Depuration of landfill leachates using fly ash as a catalyst in solar advanced oxidation processes and a compost bioreactor

Fly ash (FA) is a problematic solid waste coming, for instance, from coal combustion. Herein, FA is used in a solar photocatalytic system to deal with landfill leachates (LL), which is another challenging waste. The photocatalytic process is combined with coagulation-flocculation pretreatment and a compost-based biological post-treatment. FA was characterized initially, showing silicon dioxide (52.25%), aluminum oxide (36.99%), calcium oxide (6.92%), iron oxide (2.47%), and titanium dioxide (1.74%), as the main composing oxides. Also, FA had a low surface area (0.48 m(2)/g), and high thermal stability (<2% of weight loss at 1000 C). At the same time, the characterization of LL indicated this is a very polluted liquid, due to its high content of chemical oxygen demand (COD, 8072 mg/L), humic acids (HA, 7320 absorbance at 254 nm), iron (3.1 mg/L), copper (258 mg/L), and a slightly basic pH (8.5). After the coagulation-flocculation pretreatment, the supernatant liquid was submitted to the photocatalytic process (FA/H2O2/sunlight), which removed-70% of COD. For the photocatalytic system, the effects of pH, H2O2 concentration, and FA dose were assessed; evidencing that the pH change from 3.0 to 6.0 affected the COD and HA removals strongly. Under proper operating conditions (i.e., FA: 2.0 g/L, H2O2: 2000 mg/L, and pH: 3.0), the photocatalytic process significantly decreased the sample toxicity (-56%). Thereby, the resultant sample from the FA/H2O2/sunlight system was submitted (for 10 days) to the action of the compost-based biotreatment, comparing the use of non-adapted and adapted aerobic microorganisms. The adapted mi-croorganisms exhibited a higher action, achieving removals of-68% and-9% for the metals (Fe and Cu) and COD, respectively. The results of this research work demonstrated the good suitability of FA-photocatalysis combined with a coagulation-flocculation pre-treatment and a compost-based biological post-treatment to face problematic landfill leachates.