23. - 25. 10. 2012, Brno, Czech Republic, EU
DEVELOPMENT OF NANOFIBER SUPPORT FOR USE AS A CARRIER OF BACTERIAL BIOMASS IN WASTEWATER TREATMENT
Lucie KRIKLAVOVA a,b, Tomas Dub b, Tomas LEDERER a
a Technical university of Liberec, Centre for Nanomaterials, Advanced Technologies and Innovations, Studentska 2, 461 17, Liberec, Czech Republic, lucie.kriklavova@tul.cz
b Technical university of Liberec, Faculty of Mechatronics, Informatics and Interdisciplinary Studies, Institute of Novel Technologies and Applied Informatics, Studentska 2, 461 17 Liberec,
Czech Republic
Abstract
Nanofiber materials, mainly due to their characteristic properties (such as a high specific surface, surface morphology, small pore size, chemical and physical stability, biocompatibility) are among other notable and very suitable materials for applications in the biological wastewater treatment. The development of a modern and original technology (nano-fibrous carrier) represents a partial and parallel solution of microbiological and material engineering issues as well as the final bioengineering design. In this paper, the following characteristics of the nanofibers used as carriers of bacterial biomass were examined: the rate of colonization as a function of the used material, the rate of carrier ingrowths by relevant microorganisms (comparing short-term and long-term cultivation) and the density evaluation of the complex after an increase of biomass. Microbial biofilm formation can be greatly supported using nanofiber structures and then the whole biofilm reactor system provides stable biodegradation (faster incorporation of the biofilm, more stable biofilm, the adaptation ability to extreme conditions, decreasing of the impact of shock conditions).
Keywords:
nanofiber technology, wastewater treatment, biomass carrier, immobilization of microorganisms, biodegradation.
1. INTRODUCTION
The biomass carrier is a crucial element for wastewater treatment technology in the biofilm reactor. The material of the carrier has to be biochemically inert, chemically and physically stable, and morphologically appropriate. The specific gravity of carriers should be comparable to the wastewater even after an increase of biomass or salt precipitates. The idea is to maximize the specific surface area, thus the goal was to develop a technologically advanced and reliable type of biomass carrier.
Development of threads, respectively textiles containing nanolayers for use in the wastewater treatment has its own history. At the beginning, cotton thread was the carrying base (Figure 1, the sample A). However, there was a high degree of water absorption, high specific gravity, and high sorption; also, it was very dimensionally non-stable. Nanofibers on this thread have been applied very disproportionately (due to low experience during production). Further, the polyester yarn was used as nanofibers carrier (does not absorb water, dimensionally stable) (Figure 1, the sample B). The resulting thread already reached acceptable properties (a uniform cover of nanofibers), but the problem was during application of carriers, when nanofibers were releasing from the surface of carriers; therefore it was necessary to fix the nanofiber layer on the surface. Figure 1, the sample C was protected by the cover thread. The underlying and cover yarn was polyester fibers, nanofibers were made from polyurethane. The sample was further improved and resulted in the sample D (Figure 1), where the basic fiber is polypropylene, nanofibers made from polyurethane and the fixation is made by polyethylene fibers. Surface fixation was then much more sophisticated (more tightly wrapped nanolayers) and nanofibers are much more effectively stabilized.
Figure 1 - A brief review of the development of nanofiber yarns (samples A-E)
The development of cover thread further improved to sample E (Figure 1). Nanofiber yarn is finally composed of three parts. The basic fibre is polyester (660 dtex, air shaped), the coating is composed of polyurethane nanofibers (50-150 dtex, electrospinning method, nanofiber diameter is approx. 260 nm), everything is double-wrapped by protective polyethylene fibres (167 dtex, protected against friction during processing and subsequently during applications against disintegration of nanofibers, see [5]).
The specific surface of the resulting yarn formation with value of 50 dtex of polyurethane nanofibers determines a fixed bed structural design of more than 800 m2/m3. Thanks to a combination of several different polymers, the carrier density can be adjusted (density of approx. 900 kg/m3 to 1200 kg/m3), practically according to requirements of the application (the wastewater).
Firstly was the cotton thread as the carrying base and nanofibers. Second was the polyester yarn used as nanofibers carrier and nanofibers. Third was underlying and cover yarn polyester fibers and nanofibers were made from polyurethane. Fourth was the basic fiber polypropylene, nanofibers made from polyurethane and the fixation is made by polyethylene fibers. Finally is the nanofiber yarn composed of three parts - the basic fibre is polyester, the coating is composed of polyurethane nanofibers (approx. 260 nm); everything is double-wrapped by protective polyethylene fibres. The outline for surface formations is formed from polypropylene fiber.
2. TESTS OF NANOFIBER LAYERS COLONIZATION ON DIFFERENT POLYMERS
The main aim of these tests was to monitor the influence of the nanofibers material (type of polymer and material properties) on the possibility of microorganism’s colonization (adhesivity of bacteria to surface in the initial stages of colonization). Three types of nanofibers were made: polyether-sulfone (PES), polyurethane (PUR) and polyvinyl-butyral (PVB).
Model water containing phenol as the dominant carbon source in the order of 500 mg/l was used for these tests. The results of experiments show that polyurethane material (as a nanofiber) is more suitable. Bacteria on polyurethane nanofiber networks have the fastest kinetics growth, especially in the early days of colonization (within the first few days).
Figure 2 - Time evolution of biofilm growth on different types of nanofibers (various polymers material)
0 5 10 15 20 25 30 35 40 45
0 10 20 30 40 50
Fullness of carrier's surface by microbial biofilm [%]
Time [days]
PES PUR PVB
Figure 3 - View of the samples on the twentieth day of cultivation (PES, PUR, PVB)
3. SHORT TERM CULTIVATION OF CARRIERS (COLONIZATION)
As was published in [4], on commercial polyethylene AnoxKaldnes carriers bacterial biofilms are mounted significantly slower. This slow increase is due to insufficient adhesion of microorganisms to the surface of the carrier. Slightly better results show previously colonized carriers, where bacteria need not expend energy to disturb the surface structure. It can reach easier adhesion of microorganisms by the modification of its surface, even for the same type of carriers (by disturbances of upper layer) and thereby can obtain a faster colonization. Model water with aniline as the dominant carbon source in the order of about 100 mg/l was used for these tests.
Microorganisms settle on the nanofiber layer, even in the first days of colonization. Fiber carriers even without a nanofiber layer are, during biofilm growth, up to two times better than the commercial technology AnoxKaldnes. Significantly higher growth of microorganisms on the carrier show samples with nanolayers, where the bounded biomass is created even more than four times better. After a longer time of colonization, the microbial biomass naturally grows to places without nanofibers. The presence of nanolayers is necessary for fast colonization.
Figure 4 - Time evolution of biofilm growth on carriers (short-term cultivation)
Figure 5 - Pictures for the twentieth day of cultivation (AnoxKaldnes new and used, polyester nanofibers with and without nanolayer)
0 5 10 15 20 25
0 5 10 15 20 25 30
Fullness of carrier's surface by microbial biofilm [%]
Time [days]
AnoxKaldnes_new carrier Polyester with nanofiber AnoxKaldnes_1x used carrier Polyester without nanofiber
4. LONG TERM CULTIVATION OF CARRIERS (COLONIZATION)
A real industrial wastewater containing phenol as the dominant carbon source (hundreds of mg/l) was used for these tests; the accompanying contaminants were cresols (hundreds of mg/l), dimethylphenol (first hundred mg/l) and higher phenols (first unit mg/l); high salinity (20 g/l RAS) was also present
The experimental results show that nanofiber technology has a significantly faster character of biofilm growth on the fibers surface, especially during the first days of colonization (similar characteristics show also after the critical state, where fast revitalization of the system is needed). Two types of nanofiber thread were compared for long-term cultivation; the difference was only in the extent of nanofiber coverage (50 dtex and 100 dtex). The results show that using a higher density of nanofibers (100 dtex) does not significantly support microbial growth, compared with a mean level of coverage (50 dtex).
An important advantage of the nanofiber technology is the possibility of bacterial biofilm growth not only on the surface of fibers, but also closer to the center of the yarn, where the bacteria are much better protected against toxic effects of the environment, whilst still allowing penetration of the substrate and oxygen closer to the microorganisms.
Figure 6 - Time evolution of biofilm growth on carriers (long-term cultivation)
Figure 7 - Pictures for the hundredth day of cultivation (AnoxKaldnes new and used, polyester nanofibers 50 dtex and 100 dtex)
5. Measuring of the density of complex after biomass growth
The pictures below (Figure 8) capture the biofilms growth on the nanofiber carrier. The main idea was that the nanofibers form a biofilm skeleton that holds the biofilm together, but allow the penetration of nutrients and oxygen to the center of the biofilm. The result is a more active biofilm thickness compared to standard technologies; currently this biofilm maintains the high activity of the whole complex (carrier + biofilm) with the high efficiency of biodegradation. Laboratory experiments demonstrated the high stability of the whole complex, even at high concentrations of contaminants and at high flow rates. The population dispersed in water completely disappeared, but the biofilm on the nanofiber structures maintained their stability and high efficiency.
0 5 10 15 20 25 30 35
0 20 40 60 80 100 120
Fullness of carrier's surface by microbial biofilm [%]
Time [days]
Nanofiber carrier (50 dtex) Nanofiber carrier (100 dtex) AnoxKaldnes_new carrier
Figure 8 - Biofilm in laboratory experiment (nanofiber carrier on frames); detail of biofilm in the wet and dry state.
The nanofiber carrier was tested for colonization, where the test took almost one year. Mixed medium (activated sludge) from wastewater treatment plant was used as bacterial populations. The nanofiber carrier has been tested on real industrial wastewater containing Chloramine B.
Weight of wet sample = 1 690.124 g/m2
Weight of dry sample = 1 212.588 g/m2 (weight of one square meter of fabric = 213.5 g) Carrier density = 909 kg/m3 (the whole complex, biomass + carrier)
(Density of water ~ 1000 kg/m³, sea water ~ 1025 kg/m³)
The following photos give details of the experiment and detail of the covered carrier (one-year ongoing colonization of nanofiber carrier).
Further experiments did not show any extreme weight increase of the complex (carrier + biofilm). The complex still maintained a density comparable to wastewater. The complex still moved in suspension during the whole measurement, specifically no sedimentation on the bottom of the reactor was observed. The comparable density of the colonized carrier is important to minimize the slack of the carrier in the supporting frames.
Further experiments did not show any extreme weight increase of the complex (carrier + biofilm), where the test took almost one year. The complex still maintained a density comparable to wastewater.
Figure 9 - An image of the biomass carrier in the measuring cylinder during sedimentation tests, a nearly one year colonized carrier.
6. CONCLUSIONS
The results of the study using nanofiber technology for wastewater treatment are several variants of stable and usable biomass carriers that meet the requirements for a carrier of a bacterial biofilm. Application of nanotechnology in combination with biological methods brings distinct advantages. There are still several contentious issues, such as disintegration of nanofibers and toxicity to higher organisms, which will be further studied. The result shows that the polyurethane material is ideal for the colonization of bacterial populations. Polyurethane is a very mechanically, physically, and chemically stable polymers material. The character of nanofiber technology for use in wastewater treatment is beneficial for both short -term and long- term applications. Even after more than one year of application, the whole complex kept its advantageous characteristics.
ACKNOWLEDGEMENTS
The research was supported by the state subsidy of the Czech Republic within the ALFA project Modified biomass carriers for wastewater treatment - TA01021764. The research reported in this
paper was supported in part by the Project OP VaVpI Centre for Nanomaterials, Advanced Technologies and Innovation CZ.1.05/2.1.00/01.0005. Special thanks go to the Faculty of Textile TUL,
which provided the material for testing.
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