Biomedical Engineering Reference
In-Depth Information
Kaufman et al., 2006). The specific nitrification activity decreased with increasing SRT
(Germain et al., 2007). Oyanedel et al. (2005) found that nitrification activity kept constant
when protein was used as the biomass unit, whereas it changed with the SS or VSS as the
biomass unit. This should be considered in future studies.
In the present study, the settlement of activated sludge was found to affect the microbial
activity. With a good settlement in the G-Reactor, a relatively high SRT was maintained and a
stable nitrification was achieved. However, in the N-Reactor, a relatively high effluent SS (as
a consequence of poor settlement of activated sludge flocs) caused a low SRT in this reactor,
finally a very low biomass was maintained. Therefore, the settlement characteristics of
activated sludge flocs can affect the SRT and further affect the nitrification activity at
conditions with or without the addition of glucose.
4.2. Nitrite Accumulation Analysis
Nitrite accumulation in the nitrification process can reduce 25% of the operational cost
by lowering aeration; in addition, with a reduction of 40% in organic carbon demand of
denitrifiers for denitrifying nitrite rather than nitrate in the following denitrification process
(Hellinga et al., 1998; Blackburne et al., 2008; Dytczak et al., 2008). In our study, nitrite
accumulation was observed in both reactors with and without the addition of glucose. The
possible reasons for this are discussed as follows.
NOB is more sensitive to NH
3
than AOB (Anthonisen et al., 1976). The NH
3
inhibition
concentration for AOB ranges from 7-10 mg/l, whereas it is 0.1-5 mg/l for NOB (Anthonisen
et al., 1976; Abeling and Seyfried, 1992; Turk and Mavinic, 1989; Villaverde et al., 1997).
The inhibition of NH
3
could be also described based on the specific biomass concentration -
mg NH
3
/g biomass. Villaverde et al. (1997) and Fdz-Polanco et al. (1994) found that the
threshold concentration was 0.5 mg NH
3
/g biomass due to the nutrient limitation, while a
concentration above 2.5 mg NH
3
/g biomass caused inhibition of AOB. The NH
3
concentration
above 1.5 mg/g VSS was shown to inhibit the activity of NOB, causing the accumulation of
nitrite (Villaverde et al., 1997). In the present study, the NH
3
concentration ranged from 0 to
4.2 mg/l in the N-Reactor, and from 0 to 1.5 mg/l in the G-Reactor. The biomass based
concentration was 0 to 10.2 mg/g VSS in the N-Reactor and 0 to 1 mg/g VSS in the G-
Reactor. This showed that a higher NH
3
concentration occurred in the N-Reactor than in the
G-Reactor. However, the higher NH
3
concentration occurred during the early aerobic stage,
where a good linear regression of nitrification was obtained. Therefore, the NH
3
was not the
main reason responsible for nitrite accumulation.
pH has a great influence on nitrification by: (i) activation-deactivation of enzymes in
nitrifiers; (ii) affecting alkalinity through changing the forms of inorganic carbon; and (iii)
inhibition by substrate of NH
3
or HNO
2
(Villaverde et al., 1997). Villaverde et al. (1997)
showed that nitrification improved 13% with a pH increase from pH 5.0 to 8.5 in a submerged
biofilm system. According to the discussion by Villaverde et al. (1997), in our study, the
possible reason for nitrite accumulation could be due to the affected enzyme systems in
nitrifiers and/or alkalinity with the pH decreased from above 8 to around 6.0.
