Anaerobic treatment of Phthalates : microbiological and technological aspects

Phthalic acid isomers (dicarboxy benzenes) play an important role in our human environment as constituents of polyester fibres, films, polyethylene terephthalate (PET) bottles and other plastics. Due to the use and generation of water during phthalic acid production from the corresponding xylenes, a concentrated wastewater is generated. The generated wastewater consists of a mixture of phthalic acid isomers, acetic acid, benzoic acid, and toluic acids. The aim of the work described in this thesis was to elucidate if anaerobic biological treatment may represent an attractive alternative for conventionally applied aerobic treatment methods. With regard to the anaerobic biodegradability of the phthalate isomers it was demonstrated that all three phthalate isomers could be degraded by two types of methanogenic granular sludge and digested sewage sludge. Lag-phases prior to degradation ranged from 17 to 156 days. More reduced aromatic analogues of the phthalates were not degraded or only at extremely low rates. Kinetic properties of the anaerobic degradation of the phthalate isomers were studied using enrichment cultures obtained from the biodegradability experiments or bioreactor biomass. The phthalate isomers grown cultures were capable of degrading only one of the phthalate isomers and degraded benzoate without a lag-period. A three species kinetic model enabled the dynamic description of intermediate acetate and molecular hydrogen accumulation and final formation of methane from the phthalate isomers and benzoate. It was shown that the syntrophic biomass cultivated had a low growth rate on the phthalate isomers (μ max≈ 0.09 day -1 ). The energetic efficiency for growth on the phthalate isomers was found to be significantly smaller when compared to growth on benzoate, suggesting that an energetic inefficiency prevails in the degradation pathway of the phthalate isomers. The cultures were furthermore strongly inhibited and even deactivated by co-incubation with acetate or benzoate, or a short period of a few hours without substrate. Despite these unfavourable microbiological characteristics, it was demonstrated that highly active terephthalate degrading biomass could be cultivated at high concentrations in both UASB (Upflow Anaerobic Sludge Bed) reactors and hybrid reactors, resulting in high terephthalate removal capacities (15-20 g­COD(Chemical Oxygen Demand) · l ­1· day ­1 ). High-rate terephthalate degradation in the UASB-reactors was strictly dependent on inoculation of the reactor with granular biomass. After demonstrating that terephthalate as sole substrate could be degraded at high-rates, we studied the feasibility of a two-stage reactor concept for the treatment of terephthalic acid production wastewater, consisting of a mixture of readily degradable substrates (acetate and benzoate) and slowly degradable substrates (terephthalate and para -toluate). It was demonstrated that through pre-removal of acetate and benzoate in the first stage the lag-phase prior terephthalate degradation in the second stage could be significantly reduced (from 300 to approximately 50 days) and the wastewater could be treated at high volumetric removal rates and short hydraulic retention times (25 g­COD · l ­1· day ­1 and 6 hours respectively). For start-up of a two-stage anaerobic bioreactor system for treatment of terephthalic acid production wastewater, a gradual transition from initial operation in parallel to operation in series is suggested.