Membrane filtration recovers coating materials, process water from diluted streams

New technology concentrates recyclable coating solids up to 60%, at the same time creating a clear, high quality permeate for reuse or environmentally safe discharge.

During the paper coating process, expensive coating materials are generally lost during wash ups or cleaning of the coater, as well as in the preparation of coating formulations in the coating kitchen. Typically, diluted coating streams from the coating operation are discharged to a mill's effluent treatment plant, increasing solids load into waste treatment and losing the value of coating materials at the same time.

Filtration of diluted coating streams, however, allows a paper mill to recover concentrated coating material for recycling while reducing overall solids discharge. The treatment of diluted coating streams in most cases provides a significant cost reduction to a coated paper mill's operation.

Technological advances in membrane filtration systems, in particular, have created opportunities for coated paper mills to treat diluted coating streams for materials recovery and reduced environmental impact. For example, the Vibratory Shear Enhanced Processing (VSEP) technology developed by New Logic Research, Inc. (Emeryville, CA, USA) can very effectively filter the diluted coating process, generating a permeate stream of high quality water that can meet all discharge or reuse criteria. The resultant clear permeate can be discharged or recycled to the process. In addition, VSEP concentrates solids (coating materials) to a level suitable for recycle to the coating process. Nanofiltration or reverse osmosis filtration can also be used if smaller dissolved solids or color is an issue.

VSEP technology
In the paper mill coating process, as well as other pulp and paper processes, VSEP membrane systems can now be used in applications that have historically caused substantial fouling problems with traditional cross-flow membrane technologies. The VSEP is an attractive alternative to conventional filtration methods due to its vibrational, shear-enhanced design.

In situations where residual coating has been washed from coaters, the diluted stream is typically 1.5% to 2% (by weight) total solids (TS). VSEP can boost the TS content of such streams up to 60%, dramatically improving overall process efficiency with a highly concentrated flow of recovered coating materials and a permeate that can be reused in the process or discharged.

A second stage VSEP unit, also using an ultrafiltration membrane module with similar membranes, processes the concentrate from the first stage to recover coating material and eliminate solids and color agents from the permeate stream. The produced concentrated stream, at a flow rate of 1.5 gal/min and a TS concentration of 60%, is recycled to the coating preparation system.

VSEPs in the first stage typically generate a permeate stream of about 46 gal/min, which is recycled to the process or discharged to the sewer. The permeate stream from the second stage is about 2.5 gal/min and is sent to the sewer. The feed concentration is 1.5% to 2.0% TS. In the first stage, permeate concentration is reduced to ~1 mg/l of total suspended solids (TSS), and a low level of total dissolved solids (TDS).

Paper coatings formulations include many ingredients. Each has a very specific purpose and function. The exact mix depends on the grade or type of paper that is to be run. The most frequently used coatings include color coatings, ground and precipitated calcium carbonate, kaolin clay, and titanium dioxide.

Membrane selection is based on material compatibility, flux rates (capacity), and concentration requirements. In the example discussed above, he TSS reduction is more than 99% while the coatings are concentrated from 1.5%-2% to 60% by weight. Permeate quality from the VSEP can be controlled through laboratory selection of membrane materials available to fit the application parameters.

Using an ultrafiltration module in the VSEP system is a commercially viable option for treatment of paper mill diluted coating process streams. Nearly 95% to 97% of the water is recovered as treated water suitable for reuse or discharge, while the remaining 3% to 5% is recycled as high solids concentrate, thereby supplementing the coating material requirement by a significant margin.

Pilot trials
New Logic has conducted several successful pilot tests using VSEP technology with diluted coating streams. Depending on process temperatures, membrane selection, and the requirement for solids concentration or BOD/COD removal for effluent streams, the permeate flux rate in the VSEP can range from 15 to more than 70 gal/day/ft² (GFD).

In these trials and subsequent commercial applications, the concentration level from the second stage VSEP unit is controlled by an automatic timed control valve. This valve is set such that the concentration of the solids is held at the desired level. A multi-stage or a progressive cavity feed pump supplies the VSEP units at a pressure suitable for the membrane used.

A variable frequency electronic drive is used to set feed pressure through a PID (proportional-integral-derivative) control loop. This kind of drive acts to control the rotational speed of the pump, thus controlling the flow rate.

Economic Value
The VSEP system provides an alternative approach for processing diluted coating applications. VSEP will provide a concentrated coating stream and also reduce BOD, COD, TSS, TDS, and color to provide a high quality permeate stream for discharge or reuse in the process. In many applications, the addition of VSEP will eliminate conventional treatment process requirements and technologies.

Justification for the use of VSEP treatment system in a mill is determined through analysis of the system cost and benefits, including:

• Recovery of coating material in a concentrated stream suitable for recycle to the process
• Reduction of solids from the discharge stream and associated treatment costs
• Reduction of BOD, COD, TSS, TDS, and color for the effluent stream
• Provision of high quality water for reintroduction into the process (to offset fresh water demands and pre-treatment cost)
• Retained heat in recycled process water as a possible method to reduce energy requirements
• Simplified coating material recovery and effluent treatment with a compact, low energy system.

New Logic has successfully supplied VSEP separation technology into many industrial processes. The coating process, as well the pulp and paper industry's efforts to reduce costs and meet environmental regulations, can be dramatically enhanced through the use of membrane filtration combined with VSEP technology.

This development in coating process applications, together with the availability of new membrane materials and VSEP technology, make to treatment of more difficult streams possible, with very highly successful, economic results.

For more information and/or an economic analysis and VSEP justification study for specific mill applications, contact New Logic Research, Inc., 1295 Sixty Seventh Street, Emeryville, CA 94608. Tel: 510-655-7305. Fax: 510-655-7307. E-mail: info@vsep.com.