The efficiency of NF versus flocculation for colour removal from wastewater after the dyeing of cellulose fabric

Abstract

The objective of this research was, to study the efficiencies of two purification methods flocculation and membrane nanofiltration (NF), when treating wastewater directly after the dyeing of cellulose fabric following the exhaustion procedure. Synthetically-prepared wastewater, consisting of three mono-functional vinylsulphonic dyes, electrolytes and alkali was subjected to both methods. During the preliminary experiments with four inorganic coagulants and a commercially-available organic flocculant (guanidine; polymer with ammonium chloride and formaldehyde), the best results were achieved by the latter. Therefore, organic flocculant was chosen for further research. The influence of pH was studied, and the optimal amount of flocculant determined. Nanofiltration (NF) was carried-out with a commercial membrane, which was evaluated for membrane fouling tendency and permeate flux. Filtration experiments were conducted in a plate and frame module applying three cross-flow velocities (0.4, 0.6 and 0.8 m/s), ranging from 2-15 bars. The efficiencies of both methods were evaluated for their capabilities of removing colour, salt, and the total amount of organic carbon (TOC). The results showed that, by using commercial flocculant, 90 % removal of the colour was obtained which, by using an additional 0.45 pm fibreglass filter, increased to 99.5 %. During flocculation, no significant removal of salt from the wastewater was obtained and TOC decreased by up to 27 % of its initial value. The highest retention values were achieved at 15 bars using NFT-50 membrane. Total removal of TOC was achieved, together with complete decolourisation, and an average of 30 % salt removal.