The paper mills have two paper machines, the PM 4, producing tissue from the RCF and the PM 13, producing paper for transformers and cables from the unbleached softwood craft pulp. Excess water from PM4, PM13 and RCF factory, together with the storm water, leads on to the external wastewater treatment. When any of the paper mills has downtime, unequal outcomes are obtained, primarily with respect to pH and COD.
The aim of the project is to identify the individual steps in the processes PM 4, PM 13 and wastewater treatment re to the pH and COD changes.
Intensive sampling was carried out from twelve sampling points during thirty-two days. These samples were analyzed on the same day re to the pH, conductivity, alkalinity, COD, suspended solids, and inorganic residue. In addition, precipitation tests were carried out on the incoming wastewater with controlled doses of the ferrous sulphate, ferric chloride and aluminium sulphate. In addition methodological validation, calcium determination, and experiments with the mixed wastewater samples were carried out.
The results show that the process line PM 4 has generally higher pH, conductivity, alkalinity, COD and concentration inorganic residues, but has a lower content of suspended solids than PM 13. But the biggest difference in the two processes takes place just before and after the paper machine. After the paper machine, the water that goes on to the external wastewater treatment, where a 70% COD reduction and a pH increase of 0,75 pH equivalents for PM 13, and 30% COD reduction and a pH decrease of 1,27 pH equivalents for the PM 4 are obtained. The results suggest that the greatest pH change occurs in wastewater treatment, with 22.06%. From precipitation experiments it was found that the best pH values and lowest COD values where obtained for precipitation chemicals, ferric chloride and ferrous sulphate at the lowest dose levels, i.e. 97 g/m3 and 101 g/m3, without pH adjustment. Results from the correlation between wastewater- and mixed wastewater samples, which excludes impact of storm water, indicates that pH values are 0.01 to 0.86 pH equivalents higher in those mixed wastewater samples. Thus, there is a hint that the storm water contributes to pH reduction of the input wastewater.
To address these concerns ferric chloride should be tested at lower dose levels combined with reduced sulphuric acid addition. Eventually ferric chloride may be replaced by the iron sulphate. In addition, there should be measuring devices for pH and COD installed on the process wastewater and outgoing water, in the external wastewater treatment in order to allow for accurate dosing of chemicals precipitate from these values. Storm water should be separated from wastewater treatment and instead be diverted directly to the municipal sewage system.