The RO system concentrates the UF permeate from 7% TS to 14% solids prior to evaporation. The TS in the feed can vary significantly, and the high operating temperature of 70°C is a challenge for the membrane elements. However, the use of Desal™ Duratherm® Excel membrane elements, and the specification of a 3,8" OD, has secured good mechanical stability for the elements. It should be noted that 3,8" is a commonly used diameter in the dairy industry, but it is certainly unusual in this kind of industry. This element was chosen for its excellent mechanical strength: it is probably the sturdiest element on the market. However, it is likely that a new plant built today would use 8" elements since the designs, and mechanical strength, have improved considerably over the last few years.

The plant is built as a two-stage recirculation plant. The plant employs 400 m2 of membrane surface area, and the water removal capacity is a nominal 10,000 L/h that was in accordance to requested capacity. It turned out that the UF plant operated at considerably higher capacity than expected, which means the RO system was too small. In order to treat all the UF permeate as fast as it is produced, a significant enlargement is needed.

For many months the system performed as expected and did not show any tendency to lose flux or foul. Lately, however, the RO plant has experienced a peculiar problem of low capacity. Since UF permeate is the feed, the pre-treatment is almost perfect, and no harmful chemicals from the SSL should be reaching the RO membranes. If a UF membrane failed, raw SSL could penetrate, but it is rare to incur a failure so massive that it seriously affects the membranes.

The present problem can probably be traced to the water supply since the plant uses river water that contains humic acid. Humic acid is colloidal in nature, and there may be clusters housing microorganisms, which secrete fatty or sticky products as part of their metabolism. When such water is heated, such as for steam production, the fatty part is separated from the humic acid, and the result is water, which contains a material behaving much like mineral oil. Some of these undefined oily substances have been observed, and it has had a rather devastating effect on polyamide membranes. Since pH during cleaning is limited to 11,5, it is difficult or impossible to remove the fouling layer. We are still investigating how or if the membranes can be regenerated, and how to prevent such problems in the future.

1.3 UF and NF of White Water in the Paper Industry

White water is a term used to describe the wastewater produced during the formation of paper in the wet end of a paper machine. The water is rather white due to the high content of suspended solids.

Paper machines have grown to almost incomprehensible size and the following are a few figures that are literally breathtaking. New machines form paper measuring up to 10 meters in width. The speed of the paper is approaching 2000 meters per minute, which is equivalent to 120 km/h, or 75 mph. The largest machines produce 40 TPH paper. Now take into account that the pulp slurry contains less than 2% cellulose fibres, meaning that the volume of white water is around 1800 m3/h. The price for a machine is well over 200 million US dollars.

These numbers reveal two important aspects concerning this application. First, the paper industry's wastewater problem is serious and very large. Second, an operator of such a machine will not install any equipment that endangers the operation of production: downtime is extremely expensive.

In order to achieve the proper paper quality, it is necessary to add several chemicals to the pulp slurry, and some of those in quantity. For instance, it is common to add CaC03

as filler to obtain the glossy paper used in magazines. The pulp slurry is poured onto a fast moving mesh where the formation of paper takes place. One of the major challenges in making paper is keeping the paperforming portion of the machine in good working order. Water is drained first by gravity, and then by vacuum, leaving the pulp. After a few seconds of dewatering, the paper is lifted off and continues into the paper machine. The wire is then subjected to intense cleaning by nozzles, and is then ready again for paper formation.

Cellulose fragments some of the filler and some of the chemicals can become wedged in the wire mesh and must be removed. Otherwise, the wire will lose its ability to drain water from the paper, and machine capacity will be limited. The quantity of water needed for spraying is staggering, and it is a challenge to clean the white water by conventional methods to such an extent that it can be used in the nozzles. For years, UF has been used to do just this, and has performed well. However, low flux and low availability, combined with too little paper machine knowledge, has prevented the widespread adoption of UF.

In order to demonstrate that UF is a viable, practical process for the filtration of white water, a full-scale plant must be built since a small-scale pilot will prove nothing. Valmet Flootek (former ABB Flootek and Raisio Flootek) has, for more than 5 years, been operating full-scale UF systems on white water. It has been proven conclusively that UF meets all the criteria for use in the harsh environment. It should be noted that Valmet is one of the world's major producers of paper machines. Therefore, they can combine their paper know-how with the membrane know-how of Flootek to effectively sell a complete and proven system to the end user. This combination of knowledge is unique and necessary for the success of this application.

The filter used by Flootek is called the CR filter. It is a plate and frame type, with approximately 84 m2 per filter. The CR filter gains sufficient turbulence by mechanical means, and does not rely on pumps. The filter employs something similar to pump impellers between each plate. The advantages include:

• The volume to be treated and the linear velocity over the membrane are independent variables

• The investment is well under US $2 per m3 water treated

• The volumetric concentration ratio can be varied, and it can be very large without affecting the operation

• The module is not sensitive to suspended solids (within reason)

• High, stable flux over extended periods of time is possible.

• The disadvantages include complex module construction.

The volumetric concentration ratio is approximately 25 meaning that 100 litres of feed are divided into 96 litres of permeate and 4 litres of concentrate. It is known that the permeate recovery can be increased to >99%, however, the extra volume of water recovered is quite small. It is most probably not worth while to attempt higher levels of recovery since the solids content in the concentrate increases enough that it impairs the function of the CR filter.

The author has stated in the "Membrane Filtration Handbook" that flux is never >100 Lmh for any length of time. The CR filter has broken that rule by maintaining flux of 200 - 300 Lmh for weeks on end. This is, of course, not valid for all products, but is for most. A major reason for this occurrence is the high shear rate on the membrane surface.

There are several full-scale systems in operation. It has been proven conclusively that the UF permeate is very well suited for nozzle water and for reuse in other parts of the paper process. The CR filter has achieved an important goal in papermaking: reduction of water consumption.

1.3.1 NF of white water UF permeate The more water that is recycled in a pulp or paper mill, the more one has to watch out for salt build-up. One major step forward in the pulp industry is replacement of Cl2 bleaching by oxygen bleaching because this replacement removed a major source of chloride. Chloride is feared in all pulp and paper operations because chloride can so easily cause corrosion, that at best results in poor paper quality and at worst, can stop a complete system.

Many newer mills have recycled their water to the point that they need a method to remove salt from the system. Nanofiltration and Reverse Osmosis can achieve this objective. For several years, Flootek has been piloting NF of UF permeate and the results have always been good. However, the first priority was to gain acceptance of UF. Now that that has taken place, the next step is to gain acceptance of NF and/or RO.

As this paper is being written in 1999, a full-scale system is being commissioned. The data presented below reflects the results from the pilot test. Data from the full-scale, online system will be available during the conference in March of 2000. Tables 3 and 4 show data obtained from the full-scale UF systems and the pilot-scale NF system.

Table 3 Typical values. Kirkniemi. Papermachine I and 2
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