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Effect of the Internal Recycles on the Phosphorus Removal Efficiency of a WWTP Francisco J. Ferna´ ndez,* Jose´ Villasen˜ or, and Lourdes Rodrı´guez UniVersity of Castilla-La Mancha, Chemical Engineering Department, ITQUIMA, AVenida Camilo Jose´ Cela S/N, 13071 Ciudad Real, Spain
The aim of this work was to study the effects of the internal recycle flow rates on a University of Cape Town (UCT) process. The study was carried out by continuously feeding urban sewage to a UCT pilot-scale plant and by using batch tests. In the pilot-scale experiments, it was observed that the maximum nitrogen removal was obtained when both internal recycle flow rates ranged from 200 to 250% and that the maximum phosphorus removal was obtained when the aerobic-to-anoxic flow rate ranged from 100 to 150% and the anoxic-toanaerobic recycle flow rate ranged from 75 to 100%. In order to obtain a deeper insight on the anaerobic and anoxic processes, batch tests were carried out and the results were modeled. From the results obtained in the batch test, it was concluded that the nitrate-nitrogen concentration in the anaerobic compartment should be always lower than 1 mg L-1. Taking into account these observations and the opposite effects of the internal flow rates on the nitrogen and phosphorus removals, equilibrium internal recycle flow rates were selected in the pilot-scale plant in order to obtain the best effluent characteristics. These values ranged from 100 to 150% for the aerobic-to-anoxic compartment recycle flow rate and from 75 to 100% for the anoxic-to-anaerobic compartment recycle flow rate. stage to molecular nitrogen in four steps (eq 1), using the substrates contained in the wastewater as electron donor.7
Introduction Human population growth and resources consumption have placed increasing demands on aquatic ecosystems and have affected global biogeochemical cycles of carbon, nitrogen, and phosphorus.1 Nowadays, one of the greatest problems of the continental water resources is the cultural eutrophication, which is mainly caused by phosphorus and nitrogen compounds. Because of this, on May 1991, the European Economic Community (EEC) approved the directive 91/271, which limits the phosphorus and nitrogen contents of treated wastewater discharged on sensitive areas. The imposed restrictions can be reached by using chemicalphysical or biological processes. Nowadays, the biological nutrient removal (BNR) processes are the most used, which could be explained because of the lower cost of the bioprocesses compared to the chemical-physical processes. The BNR processes are extensions of the conventional activated sludge processes. However, the BNR processes are more complicated than conventional activated sludge processes because the simultaneous biological removal of nitrogen and phosphorus from wastewater requires a bioreactor divided, at least, into three compartmentssanaerobic, anoxic, and aerobics with selected recycling of mixed liquor between compartments.2 The most important processes occurring in these compartments are as follows: (i) In the anaerobic compartment, phosphate accumulating organisms (PAO) take up readily biodegradable chemical oxygen demand (RBCOD) and store them as polyhydroxyalcanoate (PHA).3-5 The energy required for this anaerobic process is obtained from the glycolysis of glycogen and the hydrolysis of intracellular polyphosphates coupled to the release of orthophosphate to the liquid bulk.6 (ii) In the anoxic compartment, the denitrification by heterotrophs is the main reaction. In this reaction, heterotrophic microorganisms reduce the nitrate obtained in the nitrification * To whom correspondence should be addressed. Phone: +34 902204100 (ext. 6358). Fax: +34 902204130. E-mail: FcoJesus.
[email protected].
NO3 f NO2 f NO(g) f N2O(g) f N2 (g)
(1)
(iii) In the aerobic compartment, PAO use the PHA for generating energy for growth, glycogen synthesis, and phosphate uptake. Another important reaction under aerobic conditions is the nitrification. The nitrification is accomplished in two stages by autotrophic organisms;7 these stages are outlined in eqs 2 and 3. + 2NH+ 4 + 3O2 f 2NO2 + 2H2O + 4H
(2)
2NO2 + O2 f 2NO3
(3)
Moreover, in the aerobic compartment takes place the aerobic oxidation of organic compounds and the growth of microorganisms.7 The exact sequencing and sizes of the compartments in a BNR wastewater treatment plant (WWTP) depend on the process scheme selected and the desired nitrogen and phosphorus effluent concentrations. Nowadays, there are many flow schemes to reach the required nitrogen and phosphorus removal from the wastewater, with the most commonly used being the anaerobic-anoxic-oxic (A2/O), the University of Cape Town (UCT), the Virginia Initiative Plant (VIP), and the BARDENPHO process.8 The description of each of these processes can be found in the literature.2,9 However, to reach the desired effluent concentration, it is not enough just to apply a predefined sequence of anaerobic, anoxic, and aerobic compartments; it is also necessary to optimize the operational conditions. One of the most common errors in the operation of the BNR WWTP is the application of inappropriate internal recycle flow rates. The most implemented BNR processes have two internal recycles as well as an external recycle for the biomass recirculation. One of the external recycles is for the recirculation of the mixed liquor from the aerobic to the anoxic compartment.2 This recycle must be properly defined because it supplies nitrate
10.1021/ie070407d CCC: $37.00 © 2007 American Chemical Society Published on Web 09/27/2007
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Figure 1. Schematic flow diagram of the pilot plant.
Figure 2. Return activated sludge flow rate as a function of the SVI and MLTSS concentrations.
from the nitrification stage to the denitrification stage. This recycle enhances the nitrogen removal from the wastewater by the alternation of aerobic and anoxic conditions, which allows it to consume, under anoxic conditions, the nitrogen nitrified in the aerobic stage. However, very high recycle flow rates could increase the nitrate concentration in the anaerobic compartment,10 which could reduce the phosphorus removal capacity of the system. The other external recycle is used to return mixed liquor from the anoxic to the anaerobic compartment, supplying denitrified biomass to the anaerobic stage.2 The alternation of aerobic and anaerobic conditions improves the efficiency of phosphorus removal by favoring the growth of PAO. Therefore, internal recycle flow rates optimization is an important objective because it improves the removal efficiency with minimum cost. In this context, the aim of this work was to study the influence of the internal recycle flow rates on the nutrient removal efficiency of a UCT process, with primary attention to the effects of nitrate on the phosphorus removal. To reach this objective, a UCT pilot-scale plant (850 L) was fed with real sewage under a range of different recycle flow rates. The effect of nitrate on the phosphorus removal was studied by means of continuous and batch tests and the application of the ASM2d model.11 Finally, the optimum recycle flow rates were determined. Materials and Methods Pilot Plant. The activated sludge BNR process was developed in a stainless steel pilot-scale plant working with the UCT process scheme (Figure 1). The main feature of the UCT process is that both the return activated sludge (RAS) and the
internal recirculation from the aerobic tank are recycled to the anoxic compartment. In this way, the content of the anoxic compartment is recycled to the anaerobic compartment. These recycles allow one to control the nitrate-nitrogen concentration as well as the sludge concentration in the anaerobic compartment. The pilot-scale plant was located at the full-scale Ciudad Real WWTP (Spain) and consisted of a previous equalization tank, a reactor basin, and a settler. The equalization tank was equipped with a mechanical mixer and control systems for temperature and pH. These pieces of equipment ensured approximately constant conditions for the wastewater fed to the reactor basin, which consisted of a tank divided into three consecutive environments (anaerobic, anoxic, and aerobic). All the compartments were equipped with air diffusers and mixers. Dissolved oxygen concentration was measured in all compartments and controlled in the aerobic one, with the set point in this compartment being 2.0 g m-3. The air was fed with an air compressor. Sludge settling occurred in a settler equipped with a slowly rotating scraper. Variable-speed peristaltic pumps were used for the internal recirculations between compartments (R1 and R2), for RAS to the reactor basin, and for influent wastewater to the pilot plant. The influent flow rate was intentionally kept constant so that the experimental results would reflect only the effect of the internal recycle flow rates on the performance of the process. The solid retention time (SRT) was 8 days in all experiments. The hydraulic retention times in each compartment were determined by taking into account the ranges proposed in the literature.2,9 The RAS flow rate selected was based on the expected mixed-liquor total suspended solids (MLTSS) concentration and sludge volume index (SVI) of the activated sludge and by using mass balances. A contour representation of the results of the mass balance is presented in Figure 2. Grey areas correspond to values out of the optimum range proposed in the literature. Taking into account that the recommended RAS for the UCT process ranges from 50 to 100% of the influent flow rate2,9 and that the expected MLTSS and SVI ranged from 2000 to 3000 g m-3 and from 100 to 150 mL g-1, respectively, the RAS was set to 50% of the influent flow rate (see Figure 2). More information about the hydraulic operational data and volumes of the pilot-scale plant are summarized in Table 1. Experimental Procedure. Pilot-scale experiments were carried out to evaluate the influence of the internal recycle flow rates on the nutrients and COD removal. Each experiment consisted of the continuous operation of the UCT pilot-scale plant running with selected values for the internal recycle flow rates and real sewage feed. During the length of the experiments, all the operating conditions were kept constant except the
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Table 1. Operational Data and Hydraulic Parameters of the Pilot-Scale Plant parameter h-1)
influent flow rate (L influent temperature (°C) influent pH equalization tank volume (L) anaerobic volume (L) anoxic volume (L) internal recycle R2 (%)
value
parameter
value
56.0 20 7.2-7.6 450 56 56 50-250
aerobic volume (L) internal recycle R1 (%) settler volume (L) sludge waste flow rate (L h-1) RAS flow rate (L h-1) effluent flow rate (L h-1)
224 100-320 63 1.8 28.0 54.2
Table 2. Wastewater Characteristics from the Parshall Flume and the Primary Clarifiers parameter
Parshall flume
primary clarifiers
BOD5 (g m-3) COD (g m-3) TSS (g m-3) P (g m-3) N (g m-3)
281 ( 32 784 ( 326 286 ( 117 21.2 ( 7.2 31.3 ( 12.9
215 ( 63 270 ( 183 71 ( 34 13.0 ( 4.0 23.7 ( 6.2
Table 3. Experimental Conditions of the Batch Experiments parameter m-3)
COD (g PT (g m-3) N-NH4 (g m-3) K (g m-3)
value
parameter
value
300 10 15 12
m-3)
14 5 10 2000
Na (g Mg (g m-3) Ca (g m-3) MLVSS (g m-3)
internal recycle flow rates. The pH was controlled in order to avoid phosphorus precipitation caused by high pH. However, it should be noted that only a small amount of acid and base was consumed during the experiments because of the buffer activity of the urban sewage. Once the stationary state was reached, in most of the cases about 1 month after the start of the experiment, the characteristics of the influent wastewater, the biological sludge, and the treated wastewater were daily recorded. The results presented in this work were obtained from statistical analysis (mean value and standard deviation) of daily results obtained during the stationary state of each experiment. The number of data points used in the statistical analysis of each experiment ranged from 20 to 32. Optimization Technique. The optimization technique used to select the best flow rates for the internal recycles in the pilotscale plant was based on the sequential optimization of the nitrogen and phosphorus removal efficiencies. First, the nitrogen removal was optimized in order to achieve the highest removal efficiencies, by modifying the R1 recycle flow rate, while ensuring low nitrate concentration in the anaerobic compartment. Once the nitrogen removal was optimized, the best phosphorus removal was searched by selecting the most adequate R2 recycle flow rate. Taking into account the reciprocal influences of both parameters, the selection of the best recycle flow rates for the optimum operation of the BNR in the UCT pilot scale was complex, entailing maintaining a dynamic equilibrium between the nitrogen and phosphorus removals. Wastewater. The influent wastewater fed to the pilot-scale plant was prepared by mixing sewage taken from the Parshall flume and wastewater from the overflow of the primary clarifiers of the full-scale plant in a 1:1 volumetric ratio. This mixture was employed because the future layout of the process will include a bypass of the primary clarifier. The objective of this bypass is to enhance the nitrogen removal by increasing the COD load coming into the bioreactor of the WWTP. Table 2 shows the average wastewater characteristics taken from these two different points during the whole experimental period. The characteristics of the resulting wastewater fed to the bioreactor of the pilot-scale plant are presented in Table 4(Results and Discussion section).
Analytical Techniques. The influent wastewaters were characterized by following the procedures proposed by the Dutch Foundation of Applied Water Research12 and the respirometric procedure proposed in the literature13 to determine the COD fractions. All the analyses of the sludge and settled effluent wastewater were performed using standard methods.14 Batch Experiments. In this work, several batch experiments were conducted to obtain a deeper understanding and insight into the denitrification and the phosphorus release processes occurring under anoxic and anaerobic conditions, respectively. These batch experiments were conducted by mixing activated sludge from the UCT pilot-scale plant and urban sewage, under anoxic and anaerobic conditions, in batch reactors. Each batch reactor had a total volume of 260 mL. The reactors were filled with 250 mL of the biomass with nutrients and wastewater to give the approximate nutrients and COD final concentrations presented in Table 3. At the beginning of the experiments, oxygen was purged with N2 gas. By working in this way, the oxygen transfer into the bulk liquid of the closed batch reactors was minimized, which reduced the adverse effect of oxygen on the anoxic and anaerobic processes. The pH in the batch reactors oscillated between 7.2 and 7.4, and the temperature was continuously maintained at 20 °C by means of an incubator (ISCO FTD 220). In order to maintain the sludge suspended, avoiding oxygen transfer from the gas phase, a very slow stirring rate was applied during the batch test. Experiments were carried out with a foodto-microorganisms (F/M) ratio of 0.15 gCOD‚gVSS-1. In both series of experiments, anoxic and anaerobic, two reference tests were simultaneously carried out: a maximum test and a blank test. The maximum test was carried out with sodium acetate because it is known to be a very easily biodegradable substrate that offers the highest denitrification rates15-19 and the highest phosphorus release rates.20 A simplification of the ASM2d11 model was used to determine the kinetic and stoichiometric parameters of the denitrification and the phosphorus release processes observed in all the experiments carried out with urban sewage. Model Development and Parameter Estimation. In order to study the competition for the RBCOD contained in the urban sewage by the denitrifiers and the PAO, the results obtained in the batch experiments were fitted to a simplification of the ASM2d model. The use of the model, in conjunction with the experimental results obtained in the batch test, allowed the denitrification and the phosphorus removal processes to be modeled and allowed for the determination the main parameters of both processes. The number of equations in the ASM2d model was reduced because of the experimental conditions (anaerobic or anoxic environment and presence of soluble substrates). The selected equations were simplified by taking into account that the experiments were carried out with a sufficiently high level of alkalinity, with internally stored polyphosphate, and in the absence of oxygen. These simplifications yield a set of differential equations that were used to
Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 7303 Table 4. Operational Conditions and Wastewater Characterization during the Pilot-Scale Experiments h-1)
influent flow rate (L RAS (%) R1 (aerobic to anoxic), (%) R2 (anoxic to anaerobic), (%) COD (g m-3) BOD5 (g m-3) VFA (g COD m-3) inert COD (g m-3) NT (g m-3) PT (g m-3) TSS (g m-3) N-NO3 anaerobic compartment (mg L-1) oxygen anoxic compartment (mg L-1) phosphorus content in the sludge (%)
test 1
test 2
test 3
test 4
test 5
test 6
test 7
56 50 100 50 496 ( 183 220 ( 96 79 ( 16 38 ( 8 23.9 ( 7.1 16.5 ( 4.5 369 ( 108 0.4 ( 0.1 0.3 ( 0.1 3.3 ( 0.8
56 50 100 250 586 ( 194 257 ( 101 91 ( 9 41 ( 8 26.9 ( 6.4 19.5 ( 6.0 345 ( 101 0.9 ( 0.2 0.2 ( 0.1 3.9 ( 0.9
56 50 100 100 482 ( 140 196 ( 41 73 ( 8 28 ( 4 21.9 ( 5.0 21.0 ( 4.2 320 ( 108 0.6 ( 0.1 0.2 ( 0.1 3.2 ( 0.9
56 50 150 100 524 ( 138 205 ( 83 85 ( 6 37 ( 9 32.6 ( 6.5 18.9 ( 5.7 324 ( 102 0.9 ( 0.2 0.4 ( 0.2 3.1 ( 0.8
56 50 250 100 613 ( 216 220 ( 78 101 ( 14 39 ( 10 25.0 ( 11 16.1 ( 5.2 332 ( 165 1.2 ( 0.2 0.6 ( 0.2 2.9 ( 0.5
56 50 320 100 574 ( 203 230 ( 107 92 ( 11 43 ( 12 27.9 ( 5.4 17.7 ( 5.5 402 ( 133 2.3 ( 0.3 0.8 ( 0.3 3.0 ( 0.6
56 50 250 250 451 ( 126 181 ( 32 65 ( 8 32 ( 5 20.5 ( 4.2 16.8 ( 5.0 306 ( 121 2.6 ( 0.5 0.4 ( 0.2 2.8 ( 0.5
describe the denitrification and the phosphorus removal processes. These equations were as follows, eqs 4 and 5 for the denitrification process and eqs 6 and 7 for the phosphorus release process,
(
)
dSRBCOD dt
)
Denitrification
SN-NO3 SRBCOD ‚ ‚X (4) -qDN KN-NO3 + SN-NO3 KRBCOD + SRBCOD
(
)
dSN-NO3
(
) dt Denitrification SN-NO3 SRBCOD 1 - YH ‚qDN‚ ‚ ‚X (5) 2.86 KN-NO3 + SN-NO3 KRBCOD + SRBCOD
( ) ) ( )
dSRBCOD dt dSP dt
SRBCOD ) -qPR‚ ‚X Phosphorus release KRBCOD + SRBCOD
(6)
SRBCOD ) YPO4‚qPR‚ ‚X (7) Phosphorus release KRBCOD + SRBCOD
where KH-NO3 is the saturation/inhibition coefficient for nitrate, KRBCOD is the saturation/inhibition coefficient for RBCOD, qDN is the maximum specific RBCOD consumption rate for the denitrification process, qPR is the maximum specific RBCOD consumption rate for the phosphorus release process, SRBCOD is the RBCOD concentration, SN-NO3 is the nitrate-nitrogen concentration, YH is the heterotrophic yield coefficient, YPO4 is the polyphosphate release for COD storage, and X is the biomass concentration. The pairs of equations proposed for each process, denitrification and phosphorus release, were solved simultaneously, with the objective being to adequately describe the experimental results obtained in the batch tests: nitrate and RBCOD in the denitrification process and phosphorus and RBCOD in the phosphorus release process. As results of the fitting procedure, the values of the most sensitive parameters were determined. The sensitivity of the parameters was determined by using the absolute-relative sensitivity function (eq 8). a,r )p δy,p
∂y ∂p
(8)
This function measure the change in a variable (y) for a 100% change in a parameter (p). The advantage of this function is that their units do not depend on the units of the parameter, which makes it possible to make quantitative comparisons of the effects of different parameters on a common variable.
Once the sensitivities of the parameters were defined, the values of the parameters of the model were determined by assuming an initial set of values and calculating the theoretical profiles for the RBCOD and nitrate, in the denitrification process, and for the RBCOD and phosphorus, in the phosphorus release process. These profiles were determined by simultaneously solving the pairs of equations given above (eqs 4-5 and 6-7). The Newton method was used to solve these equations with the objective function being the minimal residual sum of squared errors associated with the difference between these theoretical profiles and the experimental results obtained in the batch test. The values of the parameters associated with this minimum constitute the solution. In an effort to ensure a correct estimation, the typical ASM2d parameter values11 were taken as initial values in the parameter estimation. More information about the fitting procedure can be found elsewhere.13 Analytical Techniques. In the batch test, the samples were taken by syringe and were immediately filtered through a Millipore glass fiber filter. After filtration, samples were analyzed for the following parameters: COD, N-NO2, N-NO3, P-PO4, and N-NH4. In addition, volatile suspended solids (VSS) and phosphorus content in the sludge (XPT) were determined in the activated sludge. All the analysis of the sludge and settled effluent wastewater were performed using standard methods.14 Results and Discussion The most important operational conditions and the characteristics of the influent wastewater fed to the UCT process during each experiment are reported in Table 4. As can be seen in this table, the compositions of the influent wastewater were very similar in all the experiments carried out. This was very important, since it allowed us to isolate the effect of the internal recycle ratios on the performance of the UCT process. In general, the obtained COD fractions of the studied wastewaters were within the typical range presented by the domestic wastewaters.7 Regarding the wastewater characterization, it is important to remark on the high values obtained for the COD-to-nutrients (N and P) ratio of the studied wastewaters. These values were obtained because of the addition of wastewaters from the Parshal flume to the overflow of the primary clarifiers. The increase in the COD-to-nutrients ratio can be explained by taking into account that ∼20% of the nutrients are removed in the primary settling against the higher percentage, ∼40%, of the COD removed in this unit operation.21 The values of the COD-tonutrients ratio obtained, 22-38 mgCOD mgP-1 and 17-25 mgCOD mgN-1 were within the typical ranges proposed in the literature to achieve the nutrients removal required.9,22 This was very
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Figure 3. Experimental data points (b) and response surface plot of COD removal percentage as a function of the internal recycle flow rates.
important because the value of these ratios determined the feasibility of the nutrient removal by applying biological processes. Taking into account the values proposed in the literature and the COD-to-nutrients ratio of the studied wastewater, the biological processes were found to be suitable to achieve the nutrient removal from the studied wastewater. It is important to remark on the very low nitrogen-to-phosphorus ratio in the studied wastewater. This very low ratio has been maintained during the last few years in the Ciudad Real WWTP23-25 and could be explained because of the extended use of detergent phosphates. COD Removal. The COD removal from the wastewater was investigated for different values of the internal recycle flow rates. The results obtained are presented in Figure 3. These results indicate that, irrespective of the recycle flow rates applied, the COD removal was always more or less the same, ∼92%. The negligible effect of the internal recycle flow rates on the COD removal could be explained by taking into account that these recycles were designed to enhance the denitrification and phosphorus removal efficiencies by controlling the nitrate and biomass loads into the anoxic and anaerobic compartments, respectively. Although the nitrate and phosphorus removal processes were associated to COD removal, the amount of COD removed by these processes was negligible compared to the COD removed in the aerobic compartment, with the percentage of COD removed remaining almost unaffected. By comparing the COD concentration in the effluent wastewater and the inert COD in the influent wastewater, it was concluded that most of the effluent COD corresponded to the inert fraction, which indicated that the process was very stable and achieved a very good COD removal efficiency.23 Nitrogen Removal. Figure 4 shows the influence of the internal recycle flow rates on the nitrogen removal from the wastewaters. In this figure, it can be seen that the increase of the R1 recycle flow rate from 100% to 250% favored the nitrogen removal. This could be explained because of the higher nitrate load rate coming into the anoxic compartment; however, recycle flow rates > 250% reduced the nitrogen removal efficiency. This could be explained because of the high dissolved oxygen load coming into the anoxic compartment when the R1 flow rate was >250%, which caused an oxygen concentration of ∼0.8 g m-3. The presence of such oxygen concentration in the anoxic compartment led to an aerobic oxidation of most of
Figure 4. Experimental data points (O) and response surface plot of nitrogen removal percentage as a function of the internal recycle flow rates.
the biodegradable COD substrates contained in the wastewater, therefore reducing the denitrification capability of the system. In regard to the effect caused by the R2 recycle, the higher recycle flow rates caused the higher percentages of nitrogen removed. This could be explained by taking into account that high R2 flow rates increased the nitrate and biomass loads coming into the anaerobic compartment, leading this compartment to behave like an anoxic compartment. Under these conditions, the denitrification process could be carried out under optimum conditions because of the availability of biomass and nitrates coming from the internal recycle flow rate and RBCOD coming from the influent wastewater. This supposition was checked by measuring the nitrate concentration in the anaerobic compartment (see Table 4). In this table, tests 1-3, we can see that, for a defined R1 recycle flow rate, the higher R2 flow rate caused the higher nitrate concentration in the anaerobic compartment. Summarizing the results obtained, we can conclude that the optimum nitrogen removal efficiency was achieved by using R1 and R2 flow rates ranging from 200% to 250%. Phosphorus Removal. The results obtained for the phosphorus removal with the different recycle flow rates are presented in Figure 5. The obtained results indicate that, for a defined R1 flow rate, R2 recycle flow rates from 50% to 100% increased the phosphorus removal efficiency; however, an increase in the R2 flow rate over 100% did not enhance the phosphorus removal efficiency. This effect could be related to the biomass concentration in the anaerobic compartment when the different R2 recycle flow rates were applied. In order to check this relationship, the biomass concentrations in the anaerobic compartment were experimentally determined for different R2 recycle flow rates; the results obtained are presented in Figure 6. In this figure, it is important to remark that R2 recycle flow rates > ∼100% do not increase significantly the VSS concentration in the anaerobic compartment. This limitation controlled the biological phosphorus removal in the system, indicating that it is not interesting to work with R2 recycle flow rates > 100%. To study the effect of the R1 recycle flow rate, several flow rates were applied (see Figure 5). In Figure 5, it can be observed
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Figure 7. Phosphorus removal efficiency as a function of the nitratenitrogen concentration in the anaerobic compartment.
Figure 5. Experimental data points (O) and response surface plot of phosphorus removal percentage as a function of the internal recycle flow rates.
Figure 6. Volatile suspended solids concentration as a function of the anoxic-to-anaerobic internal recycle flow rate.
that the higher the recycle flow rate applied, the lower was the phosphorus removal obtained. Only when the flow rate applied was >250%, the phosphorus removal efficiency remained almost unaffected with an increase in the R1 flow rate. These observations could be related to the fact that the higher R1 flow rates caused the higher nitrate and oxygen loads coming into the anoxic compartment. If the nitrate or oxygen recycles to the anoxic compartments are not completely removed, some of them will be subsequently recycled to the anaerobic compartment, where they could cause adverse effects over the Biological Phosphorus Removal (BPR).26-28 Data presented in Table 4 show the clear relationship between the R1 flow rate and the nitrate concentration in the anaerobic compartment. As can be seen in this table, an inadequate selection of the R1 recycle flow rates increases significantly the nitrate concentration in the anaerobic compartment. In this context, the phosphorus removal obtained and the nitrate concentration in the anaerobic compartment seem to be related. In order to evaluate this effect, the phosphorus removal efficiency was plotted against the nitrate concentration in the anaerobic compartment (see Figure 7). The trend observed was
Figure 8. Experimental RBCOD (9), nitrate-nitrogen (O), and phosphorus (0) profiles during the anoxic and anaerobic experiments, sections a and b, respectively. Lines indicate the model fit.
a reduction in the phosphorus removal efficiency when the nitrate concentration in the anaerobic compartment was increased. This could be explained because of the competition between PAO and denitrifying bacteria for the substrates contained in the wastewater. This competition could reduce the available substrate for the BPR, therefore reducing the amount of phosphorus removed. It is important to remark that nitrate concentrations < 1 g m-3 allowed for better phosphorus removal (see Figure 7). This could be explained because the final result of the competition for the RBCOD between denitrifiers and PAO depends on the RBCOD consumption rates of both processes. In order to obtain a deeper understanding and insight into the competition for the RBCOD by the denitrifiers and the PAO, the results obtained in the batch experiments with urban sewage were modeled. The experimental RBCOD and nitrate-nitrogen in the anoxic experiments, as well as the RBCOD and the phosphorus profiles of the anaerobic experiments, were fitted to the model proposed above. The results obtained are presented in parts a and b of Figure 8.
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Table 5. Values of the Main Parameters and Average Absolute-Relative Sensitivities on the RBCOD of the Denitrification and the Phosphorus Removal Processes denitrification
phosphorus release
par.
value
sensitivity
par.
value
sensitivity
qDN KH-NO3 KRBCOD YH
16.5 (mgRBCOD gVSS-1 h-1) 1.6 (gRBCOD m-3) 4.0 (gRBCOD m-3) 0.5 (gVSS gRBCOD -1)
-19.3 (mg L-1) 5.5 (mg L-1) 1.4 (mg L-1) -6.7 (mg L-1)
qPR KRBCOD YPO4
15.9 (mgRBCOD gVSS-1 h-1) 4.0 (gRBCOD m-3) 0.32 (gP gRBCOD-1)
-27.55 (mg L-1) -1.6 × 10-5 (mg L-1) 2.71 (mg L-1)
As a result of the fitting procedure, the values of the most sensitive parameters were determined. These values and the sensitivities of the parameters are presented in Table 5. These results indicate that, for high nitrate concentrations in the anaerobic compartment, the combination of both maximum specific RBCOD consumption rates led to a quick disappearance of the RBCOD contained in the sewage. This quick consumption reduced the RBCOD available for the biological phosphorus removal process, therefore reducing the phosphorus removed by this way. Taking into account that both maximum specific RBCOD consumption rates were very similar, the RBCOD distribution between denitrifiers and PAO could be approximately on a 50%-50% basis. However, when the nitratenitrogen concentration dropped under 1 g m-3, the RBCOD consumption rate of the denitrification process decreased significantly, reaching values of ∼4.3 mgRBCOD gVSS-1 h-1. This new RBCOD consumption rate led to a new distribution of the RBCOD between denitrifiers and PAO of ∼20-80%, which explains why the biological phosphorus removal process was enhanced when the nitrate-nitrogen concentration was 1 mg L-1, only 50% of the RBCOD was consumed by PAO; however, when the nitrate concentration was
