CRUDE COD CHARACTERISTICS SIGNIFICANT FOR BIOLOGICAL P REMOVAL: A U.K. EXAMPLE

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1 CRUDE COD CHARACTERISTICS SIGNIFICANT FOR BIOLOGICAL P REMOVAL: A U.K. EXAMPLE Jarvis, S. 1, Burger, G. 2, Du, W. 2, Bye, C. 2 and Dold, P. 2 1 Thames Water Utilities Ltd., 2 EnviroSim Associates Ltd. Corresponding Author Tel dold@envirosim.com Abstract Characteristics of the influent wastewater have a significant impact on biological phosphorus removal performance, and should be considered in design and operation. A study conducted on crude sewage at the Reading STW demonstrated that the COD fractionation is similar to averages from many studies outside of the U.K. The nitrifier growth rates are favourable; that is important as this essentially sets the STW operating sludge age and the size of a nutrient removal plant. The readily biodegradable COD (RBCOD) fraction is the most important characteristic in determining nutrient removal performance. A higher particulate unbiodegradable COD fraction increases the required plant volume. If the RBCOD content is low, P removal performance can be supplemented by increasing the size of the anaerobic zone. Typically that reduces nitrogen removal performance. Keywords Wastewater characteristics; biological phosphorous removal; readily biodegradable COD; particulate unbiodegradable COD; nitrifier growth rate; solids retention time Introduction The composition of crude sewage entering municipal sewage treatment works (STWs) differs from site to site. The term wastewater characteristics refers to the partitioning of influent organic material into soluble/particulate biodegradable and unbiodegradable (inert) portions (see Fig. 1). Fractionation of influent nitrogen (N) and phosphorus (P) should be included in the concept of characterizing the crude sewage. Wastewater characteristics have a very significant impact on system performance, particularly for nutrient removal systems. For example, a single characteristic such as the readily biodegradable chemical oxygen demand (COD) fraction can determine whether or not a system designed for excess phosphorus removal will in fact remove phosphorus. A second example is the portion of unbiodegradable particulate COD in the influent wastewater; this impacts sludge production and activated sludge process volume requirements. Therefore, accurate knowledge of wastewater characteristics is extremely important for process design and analysis. This paper focuses mainly on the impact of wastewater characteristics on the performance of biological P removal systems. However, most P removal systems typically are required to nitrify (and usually denitrify). Therefore nitrification performance is an important consideration in biological nutrient removal plant design and/or process optimization. Nitrification kinetics are crucial given that they largely determine the size (and hence cost) of the secondary treatment tankage and its operation, once the flows and contaminant loads to be treated are defined.

2 Nitrification performance essentially is quantified by the maximum specific growth rates of the nitrifiers (ammonia oxidizing bacteria, AOBs, and nitrite oxidizing bacteria, NOBs) in the system. Experience has shown that nitrifier growth rates may vary substantially from plant to plant; in this sense the growth rates can be regarded as a wastewater characteristic. The implication of low nitrifier growth rates is that the system must be operated at a long SRT to avoid washout of nitrifiers. This in turn translates into an increased sludge mass in the system, resulting in either increased reactor tankage and clarifier area for new plant designs or reduced treatment capacity for existing plants. Typically, if pilot testing is not conducted to measure the plant-specific nitrification rates, design and/or capacity rating necessarily should be based on conservative (low) estimates. This in turn can have a substantial capital cost and planning implications. The main objective of this paper is to outline a number of key aspects in nutrient removal plant design, and demonstrate how certain wastewater characteristics impact P removal performance, and should be considered in design and operation. The paper also presents results from a wastewater characterization study (with nitrification rate measurement) performed recently at the Reading STW. Total Influent COD COD T,INF Biomass Biodegradable COD Unbiodegradable COD Readily Biodegradable RBCOD (S S ) Slowly Biodegradable SBCOD (X S ) Soluble Unbiodegradable S I Particulate Unbiodegradable X I Complex S S,F SCFA S S,A Colloidal S COL Particulate X SP Figure 1: Components of STW influent COD Reading STW Wastewater Characterization Study The approach used to estimate the crude wastewater characteristics and nitrification kinetics of the Reading STW essentially followed the Low F:M procedure presented in the Water Environment Research Foundation wastewater characterization report (WERF, 2003). This study likely was the first application of this procedure in the UK. The Low F:M protocol involves operating a laboratory-scale sequencing batch reactor (SBR) for several weeks to attain a quasi steady-state, and then conducting intensive monitoring over a period of approximately two weeks. Data from the intensive testing period provides estimates of: A range of wastewater characteristic fractions (e.g. unbiodegradable soluble and particulate COD; readily biodegradable COD; unbiodegradable soluble organic nitrogen, etc.); and

3 The nitrifier maximum specific growth rates (µaob, µnob). Figure 2: SBR setup in the Reading STW R&D Laboratory The SBR was located in the Thames Water Innovation Laboratory on site at the Reading STW, and operated with Reading STW crude sewage as influent. In the protocol for wastewater characterization, a single cycle in SBR operation consists of five operating modes or periods. The periods are fill, react, waste, settle, and draw (decant), in sequence. The SBR is operated on the basis of a 24-hour cycle, with a selected maximum volume (10 L). The volume of decant (effluent) withdrawn after the settle period is equal to the volume of wastewater added at each cycle (8 L), less that wasted in the previous cycle. At start up, the system was seeded with mixed liquor from the Reading STW aeration tanks. Following start up, quasi steady-state conditions were achieved by repeating the 24 hour cycle over a period of 46 days (approximately 3-4 sludge ages). The average crude sewage concentrations measured at the Reading STW during the spring of 2013 are shown in Table 1. These concentrations are typical for a medium strength municipal wastewater. Table 1: Average crude sewage concentrations measured at the Reading STW Parameter Units Value TCOD mgcod/l 432 TKN mgn/l 42.9 TP mgp/l 5.09 ISS mgiss/l 53 Table 2 presents the Reading STW crude sewage fractions measured during the study. BioWin 4.1 default values for the parameters are listed alongside the measured values as a point of reference. The defaults are averages from studies conducted on many crude wastewaters outside of the U.K. It is interesting to note the close

4 correspondence between the Reading STW data and the defaults. [One exception is the fnus fraction the soluble inert organic nitrogen content. The high value was the result of an unusual maintenance activity at an adjacent water treatment plant that happened to take place during the intensive sampling of the SBR]. Table 2: Crude sewage characterization measured at the Reading STW Parameter Reading STW BioWin 4.1 Units fbs Fraction of influent COD that is soluble readily biodegradable fus Fraction of influent COD that is soluble unbiodegradable mg COD / mg COD mg COD / mg COD fup Fraction of influent COD that is particulate unbiodegradable mg COD / mg COD fxsp Particulate fraction of influent slowly biodegradable COD mg COD / mg COD fna Fraction of influent TKN that is ammonia mg N / mg N fnus Fraction of influent TKN that is soluble unbiodegradable mg N / mg N fnox Fraction of influent TKN that is particulate organic mg N / mg N fpo4 Fraction of influent TP that is soluble phosphate mg P / mg P fcv Particulate COD/VSS ratio mg COD / mg VSS On two days during the intensive monitoring period of SBR operation, profiles of ammonia, nitrite and nitrate concentration were monitored over the first 8 hours of the react period. Analyses were performed on small sample volumes (20 ml) withdrawn from the reactor (and filtered immediately) at intervals of 30 or 45 minutes. The nitrifier maximum specific growth rates (µaob, µnob) were estimated through simulation, iteratively adjusting their values (starting with default values) to obtain a fit to the ammonia, nitrite, and nitrate profiles on the two days where profile data were gathered. An example of the model fit is shown in Fig. 3. The μaob and μnob values estimated from the Reading SBR were 1.04 and 0.80 d -1, respectively. Both values are

5 slightly higher than the default values of 0.9 (μaob) and 0.7 (μnob) d -1 in BioWin (referenced to 20 C). This indicates that there likely are no issues with inhibitory components in the crude sewage. Figure 3: Simulated (solid lines) and observed (points) ammonia, nitrite and nitrate responses over the start of an SBR cycle Important Considerations in Nutrient Removal STW Design Biological nutrient removal systems incorporate unaerated zones (or periods in SBR systems): (i) anaerobic zones for P removal systems, and (ii) anoxic zones if N removal is also an objective. As noted earlier, most nutrient removal systems are required to nitrify. Nitrifying organisms are slower growing than heterotrophs, and therefore operating at a long enough sludge age or solids retention time (SRT) to avoid washout is a basic requirement for sustaining nitrification. In addition, nitrifiers grow only under aerobic conditions, so this introduces the concept of an aerobic SRT for nitrification. Adding unaerated zones means that the total SRT for the system must be increased beyond the nitrification aerobic SRT. Increased SRT translates into increased plant size. Biological P removal is mediated by phosphorus accumulating organisms (PAOs) and is largely driven by volatile fatty acid (VFA) availability (preferably acetate) in the anaerobic zone of continuous flow processes, or during the anaerobic phase in systems such as nutrient removal SBR systems. PAO growth depends on PAOs sequestering VFAs in the anaerobic zone. VFA uptake is linked directly to release of phosphate from stored polyphosphate in PAOs. An elevated soluble phosphate concentration in the anaerobic zone is an indicator that VFA uptake by PAOs (and conversion to PHA storage products) is occurring. Subsequently the PHA is utilized for growth of PAOs in the downstream anoxic and aerobic zones to sustain the bio P removal in the system; the higher the number of PAOs, the greater the potential for P removal. Typically the amount of VFA in the influent wastewater is limited (e.g. 5 to 15 mg/l), and is insufficient for effective P removal. A rule-of-thumb is that 8 mgcod/l of acetate is required for removal of 1 mgp/l. A second source of acetate is from

6 fermentation of influent readily biodegradable COD (RBCOD) in the anaerobic zone. Ordinary heterotrophic organisms (OHOs) mediate this fermentation. Another rule-of-thumb is that approximately 15 mg/l of RBCOD is needed for removal of 1 mgp/l.; substantially more than the 8 mg/l of acetate. To understand this difference, one must recognize that fermentation is a growth process. The yield (Y) of biomass in the anaerobic process is low compared to Y for aerobic growth; typically Y = 0.10 mg biomass COD/mg COD utilized (versus 0.66 for aerobic growth). The stoichiometry of fermentation and the distribution of fermentation products depend on many factors, including dissolved hydrogen partial pressure. For low hydrogen partial pressure (typical for an anaerobic zone in a bio P system) fermentation pathways for typical RBCOD components such as glucose indicate yields (all as COD fractions for 1 unit of RBCOD consumed) of: YBIOMASS = 0.10 YH2 = 0.35 YACETATE = 0.55 For 15 mg/l of RBCOD utilized in fermentation, the acetate production based on this stoichiometry would be 15 x 0.55 = 8.25 mg/l; hence the rules-of-thumb suggesting 8 mg/l of acetate or 15 mg/l of RBCOD being required for removal of 1 mgp/l. A related point to note about fermentation is that the yield of hydrogen is significant. In certain situations some of the dissolved hydrogen could be utilized in biological reactions such as sulfate reduction. However, it is likely that much of the dissolved hydrogen is stripped in downstream aerated reactors. This translates into a COD loss in bio P systems, and the associated benefits of reduced aeration requirements and reduced sludge production. In the discussion so far, two functions of the anaerobic zone have been identified: Fermentation of RBCOD from the influent to produce VFAs. Providing conditions for sequestration of acetate (from the influent and from fermentation) and storage as PHA to support growth of PAOs. In many situations the amount of acetate directly from the influent and from fermentation of influent RBCOD still is insufficient for good P removal performance. However, there is a third source of VFAs within the anaerobic zone; namely, from fermentation of RBCOD generated from hydrolysis of slowly biodegradable particulate COD (XSP). The XSP available for hydrolysis comes from different sources: The influent wastewater; Decay of active biomass (OHOs, PAOs, AOB, NOB, etc.). In assessing particulate substrate hydrolysis as a source of RBCOD for fermentation and VFA production, several factors should be recognized: Hydrolysis of XSP should not be regarded as a process that suddenly switches on as a source for generating VFAs. Hydrolysis of XSP is occurring in the anaerobic zone in conjunction with other processes such as VFA sequestration and fermentation. XSP from the influent is immediately available for hydrolysis, particularly in the anaerobic zone at the front of the process. The amount of influent XSP remaining in the RAS is limited because the major portion has been utilized by the downstream end of the process. Organism decay is a slow process, so XSP from this source is only available at a slow rate.

7 Hydrolysis is mediated by heterotrophic organisms, so rates of hydrolysis increase with increasing mixed liquor concentration. Irrespective of where the VFAs are generated, to promote growth of PAOs and improve P removal, the VFAs must be combined with mixed liquor where the PAOs have internal stored polyphosphate. That situation enables VFA uptake with P release. [If excess VFAs are produced, this may promote growth of undesirable organisms such as GAOs]. From the foregoing discussion two important factors regarding the sizing of the anaerobic zone in biological P removal systems should be evident: If the anaerobic zone is too small then fermentation of influent RBCOD may not be complete. If the amount of influent VFA and RBCOD is limited, this mitigates for a larger anaerobic zone to promote hydrolysis of influent XSP at a location where VFA production can enhance PAO growth. The earlier discussion on SRT noted that increasing the unaerated portion of the nutrient removal system results in an increased overall SRT and a larger plant volume (for a given operating MLSS). For that reason designers would prefer a smaller anaerobic zone. However, the danger is that the zone may be undersized to meet the biological requirements for sustaining strong PAO growth. The main points to come out of this discussion is that the design, sizing, performance and optimization of nutrient removal systems is heavily dependent on influent wastewater characteristics, in particular: The influent readily biodegradable COD fraction (FBS) (and the VFA subfraction); The nitrifier maximum specific growth rates. Another important wastewater characteristic is the particulate unbiodegradable COD fraction (fup). This inert material accumulates within the system, adding to the VSS concentration, and is only removed via the surplus sludge stream. Selecting SRT for Nutrient Removal Systems The required SRT depends on the maximum specific nitrifier growth and decay rates. The design should be based on the kinetic parameters at the minimum operating temperature. The following equation for estimating the SRT incorporates the effect of having an unaerated fraction (fua) of the mixed liquor; nitrifier growth takes place only in the aerated part of the process (1 fua) but decay occurs in all zones. The equation should include a safety factor (S.F.) to make allowance for design uncertainties and diurnal loading variations. SRT = 1 (1 f UA ).μ T,min b T,min. (S. F. ) [1] The symbols μt,min and bt,min are the respective growth rate and aerobic decay rate of nitrifiers at the minimum plant operating temperature. [Note that the term unaerated fraction refers to the unaerated mass fraction and not the volume fraction. This distinction becomes important in systems where the MLSS concentration differs significantly from reactor to reactor. Examples of the latter are UCT-type configurations and MBR systems].

8 The nitrifiers are comprised of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB). The overall ammonia removal rate in nitrification essentially is determined by the AOB. Hence it is probably appropriate to apply the AOB parameters in the calculation of SRT. The values of μt,min and bt,min depend on temperature according to the following equations where θ is the Arrhenius value (θ = for μt,; θ = for bt): μ AOB,T = μ AOB,20 C θ (T 20) [2] b AOB,T = b AOB,20 C θ (T 20) [3] Taking typical values, if μaob,20 C is 0.9 d -1 and baob,20 C is 0.17 d -1, then at 12 C, μaob,12 C is d -1 and baob,12 C is d -1. Assuming an unaerated fraction (fua) of 0.45 and S.F. of 1.5, SRT is calculated as follows: SRT = 1 (1 0.45).(0.516) (1. 5) 10 d [4] As an overall comment on SRT, nutrient removal is favoured by a shorter SRT. As the SRT Increases the sludge production per unit influent load decreases. However, the N and P components in the SAS do not change with SRT. Hence, increasing SRT implies less N and P in the SAS stream, leaving more N and P to be removed from the liquid stream. Nutrient Removal STW Example An objective of this paper is to illustrate the impact of various factors on P removal. For this purpose we will consider a single nutrient removal process configuration as an example. It should be noted that many factors enter into the selection of process configuration and the design and operating parameters. Therefore the assessment below should not be considered as a rigorous design case. The flow sheet for the example plant is shown in Fig. 4 below. This is a 3-stage Bardenpho configuration with anaerobic and anoxic zones. Crude sewage enters the activated sludge system without any primary sedimentation, and the impact of return streams from solids handling is not considered. Figure 4: Layout of typical BNR plant modelled in BioWin 4.1

9 The influent flow rate for the example is 24,000 m 3 /d. The crude sewage concentrations from the Reading study listed in Table 1 were applied, but default wastewater characteristic parameters were assumed (including fbs = 0.16 and fup = 0.13). SRT for the system was based on assuming an AOB growth rate μaob,20 C of 0.7 d -1 (somewhat lower than the BioWin default) and a minimum plant mixed liquor temperature of 12 C. The unaerated fraction was set at 0.45; typically it is suggested that the aerated portion should not be less than 55%. Applying Equation (1) indicates a design SRT of 18 days at the minimum temperature. The total reactor volume was set at 18,000 m 3. For the influent load this results in a mixed liquor suspended solids (MLSS) concentration of approximately 3,700 mg/l. The underflow from the secondary clarifier is paced at 50% of the influent flow rate and the nitrified mixed liquor recycle is paced at 300% of the influent flow rate. An important part of design is deciding how to divide the unaerated fraction between the anaerobic and anoxic zones. For the base case the 0.45 fraction was split with 0.12 as anaerobic and 0.33 as anoxic. In the simulations each of the zones (anaerobic, anoxic, aerobic) is represented as several reactors in series to approximate a partial plug flow regime. The sludge mass distribution in the reactors (excluding sludge in the secondary clarifier) is shown in Fig. 5. Figure 5: Sludge mass distribution in the baseline example In the following sections all model scenarios are simulated at a temperature of 20 C with an SRT of 18 days. Obviously at the higher temperature the operating SRT could be reduced. However, for simplicity we will not investigate the impact of SRT and the many other factors that should be considered in a comprehensive study. The main objective is to illustrate the importance of wastewater characteristics. Figure 6 illustrates the inorganic nitrogen species profiles along the length of the system for the base case; ammonia in the upper section and nitrite/nitrate in the

10 lower. Figure 7 shows the soluble phosphate profile with the characteristic release in the anaerobic zone and uptake in the aerobic zone. Figure 6: Inorganic N profiles for the baseline example Figure 7: Soluble PO4-P profiles for the baseline example Impact of RBCOD on P Removal Performance The nutrient removal STW performance was simulated at steady state over a range of influent RBCOD fractions (fbs) from 0.08 to The predicted effluent TP and soluble PO4 concentrations, and reactor PAO concentration are plotted versus fbs in Fig. 8. The offset between the TP and soluble PO4 concentrations relates to the effluent suspended solids. The plot clearly shows that P removal performance is closely linked to the RBCOD concentration, improving steadily as influent RBCOD increases. As influent fbs increases, the amount of VFAs generated in the anaerobic zone increases and hence more PAOs are able to grow in the system.

11 Figure 8: Effluent TP and PO4 concentrations and reactor PAO concentration versus influent RBCOD fraction (fbs) Impact of Anaerobic Mass Fraction on P Removal Performance In the base case the effluent soluble PO4-P was 0.9 mgp/l; the effluent TP slightly exceeded 1 mgp/l. This was with an influent RBCOD fraction of 0.16 and an anaerobic mass fraction of In the earlier discussion it was noted that a possible means for compensating for an insufficient amount of influent RBCOD is to increase the anaerobic mass fraction. The STW performance was simulated at steady state with a fixed influent RBCOD fraction (fbs) of 0.16, but varying the anaerobic mass fraction over a range from 0.05 to The predicted effluent soluble PO4 concentration and reactor PAO concentration are plotted versus the anaerobic mass fraction in Fig. 9. The plot clearly shows that P removal performance can be improved by increasing anaerobic mass fraction in this case. As before, the improved performance is a result of the increased PAO concentration. For the simulations the total unaerated mass fraction was maintained at As the anaerobic mass fraction was increased, so the anoxic mass fraction was decreased. Figure 10 shows the same effluent soluble PO4 concentration plot, but this time with the effluent NOX concentration. As expected, the effluent NOX increases due to the decrease in anoxic mass fraction. Consequently more NOX is recycled in the RAS to the anaerobic zone. Denitrification of the additional NOX reduces the amount of RBCOD available for P removal. That is the main factor for the plateau in PAO concentration at increased anaerobic mass fraction shown in Fig. 9.

12 Figure 9: Effluent soluble PO4 concentration and reactor PAO concentration versus anaerobic mass fraction Figure 10: Effluent soluble PO4 and NOX concentrations versus anaerobic mass fraction

13 Impact of Particulate Unbiodegradable COD Fraction (fup) on Process Volume Requirements The particulate unbiodegradable COD fraction (fup) is important in nutrient removal plant design. This fraction does not relate directly to P removal performance; however less biodegradable COD does have an adverse impact on nutrient removal performance. The main significance of the fup fraction is the impact on process volume requirements. This inert material accumulates within the system, adding to the VSS and MLSS concentrations, and is only removed via the surplus sludge stream. Figure 11 shows a generalized plot of process volume required as a function of SRT for crude sewage fup fractions of 0.13 (typical) and The plot was generated for the influent wastewater composition from Table 1, and assuming a design MLSS of 3,700 mg/l. Nutrient removal plants generally are operated at longer SRTs, and as SRT increases the impact of a higher fup becomes more important. In this example the process volume required for a 20 d SRT increases by 33% for the increase in fup. Figure 11: Impact of particulate unbiodegradale COD fraction (fup) on process volume requirements Conclusion The paper has demonstrated that characteristics of the influent wastewater have a significant impact on biological phosphorus removal performance, and should be considered in design and operation. A study conducted on crude sewage at the Reading STW established that the COD fractionation is similar to averages from many studies outside of the U.K. The nitrifier growth rates are favourable with this wastewater. That is important because the growth rates essentially set the STW operating sludge age or SRT and the size of a nutrient removal plant.

14 The readily biodegradable COD (RBCOD) fraction is the most important characteristic in determining nutrient removal performance. P removal improves with a higher RBCOD fraction. If the RBCOD content is low, P removal performance can be supplemented by increasing the size of the anaerobic zone. Typically that reduces nitrogen removal performance. A higher particulate unbiodegradable COD fraction in the crude sewage necessarily has an adverse impact on nutrient removal. However, the main effect of a higher fup is the increased plant volume required. Reference WERF (Water Environment Research Foundation) (2003) Methods for wastewater characterization in activated sludge modeling. Project 99-WWF-3, ISBN Alexandria, Virginia.