CHARACTERIZATION AND REMOVAL OF CHLORINATED ORGANICS FROM PULP BLEACHING EFFLUENTS

Abstract

The growing environmental concern regarding the impact of bleach plant effluents has led to the progressive substitution of chlorine (C12) based conventional pulp bleaching with environment friendly elemental chlorine free (ECF) and totally chlorine free (TCF) technologies for the regulation of chlorinated organic matter, AOX (adsorbable organic halides), in the effluents. The conventional effluent treatment processes are not effective for the degradation of chloroorganics and color removal from paper mill effluents. Hence, treatment with advanced oxidation processes (AOPs), e.g. photocatalysis, is needed which leads to the mineralisation of a wide range of organics without any harmful environmental impact. The mixed hardwood kraft pulp was bleached to 87% ISO target brightness in the laboratory, for generating effluent, using D/CED, DED, DEED, and ODED sequences. The effluents generated were subjected to photocatalysis using UV/TiO2 and UV/TiO2/H202 processes. The paper mill primary clarified (PC) and biotreated (BT) effluents were also subjected to photocatalysis using UV/Ti02, Solar/Ti02, UV/TiO2/H202, and Solar/Ti02/H202 processes. The catalyst characterization, recycling, treatment process efficiency, and economic feasibility studies were also performed. The effect of bleaching on the pulp viscosity and mechanical strength properties was also studied. The bleach chemical consumption followed the order: DED > D/CED = DEED > ODED for attaining the same target brightness (87% ISO). 02 pretreatment saved 45% bleach chemical (active chlorine), while DED sequence required 14% higher as compared to D/CED sequence. ECF bleaching sequences reduced alkali consumption by 30% as compared to D/CED sequence. The pulps obtained by ECF sequences were having slightly better properties as compared to D/CED sequences. 02 pretreatment and H202 application in E stage reduced significantly the effluent pollution load. Six types of chlorophenolic compounds, i.e. chlorophenols, chloroguaiacols, chlorocatechols, chlorovanillin, chlorosyringol, and chlorosyringaldehyde, were identified in the effluents. The generation of chloroorganics is highly reduced by ECF bleaching sequences. The chlorophenolics generation was reduced by about 93, 89, and 83% by ODED, DEED, and DED sequences, respectively, as compared to D/CED sequence. Four cRFA compounds, i.e. chlorodehydroabietic acid, 12,14-dichlorodehydroabietic acid, 9,10-dichlorostearic acid, and 9,10,12,13-tetrachlorostearic acid were detected in the effluents. The generation of cRFA was reduced by about 82, 62, and 47% by ODED, DEED, and DED sequences, respectively, as compared to D/CED sequence. 02 pretreatment, complete C12 substitution by C102, and H202 application in E stage was able to sharply decrease the generation of polychlorinated compounds. DED, DEED, and ODED sequences were able to reduce AOX generation below Central Pollution Control Board (CPCB), India discharge standard (1 kg/t O.D. pulp). The pulp bleaching cost followed the order: DED > DEED > ODED > D/CED sequence. If AOX abatement at the source and end-of-pipe effluent treatment costs are considered, then the order is DED > D/CED > DEED > ODED sequence. Hence, the paper mills may look at ODED and DEED sequences to meet effluent AOX discharge standard. DEED sequence is an alternative for the mills where 02 plant installation is not possible. UV/Ti02/H202 process was able to remove the higher amount of pollutants from pulp bleaching effluents as compared to UV/TiO2 process. ODED sequence effluents were degraded to the highest extent followed by DEED, DED, and D/CED sequences. UV/Ti02/H202 process was found more efficient followed by Solar/Ti02/H202, UV/Ti02, and Solar/TiO2 processes for the remediation of paper mill effluents. Biotreated effluents were found to be more amenable to photocatalytic degradation as compared to primary clarified effluents. The biodegradability of the effluents improved substantially after photocatalysis by all the processes indicating easy removal of the pollutants by biological processes. There was no significant loss in pollutants removal efficiency after reuse of TiO2 for five subsequent effluent treatment cycles. The economical costs and environmental impacts of removing a certain quantity of COD from the effluent are very high when the processes use electric energy (UV/TiO2 and UV/TiO2/H2O2) as compared to those using solar light (Solar/TiO2 and Solar/Ti02/H202). Solar/Ti02/H202 process was found the most economically viable followed by Solar/TiO2 and UV/Ti02/H202 processes. UV/TiO2 process can not be used due to the highest treatment cost. On the basis of the pollutants removal efficiency and economic analysis, Solar/Ti02/H202 process was found suitable for paper mill effluent treatment. ii