The objective of this paper was to examine the feasibility of partial nitrification from raw domestic wastewater at ambient temperature by aeration control only. Airflow rate was selected as the sole operational parameter. A 14L sequencing batch reactor was operated at 23°C for 8 months, with an input of domestic wastewater. There was a programmed decrease of the airflow rate to 28L·h−1, the corresponding average dissolved oxygen (DO) was 0.32mg·L−1, and the average nitrite accumulation rate increased to 92.4% in 3 weeks. Subsequently, further increase in the airflow rate to 48L·h−1 did not destroy the partial nitrification to nitrite, with average DO of 0.60mg·L−1 and nitrite accumulating rate of 95.6%. The results showed that limited airflow rate to cause oxygen deficiency in the reactor would eventually induce only nitrification to nitrite and not further to nitrate and that this system showed relatively stability at higher airflow rate independent of pH and temperature. About 50% of influent total nitrogen was eliminated coupling with partial nitrification, taking the advantage of low DO during the reaction.
Currently many municipal wastewater treatment
plants (WWTPs) in China encounter the problem of
ammonia overload, especially those that mainly cater
to treatment of domestic wastewater. At the same time,
many small WWTPs are being set up to treat the domestic
wastewater decentralized in situ from residential
areas. As a result of low C/N ratio in domestic
wastewater, there is a deficiency of carbon sources,
which are essential for the nitrification-denitrification
process. As a result, improving the efficiency of processes
currently in use or exploiting new process to
convert nitrogen into harmless forms has been the
target of recent researches on nitrogen removal.
Partial nitrification to nitrite, whch is the oxidation
of ammonium to nitrite as the end-product of nitrification
(ix., nitritation), has been reported to be
technologically feasible and economically viable, especially
when wastewaters have high concentrations
of ammonium and/or low levels of organic carbon for
the process of denitrification. The main advantages of
partial nitrification when compared with complete
nitrification are lower oxygen demand during aeration,
less requirements of organic substrates for sequent
heterotrophic denitrification, and less biomass production[
13. Through appropriate regulation of the factors
such as free ammonia concentration (FA), pH, temperature,
sludge retention time (SRT), and dissolved
oxygen (DO) concentration, nitritation can be enhanced
by providing a favourable environment for
growth of ammonium oxidizing bacteria (AOB),
thereby conferring on them growth advantages, and
selectively inhibiting the growth of nitrite oxidizing
bacteria (NOB)[2]. It has been reported that FA at concentrations
of 1-5mg.L- inhibited nitrification but
not nitritation[3]. Cecen and Gonenc[4] stated that the
combined effects of high ammonia and high pH led to
nitrite accumulation by Nitrobacter inhibition. Hellinga
et aZ.[5] described the SHARON process at 35%
without sludge retention for sludge liquor treatment.
At elevated temperatures and relative high ammonia
concentration (more than 500mg NH,+ -N.L-'), nitrite
oxidation was permanently prevented and denitrification
with nitrite could begin. Ruiz et a1.[6] examined
the effects of pH and DO on partial nitrification and
achieved a nitrite accumulation rate of 65% from
simulated industrial wastewater containing 6 10 mg
NH,+ -N.L-' at DO around O.7mg.L-', whereas pH
was not a useful operational parameter to indicate nitrite
buildup. Wang and Yang[7] found that the optimal
operational parameters to realize partial nitrification
were as follows: pH, 7.5; DO, 1.5mg.L-'; and
temperature, 30"C, based on ammonia oxidation and
nitrite accumulation rate.
However, many researchers have focused on the
combined use of the above-cited methods. Some of
the conditions under which nitrite accumulation can
occur are as follows: ammonium-enriched feed inputs,
high temperatures (often above 30°C), and high pH
values (over 7.5). However, in practise, the temperature
is not susceptible to be modified and controlled in
full-scale reactors, mainly due to economic considerations.
Increasing the pH will result in increased operation
costs and complexities. To date, little attention
has been paid to the exact effect of DO alone on parr
tial nitrification, especially when the conditions in the
A practical method to achieve partial nitrification
to nitrite from raw domestic wastewater under conditions
where oxygen was limited was developed in this
study, and it was a pH-independent process under ambient
temperature. Airflow rate was the only parameterthat needed to be controlled. It was argued that there
existed a boundary value of DO concentration that
was necessary to realize nitritation. Sufficient operation
cycles were needed to accomplish full inhibition
of nitrification to nitrate. The fact that it is of relatively
rigorous condition for NOB to revive in the
system made it possible to operate this nitrification
process with nitrite accumulation under higher DO
level, as compared with that of inducing the inhibitory
conditions. Using data that were detected online for
DO, pH, and OW, the end of nitritation could be
identified through the distinctive points. Simultaneous
nitrogen removal was acheved under low-DO operation.
This strategy to achieve partial nitrification may
lead to considerable savings of aeration cost in the
nitrification phase; furthermore, less carbon dosage is
needed in the subsequent denitrification phase for nitrification
to nitrate is prevented. Ths study further
promotes the study and application of the partial nitrification/
denitrification process for domestic wastewater
treatment. On the basis of the above results, a pilot-
scale SBR with working volume of 50 cubic metres
has been set up in the Beixiao River municipal
WWTP in Beijing, to conduct further experiments.
