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Published online 1 May 2008
Published in J Environ Qual 37:977-982 (2008)
DOI: 10.2134/jeq2007.0361
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Nitrogen and Phosphorus Removal from Domestic Strength Synthetic Wastewater Using an Alternating Pumped Flow Sequencing Batch Biofilm Reactor

Michael Rodgersa, Guangxue Wua,* and Xinmin Zhanb

a Dep. of Civil Engineering and National Centre for Biomedical Engineering Science, National Univ. of Ireland, Galway, Ireland
b Dep. of Civil Engineering, National Univ. of Ireland, Galway, Ireland

* Corresponding author (wuguangxue{at}tsinghua.org.cn).

Received for publication July 9, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Nutrient removal from domestic strength synthetic wastewater by an alternating pumped flow sequencing batch biofilm reactor (APFSBBR) was investigated in this laboratory study. The APFSBBR comprised two reactor tanks (Reactors 1 and 2) with two identical biofilm modules of vertical tubular plastic media with a high specific surface area, one in each tank. The APFSBBR was operated in cycles of four phases: fill, anaerobic, aerobic, and draw. During the fill phase, Reactor 1 was half-filled with domestic strength synthetic wastewater. During the subsequent anaerobic phase, most of the phosphorus release took place from the submerged biofilm in this reactor. In the aerobic phase, the wastewater was circulated by pumps between Reactors 1 and 2, resulting in denitrification at the start of the aerobic phase due to low oxygen concentrations, followed by nitrification and luxury uptake of phosphorus when oxygen concentrations increased. During the draw phase, Reactor 2 was half-emptied of the treated water. At the chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) loading rates on the total biofilm area of 3.20 g COD, 0.33 g TN, and 0.06 g TP m–2 d–1, the removal efficiencies were 97, 85, and 92% for COD, TN, and TP, respectively.

Abbreviations: APFSBBR, alternating pumped flow sequencing batch biofilm reactor • COD, chemical oxygen demand • EBPR, enhanced biological phosphorus removal • MLVSS, mixed liquor volatile suspended solids • PAOs, polyphosphate accumulating organisms • TN, total nitrogen • TP, total phosphorus


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
DISCHARGES of untreated or partially treated wastewaters containing carbon (C), nitrogen (N), and phosphorus (P) into receiving waters can lead to eutrophication. As a result, it is necessary to develop treatment systems that efficiently and economically remove nutrients from these wastewaters. Biological nutrient removal methods have advantages over physical and chemical methods, including low waste sludge production and low capital and operational costs (Kuba et al., 1997). Biofilm wastewater treatment systems, which are characterized by their compactness, simple operation, and easy maintenance, can be more stable in treating wastewaters with high flow and substrate variations than suspended-growth activated sludge systems. In addition, biofilm systems can be more suitable for small-scale wastewater or industrial wastewater treatment than activated sludge systems.

Conventionally, biological P removal, also called enhanced biological P removal (EBPR), is achieved in alternating anaerobic/aerobic conditions, whereas biological N removal is achieved by nitrification–denitrification processes whereby NH4–N is oxidized to NO2–N or NO3–N in an aerobic phase and then the oxidized nitrogen is reduced to N2 in an anoxic phase. In biofilm systems, because of diffusion limitation, anoxic/aerobic conditions can coexist within the biofilm, facilitating simultaneous nitrification and denitrification. Various process strategies have been used to remove C, N, and P from different types of wastewaters. These strategies include (i) a long aeration period for complete nitrification to facilitate the subsequent removal of soluble N by denitrification (Altinbas, 2001; Gieseke et al., 2002); (ii) simultaneous nitrification and denitrification at the early stage of an aerobic phase when the oxygen supply rate is lower than the oxygen consumption rate, resulting in a low dissolved oxygen concentration in the reactor (Altinbas, 2001; Arnz et al., 2001); (iii) selective supply of readily biodegradable organic C to enhance the growth of polyphosphate accumulating organisms (PAOs) for P removal (Arnz et al., 2001; Goncalves and Rogalla, 1992; Helness and Ødegaard, 2001; Morgenroth and Wilderer, 1999); and (iv) regular withdrawal of excess sludge from the system by backwashing or by removing detached biomass during P removal (Li et al., 2003; Morgenroth and Wilderer, 1999).

An alternating pumped flow sequencing batch biofilm reactor (APFSBBR) system has been developed at the National University of Ireland, Galway (Rodgers et al., 2006; Zhan et al., 2006) and comprises two reactor tanks in sequence—each with an identical plastic vertical tubular biofilm module—and is operated as a sequencing batch biofilm reactor system. Two centrifugal pumps, one connected to each reactor tank, move the water from one tank to the other, resulting in the transfer of oxygen from the atmosphere to the biofilm and the reactor fluid. No air compressor is needed for the APFSBBR system. In previous studies by Rodgers et al. (2006) on the APFSBBR system, 93% chemical oxygen demand (COD) removal and 82% total N (TN) removal were achieved.

In this study, the performance of the APFSBBR system for P removal, combined with C and N removal, was investigated. In the fill phase, the influent reactor was half-filled with synthetic wastewater, and the same volume of treated wastewater was withdrawn from the effluent reactor. Phosphorus release occurred in the influent reactor during the subsequent anaerobic phase; denitrification took place at the start of the following aerobic phase when oxygen concentrations were low, and P uptake and nitrification occurred during the aerobic phase as oxygen concentrations increased. Phosphorus release was not affected by the denitrification process in the APFSBBR system because these two processes were kept separate. This study comprised two stages: During the first stage, synthetic wastewater with an influent P concentration of 42.4 mg L–1 was used to evaluate the P removal potential of the APFSBBR; during the second stage, a typical domestic strength wastewater with the influent P concentration of 23.5 mg L–1 was supplied to the system.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Laboratory-Scale APFSBBR System
The APFSBBR system was operated in a temperature-controlled room (11°C). A schematic diagram of the system is shown in Fig. 1 . The system comprised two reactor tanks (Reactors 1 and 2) with two identical vertical tubular PVC biofilm modules (200 x 200 mm in plan and 180 mm high, with a surface area of approximately 2 m2), one in each tank. The APFSBBR was operated with four phases: fill, anaerobic, aerobic, and draw. A complete operational cycle lasted 18.2 h. During the fill phase, a peristaltic pump half-filled Reactor 1 with synthetic wastewater. This phase was followed by a 6-h anaerobic phase during which P release took place, mainly in Reactor 1. After the anaerobic phase was completed, water was circulated between Reactors 1 and 2 with two magnetically coupled centrifugal pumps (Pumps A and B in Fig. 1) positioned underneath the two reactor tanks. The water circulation resulted in oxygen transfer to the biofilms and the wastewaters. At the end of the aerobic phase, most of the treated water resided in Reactor 2. This reactor was half emptied without settling having occurred so that any of detached biofilm biomass suspended in the effluent was removed. A programmable logic controller, in conjunction with water level switches, was used to control the action of pumps and the operational sequence of the APFSBBR system. The operating strategy of the system is given in Table 1 . The reactor tanks and connecting pipes were cleaned of biofilm every 2 wk. The APFSBBR system was seeded with activated sludge from the Tuam Municipal Wastewater Treatment Plant, Galway, Ireland.


Figure 1
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  		Fig. 1. Schematic diagram of the alternating pumped flow sequencing batch biofilm reactor system.

 

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  		Table 1. The operational strategy of the alternating pumped flow sequencing batch biofilm reactor (APFSBBR) system.

 
The experiment lasted 238 d and comprised two stages: Stage 1 (Days 0–107) and Stage 2 (Days 108–238). The influent P concentrations in Stages 1 and 2 were approximately 42.4 mg L–1 and 23.5 mg L–1, respectively. The synthetic wastewater was made from CH3COONa, yeast extract, dried milk, urea, NH4Cl, Na2HPO4·12H2O, KHCO3, NaHCO3, MgSO4·7H2O, FeCl3·6H2O, CaCl2·6H2O, and MnSO4·H2O (Rodgers et al., 2006). The concentrations of influent COD, TN, and total P (TP) are given in Table 2 . The average COD and TN loading rates during this study were 3.20 and 0.33 g m–2 d–1, respectively, based on the biofilm module area. The average TP loading rates were 0.11 g m–2 d–1 during Stage 1 and 0.06 g m–2 d–1 during Stage 2.


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  		Table 2. Overall performance of the alternating pumped flow sequencing batch biofilm reactor unit (SD in parentheses).

 
Batch Experiments
Batch experiments were performed to examine the nitrification, denitrification, and P release rates of the biofilm biomass taken from the two reactor tanks. For these experiments, biofilm biomasses were removed 12 cm from the bottom of the modules in Reactors 1 and 2 at the end of the aerobic phase on Day 230 (Stage 2), suspended in solutions that had the same chemical composition as the synthetic wastewater but without C or N, mixed, and added to 250-mL glass flasks. For nitrification rate analysis, NH4–N was added to the mixed liquor after it was aerated for 30 min. The variation of NH4–N in the mixed liquor was monitored against time. For denitrification rate analysis, the mixed liquor was spiked with acetate and nitrate (NO3–N), and the dynamics of the soluble NO3–N were measured. For P release analysis, the mixed liquor was pulsed with N gas for 10 min and spiked with acetate and NH4–N, after which the concentrations of soluble P in the mixed liquor were monitored. The soluble N and P concentrations (five or more data points) were fitted with linear relationships that were used to calculate biological process rates.

Analytical Methods
Mixed liquor suspended solids, mixed liquor volatile suspended solids (MLVSS), and COD were determined according to standard methods (APHA/AWWA/WPCF, 1995). We tested NH4–N, NO2–N, NO3–N, and PO4–P using a Konelab analyzer (Thermo Clinical Labsystems, Vantaa, Finland). Total N and TP were analyzed with Hach TN and TP kits (Hach, Loveland, CO). Filtration was conducted using Whatmann GF/C filter papers (1.2-µm pore size).


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Performance of the APFSBBR System
The overall performance of the APFSBBR unit in nutrient removal during the two stages is given in Table 2.

Because a settle phase was not included in the operation cycle of the APFSBBR system, concentrations of suspended solids (51 ± 31 mg L–1 during Stage 1 and 98 ± 52 mg L–1 during Stage 2, respectively) were high in the effluent, causing high total effluent nutrient concentrations. Water quality of filtered effluent, however, was very good. The average concentrations of filtered COD (CODf) in the effluent were 46 mg L–1 during Stage 1 (Days 0–107) and 36 mg L–1 during Stage 2 (Days 108–238), giving CODf removals of 96 and 97%, respectively. Complete nitrification occurred in both stages, and the removal efficiency of NH4–N was constantly above 99%. Total N was removed, with efficiencies of 81 and 85% in Stages 1 and 2, respectively. These results are similar to previous studies (Rodgers et al., 2006) and confirm that the APFSBBR system was capable of efficient N and C removal from wastewaters. The TP removal efficiency of the APFSBBR system was 70% during Stage 1 and 92% during Stage 2.

N and P Removal
It is well known that competition for readily biodegradable COD between denitrifiers and PAOs is a problem for simultaneous N and P removal from wastewater, particularly when the influent biodegradable COD is limited. In the present study, P release occurred in Reactor 1 where there was no oxidized N. Consequently, readily biodegradable COD was first used by the PAOs for P release because the competition between denitrifiers and PAOs was avoided. Because anoxic and aerobic conditions can coexist in biofilms, simultaneous nitrification and denitrification possibly occurred during the aerobic phase, particularly at the start of the phase.

The dynamics of N and P in typical cycles in Stages 1 and 2 are shown in Fig. 2 and 3 , respectively. In the anaerobic phase, the amount of P released to the bulk water was approximately 60 mg L–1 during Stage 1 and approximately 20 mg L–1 during Stage 2. In the early stage of the aerobic phase, denitrification occurred even though the oxygen concentration in the bulk water was above 2 mg L–1 (Fig. 3); this could have been due to the probable existence of aerobic, anoxic, and anaerobic conditions simultaneously in the biofilms. This phenomenon was also observed in other studies (Altinbas, 2001; Arnz et al., 2001). For the nitrification process in the aerobic phase, there was a small amount of NO2–N produced during the first 5 to 7 h, but after that, NO2–N and all the NH4–N were oxidized to NO3–N. Hence, an aeration time of about 9 h for this study's conditions—down from the 12 h used in the study—should achieve complete nitrification, which means the nutrient loading rate per day could be increased without adversely affecting the effluent quality. During the aerobic phase, soluble P uptake with time (t) can be described by the following exponential equations: P = 137.9e–0.16t (R2 = 0.99) and P = 770.9e–0.49t (R2 = 0.99) for Stages 1 and 2, respectively, which are similar to equations by Beun et al. (2002) for the degradation of intracellular stored C that mainly controls P uptake.


Figure 2
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  		Fig. 2. Dynamics of filtered NH4–N, NO3–N, and PO4–P in a typical cycle during Stage 1.

 

Figure 3
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  		Fig. 3. Dynamics of filtered NH4–N, NO3–N, and PO4–P in a typical cycle during Stage 2.

 
In the APFSBBR unit, the biofilms in Reactors 1 and 2 experienced different nutrient loading rates, resulting in different microbial communities and corresponding metabolism kinetics in the two reactors. The rates of nitrification, denitrification, and P release in the two reactors (obtained using batch experiments in Stage 2) are given in Fig. 4 . The results showed that biofilm biomasses in both reactors were capable of nitrification, denitrification, and P release. The rates per gram of MLVSS were lower in Reactor 1 than in Reactor 2.


Figure 4
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  		Fig. 4. Nitrification, denitrification, and phosphorus release rates per gram of mixed liquor volatile suspended solids (MLVSS) per hour for the biofilm biomass taken from Reactors 1 and 2 in Stage 2.

 
Effect of Influent P Concentrations on P Removal Efficiency
In Stage 1 of the study, the influent P concentration was 42.4 mg L–1, and the P loading rate was 0.11 g m–2 d–1. In Stage 2, the influent P concentration was changed to 23.5 mg L–1, and the P loading rate was decreased to 0.06 g m–2 d–1. After the operation was shifted from Stage 1 to Stage 2, the soluble effluent P concentration (Peffluent) decreased with time (t in days) (Fig. 5 ), and this decrease can be described by Eq. [1] as follows:

Formula 1[1]
The effluent P concentration was below 1 mg L–1 after 84 d into Stage 2 (Fig. 5). The removed COD/TN/TP ratio was 100:9:2.4 during Stage 1 and 100:9.3:1.7 during Stage 2. This showed less P was removed during Stage 2 even though the COD used was the same. The P release rate during the anaerobic phase decreased when the influent P concentration was changed from 42.4 mg L–1 to 23.5 mg L–1, with average volumetric release rates of 13.5 ± 1.7 mg L–1 h–1 (six tests) and 6.8 ± 2.2 mg L–1 h–1 (seven tests) in Stages 1 and 2, respectively.


Figure 5
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  		Fig. 5. Dynamics of the effluent soluble phosphorus concentration (P) against time (t) during Stage 2.

 
A higher P/C ratio has been used to acclimatize PAOs and consequently induce EBPR (Lin et al., 2003; Liu et al., 1997). Decreasing the P/C ratio from 1:10 to 1:50 has shown that the polyphosphate content in the biomass and the proportion of PAOs to total bacteria decreased (Kong et al., 2002; Liu et al., 2000). The lower P release rate and lower amount of P removed during Stage 2 compared with Stage 1 was probably due to the lower PAO activity under lower influent P conditions.

Excess P Removal in the Biofilm System
Enhanced biological P removal can only be achieved by removing excess sludge with high P content from activated sludge systems. Controlling biomass removal is a key parameter in achieving biological P removal in biofilm systems (Morgenroth and Wilderer, 1999). Rogalla et al. (2006) proposed that a system combining suspended-growth activated sludge with biofilms should be more efficient for P removal. Phosphorus removal, however, can be achieved in biofilm systems by removing detached biofilm biomass from the system or by backwashing (Li et al., 2003; Morgenroth and Wilderer, 1999). In this study, excess P removal was achieved by allowing detached biomass exit from the system during draw-off of the non-settled treated water. There was a good relationship between the effluent mixed liquor suspended solids and the TP in the detached biomass during Stage 2 (Fig. 6 ). The P content in the suspended solids in the effluent was 8.7% in Stage 1 and 7.1% in Stage 2, showing that the P content in the removed biomass was similar to that in EBPR wasted activated sludge, which can be in the range of 4 to 15% (Crocetti et al., 2000; Lin et al., 2003).


Figure 6
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  		Fig. 6. Relationship between the effluent total phosphorus (TP) in the detached biomass and the effluent suspended solid (SS) during Stage 2.

 
The stability of N and P removal in biofilm systems can be greatly affected if too much biomass is removed from the system because the nitrification process, performed by low growing nitrifiers, will be impaired. Also, in one-tank systems, heterotrophs can dominate and inhibit the nitrification process, reducing dependent denitrification rates (van Benthum et al., 1998). The separation of P release from the denitrification, nitrification, and P uptake processes, as in the two-tank APFSBBR system, has operational advantages over activated sludge and sequencing batch biofilm reactor single tank systems. The exposure of the biofilms to the atmosphere—by recirculating the wastewater from reactor to reactor—produces a high oxygen transfer gradient for the biofilm microorganisms. From the batch studies, the surprisingly higher rates of P release in Reactor 2 suggest that the influent and effluent reactors could be alternated beneficially after a period of time, but this needs to be investigated and carefully considered because the nitrification and denitrification rates are also higher in Reactor 2.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
At loading rates of 3.20 g COD m–2 d–1, 0.33 g TN m–2 d–1, and 0.06 g TP m–2 d–1, the removal efficiencies of COD, NH4–N, TN, and TP in the APFSBBR system were 97, 99, 85, and 92%, respectively. The influent P concentration of 42.4 mg L–1 in Stage 1 produced a higher rate of P release in the anaerobic phase than the influent P concentration of 23.5 mg L–1 in Stage 2. There were higher rates (per gram of MLVSS) of nitrification, denitrification, and P release in Reactor 2 than in Reactor 1. The use of the two reactor tanks facilitated the removal of organic C, N, and P and demonstrated the flexibility and simplicity of the APFSBBR system.


   ACKNOWLEDGMENTS
 
This research was supported by the Irish Higher Education Authority through the National Centre for Biomedical Engineering Sciences (NCBES), NUI Galway. The authors are grateful to Edmond O' Reilly for his expertise and technical help.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 





This Article
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Right arrow Articles by Rodgers, M.
Right arrow Articles by Zhan, X.
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