this work package were to develop an optimised design for the UV photocatalytic reaction chamber, incorporating both the UV LED sources and the immobilised photocatalyst.
The design of the photoreactor has been based on the simultaneous consideration of the optical design (reported in D4.1) the incorporation of the photocatalyst and design of the photoreactor (D4.2) combined together and a prototype delivered for testing (D4.3). The design process therefore went through a number of iterations, with kinetic, optical and useability considerations leading to a final designprototype.
The advantages of LED illumination are a compact form factor and low power consumption, but the ability to provide a high illumination power in the desired wavelength range from a single LED results in a limited catalyst surface area, and thus a limited reactor size. In order to facilitate photocatalytic removal of pollutants or disinfection in a single pass process, a large surface area would be required, along with large numbers of LEDs, and prohibitive costs. The consortium decided to develop a low cost, continuous flow, multiple pass photoreactor based on the illumination level provided by a single LED. By optimising this compact photoreactor, an effective LED based solution could be produced. It should be possible to scale up this system using larger numbers of LEDs.
A compact, multiple pass photoreactor will have applications where a body of water is, or can be, stored for a period of time. Water tanks in properties that are vacant for extended periods, such as holiday homes are one example, another example would be ornamental fish tanks. While a number of applications exist, for the purposes of the initial design brief, the application to ornamental fish tanks has been agreed as one of the more promising initial target markets.
Design Brief:
The photoreactor was to be illuminated using a single UV LED with peak emission at 365-370 nm. These LEDs are available in a range of power outputs, but the most likely candidates for incorporation into the photoreactor are of 1 W and 250 mW nominal output. The photocatalyst will be a TiO2 material, with a number of types being compared in WP 3 in order to identify the optimum material.
Initial discussions suggested a flow rate of up to 1 L/min, and a single pass, continuous flow reactor configuration was proposed as ideal for a range of applications. This would be capable of delivering purified water at point of use. The kinetics of the photocatalytic reaction necessitates either long residence times, or very large illuminated surface areas in order to purify water in a reactor of this single pass configuration. Subsequent evaluation suggested that a multiple pass system would be more appropriate to the small form factor and low power nature of LED illumination. The final design should therefore be a continuous flow, multiple pass reactor, that should be inexpensive and fit into a small enclosure, suitable for fitting to a fish tank or small water storage tank. The system should be suitable for the degradation of organic or inorganic pollutants as well as for disinfection. Concentrations of pollutants and / or pathogens will typically be low, and the system designed to improve and maintain water quality in a stored body of water, rather than to treat a continuous stream of contaminated water.
Design Process Summary
Photocatalytic Reaction Kinetics:
The kinetics of photocatalytic degradation in a continuous flow reactor can be approximated by the following equation, in this case written to determine the reactor volume:
Where Q is the volumetric flow rate of water, [A] is the initial concentration of the pollutant, X is the fractional decomposition required, ICD is the illuminated catalyst density and AMP is the overall reaction rate, including both kinetic and mass transport effects.
Assuming the flow rate of 1 L/min, a 90 % decomposition requirement from an initial concentration of 12 ppm TOC, and AMP of 3 x 10-5 mol/m2.s initial calculations show that the illuminated surface area required for single pass, 90 % conversion is 1250 cm2. To illuminate this at the required illumination density of >5 mW/cm2 requires 6.25 W of illuminated power, assuming no losses. At least seven 1 W LEDs would be required. Due to both the expense and the difficulty in integrating several LEDs to illuminate a given reactor configuration, a single pass continuous flow system was deemed to be impractical. This led the consortium to decide on a single LED reactor that could be scaled up by adding reactors in series or in parallel.
Optical Design:
Having decided on a reactor that should use a single LED for illumination, the most obvious optical design, based on those considered in WP2 was a circular flat plate type reactor. This matches the illumination pattern of the LED and therefore minimises are issues related to reflectors or losses. The optical design was modelled, and a number of modifications investigated, including the use of lenses, the impact of borosilicate glass beads and adding a reflector behind the LED. While it was clear that the beads may not be effectively illuminated on both sides, they still provided a significant increase in surface area, and remained worth investigating experimentally.
Lenses could provide a more uniform illumination of the circular reactor, reducing the peak illumination intensity in the central region by spreading this out across the whole disc, but all additional components in the light path will absorb some of the UV light, and the additional cost of the lens would add a significant amount to the total product cost. A reflector did not show any significant improvement in illumination compared with the LED without a reflector.
CFD Modelling:
The illumination pattern from a single LED is circular, with the highest intensity in the centre and a decrease in intensity with increasing radius. A disk is therefore the optimum surface to be illuminated using a single LED. The photoreactor design also needs to allow for a reasonable residence time, proximity of the pollutants to the catalyst surface and good mass transport. The inclusion of a spiral channel in the disc can provide a long tube reactor in a flat circular configuration, as shown in figure 9.
Figure 9: Fluidic disc design drawings including a UV reflector if required.
In the disc reactor model, the photocatalyst is coated on a flat borosilicate glass plate, which is sealed across the surface of the channels. The catalyst is therefore illuminated from the back. As well as a disc reactor with open channels, a similar reactor in which the spiral channel was large enough to contain glass bead packing material was also evaluated using CFD.
Combining kinetic and optical models:
The photocatalytic degradation rate for both organic pollutants and for bacteria is dependent on the illumination intensity. In order to evaluate the effect of the radially variable illumination intensity, custom field functions were developed within fluent to incorporate the experimentally derived reaction rate based on the illumination analysis provided by CIT as part of D4.1. Initial calculations were performed using formic acid as a model pollutant using experimental reaction rate data; it was therefore possible to determine the effect of the variable illumination pattern on the overall reaction rate in the disc reactor. It is important to note that this custom field function is based on the reaction rate from a stirred tank reactor, and thus assumes no mass transport limitations. If the reaction is mass transport limited, then the overall reaction rate will be slower than the modelled rate.
Disc reactor with a gap between the channels and the sealing plate
The existing disc designs were based on the channels being fabricated in one part and sealed using a borosilicate glass plate. It would be difficult to fabricate a reactor that will seal effectively, so there is likely to be a gap between the top of the channels and the glass plate. The existence of a gap will also allow the whole of the glass surface to be both illuminated and exposed to water, while if there is no gap, the area of the glass plate that is in contact with the tops of the channels will not be in contact with water, and thus the illuminated catalyst surface area will be slightly reduced.
CFD models were constructed to include a gap between the tops of the channels and the catalyst coated glass plate. It showed that as well as the spiral velocity, there is a linear velocity component between the inlet and the outlet for the reactor with a 0.3 mm gap. It was considered that this could disrupt the laminar flow in the channels and improve the mixing within the reactor.A portion of the fluid that travels through the 0.3 mm gap at the glass plate face creates a spiralling flow within the channel, which will result in significantly better mixing.
The reaction rate was compared for the sealed channel reactor and the channel with a 0.3 mm gap. The calculations are based on the assumption that there are no mass transfer limitations, the resultant difference in reaction rate is because the illuminated surface area is higher for the reactor that includes the 0.3 mm gap. As a result of the increase in turbulent kinetic energy, resulting in good mixing properties in the channel, and the increased catalyst surface area, the disc reactor with a 0.3 mm gap between the top of the channels and the borosilicate glass top plate was chosen for pre-prototype development and testing.
A number of pre-prototype designs were fabricated. The channels were machined from a 10 mm thick PMMA sheet, and a borosilicate glass plate sealed to this using a PMMA backing ring and a silicone gasket. The assembly was mounted on four M8 threaded rods. The LED assembly was attached to a third PMMA sheet and mounted on the same threaded rods, the spacing between the LEDs and the reactor being easily adjustable. Many of the design questions posed have been answered based on the kinetic calculations and CFD models reported above, but many other decisions have been based on the application of sound engineering design principles.
Reactor Prototype:
An early prototype reactor (Mark 1a) was built and tested, based on the spiral channel with a photocatalyst coated borosilicate glass plate, with the catalyst back-side illuminated. At this point, experimental data was sought to determine the optimum configuration for removal of organic pollutants and for disinfection. The Mark 1a reactor was effective for the degradation of organics but not for disinfection of E-Coli.
During this experimentation, reported in D3.4 a number of improvements were made to the reactor including the addition of a Peltier cooling system to maintain the LED temperature and thus maximise the illumination output, The Mark 1b photoreactor had the photocatalyst coated on borosilicate glass beads within the spiral flow channel, and was illuminated through a clear borosilicate glass plate, along with the Peltier cooler. The Peltier cooler resulted in approximately double the illumination intensity, when compared with air cooling. The Mark 1b photoreactor was shown to be effective both for degradation of organics and for disinfection of E-Coli.
Extended testing of the Mark 1b photoreactor showed that it was effective in photocatalytic degradation and disinfection in a 20 Litre tank. This prototype system was therefore chosen as the basis for the Mark 2 system that was to be validated in WP5.
Building and Distributing Photoreactors:
For validation purposes, a number of systems were designed and built. The basis of the Mark 1b system was re-designed to fit in a DIN rail enclosure, with the required power supplies housed at one side and the fluidic parts at the other. An aquarium pump is contained within the enclosure with silicone tubing to deliver the water to and from the aquarium or test reservoir. Power supplies for the pump, for the peltier cooler and fan as well as for the LED are contained within the enclosure and both are isolated through the main switch. Once assembled and running, the system is self-contained and can be run continuously. The assembled prototype reactor can be seen in figure 10. Systems were supplied to TI for validation and testing as part of WP5 and to other consortium partners.
Figure 10: A) Prototype photoreactor for validation and testing and B) Prototype photoreactor connected to a 20Litre aquarium for demonstration purposes
Conclusions:
WP4 contained three parts. The optical reactor design, built on WP2 and contained within D4.1 The photoreactor design process, based on the optimised photocatalyst from WP3 and the CFD models and prototype testing in D4.2 and then the final prototype delivery of D4.3. All milestones were met on-time and within budget, and assistance provided to all partners, in particular to TI in setting up, testing and validating the performance of the prototype photoreactor. A functional system, capable of maintaining the water quality in a small aquarium was designed, tested, built and provided for validation testing.
WP 5 Integration, Validation and Testing (M16-M24) WP leader TI
Summary:
The prototype of the photo catalysis reactor developed in AQUA-PULSE has been validated at TI from mid June 2013 until the end of the project period based on recommendations presented in Deliverable 4.3. In Accordance with the Amendment of the Grant Agreement, the consortium had also decided to use waste water from the Bergen Aquarium for tests run at TI laboratory. The test water included aquariums for species of cold and warm fresh water as well as seawater. Wastewater was collected from the holding tank of the recirculation system at the aquarium before the water goes to the treatment plant. In addition to performance tests, toxicity test that involved acute toxicity on Daphnia Magna and growth inhibition on pseudokirchneriella subcapitata was also carried out. Further to this extended tests on longevity of the photocatalyst and potential catalyst stripping trials have been carried out.
The reactor is designed as a multiple pass system with a maximum capacity of 1.4 l/min. For the performance tests, the flow rate was set to 100 ml/min in order to attain a complete degradation of pollutants after multiple pass treatment cycle of 72 hours. Performance tests were run for investigating the reactor’s capability of degrading organic and microbiological pollutants as well as nitrogenous compounds in the aquarium water. During the tests, both control and treated water samples were also analysed for physical water quality parameters including pH, temperature and conductivity in order to evaluate potential impact of the photocatalysis process on these parameters.
The trial results show that the Aqua-Pulse system has capability of degrading microbiological and organic pollutants effectively, also in saline water media. However, no effect is found on nitrogenous compounds. The photocatalysis of the system has also shown no generation of toxic substances, and does not impact on the physical parameters of the aquarium water. There is no significant stripping of TiO2 as a result of water flow through the reactor.
Background:
According Annex 1 of the Grant Agreement the Aqua-Pulse project has been designed for purification of water without specifying the industrial sector of application. However, the consortium has continuously evaluated several applications in the process of developing the technology. The initial DOW had indicated that the reactor prototype would be integrated with one of Unik’s water purification units, which are designed primarily for treatment of drinking water. Thus, Drinking Water has been one of the main focus sectors for application until the first version of the reactor has been developed and subjected to preliminary laboratory tests.
For Aqua-Pulse, compactness and low power consumption has been a vital factor as highlighted in the DOW. However, the ability to provide a high illumination power in the desired wavelength range from a single LED has been found to be limited. A detailed explanation of this issue is presented in Deliverables 4.2 & 4.3. To attain adequate purification, the consortium thus decided to develop a low cost, continuous flow, multiple pass photoreactor based on the illumination level provided by a single LED with possibility of scaling up later using larger numbers of LEDs. While the reactor that is based on such multi pass process can have several applications, the primary market selected by the SMEs in Aqua-Pulse was small scale aquariums.
The fact that Unik’s existing water purification units are applicable only for the water supply industry and have no relevance neither to the selected application nor the volume of water that the prototype is capable of treating resulted in amendment of the Grant Agreement that excluded integration of the prototype to a purification system. Instead, it was decided to run functionality tests and validation of the reactor prototype using wastewater extracted from a major aquarium in Norway, and modeling the water recirculation from a home aquarium. For this purpose, the Bergen Aquarium that has rearing tanks of both fresh and seawater has been selected.
The Aqua-Pulse prototype is the outcome of the work performed in several work packages especially WP2 - UV LED source development, WP3 – Photocatalyst development and WP4. – Development of the photocatalyst reactor.as well as Task 5.1. Thus, the work performed in Task 5.2 and reported in this deliverable is an extension from previous WPs.
Materials and methods:
In the Aqua-Pulse project, none of the RTD participants are licensed to experiment with living animals. As this has created ethical limitations for use of a living home aquarium in the validation, the requested amendment of the Grant Agreement has also addressed this issue and suggested use of wastewater from rearing tanks of a large aquarium for the validation of the reactor prototype.
The aquarium selected for extraction of wastewater for the validation is located in the centre of Bergen city at the west coast of Norway about 500 km from Oslo. This aquarium was selected due to its proximity to Unik who have been responsible for collecting and shipping of samples to TI laboratory during the validation period. Even if the company is not part of the project, the management of the aquarium has been benevolent and cooperated throughout the trial period.
The Bergen aquarium has more than 60 large and small rearing tanks, 3 of which were selected for the Aqua-Pulse validation. Water samples at each aquarium were collected from the holding tank of wastewater before it is pumped to the treatment plant. This approach was preferred as water returning from the treatment plant is of high quality and may not contain traceable pollutants for validating the functionality of Aqua-Pulse. Typically, the wastewater treatment includes coarse/fine filtration, bio-filtration for control of nitrogenous compounds and UV-irradiation as final treatment stage. The aquarium tanks are continuously oxygenated and the oxygen saturation level during the validation period has fluctuated between 78 and 100%.
Test set up:
The set-up consisted of the final version of the Aqua-Pulse prototype and a 20 L aquarium filled with 10-20L of waste water from Bergen aquarium for the validation. The water is pumped by a submerged aquarium pump via the silicon tubing 1 to the photocatalytic reactor and treated water returns to the aquarium via tubing 2. The water in the aquarium is continuously aerated with a diffusor pump via tubing 3 to supply oxygen and maintain maximized effect of the photocatalysis process. Flow rate through the reactor is set to 100 ml/min. Samples for analysis of different parameters are collected with an interval of 2 hr from the aquarium.
Figure 11: Test set-up for validation of the Aqua-Pulse reactor
Parameters measured during the validation:
Having established the application of Aqua-Pulse, the two important parameters selected as target pollutants for validation of the prototype are organic substances, organic substances and micro-organisms. In the validation organic substances are measured as TOC/COD and micro-organisms as total coliform bacteria whereas nitrogenous compounds are analysed as NH4, nitrites and nitrates. During the trial period, water samples collected from the Bergen aquarium are shipped to the TI laboratory for delivery within a maximum of 12 hours. Samples are then stored at a temperature of 4-50C before being subjected to photocatalysis. Each cycle of the photocatalysis test is run for 72 hours continuously. The background for 72 hour test cycles are preliminary tests run at Ulster that have shown this test duration for attaining sufficient degradation. Samples for analysis are collected only at day time.
During the validation, iteration of the tests were made with improved procedures, the main one being running the tests in a temperate room of about 7 - 100C rather than at room temperature of about 22 0C as the population of the coliforms grew exponentially when the temperature of the test sample rises from 4 to above 200C. Further to this, aeration of the aquarium water has been increased by using two air diffusers rather than one as used at the initial phase of the trials. In addition, the sampling procedure of treated water has been improved which resulted in more representative samples.
Even though organic pollutants and bacteria are the prime pollutants for the performance test of the reactor, there are other parameters that have impact on fish health and have to be controlled. A full list of these is presented in Deliverable 3.5. These include:
- Nitrites
- Nitrates
- NH4
- pH
- Conductivity
- Water temperature
In addition, the concentration of dissolved oxygen has been monitored during the test runs.
In some of the samples collected from the Bergen aquarium, a few parameters have not been traceable as concentrations were below detection level. In addition to measurement of these parameters a separate toxicity test has been run for analysing formation of intermediates that may potentially have toxicity effects. In addition, potential release of TiO2 has been evaluated. Finally, a longevity test was performed for 6 days using new photocatalyst and examining degradation of phenol with concentration of 15 ppm dissolved in tap water and at a flow rate of at 100ml/min.
A variety of difference parameters are measure during the validation phase. These include
• Dissolved Oxygen
• Temperature
• pH
• Conductivity
• Nitrites and nitrates
• Ammonium
• Chemical Oxygen demand
• Total Organic Carbon
• Coliform bacteria
The measurement process is carried out in accordance with a number of specific standards and measures. These are available for consultation in D5.1 and D5.2
Test results:
Results of the tests are shown in chart diagrams for all three types of water from the aquarium. Results of measured total suspended substances, TSS are not presented in this report as the concentration has been very low in all samples with little variation as a result of the photocatalysis process. The reactor showed a good reduction in the levels of Coliform bacteria and TOC/COD over the duration of the 72hr test. More in-depth details are available in D5.2.
To investigate if the photocatalysis process has any effect on degradation of nitrogenous compounds or changes the water quality parameters measurement of nitrites, nitrates, NH4, pH and conductivity have been made. In addition, the concentration of dissolved oxygen has been monitored to verify that there is sufficient aeration of the aquarium water during the test runs. Results of measurements showed that pH, dissolved O2, conductivity and NH4 levels all remained stable during the testing phase.
To check whether the bacteria are eliminated due to factors other than photocatalysis or if the bacteria flora contribute to degradation of organic substances in the test water, 1L control samples of both fresh and sea aquarium water were set beside the reactor for 72 hours and samples were taken for analysis twice a day. In addition to organic substances and bacteria, measurements of other parameters, including nitrogenous compounds (nitrites and ammonium), conductivity, O2 concentration, pH level and temperature were made.
Toxicity test:
Toxicity testing was performed using samples from fresh water aquarium of cold water species. Tests were run for acute toxicity on Daphnia magna and growth inhibition on pseudokirchneriella subcapitata at NIVA Laboratory for Eco-toxicology in Oslo.
Acute toxicity test:
The acute toxicity test method is in accordance with the OECD Guideline 202, “Daphnia sp. acute immobilization test". The test organism is Daphnia magna maintained in Elendt M7 and fed with Pseudokirchneriella subcapitata grown in 10% Z8 nutrient salt solution. Age at start of test was < 24 hours.
The test period lasted from the 20th to the 22nd of August. For the test ,1 liter samples of untreated (control) and treated aquarium water for cold fresh water species was sent to the laboratory. Pretreatment of the samples in the laboratory included filtration with a filter of 0.45µm size and addition of nutrient salts (ISO 6341). After pretreatment, dilution was performed with the untreated water to test concentrations of 10; 18; 32; 56; 100 %. Replicates were prepared in 4 vessels for each concentration, with 5-7 test organisms pr vessel.
The test parameter data are as follows:
Temperature Min: 20.2 Max: 20.4
pH in control Start: 7.86 End: 7.88
pH in highest cons. Start: 8.02 End: 7.93
Dissolved O2, 48 t Control: 8.71 Highest concentration: 8.69
Calculation of EC50 Not preformed
Calculation of NOEC Cochran-Armitage step down trend test
Results:
There was mortalities in the control, 18 %, 56 % and 100 % at 24 hours (table 5), but there was not a concentration dependent response (Figure X). Less than 10 % mortality is acceptable within the criteria of OECD 202. Increase in pH and dissolved O2 was also within OECD 202 criteria.
Table 5: There was no significant difference in mortality in the highest concentration from control, see Cochran-Armitage step down trend test below
Table 6: Summary of test results for water treated with the Aqua-Pulse reactor
Algae growth inhibition test:
The algae growth inhibition test method is in accordance with the OECD Guideline 201. The test organism is Pseudokirchneriella subcapitata cultivated for 72 hours starting on the 20th of August 2013. Water samples used are same as the one used for the acute toxicity investigations. The stock culture was semi continuous. Pretreatment included filtration with a 0,45µ mesh and addition of stock nutrient according to ISO 8692. Distilled water containing ISO 8692 was also used as addition control sample along with the control sample of the aquarium water. Concentrations were prepared with 10; 18; 32; 56; 100 % , in addition to the extra control sample prepared from distilled water to find out if there is any difference between the distilled and aquarium water control samples. Replicates included 3 per diluted sample, 6 in the control and undiluted samples.
Incubation was undertaken in a temperature controlled incubator (19.7-21.10C with orbital shaking (90rpm). As test vessel, 30 ml glass vials with approximately 15 ml sample were used. Illumination was made using Gro-lux fluorescent bulbs from Sylvania.
Results:
Exponential growth was observed throughout the test, though growth was slightly higher in the separate ISO8692 control during the first two days. The test met the criteria defined in OECD 201. After 72 hours there was no reduction in growth compared to the control in any of the test concentrations. A more extensive explanation and analysis of the control, algae and toxicity tests is available in D5.1-5.2
Evaluation of Catalyst Stripping over prolonged operation times and release of nanoparticles from the reactor:
Release ofTiO2 particles during operation of the reactor can be an indicator to the lifetime of the catalyst, release of TiO2 as well as potential impact to the stock in the aquarium. Therefore two analyses have been carried out to investigate any catalyst stripping and concentration of TiO2 particles in the water exposed to photocatalysis.. The first trial was run at Ulster using distilled water at a flow rate three times higher than the set flow rate used for other experiments. In the second phase, measurement of TiO2 was performed in connection with the performance tests with aquarium water run at TI using same analysis technique.
The same type of catalyst has been used in the trials at TI and calibration curve was generated from change of absorbance in the same wave length. Samples were taken and analysed every 72 hour for a period of 18 days. In the first experiment, there is a small increase in catalyst stripping up to around 100 min from where there is a decrease in the TiO2 concentration. As the flow rate in the second experiment is much lower and samples are analyzed much less frequently, the initial increase in TIO2 increase has not been observed. However, the total result observed mirrors what has been recorded in the initial experiment. The consistent decrease in TiO2 concentration can be explained in that as the beads are loaded into the reactor there is mechanical damage of the coating and some particles are released. It has been observed that these stick to the plastic tubing and as a result are not free to enter the water system. As there is no continued release of particles, it is suggesting that after loading of the beads a washing stage to be carried out to reduce the potential threat of nanoparticle release into the environment.
Longevity test:
The longevity test was performed by rerunning the experiment 6 times using tap water with a new set of catalyst. At the start of the experiment phenol was added to the water until the concentration was brought to 15ppm. The degradation of phenol is measured after 24 hours, and then more phenol is added to the reservoir and the phenol concentration is elevated to about 15 ppm at the start of Day 2. The cycle is then repeated five times. The test result is presented in Table 7.
Table 7: Longevity test result
The longevity data demonstrates the continued removal of the pollutant. As the concentration and pollutant used in the test is higher than the average organic contamination in the aquarium water used for the other trials, this test exaggerates the expected working conditions. In spite of this, the longevity test demonstrates that the photocatalyst can be expected to be robust. This is also demonstrated by the performance trial and the TiO2 stripping tests.
Discussion:
Results of the trials conducted for evaluating the performance of Aqua-Pulse in purification of aquarium water and the control tests show that the technology works well for degrading microbiological and organic pollutants both in fresh and seawater.
While a number of other studies have demonstrated that photocatalysis is emerging as a potent method in environmental protection due to its ability to oxidise organic and inorganic substrates in fresh water, only a few reports are available regarding its ability to decompose pollutants in saline water. Although it has not been possible to get information related to photocatalysis in aquarium water for seawater species, there are however studies that have shown degradation of organic substances in seawater. Unlike the findings in the trials of Aqua-Pulse, most of the above reports claim that the decomposition rate of organics in seawater was slow compared with fresh water media. However, none of the studies found that chlorinated compounds have been detected during the irradiation and after complete decomposition was achieved. The toxicity test carried out using fresh water from the Bergen aquarium has also confirmed that the tested prototype does not generate toxic substances. Thus, AquaPulse can be a safe technology for the target market – home aquariums. The longevity test and analysis performed for investigation of catalyst stripping have shown that the photocatalyst used in the prototype is expected to be robust.
The trials have not shown any degradation effect by the Aqua-Pulse system on the nitrogenous compounds of the aquarium water. Therefore, the tests cannot confirm that the technology can substitute existing methods for control of ammonium and nitrites. Regarding physical water quality parameters, the test results have not shown any impact on the temperature, pH level and conductivity of both fresh and seawater subjected to photocatalysis.
The process of the tested prototype is slow compared to traditional methods, and further work may be needed before AQUA-PULSE can be a complete substitution of the products in the market. Its functionality in seawater media is however promising as a pre-treatment method of desalination process such as reverse osmosis technique.
Potential Impact:
Attached PDF of Dissemination activities.
List of Websites:
www.aqua-pulse.org