Pes-enzyme immobilised membranes for the simultaneous biodegradation and filtration of natural organic matter (NOM)

Abstract

Natural organic matter (NOM) removal is one of the challenges encountered by water treatment utilities in water treatment. NOM is found naturally in all surface waters and consists of compounds with complex structures emanating from decomposing plants and animal remains. NOM presence in water sources is a concern due to its ability to generate carcinogenic disinfection by-products (DBPs) with chlorine-based disinfectants, its ability to enable bacterial re-growth in distribution pipelines and introduction of organoleptic problems such as bad taste, odour and colour in drinking water. In attempt to remove NOM from water, conventional water treatment processes such as coagulation-flocculation have been employed. However, this can be achieved by optimising the coagulant dose and pH which not only increase the treatment costs but also time consuming. Therefore, fungal degradation of NOM into smaller fragments which can be easily removed by advance treatment methods such as membrane filtration could be a viable alternative to remove NOM from water. This work was aimed at developing and applying white rot fungi (WRF) enzyme-modified membranes for NOM degradation and removal in water. The phase inversion method was used to prepare Polyethersulfone (PES) membranes using water as a non-solvent additive, N-methly-2-pyrrolidone (NMP) as a solvent and polyethylene glycol (PEG) as the pore-forming agent. This was carried out by varying the dope solution composition and concentration of the coagulation bath. Scanning electron microscopy (SEM) was used to study the changes in membrane structural arrangement and rejection studies were carried out using humic acid (HA) and bovine serum albumin (BSA) as NOM model compounds. Furthermore, eighteen (18) WRF isolates were collected from decaying wood and screened for their ability to produce lignolytic enzymes. Isolates (D, L and R) which tested positive for laccase were further screened for HA degradation from liquid media. Moreover, production of the desired enzymes in high quantities were carried out using solid-state fermentation (SSF) method and purified using 80 % ammonium sulfate. Thereafter, crude enzyme laccase was extracted and immobilised on the most promising PES membrane as a support material and 4-hydroxybenzoic acid vii was used as a substrate for laccase stability. The changes in morphology of the modified membranes was also used as a confirmation for the successful immobilisation of the enzyme. SEM micrographs revealed that the non-solvent additive impacts the morphology of the membrane pores and ultimately the permeation of water. High water flux was achieved from the membrane (M.5) with 0 wt.% water and 36 wt.% PEG. High flux performing membranes were selected for coagulation bath experiments. The highest water flux achieved for M.5 with 15 wt.% NMP was 568.66 L.m-2.h-1. The most permeable membranes recorded solute rejections as high as 88 % for BSA and 69 % for HA. Only three isolates (D, L, and R) identified as Perenniporia sp, Trichoderma koningiopsis and Polyporaceae sp, respectively, tested positive for laccase production. Colour change after fungal treatment was observed in all three isolates indicating the degradation of HA by the fungi. Ultraviolet visible (UV-vis) at 254 nm and dissolved organic carbon (DOC) measurement analysis revealed the highest percentage removal of (DOC) 52 % and (UV254) 27.3 % for isolate R. After enzyme purification, results indicated laccase activity of 0.297 U/mL for isolate R and 0.152 U/mL for isolate D. In immobilisation studies, the interaction amongst the enzymes, the sulfonate groups and the substrate were confirmed by an intense colour change on the membrane surface. SEM images revealed the presence of spherically shaped structures on the surfaces of enzyme-modified membranes which in turn confirmed the successful immobilisation of the enzyme laccase. Moreover, a reduction in the permeability of the modified membranes served as confirmation for the immobilisation of the enzyme. Rejection studies suggest that the enzyme-modified membranes can be used for HA removal and other similar pollutants from water with a 95 % HA removal using membrane modified with enzyme laccase from isolate R. Therefore, enzyme-modified membranes can be recommended for NOM removal from real water samples. However, the application of the enzyme-modified membranes in a large-scale water treatment is limited by challenges emanating with enzyme extraction, and high operational costs associated with higher pressure conditions.