aquifas 3 tertiary paper

MINIMIZING AEROBIC AND POST ANOXIC VOLUME REQUIREMENTS IN TERTIARY INTEGRATED FIXED-FILM ACTIVATED SLUDGE (IFAS) AND MOVING BED BIOFILM

REACTOR (MBBR) SYSTEMS USING THE AQUIFAS MODEL

Dipankar Sen*, Sudhir Murthy**, Heather Phillips***, Vikram Pattarkine****, Rhodes R. Copithorn*5, Clifford Randall*6,

*Santa Clara Valley Water District

1290 Bryant Avenue Mountain View, CA 94040

**DC WASA, Washington DC

***Black & Veatch, Kansas City, MO ****Brinjac Engineering, Harrisburg, PA

*5Stearns & Wheler, Bowie, MD *6Virginia Tech, Blacksburg, VA

ABSTRACT Research was undertaken to calibrate and verify the Aquifas semi-empirical and biofilm 1D models against a full scale Integrated Fixed-Film Activated Sludge (IFAS) system. The model was then used to evaluate and identify alternatives to upgrade the performance of tertiary IFAS system and Moving (mobile) Bed Biofilm Reactor (MBBR) systems that could minimize volume required for nitrogen removal. For the verification, the model was operated in a dynamic simulation mode over several 31 day periods was evaluated against observations from a full scale IFAS system. The evaluation was performed for the semi-empirical and the biofilm 1D versions. While both versions were able to predict the average effluent ammonium-N, oxidized-N, reactor MLSS, MLVSS and waste sludge production accurately (within 10% for MLSS, MLVSS, WAS and N, 20% at N concentrations less than 1 mg/L), the biofilm 1D model was able to simulate the day-to-day variations in nitrogen forms better. Also, the biofilm 1D model was able to predict the biofilm thickness and growth to within 20% in each of the aerobic cells operated with media. Because of the significantly longer time required to run Biofilm 1D models and similarity in the results for steady state and average 31 day averages in a dynamic simulation mode, the semi-empirical model was used to analyze alternatives several alternatives for tertiary removal together with with limited number of runs of Biofilm 1D model. Several configurations were evaluated for an existing tertiary activated sludge system. The conditions evaluated were similar to those observed at the DCWASA (Blue Plains) plant. The system was operated with 40 to 60% aerobic volume and 40% post-anoxic volume (with methanol addition) at MLSS MCRTs (mean cell residence time) of 8 to 15 days. The plant can achieve near complete nitrification but suffers from partial loss of denitrification in winter (NOxN increases from 4 to 12 mg/L). Winter temperatures drop to a range between 13 and 15 C. The TSS, VSS and BOD5 loadings to the tertiary system can increase during wet weather conditions. This necessitates evaluation of both normal and wet-weather conditions for tertiary systems and coupling them to the performance of upstream secondary systems. The configurations evaluated included (a) activated sludge, (b) activated sludge with media added to the anoxic cells, and (c) activated sludge with media added to both anoxic and aerobic cells. Following the application of media with a biofilm surface area of 350 m2/m3, the model showed

that the effluent oxidized-N could be reduced from 11 mg/L to 5 mg/L, as simulated under winter wet weather conditions. These conditions were evaluated at denitrification rates observed for methanogenic bacteria (0.7 to 0.75 d-1 at 15 C). Additionally, methanol dosing can be increased by 20 % during winter wet weather conditions without increasing the effluent COD to enhance the denitrification rates without impacting the effluent soluble COD (SCOD). Several additional alternatives were considered for plants where a new tertiary tank can be added downstream of a high rate secondary treatment system similar to Harrisburg, PA. These alternatives include a new MBBR dedicated to nitrification and discharging directly to a denitrification filter (no clarifiers downstream of a MBBR), nitrification and denitrification cells within the same MBBR volume (40% aerobic, 40% anoxic, 20% reaeration), and nitrification and denitrification in an IFAS operated at a MLSS MCRT of 4 days within the same volume. In the MBBR, the effluent MLSS increased from 25 to 35 mg/L when methanol was added and the effluent oxidized-N decreased from 12 to 6 mg/L, while maintaining an effluent ammonium-N below 1.5 mg/L at 10 C. The biofilm specific surface area had to be increased from 150 to 225 m2/m3. The IFAS system, which can nitrify and denitrify in both the biofilm and the mixed liquor had 1000 mg/L MLSS and achieved better performance ( 0.5 mg/L in the tank. Increasing the post-anoxic volume and HRT (to compensate for the lower denit rates) at the expense of some aerobic volume affects the ability to fully nitrify. Therefore, at land-locked facilities, there is a need to improve both the nitrification rates in the aerobic zone and the denitrification rates in the post-anoxic zone. This study was undertaken to evaluate process configurations and design tools that could help identify solutions and improve the reliability of designs. The focus of this paper is on tertiary systems but the results can be applied to secondary systems with post-anoxic zones operated for ENR.

OBJECTIVES The objectives of this research were to:

1. Calibrate and verify the Aquifas model in a dynamic simulation mode against extended periods of input and output data from a full-scale IFAS facility

2. Identify techniques to maximize nitrification and denitrification in existing or new tertiary tanks originally sized for nitrification

3. Evaluate techniques to combine nitrification and denitrification within a multi-cell reactor 4. Address specific challenges in tertiary systems such as changes in the loadings from the

secondary treatment system during winter high flows. METHODS Identify Representative Plants The researchers identified two representative plants that could serve as templates to illustrate the challenges faced in tertiary nitrogen removal, and develop and evaluate alternatives. The first was the Blue Plains WWTP of the District of Columbia Water and Sewer Authority (DCWASA) that receives 1,400,000 m3/d of flow. This is a two stage activated sludge system: first stage with a 2 hour hydraulic retention time (HRT) for BOD removal and second stage with a 3.6 hour HRT for nitrification. The first and second stage cannot be operated in parallel. The second stage has five cells in series. The plant has been improving nitrogen removal each year by aerating the first two stages, using the third as a swing (switch zone) that is aerated only during certain periods of ammonia bleedthrough in winter, and converting the fourth and fifth stage to an anoxic cell with methanol feed to the fourth stage (Figure 1). The mixed liquor is reaerated in the channel downstream of the reactor. This configuration has worked well in warmer weather with the plant achieving full nitrification and as low as 4 mg/L oxidized-N. However, in colder weather, when the upstream secondary system discharges higher TSS and BOD loadings during wet weather, the tertiary system has difficulty maintaining good denitrification. The effluent nitrate-N can increase from 8 to 12 mg/L. This has been attributed to the washout of denitrifriers when when the tertiary system MCRT decreased at higher loadings. The second facility is Harrisburg, PA, which receives 75,000 m3/d of flow and has a capacity of 120,000 m3/d. The original facility is designed for BOD5 removal. New tanks are being considered for nitrification in MBBRs followed by denitrification in denitrification filters. The effluent from the MBBR is not followed up with clarifiers, instead it goes directly to the denitrification filters. This paper examines what the volume (and clarification) requirements would be if a nitrifying MBBR were to be upgraded as a MBBR for nitrification and denitrification using the same tank volume and as an IFAS. The intent is not to recommend a process but to evaluate options that could be available to facilities considering tertiary nitrification and denitrification. Developing and Verifying Model for IFAS and MBBR One of the challenges in modeling IFAS and MBBR systems using Aquifas, BIOWINTM or GPS-X is the limited full-scale verification. For this reason, during this research, full scale verification was undertaken as the first step. The results of modeling with Aquifas are summarized here.

The Aquifas model was run for several 31-day periods of data from the Broomfield WWTP, CO (Rutt et al., 2006, Sen et al., 2006). This facility is an IFAS plant operated with anaerobic, anoxic and aerobic cells (Figure 2). The daily data of the reactor influent (primary effluent) are shown in Table 1. The results of the modeling are shown in Figures 3, 4, and 5.

Aerobic Aerobic Unaerated

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Figure 1: Schematic of Tertiary Activated Sludge or IFASfor Nitrification and Denitrification

RAS (30 to 50%)

Nitrate Recycle (0 to 200%)

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Anox Kaldnes K1 Media at 30% Media Fill FractionBiofilm (Effective) specific surface area of 150 m2/m3

Figure 2: Layout of Broomfield WWTP, CO, USA

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Plant Description The Broomfield plant is designed with influent screening and partial flow equalization in the headworks. It has primary clarifiers. The primary effluent is sent to the secondary treatment system that is operated in the A2O or modified Johannesburg configuration. The secondary treatment system is operated with Kaldnes K1 media in the aerobic cells at a 30 percent fill. This results in a biofilm specific surface area of 150 m2/m3 (when the media is sufficiently loaded in the winter season to create a biofilm on it). The mixed liquor is operated at around a 5 day MLSS MCRT of which 3.25 days is aerobic. The plant has secondary clarifiers.

The total volume of the activated sludge tanks is 7015 m3. Of this, 15 percent is in the two anaerobic cells (also designated as one preanoxic and one anaerobic) that receive the primary effluent and the RAS, 20 percent is in the two pre-anoxic cells that also receive the mixed liquor recycle, and 65 percent (4550 m3) is in the two aerobic cells. Both aerobic cells have the Kaldnes K1 media that is currently at 30 percent fill (media fill fraction, mf = 30%). Bare media surface area is 750 m2/m3 on the inside of the cylinder and on the two cross vanes. The biofilm surface area is 500 m2/m3 based on 1 mm thickness and growth on the inner surface. The aerobic cells are 4.5 m deep. They are aerated with coarse bubble diffusers that are installed 0.25 m above the floor and concentrated at certain points within a grid on the floor (approximately 10 percent of the floor is covered with diffusers). This results in a roll pattern that is similar to a quarter point arrangement (WEF Aeration Manual of Practice, 1988; Rooney and Huibregtse, 1980). The media is retained by screens. The plant has two similar parallel trains. The plant is operated similar to an activated sludge system with additional monitoring for the media. The plant takes samples of the mixed liquor, strains it through a metal sieve to separate the media from the MLSS, and measures the MLSS, MLVSS and the amount of growth on the media (referred to as fixed growth below). To measure the growth, the media with the biofilm is dried and weighed. Its weight is compared to the weight of bare media without the biofilm. Alternatively, the oven dried biomass (at 105 C) can be removed by physical scraping or chemical rinsing and the dry media weighed again to obtain the weight of the biomass. While the plant does not measure the biofilm thickness on a weekly basis, visual observations show that the biofilm is thinner in summer than in winter. It is thicker in the first aerobic cell (~ 1 +/- 0.2 mm) than in the second cell (~ 0.6 mm +/- 0.2 mm). Plant Data The operating data for December are shown in Table 1. The primary effluent flow averaged 20,000 m3/d (5.3 MGD). The RAS was 40 percent of this flow (8,000 m3/d). The operator estimated the nitrate recycle to be 150 percent (30,000 m3/d). Primary Effluent The plant measures primary effluent flow and temperature daily; it measures the BOD5, TSS, NH4N, NO2N and NO3N five times each week; and alkalinity once a week. (Note: in this paper, NH4N is the sum total of NH3N and NH4N, at a pH of 7, most of it exists as NH4N). The plant measures the COD/BOD5 ratio once a month. This ratio averaged 2.7 for three years of data (2004-2006) and was 2.67 for a single sample in December. While there was no measurement of the TKN in the primary effluent, discussions with AnoxKaldnes indicated that their samples showed an average TKN of 40 mg/L for the typical primary effluent (110 to 120 mg/L BOD5). A TKN/NH4N ratio and a TSS/Particulate Organic N ratio were used to generate the TKN levels for December 2006 (Table 1). Additionally, there were a few measurements which showed that 80 percent of the COD measured was filtered COD. There were no data on the VSS in the primary effluent. Based on typical data, primary effluent VSS was estimated to be 80-85 % of the TSS. Aerobic Cells The plant measures MLSS and MLVSS in the aerobic cells and the amount of biomass in the biofilm. In December 2006, the MLSS averaged 1630 mg/L and the MLVSS averaged 1270 mg/L,

resulting in a percent VSS of 78%. The fixed biomass averaged 2050 mg/L in the first aerobic cell and 1104 mg/L in the second aerobic cell. For a cell volume of 2271 m3 and a specific surface area of 150 m2/m3, this quantity of fixed biomass equates to 13.7 and 7.4 kg/1000 m2 of biofilm surface, respectively. At a biofilm density of 12.5 kg/m3 (12,500 mg/L), the biofilm thickness would be 1.1 mm in the first aerobic cell and 0.6 mm in the second cell. The mixed liquor temperature dropped from 19 C at the beginning of the month to 13 C at the end of the month. The DO averaged 4.2 mg/L in the first aerobic cell and 5.6 mg/L in the second aerobic cell. The NH4N and NOxN levels were not measured in the anaerobic, anoxic and aerobic cells. Secondary / Plant Effluent The plant achieved complete nitrification in December 2006 with effluent averages less than 0.2 mg/L for NH4N. The NOxN was 14.4 mg/L of which less than 0.1 mg/L was as NO2N. The NO3N was higher than expected, the reasons for which are discussed later. The effluent TSS and VSS in the plant effluent were less than 5 mg/L. Modeling IFAS in Aquifas Aquifas can model the biofilm in the IFAS system by two different approaches

a) semi-empirical equations b) 1 and 2 D biofilm modeling

The advantage of the semi-empirical equations is that computations are much faster, especially in for dynamic simulation of 31 days of data. The advantage of the 1 and 2D biofilm modeling, where Aquifas breaks the biofilm up into 12 layers, is that it provides additional data such as the the biofilm thickness for each cell and biofilm yield. Also, it provides a second method of computation of flux rates. Since the semi-empirical equations are calibrated to real life conditions, they provide the requisite accuracy. The section below presents the results of both models and compares them to each other and to the plant data. Aquifas can be run with different levels of primary effluent characterization. For this run, the influent was characterized as described above, which is similar to requirements for Biowin and GPS-X models. The denitrification kinetics for the MLVSS (in the absence of media in the anoxic cells) were based on the estimated value of flocculated filtered biodegradable COD (40% of the total COD, half the filtered COD) (Mamais et al, 1993). The phosphorus removal mechanisms in the anaerobic cells were driven by the VFAs in the primary effluent and those generated through fermentation in the anaerobic cells. Results from Aquifas Table 2 shows the average values for December 2006 from the plant and those predicted by a 31 day dynamic simulation using the semi-empirical and biofilm 1D models. The input to the model is the data in Table 1. The results of the 31 day dynamic simulation (Table 2 and Figure 3) showed that both the semi-

empirical model and the Biofilm 1D model were able to predict the effluent NH4N, NOxN, alkalinity and phosphorus. However, there was a higher degree of granularity in the ammonium-N prediction on a day to day basis with the biofilm 1D model. Additionally, the models were accurate in their prediction of the average MLSS, MLVSS, percent VSS and WAS production. The COD, NH4N, NO3N profiles from the semi-empirical model and biofilm 1D models are shown in Figure 4. Both models predict similar profiles. The removals in the biofilm and the mixed liquor are shown in Figure 5. The semi-empirical model shows a slightly higher nitrification in the biofilm while the Biofilm 1D model predicts a slightly higher denitrification in the biofilm. This explains the slightly better nitrification predicted by the semi-empirical model (closer to the level measured at the plant) and the slightly higher denitrification in the Biofilm 1D model (lower effluent NO3N compared to the plant data) shown in Table 2. While additional calibration of the models to this months data would eliminate this difference for December 2006, the calibration parameters were selected because they were also appropriate for other months modeled (such as December 2005 when the plants effluent NO3N was lower 10.5 mg/L). The analysis of the data for the semi-empirical model showed that 32 % of the TKN taken up across the reactor (for cell synthesis and nitrification) was taken up by the biofilm as compared to just 4 % for COD. Therefore, 68 % of the TKN and 96 % of the COD was converted/consumed by biomass in the mixed liquor VSS. The semi-empirical model showed that 47 % of the nitrification was in the biofilm (35% for the Biofilm 1D); the remaining 53 % was in the mixed liquor VSS. In the aerobic zone, the contribution of the biofilm towards denitrification was insignificant. Only 2.5 % of the overall denitrification was in the biofilm (12% for the Biofilm 1D). This is because of the high DO in the aerobic cells and a fairly high turbulence associated with coarse bubble diffusers. The turbulence can reduce the depth of the stagnant liquid layer and increase the DO levels within the biofilm (Figure 6). The quantity of growth (fixed biomass) measured on media in the two aerobic cells in series compared satisfactorily to the computed values from the Biofilm 1D model (Table 2). The substrate, electron acceptor and fraction VSS profiles within the 12 layers inside the biofilm in aerobic cells 1 and 2 are shown in Figure 6. Key inputs to Aquifas Biofilm 1D The user has to provide the model with the information on the MLSS concentration of the biofilm . This should be based on the measurement of fixed solids and thickness of the biofilm on the carrier particle in each cell. In this plant, the growth on the carrier particle is measured once each week in each aerobic cell. The Aquifas Biofilm 2D model requires information on biofilm surface area changes on each carrier particle based on substrate conditions and biofilm thickness. Figure 6b shows how the surface area can change with the biodegradable soluble COD (SCODbio) concentrations. Discussion of Aquifas Model and the Accuracy of Results In general, the Aquifas semi-empirical model was able to predict the effluent satisfactorily and is significantly faster to run than other Biofilm 1D models. In the Aquifas biofilm 1D (also called Aquifas 4), the biofilm is broken up into 12 layers. The layers that are close to the surface are kept thinner than the deeper layers because substrate

concentrations can change rapidly in the surface layers (Figure 6). The total number of layers is higher than other models; the variation in thickness across the depth of the biofilm as a function of the steepness of the substrate profile is an additional feature. The fraction of nitrifiers and VSS are allowed to vary from layer to layer. The model computes the thickness of the biofilm based on the substrate and electron acceptor conditions: Algorithm to compute biofilm thicknessif NO3N>0.5, then Thickness = Multiplier for Anoxic Thickness * [ if (SCODbio * NO3N) > 50, 50, (SCODbio * NO3N) ] * 10,000 / MLVSS * if (NH4N < 0.25, 4* NH4N, 1) / Shear Factorif DO>1, then Thickness = Multiplier for Aerobic Thickness * [ if (SCODbio * DO) > 50, 50, (SCODbio * DO) ] * 10,000 / MLVSS * if (NH4N < 0.25, 4* NH4N, 1) / Shear Factor

Max Aerobic Thickness at (SCODbio x DO) of 50

Max Anoxic Thickness at (SCODbio x NO3N) of 50Multiplier for aerobic thickness 0.1Multiplier for anoxic thickness 0.05 (umhanx/ umhaer)*multiplier for aerobic thickness; will vary with umH from preanoxic to post anoxicShear factor for aerobic 4 Values range from 1 to 5 for aerobic. Increase with turbulenceShear factor for anoxic 2 Values range from 0.5 to 2.5 for anoxic; function of mixing and liquid velocities relative to aerobic zone The algorithm allows biofilm thickness to increase in the aerobic cells with the SCODbio and DO up to an upper limit (value of 50). In the anoxic cells, the biofilm thickness increases with SCODbio and NOxN in the anoxic cells up to its upper limit. The upper limit is based on experiences with municipal wastewater treatment systems whose primary effluent COD was less than 500 mg/L. It may have to be relaxed for industrial wastewater treatment systems that have higher organic concentrations. The formula is standardized to a biofilm MLVSS of 10,000 mg/L. The thickness increases inversely in proportion to the MLVSS of the biofilm. The multiplier for anoxic thickness will decrease based on the ratio of m,anx to m,aer. If the m,anx for biofilm MLVSS is much lower in the post-anoxic cell than in the pre-anoxic cells, the thickness will be less. This is consistent with the observations at the pilot study for the Mamaroneck WWTP (Johsnson et al., 2007). The algorithm has been evaluated against the thickness observed at the Broomfield, CO, full scale IFAS plant (Sen et al., 2007a). The shear factor in aerobic cells varies from 1 to 5. It is 1 for systems with low mixing or a media whose design is conducive to the development of a thick biofilm. It is 5 for systems with very vigorous mixing and an open media structure (such as the AgarTM media which has rings instead of a cylinder). Typically, the shear factor applied to the anoxic cells is half the value applied to the aerobic cells. This can be based on measurement of the velocity of the media along the surface of the tank relative to the velocity measured in the aerobic cells. (This is done by measuring the time the media takes to move from point A to B along the surface of the aerobic and anoxic cells). Finally, the formula adjusts the thickness when NH4N concentrations in a cell drop below 0.25 mg/L. This is to simulate thickness of biofilms that are starved of NH4N. The results for biofilm growth and thickness are shown in Table 3. Based on these findings and calibration, the Aquifas model was used in the semi-empirical mode for running several scenarios. The biofilm 1D model was run for one or two selected scenarios to verify that the output of the semi-empirical model was consistent with the biofilm 1D model.

Table 1: Primary Effluent and Secondary Effluent Data (31 day period, December 2006) Flow Flow TSS BOD5 COD NH4N SKN TKN NO2N NO3N Alkalinity Mixed Liquor NH4N NO3N NO2N T MLSS % VSS MLVSS Alkalinity

MLSS Tm3/d MGD mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L C mg/L mg/L mg/L F mg/L mg/L mg/L

18889 4.99 103 122 325.74 27.8 32.0 41.7103 122 325.74 27.8 32.0 41.787 122 325.74 25.8 29.7 37.9

103 122 325.74 27.8 32.0 41.7103 122 325.74 27.8 32.0 41.785 107 285.69 25 28.8 36.8

103 122 325.74 27.8 32.0 41.7103 122 325.74 27.8 32.0 41.791 144 384.48 27.3 31.4 40.0

103 122 325.74 27.8 32.0 41.7103 122 325.74 27.8 32.0 41.7164 189 504.63 28 32.2 47.8

103 122 325.74 27.8 32.0 41.7103 122 325.74 27.8 32.0 41.797 137 365.79 28 32.2 41.4

Average concentrations were used on days with no data. These are shown in red.

19.4 67 1560 121423583 6.23 19.4 67 1660 129120252 5.35

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Figure 3 Aquifas output: The figures on the column to the left are from the Semi-Empirical Model; the figures in the column to the right are from the Biofilm 1D model. The diffusion model can offer a higher degree of precision in its ability to predict day to day variations in a dynamic simulation but takes substantially longer to run. Both models are able to predict the diurnal and 31 day average.

COD Profile

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Figure 5.COD Uptake, NH4N uptake and Denitrification in the Biofilm and MLVSS. The left hand column is from Semi-Empirical Model. The right hand column is from the Biofilm 1D Model.

Table 2 Comparison of 31 Day Average from Dynamic Simulation against Plant Data

Aquifas Semi-Empirical and Biofilm 1D Models

Semi Empirical

Biofilm 1D

Plant Data Notes

Primary Effluent (Input) Flowrate, m3/d 19,512 19,512 19,512Flowrate, MGD 5.33 5.33 5.33 BOD, mg/L 122 122 122 TSS, mg/L 103 103 103 VSS (%) 82 82 COD, mg/L 326 326 326 SCOD, mg/L (flocculated) 130 130 TKN, mg/L 41.7 41.7 NH3N, mg/L 27.8 27.8 27.8 TP, mg/L 5 5 PO4P, mg/L 3 3

It was assumed that the filtered flocculated COD was 50% of measured soluble COD (SCOD)

IFAS Process (Output) MLSS, mg/L 1650 1650 1673 MLVSS, mg/L 1308 1310 1271 VSS (%) 79 79 78 DO (First Cell), mg/L 4.2 4.2 4.2 DO (Second Cell), mg/L 5.6 5.6 5.6 MLSS SRT (Oxic), days 3.3 3.3 3.3 Fixed Biomass (First Cell), g/m2 11.7 13.7 Fixed Biomass (Second Cell), g/m2 8.4 7.4 WAS TS, lb/d 4802 4806 4800

Other than the biomass on the biofilm, which is computed at 14 C, the rest of the data are the average of 31 day dynamic simulation.

Secondary Effluent (Output) BOD, mg/L 5.0 5.4 2.6 TSS, mg/L 5.0 5.0 4.5 TKN, mg/L 1.3 1.4 NH4N, mg/L 0.24 0.26 0.19 NO3N, mg/L 13.8 12.5 14.4 TP, mg/L 0.1 0.1 0.1 PO4P, mg/L 0.1 0.1 0.1 Alkalinity (mg/L as CaCO3) 100 104 90

Effluent TKN is based on an assumed value of 0.5 mg/L non biodegradable SKN

Table 3 Measured and Computed values of Biofilm Growth (Aquifas Biofilm 1D)

Biofilm growth (measured) kg/1000m2/d 13.7 7.4Biofilm growth (computed) kg/1000m2/d 11.7 8.4

Thickness as computed um 932 679

Heterotrophic Yield 0.290 0.249Autotrophic Yield 0.038 0.007fraction nitrifiers in layer1 of biofilm as used in the model 46.29% 41.13%Biofilm MCRT days 24.3 51.1

-202468

101214

-0.5 0 0.5 1

Distance from Surface of biofilm, mm

mg/

LDONH4NNOxNSCODbio%VSS /10

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DONH4NNOxNSCODbio% VSS / 10

Figure 6. Substrate profiles (DO, NH4N, NOxN, SCODbio, and Percent VSS) inside the biofilm in Aerobic Cell 1 (top) and Aerobic Cell 2 (bottom)

Biofilm surface area is the inner surface of the brown biofilm and outer surface of biofilm between fins/ridges/protruberances

10 mm

Cylinder Dimensions:Outer Diameter = 10 mmLength = 10 mmThickness of annular ring = 1 mmThickness of Cross Vane = 1mmHypothetical Carrier Particle has one vane per cylinderhas several fins/ridges/protruberances

Vane

Figure 2b. Magnitude of difference in biofilm surface area for thin and "thick"biofilms on the same carrier particle of moving bed or IFAS system.

Thick biofilmThin biofilm

Fin/Ridge/Protruberance

Figure 6b: Effect of substrate concentrations (in this instance, SCOD) on biofilm surface area on a carrier particle

RESULTS Tertiary IFAS Configuration (DC WASA type of configuration) The results shown are for 14,000 m3/d facility with 2090 m3 volume in tertiary tanks, resulting in a HRT of 3.6 hours (which is 1/100th size of DCWASA / Blue Plains). Several configurations were evaluated of which four are presented in this paper (Table 4):

1. Configuration A with no media, which is how the plant operates today with three aerobic cells (the third cell is operated with mixing and no aeration except when there is some ammonium-N bleedthrough in winter), two anoxic cells and reaeration in the channel (Figure 1);

2. Configuration B, where media with a specific surface area of 350 m2/m3 was added to the fourth and fifth anoxic cells (Table 4);

3. Configuration C, where media was also added to the second aerobic cell (Table 4); and 4. Configuration D, in which Configuration C was operated with 20 percent higher methanol dose in

winter to increase the denitrification rates in the biofilm and the mixed liquor, thereby increasing the denitrification in the biofilm while maintaining an effluent biodegradable SCOD (SCODbio) level below that in Configuration A.

The secondary feed strength can vary from summer to winter (Table 5). The results shown in Table 6 are for the average temperature in March 2005 (15 C liquid temperature) for the influent conditions in Table 5. The results are shown for normal flow and concentrations in the secondary effluent and for conditions experienced during wet-weather when the secondary clarifiers release a higher level of TSS and BOD5. It is important to analyze, understand and evaluate the accuracy of the model under both normal and wet-weather conditions because the observations at the plant show an increase in the tertiary effluent NOxN during and following such wet weather conditions in winter. This has been hypothesized to be the result of higher concentrations of TSS and BOD5 in the tertiary influent and the operating mode in which the operators try to maintain the same level of MLSS during all flow conditions, thereby reducing the F/M ratio (where F includes the COD of the VSS in the tertiary influent). The researchers modeled this condition, in addition to the normal flow condition and were able to confirm the hypothesis. The results of the modeling were compared with actual observations. For example, the MLSS MCRT of 10 days in winter and 12.5 to 15 days in summer at MLSS levels less than 2000 mg/L (MLVSS of 1400 mg/L), as computed by the model, were in line with the MLSS and MCRTs observed. In the model, the maximum post-denitrification rate in the post-anoxic cells in the MLVSS was reduced to 1 kg COD utilized/kg VSS/d at 25 C with a temperature adjustment coefficient of 1.03 in the Arrhenius equation. This is lower than the default value of 2 kg COD utilized/kg VSS/d at 25 C used for post anoxic dentrification. This resulted in a rate of 0.74 /d at 15 C, which was close to value of 0.70 /d computed using the formula presented by Dold et al. (2007) for the DC WASA facility. (Dold et al., 2007 observed a rate of 1.3/d at 20 C and a temperature coefficient of 1.13). When the biofilm was introduced, the post-anoxic rate was kept at 1.1/d at 15 C, based on rates observed in various denitrification studies with media. The sensitivity of the computations to rates as low as 0.7/d was also evaluated. The changes in performance from Configuration A to D are presented in Figures 7 and 8. The addition of media helped improve the performance substantially under wet-weather conditions in winter. The effluent oxidized-N decreased from 10.8 mg/L to 4.3 mg/L. The sensitivity of the IFAS configuration to further reduction in denitrification rates was evaluated in both the MLVSS and the biofilm was modeled. Reducing the rate to 0.7/d resulted in an increase in effluent NOxN to 5.6 mg/L. To understand the changes in performance, the COD, NH4N and NOxN profiles and uptake rates are presented in Figures 9, 10, and 11. Figure 9 shows the improvement in COD removal in the anoxic cells

following addition of media. Figure 10 shows that the addition of media to the second aerobic cell in Configuration C increased its nitrification rate from 80 kg/d to 150 kg/d. Figure 11 shows that the addition of media increased the denitrification rate from 80 kg/d to 105 kg/d (total of two cells) from Configuration A to B. Finally, Figure 11 shows that the increase in methanol dosing from 62.5 mg/L to 75 mg/L as COD (dose is relative to the influent flow rate) increased the denitrification in the post-anoxic cells from 105 kg/d to 140 kg/d. Despite the additional COD addition with the higher methanol dose, the effluent SCOD remained below the current configuration (Table 6 and COD profiles in Figures 9) because the addition of biofilm surface area in the anoxic zones allowed the system to assimilate the methanol. The supplemental methanol addition is recommended in winter months after high flows are experienced. It helps drive the biofilm kinetics up and compensate for the loss of methanogens in the MLVSS. Figure 7 shows the performance of Configuration C at warmer temperatures (20 C and higher). At the average and summer temperatures, the normal methanol dose maintains an effluent NOxN of 4 mg/L. The model was also run in a dynamic simulation mode with the daily temperature and flow conditions for March 2006. The results were consistent with the findings at Broomfield where the model predicted the effluent conditions over the 31 day period (Figure 3). Table 4. Evaluation of "Blue Plains" Configuration - Tertiary IFAS with Nitrification and Denitrification

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5Methanol

Current Config A current configuration aerobic aerobic no air anoxic, C anoxic

Media Config B aerobic aerobic no airanoxic, C,

mediaanoxic, media

Media Config C aerobicaerobic, media no air

anoxic, C, media

anoxic, media

Media Config D20% higher Methanol in

winter aerobicaerobic, media no air

anoxic, C, media

anoxic, media

Notes:1 Media evaluated at 350 m2/m3 biofilm surface area2 Cells with Media are in yellow3 All cells occupy 19.5% of the volume; there is reaeration in a channel that occupies 2.5% of volume4 DO levels are 2.0 to 2.5 mg/L in Aerobic Cells5 In Configurations A and B, Cell 3 is aerated during winter high flows to help maintain nitrification

Table 5. Tertiary System Influent Characteristics Modeled in Steady State and Dynamic Simulation

Normal High Flow Daily Maxin 30 day period modeled

Flow m3/d 14008 15398 18927TSS mg/L 20 30 50BOD5 mg/L 15 24 40TKN mg/L 18 19 21

Flows modeled are 1/100 of actual flows at the Blue Plains (DC WASA) plant

Table 6. Results from Aquifas Modeling, Blue Plains Type Tertiary IFAS SystemNormal Flow, 15 C During wet weather, Winter (15 C) Comments

MCRT SCODbio NH4N NOxN MCRT SCODbio NH4N NOxNdays mg/L mg/L mg/L days mg/L mg/L mg/L

Current Configuration A 10 12.3 0.4 9.4 8 10.5 0.8 10.8 Third cell aerated during higher loadings in

Media Configuration B 10 7.3 0.6 5.9 8 6.5 0.4 8.7 Third cell aerated during higher loadings in

Media Configuration C 10 7 0.1 7.4 8 6.5 0.3 7.4 Only first two cells are aerated at all flows; N

Media Configuration D 8 8.5 0.2 4.8 6 9 1.1 4.3 Only first two cells are aerated at all flows; N20% additional methanol dose in winter (16

Plant maintains MLVSS between 1400 and 1500 mg/L in all configurations and at all temperatures

Notes:1 Plant has observed loss of denitrification during high flows. Measured value of NOxN increases to 12 mg/L2 Performance improves to 5.4 mg/L NOxN under average conditions at 20 C3 The plant requires media in one aerobic cell to maintain complete nitrification at high flows while operating with 40% aerobic volume

Tertiary Effluent, IFAS Nit-Denit in One Multi-Cell ReactorAverage Winter Month, 15 C, Normal Flow Conditions

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A B C D

Plant Category

SCO

D, N

OxN

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L

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7SCODbioNOxNNOxN 20 CNH4N

NH4N, mg/L

Figure 7. A comparison of effluent conditions under average flow and load conditions at 15 C (winter).

Configuration A represents the existing condition Data on NOxN at 20 C is shown for Configuration C. This is to illustrate that normal methanol dose can maintain low levels of NOxN (between 3 and 5 mg/L) outside the winter months.

Tertiary Effluent, IFAS Nit-Denit in One Multi-Cell ReactorWinter Month, 15 C, High Flow & Load Conditions

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Plant Category

SC

OD

, NO

xN, m

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1

1.2SCODbioNOxNNH4N

NH4N, mg/L

Figure 8. A comparison of effluent conditions under wet weather flow and load conditions at 15 C (winter).

Configuration A represents the existing condition The increase in NH4N from 0.3 to 1.1 mg/L from Configuration C to D is because of the reduction in MLSS MCRT from 8 to 6 days. The NOxN decreases from 7.4 to 4.3 mg/L because of the higher methanol dose.

COD Profile

0.05.0

10.015.020.025.030.035.040.045.0

Aerobic Aerobic Aerobic Anoxic Anoxic

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SCODbioSCOD


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ke, k

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COD Profile

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COD Profile

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COD Profile

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SCODbioSCOD


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ake,

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Figure 9. Configurations A through D (top to bottom) - Substrate profiles and uptake rates for COD in the MLVSS and the Biofilm, as identified by modeling conditions observed during and after high flows in winter Configuration A: No media in reactor; Cells 1, 2 and 3 operated aerobically, Cells 4 and 5 are anoxic with methanol added to 4 Configuration B: Cells 1 and 2 are operated aerobically with no media, Cells 4 and 5 are operated anoxically and with media. Methanol is added to Cell 4). Note the increase in COD uptake associated with denitrification following the addition of media. Configuration C: Cells 1 and 2 are operated aerobically; Cells 4 and 5 are anoxic; media is installed in Cells 2, 4 and 5; methanol added to Cell 4. Media contributes towards nitrification in Cell 2 (Figure 10) which allows complete nitrification in two cells. Configuration D - In this configuration, an additional 20% methanol is added in winter to enhance the kinetics of denitrification (75 mg/L of COD supplement instead of 62.5 mg/L in Configuration C). COD uptake rates in the biofilm in Anoxic Cell 4 increased from 180 to 250 kg/d, resulting in improvement in effluent NOxN. The COD profiles show that because of the presence of media, the effluent SCOD in Configuration D decreased despite the higher methanol feed.


0.01.02.03.04.05.06.07.08.09.0


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NH4NSKN


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NH4NSKN


0.00010.00020.00030.00040.00050.00060.00070.00080.00090.000

100.000


NH4

N U

ptak

e, k

g/d


Figure 10. Configurations A through D (top to bottom) - Substrate profiles and uptake rates for NH4N in the MLVSS and the Biofilm, as identified by modeling conditions observed during and after high flows in winter Configuration A: No media in reactor; Cells 1 and 2 are operated aerobically, Cells 4 and 5 are anoxic; methanol added to Cell 4 Configuration B: Cells 1 and 2 are operated aerobically, Cells 4 and 5 are operated anoxically and with media; methanol is added to Cell 4. Configuration C: Cells 1 and 2 are operated aerobically; Cells 4 and 5 are anoxic; media is installed in Cells 2, 4 and 5; methanol added to Cell 4. Note that media contributes towards nitrification in Cell 2 which allows complete nitrification in two cells (differences between Configuration B and C). Configuration D - In this configuration, an additional 20% methanol is added in winter to enhance the kinetics of denitrification. 75 mg/L of COD supplement is added instead of 62.5 mg/L in Configuration C. MLSS MCRT has to be reduced to 6 days to maintain the operating MLSS. This increases the effluent NH4N during winter wet weather.

Oxidized N Profile

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NO3N


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Oxidized N Profile

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Oxidized N Profile

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Oxidized N Profile

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Figure 11. Configurations A through D (top to bottom) - Substrate profiles and uptake rates for NOxN in the MLVSS and the Biofilm, as identified by modeling conditions observed during and after high flows in winter Configuration A: No media in reactor; Cells 1 and 2 are operated aerobically, Cells 4 and 5 are anoxic with methanol added to 4 Configuration B: Cells 1 and, 2 are operated aerobically with no media, Cells 4 and 5 are operated anoxically; media is installed in Cells 4 and 5. Note the increase in denitrification from 80 to 110 kg/d (sum total of two anoxic cells) following the addition of media. Configuration C: Cells 1 and 2 are operated aerobically; Cells 4 and 5 are anoxic; media is installed in Cells 2, 4 and 5; methanol added to Cell 4. Media contributes towards nitrification in Cell 2 (Figure 10) which allows complete nitrification in two cells. Configuration D - In this configuration, an additional 20% methanol is added in winter to enhance the kinetics of denitrification (75 mg/L of COD supplement instead of 62.5 mg/L in Configuration C). In Cell 4, denitrification rates in the biofilm in Anoxic Cell 4 increased from 33 to 44 kg/d; denitrification rates in the MLSS increased from 52 to 62 kg/d; improvement in denitrification rates result in an improvement in effluent NOxN from 7.4 to 4.3 mg/L.

Tertiary MBBR and IFAS as alternatives in a New Tertiary Reactor In the second example that is based on a plant in Harrisburg, PA, a new reactor can be added downstream of high rate system for BOD removal. Table 7 shows three of the alternatives that were evaluated. In Alternative 1, the new nitrification tank is to be designed as a nitrification MBBR. In Alternative 2, the same volume is to be used for nitrification and denitrification in the MBBR. In Alternative 3, a RAS is added to Alternative 2 and the facility is operated as an IFAS system. Supplemental carbon is added at a dose of 50 mg/L of COD in terms of the influent flow to the third cell. Table 8 shows the tertiary influent conditions that were modeled. These strengths are more typical of winter conditions when the secondary treatment system is not operating as effectively as in summer and the plant may be operated at higher flows. Table 7. Evaluation of Tertiary MBBR and IFAS in One Multi-Cell Reactor for Nitrification and Denitrification

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5

Alternative 1 Nitrification MBBR, no denit aerobic aerobic aerobic aerobic aerobicAlternative 2 MBBR with Nit and Denit aerobic aerobic anoxic, C anoxic aerobicAlternative 3 IFAS with Nit and Denit aerobic aerobic anoxic, C anoxic aerobic

Notes:1 Media evaluated at 150 m2/m3 biofilm surface area (30 % fill) in Alternatives 1 and 32 Capacity can be increased in the future by increasing the fill to 66% 3 Supplemental Carbon is added to Cell 3 in Alternatives 2 and 34 Media evaluated at 200 and 250 m2/m3 biofilm surface area (40 to 50% fille) in Alternative 2

Table 8. Tertiary System Influent Characteristics Modeled in Steady State and Dynamic Simulation

Flow m3/d 14000TSS mg/L 25BOD5 mg/L 25NH4N mg/L 12TKN mg/L 16NOxN mg/L 0 Table 9. Results from Aquifas Modeling of Tertiary MBBR and IFAS alternatives for N removal integrated in one tankAverage Load, 10 C SCOD added ltered effluent Comments

MLSS MCRT HRT MLSS Media SSA as Methanol SCODbio NH4N NOxN MCRT, days days mg/L m2/m3 mg/L mg/L mg/L mg/L

Alternative 1 Nit MBBR 0.15 0.15 29 150 0 13 0.4 11.5 All cells are aerobic

Alternative 2a Nit-Denit MBBR 0.15 0.15 43 200 50 17.8 1.5 5.8 First two cells aerobic, next two cells anAlternative 2b Nit-Denit MBBR 55 250 62.5 20.2 0.7 4.7 Media specific surface area increased t

Alternative 3a Nit-Denit IFAS 4.00 0.15 990 150 50 3.5 0.2 6.6 First two cells aerobic, next two cells anAlternative 3b Nit-Denit IFAS 4.00 0.15 1070 150 62.5 3.8 0.2 4.5

Notes:1 Effluent SBOD5 estimated to be 67% of SCODbio2 Effluent NOxN can be reduced in MBBR by increasing anoxic biofilm specific surface area together with the methanol dose3 MBBRs need chemical coagulation and clarification; IFAS needs clarifiers

Reactor

Table 9 shows the reactor HRT and MLSS MCRT, media specific surface area, MLSS and effluent quality computed for the three alternatives. The media specific surface area was the same in all cells. These results at 10 C and are summarized in Figure 12.

In Alternative 1, the MBBR achieves complete nitrification and discharges an effluent NH4N less than 1 mg/L and NOxN of 11.5 mg/L. The MLSS in the effluent from the MBBR is 25 mg/L for the tertiary influent conditions. The MLSS is such that a plant can consider the option of discharging directly to a denitrification filter designed to handle 20 to 30 mg/L TSS.

In Alternative 2, the MBBR was modified to operate with two aerobic cells and two anoxic cells followed by the reaeration cell. The media fill had to be increased to increase the biofilm specific surface area from 150 to 200 m2/m3. The effluent NH4N was 1.5 to 2 mg/L. The performance can be improved further to decrease the effluent NH4N to less than 1 mg/L by increasing the specific surface area to 250 m2/m3. The addition of methanol increased the MLSS in the reactor effluent from 25 mg/L to 35 mg/L. Removing the additional MLSS on a filter (instead of settling in a tertiary clarifier) can be a challenge unless the filter is designed to accommodate the higher loadings. Typically, a plant would use a coagulant and settle the MLSS in tertiary clarifiers. One of the concerns is the effluent soluble COD from 13 to 18 mg/L. This can be reduced by increasing the biofilm specific surface area in the reaeration cell. At the same time, it should be noted that 10 C is the lowest temperature. The performances are substantially better at higher temperatures.

In Alternative 3, a RAS was introduced to increase the MLSS MCRT from 0.15 day to 4 days by converting to an IFAS configuration. The media specific surface area was kept the same as in Alternative 1 (150 m2/m3). This helped reduce the effluent soluble biodegradable COD from 15+ mg/L in Alternative 2 to less than 4 mg/L (Table 9). The effluent NH4N improved to less than 0.5 mg/L. The plant achieved good denitrification and can achieve between 2 and 7 mg/L NOxN, depending on the methanol dose added. The disadvantage of Alternative 3 is that it requires a secondary clarifier. This is a tradeoff against a denitrification filter which is not necessary with the IFAS system. The activated sludge portion is a proven element (DC WASA) and the biofilm is a recently proven element (Broomfield).

Figures 13, 14, and 15 show the COD, NH4N and NOxN profiles and the uptake rates for the three alternatives. The combination of media with MLVSS in the IFAS configuration in Alternative 3 increases the ammonium-N uptake rates more than 50 percent in the first two aerobic cells over that computed in Alternative 1 where the removal in the MLVSS is insignificant. Both of these alternatives are operated at the same media specific surface area.

Tertiary Effluent from Nitr MBBR (1), Nit-Denit MBBR (2a & 2b), Nit-Denit IFAS (3a & 3b)in One Multi-Cell Reactor (Total Volume is Constant for all Alternatives)

Temperature of 10 C

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1 Nit MBBR 2a Nit-Denit MBBR 2b Nit-Denit MBBR, higherMethanol + Media

3a Nit-Denit IFAS 3b Nit+Denit IFAS, higherMethanol

Plant Category

SCO

D, N

H4N

, NO

xN, m

g/L

SCODbioNH4NNOxN

25 33 45 990 1060 MLSS

Figure 12. Effluent quality and MLSS resulting from conversion of a MBBR sized for Nitrification (Alternative 1) to a MBBR Nitrification and Denitrification (Alternatives 2a and 2b) and an IFAS (3a and 3b) while operating with the same volume.

Note that while the effluent NOxN improves from Alternative 1 to 3b, the effluent MLSS discharged from the reactor also increases. This has implications on the clarifier requirements.

COD Profile

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Figure 13. COD profiles and COD uptake in Alternatives 1 (top), 2 (middle) and 3 (bottom) at 10 C. Note:

1. The COD uptake rates in Cell 3 increased following addition of methanol from Alternative 1 (140 kg/d, Cell 3 is Aerobic) to Alternative 2 (300 kg/d, Cell 3 is Anoxic) and Alternative 3 (420 kg/d, Anoxic IFAS)

2. The COD uptake rates in the MLVSS exceed the biofilm following conversion from MBBR (Alternative 2) to IFAS (Alternative 3)

3. The soluble biodegradable COD in the last aerobic cell (which corresponds to the effluent) decreased from 15+ mg/L to < 5 mg/L following conversion to IFAS


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60.000


NH4N

Upt

ake,

kg/

d



0.01.02.03.04.05.06.07.08.09.0

10.0


Cells in System

N, m

g/L

NH4NSKN


0.000

10.000

20.000

30.000

40.000

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60.000

70.000


NH4

N U

ptak

e, k

g/d



0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0


Cells in System

N, m

g/L

NH4NSKN


0.000

10.000

20.000

30.000

40.000

50.000

60.000


NH4N

Upt

ake,

kg/

d


Figure 14. NH4N profiles and uptake in Alternatives 1 (top), 2 (middle) and 3 (bottom) at 10 C. Note:

1. The NH4N uptake rate in Aerobic Cells 1 and 2 increased almost in proportion to the increase in the biofilm specific surface area when it was increased from 150 m2/m3 in Alternative 1 to 200 m2/m3 in Alternative 2.

2. The NH4N uptake rate in the MLVSS increased following conversion from MBBR (Alternative 2) to IFAS (Alternative 3). The sum total of NH4N uptake in the MLVSS and biofilm is higher in the IFAS (165 kg/d in Aerobic Cells 1 and 2) than the Nitrification MBBR (100 kg/d)

3. The effluent NH4N decreased following conversion to IFAS.

Oxidized N Profile

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0


Cells in System

N, m

g/L

NO3N


0.000

1.000

2.000

3.000

4.000

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6.000


Oxi

dize

d N

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itrifi

ed, k

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Mixed Liquor VSS

Oxidized N Profile

0.0

1.0

2.0

3.0

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8.0


Cells in System

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NO3N


0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

40.000

45.000


Oxi

dize

d N

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itrifi

ed, k

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Mixed Liquor VSS

Oxidized N Profile

0.0

1.0

2.0

3.0

4.0

5.0

6.0

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8.0

9.0


Cells in System

N, m

g/L NO3N


0.000

5.000

10.000

15.000

20.000

25.000

30.000


Oxi

dize

d N

Deni

trifie

d, k

g/d Biofilm

Mixed Liquor VSS

Figure 15. Oxidized N profiles and uptake rates in Alternatives 1 (top), 2 (middle) and 3 (bottom) at 10 C. Note

1. The denitrification in Cell 3 increased from 4 to 40 kg/d following its conversion from aerobic (Alternative 1) to anoxic (Alternative 2) and increasing the biofilm specific surface area from 150 to 200 m2/m3.

2. The denitrification rates in the MLVSS increased following conversion from MBBR (Alternative 2) to IFAS (Alternative 3). In Alternative 3, the rates were limited by the availability of biodegradable SCOD (also see Figure 13).

3. Increasing the methanol dose to the IFAS configuration in Alternative by 25 percent (from 50 to 62.5 mg/L) reduced the effluent oxidized-N from 6.6 to 4.5 mg/L (shown in Table 9).

DISCUSSION There are several reasons why biofilm support media is attractive for denitrification in post-anoxic zone of an existing tertiary activated sludge system and also in secondary systems. In post-anoxic application, it is possible to use higher high fill volume fraction of moving bed media because of the absence of a forward flux from a nitrate recycle flow passing through the cell. In pre-anoxic cells, nitrate recycle increases forward flux of media, causing media to accumulate at the downstream end. This makes it difficult to apply higher fill volume fractions (> 50%) in pre-anoxic cells. In an IFAS mode, the combination of MLVSS and biofilm enhances the oxygen uptake rate and drives down the DO. This improves denitrification rates. Overall, the system can maintain a higher fraction of methanogenic denitrifiers in the MLVSS because of continuous sloughing and seeding from the biofilm. Further, the ability to grow bacteria in the biofilm in the post-anoxic cells may allow the methanogenic denitrifier populations to build up the requisite enzyme system and seed the MLVSS through their sloughing. The system may be less sensitive to temperature than activated sludge systems. In the instance where new nitrification tanks are being added, the choices for the process configuration depend on the operator and the engineer. One could go with a nitrification MBBR followed by a denitrification filter (thereby eliminating the tertiary clarifier) if the denitrifying filter is designed to handle 25 to 30 mg/L of TSS. One could also go with an IFAS system that achieves high levels of nitrification and denitrification and requires clarifiers that are sized similar to the clarifiers in the secondary treatment system. The IFAS system has the ability to achieve higher levels of performance in terms of both ammonium-N and oxidized-N removal and achieve lower levels of effluent soluble BOD. Alternatively, one could stay with the MBBR for both nitrification and denitrification in a multi-cell reactor and design high rate clarifiers with lamella plates of tubes when space is a constraint if the MLSS levels coming out of the anoxic MBBR are too high for filters. Unlike an IFAS, the MBBRs may need a chemical coagulant to coalesce residual fine particulates and biomass sloughed off the bioflm to improve the settling. These findings can also be extended to secondary treatment systems operating in an ENR configuration. It is possible that the quantity of denitrification achieved in the post-anoxic cells can be increased significantly following the addition of media. This is not only because of denitrification by the biofilm but also because of the additional and more stable population of methanogenic denitrifiers in the ENR system. The methanol dose requirements may begin to approach the stoichiometric levels when the media is added to secondary ENR systems. CONCLUSIONS

1. Introduction of moving or mobile bed biofilm media with specific surface areas of 300 to 350 m2/m3 in post-anoxic zones of activated sludge plants can increase the volumetric denitrification rates by 30 to 50% (activated sludge versus IFAS configuration).

2. Conversion from a nitrification MBBR to an IFAS mode can increase the volumetric nitrification rate by 150 to 200 percent. This increase can reduce the aerobic volume requirements in half. The magnitude of the increase depends on the media specific area. However, unlike a tertiary MBBR for nitrification which may go directly to a denitrification filter, IFAS may need its own clarifier.

3. Integration of biofilm and activated sludge in the post anoxic cell of secondary treatment facilities can be an attractive alternative for land-locked plants that have to upgrade their performance for Enhanced Nutrient Removal (reduce TN from 10 to 3 mg/L). The volumes required for nitrification and denitrification can be reduced.

ACKNOWLEDGMENT The authors would like to acknowledge the help and assistance from the staff of the Broomfield WWTP, the process engineering team at Black & Veatch, and from Anox Kaldnes. The authors would also like to acknowledge the help from DCWASA, and the funding and support of nitrogen evaluation alternatives in Pennsylvania from USEPA Chesapeake Bay Program, PA DEP and Virginia Tech. REFERENCES AQUASIM 2.1 Model (2007) http://www.aquasim.eawag.ch/ Aquifas Model (2007) www.Aquifas.com BioWin Model (2007) www.envirosim.com GPSX Model (2007) http://www.hydromantis.com Aeration (1988) Manual of Practice FD-13; Water Pollution Control Federation, Alexandria, VA District of Columbia Water and Sewer Authority. Comprehensive Financial Report, 2000. Paul Bender. Dold, P., Takacs, I., Mokhayeri, Y., Nichols, A., Hinojosa, J., Riffat, R., Bailey, W., Murthy, S. (2007)

Denitrification with Carbon Addition Kinetic Considerations. Nutrient 2007; Water Env Federation, 218-238, Baltimore, MD.

Johnson, T. L.; Shaw, A.; Landi, A.; Lauro, T.; Butler, R; Radko, L. (2007). A Pilot-Scale Comparison of IFAS and MBBR to Achieve Very Low Total Nitrogen Concentrations. Submitted to Water Practice, 2007; Proceedings: Nutrient 2007. Water Env Federation, 521-535, Baltimore, MD.

Reichert, P. (1998a) AQUASIM 2.0. User Manual. Swiss Federal Institute for Environmental Science and Technology (EAWAG), Dbendorf, Switzerland.

Reichert, P. and Wanner, O. (1997) Movement of solids in biofilms: significance of liquid phase transport. Water Science and Technology 36(1) pp. 321-328.

Rooney, T. C.; Huibregtse, G. L. (1980) Increased Oxygen Transfer Efficiency with Coarse Bubble Diffusers. J. Water Pollut. Control Fed. 52, 9, 2315-2326.

Rutt, K.; Seda, J.; Johnson, C. H (2006) Two Year Case Study of Integrated Fixed Film Activated Sludge (IFAS) at Broomfield, CO, WWTP. Proceedings of the 79th Annual Water Environment Federation Technical Exposition and Conference [CD-ROM]; Dallas, Oct 21-25; Water Env. Fed.:Alexandria, VA, 225-239.

Mamais, D.; Jenkins, D.; Pitt, P. (1993) A rapid physical-chemical method for the determination of readily biodegradable soluble COD in municipal wastewater. Water Research 27 pp. 195-197.

Sen, D.; Copithorn, R. R.; Randall, C. W. (2006) Successful Evaluation of Ten IFAS and MBBR facilities by Applying the Unified Model to Quantify Biofilm Surface Area Requirements for Nitrification, Determine its Accuracy in Predicting Effluent Characteristics, and Understand the Contribution of Media towards Organics Removal and Nitrification. Proceedings of the 79th Annual Water Environment Federation Technical Exposition and Conference [CD-ROM]; Dallas, Oct 21-25; Water Environment Federation, Alexandria, VA, 185-199.

Sriwiriyarat, T.; Randall, C.W; Sen, D. (2005) Computer Program Development for the Design of Integrated Fixed Film Activated Sludge Processes. Journal of Environmental Engineering, Vol. 131, No. 11, November 2005, 1540-1549

Wanner, O; Eberl, H; Morgenroth, E; Noguera, D; Picioreanu, C; Rittman, B; Loosdrecht, M.V. (2006). Mathematical Modeling of Biofilms. IWA Task Group on Biofilm Modeling. Scientific and Technical Report 18. IWA Publishing, London.

Wanner, O. and Reichert, P. (1996) Mathematical modeling of mixed-culture biofilms. Biotechnolology and Bioengineering 49 pp. 172-184.

WERF (2003) Melcer, H.; Dold, P.L.; Jones, R.M.; Bye, C.M.; Takacs, I.; Stensel, H.D.; Wilson, A.W.; Sun, P.; Bury, S.; Methods for Wastewater Characterization in Activated Sludge Modeling. Final Report. Project 99-WWF-3.

CONCLUSIONSACKNOWLEDGMENTREFERENCES

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