Biomedical Engineering Reference
In-Depth Information
sequentially by ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB).
Both AOB and NOB are named as “nitrifiers”. Nitrifiers are slow-growing bacteria and have
yield rates of 0.21 g biomass/g N (Rittmann and McCarty, 2001). In addition, nitrifiers are
sensitive to shock-loading and toxins (Wagner et al., 1995; Okabe et al., 2004). Therefore,
nitrification is usually the limiting process in wastewater treatment. As a result, more
attention is being paid to the improvement of the efficiency and stability of nitrification.
Biofilm systems or bio-augmentation processes, wherein the sludge retention time (SRT) is
separated from the hydraulic retention time (HRT) to maintain low-growth nitrifiers, are
commonly used to improve nitrification (Park et al., 2002; Head and Oleszkiewicz, 2004).
Nitrification can be hindered by heterotrophic activities, because heterotrophs out-
compete nitrifiers for space and nutrients (Hanaki et al., 1990; Nogueira et al., 2002). The
carbon/nitrogen (C/N) ratio is one of the factors that determine the heterotroph/autotroph
population ratio in wastewater treatment systems. In general, a high carbon substrate
concentration can cause the inhibition of nitrification in activated sludge systems and biofilm
systems (Hanaki et al., 1990; Nogueira et al., 2002). Lindemann and Wiesmann (2000) found
that aerobic nitrifiers were out-competed by heterotrophs at a C/N ratio of 1.8. Komorowska-
Kaufman et al. (2006) observed that a chemical oxygen demand/nitrogen (COD/N) ratio
below 4 encouraged nitrification and a value above 4 caused instability of nitrification,
especially at low temperatures. Nogueira et al. (2002) found that, at long HRTs, nitrification
efficiency decreased drastically when acetate was added. However, some studies have shown
that nitrification may be improved with the existence of a heterotrophic protective layer
(Germain et al., 2007). Compared with a biological oxygen demand (BOD) loading rate of
410-450 mg/l·d, at the BOD loading rate of 290 mg/l·d, nitrification activity was reduced,
which was caused by the diminishment of the protective heterotrophic layer for nitrifiers at
low temperatures ranging from 9 to 14°C (Germain et al., 2007). Oyanedel et al. (2005)
observed that, for the suspension biomass, the nitrification capacity was improved with COD
addition compared with no COD addition.
The settlement characteristics of activated sludge flocs are very important for the design
and operation of wastewater treatment processes. Several studies have examined the
settlement characteristics of activated sludge flocs in nitrifying activated sludge systems
(Yoshinori et al., 1999; Martinez et al., 2004; Texier and Gemez, 2004). In these studies, the
sludge volume index (SVI) values were below 22 ml/g, indicating good settleability of
nitrifying activated sludge. In addition, the accelerated nitrification activity could improve the
activated sludge settleability. However, few studies have investigated the effect of carbon
substrate on the settleability of nitrifying activated sludge flocs. In the study of Wu et al.
(2008a), the biomass concentration decreased drastically in a nitrifying reactor without the
addition of organic carbon substrate and there was little increase in the biomass concentration
even without desludging; however, the settlement characteristics of activated sludge were not
investigated and no explanation was given for the decrease in the biomass concentration.
Therefore, in this study, the effects of glucose addition on nitrification and activated
sludge settlement were investigated in a short-term experiment. The activity of nitrification
and the characteristics of activated sludge settlement were examined under conditions with
and without glucose addition. The underlying mechanisms were analyzed.
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