Closing Pulp and Paper Mill Water Circuits with Membrane Filtration

Abstract

In this study membrane filtration, ultrafiltration, and nanofiltration alone and as part of hybrid processes are considered as means to purify pulp and paper mill process waters suitable for reuse. Thermophilic aerobic biological treatment, pH adjustment, flocculation, and ozonation were tested as pretreatment methods on pilot or on laboratory scale. The aim was to increase flux and reduce fouling by various pretreatment steps and, thus, increase the competitiveness of the membrane process. The results were also evaluated by comparing the benefits obtained against the costs. It was discovered that benefits could be obtained with all the pretreatments tried. Thermophilic aerobic biology assisted in the removal of organic material and increased flux significantly, but the costs were the highest. The most cost-effective processes, however, seem to be pH-adjusted nanofiltration and flocculation nanofiltration hybrid processes, which is understandable because of their significantly lower investment costs compared to, for example, those of biological process. The pH adjustment increased the electrostatic repulsion between negatively charged solutes and membrane, thereby increasing the flux. Flocculation removed the foulants effectively from the feed and it both increased flux and reduced fouling. Yet, many noteworthy benefits were obtained also with ultrafiltration and ozonation. All of the hybrid processes tested could be applied at various points of the water circuit of an integrated pulp and paper mill for purification purposes. The eventual superiority and cost-effectiveness of the applied process remains to be determined case by case.

Membrane treatment of alkaline bleaching effluents from elementary chlorine free kraft softwood cellulose production

“…However, an intrinsic solution to the problem of membrane fouling could be the concept of critical or sustainable flux. [35] Some authors [3] pointed out that operating below the critical flux may not be sufficient to avoid long-term fouling. These authors introduced the concept of sustainable flux, at which the desired separation can be operated in a profitable manner, only minimizing but not eliminating fouling entirely.…”

“…Microfiltration, ultrafiltration (UF) and reverse osmosis (RO) have been tested in the treatment of effluents from recycled paper mills, [1] and integrated pulp and paper plants. [2][3][4] Membrane processes have also been applied *Corresponding author. Email: eonatece@gmail.com to bleaching effluents from traditional chlorine-based bleaching plants and modern elementary chlorine free (ECF) and totally chlorine free mills.…”

Pulp and Paper Chemicals∗∗Some excerpts taken from Bajpai, P., 2004. Emerging Technologies in Sizing. PIRA Technology Report, Smithers PIRA, the worldwide authority on the Packaging, Print and Paper supply chains.

“…Circular sedimentation tanks are common. Sometimes the use of semipermeable membranes, especially microfilters, are found to be helpful in separating very fine suspended particles (Nuortila-Jokinen et al, 2003). Removal efficiency varies according to the type of paper produced but removal rates in excess of 80% are usually obtained.…”

“…Most paper mills do not add coagulant to their effluent as the solids have good settling characteristics. Efficiency, measured using colloidal and dissolved substances can partially be removed from wastewater by membrane filtration, known as micro-, nano-, and ultrafiltration (Nuortila-Jokinen et al, 2003). The difference in performance is because of the nature of the solids discharged from the different types of mill.…”

Efficient water use in industries: Cases from the Indian agro-based pulp and paper mills

“…This, in turn, results in increased fouling and corrosion, and has a negative impact on the final product quality. In this context, it is necessary to treat the recirculation streams appropriately (Nuortila-Jokinen et al, 2003).…”