- Email: [email protected]
Potential phosphorus recovery in a WWTP with the BCFS® process: Interactions with the biological process
ARTICLE IN PRESS WAT E R R E S E A R C H
40 (2006) 3507 – 3516
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Potential phosphorus recovery in a WWTP with the BCFSs process: Interactions with the biological process R. Barata,, M.C.M. van Loosdrechtb a
Department of Hydraulic Engineering and Environment, Polytechnic University of Valencia, Camino de Vera s/n. 46022, Valencia, Spain Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands
b
art i cle info
A B S T R A C T
Article history:
The BCFSs process was developed to optimize the activity of denitrifying and P-removing
Received 3 March 2006
bacteria. In this technology in combination with optimal operating conditions for biological
Received in revised form
nitrogen removal, chemical precipitation of phosphorus is used to ensure compliance with
6 August 2006
effluent standards regarding phosphorus. This work addresses the potential of the BCFSs
Accepted 14 August 2006
technology for phosphorus recovery and the interactions with the biological process. The
Available online 29 September 2006
TUD model calibrated for the Hardenberg WWTP was used. Nitrification was the biological
Keywords:
process most influenced by the P stripper operation; however, further research is needed
BCFS
s
technology
into the effect of limiting phosphate concentrations. Phosphate removal in the anaerobic
Biological wastewater treatment
reactor causes a decrease in the sludge poly-P content. The evaluation of the process
Modelling
operation under dynamic conditions showed that the P stripper use for phosphate recovery
Nitrification
does not imply complicated control strategies. The use of the BCFSs for phosphate
Phosphate recovery
recovery implies a change in the design philosophy not only to achieve the effluent requirements but also to maximize the anaerobic phosphate release and thereby recovery. & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Phosphate is the limiting component for growth in most ecosystems. The discharge of phosphates in surface waters leads to eutrophication and blooming of algae. Therefore, it is essential to control the phosphate emissions. Nowadays, biological phosphorus removal (Mino et al., 1998) is often the preferred technology to achieve the effluent standards (typically in the range of 0.5–1 gP m3). However, if COD/P ratios in the influent are too low it is necessary to support the biological activity by chemical precipitation (Kiuru and Rautiainen, 1998; Taka´cs et al., 2005) to achieve the effluent phosphorus (P) requirements. The efficient combination of chemical and biological phosphorus removal was one of the reasons for the development of the BCFSs process (Biological–chemical phosphorus and nitrogen removal, van Loosdrecht et al. (1998)). In this technology in
combination with optimal operating conditions for biological nitrogen removal, chemical precipitation of phosphorus is used to ensure compliance with effluent standards regarding phosphorus. The key issue is where to carry out this chemical precipitation. Precipitation at high phosphate concentration (end of anaerobic phase) is preferential since it minimizes coprecipitation of other minerals. If the chemicals dose takes place in the sludge reactor, precipitates will accumulate in the activated sludge. This will lead to a lower sludge age and consequently higher sludge production and lower nitrogen removal efficiencies. Therefore, the phosphate is precipitated in a separate reactor after a sludge–water separation in a simple integrated settler in the anaerobic zone. This technology presents another opportunity, not studied until now, which besides achieving the phosphate effluent standards also recovers phosphorus from wastewater (Doyle and Parsons, 2002). The aim of this paper is to study the
Corresponding author. Tel.: +34 963879618; fax: +34 963877617.
E-mail addresses: [email protected] (R. Barat), [email protected] (M.C.M. van Loosdrecht). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.08.006
ARTICLE IN PRESS 3508
WAT E R R E S E A R C H
40 (2006) 3507– 3516
potential of phosphorus recovery in a WWTP operated for biological COD, N and P removal with the BCFSs technology and the interactions of this phosphate stripping with the biological process. As an example we will use the Hardenberg WWTP (Meijer et al., 2001) since a calibrated model was already available for this treatment plant. The study was based on the Hardenberg WWTP in the Netherlands, which included:
upgraded to a BCFSs process (Fig. 1), which is a modified UCT type design. Typical process characteristics are SVI below 100 ml g1 without addition of chemicals, high activity of denitrifying EBPR and effluent orthophosphate below 0.5 gP m3. The chemical phosphate stripper, as shown in Fig. 1, was not built, however optional. An elaborate description of BCFS and the upgrading philosophy can be found in van Loosdrecht et al. (1998).
Hardenberg WWTP model implementation in the simula2.1.
tion software AQUASIM.
Evaluation of the implemented model. Study of the potential phosphorus recovery and the interactions with the biological process for different scenarios: J Steady state simulations: different phosphate stripper flow, sludge loading rate and temperature. J Dynamic simulations: influent flow and wastewater composition variables throughout two weeks of dry weather.
2.
Methods
WWTP Hardenberg is one of the seven WWTPs managed by the Dutch water board ‘‘Groot Salland’’. In 1998, it was
Water line
The WWTP includes an anaerobic reactor (R1) and a selector reactor (S) both with plug-flow characteristics, an anoxic reactor (R2) supposed to be completely mixed, followed by two oxidation ditches (R3 and R4) in line (Fig. 1a). R3 is only aerated if the dissolved oxygen (DO) in R4 drops below a certain set point (typically 0.5 gO2 m3) and R4 was fully aerated. The anaerobic tank, the selector and the anoxic tank are configured in concentric rings (Fig. 1b). The wastewater is fed into the anaerobic tank together with the recycle stream from the anoxic tank (QA), after passing through a screen and grit chamber. After this, the sludge/water mixture flows to the selector (contact tank) and the anoxic tank, and from there to the aeration reactors (not shown). In simulation the oxygen concentration in R3 and R4 (DOR3 and DOR4, respectively) was controlled on, respectively, 0.6 and 2.8 gO2 m3 (Meijer et al., 2001). In the clarifiers (CL), activated sludge was separated
QA
QC CL
Influent
R1
S
R2
R3 QB
P recovery
QRAS Phosphate Stripper
(a)
Effluent
R4
Anaerobic
WAS
Overflow thickener
Anoxic
Anoxic/Aerated
Aerated
Settler
Thickener Anaerobic reactor (R1) Selector (S) Influent
QA
Anoxic reactor (R2) R3
Return sludge
QB (b) Fig. 1 – WWTP Hardenberg. (a) Schematic layout, adapted from Meijer et al. (2001) and (b) configuration of the anaerobic–anoxic reactors.
ARTICLE IN PRESS WAT E R R E S E A R C H
from the liquid phase. Cyclic process conditions were provided by the return activated sludge (RAS) from the clarifier underflow (QRAS) and three recycle pump flows QA, QB and QC. During the record period for model calibration, dry weather influent caused QA, QB, QC and QRAS to be more or less constant. In Table 1 operational data and reactor volumes are listed.
2.2.
Sludge line
The excess sludge is discharged from the return stream and thickened in a gravity thickener. The thickened sludge is pumped to a sludge equalization tank, where further thickening takes place. From the equalization tank the sludge is dewatered in a filter press.
2.3.
3509
4 0 (200 6) 350 7 – 351 6
Influent characterization
Influent was measured with flow proportional sample collector. During the recorded period no rain events occurred. Concentration variations in the measured influent were relatively small. Variations in the influent loads (concentration flow), therefore, were primarily caused by flow dynamics. To decrease simulation time, the influent was modelled with average concentrations and actual flow data. The influent characterization was performed according to Roeleveld and Van Loosdrecht (2001). Table 2 shows the
characterized model influent composition obtained from the average influent data (not shown).
2.4.
The Hardenberg WWTP model
All simulations were performed with the TUD model (Meijer, 2004). AQUASIM 2.0 was used as simulation platform (Reichert, 1994). In AQUASIM models are constructed by connecting completely stirred reactors (CSTRs) in a desired process configuration. The WWTP was modelled according to the flow scheme in Fig. 1 and data from Table 1. R1 was modelled as three CSTRs in series. S and R2 were modelled as single CSTRs. R3 and R4 were modelled as six CSTRs with internal recycle flows (QR3IN and QR4IN). With this set-up, measured longitudinal oxygen gradients in R3 and R4 were simulated sufficiently. CL separated particulate matter from the liquid phase ideally. A residual effluent solids concentration (XEF) was modelled as a percentile loss of the RAS. Biological conversions in the clarifier were simulated by a non-aerated CSTR placed in the RAS flow. DOR3 and DOR4 were controlled on respectively 0.6 and 2.8 gO2 m3. In the model, QA, QB, QC and QRAS were constant. The particulate COD (XR4) was controlled on 4700 gCOD m3 by regulating the waste sludge flow (QW) in order to simulate the manual operation by the plant operators. This control was implemented in AQUASIM as a proportional integral controller (Eq. (1)), where QWt and QW0 are the waste sludge
Table 1 – Operational data and hydraulic parameters of the WWTP used for model calibration (Meijer et al., 2001) Flow
Average7SD, m3 d1
Process unit
Volume, m3
Depth, m
HRT, h
685572387 102a 673872516 219a 22,415a 44,536a 59 17473021 1.2 Qina 635,040
Anaerobic reactor, R1 Selector reactor, S Anoxic reactor, R2 Alternate aerated carrousel, R3 Aerated carrousel, R4 Clarifier, CL (2 2625 m3) Sludge blanket volume
1480 740 2290 4190 4190 5250 150b
5.0 5.0 5.0 2.5 2.5 — —
1.2 0.5 0.7 0.7 0.7 3.4 0.4
Total WWTP
18,140
—
62.6
Influent, QIN Overflow thickener to R2, QOF Effluent, QEF Waste activated sludge, QW Pump flow A, QA Pump flow B, QB Pump flow C, QC Recycle activated sludge, QRAS Internal recycles, QR4 in and QR3 in a b
Estimated from mass balances. Estimated from denitrification in the RAS.
Table 2 – Characterized model influent calculated according to Roeleveld and van Loosdrecht (2001) Soluble compounds Dissolved oxygen Fermentable COD Fatty acid Ammonium Nitrate Orthophosphate Inert COD Alkalinity, HCO3
So SF SA SNH SNO SPO SI SHCO
Value
Unit
0 109 91 52 0 3.7 40 8
gO2 m3 gCOD m3 gCOD m3 gN m3 gN m3 gP m3 gCOD m3 mole l1
Particulate compounds Inert COD Solid COD Autotrophic micro-organisms Heterotrophic micro-organisms P-accumulating micro-organisms Poly-phosphate Ply-hidroxybutirate Glycogen
XI XS XA XH XPAO XPP XPHA XGLY
Value
Unit
174.8 188.7 0.01 0.01 0.01 0.001 0.001 0.001
gCOD m3 gCOD m3 gCOD m3 gCOD m3 gCOD m3 gP m3 gCOD m3 gCOD m3
ARTICLE IN PRESS 3510
WAT E R R E S E A R C H
40 (2006) 3507– 3516
flow at time t and initial respectively; XR4t is the simulated suspended COD at time t; XR4sp is the suspended COD set point and KP and KI are the proportional and integral constants, respectively. In this controller the control command to the waste sludge flow (QWt) at any time t is proportional to the deviation of the controlled variable (XR4) from the set point and to the sum of all set-point deviations. t X ½KI ðXR4t XR4sp Þ. Q Wt ¼ Q W0 þ KP XR4t XR4sp þ
Evaluation results
Fig. 2 shows the simulation results compared with the experimental values obtained in R1 (Fig. 2a) and R2 (Fig. 2b). As can be seen good agreement was obtained between simulated and experimental values confirming the good model performance. Table 3 shows the model parameters calibrated by Meijer et al. (2001) with values differing from the TUD Model default ones.
(1)
t¼0
3.
3.1.
3.2.
Results
After implementing the Hardenberg model in AQUASIM we evaluated the model performance. For the evaluation purpose we used the wastewater characterization and the Hardenberg calibrated model as described by Meijer et al. (2001).
Phosphate recovery study
After implementation of the model and evaluation with the experimental data, the P-stripper unit was introduced in order to evaluate by simulation the potential phosphorus recovery and the interactions between this recovery and the biological process. For this study all simulations were performed on the basis of the calibrated simulated pseudo steady state model of WWTP Hardenberg. The simulations were divided into two parts: steady state simulations, considering constant influent flow and concen-
50
5000 4000
35 30
3000
25 20
2000
15 10
1000
(gCOD m-3)
Phosphate (gP m-3)
40
Total suspended solids
45
5 23/06/1998 21:00
Ammonium (gN m-3)
(a)
25/06/1998 9:00
0 25/06/1998 21:00
12
24
10
20
8
16
6
12
4
8
2
4
0 23/06/1998 9:00
(b)
24/06/1998 24/06/1998 9:00 21:00 Date/Time
23/06/1998 21:00
24/06/1998 24/06/1998 9:00 21:00 Date/Time
25/06/1998 9:00
Phosphate (gP m-3)
0 23/06/1998 9:00
0 25/06/1998 21:00
Fig. 2 – Dynamic measurements and simulations: (a) actual measurements in the anaerobic reactor R1 of phosphate (white dots) and suspended solids (black dots), simulation of the calibrated model is plotted in lines and (b) actual measurements in the anoxic reactor R2 of ammonium (black dots) and phosphate (white dots), simulation of the calibrated model is plotted in lines.
ARTICLE IN PRESS WAT E R R E S E A R C H
3511
4 0 (200 6) 350 7 – 351 6
Table 3 – Hardenberg WWTP calibrated parameters values (Meijer et al., 2001) Parameter KN,PO KO gPP ZPNO
Description
Unit
Calibrated
TUD model
Saturation coefficient for phosphate (nutrient) Inhibition coefficient for oxygen (fermentation and anoxic growth) Saturation reduction factor for PP formation Anoxic PAO reduction factor
gP m3 gO2 m3 — —
0.001 0.7 0.25 0.8
0.01 0.2 0.1 0.5
trations; dynamic simulations, considering variable influent flow and concentrations.
3.3.
Steady state simulations
The following parameters were varied in this study:
Fraction of the influent flow diverted from the anaerobic reactor (R1) to the P-stripper (fstripper) (see Fig. 1a): 0–1.
Sludge loading rate (SLR): 0.042–0.167 kgCOD kgVSS1 d1,
corresponding to an influent flow between 4000 and 16,000 m3 d1, respectively. Temperature: 10–20 1C.
The potential phosphate recovery was studied by means of the total phosphorus mass balance. The amount of phosphorus content in the effluent (Peff), waste activated sludge (Pwas) and effluent stripper (Precovery) streams were calculated as a percentage of the influent total phosphate load. To evaluate the P-stripper effect on the biological process, the next effluent requirements were used: total nitrogen (TN) o10 gN m3, total phosphorus (TP) o1 gP m3 and complete nitrification (NH+4 o1 gNH+4 N m3). Fig. 3 shows the results of the steady state simulation at different sludge loading rate and fraction of inflow diverted to the phosphate stripper (fstripper). As can be seen in Fig. 3a and c, TP and TN concentration remain below the effluent requirements (o1 gP m3 and o10 gN m3) independent of the stripper flow rate. However, at low load conditions and without the stripper working a significant deterioration in the phosphate removal can be observed. This is due to the low load and, therefore, high SRT at constant sludge content in the reactors. Under these conditions the phosphate uptake is limited by a maximum poly-P content in the sludge. Fig. 3d shows a significant increase in the effluent ammonia concentration at higher fstripper values. This effect is more significant at high SLR, resulting in concentrations above the limits. This deterioration is due to the low phosphate concentration that inhibits the autotrophic organisms growth and hence the nitrification. However, the value of the phosphate half saturation constant for nitrifiers growth (KP) is not well known and the model results are very sensitive to this parameter value. The potential phosphate recovery results (the fraction of influent phosphorus that is in the stripper outflow, Fig. 3e) show that the higher fstripper value the higher potential phosphate
recovery. However, this phosphate recovery is limited by its effect on the biological process performance regarding the effluent limits. The P-stripper overuse causes a phosphate limitation in the downstream biological processes. The bold line represents the highest phosphate recovery achieved at each SLR accomplishing the effluent limits. Therefore, the information provided in this graph can be used to evaluate the operation of a stripper to maximize the phosphate recovery without affecting the biological process. As can be seen, it is possible to recover up to 70% of the influent phosphate at low SLR. However, this potential phosphate recovery is reduced at high SLR due to the important deterioration of the biological process. At high SLR the ammonia effluent concentration is higher because of the ammonia overload. Nevertheless, for phosphate recovery purposes the soluble phosphate concentration in the stripper flow is also very relevant (Fig. 3b). Fig. 3b shows the phosphate concentration in the anaerobic reactor at different fstripper and SLR. The bold line represents the orthophosphate concentration that corresponds to the high phosphate recovery achieved at each SLR. As can be seen, the orthophosphate concentration achieved at high SLR is 25 gP m3 opposite to the 13 gP m3 achieved at low SLR. It was noticed that the concentration of fermentable substrate (SF) and particulate organic matter (XS) at the end of the anaerobic reactor remains quite high (around 30 and 80 gCOD m3, respectively). Therefore, in order to increase the amount of phosphate released to maximize the phosphate recovery is important to optimize the anaerobic process, that is, to increase the XS hydrolysis and volatile fatty acids production by anaerobic fermentation. This could be obtained introducing the fermentation–elutriation process (Bouzas et al., 2002) or increasing the anaerobic zone. Fig. 4 shows the effect of the anaerobic reactor volume on the amount of phosphate released during the anaerobic phase and on the effluent composition. The anaerobic reactor volume increase intensifies the hydrolysis and fermentation without deteriorating the effluent composition. Fig. 5a shows the temperature effect on the stripper performance. The increase of temperature stimulates the biological activity thereby increasing the phosphate release in the anaerobic reactor by polyphosphate accumulating organisms (PAOs), which increases the potential P recovery. The increase of fstripper (removing phosphate) causes a significant reduction in the ratio Poly-P/PAO. At low Poly-P/PAO ratio this effect is naturally less signifficant. The temperature has also an important effect on the biological process, with deterioration of the nitrification process at low temperatures
ARTICLE IN PRESS 3512
WAT E R R E S E A R C H
40 (2006) 3507– 3516
Fig. 3 – Steady state simulations at different fstripper and SLR values. (a) effluent total phosphate, (b) phosphate concentration in the anaerobic reactor, (c) effluent total nitrogen, (d) effluent ammonia, (e) potential phosphate recovery and (f) fraction of the influent phosphate in the waste sludge.
(Fig. 5b and d) because of the influence of temperature on nitrifiers. Therefore, temperature increase allows improving the stripper performance for phosphate recovery
owing to the increase of phosphate concentration in the stripper combined with an improvement of the biological nitrogen removal.
ARTICLE IN PRESS WAT E R R E S E A R C H
3513
4 0 (200 6) 350 7 – 351 6
4
35
VR1 = VR1o × f
3.5
30
VR1o = 1480 m3
3
25
2.5
20
2
15
1.5
10
1
5
NH4+ eff - TP eff (g m-3)
PO4+ R1 - TNeff (g m-3)
40
0.5
0
0 0.5
1
1.5
2
2.5
3
3.5
f PO4 R1
TN eff
NH4 eff
TP eff
Fig. 4 – Effect of the anaerobic reactor R1 volume on the phosphate released and effluent concentrations.
30.0
45 40
22.0
35
20.0
30
18.0
25 20
-
16.0
15
14.0
10
-
12.0
P recovery (%)
50
24.0
NH4+ effluent (gN m-3)
55
+
T
26.0
PO4 R1 (gPm-3)
6.0
60
+
28.0
-
4.0 3.0
T
2.0 1.0
5
10.0 0.05
0.1
0.15
(a)
0.2
0.25
0.3
0.35
0.4
0.45
+
0.0
0 0
5.0
0.5
0
0.05
0.1
0.15
0.2
(b)
f stripper
0.25
0.3
0.4
0.45
0.5
-
12.0
1.2
0.35
f stripper
Total N effluent (gN m-3)
Total P effluent (gPm-3)
11.0
1.0 0.8
0.6
T 0.4
+
0.2
10.0
T
9.0 8.0 7.0 6.0 5.0
+
4.0 3.0 2.0 1.0 0.0
0.0 0
0.05
0.1
0.15
(c)
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0
0.05
0.1
0.15
(d)
f stripper
T=10°C
T=13°C
0.2
0.25
0.3
0.35
0.4
0.45
0.5
f stripper
T=17°C
T=20°C
Fig. 5 – Temperature effect on: (a) the potential phosphate recovery and orthophosphate concentration achieved in the anaerobic reactor, (b) ammonia effluent concentration, (c) total P effluent concentration and (d) total N effluent concentration.
3.4.
Dynamic simulations
3.4.1.
Influent data
The influent data for dynamic simulations used in this work were the ones proposed in the COST 624 benchmark by Vanhooren and Nguyen (1996). These influent data was rescaled up or down from the proposed values to the actual average Hardenberg WWTP data. The values were scaled by performing a linear regression keeping the proportional average deviations. We used two weeks of dry weather influent flow and water composition. Variations in the hydraulic loading from Mon-
day to Friday were decreased by 3% per day. The influent during the weekend was also reduced by 15% from the flow during the weekday. The resulting influent flow is depicted in Fig. 6. For the particulate components (Fig. 6a), it was assumed that the suspended solids concentration increases with high hydraulic loading because resuspension of solids occurs during high flows (Verbanck, 1995). As a result the influent suspended solids concentration follows the hydraulic loading variations that occur during the week. The influent soluble substrate data are depicted in Fig. 6b. The volatile fatty acids (SA) and phosphate (SPO4) profiles were
ARTICLE IN PRESS 3514
WAT E R R E S E A R C H
40 (2006) 3507– 3516
Influent Flow (/10) - Particulate Components
Qin/10 (m3 d-1) - XC (gCOD m-3)
1200 1000 800 600 400 200 0 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time (d) Xi
(a)
Xs
Qin
Soluble Components 225 200 175
SC (g/m3)
150 125 100 75 50 25 0 0
1
2
3
4
(b)
5
Sf
6
7 Time (d)
Sa
8
Snh4
9
10
11
12
13
14
Spo4
Fig. 6 – (a) Influent flow scaled to the actual average flow (6855 m3 d1) and influent particulate concentrations scaled to the actual average concentration: XI ¼ 174.8 gCOD m3, XS ¼ 188.7 gCOD m3 and (b) Influent readily fermentable substrate, volatile fatty acids, ammonia and phosphate scaled to the actual average concentration: SF ¼ 109 gCOD m3, SA ¼ 91 gCOD m3, SNH4 ¼ 52 gN m3, SPO4 ¼ 6.6 gP m3.
considered to be proportional to fermentable substrate (SF) and ammonia (SNH4), respectively.
3.4.2.
Phosphate recovery under dynamic conditions
After the study of the potential phosphate recovery at steady state, different control strategies under dynamic conditions were evaluated. This study was performed with the influent composition showed above, average inflow of 10,000 m3 d1 and 18 1C. The stripper operation strategies were:
Case 1: without control, constant Q stripper proportional to the
phate concentration in the anaerobic reactor (PO4 R1 ): f stripper 0 ¼ 0:45, Kp ¼ 0:03; f stripper ¼ f stripper 0 þ KP ðPO4 R1t PO4 R1 min Þ.
average inflow Q in , Q stripper ¼ f stripper Q in : fstripper ¼ 0.45. Case 2: variable Qstripper proportional to Qin, Q stripper ¼ f stripper Q in , with constant f stripper : f stripper ¼ 0:45. Case 3: variable Qstripper proportional to Qin, Q stripper ¼ f stripper Q in , with variable f stripper as function of the phos-
Table 4 and Fig. 7 show the dynamic simulation results of different control strategies. As can be seen without stripper control, working at fixed flow (Case 1) the potential phosphate recovery is 48.6%. In this case the phosphate load in the stripper (Fig. 7) varies due to the influent phosphate load fluctuations. The introduction of a variation in the stripper flow, proportional to the influent phosphate load (Case 2), caused a slight increase in the potential phosphate recovery (50.7%). Finally, the introduction of fstripper variation (Case 3) led to increase the potential phosphate recovery to 55.7%. In this case the fstripper was changed with time in order to maximize stripper flow at high phosphate concentration in the anaerobic reactor. As can be
ARTICLE IN PRESS
seen in Fig. 7, with this stripper control the amount of potential phosphate recovery increases at high phosphate concentration in R1 and decreases at low phosphate concentration.
4.
Discussion
The steady state simulations at different SLR and fstripper values showed the potential use of the BCFSs technology to recover phosphate. The results suggest that the biological process most influenced by the P-stripper operation might be the nitrification due to phosphate limitation. However, further research is needed due to the lack of knowledge of the nitrification process under limiting phosphate concentration. There are indications in literature that nitrifying bacteria can accumulate polyphosphate (Terry and Hooper, 1970; Eigener and Bock, 1972; Chain et al., 2003); however, their role in the metabolism is not well studied. It might, therefore, be that nitrifying bacteria will
Table 4 – Dynamic simulations results under different P stripper control strategies %P recoverya
Case
1 2 3
Average P load in the stripper flow (kgP d1)
48.6 50.7 55.7
3515
4 0 (200 6) 350 7 – 351 6
60.6 63.2 69.4
a
Potential phosphate recovery in the stripper calculated as a percentage of the influent phosphate.
respond to temporary absence of phosphate (final stage of aerobic phase) by accumulating polyphosphate in the presence of phosphate (early stage of aerobic phase). This could be included relatively simply in the present activated sludge models. For simplicity one could also leave out the P-limitation for nitrifying growth, assuming that there is always enough phosphate for growth of the nitrifiers in normal wastewater and that the mass of phosphate taken up by nitrifiers in activated sludge systems is insignificant. Increasing the effluent standards demands to further investigate the effect of P-limitations in EBPR systems on nitrification. Furthermore, the simulation at low load showed that under these conditions the P-stripper can be used as originally was envisaged, that is, to assist the biological process in order to achieve the effluent TP legal requirements. Therefore, the BCFSs technology can be potentially used to assist the biological process and to recover phosphate from wastewater. Besides the phosphorus recovery, this new use of the BCFSs technology has another advantage. The phosphate removal in the anaerobic reactor causes the decrease of the phosphorus content as poly-P in the sludge (Fig. 3f). Therefore during the downstream sludge treatment (anaerobic and aerobic sludge digestion) less phosphate is released decreasing the possible phosphate precipitation that can cause very important problems (Doyle and Parsons, 2002). According to Fig. 3e, the recovery of phosphate presents two possibilities: higher phosphate recovery in a stream with low phosphate concentration or lower phosphate recovery in a stream with high phosphate concentration. The WWTP operators must decide the P-stripper operation depending on the technology used to recover phosphate.
35
3.00E+05
30
2.50E+05
25
2.00E+05
20
1.50E+05
15
1.00E+05
10
5.00E+04
5
gP d-1
3.50E+05
0.00E+00
0 365
367
369
371
373
375
377
379
Time (d)
P influent
P recovery, Case 1
P recovery, Case 2
P recovery, Case 3
Fig. 7 – Potential phosphate recovery under different P-stripper control strategies.
PO4 R1
PO4-3 R1 (gP m-3)
WAT E R R E S E A R C H
ARTICLE IN PRESS 3516
WAT E R R E S E A R C H
40 (2006) 3507– 3516
High temperatures (Fig. 5a) cause the increase of the amount of phosphate released during the anaerobic stage. Hence, the potential phosphate recovery will be higher during summer season than during low temperatures periods. On the other hand, anaerobic zone volume (Fig. 4) increase allows maximizing the amount of phosphate recovery at the same P-stripper operation conditions. Furthermore, pre-fermentation of primary or return sludge (Bouzas et al., 2002; Jo¨nsson and Jansen, 2005; Vollertsen et al., 2005) could be an interesting operation strategy to increase the readily biodegradable COD in order to have extra phosphate release. Therefore, the use of phosphate recovery implies a change in the design philosophy not only to achieve the effluent requirements but also to maximize the anaerobic phosphate release and thereby recovery. The evaluation of the P-stripper operation under dynamic conditions showed that the introduction of control strategies that try to increment the stripper flow at high influent phosphate load and high phosphate concentration in the anaerobic reactor (Cases 2 and 3), increase the potential of phosphate recovery (Table 4). However, there are not significant variations in the amount of phosphate recovery between the simulated control strategies. This is most likely due to the fact that the most dominant factor will be the Prelease, which depends on the influent load. These results suggest that the P-stripper use for phosphate recovery does not imply complicated control strategies, which is an important aspect from an operational point of view.
5.
Conclusions
This study indicated the potential of P-recovery in a biological P-removal plant. Approx. 60% of the influent load can be recovered with a simple adaptation of the process. Complex process control does not seem to be able to improve results. The potential limitation of phosphate in the aerobic phase for especially nitrifiers needs further investigation.
Acknowledgements This research work has been supported by the Conselleria de Empresa, Universidad y Ciencia de la Generalitat Valenciana ´ rea de Polı´tica Cientı´fica y Tranferencia Tecnolo´gica), which (A is gratefully acknowledged. Special acknowledgements to the Department of Biotechnology of the Faculty of Applied Sciences, TU Delft. R E F E R E N C E S
Bouzas, A., Gabaldon, C., Marzal, P., Penya-Roja, J.M., Seco, A., 2002. Fermentation of municipal primary sludge: effect of SRT
and solids concentration on volatile fatty acid production. Environ. Technol. 23 (8), 863–875. Chain, P., Lamerdin, J., Larimer, F., Regala, W., Lao, V., Land, M., Hauser, L., Hooper, A., Klotz, M., Norton, J., Sayavedra-Soto, L., Arciero, D., Hommes, N., Whittaker, M., Arp, D., 2003. Complete genome sequence of the ammonia-oxidizing bacterium and obligate chemolithoautotroph Nitrosomonas europaea. J. Bacteriol. 185 (9), 2759–2773. Doyle, J.D., Parsons, S.A., 2002. Struvite formation, control and recovery. Water Res. 36 (16), 3925–3940. Eigener, U., Bock, E., 1972. Auf- und abbau der polyphosphatfraktion in zellen von Nitrobacter winogradskyi buch. Arch. Microbiol. 81 (4), 367–378. Jo¨nsson, K., Jansen, J.C., 2005. Hydrolysis of return sludge for production of easily biodegradable carbon—effect of pretreatment, sludge age and temperature. Conference proceedings of the IWA Specialized Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Krakow (Poland). Kiuru, H.J., Rautiainen, J.A., 1998. Biological nutrient removal at a very low-loaded activated sludge plant with high biomass concentrations. Water Sci. Technol. 38 (1), 63–70. Meijer, S.C.F., van Loosdrecht, M.C.M., Heijnen, J.J., 2001. Metabolic modelling of full-scale biological nitrogen and phosphorus removing WWTP’s. Water Res. 35 (11), 2711–2723. Meijer, S.C.F., 2004. Theoretical and practical aspects of modelling activated sludge process. Thesis, Technical University of Delft, The Nederlands. Mino, T., van Loosdrecht, M.C.M., Heijnen, J.J., 1998. Microbiology and biochemistry of the enhanced biological phosphate removal process. Water Res. 32 (11), 3193–3207. Reichert, P., 1994. Aquasim—a tool for simulation and dataanalysis of aquatic systems. Water Sci. Technol. 30, 21–30. Roeleveld, P.J., Van Loosdrecht, M.C.M., 2001. Experiences with guidelines for wastewater characterisation in The Netherlands. Water Sci. Technol. 45 (6), 145–156. Taka´cs, I., Murthy, S., Smith, S., McGrath, M., 2005. Chemical phosphorus removal to extremely low levels-experience of two plants in the Washington, D.C. area. IWA Specialized Conference: Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Krakow (Poland). Terry, K.R., Hooper, A.B., 1970. Polyphosphate and orthophosphate content of Nitrosomonas europaea as a function of growth. J. Bacteriol. 103 (1), 199–206. Vanhooren, H., Nguyen, K., 1996. Development of a simulation protocol for evaluation of respirometry-based control strategies. COST 624 Technical Report. Van Loosdrecht, M.C.M., Brandse, F.A., de Vries, A.C., 1998. Upgrading of wastewater treatment processes for integrated nutrient removal—the BCFSs process. Water Sci. Technol. 37 (9), 209–217. Verbanck, M., 1995. Variabilte´ des charges solides en suspension a` l’exutoire des re´seaux de collecte. Journe´e d’e´tude CB-IAWQ, Lie`ge. Vollertsen, J., Petersen, G. Borregaard, V.R., 2005. Hydrolysis and fermentation of activated sludge to enhance biological phosphorus removal. Conference proceedings of the IWA Specialized Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Krakow (Poland).
40 (2006) 3507 – 3516
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Potential phosphorus recovery in a WWTP with the BCFSs process: Interactions with the biological process R. Barata,, M.C.M. van Loosdrechtb a
Department of Hydraulic Engineering and Environment, Polytechnic University of Valencia, Camino de Vera s/n. 46022, Valencia, Spain Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands
b
art i cle info
A B S T R A C T
Article history:
The BCFSs process was developed to optimize the activity of denitrifying and P-removing
Received 3 March 2006
bacteria. In this technology in combination with optimal operating conditions for biological
Received in revised form
nitrogen removal, chemical precipitation of phosphorus is used to ensure compliance with
6 August 2006
effluent standards regarding phosphorus. This work addresses the potential of the BCFSs
Accepted 14 August 2006
technology for phosphorus recovery and the interactions with the biological process. The
Available online 29 September 2006
TUD model calibrated for the Hardenberg WWTP was used. Nitrification was the biological
Keywords:
process most influenced by the P stripper operation; however, further research is needed
BCFS
s
technology
into the effect of limiting phosphate concentrations. Phosphate removal in the anaerobic
Biological wastewater treatment
reactor causes a decrease in the sludge poly-P content. The evaluation of the process
Modelling
operation under dynamic conditions showed that the P stripper use for phosphate recovery
Nitrification
does not imply complicated control strategies. The use of the BCFSs for phosphate
Phosphate recovery
recovery implies a change in the design philosophy not only to achieve the effluent requirements but also to maximize the anaerobic phosphate release and thereby recovery. & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Phosphate is the limiting component for growth in most ecosystems. The discharge of phosphates in surface waters leads to eutrophication and blooming of algae. Therefore, it is essential to control the phosphate emissions. Nowadays, biological phosphorus removal (Mino et al., 1998) is often the preferred technology to achieve the effluent standards (typically in the range of 0.5–1 gP m3). However, if COD/P ratios in the influent are too low it is necessary to support the biological activity by chemical precipitation (Kiuru and Rautiainen, 1998; Taka´cs et al., 2005) to achieve the effluent phosphorus (P) requirements. The efficient combination of chemical and biological phosphorus removal was one of the reasons for the development of the BCFSs process (Biological–chemical phosphorus and nitrogen removal, van Loosdrecht et al. (1998)). In this technology in
combination with optimal operating conditions for biological nitrogen removal, chemical precipitation of phosphorus is used to ensure compliance with effluent standards regarding phosphorus. The key issue is where to carry out this chemical precipitation. Precipitation at high phosphate concentration (end of anaerobic phase) is preferential since it minimizes coprecipitation of other minerals. If the chemicals dose takes place in the sludge reactor, precipitates will accumulate in the activated sludge. This will lead to a lower sludge age and consequently higher sludge production and lower nitrogen removal efficiencies. Therefore, the phosphate is precipitated in a separate reactor after a sludge–water separation in a simple integrated settler in the anaerobic zone. This technology presents another opportunity, not studied until now, which besides achieving the phosphate effluent standards also recovers phosphorus from wastewater (Doyle and Parsons, 2002). The aim of this paper is to study the
Corresponding author. Tel.: +34 963879618; fax: +34 963877617.
E-mail addresses: [email protected] (R. Barat), [email protected] (M.C.M. van Loosdrecht). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.08.006
ARTICLE IN PRESS 3508
WAT E R R E S E A R C H
40 (2006) 3507– 3516
potential of phosphorus recovery in a WWTP operated for biological COD, N and P removal with the BCFSs technology and the interactions of this phosphate stripping with the biological process. As an example we will use the Hardenberg WWTP (Meijer et al., 2001) since a calibrated model was already available for this treatment plant. The study was based on the Hardenberg WWTP in the Netherlands, which included:
upgraded to a BCFSs process (Fig. 1), which is a modified UCT type design. Typical process characteristics are SVI below 100 ml g1 without addition of chemicals, high activity of denitrifying EBPR and effluent orthophosphate below 0.5 gP m3. The chemical phosphate stripper, as shown in Fig. 1, was not built, however optional. An elaborate description of BCFS and the upgrading philosophy can be found in van Loosdrecht et al. (1998).
Hardenberg WWTP model implementation in the simula2.1.
tion software AQUASIM.
Evaluation of the implemented model. Study of the potential phosphorus recovery and the interactions with the biological process for different scenarios: J Steady state simulations: different phosphate stripper flow, sludge loading rate and temperature. J Dynamic simulations: influent flow and wastewater composition variables throughout two weeks of dry weather.
2.
Methods
WWTP Hardenberg is one of the seven WWTPs managed by the Dutch water board ‘‘Groot Salland’’. In 1998, it was
Water line
The WWTP includes an anaerobic reactor (R1) and a selector reactor (S) both with plug-flow characteristics, an anoxic reactor (R2) supposed to be completely mixed, followed by two oxidation ditches (R3 and R4) in line (Fig. 1a). R3 is only aerated if the dissolved oxygen (DO) in R4 drops below a certain set point (typically 0.5 gO2 m3) and R4 was fully aerated. The anaerobic tank, the selector and the anoxic tank are configured in concentric rings (Fig. 1b). The wastewater is fed into the anaerobic tank together with the recycle stream from the anoxic tank (QA), after passing through a screen and grit chamber. After this, the sludge/water mixture flows to the selector (contact tank) and the anoxic tank, and from there to the aeration reactors (not shown). In simulation the oxygen concentration in R3 and R4 (DOR3 and DOR4, respectively) was controlled on, respectively, 0.6 and 2.8 gO2 m3 (Meijer et al., 2001). In the clarifiers (CL), activated sludge was separated
QA
QC CL
Influent
R1
S
R2
R3 QB
P recovery
QRAS Phosphate Stripper
(a)
Effluent
R4
Anaerobic
WAS
Overflow thickener
Anoxic
Anoxic/Aerated
Aerated
Settler
Thickener Anaerobic reactor (R1) Selector (S) Influent
QA
Anoxic reactor (R2) R3
Return sludge
QB (b) Fig. 1 – WWTP Hardenberg. (a) Schematic layout, adapted from Meijer et al. (2001) and (b) configuration of the anaerobic–anoxic reactors.
ARTICLE IN PRESS WAT E R R E S E A R C H
from the liquid phase. Cyclic process conditions were provided by the return activated sludge (RAS) from the clarifier underflow (QRAS) and three recycle pump flows QA, QB and QC. During the record period for model calibration, dry weather influent caused QA, QB, QC and QRAS to be more or less constant. In Table 1 operational data and reactor volumes are listed.
2.2.
Sludge line
The excess sludge is discharged from the return stream and thickened in a gravity thickener. The thickened sludge is pumped to a sludge equalization tank, where further thickening takes place. From the equalization tank the sludge is dewatered in a filter press.
2.3.
3509
4 0 (200 6) 350 7 – 351 6
Influent characterization
Influent was measured with flow proportional sample collector. During the recorded period no rain events occurred. Concentration variations in the measured influent were relatively small. Variations in the influent loads (concentration flow), therefore, were primarily caused by flow dynamics. To decrease simulation time, the influent was modelled with average concentrations and actual flow data. The influent characterization was performed according to Roeleveld and Van Loosdrecht (2001). Table 2 shows the
characterized model influent composition obtained from the average influent data (not shown).
2.4.
The Hardenberg WWTP model
All simulations were performed with the TUD model (Meijer, 2004). AQUASIM 2.0 was used as simulation platform (Reichert, 1994). In AQUASIM models are constructed by connecting completely stirred reactors (CSTRs) in a desired process configuration. The WWTP was modelled according to the flow scheme in Fig. 1 and data from Table 1. R1 was modelled as three CSTRs in series. S and R2 were modelled as single CSTRs. R3 and R4 were modelled as six CSTRs with internal recycle flows (QR3IN and QR4IN). With this set-up, measured longitudinal oxygen gradients in R3 and R4 were simulated sufficiently. CL separated particulate matter from the liquid phase ideally. A residual effluent solids concentration (XEF) was modelled as a percentile loss of the RAS. Biological conversions in the clarifier were simulated by a non-aerated CSTR placed in the RAS flow. DOR3 and DOR4 were controlled on respectively 0.6 and 2.8 gO2 m3. In the model, QA, QB, QC and QRAS were constant. The particulate COD (XR4) was controlled on 4700 gCOD m3 by regulating the waste sludge flow (QW) in order to simulate the manual operation by the plant operators. This control was implemented in AQUASIM as a proportional integral controller (Eq. (1)), where QWt and QW0 are the waste sludge
Table 1 – Operational data and hydraulic parameters of the WWTP used for model calibration (Meijer et al., 2001) Flow
Average7SD, m3 d1
Process unit
Volume, m3
Depth, m
HRT, h
685572387 102a 673872516 219a 22,415a 44,536a 59 17473021 1.2 Qina 635,040
Anaerobic reactor, R1 Selector reactor, S Anoxic reactor, R2 Alternate aerated carrousel, R3 Aerated carrousel, R4 Clarifier, CL (2 2625 m3) Sludge blanket volume
1480 740 2290 4190 4190 5250 150b
5.0 5.0 5.0 2.5 2.5 — —
1.2 0.5 0.7 0.7 0.7 3.4 0.4
Total WWTP
18,140
—
62.6
Influent, QIN Overflow thickener to R2, QOF Effluent, QEF Waste activated sludge, QW Pump flow A, QA Pump flow B, QB Pump flow C, QC Recycle activated sludge, QRAS Internal recycles, QR4 in and QR3 in a b
Estimated from mass balances. Estimated from denitrification in the RAS.
Table 2 – Characterized model influent calculated according to Roeleveld and van Loosdrecht (2001) Soluble compounds Dissolved oxygen Fermentable COD Fatty acid Ammonium Nitrate Orthophosphate Inert COD Alkalinity, HCO3
So SF SA SNH SNO SPO SI SHCO
Value
Unit
0 109 91 52 0 3.7 40 8
gO2 m3 gCOD m3 gCOD m3 gN m3 gN m3 gP m3 gCOD m3 mole l1
Particulate compounds Inert COD Solid COD Autotrophic micro-organisms Heterotrophic micro-organisms P-accumulating micro-organisms Poly-phosphate Ply-hidroxybutirate Glycogen
XI XS XA XH XPAO XPP XPHA XGLY
Value
Unit
174.8 188.7 0.01 0.01 0.01 0.001 0.001 0.001
gCOD m3 gCOD m3 gCOD m3 gCOD m3 gCOD m3 gP m3 gCOD m3 gCOD m3
ARTICLE IN PRESS 3510
WAT E R R E S E A R C H
40 (2006) 3507– 3516
flow at time t and initial respectively; XR4t is the simulated suspended COD at time t; XR4sp is the suspended COD set point and KP and KI are the proportional and integral constants, respectively. In this controller the control command to the waste sludge flow (QWt) at any time t is proportional to the deviation of the controlled variable (XR4) from the set point and to the sum of all set-point deviations. t X ½KI ðXR4t XR4sp Þ. Q Wt ¼ Q W0 þ KP XR4t XR4sp þ
Evaluation results
Fig. 2 shows the simulation results compared with the experimental values obtained in R1 (Fig. 2a) and R2 (Fig. 2b). As can be seen good agreement was obtained between simulated and experimental values confirming the good model performance. Table 3 shows the model parameters calibrated by Meijer et al. (2001) with values differing from the TUD Model default ones.
(1)
t¼0
3.
3.1.
3.2.
Results
After implementing the Hardenberg model in AQUASIM we evaluated the model performance. For the evaluation purpose we used the wastewater characterization and the Hardenberg calibrated model as described by Meijer et al. (2001).
Phosphate recovery study
After implementation of the model and evaluation with the experimental data, the P-stripper unit was introduced in order to evaluate by simulation the potential phosphorus recovery and the interactions between this recovery and the biological process. For this study all simulations were performed on the basis of the calibrated simulated pseudo steady state model of WWTP Hardenberg. The simulations were divided into two parts: steady state simulations, considering constant influent flow and concen-
50
5000 4000
35 30
3000
25 20
2000
15 10
1000
(gCOD m-3)
Phosphate (gP m-3)
40
Total suspended solids
45
5 23/06/1998 21:00
Ammonium (gN m-3)
(a)
25/06/1998 9:00
0 25/06/1998 21:00
12
24
10
20
8
16
6
12
4
8
2
4
0 23/06/1998 9:00
(b)
24/06/1998 24/06/1998 9:00 21:00 Date/Time
23/06/1998 21:00
24/06/1998 24/06/1998 9:00 21:00 Date/Time
25/06/1998 9:00
Phosphate (gP m-3)
0 23/06/1998 9:00
0 25/06/1998 21:00
Fig. 2 – Dynamic measurements and simulations: (a) actual measurements in the anaerobic reactor R1 of phosphate (white dots) and suspended solids (black dots), simulation of the calibrated model is plotted in lines and (b) actual measurements in the anoxic reactor R2 of ammonium (black dots) and phosphate (white dots), simulation of the calibrated model is plotted in lines.
ARTICLE IN PRESS WAT E R R E S E A R C H
3511
4 0 (200 6) 350 7 – 351 6
Table 3 – Hardenberg WWTP calibrated parameters values (Meijer et al., 2001) Parameter KN,PO KO gPP ZPNO
Description
Unit
Calibrated
TUD model
Saturation coefficient for phosphate (nutrient) Inhibition coefficient for oxygen (fermentation and anoxic growth) Saturation reduction factor for PP formation Anoxic PAO reduction factor
gP m3 gO2 m3 — —
0.001 0.7 0.25 0.8
0.01 0.2 0.1 0.5
trations; dynamic simulations, considering variable influent flow and concentrations.
3.3.
Steady state simulations
The following parameters were varied in this study:
Fraction of the influent flow diverted from the anaerobic reactor (R1) to the P-stripper (fstripper) (see Fig. 1a): 0–1.
Sludge loading rate (SLR): 0.042–0.167 kgCOD kgVSS1 d1,
corresponding to an influent flow between 4000 and 16,000 m3 d1, respectively. Temperature: 10–20 1C.
The potential phosphate recovery was studied by means of the total phosphorus mass balance. The amount of phosphorus content in the effluent (Peff), waste activated sludge (Pwas) and effluent stripper (Precovery) streams were calculated as a percentage of the influent total phosphate load. To evaluate the P-stripper effect on the biological process, the next effluent requirements were used: total nitrogen (TN) o10 gN m3, total phosphorus (TP) o1 gP m3 and complete nitrification (NH+4 o1 gNH+4 N m3). Fig. 3 shows the results of the steady state simulation at different sludge loading rate and fraction of inflow diverted to the phosphate stripper (fstripper). As can be seen in Fig. 3a and c, TP and TN concentration remain below the effluent requirements (o1 gP m3 and o10 gN m3) independent of the stripper flow rate. However, at low load conditions and without the stripper working a significant deterioration in the phosphate removal can be observed. This is due to the low load and, therefore, high SRT at constant sludge content in the reactors. Under these conditions the phosphate uptake is limited by a maximum poly-P content in the sludge. Fig. 3d shows a significant increase in the effluent ammonia concentration at higher fstripper values. This effect is more significant at high SLR, resulting in concentrations above the limits. This deterioration is due to the low phosphate concentration that inhibits the autotrophic organisms growth and hence the nitrification. However, the value of the phosphate half saturation constant for nitrifiers growth (KP) is not well known and the model results are very sensitive to this parameter value. The potential phosphate recovery results (the fraction of influent phosphorus that is in the stripper outflow, Fig. 3e) show that the higher fstripper value the higher potential phosphate
recovery. However, this phosphate recovery is limited by its effect on the biological process performance regarding the effluent limits. The P-stripper overuse causes a phosphate limitation in the downstream biological processes. The bold line represents the highest phosphate recovery achieved at each SLR accomplishing the effluent limits. Therefore, the information provided in this graph can be used to evaluate the operation of a stripper to maximize the phosphate recovery without affecting the biological process. As can be seen, it is possible to recover up to 70% of the influent phosphate at low SLR. However, this potential phosphate recovery is reduced at high SLR due to the important deterioration of the biological process. At high SLR the ammonia effluent concentration is higher because of the ammonia overload. Nevertheless, for phosphate recovery purposes the soluble phosphate concentration in the stripper flow is also very relevant (Fig. 3b). Fig. 3b shows the phosphate concentration in the anaerobic reactor at different fstripper and SLR. The bold line represents the orthophosphate concentration that corresponds to the high phosphate recovery achieved at each SLR. As can be seen, the orthophosphate concentration achieved at high SLR is 25 gP m3 opposite to the 13 gP m3 achieved at low SLR. It was noticed that the concentration of fermentable substrate (SF) and particulate organic matter (XS) at the end of the anaerobic reactor remains quite high (around 30 and 80 gCOD m3, respectively). Therefore, in order to increase the amount of phosphate released to maximize the phosphate recovery is important to optimize the anaerobic process, that is, to increase the XS hydrolysis and volatile fatty acids production by anaerobic fermentation. This could be obtained introducing the fermentation–elutriation process (Bouzas et al., 2002) or increasing the anaerobic zone. Fig. 4 shows the effect of the anaerobic reactor volume on the amount of phosphate released during the anaerobic phase and on the effluent composition. The anaerobic reactor volume increase intensifies the hydrolysis and fermentation without deteriorating the effluent composition. Fig. 5a shows the temperature effect on the stripper performance. The increase of temperature stimulates the biological activity thereby increasing the phosphate release in the anaerobic reactor by polyphosphate accumulating organisms (PAOs), which increases the potential P recovery. The increase of fstripper (removing phosphate) causes a significant reduction in the ratio Poly-P/PAO. At low Poly-P/PAO ratio this effect is naturally less signifficant. The temperature has also an important effect on the biological process, with deterioration of the nitrification process at low temperatures
ARTICLE IN PRESS 3512
WAT E R R E S E A R C H
40 (2006) 3507– 3516
Fig. 3 – Steady state simulations at different fstripper and SLR values. (a) effluent total phosphate, (b) phosphate concentration in the anaerobic reactor, (c) effluent total nitrogen, (d) effluent ammonia, (e) potential phosphate recovery and (f) fraction of the influent phosphate in the waste sludge.
(Fig. 5b and d) because of the influence of temperature on nitrifiers. Therefore, temperature increase allows improving the stripper performance for phosphate recovery
owing to the increase of phosphate concentration in the stripper combined with an improvement of the biological nitrogen removal.
ARTICLE IN PRESS WAT E R R E S E A R C H
3513
4 0 (200 6) 350 7 – 351 6
4
35
VR1 = VR1o × f
3.5
30
VR1o = 1480 m3
3
25
2.5
20
2
15
1.5
10
1
5
NH4+ eff - TP eff (g m-3)
PO4+ R1 - TNeff (g m-3)
40
0.5
0
0 0.5
1
1.5
2
2.5
3
3.5
f PO4 R1
TN eff
NH4 eff
TP eff
Fig. 4 – Effect of the anaerobic reactor R1 volume on the phosphate released and effluent concentrations.
30.0
45 40
22.0
35
20.0
30
18.0
25 20
-
16.0
15
14.0
10
-
12.0
P recovery (%)
50
24.0
NH4+ effluent (gN m-3)
55
+
T
26.0
PO4 R1 (gPm-3)
6.0
60
+
28.0
-
4.0 3.0
T
2.0 1.0
5
10.0 0.05
0.1
0.15
(a)
0.2
0.25
0.3
0.35
0.4
0.45
+
0.0
0 0
5.0
0.5
0
0.05
0.1
0.15
0.2
(b)
f stripper
0.25
0.3
0.4
0.45
0.5
-
12.0
1.2
0.35
f stripper
Total N effluent (gN m-3)
Total P effluent (gPm-3)
11.0
1.0 0.8
0.6
T 0.4
+
0.2
10.0
T
9.0 8.0 7.0 6.0 5.0
+
4.0 3.0 2.0 1.0 0.0
0.0 0
0.05
0.1
0.15
(c)
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0
0.05
0.1
0.15
(d)
f stripper
T=10°C
T=13°C
0.2
0.25
0.3
0.35
0.4
0.45
0.5
f stripper
T=17°C
T=20°C
Fig. 5 – Temperature effect on: (a) the potential phosphate recovery and orthophosphate concentration achieved in the anaerobic reactor, (b) ammonia effluent concentration, (c) total P effluent concentration and (d) total N effluent concentration.
3.4.
Dynamic simulations
3.4.1.
Influent data
The influent data for dynamic simulations used in this work were the ones proposed in the COST 624 benchmark by Vanhooren and Nguyen (1996). These influent data was rescaled up or down from the proposed values to the actual average Hardenberg WWTP data. The values were scaled by performing a linear regression keeping the proportional average deviations. We used two weeks of dry weather influent flow and water composition. Variations in the hydraulic loading from Mon-
day to Friday were decreased by 3% per day. The influent during the weekend was also reduced by 15% from the flow during the weekday. The resulting influent flow is depicted in Fig. 6. For the particulate components (Fig. 6a), it was assumed that the suspended solids concentration increases with high hydraulic loading because resuspension of solids occurs during high flows (Verbanck, 1995). As a result the influent suspended solids concentration follows the hydraulic loading variations that occur during the week. The influent soluble substrate data are depicted in Fig. 6b. The volatile fatty acids (SA) and phosphate (SPO4) profiles were
ARTICLE IN PRESS 3514
WAT E R R E S E A R C H
40 (2006) 3507– 3516
Influent Flow (/10) - Particulate Components
Qin/10 (m3 d-1) - XC (gCOD m-3)
1200 1000 800 600 400 200 0 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Time (d) Xi
(a)
Xs
Qin
Soluble Components 225 200 175
SC (g/m3)
150 125 100 75 50 25 0 0
1
2
3
4
(b)
5
Sf
6
7 Time (d)
Sa
8
Snh4
9
10
11
12
13
14
Spo4
Fig. 6 – (a) Influent flow scaled to the actual average flow (6855 m3 d1) and influent particulate concentrations scaled to the actual average concentration: XI ¼ 174.8 gCOD m3, XS ¼ 188.7 gCOD m3 and (b) Influent readily fermentable substrate, volatile fatty acids, ammonia and phosphate scaled to the actual average concentration: SF ¼ 109 gCOD m3, SA ¼ 91 gCOD m3, SNH4 ¼ 52 gN m3, SPO4 ¼ 6.6 gP m3.
considered to be proportional to fermentable substrate (SF) and ammonia (SNH4), respectively.
3.4.2.
Phosphate recovery under dynamic conditions
After the study of the potential phosphate recovery at steady state, different control strategies under dynamic conditions were evaluated. This study was performed with the influent composition showed above, average inflow of 10,000 m3 d1 and 18 1C. The stripper operation strategies were:
Case 1: without control, constant Q stripper proportional to the
phate concentration in the anaerobic reactor (PO4 R1 ): f stripper 0 ¼ 0:45, Kp ¼ 0:03; f stripper ¼ f stripper 0 þ KP ðPO4 R1t PO4 R1 min Þ.
average inflow Q in , Q stripper ¼ f stripper Q in : fstripper ¼ 0.45. Case 2: variable Qstripper proportional to Qin, Q stripper ¼ f stripper Q in , with constant f stripper : f stripper ¼ 0:45. Case 3: variable Qstripper proportional to Qin, Q stripper ¼ f stripper Q in , with variable f stripper as function of the phos-
Table 4 and Fig. 7 show the dynamic simulation results of different control strategies. As can be seen without stripper control, working at fixed flow (Case 1) the potential phosphate recovery is 48.6%. In this case the phosphate load in the stripper (Fig. 7) varies due to the influent phosphate load fluctuations. The introduction of a variation in the stripper flow, proportional to the influent phosphate load (Case 2), caused a slight increase in the potential phosphate recovery (50.7%). Finally, the introduction of fstripper variation (Case 3) led to increase the potential phosphate recovery to 55.7%. In this case the fstripper was changed with time in order to maximize stripper flow at high phosphate concentration in the anaerobic reactor. As can be
ARTICLE IN PRESS
seen in Fig. 7, with this stripper control the amount of potential phosphate recovery increases at high phosphate concentration in R1 and decreases at low phosphate concentration.
4.
Discussion
The steady state simulations at different SLR and fstripper values showed the potential use of the BCFSs technology to recover phosphate. The results suggest that the biological process most influenced by the P-stripper operation might be the nitrification due to phosphate limitation. However, further research is needed due to the lack of knowledge of the nitrification process under limiting phosphate concentration. There are indications in literature that nitrifying bacteria can accumulate polyphosphate (Terry and Hooper, 1970; Eigener and Bock, 1972; Chain et al., 2003); however, their role in the metabolism is not well studied. It might, therefore, be that nitrifying bacteria will
Table 4 – Dynamic simulations results under different P stripper control strategies %P recoverya
Case
1 2 3
Average P load in the stripper flow (kgP d1)
48.6 50.7 55.7
3515
4 0 (200 6) 350 7 – 351 6
60.6 63.2 69.4
a
Potential phosphate recovery in the stripper calculated as a percentage of the influent phosphate.
respond to temporary absence of phosphate (final stage of aerobic phase) by accumulating polyphosphate in the presence of phosphate (early stage of aerobic phase). This could be included relatively simply in the present activated sludge models. For simplicity one could also leave out the P-limitation for nitrifying growth, assuming that there is always enough phosphate for growth of the nitrifiers in normal wastewater and that the mass of phosphate taken up by nitrifiers in activated sludge systems is insignificant. Increasing the effluent standards demands to further investigate the effect of P-limitations in EBPR systems on nitrification. Furthermore, the simulation at low load showed that under these conditions the P-stripper can be used as originally was envisaged, that is, to assist the biological process in order to achieve the effluent TP legal requirements. Therefore, the BCFSs technology can be potentially used to assist the biological process and to recover phosphate from wastewater. Besides the phosphorus recovery, this new use of the BCFSs technology has another advantage. The phosphate removal in the anaerobic reactor causes the decrease of the phosphorus content as poly-P in the sludge (Fig. 3f). Therefore during the downstream sludge treatment (anaerobic and aerobic sludge digestion) less phosphate is released decreasing the possible phosphate precipitation that can cause very important problems (Doyle and Parsons, 2002). According to Fig. 3e, the recovery of phosphate presents two possibilities: higher phosphate recovery in a stream with low phosphate concentration or lower phosphate recovery in a stream with high phosphate concentration. The WWTP operators must decide the P-stripper operation depending on the technology used to recover phosphate.
35
3.00E+05
30
2.50E+05
25
2.00E+05
20
1.50E+05
15
1.00E+05
10
5.00E+04
5
gP d-1
3.50E+05
0.00E+00
0 365
367
369
371
373
375
377
379
Time (d)
P influent
P recovery, Case 1
P recovery, Case 2
P recovery, Case 3
Fig. 7 – Potential phosphate recovery under different P-stripper control strategies.
PO4 R1
PO4-3 R1 (gP m-3)
WAT E R R E S E A R C H
ARTICLE IN PRESS 3516
WAT E R R E S E A R C H
40 (2006) 3507– 3516
High temperatures (Fig. 5a) cause the increase of the amount of phosphate released during the anaerobic stage. Hence, the potential phosphate recovery will be higher during summer season than during low temperatures periods. On the other hand, anaerobic zone volume (Fig. 4) increase allows maximizing the amount of phosphate recovery at the same P-stripper operation conditions. Furthermore, pre-fermentation of primary or return sludge (Bouzas et al., 2002; Jo¨nsson and Jansen, 2005; Vollertsen et al., 2005) could be an interesting operation strategy to increase the readily biodegradable COD in order to have extra phosphate release. Therefore, the use of phosphate recovery implies a change in the design philosophy not only to achieve the effluent requirements but also to maximize the anaerobic phosphate release and thereby recovery. The evaluation of the P-stripper operation under dynamic conditions showed that the introduction of control strategies that try to increment the stripper flow at high influent phosphate load and high phosphate concentration in the anaerobic reactor (Cases 2 and 3), increase the potential of phosphate recovery (Table 4). However, there are not significant variations in the amount of phosphate recovery between the simulated control strategies. This is most likely due to the fact that the most dominant factor will be the Prelease, which depends on the influent load. These results suggest that the P-stripper use for phosphate recovery does not imply complicated control strategies, which is an important aspect from an operational point of view.
5.
Conclusions
This study indicated the potential of P-recovery in a biological P-removal plant. Approx. 60% of the influent load can be recovered with a simple adaptation of the process. Complex process control does not seem to be able to improve results. The potential limitation of phosphate in the aerobic phase for especially nitrifiers needs further investigation.
Acknowledgements This research work has been supported by the Conselleria de Empresa, Universidad y Ciencia de la Generalitat Valenciana ´ rea de Polı´tica Cientı´fica y Tranferencia Tecnolo´gica), which (A is gratefully acknowledged. Special acknowledgements to the Department of Biotechnology of the Faculty of Applied Sciences, TU Delft. R E F E R E N C E S
Bouzas, A., Gabaldon, C., Marzal, P., Penya-Roja, J.M., Seco, A., 2002. Fermentation of municipal primary sludge: effect of SRT
and solids concentration on volatile fatty acid production. Environ. Technol. 23 (8), 863–875. Chain, P., Lamerdin, J., Larimer, F., Regala, W., Lao, V., Land, M., Hauser, L., Hooper, A., Klotz, M., Norton, J., Sayavedra-Soto, L., Arciero, D., Hommes, N., Whittaker, M., Arp, D., 2003. Complete genome sequence of the ammonia-oxidizing bacterium and obligate chemolithoautotroph Nitrosomonas europaea. J. Bacteriol. 185 (9), 2759–2773. Doyle, J.D., Parsons, S.A., 2002. Struvite formation, control and recovery. Water Res. 36 (16), 3925–3940. Eigener, U., Bock, E., 1972. Auf- und abbau der polyphosphatfraktion in zellen von Nitrobacter winogradskyi buch. Arch. Microbiol. 81 (4), 367–378. Jo¨nsson, K., Jansen, J.C., 2005. Hydrolysis of return sludge for production of easily biodegradable carbon—effect of pretreatment, sludge age and temperature. Conference proceedings of the IWA Specialized Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Krakow (Poland). Kiuru, H.J., Rautiainen, J.A., 1998. Biological nutrient removal at a very low-loaded activated sludge plant with high biomass concentrations. Water Sci. Technol. 38 (1), 63–70. Meijer, S.C.F., van Loosdrecht, M.C.M., Heijnen, J.J., 2001. Metabolic modelling of full-scale biological nitrogen and phosphorus removing WWTP’s. Water Res. 35 (11), 2711–2723. Meijer, S.C.F., 2004. Theoretical and practical aspects of modelling activated sludge process. Thesis, Technical University of Delft, The Nederlands. Mino, T., van Loosdrecht, M.C.M., Heijnen, J.J., 1998. Microbiology and biochemistry of the enhanced biological phosphate removal process. Water Res. 32 (11), 3193–3207. Reichert, P., 1994. Aquasim—a tool for simulation and dataanalysis of aquatic systems. Water Sci. Technol. 30, 21–30. Roeleveld, P.J., Van Loosdrecht, M.C.M., 2001. Experiences with guidelines for wastewater characterisation in The Netherlands. Water Sci. Technol. 45 (6), 145–156. Taka´cs, I., Murthy, S., Smith, S., McGrath, M., 2005. Chemical phosphorus removal to extremely low levels-experience of two plants in the Washington, D.C. area. IWA Specialized Conference: Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Krakow (Poland). Terry, K.R., Hooper, A.B., 1970. Polyphosphate and orthophosphate content of Nitrosomonas europaea as a function of growth. J. Bacteriol. 103 (1), 199–206. Vanhooren, H., Nguyen, K., 1996. Development of a simulation protocol for evaluation of respirometry-based control strategies. COST 624 Technical Report. Van Loosdrecht, M.C.M., Brandse, F.A., de Vries, A.C., 1998. Upgrading of wastewater treatment processes for integrated nutrient removal—the BCFSs process. Water Sci. Technol. 37 (9), 209–217. Verbanck, M., 1995. Variabilte´ des charges solides en suspension a` l’exutoire des re´seaux de collecte. Journe´e d’e´tude CB-IAWQ, Lie`ge. Vollertsen, J., Petersen, G. Borregaard, V.R., 2005. Hydrolysis and fermentation of activated sludge to enhance biological phosphorus removal. Conference proceedings of the IWA Specialized Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams. Krakow (Poland).