Financial support: FONDECYT Grants 1010644 and 1040987.

Keywords: anaerobic biodegradation, DTPA, kraft mill effluent.

Kraft mills are responsible for the massive discharge of highly polluted effluents, and new bleaching processes (i.e. Total Chlorine Free (TCF)) is presented as a feasible option to reduce this environmental impact. However, increased TCF pulp production is accompanied by an increase in chelate use. The most commonly used chelates, ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DPTA), are considered to be relatively persistent substances in water treatment plants, and consequently environmentally critical compounds. The purpose of this work is to investigate DPTA behavior in an anaerobic system. An Anaerobic Filter (AF) was operated with three different DPTA load rates and the operating strategy was to maintain the anaerobic system stable during the entire operation. The AF's maximum Chemical Oxygen Demand (COD) removal was 59%, whereas the Biological Oxygen Demand (BOD₅) was around 95%. However, only 5% of DPTA removal was observed under anaerobic conditions during the first operating period.

The TCF bleaching process employs hydrogen peroxide (H₂O₂) instead of chlorine compounds to remove residual lignin. However, the heavy metals contained in wood (Kaluza et al. 1998) and water catalyze the production of OH^- radicals, which then readily react with pulp carbohydrates, resulting in yield losses and a decrease in pulp strength (Rodriguez et al. 1999).

Chelating agents are used in pulp and paper processing to control the action of different metal ions. Although TCF pulp production eliminates the use of chlorine, EDTA (ethylenediaminetetracetic acid) and DPTA (diethylenetriaminepentaacetic acid) concentrations will increase in TCF pulp production wastewaters. EDTA and DTPA concentrations in pulp mill effluent treatment outfalls have found between 10 and 60 mg/L (Suss and Nimmerfroh, 1993). Chelating agents are often considered as a compound that cannot be biologically treated (Alder et al. 1990; Saunamäki, 1995); nevertheless, recent studies have shown that EDTA has been degraded at slightly alkaline pH values with activated sludge treatment (Van Ginkel et al. 1997;Van Ginkel et al. 1999).

Influent

The TCF synthetic influent was prepared to according to Rodríguez et al. (1999).

Anaerobic filter

An AF (240 ml) was continuously operated for 200 days. The reactor was spiked with 5 gVSS/L of anaerobic flocculent sludge, and placed in a thermostatic chamber at a constant temperature of 37ºC. The sludge's maximum specific methanogenic activity was 0.9 gCOD/gVSS·d. The flow rate, pH, Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD₅), DPTA, Total Alkalinity (TA) and alkalinity ratio were measured throughout the operation.

Analytical methods

All the analytical methodswere measured using Standard Methods (APHA, 1985). Whereas DPTA was measured following Bhattacharya and Kundu (1971), adapting the methodology to EDTA.

The biodegradation feasibility of Fe-DPTA-containing TCF bleaching effluent was studied using an AF reactor. To prevent methanogenic inhibition during reactor start-up and to promote gradual biomass acclimatization, the Organic Load Rate(OLR) was gradually increased from 0.17 to 1.4 gCOD/L·d, whereas the Fe-DPTA Load Rate (LR_DPTA),increased from 0.07 to 0.28 gDTPA/L·d and Hydraulic Retention Time (HRT) from 34.6 to 6.1 hr after 200 days.

During operation, the maximum BOD₅ removal was above 95%, whereas COD was below to 59%. BOD₅ removal remained stable during the entire operation. However, COD efficiency gradually increased from 40% to 59% as the anaerobic bacteria adapted. On the other hand, DPTA was accumulated in the reactor due to DPTA adsorption/desorption on the biomass. However, during the first operating period (1 – 64 days), only 5% of Fe-DPTA was removed, and this was probably due to adsorption. The recalcitrant COD fraction can be explained by the non-mineralized DPTA fraction under anaerobic conditions because EDTA and DPTA biodegradability depends on metal complex stability. If Ca and Mg ions exist in stoichiometric excess, the composition of the metal complexes will slowly be displaced in favor of the more degradable Ca and Mg complexes (Henneken et al. 1996). However, other evidence indicates that pure Fe-EDTA and Fe-DPTA complex is biodegradable under aerobic conditions (Lauff et al. 1990), although there is no evidence on the anaerobic biodegradation of these kraft mill effluent compounds. However, combined technologies, including advanced oxidation processes and biological treatment (Rodriguez et al. 1999), may remove Fe-DPTA compounds. Moreover, Virtapohja and Alén (1999) found that the DPTA's photochemical conversion is faster than that of EDTA. Theoretical half-lives of 8.0 and 11.3 min were found for the DPTA and EDTA iron (III) complexes, respectively.

Continuous TCF effluent biodegradation at different organic load rates (0.17 – 1.4 g COD/L×d) indicates incomplete COD biodegradation (up to 59%) due to DPTA's recalcitrant behavior. Still, BOD₅ biodegradation was above 95%. The recalcitrant biodegradation of Fe-DPTA is related with complex stabilization and also with chemical stoichiometry. Advanced treatment could be the best process to degrade this compound.

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