Denitrification of phenolic wastewater by immobilized sludge

Wastewater containing NO3--N (223 mg l(-1)), phenol(100 mg l(-1)) and m-cresol (50 mg l(-1)) was treated at 30 degrees C in an upflow reactor packed with polyvinyl-alcohol (PVA) beads entrapped with acclimate sludge. An additional 500 mg l(-1) of sucrose was added as co-substrate, keeping the COD/NO3--N ratio at 4.3/1. The experiment was conducted by lowering the hydraulic retention time (HRT) stepwise from 12.2 to 0.52 hour. Results showed that nitrate was completely denitrified to less than 0.1 mg l(-1) and all the phenol, m-cresol and sucrose were degraded to less than 1 mg l(-1) even at the HRT of 0.52 hour. At this HRT, the degradation rates were 4.3 g.l(-1).d(-1) for phenol and 2.1 g.l(-1).d(-1) for m-cresol, and the denitrification rate was 10.1 g-NO3--N l(-1) d(-1). Throughout the experiment, there was no noticeable disintegration of PVA beads, and the total solid content in the effluent never exceeded 112 mg l(-1). Each gram of NO3--N reduction required an average of 4.14 grams of GOD, and the average sludge yield was estimated as 0.22 g-VSS.g-COD-1. Operated at a COD/NO3--N ratio of 5.0/1, part of the excess COD was converted to methane; whereas at 3.1/1, the excess NO3--N was converted to N2O. Scanning electron microscopic examination showed that bacteria were mostly populated on the immobilized beads surface due to the availability of nitrate and substrate.