Fig. 3 illustrates the percentage values of AOB concentrations with respect to VSS concentrations in biofilm samples from the full-bed reactor.

Fig. 3. Percentage values of AOB in the biofilm samples of the full-bed reactor

The highest percentage of AOB was found in a sample from the middle of the full-bed reactor (0.0829%), followed by the top part (0.0295%), whilst very little was found in the bottom part (0.0216%). A low percentage of AOB was obtained at the bottom despite the fact that the substrate and oxygen sources were supplied from here. This anomaly could best be explained by the fact that competition between heterotrophic and nitrifying bacteria for substrates (oxygen and ammonia) and space in the biofilms resulted in the fast-growing heterotrophic bacteria dominating the bottom part of the reactor. Plate 8 of biofilm sample from the bottom of the full-bed reactor show that AOB clusters are not dense as in Plates 1- 2 of the top and the middle positions.

The trend of AOB growth in the biofilm samples of the full-bed reactor was followed through for the partial-bed reactor (Fig. 4):

Fig. 4. Percentage values of AOB in the biofilm samples of the partial-bed reactor

The same argument of competition for substrates and space between heterotrophic bacteria and nitrifiers explained the lower percentage of AOB obtained in the middle (0.1019%) compared to the top part of the partial-bed reactor (0.2151%).

To validate the hypothesis made on AOB distribution in both the full and partial-bed reactors, a previous work by Wijeyekoon et al (2000) was used to investigate the effect of organic loading rates on nitrification activity. Table 5 summarizes the reactor conditions of their study.

The three reactors, packed with the same weights of anthracite, were equipped with sampling ports at depths of 6 cm (port 1), 18.5 cm (port 2) and 37.5 cm (port 3) from the top end of the filters. The specific rate of NH4+-N oxidation in the reactors was determined by the biomass extracted from those ports. It was discovered that the highest rates in filter A and B were obtained at the effluent ends of the reactors, but in filter C, the rates were comparably high from all ports. Also, among the three reactors, filter C produced the highest rates, with an average of 48.1 and 56.4 g N/ (mg protein. hr) for ports 1 and 2 respectively. The conclusion derived from the study is that at high organic carbon loadings nitrifiers are non-uniformly distributed along the length of a filter, with excessive growth of heterotrophs near the feed end and nitrifiers at the effluent end under the influence of
comparatively higher organic loading. Meanwhile, at low organic loadings, the heterotrophs and autotrophs can coexist. Filter C had the lowest organic carbon loading and consequently had the lowest biomass density. Therefore, the nitrifiers in filter C may have experienced less competitive pressure from the faster-growing heterotrophic organisms for oxygen and space. The displacement of the nitrifying population by the heterotrophs is caused by the varying ratio of carbon and nitrogen entering the reactor.

The carbon loading used in this part of study, 5.71 ±0.16 kg COD/m3.d, was much higher than the loadings used by Wijeyekoon (Table 9.4), and therefore nitrifiers were not only displaced further away from the feed source, but also buried deeper into the biofilm (Ohashi et al 1995). Fdz-Polanco et al (2000) also observed that as the amount of organic carbon entering the filter increases, the nitrification activity is displaced to the upper part of the filter in an upflow process. Quyang et al (2000) also argued that the differences in biological activity at different filter heights were due to their varying loadings.

Rowan et al (personal communication) also investigated the percent value of AOB in a full — scale BAF plant treating municipal wastewater and obtained a value of 0.65%. This value is almost three times higher than the highest percentage obtained in this study (0.2151% from Figure 9.4). The difference in values could be attributed to a number of factors including carbon loading, nitrogen loading, pH, DO, media type and size, direction of flow, backwashing regime and thus mean SRT and biofilm attachment characteristics.

4. Conclusion

The extent of comparable nitrogen removal in the two reactor configurations needs further microbiological evidence, specifically that of the existence of AOB. The formation of a dense biofilm as a result of higher turbulence would account for the higher number of AOB cells enumerated in the biofilm samples from the partial-bed reactor (4.259 x 105 ±1.881 x 105 no of AOB cells/ml sample) as compared to those from the full-bed reactor (1.523 x 105 ±7.979 x 104 no of AOB cells/ml sample). Although biomass was washed out in the treated effluent and during backwash operation, the SRT at the high organic loading of 5.71±0.16 kg COD/m3.d was still maintained at 4.2 days for the partial-bed reactor and 7.6 days for the full-bed reactor. These SRTs were still longer than the limit noted by Sastry et al (1999), who claimed that a mean cell residence time > 3 days is desirable for nitrifiers to reach a stable population for effective nitrification, and Gergeker (2002) who recorded a loss of nitrification below 2.5-2.7 days at an OLR of 5 kg COD/m3.d and a temperature of 25oC.

5. Acknowledgement

This chapter of the book could not have been written without the help of my PhD supervisor Prof Tom Donnelly who not only served as my supervisor but also encouraged and challenged me throughout my academic program. He and the other faculty members, Dr. Davenport and Dr Joana of University of Newcastle upon Tyne guided me through the process, never accepting less than my best efforts. I thank them all. And last but not least the Government of Malaysia for the sponsorship of my study.