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Wastewater recycling and sludge removal in the paper industry account for about 60 % of all process costs. New and environmentally friendly wastewater treatment techniques are, therefore, continually being developed. Cavitation exploitation is currently a well-investigated topic that is also interesting for the paper production industry. This study investigates efficiency of hydrodynamic cavitation alone and in combination with the addition of NaOH for the treatment of secondary pulp and paper mill sludge in order to enhance nutrient release. First, two laboratory-scale devices were tested: the blow-through and the rotating hydrodynamic cavitation generator. The latter set-up proved to be more efficient: therefore, further experiments were performed on its pilot-scale version. The results showed an increase of soluble chemical oxygen demand (CODs) by 514 mg/L, total nitrogen (Nt) by 17.4 mg/L, and total phosphorous (Pt) by 2.3 mg/L. To further increase nutrient release combination of cavitation and sludge sample alkalinization was tested. The addition of NaOH and 30 min cavitation of secondary sludge (500 L) significantly improved CODs and Nt release by 2400 mg/L and 120 mg/L, respectively. Microbiological photos revealed a definite disintegration of sludge flocks. According to our estimates, 1.9 kg of released CODs from alkaline pre-treated and cavitated sludge would cost only one euro.

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Biological treatment, especially nitrogen removal, in an upflow packed bed reactor with immobilised biomass was studied. As a support material granulated activated carbon (GAC), ceramic material (crush brick), brown coal and cut polyethylene tubes were used. The size of the support material was 4 to 6 millimetres. In laboratory scale studies pharmaceutical wastewater was treated. Volumetric loading was from 6.8 to 7.6 g/l/day. Results of investigations showed us that aerated packed bed reactors are usable for nitrogen removal because in the reactor originates aerobic and anoxic conditions. In anoxic cones the denitrification of nitrate, nitrite and ammonia ions takes place. The most effective systems for nitrogen removal were reactors filled with granulated activated carbon and with ceramic material. In the reactor with granulated activated carbon also colour from wastewater was degraded.