20,000 mg/1

<150 mg/1

200,000 mg/1

1. Filter cake, which is incinerated

2. RO concentrate, which is used as glue

3. RO permeate, which is reused in the factory.

1.1.1. No water or solids are wasted The plant design employs the traditional multistage recirculation design (Figure 1). Although it has been argued that the single-pass design used for water treatment is an alternative; it is, in the author's opinion, not a viable alternative since flux must be stable over time in order for single-pass designs to succeed. The flux in this application is quite variable, which would make the operation of a single-pass system extremely difficult.

Pumps are, as usual, also a challenge. The feed pump(s) is the easiest to specify since it simply supplies a specified flow at a given pressure. Grundfos pumps were chosen and have operated fine. Although the mechanical seals have been problematic, they have actually performed better than expected considering the pump was manufactured to pump cold, clean water only.

Figure 1 Membrane Skid Figure 2 Plant Overview

The recirculation pumps, on the other hand, have been a problem. It is tempting to use borehole pumps, such as the BM type from Grundfos, since they are inexpensive. But in the long run the problems exceed the savings and they eventually were abandoned. Problems were as follows: First, the system operating temperature was on the high side for the pump, although within specifications. Since the liquid it pumps cools the pump, it is likely that the warm water caused the motor to overheat. Second, the seal between the pump motor and the product leaked occasionally, allowing MDF liquid to enter the motor which is obviously undesirable. Lastly, the cables were not sufficiently insulated, resulting in short circuits and motor burnout. More conventional pumps, albeit with quite expensive mechanical seals, have proven to operate better.

An indication of this application's success is the fact that Esmil has installed a total of eight plants in Europe since the first plant was installed in Chirk (see Figure 1 and 2), and the number is still growing.

1.2 UF and RO of SSL

Pulp production by cooking with highly acidic Ca-sulphite is the first and oldest method used industrially. Today, there are very few of these mills because the sulphite process has largely been replaced by alkaline cooking, otherwise called Kraft pulping. Sulphite pulping survives because cellulose made from the sulphite process produces a specific type of cellulose, which is difficult or impossible to produce by other processes.

For every ton of pulp produced, one ton of solids are dissolved. This means that a factory producing 300.000 TPY of pulp also generates 300.000 TPY of waste water containing approximately 55% lignosulphonate (LS), 25% various sugars and 20% ash by products. The sheer tonnage can flood most markets since there are few buyers that can absorb so much product. The problem of selling huge amounts of a byproduct and the difficulty of finding a market for materials such as lignosulphonate has resulted in the limited use of membrane technology. Borregaard (Norway) is the exception because they employ membrane filtration plants world wide and dominate the lignosulphonate market. There are other markets, but they have for non-technical reasons not been exploited yet.

Nevertheless, there are a few RO systems treating Spent Sulphite Liquor that can be reported upon. RO of SSL can potentially replace evaporators, and can certainly enhance the performance of existing evaporators. For many reasons it is the latter which has been most successful, although few systems have been installed.

One particular application concentrates permeate from a UF plant treating SSL. An RO membrane system may either treat SSL or UF permeate, even though the composition of SSL varies considerably. Table 2 shows typical values for SSL and for UF permeate.

The UF plant treating SSL is a tubular system from PCI. The system incorporates 12 mm tubes, and the main advantage of the tubular system is its ability to handle suspended solids, especially fibres, and to do so with a minimum of pre-treatment. It is common to use a 20K MWCO PSO membrane in such an application. The operating temperature is around 70°C, and the majority of the LS are rejected by the UF membrane - only the very low MW LS may pass through the membrane. However, LS is not of great concern since LS is easy to wash away and does not influence flux very much.

Table 2. Typical data for the RO plant
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