Combined biological and advance oxidation processes for paper and pulp effluent treatment
Recently, the South African paper and pulp industry has become increasingly interested in the development of suitable wastewater treatment technologies able to assist in the closure of the water network and also to minimize their environmental footprint at their sites. Factors such as the rising cost of fresh water, stricter environmental legislation and socio-political pressure have forced water intensive users to become less dependent on the municipalities. The research described here addresses wastewater problems from two separate mills. Mill X (Case A) is relying on the municipality for fresh water and the treatment of their effluent. The mill wants to become less dependent on the municipality by closing the water network (zero effluent discharge). A wastewater treatment plant (WWTP) would be required to reduce the chemical oxygen demand (COD), total suspended solids (TSS) and colour before any processes water could be reused. Mill Y (Case B) is currently using their effluent for the irrigation of the local plantation. The mill would like a wastewater treatment plant able to reduce the biodegradable material prior to irrigation. Excessive amounts of biodegradable organics in the effluents can cause bacterial and fungal growth in the irrigations systems and consequently clogging problems. More advanced treatment steps would also be required to lower the bio-recalcitrant COD to environmental discharge limits (<400 mg/L). As a result, this study investigated the potential of combining biological and advanced oxidation processes (AOP) for effluent treatment at both mill effluents. An extensive literature study on the treatment of paper and pulp mill effluents was conducted to get a comprehensive understanding of the treatment technologies/combinations. The treatment of paper and pulp mill effluents can be divided into three distinct treatment stages namely: Primary treatment: For the removal of the total suspended solids (TSS) Secondary treatment: For the removal of the biochemical oxygen demand (BOD) Tertiary treatment: Mainly for the removal of bio-recalcitrant chemical oxygen demand (COD) and colour Mill X and Mill Y already contained primary clarifiers to remove the majority of the total suspended solids (TSS). Consequently, the secondary and tertiary treatment steps were evaluated. A detailed technology selection assessment was done to select the best suited secondary and tertiary treatment technologies for the purpose of this project. The work demonstrated that an aerobic MBBR could be used in combination with Fenton related treatment technologies in order to comply with the individual mill specifications. The applicability of both these biological and AOP treatment solutions was therefore extensively investigated. The results indicated that the aerobic moving bed biofilm reactor (MBBR) was able to remove the majority of the biodegradable organics from the recycle and neutral semisulfite chemical pulping mill effluents. The optimal COD removal efficiency ranged between 46% and 57% for the various effluents. The effluent from Mill X was generally found to be more readily biodegraded than the effluents from Mill Y. Experimental results indicated that certain effluents contain organics that display antimicrobial properties. The maximum substrate removal rate decreased linearly with an increase in phenols. As a result, it was therefore assumed that lignin derived alkyl phenols might have inhibited aerobic and anaerobic microbial digestion processes. The results indicate that the MBBR system was not fully acclimatized for high phenolic wastewaters. It is therefore recommended that future experimental studies consider the effects of phenolic content and employ longer acclimatization periods. A significant fraction of the paper and pulp mill effluents were considered to be bio-recalcitrant and required tertiary treatment to be removed. It was found that both the Fenton (Fe3+/H2O2) and Fenton-like (Fe3+/H2O2) oxidation processes can remove bio-recalcitrant organics from biologically treated mill effluents (BTME). However, preliminary experimental results indicated that the Fenton process had a faster oxidation rates. For the Fenton process, the optimal COD removal efficiencies ranged between 40% and 67% for the BTMEs. The experimental results also demonstrated that a combination of Fenton oxidation and slaked lime treatment can effectively remove the colour of BTMEs (97%). The COD removal rates for the neutral sulfite semi-chemical (NSSC) effluents were found to be higher than that of the recycle mill effluent (RME). The aromatic and volatile organic acid (VOA) content of the BTMEs had an important role in the oxidizing processes. The BTMEs with a higher volatile organic acid (VOA) content generally had slower oxidizing rates. The experimental results indicated that the combination of an aerobic MBBR and Fenton process can be implemented at both paper and pulp mills to assist with their individual treatment requirements. An economic study for Case A (Mill X) was also conducted. The data obtained throughout this study was linked to previous water optimization work done at the mill. The economic analysis demonstrated that the aerobic moving bed biofilm reactor (MBBR) and Fenton treatment combination could treat the recycle mill effluent for reuse in a cost-effective manner. The total capital investment cost of the treatment plant was estimated to be R28.5 million and the operational cost was found to be R12.21/m3 of wastewater. The implementation of this treatment solution on the water network could save the mill approximately R 1.25 million/year. The rising cost of fresh water and discharge might increase the economic feasibility of such a WWTP in the near future.
A thesis submitted in fulfilment of the requirements f Master of Science in Engineering to the School of Chemical and Metallurgical Engineering, Faculty of Engineering, University of the Witwatersrand, Johannesburg, 2017
Brink, Antonie (2017) Combine biological and advanced oxidation processes for paper and pulp effluent treatment, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/24952>