RESUMO
A Brazilian thermo-mechanical pulp mill (TMP) was evaluating the installation of a proposed bleaching process, with changes in the qualitative and quantitative characteristics of the wastewaters and the Effluent Treatment Plant (ETP). The objectives of this research were to evaluate the treatment plant configuration for the future industrial effluent, consisting of a flotation unit followed by an upflow anaerobic sludge blanket reactor (UASB), an activated sludge process and nanofiltration (NF) using polymeric membranes, and to study the technical feasibility of recycling the treated effluents in the industrial process. The possible options for recycling the treated effluent were determined through a water balance of the mill. The pulp quality was evaluated in laboratory bleaching assays, based on brightness and brightness reversion tests after the recycling of 50%, 75% and 100% of the treated effluent. The buildup of the non-process elements (NPE) in the industrial water cycle after each effluent recycling proportion was evaluated through computer simulation, using the Aspen Plus® simulator software. The future mill effluent, considering the implementation of a proposed bleaching stage with hydrogen peroxide, was generated in the laboratory and treated in a bench-scale effluent plant, simulating the future configuration. The treatment plant removed 99.8%, 99.2% and 61.6% of soluble COD, BOD5 and color, respectively. The water consumption was highest in the bleaching plant and, therefore, the recycling of 50%, 75% and 100% of the treated effluent for washing the pulp was simulated. The brightness and brightness reversion of the pulp, with 100% of the treated effluent used in the bleaching process, were similar to those provided by fresh water. The recycling of 100% of the treated effluent in the proposed treatment plant was possible in the TMP pulp mill without decreasing the pulp quality.
RESUMO
The electrostatic precipitators ash leachate (EPAL) from recovery boilers of kraft pulp mills is generated in processes dedicated to the removal of potassium and chloride salts seeking to avoid fouling and corrosion in pipes and equipment. However, the EPAL is rich in salts and has high electrical conductivity. Whenever it is sent to the effluent treatment plant (ETP), it can cause negative impacts to the biological process. It may jeopardize particularly the microbial community and the sludge bio-flocculation, causing sludge losses in the secondary clarifiers, reducing the overall efficiency of the effluent treatment plant. The objective of this research was to evaluate the influence of EPAL from the recovery boilers added to a typical effluent from a bleached eucalyptus kraft pulp mill in gradually increased doses and in sudden doses (shock loads) in the efficiency of the biological treatment. Three biological sequential batch reactors (R1, R2, R3) with a volume of 1000â¯ml each were tested. The Reactor R1 was operated as a reference and fed only with typical effluent from the pulp mill; Reactor R2 was fed with mill effluent with gradually increased doses of EPAL in fixed proportions until reaching a real industrial condition; Reactor R3 was fed with mill effluent and with sudden doses of EPAL. The reactors were operated in similar conditions, i.e., the concentration of dissolved oxygen (DO) was always kept higher than 2â¯mgâ¯L-1, COD:N:P ratio equal to 250:5:1 and hydraulic retention time of 12â¯h. Depending on the dose which was applied, the EPAL inhibited microbiological activity, reduced the efficiency of COD removal of the biological treatment (10% when EPAL was gradually applied and 15% in sudden doses) and increased the sludge carryover in the sedimentation phase (148% when EPAL was gradually applied and 170% in sudden dosages). Furthermore, the reduced efficiency and suspended solid carryover were more significant when sudden doses were applied compared to the application of gradual doses of the EPAL.