Ozonation and wet oxidation in the treatment of thermomechanical pulp (TMP) circulation waters

Abstract

Ozonation and wet oxidation were studied as means of the treatment of thermomechanical pulp (TMP) processing waters. There were two objectives of the experimental research concerning circulation waters: to reduce concentration of lipophilic wood extractives (LWEs) and to treat the concentrated residues from evaporation and membrane filtration by low-pressure catalytic wet oxidation. Ozone was found to selectively react with LWEs. The selectivity of ozonation defined as the ratio of the rate coefficients of the reactions with LWEs and other organic matter was found to be from 3 to 20. The reaction was always found to be of fast or moderate rate and, therefore, no dissolved ozone could be detected in water during ozonation. However, the ozone dose necessary to remove 90% of the LWEs in TMP water was found to be rather high, from 500 to 800 mg O 3 /l. This was determined by high content of organic matter, COD ranged between 3 and 6 kg O 2 /m 3 . The implementation of ozone for LWEs removal is therefore problematic from economical point of view. Catalytic low pressure-wet oxidation of membrane and evaporation concentrates was found to be effective: 50% of COD were removed under 150°C with subsequent increase of the wastewater biodegradability.