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Ozonation reduces sludge production and improves denitrification
ARTICLE IN PRESS WAT E R R E S E A R C H
41 (2007) 543 – 550
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Ozonation reduces sludge production and improves denitrification Magdalena A. Dytczaka, Kathleen L. Londryb, Hansruedi Siegristc, Jan A. Oleszkiewicza, a
Department of Civil Engineering, University of Manitoba, Winnipeg, Canada MB R3T 5V6 Department of Microbiology, University of Manitoba, Winnipeg, Canada MB R3T 2N2 c EAWAG, CH-8600 Du¨bendorf, Switzerland b
art i cle info
A B S T R A C T
Article history:
The effectiveness of partial ozonation of return activated sludge for enhancing denitrifica-
Received 12 March 2006
tion and waste sludge minimization were examined. A pair of nitrifying sequencing batch
Received in revised form
reactors was operated in either aerobic or alternating anoxic/aerobic conditions, with one
20 October 2006
control and one ozonated reactor in each set. The amount of solids produced decreased
Accepted 7 November 2006
with the ozone dose. Biomass in the anoxic/aerobic reactor was easier to destroy (up to 25%
Available online 26 December 2006
of the initial excess sludge) than in the aerobic (10%) one, generating approximately twice
Keywords:
as much soluble COD by cell lysis. Denitrification rate improved up to 60% due to additional
Sludge minimization
carbon released by ozonation. Nitrification rates deteriorated much more in the aerobic
Wastewater
than in the alternating reactor, possibly as a result of direct destruction of nitrifying
Ozone
autotrophs as well as competition created by growth of heterotrophs receiving the
Denitrification
additional COD. Overall, ozonation provided the expected benefits in denitrification and
Nitrification
had less impact on nitrification in the alternating reactors. & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Activated sludge treatment produces excess biomass as waste activated sludge (WAS) that is difficult and expensive to dispose of, particularly in plants adapted for biological nutrient removal (BNR), as return sludge liquors require costly treatment. Technologies for reducing WAS biomass production are gaining attention (Low and Chase, 1999; Liu and Tay, 2001; Wei et al., 2003). One promising technology is ozonation of return activated sludge (RAS). Ozone is a strong chemical oxidant that destroys the cell walls of microorganisms, which leads to the release of cytoplasm, as well as oxidation of many recalcitrant organic compounds; for that reason it is often employed for disinfection and post-treatment (Beltran, 2003). Ozonation has recently been adopted as a pretreatment before anaerobic sludge digestion (Muller et al., 1998; Weemaes et al., 2000; Goel et al., 2003)—mainly to hydrolyze Corresponding author. Tel.: +204 474 8722; fax: +204 474 7513.
E-mail address: [email protected] (J.A. Oleszkiewicz). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.11.009
solids and increase biogas production. To reduce excess biomass production, ozonation has been introduced to the RAS line in activated sludge systems (Yasui and Shibata, 1994; Sakai et al., 1997). The process consists of sludge (biomass) ozonation stage followed by a biodegradation stage. A fraction of recycled sludge passes through the ozonation unit, where both solubilization to biodegradable organics (due to disintegration of suspended solids) and to a lesser extent mineralization to CO2 and H2O (due to oxidation of soluble organic matter), occur (Kamiya and Hirotsui, 1998). The treated sludge is then recycled to the aeration tank (Yasui and Shibata, 1994; Boehler and Siegrist, 2004). Full-scale studies suggest that ozonation could even lead to a process (of carbonaceous BOD removal) with no excess sludge production (Yasui et al., 1996). The disintegration of the sludge biomass, reflected in a decrease of both total suspended solids (TSS) and volatile suspended solids (VSS) in the ozonated sludge and the
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receiving reactors (Weemaes et al., 2000) is one of the key objectives of sludge ozonation. A decrease in the ratio of VSS/ TSS of treated sludge has been observed, and accumulation of inorganic solids in the aeration tank was considered negligible (Sakai et al., 1997) and may even be reduced by ozonation (Deleris et al., 2002). However, sludge ozonation has been reported to cause a slight increase of total organic carbon (TOC) and TSS in the final effluent in some studies (Sakai et al., 1997). More importantly, ozonation could potentially impact biological rates of carbon, nitrogen, or phosphate removal when treated RAS is introduced back into the process. This could be due to the destruction or inhibition of the specialized microbial populations. One could anticipate that reduction of the ammonia utilization rate (AUR) after ozonation should be proportional to the sludge reduction. Some researchers have shown, however, that the resulting AUR remained constant—i.e. there was no inhibitory effect of ozonation on nitrification (Sakai et al., 1997). It was suggested that nitrifiers, overgrown by the faster growing heterotrophs, may be protected inside the sludge floc and not be exposed to ozone as much as the surface-hugging heterotrophs (Boehler and Siegrist, 2004). The destruction of biomass by ozonation can improve the overall process by indirectly impacting the solids residence time (SRT). Reduced WAS production leads to an apparently higher SRT (defined as the mass of solids in the aeration tank divided by solids removed) if the mixed liquor suspended solids concentration (MLSS) remains constant (Boehler and Siegrist, 2004). Loss of nitrifiers due to ozonation seems to be partly compensated by this increased apparent SRT. Furthermore, there are potential benefits of ozonation for biological processes. In a denitrifying plant, the recycle of the ozonated RAS to the anoxic zone improves denitrification as the solubilized degradable organic compounds become an additional carbon source (Ahn et al., 2002; Park et al., 2004). The efficiency of ozonation as a method of excess sludge reduction, and its impacts on nitrification and denitrification,
41 (2007) 543– 550
are highly dependent on process operating parameters. Therefore, research is needed for the optimization of proper ozone dosage and dosing method (Liu and Tay, 2001). The objective of this research was to examine the influence of ozonation on sludge minimization and process performance under strictly aerobic and alternating anoxic/aerobic conditions. Although a number of earlier studies employed sequencing batch reactors (SBR) with strictly aerobic and alternating anoxic/aerobic conditions (Boehler and Siegrist, 2004; Deleris et al., 2002) this is the first study aiming to compare both processes and provide a unique perspective on the effects of ozonation on releasing biodegradable carbon. Potential benefits and limitations of partial RAS ozonation are discussed.
2.
Material and methods
2.1.
Sequencing batch reactors
Four SBRs were established (Table 1) with a total liquid volume of 3 L each, operated at 2071 1C. The SRT was 12 d (real SRT including solids in the effluent was calculated as 11.270.4 d), and a hydraulic retention time (HRT) of 36 h. The reactors had a sequence of fill, react, settle, and decant and were operated as two pairs of aerobic and alternating anoxic/ aerobic conditions, as per Table 1. The ratio between the anoxic period and total time of the cycle in the alternating reactors was 30%. Dissolved oxygen (DO) was controlled at 5 mg O2/L during aerobic conditions with the residual DO at 0.1 mg/L during the anoxic periods of the alternating reactors. The alkalinity remaining in the effluent was high enough to ensure unhindered nitrification (above 120 mg CaCO3/L). In each pair, one reactor served as a control and the other went through the ozone treatment. All reactors were acclimated for more than 5 SRT periods before testing commenced. Wasting of approximately 250 mL from the reactors
Table 1 – Operating conditions of aerobic and alternating anoxic/aerobic reactors Operation
Time
Air
Mixing
Aerobic reactors Feeding (2 L) Aerobic reaction Wasting (0.25 L) Settling Decanting (1.75 L) Ozonation of RAS (0.2 L)
5 min 22 h+10–25 min 10 min 1 h 05 min 10 min 5–20 min
— d — — — Batch
d d d — —
Alternating anoxic/aerobic reactors Feeding (2 L) Anoxic reaction Aerobic reaction Anoxic reaction Aerobic reaction Wasting (0.25 L) Settling Decanting (1.75 L) Ozonation of RAS (0.2 L)
5 min 3 h+10–25 min 7h 3 h 30 min 8 h 30 min 10 min 1 h 05 min 10 min 5–20 min
— — d — d — — — Batch
d d d d d d — —
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took place at the end of the reaction period. After settling of biomass and decanting 1.75 L of supernatant (final effluent), 1 L of sludge was left in the reactors, which was the equivalent of RAS. For the ozonated reactors, 20% of the sludge was withdrawn, treated by ozone, and then returned to the activated sludge reactor just before feeding. Ozone was produced from air (compressed air lab line) by an ozone generator (OZO 1 VTT Ozomax Ltd.). Ozone was applied to the RAS in a gas-washing bottle by bubbling the ozone gas (flow 10.5 mg/min) directly into the sludge for different lengths of time to achieve greater ozone doses (maximum retention time in the ozone contactor 16 min). The ozone dose consumed by sludge was analyzed each time by calculating the inlet gas and the off-gas trapped by potassium iodide solution (Standard Method 2350E). During the experiments, doses of 7–33 mg O3 consumed per reactor and day were applied, corresponding to doses of 0.006–0.030 mg O3/mg TSS of ozonated sludge and 0.015–0.080 mg O3/mg TSS of initial excess sludge. Doses were applied from smallest to largest, for a period of 1 SRT for each target dose, increasing in increments of 0.005 mg O3/mg TSS.
2.2.
Operating conditions
At the beginning of each daily cycle (following ozonation of RAS as applicable), the reactors were fed a synthetic wastewater consisting of beef and yeast extract as a carbon source (resultant total TCOD 740 mg/L) and ammonium chloride as ammonia source (resultant NH3 40 mg NHþ 4 -N/L). The alternating anoxic/aerobic reactors also received 10 mL of 40 g/L NaNO3 solution at the beginning of the anoxic reaction phase to create a sufficient supply of NO 3 for denitrification. TSS and VSS were measured in RAS from ozonated reactors directly before and after ozonation as well as in RAS from control reactors. Additionally, TSS was measured daily in samples from all reactors before wasting. In the ozonated reactors, the wasting was adjusted daily to maintain TSS at 1800 mg/L at the beginning of the reaction period, similar to the controls, resulting in a higher apparent SRT in ozonated reactors (SRT up to 14.970.7 d).
2.3.
Chemical analysis
Soluble COD was measured in quintuplicate in the portion of ozonated RAS directly before and after ozonation as well as in RAS from control reactors. Soluble and total COD were measured in the influent and effluent daily. The AUR, nitrite+nitrate utilization rate (NUR) and nitrite+nitrate production rate (NPR) were calculated based on changes in measured ammonium or nitrite+nitrate concentration as described previously (Lee and Oleszkiewicz, 2003). VSS, TSS, NHþ 4 , NO3 + NO2 and COD were analyzed in accordance with Standard Methods (APHA, 1998). NO 3 and NO2 were measured separately by ion chromatography using a Dionex IC system with an electrochemical detector, an ASII column, an eluent of NaOH at 2 mL/min, and quantification by comparison of peak areas to external standers curves. Floc structure was examined by contrast phase microscopy.
41 (20 07) 54 3 – 550
3.
Results and discussion
3.1.
Excess sludge reduction
545
The RAS that was ozonated experienced a large decrease in biomass that was proportional to ozone dose, for the highest doses up to 55 and 112 mg for aerobic and alternating reactor, respectively. Ozonation decreased the pH from 7.5970.07 to 7.2870.09 in the ozonated portion of RAS from aerobic reactor and from 7.8870.11 to 7.6370.08 for the alternating reactor. However, after returning the ozonated batch to the main reactor together with feed, a difference in pH between control and ozonated reactor in the pair was insignificant. Ozonation of 20% of the RAS also consistently reduced the amount of excess solids in the reactors receiving the ozonated RAS compared to the controls. This reduction was proportional to the ozone dose over the range tested for both reactors (R2 ¼ 0:5) and much greater in the alternating anoxic/ aerobic reactor than the aerobic one (Fig. 1). This was probably related to the different structures of the flocs in the alternating and aerobic reactors. Biomass in the aerobic reactors consisted of abundant protozoa, with a backbone of filamentous bacteria in dense, spherical, compact flocs, and appeared to be more resistant to higher doses of ozone. The biomass in the alternating reactors, consisting mostly of bacteria with fewer filaments, inhabiting weak, thin and elongated flocs (chain-type structure), seemed more susceptible to destruction. For a moderate dose of 0.05 mg O3/mg TSS of initial excess sludge (24 mg consumed O3/reactor) the average decrease in excess sludge was 6.3% and 14.7% of initial excess sludge in the aerobic and alternating reactor, respectively, compared to their controls. Reduction of solids resulted in a decreasing amount of activated sludge that had to be wasted daily up to 25%. This is comparable to previous research by Boehler and Siegrist (2004), where excess sludge reduction of 25–30% was observed up to an optimal dosage of 0.05 g O3/g TSS. During 7 months of operation of the ozonated reactors with higher apparent SRT, the average VSS/TSS ratio of the mixed liquor dropped slightly from 0.8570.05 to 0.8370.06, and from 0.7970.056 to 0.7770.05, in the aerobic and alternating reactors, respectively. This decrease in the VSS/TSS ratios indicated an accumulation of inorganic solids in the reactors. During ozone treatment, part of particulate matter was solubilized and this soluble matter could be utilized during biological treatment. In terms of final effluent quality, no statistical difference between the amount of TSS in the effluent from control and ozonated reactors was observed for either type of reactor, as all four reactors were quite variable in effluent TSS (average 1677.8 mg/L).
3.2.
Soluble COD production
Ozonation was expected to generate soluble organic matter by oxidation of organic polymers. Production of soluble COD (sCOD) during ozone treatment was confirmed, and it correlated with suspended solids destruction. Fig. 2 shows the increase of sCOD content due to ozonation, by comparing the RAS before and after treatment. The effect of ozonation
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41 (2007) 543– 550
30%
Decrease in TSS (WAS) in ozonated reactors in comparison to the controls (%)
y = 2.94x R2= 0.50
Aerobic Anoxic/aerobic
25%
20%
15%
10% y = 1.27x R2= 0.50
5%
0% 0.00
0.02
0.04
0.06
0.08
0.10
ozone dose (mg O3/mg TSS initial excess sludge) Fig. 1 – Decrease in WAS in the aerobic and alternating reactors after ozone treatment in comparison to the control reactors.
500%
Aerobic Anoxic/aerobic
Increase of sCOD in ozonated RAS (%)
400%
y = 40.18x
300%
R2 = 0.55
200%
y = 25.10x 100%
0% 0.00
R2 = 0.25
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
ozone dose (mg O3/mg TSS initial excess sludge) Fig. 2 – Increase in soluble COD production in RAS for aerobic and alternating ozonated reactors after ozone treatment.
was much greater in the alternating reactor than the aerated one; sCOD in the ozonated portion of RAS averaged 120751 mg/L for aerobic and 161757 mg/L for alternating reactor. The sCOD increased with the ozone dose in ozonated RAS, and for the moderate dose of 0.05 mg O3/mg TSS of initial excess sludge, the increase was 125% and 200% for the aerobic and alternating reactors, respectively, causing the increase in SCOD in influent to the reactors (Table 2). There was a
reasonable correlation between the destruction of TSS and increase in sCOD in the reactors (Fig. 3). In spite of additional sCOD loads to the ozonated reactors, the amount of TCOD in the effluent did not increase, and averaged 46717 mg/L (Table 2). To test whether ozonation affected the biodegradability of the COD in the final effluent, the effluent from all four reactors was transferred to separate aerated vessels, incubated at 2071 1C (no additional feed was
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41 (20 07) 54 3 – 550
Table 2 – Chemical oxygen demand removal in control and ozonated reactors (average for doses 0.05–0.08 mg O3/mg TSS initial excess sludge) Parameter
Reactor Aerobic control
Aerobic ozonated
Alternating control
Alternating ozonated
495748 47714 32710 90.573.7 47.2712.4
544751 46718 30711 91.574.1 53.5712.3
495748 46716 3477 90.774.1 50.479.9
594755 46718 3378 92.373.8 53.677.5
sCOD influent (mg/L) TCOD effluent (mg/L) sCOD effluent (mg/L) % TCOD removal (full reaction) % TCOD removal (first 15 min)
500%
increase in sCOD (ozonated RAS) (%)
400%
300%
y = 19.04x R2 = 0.26
y = 13.56x R2 = 0.47
200%
100%
Aerobic Anoxic/aerobic
0% 0%
5%
10%
15%
20%
25%
30%
decrease in TSS (WAS) (%) Fig. 3 – Increase in soluble COD in ozonated RAS vs. decrease in WAS for doses 0.01–0.08 mg O3/mg TSS of initial excess sludge.
added) and COD was analyzed daily. No further COD removal was observed over 5 days, indicating that ozonation did not significantly impact the quality of the COD in the effluent. In the four reactors, the percentage of TCOD removal averaged 90.5% in both control reactors and achieved 91.5% and 92.3% removal in the ozonated aerobic and alternating reactors, respectively. Therefore, ozonation did not negatively impact carbon removal during the treatment process. This carbon removal would consist of both biodegradation and biosorption. Physical–chemical phenomena such as flocculation of particulate and colloidal material released from cells, and sorption of dissolved organics mitigate the transfer of organic matter (COD) from the wastewater to the activated sludge floc surface. After sorption, the colloids are hydrolyzed by extracellular bacterial enzymes and further COD assimilation
occurs. Biosorption capacity, expressed here as percent of TCOD removed from the bulk solution during the first 15 min of feed—biomass contact (Table 2) was close to the value of 45% reported previously (Guellil et al., 2001). Biosorption was higher in alternating reactors, probably due to the larger active surface of the flocs (open structure) compared to the compact flocs in the aerobic reactors. Biosorption capacity increased slightly after ozone treatment, probably due to enlargement of the floc surface and increased porosity during floc destruction.
3.3.
Denitrification
The production of soluble COD from partial ozonation of RAS has the potential to provide a good carbon source for
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NUR improvement in comparison to the control (%)
70% 60% 50% 40% 30%
y = 9.12x
20%
R2 = 0.95
10% 0% 0
0.02
0.04
0.06
0.08
ozone dose (mg O3/ mg TSS initial excess sludge) Fig. 4 – Improvement of denitrification in alternating ozonated reactor in comparison to the control (NUR rate increase).
a
NO3-
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denitrification and lead to substantial cost savings in reactions that are carbon-limited. Here, the alternating reactors were exposed to anoxic conditions first, right after ozonation of 20% of the sludge and the subsequent addition of feed (containing nitrate). As such, the soluble COD generated by ozonation could potentially be used for denitrification. Conversely, ozonation of the RAS could decrease the population of denitrifiers and could decrease initial rates of denitrification. Removal of nitrate and nitrite was compared to the control where RAS was not ozonated. Denitrification during the initial anoxic phase in the alternating reactors was enhanced rather than inhibited by ozonation of 20% of the RAS. In the ozonated alternating reactor, the level of nitrate in the final effluent decreased in comparison to the control reactor, in proportion to the ozone dose. This drop was linear from 5% for the lowest dose of
NH4+
NO2-
45
NO3-, NO2-, NH4+-N (mg/L)
aerobic
anoxic
40 35 30 25 20 15 10 5 0 0
1
2
b
3 NO3-
4 time (h)
5
NH4+
6
7
8
NO2-
45 anoxic
NO3-, NO2-, NH4+-N (mg/L)
40
aerobic
35 30 25 20 15 10 5 0 0
1
2
3
4 time (h)
5
6
7
8
Fig. 5 – Profiles of nitrogen changes (time 0–feeding); (a) alternating reactor control, (b) alternating reactor ozonated, dose 0.037 mg O3/mg TSS of initial excess sludge (18.9 mg consumed/reactor).
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3.4.
Nitrification
Nitrification occurred in all four reactors, with complete ammonium oxidation in both aerobic reactors and in alternating reactors during the aerobic phase that followed the denitrification phase (Fig. 5). The ammonium nitrogen in the effluent was always below the detection limit of 0.3 mg/L, regardless of ozonation. This was in spite of additional ammonia released from RAS after ozone treatment: approximately 2.7% and 5.9% more in the aerobic and alternating reactor, respectively. Nitrite was not detected in the effluent, indicating complete nitrification in both sets of reactors. Furthermore, nitrate accumulated in the aerobic reactors and during the aerobic phase in the alternating reactors. Alternating reactors had higher AUR and NPR than aerobic reactors, which corroborate earlier findings by Lee and Oleszkiewicz (2003). In both reactors, nitrification rates decreased proportionally to increasing ozone dose. In the alternating anoxic/aerobic reactor, nitrification (AUR, NPR) decreased with ozonation (up to 8%), but was affected far less than the aerobic reactor (25%) (Fig. 6). It has been suggested previously that nitrifying bacteria may be protected within floc structures. Boehler and Siegrist (2004) reported that their survival is inversely proportional to solids’ destruction. However, in these reactors, the alternating reactors had flocs that were smaller and easier to destroy (based on sCOD release and microscopy). Therefore, there was less of an impact on nitrification in spite of the fact that the solids destruction was even greater in the alternating reactors. There must be another mechanism to explain why nitrification was affected more in the aerobic reactors than in the alternating reactors. The nitrifiers in the remaining 80% of the RAS that was not ozonated were probably responsible for most if not all of the nitrification observed in the reactors. In the ozonated reactors, the population of nitrifiers could actually be higher than in the control reactors due to the longer SRT that was maintained to compensate for solids destruction. This additional amount of nitrifier biomass would be even higher in the alternating reactor than in the aerobic reactor. Either way, the abundance of nitrifiers does not seem to control nitrification rates in these reactors.
8 AUR ( mgNH4+-N/g VSS h).
0.01 mg O3/mg TSS of initial excess sludge up to 20% for the highest dose of 0.08 mg O3/mg TSS of initial excess sludge, indicating better total denitrification. Kinetic studies where then conducted, and the rate of denitrification NUR was significantly improved in the ozonated alternating reactor compared to the control, as indicated by an increase in the specific NUR (Fig. 4). Fig. 5 shows the profiles of NHþ 4 , NO3 and NO2 changes over the initial reaction time in alternating reactor. Ammonium did not decrease during anoxic phase, its removal occurred after switching to aerobic conditions (3.5 h). Nitrate concentration decreased during denitrification and its removal was greater in rate and extent in the ozonated reactor. Nitrite accumulated and then decreased during denitrification stage. In the ozonated reactor, this accumulation was less pronounced, and removal of both ammonium and nitrite were faster. The ratio of g additional nitrate+nitrite removed per g excess sludge reduced was 0.161.
549
41 (20 07) 54 3 – 550
Aerobic
Anoxic/aerobic
7 6 y = -8.27x + 6.25 R2 = 0.01
5 4 y = -23.69x + 5.08 R2 = 0.53
3 0
0.02
0.06
0.04
0.08
ozone dose (mg O3/mg TSS initial excess sludge) 10 Aerobic
9 NPR (mg NOx--N/gVSS h).
WAT E R R E S E A R C H
Anoxic/aerobic
8 7 y =-6.02x + 7.75
6
R2 = 0.10
5 4
y = -29.57x + 5.94 R2 = 0.18
3 0
0.02
0.04
0.06
0.08
ozone dose (mg O3/mg TSS initial excess sludge) Fig. 6 – Impact of O3 on nitrification—changes in AUR and NPR rates in ozonated reactors in comparison to the controls, over a range of ozone doses.
Further research is needed to quantify the relative impacts of the different SRTs and the ozone doses on nitrifying populations in the reactors. A more intriguing explanation is that deterioration of AUR and NPR in the aerobic reactor was caused not only by the direct influence of ozone on nitrifiers and floc structure, but also by the higher sCOD released during this treatment. Autotrophic nitrifiers could have been suppressed by faster growing heterotrophs. Nitrification rate was hindered more in the ozonated aerobic reactor due to more COD present in the beginning of the cycle (at the same time feed was added), and the time needed for its utilization (Table 1). Both nitrification and heterotrophic COD removal occur simultaneously there; in the presence of organic matter, nitrifiers can be easily outcompeted by heterotrophs for oxygen and ammonia. On the other hand, nitrification in alternating reactors started only after the onset of aeration, 3.5 h after feeding; by that time only non-easily biodegradable COD would be present in the bulk liquid (94710 mg/L), which creates more favorable conditions for nitrification. The period of the anoxic reaction, during which the readily biodegradable COD is removed from solution, not only reduces the competition with faster growing heterotrophs for oxygen, but also creates conditions
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that lead to higher nitrification rates than in purely aerobic reactors. The period of heterotrophic denitrification in alternating reactors also increased the pH (from 7.7070.11 to 8.2070.07) and alkalinity in the solution, which could further stimulate the rate of nitrification.
4.
Conclusions
Ozonation of 20% of RAS had no negative impact on the final effluent quality from the SBRs, as the total COD and suspended solids in the effluent remained similar controls and nitrification was complete in both sets of reactors regardless of ozone dose. Ozonation significantly reduced the production of excess sludge in the reactors. The reduction of suspended solids in the bioreactor after ozone treatment was proportional to the ozone dose. After prolonged operation, some accumulation of inorganic solids in the ozonated reactors was observed, however it was not considered significant. Ozonation of sludge increased the soluble COD in the sludge. The extra supply of sCOD was proportional to ozone dose and greatly increased the denitrification rate in alternating reactor. Ozonation had less of an adverse effect on nitrification rate in the alternating reactor, indicating possible negative impact of the increased sCOD on the autotrophic population in the aerobic reactor. However, in spite of additional ammonia released by ozonation, nitrification in the reactors was complete.
Acknowledgements This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), City of Winnipeg—Water and Waste Department and Manitoba Conservation—Sustainable Development Innovation Fund. R E F E R E N C E S
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Ozonation reduces sludge production and improves denitrification Magdalena A. Dytczaka, Kathleen L. Londryb, Hansruedi Siegristc, Jan A. Oleszkiewicza, a
Department of Civil Engineering, University of Manitoba, Winnipeg, Canada MB R3T 5V6 Department of Microbiology, University of Manitoba, Winnipeg, Canada MB R3T 2N2 c EAWAG, CH-8600 Du¨bendorf, Switzerland b
art i cle info
A B S T R A C T
Article history:
The effectiveness of partial ozonation of return activated sludge for enhancing denitrifica-
Received 12 March 2006
tion and waste sludge minimization were examined. A pair of nitrifying sequencing batch
Received in revised form
reactors was operated in either aerobic or alternating anoxic/aerobic conditions, with one
20 October 2006
control and one ozonated reactor in each set. The amount of solids produced decreased
Accepted 7 November 2006
with the ozone dose. Biomass in the anoxic/aerobic reactor was easier to destroy (up to 25%
Available online 26 December 2006
of the initial excess sludge) than in the aerobic (10%) one, generating approximately twice
Keywords:
as much soluble COD by cell lysis. Denitrification rate improved up to 60% due to additional
Sludge minimization
carbon released by ozonation. Nitrification rates deteriorated much more in the aerobic
Wastewater
than in the alternating reactor, possibly as a result of direct destruction of nitrifying
Ozone
autotrophs as well as competition created by growth of heterotrophs receiving the
Denitrification
additional COD. Overall, ozonation provided the expected benefits in denitrification and
Nitrification
had less impact on nitrification in the alternating reactors. & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Activated sludge treatment produces excess biomass as waste activated sludge (WAS) that is difficult and expensive to dispose of, particularly in plants adapted for biological nutrient removal (BNR), as return sludge liquors require costly treatment. Technologies for reducing WAS biomass production are gaining attention (Low and Chase, 1999; Liu and Tay, 2001; Wei et al., 2003). One promising technology is ozonation of return activated sludge (RAS). Ozone is a strong chemical oxidant that destroys the cell walls of microorganisms, which leads to the release of cytoplasm, as well as oxidation of many recalcitrant organic compounds; for that reason it is often employed for disinfection and post-treatment (Beltran, 2003). Ozonation has recently been adopted as a pretreatment before anaerobic sludge digestion (Muller et al., 1998; Weemaes et al., 2000; Goel et al., 2003)—mainly to hydrolyze Corresponding author. Tel.: +204 474 8722; fax: +204 474 7513.
E-mail address: [email protected] (J.A. Oleszkiewicz). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.11.009
solids and increase biogas production. To reduce excess biomass production, ozonation has been introduced to the RAS line in activated sludge systems (Yasui and Shibata, 1994; Sakai et al., 1997). The process consists of sludge (biomass) ozonation stage followed by a biodegradation stage. A fraction of recycled sludge passes through the ozonation unit, where both solubilization to biodegradable organics (due to disintegration of suspended solids) and to a lesser extent mineralization to CO2 and H2O (due to oxidation of soluble organic matter), occur (Kamiya and Hirotsui, 1998). The treated sludge is then recycled to the aeration tank (Yasui and Shibata, 1994; Boehler and Siegrist, 2004). Full-scale studies suggest that ozonation could even lead to a process (of carbonaceous BOD removal) with no excess sludge production (Yasui et al., 1996). The disintegration of the sludge biomass, reflected in a decrease of both total suspended solids (TSS) and volatile suspended solids (VSS) in the ozonated sludge and the
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receiving reactors (Weemaes et al., 2000) is one of the key objectives of sludge ozonation. A decrease in the ratio of VSS/ TSS of treated sludge has been observed, and accumulation of inorganic solids in the aeration tank was considered negligible (Sakai et al., 1997) and may even be reduced by ozonation (Deleris et al., 2002). However, sludge ozonation has been reported to cause a slight increase of total organic carbon (TOC) and TSS in the final effluent in some studies (Sakai et al., 1997). More importantly, ozonation could potentially impact biological rates of carbon, nitrogen, or phosphate removal when treated RAS is introduced back into the process. This could be due to the destruction or inhibition of the specialized microbial populations. One could anticipate that reduction of the ammonia utilization rate (AUR) after ozonation should be proportional to the sludge reduction. Some researchers have shown, however, that the resulting AUR remained constant—i.e. there was no inhibitory effect of ozonation on nitrification (Sakai et al., 1997). It was suggested that nitrifiers, overgrown by the faster growing heterotrophs, may be protected inside the sludge floc and not be exposed to ozone as much as the surface-hugging heterotrophs (Boehler and Siegrist, 2004). The destruction of biomass by ozonation can improve the overall process by indirectly impacting the solids residence time (SRT). Reduced WAS production leads to an apparently higher SRT (defined as the mass of solids in the aeration tank divided by solids removed) if the mixed liquor suspended solids concentration (MLSS) remains constant (Boehler and Siegrist, 2004). Loss of nitrifiers due to ozonation seems to be partly compensated by this increased apparent SRT. Furthermore, there are potential benefits of ozonation for biological processes. In a denitrifying plant, the recycle of the ozonated RAS to the anoxic zone improves denitrification as the solubilized degradable organic compounds become an additional carbon source (Ahn et al., 2002; Park et al., 2004). The efficiency of ozonation as a method of excess sludge reduction, and its impacts on nitrification and denitrification,
41 (2007) 543– 550
are highly dependent on process operating parameters. Therefore, research is needed for the optimization of proper ozone dosage and dosing method (Liu and Tay, 2001). The objective of this research was to examine the influence of ozonation on sludge minimization and process performance under strictly aerobic and alternating anoxic/aerobic conditions. Although a number of earlier studies employed sequencing batch reactors (SBR) with strictly aerobic and alternating anoxic/aerobic conditions (Boehler and Siegrist, 2004; Deleris et al., 2002) this is the first study aiming to compare both processes and provide a unique perspective on the effects of ozonation on releasing biodegradable carbon. Potential benefits and limitations of partial RAS ozonation are discussed.
2.
Material and methods
2.1.
Sequencing batch reactors
Four SBRs were established (Table 1) with a total liquid volume of 3 L each, operated at 2071 1C. The SRT was 12 d (real SRT including solids in the effluent was calculated as 11.270.4 d), and a hydraulic retention time (HRT) of 36 h. The reactors had a sequence of fill, react, settle, and decant and were operated as two pairs of aerobic and alternating anoxic/ aerobic conditions, as per Table 1. The ratio between the anoxic period and total time of the cycle in the alternating reactors was 30%. Dissolved oxygen (DO) was controlled at 5 mg O2/L during aerobic conditions with the residual DO at 0.1 mg/L during the anoxic periods of the alternating reactors. The alkalinity remaining in the effluent was high enough to ensure unhindered nitrification (above 120 mg CaCO3/L). In each pair, one reactor served as a control and the other went through the ozone treatment. All reactors were acclimated for more than 5 SRT periods before testing commenced. Wasting of approximately 250 mL from the reactors
Table 1 – Operating conditions of aerobic and alternating anoxic/aerobic reactors Operation
Time
Air
Mixing
Aerobic reactors Feeding (2 L) Aerobic reaction Wasting (0.25 L) Settling Decanting (1.75 L) Ozonation of RAS (0.2 L)
5 min 22 h+10–25 min 10 min 1 h 05 min 10 min 5–20 min
— d — — — Batch
d d d — —
Alternating anoxic/aerobic reactors Feeding (2 L) Anoxic reaction Aerobic reaction Anoxic reaction Aerobic reaction Wasting (0.25 L) Settling Decanting (1.75 L) Ozonation of RAS (0.2 L)
5 min 3 h+10–25 min 7h 3 h 30 min 8 h 30 min 10 min 1 h 05 min 10 min 5–20 min
— — d — d — — — Batch
d d d d d d — —
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took place at the end of the reaction period. After settling of biomass and decanting 1.75 L of supernatant (final effluent), 1 L of sludge was left in the reactors, which was the equivalent of RAS. For the ozonated reactors, 20% of the sludge was withdrawn, treated by ozone, and then returned to the activated sludge reactor just before feeding. Ozone was produced from air (compressed air lab line) by an ozone generator (OZO 1 VTT Ozomax Ltd.). Ozone was applied to the RAS in a gas-washing bottle by bubbling the ozone gas (flow 10.5 mg/min) directly into the sludge for different lengths of time to achieve greater ozone doses (maximum retention time in the ozone contactor 16 min). The ozone dose consumed by sludge was analyzed each time by calculating the inlet gas and the off-gas trapped by potassium iodide solution (Standard Method 2350E). During the experiments, doses of 7–33 mg O3 consumed per reactor and day were applied, corresponding to doses of 0.006–0.030 mg O3/mg TSS of ozonated sludge and 0.015–0.080 mg O3/mg TSS of initial excess sludge. Doses were applied from smallest to largest, for a period of 1 SRT for each target dose, increasing in increments of 0.005 mg O3/mg TSS.
2.2.
Operating conditions
At the beginning of each daily cycle (following ozonation of RAS as applicable), the reactors were fed a synthetic wastewater consisting of beef and yeast extract as a carbon source (resultant total TCOD 740 mg/L) and ammonium chloride as ammonia source (resultant NH3 40 mg NHþ 4 -N/L). The alternating anoxic/aerobic reactors also received 10 mL of 40 g/L NaNO3 solution at the beginning of the anoxic reaction phase to create a sufficient supply of NO 3 for denitrification. TSS and VSS were measured in RAS from ozonated reactors directly before and after ozonation as well as in RAS from control reactors. Additionally, TSS was measured daily in samples from all reactors before wasting. In the ozonated reactors, the wasting was adjusted daily to maintain TSS at 1800 mg/L at the beginning of the reaction period, similar to the controls, resulting in a higher apparent SRT in ozonated reactors (SRT up to 14.970.7 d).
2.3.
Chemical analysis
Soluble COD was measured in quintuplicate in the portion of ozonated RAS directly before and after ozonation as well as in RAS from control reactors. Soluble and total COD were measured in the influent and effluent daily. The AUR, nitrite+nitrate utilization rate (NUR) and nitrite+nitrate production rate (NPR) were calculated based on changes in measured ammonium or nitrite+nitrate concentration as described previously (Lee and Oleszkiewicz, 2003). VSS, TSS, NHþ 4 , NO3 + NO2 and COD were analyzed in accordance with Standard Methods (APHA, 1998). NO 3 and NO2 were measured separately by ion chromatography using a Dionex IC system with an electrochemical detector, an ASII column, an eluent of NaOH at 2 mL/min, and quantification by comparison of peak areas to external standers curves. Floc structure was examined by contrast phase microscopy.
41 (20 07) 54 3 – 550
3.
Results and discussion
3.1.
Excess sludge reduction
545
The RAS that was ozonated experienced a large decrease in biomass that was proportional to ozone dose, for the highest doses up to 55 and 112 mg for aerobic and alternating reactor, respectively. Ozonation decreased the pH from 7.5970.07 to 7.2870.09 in the ozonated portion of RAS from aerobic reactor and from 7.8870.11 to 7.6370.08 for the alternating reactor. However, after returning the ozonated batch to the main reactor together with feed, a difference in pH between control and ozonated reactor in the pair was insignificant. Ozonation of 20% of the RAS also consistently reduced the amount of excess solids in the reactors receiving the ozonated RAS compared to the controls. This reduction was proportional to the ozone dose over the range tested for both reactors (R2 ¼ 0:5) and much greater in the alternating anoxic/ aerobic reactor than the aerobic one (Fig. 1). This was probably related to the different structures of the flocs in the alternating and aerobic reactors. Biomass in the aerobic reactors consisted of abundant protozoa, with a backbone of filamentous bacteria in dense, spherical, compact flocs, and appeared to be more resistant to higher doses of ozone. The biomass in the alternating reactors, consisting mostly of bacteria with fewer filaments, inhabiting weak, thin and elongated flocs (chain-type structure), seemed more susceptible to destruction. For a moderate dose of 0.05 mg O3/mg TSS of initial excess sludge (24 mg consumed O3/reactor) the average decrease in excess sludge was 6.3% and 14.7% of initial excess sludge in the aerobic and alternating reactor, respectively, compared to their controls. Reduction of solids resulted in a decreasing amount of activated sludge that had to be wasted daily up to 25%. This is comparable to previous research by Boehler and Siegrist (2004), where excess sludge reduction of 25–30% was observed up to an optimal dosage of 0.05 g O3/g TSS. During 7 months of operation of the ozonated reactors with higher apparent SRT, the average VSS/TSS ratio of the mixed liquor dropped slightly from 0.8570.05 to 0.8370.06, and from 0.7970.056 to 0.7770.05, in the aerobic and alternating reactors, respectively. This decrease in the VSS/TSS ratios indicated an accumulation of inorganic solids in the reactors. During ozone treatment, part of particulate matter was solubilized and this soluble matter could be utilized during biological treatment. In terms of final effluent quality, no statistical difference between the amount of TSS in the effluent from control and ozonated reactors was observed for either type of reactor, as all four reactors were quite variable in effluent TSS (average 1677.8 mg/L).
3.2.
Soluble COD production
Ozonation was expected to generate soluble organic matter by oxidation of organic polymers. Production of soluble COD (sCOD) during ozone treatment was confirmed, and it correlated with suspended solids destruction. Fig. 2 shows the increase of sCOD content due to ozonation, by comparing the RAS before and after treatment. The effect of ozonation
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30%
Decrease in TSS (WAS) in ozonated reactors in comparison to the controls (%)
y = 2.94x R2= 0.50
Aerobic Anoxic/aerobic
25%
20%
15%
10% y = 1.27x R2= 0.50
5%
0% 0.00
0.02
0.04
0.06
0.08
0.10
ozone dose (mg O3/mg TSS initial excess sludge) Fig. 1 – Decrease in WAS in the aerobic and alternating reactors after ozone treatment in comparison to the control reactors.
500%
Aerobic Anoxic/aerobic
Increase of sCOD in ozonated RAS (%)
400%
y = 40.18x
300%
R2 = 0.55
200%
y = 25.10x 100%
0% 0.00
R2 = 0.25
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
ozone dose (mg O3/mg TSS initial excess sludge) Fig. 2 – Increase in soluble COD production in RAS for aerobic and alternating ozonated reactors after ozone treatment.
was much greater in the alternating reactor than the aerated one; sCOD in the ozonated portion of RAS averaged 120751 mg/L for aerobic and 161757 mg/L for alternating reactor. The sCOD increased with the ozone dose in ozonated RAS, and for the moderate dose of 0.05 mg O3/mg TSS of initial excess sludge, the increase was 125% and 200% for the aerobic and alternating reactors, respectively, causing the increase in SCOD in influent to the reactors (Table 2). There was a
reasonable correlation between the destruction of TSS and increase in sCOD in the reactors (Fig. 3). In spite of additional sCOD loads to the ozonated reactors, the amount of TCOD in the effluent did not increase, and averaged 46717 mg/L (Table 2). To test whether ozonation affected the biodegradability of the COD in the final effluent, the effluent from all four reactors was transferred to separate aerated vessels, incubated at 2071 1C (no additional feed was
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Table 2 – Chemical oxygen demand removal in control and ozonated reactors (average for doses 0.05–0.08 mg O3/mg TSS initial excess sludge) Parameter
Reactor Aerobic control
Aerobic ozonated
Alternating control
Alternating ozonated
495748 47714 32710 90.573.7 47.2712.4
544751 46718 30711 91.574.1 53.5712.3
495748 46716 3477 90.774.1 50.479.9
594755 46718 3378 92.373.8 53.677.5
sCOD influent (mg/L) TCOD effluent (mg/L) sCOD effluent (mg/L) % TCOD removal (full reaction) % TCOD removal (first 15 min)
500%
increase in sCOD (ozonated RAS) (%)
400%
300%
y = 19.04x R2 = 0.26
y = 13.56x R2 = 0.47
200%
100%
Aerobic Anoxic/aerobic
0% 0%
5%
10%
15%
20%
25%
30%
decrease in TSS (WAS) (%) Fig. 3 – Increase in soluble COD in ozonated RAS vs. decrease in WAS for doses 0.01–0.08 mg O3/mg TSS of initial excess sludge.
added) and COD was analyzed daily. No further COD removal was observed over 5 days, indicating that ozonation did not significantly impact the quality of the COD in the effluent. In the four reactors, the percentage of TCOD removal averaged 90.5% in both control reactors and achieved 91.5% and 92.3% removal in the ozonated aerobic and alternating reactors, respectively. Therefore, ozonation did not negatively impact carbon removal during the treatment process. This carbon removal would consist of both biodegradation and biosorption. Physical–chemical phenomena such as flocculation of particulate and colloidal material released from cells, and sorption of dissolved organics mitigate the transfer of organic matter (COD) from the wastewater to the activated sludge floc surface. After sorption, the colloids are hydrolyzed by extracellular bacterial enzymes and further COD assimilation
occurs. Biosorption capacity, expressed here as percent of TCOD removed from the bulk solution during the first 15 min of feed—biomass contact (Table 2) was close to the value of 45% reported previously (Guellil et al., 2001). Biosorption was higher in alternating reactors, probably due to the larger active surface of the flocs (open structure) compared to the compact flocs in the aerobic reactors. Biosorption capacity increased slightly after ozone treatment, probably due to enlargement of the floc surface and increased porosity during floc destruction.
3.3.
Denitrification
The production of soluble COD from partial ozonation of RAS has the potential to provide a good carbon source for
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NUR improvement in comparison to the control (%)
70% 60% 50% 40% 30%
y = 9.12x
20%
R2 = 0.95
10% 0% 0
0.02
0.04
0.06
0.08
ozone dose (mg O3/ mg TSS initial excess sludge) Fig. 4 – Improvement of denitrification in alternating ozonated reactor in comparison to the control (NUR rate increase).
a
NO3-
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denitrification and lead to substantial cost savings in reactions that are carbon-limited. Here, the alternating reactors were exposed to anoxic conditions first, right after ozonation of 20% of the sludge and the subsequent addition of feed (containing nitrate). As such, the soluble COD generated by ozonation could potentially be used for denitrification. Conversely, ozonation of the RAS could decrease the population of denitrifiers and could decrease initial rates of denitrification. Removal of nitrate and nitrite was compared to the control where RAS was not ozonated. Denitrification during the initial anoxic phase in the alternating reactors was enhanced rather than inhibited by ozonation of 20% of the RAS. In the ozonated alternating reactor, the level of nitrate in the final effluent decreased in comparison to the control reactor, in proportion to the ozone dose. This drop was linear from 5% for the lowest dose of
NH4+
NO2-
45
NO3-, NO2-, NH4+-N (mg/L)
aerobic
anoxic
40 35 30 25 20 15 10 5 0 0
1
2
b
3 NO3-
4 time (h)
5
NH4+
6
7
8
NO2-
45 anoxic
NO3-, NO2-, NH4+-N (mg/L)
40
aerobic
35 30 25 20 15 10 5 0 0
1
2
3
4 time (h)
5
6
7
8
Fig. 5 – Profiles of nitrogen changes (time 0–feeding); (a) alternating reactor control, (b) alternating reactor ozonated, dose 0.037 mg O3/mg TSS of initial excess sludge (18.9 mg consumed/reactor).
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3.4.
Nitrification
Nitrification occurred in all four reactors, with complete ammonium oxidation in both aerobic reactors and in alternating reactors during the aerobic phase that followed the denitrification phase (Fig. 5). The ammonium nitrogen in the effluent was always below the detection limit of 0.3 mg/L, regardless of ozonation. This was in spite of additional ammonia released from RAS after ozone treatment: approximately 2.7% and 5.9% more in the aerobic and alternating reactor, respectively. Nitrite was not detected in the effluent, indicating complete nitrification in both sets of reactors. Furthermore, nitrate accumulated in the aerobic reactors and during the aerobic phase in the alternating reactors. Alternating reactors had higher AUR and NPR than aerobic reactors, which corroborate earlier findings by Lee and Oleszkiewicz (2003). In both reactors, nitrification rates decreased proportionally to increasing ozone dose. In the alternating anoxic/aerobic reactor, nitrification (AUR, NPR) decreased with ozonation (up to 8%), but was affected far less than the aerobic reactor (25%) (Fig. 6). It has been suggested previously that nitrifying bacteria may be protected within floc structures. Boehler and Siegrist (2004) reported that their survival is inversely proportional to solids’ destruction. However, in these reactors, the alternating reactors had flocs that were smaller and easier to destroy (based on sCOD release and microscopy). Therefore, there was less of an impact on nitrification in spite of the fact that the solids destruction was even greater in the alternating reactors. There must be another mechanism to explain why nitrification was affected more in the aerobic reactors than in the alternating reactors. The nitrifiers in the remaining 80% of the RAS that was not ozonated were probably responsible for most if not all of the nitrification observed in the reactors. In the ozonated reactors, the population of nitrifiers could actually be higher than in the control reactors due to the longer SRT that was maintained to compensate for solids destruction. This additional amount of nitrifier biomass would be even higher in the alternating reactor than in the aerobic reactor. Either way, the abundance of nitrifiers does not seem to control nitrification rates in these reactors.
8 AUR ( mgNH4+-N/g VSS h).
0.01 mg O3/mg TSS of initial excess sludge up to 20% for the highest dose of 0.08 mg O3/mg TSS of initial excess sludge, indicating better total denitrification. Kinetic studies where then conducted, and the rate of denitrification NUR was significantly improved in the ozonated alternating reactor compared to the control, as indicated by an increase in the specific NUR (Fig. 4). Fig. 5 shows the profiles of NHþ 4 , NO3 and NO2 changes over the initial reaction time in alternating reactor. Ammonium did not decrease during anoxic phase, its removal occurred after switching to aerobic conditions (3.5 h). Nitrate concentration decreased during denitrification and its removal was greater in rate and extent in the ozonated reactor. Nitrite accumulated and then decreased during denitrification stage. In the ozonated reactor, this accumulation was less pronounced, and removal of both ammonium and nitrite were faster. The ratio of g additional nitrate+nitrite removed per g excess sludge reduced was 0.161.
549
41 (20 07) 54 3 – 550
Aerobic
Anoxic/aerobic
7 6 y = -8.27x + 6.25 R2 = 0.01
5 4 y = -23.69x + 5.08 R2 = 0.53
3 0
0.02
0.06
0.04
0.08
ozone dose (mg O3/mg TSS initial excess sludge) 10 Aerobic
9 NPR (mg NOx--N/gVSS h).
WAT E R R E S E A R C H
Anoxic/aerobic
8 7 y =-6.02x + 7.75
6
R2 = 0.10
5 4
y = -29.57x + 5.94 R2 = 0.18
3 0
0.02
0.04
0.06
0.08
ozone dose (mg O3/mg TSS initial excess sludge) Fig. 6 – Impact of O3 on nitrification—changes in AUR and NPR rates in ozonated reactors in comparison to the controls, over a range of ozone doses.
Further research is needed to quantify the relative impacts of the different SRTs and the ozone doses on nitrifying populations in the reactors. A more intriguing explanation is that deterioration of AUR and NPR in the aerobic reactor was caused not only by the direct influence of ozone on nitrifiers and floc structure, but also by the higher sCOD released during this treatment. Autotrophic nitrifiers could have been suppressed by faster growing heterotrophs. Nitrification rate was hindered more in the ozonated aerobic reactor due to more COD present in the beginning of the cycle (at the same time feed was added), and the time needed for its utilization (Table 1). Both nitrification and heterotrophic COD removal occur simultaneously there; in the presence of organic matter, nitrifiers can be easily outcompeted by heterotrophs for oxygen and ammonia. On the other hand, nitrification in alternating reactors started only after the onset of aeration, 3.5 h after feeding; by that time only non-easily biodegradable COD would be present in the bulk liquid (94710 mg/L), which creates more favorable conditions for nitrification. The period of the anoxic reaction, during which the readily biodegradable COD is removed from solution, not only reduces the competition with faster growing heterotrophs for oxygen, but also creates conditions
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that lead to higher nitrification rates than in purely aerobic reactors. The period of heterotrophic denitrification in alternating reactors also increased the pH (from 7.7070.11 to 8.2070.07) and alkalinity in the solution, which could further stimulate the rate of nitrification.
4.
Conclusions
Ozonation of 20% of RAS had no negative impact on the final effluent quality from the SBRs, as the total COD and suspended solids in the effluent remained similar controls and nitrification was complete in both sets of reactors regardless of ozone dose. Ozonation significantly reduced the production of excess sludge in the reactors. The reduction of suspended solids in the bioreactor after ozone treatment was proportional to the ozone dose. After prolonged operation, some accumulation of inorganic solids in the ozonated reactors was observed, however it was not considered significant. Ozonation of sludge increased the soluble COD in the sludge. The extra supply of sCOD was proportional to ozone dose and greatly increased the denitrification rate in alternating reactor. Ozonation had less of an adverse effect on nitrification rate in the alternating reactor, indicating possible negative impact of the increased sCOD on the autotrophic population in the aerobic reactor. However, in spite of additional ammonia released by ozonation, nitrification in the reactors was complete.
Acknowledgements This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), City of Winnipeg—Water and Waste Department and Manitoba Conservation—Sustainable Development Innovation Fund. R E F E R E N C E S
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