Basic mathematical modelling for polymer woven fabric performance suitable for low energy filtration systems

Abstract

Water is one of the most important and essential resources that people usually misuse and take for granted until it is either gone or unsuitable to be utilized for domestic, industrial or agricultural purposes. The need to explore affordable purification technologies is essential. The filtration processes are innovative technologies that can be employed in water treatment systems or water purification technologies. However, the filtration technologies have one prime limitation factor of which is fouling and biofilm formed on the membrane surface sometimes internal.

Recent advancements in polymer science and textiles have led to developing fabric material that can be used as membranes suitable for emerging economies. For years’ people do use fabric to purify river water especially women from rural areas. Yet non-woven materials are used as a membrane by industries as compared to woven fabrics. However, most non-woven fabrics are easily damaged when cleaned with a polymer brush and require periodical replacement. The tapeline and filter manufacture use a woven fabric as a backer before casting or putting a filter on the weave fabric. These prove the fact that any woven fabric can be modified for optimal use.

On the other hand, most Engineers and scientists have not given much attention to woven fabrics as a result, woven fabrics are not employed as membranes. Some scientists and engineers believe that woven fabrics are not suitable for treating water for domestic use. Some believe that some woven fabrics can be used as membranes provided they are capable to remove unwanted materials like bacteria and pathogen.

The aim of this study is to create a full understanding of the factors that affect the fabrics when used as membranes, especially when the polymer woven fabrics are used as filters to treat water and wastewater. It is essentially important to develop standardized procedures or models that accurately describe the textile woven fabrics behaviour when used as filters. The standardized models or procedures will assist engineers and scientists when developing filtration systems using woven fabrics.

The first objective was to evaluate and compare the fabric types that can be used as filters or membranes in water and wastewater treatment processes. The second objective was to identify the applications for woven fabric membranes and evaluate the factors that play a critical role during the filtration process and relationship between those factors. The experimental investigations conducted were to evaluate the (1) main objectives; (2) effect of membrane orientation; (3) effect of feed quality on membrane performance; (4) effect on stable flux quality and quantity of the selected fabrics; (5) effect of fabric type on filtration or microfiltration processes; (6) effect of membrane fouling on membrane performance; (7) develop the basic model suitable in identifying the right fabric for any filtration system operating at low energy.

The experimental investigations conducted were to evaluate the selected woven fabrics that were manufactured in South Africa, easy to clean with a polymer brush. Those woven fabrics were tested using South African river water and wastewater from treatment plants. When evaluating different feed solutions, bio-fouling was considered to be the major limiting factor of woven fabrics, but the feed with a lot of bio impurities can be modified for optimization processes.

Laboratory apparatus and field apparatus was developed to analyze and evaluate the effect and behaviour of fabrics performance, and cake formed on the fabrics. The result clearly states that a solution or wastewater with a lot of biological organisms produce lower flux and also produces a lower critical/stable flux when compared with the solution with more incompressible solids or impurities. The result clearly shows that all selected fabrics can be used as filters however; the polyester fabric was the only fabric that can be used for microfiltration processes suitable to clean water for domestic use. This polyester fabric removes 99.995% of impurities from the polluted waters. The Permeate water quality coming from this polyester fabric was less than 1NTU, before and after stable flux. Other fabrics can be used as filters but not for microfiltration. These three fabrics are not capable of removing micro-impurities (less than 20 micrometres). The basic mathematical modelling Equation developed, proved that the membrane pore size, driving force, impurities size in polluted water, impurities nature and impurities concentration play major roles in the filtration process especially in stable flux formation. The simple Equation

F = Ae−Bt + C was discovered to be suitable to evaluate the fabric performance, where C is the constant flux value, A is the maximum flux value and B is the part of the critical area or rate change. The Equation can be applied to most fabrics that are used as filters.

Testing the maximum flux value was critical and achievable when using pure and clean water especially the distilled water. The results show that most solutions with high compressible impurities will take less time to reach a critical or stable flux. The solution or effluent or river water with more bio impurities and more bacteria will have less flux when compared with a solution with more incompressible impurities.

Most polymer woven fabrics do not require any sophisticated technologies or additional chemicals to clean. It can be easily brushed with a polymer brush. Brushing the surface of the fabric with balanced tensile strengths in both warp and weft yarns will not rearrange, damage, or affect the pore size. Only sharp objects can damage the polymer fabrics.

The knowledge of this report will assist in optimising the filtration system operation at low energy when using woven polymer fabrics as membranes for filtration. The basic mathematical model can be useful to engineers and scientists willing to use woven fabrics as membranes. Hence, mathematical modelling is one of the important tools of engineering optimization and design.

This study focuses on the low energy (gravity-driven) systems that treat water and wastewater like Household Point of Use (POU) systems. Other POU systems were tested and compared to POU systems that are made of the Polymer woven fabric. Based on results, it can be concluded that POU's that uses polyester membranes (PWF-POU) are good prospects for area without sophisticated water or wastewater treatment systems since it removes almost all bacteria and impurities. Polyester woven fabrics can be used as a microfiltration membrane not only to process water or wastewater but also to process chemicals, oils, etc. The other selected fabrics that were made of polypropylene filaments need to be modification in order to operate at optimum when cleaning water for domestic and tertiary use. When modifying these polypropylene fabrics, the quality do improved.