Denitrification of phenolic wastewater by immobilized sludge

Abstract

Wastewater containing NO3 .-N (223 mg l-1), phenol (100 mg l-1) and m-cresol (50 mg l-1) was heated at 30°C in an upflow reactor packed with polyvinyl-alcohol (PVA) beads entrapped with acclimated 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.