Abstract
Dynamic behavior of Nitrosomonas cells immobilized in PVA gel pellets was investigated by fluorescent in situ hybridization (FISH) of oligonucleotide probes. Two laboratory-scale reactors were seeded with PVA-immmobilized Nitrosomonas cells, and then operated for 160 days by feeding with either a synthetic wastewater containing ammonium solely or a synthetic wastewater composed of ammonium and organic source (acetate). The nitrogen loading on the first reactor (organic-free) was varied from 0.2 up to 0.6kgN·m-3·d-1 by stepwise reducing hydraulic retention time, while the second reactor was operated by varying the C/N ratio of the feed from 0 to 0.8.
The first reactor (organic-free) achieved 99% of ammonia removal at 300mg-N·l-1·d-1, and ammonia-oxidation activity per unit pellet volume remained unchanged above 4.5 mg-N·ml-1·d-1 throughout 160 days operation. On the other hand, a severe deterioration in ammmonia oxidation was observed in the second reactor (organic-fed) when the feed C/N ratio was increased to 0.8. For the both reactors, the presence ratio of Nitrososnonas cells to DAPI-stained total cells decreased to below 2% within the initial 90 days operation, regardless of the pressence of organic source in the feed.
Nitrosomonas cells were present only up to 50μm-depth of the pellets in the form of large cell clusters. The cluster-forming growth of Nitrosomonas tended to break the lattice-structure of PVA gel material, causing the disappearance of these bacteria from the pellets.
