Improvement of nutrient removal and phosphorus recovery in the anaerobic/oxic/anoxic process combined with sludge ozonation and phosphorus adsorption

The effects of ozonation conditions on the performance of a continuous anaerobic/oxic/anoxic (A/O/A) process with sludge ozonation and phosphorus adsorption were investigated. In this system, excess sludge was ozonated by microbubble ozonation, and then the supernatant of the ozonated sludge was flowed into a phosphorus adsorption column packed with zirconium-ferrite adsorbent. The effluent from the column and the settlings of the ozonated sludge were recirculated in the A/O/A process. Long-term operation of a lab-scale system treating rural wastewater showed that ozonation affected not only the sludge reduction efficiency but also the nitrogen removal efficiency. When the amount of sludge to be ozonated was set at 16% of total MLSS per day, no excess sludge was withdrawn, but the nitrogen removal efficiency was deteriorated. Decreasing the amount of sludge to be ozonated (to 9.4% of total MLSS per day) resulted in efficient nitrogen removal, but the MLSS concentration increased slightly. Phosphorus accumulated in the sludge was re-solubilized by ozonation, and a large part of the solubilized phosphorus consisted of Pi. Almost all Pi was recovered in the phosphorus adsorption column.

Improvement of nutrient removal and phosphorus

recovery in the anaerobic/oxic/anoxic process combined with sludge ozonation and phosphorus adsorption

Takashi KONDO*, Satoshi TSUNEDA**, Yoshitaka EBIE*, Yuhei INAMORI***, and Kaiqin XU*

* Research Center for Material Cycles and Waste Management, National Institute for

Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan

** Department of Life Science and Medical Bioscience, Waseda University, 2-2 Wakamatsu-cho, Shinjuku-ku, Tokyo 162-8480, Japan

*** Faculty of Symbiotic Systems Science, Fukushima University, 1 Kanayagawa, Fukushima, Fukushima 960-1296, Japan

ABSTRACT

The effects of ozonation conditions on the performance of a continuous anaerobic/oxic/anoxic (A/O/A) process with sludge ozonation and phosphorus adsorption were investigated In this system, excess sludge was ozonated by microbubble ozonation, and then the supernatant of the ozonated sludge was flowed into a phosphorus adsorption column packed with zirconium-ferrite adsorbent The effluent from the column and the settlings of the ozonated sludge were recirculated in the A/O/A process Long-term operation of a lab-scale system treating rural wastewater showed that ozonation affected not only the sludge reduction efficiency but also the nitrogen removal efficiency When the amount of sludge to be ozonated was set at 16% of total MLSS per day, no excess sludge was withdrawn, but the nitrogen removal efficiency was deteriorated Decreasing the amount of sludge to be ozonated (to 9.4% of total MLSS per day) resulted in efficient nitrogen removal, but the MLSS concentration increased slightly Phosphorus accumulated in the sludge was re-solubilized by ozonation, and a large part of the solubilized phosphorus consisted of Pi Almost all Pi was recovered in the phosphorus adsorption column

Keywords: Ozonation; phosphorus recovery; sludge reduction

INTRODUCTION

In recent years, biological nutrient removal processes, such as the anaerobic/anoxic/oxic (A/A/O) process, have been widely introduced in wastewater treatment plants (WWTPs) In these systems, nitrogen is removed by nitrifying and denitrifying bacteria, and phosphorus is removed by polyphosphate-accumulating organisms (PAOs) Although the A/A/O process achieves effective nutrient removal, competition for energy sources between PAOs and denitrifying bacteria critically affects the nutrient removal efficiency To avoid this competition, some new systems employing denitrifying PAOs (DNPAOs), which are capable of utilizing nitrate/nitrite as electron acceptors unlike

PAOs, have been proposed (Ahn et al., 2002a, b; Kuba et al., 1996, 1997; Soejima et

al., 2006; Tsuneda et al., 2006) Previously, Tsuneda et al (2006) described an

anaerobic/oxic/anoxic (A/O/A) process and succeeded in causing DNPAOs to take an active part in simultaneous nitrogen and phosphorus removal in an acetate-fed sequencing batch reactor (SBR) in which additional acetate was required for inhibition

of phosphorus uptake in the oxic period

Treatment and disposal of excess sludge have been serious problems in WWTPs; the

Address correspondence to Satoshi Tsuneda, Department of Life Science and Medical Bioscience,

treatment of excess sludge may account for as much as 25% to 65% of total plant operating costs (Liu, 2003) One method to reduce excess sludge is solubilization of the excess sludge by using ozone, which is a strong oxidant, and recirculation of the solubilized excess sludge, containing readily biodegradable carbon, in a biological

treatment system (Chu et al., 2008; Cui and Jahng, 2004; Kamiya and Hirotsuji, 1998; Nagare et al., 2008; Sakai et al., 1997; Saktaywin et al., 2005; Suzuki et al., 2006; Yasui

et al., 1996; Yasui and Shibata, 1994) Introduction of an ozonation system for excess

sludge reduction is advantageous in terms of total energy consumption in WWTPs

(Nagare et al., 2008) Additionally, Chu et al (2008) reported that the efficiencies of

ozone utilization and sludge solubilization were improved by using a microbubble ozonation system In this system, microbubbles with a diameter less than several tens of micrometers are generated by a mixing pump that mixed gas and liquid at high speed

(Chu et al 2008) In another system, phosphorus accumulated in excess sludge is

solubilized by ozonation, and then the solubilized phosphorus can be easily recovered

by crystallization or with a phosphorus adsorbent (Saktaywin et al., 2005; Suzuki et al.,

2006)

Against this background, we previously proposed an advanced system involving a continuous A/O/A process combined with sludge reduction by ozonation and

phosphorus recovery by phosphorus adsorbent (Suzuki et al., 2006) (Fig 1A) In that

system, nitrogen and phosphorus were removed effectively without any sludge production; however, the TOC removal efficiency was deteriorated due to the circulation of ozonated sludge Furthermore, the ozonated sludge was not suitable as an additional carbon source to inhibit oxic phosphorus uptake

In the present study, to achieve efficient nutrient removal, the previous system was improved by introducing a microbubble ozonation system and by changing the ozonated sludge recirculation lines (Fig 1B) During 152 days operation of a lab-scale reactor, the operational conditions were optimized by changing the amount of sludge to be ozonated and the recirculation ratio of the residual liquid from the physico-chemical processes to the anaerobic and oxic tanks

Figure 1 Schematic diagrams of the A/O/A process with ozonation and phosphorus

adsorption: A, previous study (Suzuki et al., 2006); B, this study

Influent Anaerobic tank Oxictank Anoxictank

Effluent

Sludge return Ozonation

process

Phosphorus adsorption column

Influent Anaerobic tank Oxictank Anoxictank

Effluent

Sludge return Phosphorus

adsorption column

Microbubble ozonation process

MATERIALS AND METHODS

A lab-scale continuous A/O/A process with a working volume of 36 L (anaerobic tank, 10.3 L; oxic tank, 10.3 L; anoxic tank, 15.4 L), which was the same as that used in our

previous study (Suzuki et al., 2006), was operated for 152 days The microbubble

ozonation system and the phosphorus adsorption column were introduced to the A/O/A process (Fig 1B) The microbubble ozonation system was a cylindrical reactor with an internal diameter of 0.2 m and a height of 0.8 m (effective volume of 20 L) The ozonation system received sludge withdrawn continuously from the end of the anoxic tank When the amount of sludge reached approximately 10 L (once in 1 to 3 days), ozonation was conducted under specified ozonation conditions, as shown in Table 1 Ozone gas was generated by an ozone generator (PO-10; Fuji Electric, Japan), and the applied ozone concentration was monitored with a UV ozone monitor (PG-620HA; Ebara Jitsugyo, Japan) Ozone gas and sludge from the anoxic tank were mixed in a turbulent flow by a turbine pump (Nikuni swirling current pump M15NPD02S; Nikuni Co., Japan), and then the mixture of microbubble ozone and sludge was circulated back into the reactor After ozonation, the supernatant with unsettled microsolids was flowed into the phosphorus adsorption column (Fig 1B) The phosphorus adsorption column (internal diameter, 75 mm; height, 0.7 m; effective volume, 2 L) was filled with 1.5 L of spherical zirconium-ferrite (ZrFe2(OH)8) adsorbent with an effective diameter of 0.7

mm (Japan Enviro Chemicals, Japan) The flow rate of the supernatant was set at 25 mL/min After phosphorus adsorption, the column was backwashed with 15 L of tap water to remove residual suspended solids (SS) Then, the residual liquid from the physico-chemical processes, which was the mixture of the effluent from the phosphorus adsorption column and the backwash water, was circulated back to the anaerobic tank and the oxic tank at an appropriate rate (Table 1)

Table 1 Operational conditions

The hydraulic retention time (HRT) for influent wastewater was adjusted to 10 h The sludge return ratio from the settling tank to the anaerobic tank was controlled at 80% The reactor was inoculated with activated sludge (initial MLSS was adjusted to 4,000 mg/L), which was collected from a WWTP (A/A/O process) with efficient biological phosphorus removal Raw rural wastewater, which was collected from a rural sewage treatment plant daily, was flowed into the system The characteristics of the influent wastewater were as follows: 160–200 mg/L of SS, 55–80 mg/L of TOC, 45–55 mg/L of T-N, and 4.0–5.5 mg/L of T-P In this study, four different operational conditions were used to determine the most appropriate ones (Table 1) The nutrient recovery efficiency

in each phase, except for Phase 1, was evaluated more than 25 days after changes to prevent effects of the previous operational conditions

MLSS was measured according to the Standard Methods (1995) To determine soluble TOC (S-TOC), NH4-N, NO2+3-N, NO2-N, and PO4-P, water samples were filtered using

a glass-fiber filter (GF/C, Whatman Japan KK, Japan) TOC and S-TOC were measured with a SHIMADZU TOC-VSCH (Shimadzu, Japan) Total nitrogen (T-N), total phosphorus (T-P), soluble T-N (ST-N), soluble T-P (ST-P), NH4-N, NO2+3-N, NO2-N, and PO4-P were measured with a TRAACS 2000 (Bran+Luebbe, Japan)

RESULTS AND DISCUSSION

The A/O/A process combined with the microbubble ozonation system and the phosphorus adsorption column was operated for 152 days Water quality in the effluent and MLSS concentration in the reactor were affected by the ozonation conditions (Figure 2) In Phase 1, 23% of total MLSS per day was withdrawn, and the sludge was ozonated every day This operational condition resulted in a dramatic decrease in MLSS concentration On day 23, the amount of sludge to be ozonated was reduced from 23%

to 16% of total MLSS per day (Phase 2), and as a result, MLSS concentration was maintained at around 3,000 mg/L

Figure 2 Time course of TOC, T-N, and T-P concentrations in the effluent and MLSS in the reactor

Figure 3 shows the water quality profiles at day 49 (Phase 2), day 78 (Phase 3), and day

147 (Phase 4) In Phase 2, the effluent T-N concentration increased to approximately 20 mg/L, whereas no excess sludge was withdrawn in Phase 2 (Fig 3) Effluent T-N was mostly composed of NH4-N, indicating deterioration of nitrification performance Considering that organic carbon generally induces dissolved oxygen competition between nitrifying bacteria and heterotrophic bacteria, organic carbon loading by circulation of ozonated sludge might have been the cause of the deteriorated nitrification performance Then, to reduce the organic carbon loading to the oxic tank, the circulation ratio of the residual liquid from the physico-chemical processes was changed (Phase 3) However, nitrification performance was not improved In Phase 4, to maintain the population density of slow-growing nitrifying bacteria, the amount of sludge to be ozonated was reduced to 9.4 % of total MLSS per day (Table 1) As a result, the effluent T-N concentration decreased to around 10 mg/L This efficient nitrogen removal was achieved over 2 months with a slight increase of MLSS concentration (Fig 2) The

sludge yield was roughly estimated from the slope in Fig 2 to be 17 mg-MLSS/L/day This was 50 % of the sludge yield in the A/O/A process without the physico-chemical processes, operated under the same experimental conditions (data not shown)

Figure 3 Water quality profiles at day 49 (Phase 2), day 78 (Phase 3), and day 147 (Phase 4)

In our previous study, effluent TOC was deteriorated when an ozonation system was

introduced to the A/O/A process (Suzuki et al., 2006) This deterioration might be due

to slowly biodegradable materials derived from ozonated sludge (Yasui and Shibata, 1994) In this study, TOC was effectively removed and deterioration was not observed

(Fig 2) Chu et al (2007, 2008) reported that microbubble ozonation improved the

mass transfer of ozone, and the high inner pressure in the bubbles could accelerate the formation of hydroxyl radicals Therefore, some of the biorefractory and/or slowly biodegradable materials might be oxidized to easily biodegradable materials by changing the ozonation system

Phosphorus was removed effectively in all phases (Fig 3) The phosphorus concentration increased in the anaerobic phase due to phosphorus release from PAOs Part of the released phosphorus was then accumulated in the sludge in the subsequent oxic tank by normal oxygen-utilizing PAOs The residual phosphorus was removed in the final anoxic tank without oxygen Especially in Phase 4, phosphorus was removed without a decrease in TOC concentration in the anoxic tank, indicating that phosphorus

was accumulated by DNPAOs, like previous studies (Ahn et al., 2002a, b; Kuba et al.,

1996, 1997; Soejima et al., 2006; Tsuneda et al., 2006) Therefore, it was suggested that

DNPAOs contributed to not only phosphorus removal but also denitrification In the previous system, most of the phosphorus was removed in the oxic tank by the PAOs even though ozonated sludge was added to the oxic tank to inhibit oxic phosphorus

uptake (Suzuki et al., 2006) Therefore, the residual liquid from the physico-chemical

processes might contain an organic carbon source that was available for the PAOs

Phosphorus accumulated in the sludge was re-solubilized by ozonation The phosphorus concentrations in the influent (after ozonation) and effluent of the phosphorus adsorption column are shown in Fig 4 During the operation, about 70% of the phosphorus in the sludge was solubilized by ozonation, and a large part of the solubilized phosphorus consisted of PO4-P Over 90% of the solubilized phosphorus was absorbed, and the effluent PO4-P concentration was maintained at less than 1 mg/L until day 119 (Fig 4) After 119 days operation, the effluent PO4-P concentration reached 1 mg/L (Fig 4) Then, the phosphorus adsorbent was subjected to phosphorus desorption

by using an alkali solution and subsequent reactivation by using an acid solution,

according to a method described in the literature (Ebie et al., 2008) The reactivated

adsorbent was packed into the column again, and phosphorus was effectively adsorbed

Figure 4 Concentration of each type of phosphorus in the influent and effluent of the phosphorus adsorption column The small graph at the right shows the breakthrough point (PO4-P concentration: 1 mg/L) of the adsorbent at day 119

As the amount of excess sludge increases, regulations for its disposal have become increasingly stringent (Liu, 2003; Ødegaard, 2004) Many attempts have been made to reduce excess sludge production In this study, a microbubble ozonation system was combined with the A/O/A process In Phase 1, the MLSS concentration in the reactor was dramatically decreased due to excess sludge reduction MLSS concentration was maintained at around 3,000 mg/L in Phases 2 and 3; however, the nitrification efficiency was deteriorated Subsequently, reducing the amount of sludge to be ozonated (9.4% of total MLSS in the reactor) improved the nitrification efficiency in Phase 4

Ozonated sludge also acts as nutrient loading in the A/O/A process Cui and Jahng (2004) reported that nitrogen derived from ozonated sludge was not completely reduced

to nitrogen gas by energy sources originating from the ozonated sludge In the present study, although nitrogen components in the ozonated sludge were not analyzed, deterioration of the nutrient removal efficiency was not observed in Phase 4 (Figs 2 and 3)

Reduction of growth yield potential of microorganisms in the sludge is also important

for excess sludge reduction Kuba et al (1996) reported that sludge production

removed both in the oxic and anoxic tanks, suggesting that both PAOs and DNPAOs contributed to phosphorus removal Therefore, DNPAOs in the reactor might contribute

to both nutrient removal and sludge reduction

It is important for practical applications to estimate the benefits in terms of reduced energy consumption (operating costs); however, they were not estimated in this study

Chu et al (2008) reported that effective ozone utilization and sludge solubilization

could be achieved However, operation of an additional turbine pump that is required would increase the energy consumption Additionally, energy consumption is likely to

be increased compared with conventional WWTPs because of the introduction of the phosphorus adsorption column Therefore, energy balance and life cycle assessment analyses will be necessary in a future study

CONCLUSIONS

In order to meet the increasingly stringent requirements for environmental protection and phosphorus resource recovery, an A/O/A process involving microbubble ozonation and phosphorus adsorption was tested under various ozonation conditions The main conclusions are as follows

(1) The amount of sludge to be ozonated affected the nitrogen removal efficiency, whereas TOC and phosphorus removal were not affected by ozonation Under the optimum ozonation conditions (9.4% of total MLSS per day), efficient nutrient removal was achieved with a slight increase in MLSS concentration

(2) Phosphorus in the sludge was accumulated by not only normal oxygen-utilizing PAOs in the oxic tank but also by DNPAOs in the anoxic tank It was suggested that DNPAOs contributed to both phosphorus removal and denitrification

(3) Most of the phosphorus in the sludge was solubilized by ozonation, and a large part

of the solubilized phosphorus consisted of PO4-P Over 90% of the PO4-P was absorbed in the phosphorus adsorption column

REFERENCES

Ahn, J., Daidou, T., Tsuneda, S and Hirata, A (2002a) Characterization of denitrifying phosphate-accumulating organisms cultivated under different electron acceptor conditions using polymerase chain reaction-denaturing gradient gel electrophoresis

assay, Water Res., 36(2), 403-412

Ahn, J., Daidou, T., Tsuneda, S and Hirata, A (2002b) Transformation of phosphorus and relevant intracellular compounds by a phosphorus-accumulating enrichment

culture in the presence of both the electron acceptor and electron donor, Biotechnol

Bioeng., 79(1), 83-93

Chu, L.B., Xing, X.H., Yu, A.F., Zhou, Y.N., Xun, X.L and Jurcik, B (2007) Enhanced

ozonation of dyestuff wastewater by microbubble technology, Chemosphere, 68(10),

1854-1860

Chu, L.B., Yan, S.T., Xing, X.H., Yu, A.F., Sun, X.L and Jurcik, B (2008) Enhanced

sludge solubilization by microbubble ozonation, Chemosphere, 72(2), 205-212

Cui, R and Jahng, D (2004) Nitrogen control in AO process with recirculation of

solubilized excess sludge, Water Res., 38(5), 1159-1172

Ebie, Y., Kondo, T., Kadoya, N., Mouri, M., Maruyama, O., Noritake, S., Inamori, Y

and Xu, K (2008) Recovery oriented phosphorus adsorption process in

decentralized advanced Johkasou, Water Sci Technol., 57(12), 1977-1981

Kamiya, T and Hirotsuji, J (1998) New combined system of biological process and

intermittent ozonation for advanced wastewater treatment, Water Sci Technol.,

38(8-9), 145-153

Kuba, T., van Loosdrecht, M.C.M and Heijnen, J.J (1996) Phosphorus and nitrogen removal with minimal COD requirement by integration of denitrifying

dephosphatation and nitrification in a two-sludge system, Water Res., 30(7),

1702-1710

Kuba, T., van Loosdrecht, M.C.M., Brandse, F.A and Heijnen, J.J (1997) Occurrence

of denitrifying phosphorus removing bacteria in modified UCT-type wastewater

treatment plants, Water Res., 31(4), 777-786

Liu, Y (2003) Chemically reduced excess sludge production in the activated sludge

process, Chemosphere, 50(1), 1-7

Nagare, H, Tsuno, H., Saktaywin, W and Soyama, T (2008) Sludge ozonation and its application to a new advanced wastewater treatment process with sludge

disintegration, Ozone: Science & Engineering, 30, 136-144

Ødegaard, H (2004) Sludge minimization technologies - an overview, Water Sci

Technol., 49(10), 31-40

Sakai, Y., Fukase, T., Yasui, H and Shibata, M (1997) An activated sludge process

without excess sludge production, Water Sci Technol., 36(11), 163-170

Saktaywin, W., Tsuno, H., Nagare, H., Soyama, T and Weerapakkaroon, J (2005) Advanced sewage treatment process with excess sludge reduction and phosphorus

recovery, Water Res., 39(5), 902-910

Soejima K., Oki, K., Terada, A., Tsuneda, S and Hirata, A (2006) Effects of acetate and nitrite addition on fraction of denitrifying phosphate-accumulating organisms

and nutrient removal efficiency in anaerobic/aerobic/anoxic process, Biopro Biosys

Eng., 29(5-6), 305-313

Standard Methods for the Examination of Water and Wastewater (1995) 19th edn,

American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC, USA

Suzuki, Y., Kondo, T., Nakagawa, K., Tsuneda, S., Hirata, A., Shimizu, Y and Inamori,

Y (2006) Evaluation of sludge reduction and phosphorus recovery efficiencies in a new advanced wastewater treatment system using denitrifying polyphosphate

accumulating organisms, Water Sci Technol., 53(6), 107-113

Tsuneda, S., Ohno, T., Soejima, K and Hirata, A (2006) Simultaneous nitrogen and phosphorus removal using denitrifying phosphate-accumulating organisms in a

sequencing batch reactor, Biochem Eng J., 27(3), 191-196

Yasui, H and Shibata, M (1994) An innovative approach to reduce excess sludge

production in the activated sludge process, Water Sci Technol., 30(9), 11-20

Yasui, H., Nakamura, K., Sakuma, S., Iwasaki, M and Sakai, Y (1996) A full-scale operation of a novel activated sludge process without excess sludge production,

Water Sci Technol., 34(3-4), 395-404