Merits of ozonation and catalytic ozonation pre-treatment in the algal treatment of pulp and paper mill effluents.

Since the performance of algal treatment for pulp mill effluent decreases with increasing color intensity and AOX content, which mainly originate from the chlorine bleaching of Kraft pulp, the separated CEH bleaching effluent was pre-treated by both the conventional and the heterogeneous catalytic ozonation processes. An increase in the BOD(5)/COD ratio from 0.11 to 0.28 and 87% color abatement in terms of Pt-Co were achieved by catalytic ozonation, which had the best treatment performance. Biodegradability enhancement of the CEH effluent correlated well with a decrease in toxicity, high-molecular-weight-compound content, and AOX abatement. By the pre-treatment of the CEH bleaching effluent, the overall efficiencies of algal treatment of the combined pulp mill effluent in terms of the fractional removal of COD and color were increased from 76% and 53% to 86-90% and 96-99%, respectively. Effects of both the conventional and the catalytic ozonation pre-treatments on subsequent biological treatment were close to each other and they reduced the filling period of the Sequential Batch Reactor (SBR) cycle from 8 to 5 days.

Algal treatment of pulp and paper industry wastewaters in SBR systems.

Highly colored and highly polluted pulp and paper industry wastewaters are proposed to be treated by using algae in sequential batch reactors (SBR). Results of batch studies revealed that up to 74% COD; 74% color removal could be attained in about 40 days of incubation. From the preliminary SBR experiments, filling period was found to be a critical step affecting the overall efficiency when mixing and aeration is applied during filling. Therefore, 5 different filling periods (4, 6, 8, 10 and 12 days) were studied with a total SBR cycle of 15 days. For all filling periods; COD, color and AOX removal efficiencies increased with increasing filling time. Maximum removal efficiencies achieved were 60 to 85% for COD, 42 to 75% for color and 82 to 93% for AOX for the filling periods of 4 to 12 days. For 8 days or longer filling periods, no additional reaction time was required. Results showed that, organics in the wastewater were both chlorinated and non-chlorinated; algae removed these mainly by metabolism; and chlorine cleavage from chlorinated organic molecules was more rapid than the degradation of non-chlorinated and colored organics. Adsorbed lignin on algal biomass was found to be varying between 10-20% depending on filling period applied.

Improvement of primary settling performance with activated sludge.

In biological treatment plants employing activated sludge processes, it is possible to recirculate some portion of the waste activated sludge that is not sent to the aeration basin, to the inlet of the primary sedimentation tanks. But in the literature there is no detailed information about the conditions, ratios and the characteristics of the waste sludge that can be recirculated back. However, depending on its settling characteristics, the addition of waste activated sludge to raw wastewater may improve primary settling. Settling tests have shown that the effect of waste activated sludge on primary settling is strongly dependent on the mean cell residence time (or sludge age), theta(c), of the waste activated sludge and also on the suspended solids concentration. Different sludge ages of 4, 6, 8, 10, 14, 20 and 26 days, and for each sludge age at least five different initial suspended solids concentrations were studied. A sludge age of 8-10 days achieved the optimum efficiency in terms of the remaining suspended solids concentration as well as percent-suspended solids removal. Also, the settled sludge volumes were measured throughout the experiments; so, the comparison was made between settled sludge volumes, initial suspended solids (SS) concentrations and theta(c).