The effluents of conventional pulp bleaching represent the main problem of waste water disposal of the pulp industry because of their non-biodegradability. Nowadays the demanded quality of discharges requires new methods exceeding the conventional biological ones.

Experience has shown that for purifying pulp bleaching sewages, only an extra stage treatment-process seems to be successful. In a first stage (e.g. precipitation/flocculation, oxidation with ozone, high-energy radiation) the resistant organic substances must be changed to biodegradable substances. The main elimination should be done in a biological stage, and remaining concentrations could be eliminated by a final treatment,e.g.AC-adsorption.The authors' research works are dealing with all the mentioned stages, and some results of these works are presented in this paper.

With different types of activated sludge the following elimination rates could be achieved: ~ 90 % BOD5, 30 - 40 % COD and 20 - 30 % AOX. Precipitation with lime in addition to flocculation resulted in reduction rates of 40 % COD, 50 % AOX and 70 % of colour. Such a pretreatment and a following biological treatment increased the reduction rates to > 95 % BOD5, 70 - 80 % COD and 70-85 % colour.

The treatment with ozone with and without simultaneous γ-irradiation firstly was carried out with native bleaching effluents at different temperatures and at different pH. These preliminary tests turned out to be unsatisfactory probably because of the high amount of easily biodegradable substances in this sewage, which reduce the effectivity of ozone/radiation. Therefore the research programme was directed to biologically pretreated bleaching effluents.

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