Water in Pulp & Paper – The Application
The paper industry requires large amounts of water due to the production conditions. The German Waste Water Ordinance implied, that the resulting effluent has to be tested on the content of chemical oxygen demand (COD). In comparison with conventional testing methods the use of online COD analysers offers high savings and also enables an efficient water management.
In many areas of life paper is used, either as graphic paper, as a technical paper or special papers as hygienic paper or packing material.
The diversity of the finished paper and the qualitative differences are caused by the selection, coordination and type of processing of basic materials. By using further ingredients or processing methods, different variations are created. Today, each paper basically contains wood fibres, in the form of wood pulp and / or cellulose, paper, fillers to improve the structure and water. Rags are only used in a few special cases, such as for banknotes.
In the manufacturing process of pulp and paper a lot of water is required. The major part of passes in a re-using cycle. To avoid an increasing concentration and a loss of quality of the ingredients, a small amount of waste water is discharged and replaced by fresh water. The entire process is characterized by a water temperature of about 35 °C and a COD content of 30,000-40,000 mg / L.
Within the manufacturing process, the parameter COD has no significance. Only when the water leaves the process, it becomes waste water and then, it is supplied to a municipal or the plant’s own waste water treatment plant. The treatment plants of large paper mills are designed for 100,000-200,000 PE (population equivalent).
In recent years, the industry was able to reduce the water requirements for the manufacturing process significantly. However, there are 10-11 l waste water / kg product in the course of production on average. A complete avoidance of waste water is not possible even in a closed circuit.
Quality requirements as well as problems such as salting or odor formation makes the drain of waste water necessary. Generally, the waste water is warm, mixed with fibers, chemically highly contaminated and relativly low in quantity.
Due to the high loads an anaerobic biological waste water treatment is recommended. Sewage treatment plants in the paper industry show high intakes of loads by 3,000-6,000 mg / l COD.
In the effluent of a paper mill, the monitoring of COD is very important. It is the only way to identify production losses easily and buffer problematic loads. The waste water treatment plant operators can better estimate the required cleaning performance and control it efficiently. The COD can also be determined by correlation with TOC (total organic carbon).
Additionally, the parameter TNb (total bound nitrogen) is eecommended for monitoring because an optimal cleaning performance needs a certain ratio of nitrogen and phosphorus. Further, the waste water toxicity should be tested. Chemicals such as biocides or chlorine, which are added to avoid mold, can significantly affect the biology of a waste water treatment plant.
Conventional COD test methods are time-consuming and cost-intensive. Hence, they are unsuitable for an efficient water management. Online measuring systems provide a remedy.
Depending on the requirements, LAR offers the best solution as its methods operate analytically correct, fast and clean. Due to the ultra-high temperature of 1,200 °C, the complete oxidation of the sample is guaranteed and the XY injection system prevents clogging and carry-over effects.
Using LAR analysers operators optimize their water management significantly. Firstly, it is possible to control the discharge of waste water into the waste water treatment plants or buffer tanks. Secondly, the labs will be considerably relieved, since the enormous working efforts and expenses for the COD analysis are no longer needed.