Physico-chemical pre-treatments of anaerobic digestion liquor for aerobic treatment

Abstract

Centrifugation of anaerobically digested sewage sludge gives rise to a solid phase, which could be employed as a fertilizer, and a liquid fraction (ADL), which should be treated before being spilled out. This is not an easy task because this liquor is characterized for presenting high COD (~16000 mg O₂/L), high ammonium content (~4000 mg/L) and low biodegradability (BOD₅/COD ~0.2). With the objective to pre-treat this aqueous waste before its treatment by means of more traditional aerobic processes, different physico-chemical methods (ultrasound, ozonation, hydrolysis and wet air oxidation) were assessed in this work. Ultrasound and thermal hydrolysis gave solubilizations around 47% and 68% respectively. The best results in terms of total COD removal were obtained when wet air oxidation (8 h, 160 C-200 °C and 6.0 MPa) and ozonation (8 h, 25 °C, 12 g/h O₃) techniques were employed achieving COD degradations of 71% and 38%, respectively. The pre-treatment of ADL with the four assayed techniques improved considerably the biodegradability (BOD₅/COD) of the effluent, with values around 0.3-0.4, depending on the treatment. The experimental data were successfully fitted by kinetic models and the kinetic constants for the solubilization and degradation steps were obtained. Application of the proposed models can be of interest for the optimization and selection of the most suitable techniques and operational conditions, in each particular case.

Keywords: Advanced oxidation processes; Anaerobic digestion liquor; Biodegradability; Hydrolysis; Ultrasound; Wastewater pre-treatment.