Biomedical Engineering Reference
In-Depth Information
The ratio of COD/N and the applied loading rate should be both considered when
discussing the effect of carbon substrate on nitrification. For example, even at high COD
concentrations, if there still has adequate time for nitrification to process after carbon
oxidation, nitrification will not be much affected. Therefore, in order to achieve stable
nitrification, an adequate nitrification duration should be maintained and also an enough
loading rate should be adopted to maintain the growth of nitrifiers (Rothman, 1998; Altinbas,
2001; Gieseke et al., 2002).
With glucose addition, a higher biomass concentration (including heterotrophs and
protozoa concentrations) was obtained compared with the condition without the addition of
glucose. Nitrification capacity was found to increase with increasing protozoa concentrations,
which could be due to the following protozoa activities: (1) supplying nutrients through
excretion; and (2) predating heterotrophs (Petropoulos and Gilbride, 2005). Therefore, by
ensuring a high biomass concentration and an adequate nitrification duration, nitrification
could be improved with the addition of organic carbon substrates.
4.1.2. Effect of Microbial Distribution
The distributions (suspended or attached) of heterotrophs and autotrophs can be affected
by the C/N ratio. At conditions without organic carbon addition, heterotrophs and nitrifiers
co-existed in the outermost biofilm layers and heterotrophs dominated in the innermost
biofilm layers; while, at the C/N ratio of 1.5, heterotrophs dominated in the outermost biofilm
layers and nitrifiers were only present in the innermost biofilm layers (Okabe et al., 1996). In
addition, other environmental conditions like HRT can also affect the distribution of
microbial communities. When HRT is about an order of magnitude larger than the reciprocal
of the net maximum growth rate, the majority of the active biomass will grow suspended in
the mixed liquid rather than in the biofilm or granules (Van Benthum et al., 1997; Villaverde
et al., 2000). Nitrifiers existing at the top layer of biofilm or flocs can increase the nutrient
supply and enhance the nitrification activity.
The distribution of nitrifiers (within the same flocs or biofilms) can contribute to the
different nitrification rates at conditions with and without the addition of glucose. AOB were
mainly in a form of clusters in the activated sludge flocs or biofilm systems without the
addition of carbon substrate, but were well dispersed throughout the flocs or biofilms with the
addition of carbon substrate (Aoi et al., 2000; Wu et al., 2008a). The dispersed distribution of
AOB can improve the nutrient availability for these microbial communities within activated
sludge flocs, where nutrient diffusion may be limited. As a result, nitrification activity can be
improved by dispersed distribution of AOB with the addition of organic carbon substrate.
4.1.3. Effect of SRT
Several other conditions that contribute to the different nitrification activities may be due
to the different SRTs caused by different environmental conditions, such as temperature and
settlement characteristics of activated sludge flocs.
Temperature affects the microbial activity, doubling with increasing temperatures in
10°C increments. Temperatures ranging from 14-27°C had no significant influence on the
nitrification activity (Zhu and Chen, 2002; Germain et al., 2007). With a temperature above
15°C, nitrification performed efficiently when the SRT was above 10 days (Komorowska-
