Water Research 36 (2002) 2541–2546
Influence of aeration and sludge retention time on ammonium oxidation to nitrite and nitrate Alfieri Pollicea,*, Valter Tandoib, Carmela Lestingia a
CNR, Istituto di Ricerca Sulle Acque, Viale F. de Blasio 5 (Zona Ind.) 70123 Bari, Italy b CNR, Istituto di Ricerca Sulle Acque, Via Reno 1–00198 Roma, Italy Received 3 May 2001; accepted 5 October 2001
Abstract Partial nitrification to nitrite was reported to be technically feasible and economically favourable, especially when wastewater with high ammonium concentrations or low C/N ratios are treated. Nitritation can be obtained by selectively inhibiting nitrite oxidizing microrganisms through appropriate regulation of the system’s pH, temperature, and sludge retention time. In addition to already known methods, the work showed that aeration patterns may play a relevant role too. Nitrification tests were performed in two lab-scale reactors operated under continuous and intermittent aeration, respectively. In both plants, temperature was maintained at 321C and pH was regulated at 7.2 by providing external buffer capacity when needed. The results showed that partial nitrification to nitrite was steadily obtained under oxygen limitation, independent of the sludge age. Therefore, the aeration pattern is proposed as an alternative parameter to the sludge retention time for controlling ammonium oxidation to nitrite. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Nitritation; Partial nitrification; Ammonium oxidation; Autotrophic bacteria; Nitrite
1. Introduction Recent research on nitrogen removal was mostly oriented either towards improvement of efficiencies and energy savings in traditional pathways or towards identification of new processes (and possibly new microrganisms) able to convert ammonium and/or oxidated nitrogen into harmless forms. These issues are especially relevant when high ammonium concentrations need to be removed, e.g. landfill leachates, supernatants from sludge digestion, effluents from anaerobic treatment plants, agro-industrial wastewaters [1–3]. This work deals with biological nitrification, with a special attention to the production of nitrite (NO2 ), which was reported as the ‘‘weakest link’’ in the understanding of nitrogen removal processes, both from *Corresponding author. Tel.: +39-080-582-05-31; fax: +39080-531-33-65. E-mail address:
[email protected] (A. Pollice).
a biochemical and physiological standpoint [4]. Already in the 1970s, denitrification was observed to occur at higher rates when nitrite was the electron acceptor rather than nitrate [5]. Partial oxidation of NH+ 4 to NO2 and subsequent reduction of the latter to molecular nitrogen (N2 gas) was seen as a favourable short-cut, especially for treatment of wastewaters having low C/N ratios [5–8,2]. The main advantages of partial with respect to complete nitrification were reported to be lower oxygen demand (up to 25% energy savings during aeration), reduced organic substrate requirements for heterotrophic denitrification (up to 40%), lower biomass production (up to 300%), and increased denitrification kinetics [6,7]. A disadvantage of partial nitrification is the accumulation of nitrite and its presumed toxic effect on the biomasses even at relatively low concentrations (10– 30 mg N-NO2 L 1, [6,9]). However, inhibition due to nitrite was not consistently confirmed, and it is probably linked to pH, which regulates the equilibrium between
0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 4 6 8 - 7
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nitrite NO2 and nitrous acid HNO2 [9–12]. Furthermore, pH values >7 have the double effect of limiting the conversion of nitrite into nitrous acid (which causes inhibition of ammonium oxidizers) and ensuring concentrations of free ammonia that would selectively inhibit nitrite oxidizers, thus favouring the stability of partial nitrification [12,13]. However, long-term stability of the process was obtained when it was noticed that, at temperatures higher than 301C and pH values >7, nitrite-oxidizing organisms have slower growth rates than ammonium-oxidizers [14,15]. Under these conditions, it is possible to select biomasses capable of partial nitrification to nitrite by operating on the average cell residence time in the aeration tanks, i.e. the sludge age or sludge retention time (SRT). These considerations brought to the recent developement of the SHARON process for partial oxidation of ammonium to nitrite [16,17]. The main features of this process are high operational temperatures (30–351C), and very short sludge age (1–3 d), or also absence of sludge recirculation (SRT=HRT). The present work describes a laboratory-scale investigation on the role of sludge age and oxygen supply on nitritation (the latter parameter is not normally investigated for this purpose). The main objectives were to steadily obtain nitritation in batch tests, to evaluate ammonium oxidation and nitrite formation rates, to investigate partial nitrification with respect to the sludge age, and to compare the effects of intermittent versus continuous aeration.
2. Materials and methods Two jacketed, completely stirred, laboratory scale sequencing batch reactors (SBRs) were set up and operated at 321C constant temperature. Each reactor was equipped with a pH control unit, and pH was maintained above 7.2 by dosing sodium carbonate (Na2CO3 0.5 M). The larger reactor (named reactor L) had 10 L operating volume and was areated continuously in order to provide at least 2.0 mg O2 L 1. The smaller reactor (named reactor S) had 5 L operating volume and was areated 10 min every 20 min. Occasional oxygen measurements in reactor S showed that in-reactor concentrations after interruption of aeration decreased from 2.0 mg O2 L 1 to values below the detection limit of the instrument (0.01 mg O2 L 1) in o5 min, also depending on the oxidation kinetics and considering the low value of the specific oxygenation capacity of slow stirring (OC=0.25 mg O2 L 1 h 1, as measured during preliminary tests after flushing the reactor with nitrogen). Both plants were inoculated with nitrifying activated sludge from a full scale plant treating combined industrial/domestic sewage. The synthetic feed was
composed of ammonium bicarbonate (NH4HCO3), potassium phosphate (KH2PO4) and Nutriflokt (Proviron Ind. NV, [18]). The first two compounds were dosed in order to obtain a molar ratio of about 6:1, while the commercial flocculant was provided in concentrations of about 0.08 gL 1. No organic substrates were added to the biomass, in order to favour the enrichment of autotrophic nitrifying bacteria. Biomass selection and acclimatization was obtained by providing the plants with increasing ammonium concentrations for a period of 4 months. Then, the sludge age was progressively brought to conventional operational values (i.e. below 40 d) by increasing biomass wastage. Excess sludge was removed daily as mixed liquor suspended solids (MLSS) in order to obtain the desired sludge age. The sludge was then separated from the liquid phase by centrifugation (5 min at 1800 rpm), and the latter was returned to the plant. The sludge age in the smaller reactor was calculated by considering that autotrophic growth could occur only when aeration was provided. Theoretical SRT (ratio between the total volume and the volume extracted daily) was then multiplied by the aeration period (i.e. 1/3 for reactor S) in order to obtain the SRT referred to the time of active metabolism. Concentrations of SS in the effluent were observed to be negligible due to the excellent settling properties of the sludge and their contribution did not affect the calculation of the sludge age. The reactors were operated by replacing about 90% of the liquid-phase (after sludge settling) with an equal volume of fresh feed, when at least 80% of the ammonium had been oxidized. Samples from the two reactors were withdrawn daily and analyzed for pH, ammonium, nitrite and nitrate. The consumptions of Na2CO3 were also recorded. Sludge concentration in terms of total and volatile suspended solids (TSS and VSS) was evaluated every 10– 15 d [19]. Ammonium and nitrite were analyzed according to methods 4500-NH3-C and 4500-NO2-B, respectively [19]. Nitrate was analyzed by chromatographic separation (Dionex mod. 4000i) and UV detection (220 nm).
3. Results and discussion 3.1. Reactor L The larger reactor was operated by repeating a number of cycles of fill-and-draw at each different value of sludge age tested. The NH4HCO3 feed concentrations ranged between 400 and 500 mg N L 1 and the sludge retention time was brought to 40 d during the initial runs and was then progressively lowered to reach 10 d.
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Table 1 Operational parameters of reactor L at the different SRTs tested Unit 1
NH+ 4
in the feed TSS VSS Sludge loading Number of tests Avg oxidation rate SD
mg N L g TSS L 1 g VSS L 1 mg N-NH+ 4 mg VSS mg N-NH+ 4 g VSS
1
1
h
1
SRT 40 d
mgN-NH4/L
500
SRT 14 d
400
SRT 10 d
300 200 100 0 0
50
100
150
200
250
SRT 14 d
SRT 10 d
400 1.64 0.40 1.0 4 4.4 1.1
500 0.96 0.31 1.6 4 6.1 1.1
500 0.34 0.11 4.6 4 25.8 12.4
sludge loading (mgN-NH4/mg VSS)
600
SRT 40 d
300
5
50
4
40
3
30
2
20
1
10
0
0 80
20
0
Fig. 2. The decrease of sludge age results in an increase of sludge loading and oxidation rate.
SRT 40 d
500
mgN-NO2/L
40
SRT (d)
time (h) 600
SRT 14 d
400
SRT 10 d
300 200 100 0 0
50
100
150
200
250
300
200
250
300
time (h) 600 SRT 40 d
500
mgN-NO3/L
60
max oxidation rate (mgN-NH4/g VSS*h)
Parameter
SRT 14 d
400
SRT 10 d 300 200 100 0 0
50
100
150
time (h) Fig. 1. Ammonium oxidation, nitrite and nitrate production at three different SRTs under continuous aeration (Reactor L).
of nitrogen under observation in typical tests with different sludge retention times. With a sludge retention time of 40 d, nitrite concentrations close to zero were detected in all the experimental runs and complete nitrification to nitrate was observed. On the contrary, stable conversion of ammonium to nitrite occurred in all the tests performed with a sludge age of 10 d. These results suggest that longer SRTs tend to favour nitrite oxidizers when oxygen is not limiting. Defined SRTs are maintained in an SBR by withdrawing fractions of the mixed liquor without settling the solids. Short sludge ages can only be obtained in such systems by daily removing relevant volumes of mixed liquor (e.g. daily removal of 25% of the reactor volume provides an SRT of 4 d). Decrease of biomass concentration reflected in increase of sludge loading and, if appropriate operational conditions were maintained, increase of the oxidation rate, as shown in Fig. 2. 3.2. Reactor S
Table 1 reports the main operational features of this plant during the tests. This set of experiments confirmed previous findings and showed that it is possible to obtain partial nitrification to nitrite even when oxygen is not limiting, i.e. under continuous aeration, by operating at relatively low sludge ages. The progressive SRT decrease also caused a strong increase of the ammonium oxidation rate. Fig. 1 shows the concentrations of the three forms
The smaller reactor was operated with alternated aeration and ammonium bicarbonate feed concentrations ranging between 350 and 500 mg N L 1. The sludge retention time was reduced to 24 d before beginning the experimental runs, and several sets of experiments were performed at decreasing values of the sludge age. Table 2 reports the main operational features of Reactor S during the tests.
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Table 2 Operational parameters of reactor S at the different SRTs tested Parameter
Unit 1
NH+ 4
in the feed TSS VSS Sludge loading Number of tests Avg oxidation rate SD
mg N L g TSS L 1 g VSS L 1 mg N-NH+ 4 mg VSS mg N-NH+ 4 g VSS
1
1
h
1
600 SRT 24 d
mgN-NH4/L
500
SRT 14 d
400
SRT 5 d
300
SRT 3 d
200 100 0 0
50
100
150
200
150
200
150
200
time (h) 600 SRT 24 d
mgN-NO2/L
500
SRT 14 d
400
SRT 5 d
300
SRT 3 d
200 100 0 0
50
100
time (h)
mgN-NO3/L
600 SRT 24 d
500
SRT 14 d
400
SRT 5 d
300
SRT 3 d
200 100 0 0
50
100
time (h) Fig. 3. Ammonium oxidation, nitrite and nitrate production at four different SRTs under alternated aeration (Reactor S).
These experiments aimed at evaluating the effect of alternated aeration on partial nitrification at different SRTs. Complete ammonium oxidation to nitrite was obtained under oxygen limitation in all the tests performed, independent of the sludge age. The latter parameter only affected the conversion rate, as shown in Fig. 3 where ammonium oxidation, nitrite and nitrate formation are plotted. In all the cases, the nitrate
SRT 24 d
SRT 14 d
SRT 5 d
SRT 3 d
350–450 2.23 0.72 0.5–0.6 7 3.2 0.7
400 2.12 0.48 0.8 6 4.8 0.6
500 1.00 0.26 1.9 4 10.0 1.9
500 0.35 0.10 5.0 4 22.9 9.5
produced at the end of each experiment never exceeded 8% of the total nitrogen. Moreover, under these conditions sludge ages as low as 5 and 3 d were tested. When the SRT was decreased by increasing the wasted sludge, the concentration of biomass only suffered an initial drop and then stabilized. The decrease of sludge age resulted, as expected, in a decrease of VSS, which in turn reflected in higher bacterial activity as indicated by the specific ammonium oxidation rates. Comparison of the specific oxidation rates of the two reactors shows that similar values were obtained when the sludge age of reactor L was similar to the theoretical sludge age of reactor S (calculated over both aerated and non-areated periods). Therefore, the influence of non aerated periods on the activity and physiological state of nitrifiers may deserve further investigation. These results suggest that controlled oxygen supply may be the main parameter for sustaining ammonium oxidizers under the tested conditions, independent of the sludge age. Further experimental investigations are needed to assess the effects of different aeration procedures on ammonium oxidation at a given sludge age. However, the results reported here show the possibility of obtaining stable conversion of ammonium to nitrite by operating on the aeration instead of the SRT. This may result in energy savings in the whole process of nitrogen removal. Moreover, at the operational level, the air supply to the oxidation tanks may be easier to control and may allow more flexibility than the sludge retention time, which should be controlled very carefully in order to reach a steady state. Also in the case of alternated aeration, lower sludge age reflects in decrease of biomass concentration which causes increase of sludge loading and, under appropriate operational conditions, increase of the oxidation rate, as shown in Fig. 4. Alternated aeration as a strategy to limit the oxidation of ammonia to nitrite may also favour the processes of simultaneous ammonia and nitrite removal (such as Anammox or Oland, [20,21]). In the present experience, nitrogen losses observed in all the kinetic tests were in the range 3–7%, and they seemed more likely to be
5
50
4
40
3
30
2
20
1
10
0 100
0 80
60
40
20
max oxidation rate (mgN-NH4/g VSS*h)
sludge loading (mgN-NH4/mg VSS)
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Acknowledgements The authors acknowledge Dr. R. Ramadori (CNR IRSA) and Prof. A. Rozzi (Politecnico di Milano) for inspiring the work and revising the manuscript, Mr. N. Limoni and Mr. G. Evaristo (CNR IRSA) for building and assembling the lab-scale reactors.
0
SRT (d) Fig. 4. The decrease of sludge age results in an increase of sludge loading and oxidation rate.
caused by analytical reasons rather than suggest the above mentioned mechanisms.
4. Conclusions In the present work, the effects of sludge age and aeration on ammonium oxidation to nitrite were experimentally investigated at the laboratory scale. Tests were performed at different sludge retention times both under continuous aeration and under oxygen limitation. The results indicate that, at given temperature and pH, the sludge age is the critical parameter for partial nitrification when the oxygen supply is not limiting. Ammonium oxidation to nitrite was successfully obtained for SRTs around 10 d, although a certain instability was observed which turned into nitrate formation as soon as the sludge age slightly increased. Under limited oxygen supply, i.e. under alternated aeration, complete and stable conversion of ammonium into nitrite was obtained, independent of the sludge age. The latter parameter only showed some influence on the kinetics of ammonium oxidation under oxygen limitation. This behaviour should be confirmed by further experiments aimed at limiting the oxygen supply by regulating the air flowrate, rather than the aeration time. However, the results reported here indicate dissolved oxygen as an alternative parameter for controlling nitrification to nitrite. The possibility of converting ammonium into nitrite by modifying the aeration patterns instead of the SRT is attractive considering potential energy savings and looking at the operational practices. Air supply is probably an easier parameter to control in SBRs with respect to the sludge age, and allows better flexibility to plant operations. Moreover, the adoption of this parameter would decrease the risks of biomass washout, which may be relevant when operating at very short SRTs. Finally, alternated aeration as a strategy to limit the oxidation of ammonia to nitrite may also favour the processes of autotrophic simultaneous ammonia and nitrite removal.
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