The impact of effluents of Pitkäranta pulp mill on the water quality of Lake Ladoga: a model study

Abstract

The aim of this study is to show the effects of the Pitkäranta pulp mill on the water quality of Northern Ladoga by using water quality models. The effluent loading of the pulp mill with its full production capacity is as follows:

• Water Flow 85 000 m³ d^-1

• BOD₅ 2.4 t d^-1

• Suspended solids 4.1 t d^-1

• tot-N 330 kg d^-1

• tot-P 68 kg d^-1

• COD(Cr) 14.4 t d^-1

• Org. C 6 t d^-1

• Lignosulphonates 9.4 t d^-1

Loadings of lignosulphonates and organic C are estimations. Lignosulphonate concentrations of 10.5 mg l⁻¹ have been reported in the region of Pitkäranta. The study area of northern Ladoga near the pulp mill was divided into three zones (I, II and III). The mean depth of each of them is considered as 10 m, their respective areas 5, 10 and 50 km², and volumes 50, 100 and 500 million m³. The estimated discharges of the zones are 10, 20 and 50 m³ s⁻¹, respectively. With the aid of simple water quality models the effects of the pulp mill effluents on the concentrations of oxygen, total phosphorus, lignin, COD(Mn) and Secchi disk depths in each of the zones were estimated. Estimations were made during full production capacity and half production capacity of the pulp mill. The modelling results were compared with the preliminary water analysis results of the Finnish-Russian joint research expedition into Lake Ladoga in August 1993. The results show that near the pulp mill (zone I) phosphorus concentrations are high even with half production capacity. Also lignin and COD(Mn) contents have increased, and oxygen concentrations are low both in the summer and during wintertime. Farther away in the open water (zone III) the pollution effects are low. Eutrophication, indicated by high total phosphorus concentrations, is the main effect of the pulp mill effluents.