The recycled paper group of KNP-BT operates in total 12 paper and board mills in Western Europe. All together the output of recycled product is 1.5 million tpy, of which approx. 50% is testliner and corrugated medium. The company has been a pioneer in process water closure for more than 20 years and several of its mills were already operating totally effluent free, long before the term “zero effluent” was invented.
Some advantages of this way of operation are energy conservation and no charges for effluent discharge. Disadvantages are amongst others, accumulation of organic compounds, increased salt content, water hardness and volatile fatty acid build up.
One of the fluting and testliner mills in the group is Zülpich Papier, which is currently going through an extension phase from 170.000 tpy to 340.000 tpy. This particular mill has been operated under complete effluent free circumstances almost 20 years. However, even though the new machine is to be put into operation in October 1996, a biological process water regeneration plant has been put on stream already in September 1995 in order to collect experience with the internal treatment and to have everything running smoothly by the time the new machine is started.
The in line treatment consists of an anaerobic and an aerobic stage. During the first stage the dissolved organic components like carbohydrates and volatile fatty acids are converted into methane gas. At the same time, sulphate reducing bacteria convert sulphate into hydrogen sulphide, which is stripped out by the biogas. During the subsequent aerobic stage, residual organic components are oxidized into carbonate, which precipitates with calcium ions into the aerobic sludge and accumulates as calcium carbonate. These are a few examples of what is being achieved. Due to several favourable reactions in the biological stage, the process water quality has improved drastically for the following criteria:
Odour, corrosion properties, salt content (conductivity), hardness, viscosity, COD-level and pH. Also components like stickies, waxes and colloidal material are presumably being degraded during the biological regeneration process.
In the paper a description is given of the plant, as well as an overview of the main improvements of the process water quality during the first four months of in line biological treatment.