TWO YEAR CASE STUDY OF INTEGRATED FIXED FILM … · TWO YEAR CASE STUDY OF INTEGRATED FIXED FILM...

TWO YEAR CASE STUDY OF INTEGRATED FIXED FILM ACTIVATED SLUDGE (IFAS) AT BROOMFIELD, CO WWTP

Mr. Ken Rutt1, Jim Seda1 and Mr. Chandler H. Johnson2

1City & County of Broomfield 2985 West 124th Street Broomfield, CO 80020

2AnoxKaldnes, Inc.

ABSTRACT

The City & County of Broomfield wastewater treatment plant secondary treatment processes were upgraded to a new biological nutrient removal process described as Integrated Fixed Film Activated Sludge (IFAS) back in 2003 using a moving bed plastic carrier element to help grow the biomass. This allows the existing aerobic reactors to maintain nitrification during year round operation while still operating near conventional activated sludge solids retention times (SRT) one would find in just carbonaceous treatment plants. The upgraded IFAS system includes anaerobic and anoxic reactors to help meet new effluent Total Nitrogen and Total Phosphorus limits. The performance of the entire system since July 2003 averaged effluent concentrations of 2.4 mg/L for BOD, 4.2 mg/L for TSS, 0.3 mg/L for NH3-N, 8.3 mg/L for NOx-N and 1.25 mg/L for TP while operationally during this period of time the aerobic suspended MLSS SRT averaged 4 days, the MLSS concentration averaged 1,718 mg/L and the SVI averaged 120 mL/L. The plant has operated with one set point for both the RAS and WAS rates for months at a time while meeting these very consistent effluent results.

KEYWORDS

IFAS, municipal wastewater, nutrient removal, Moving Bed™ biofilm process, MBBR, nitrification.

INTRODUCTION

Following an expansion in 1988, the City & County of Broomfield wastewater treatment plant secondary treatment processes consisted of a roughing biofilter (Trickling Filter) followed by activated sludge for meeting discharge limits of 25 mg/L cBOD and 30 mg/L TSS at a flow rate of 5.4 MGD.

Due to population increases & new regulations with respect to NH3-N, the city needed to upgrade its facility. To further add to the design and treatment requirements, the City was also adding treatment processes to reuse a large portion of its wastewater as reuse irrigation water. The storage system for the reuse water includes converting a former water supply reservoir into a water reuse reservoir. Therefore, the added treatment requirements of lower total phosphorus (TP) and total nitrogen (TN) levels became part of the overall plan. Table 1 shows the effluent requirements the City’s WWTP would have to meet for the reuse water storage standards.

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Copyright 2006 Water Environment Foundation. All Rights Reserved©

Table 1 – City of Broomfield WWTP Effluent Requirements

Parameter Effluent Requirement

BOD <10 mg/L TSS <10 mg/L

NH3-N <1.5 mg/L – Summer <3.0 mg/L - Winter

TIN <10 mg/L TP <1.0 mg/L

The City & County of Broomfield evaluated six (6) options put forward by their engineer. Each option was evaluated based on a) future expansion, b) similar treatment process, c) land usage, as the selected approach needed to leave room at the site for eventual expansion to 16 MGD and d) overall cost. The City selected an IFAS (Integrated Fixed Film Activated Sludge) type process based on free floating plastic media.

The first process train was converted during the summer months of 2002. Wastewater was treated through the process train in September 2002 and media was first added in November 2002. The second process train was converted in the summer of 2003 with final media addition by fall 2003. After 2 years of operation, the staff has had the opportunity to play and learn from the new IFAS system.

Figure 1 - IFAS principle with biofilm carrier (K1) used at the Broomfield WWTP

INTEGRATED FIXED FILM ACTIVATED SLUDGE

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In the simplest definition, IFAS takes a conventional activated sludge wastewater treatment plant and adds into its existing aerobic basins some form of media to help the slower growing bacteria, mainly nitrification type bacteria, to inhabit the media. The overall goal is to take the investment provided in the existing system (to perform BOD removal and potentially a portion of nitrification) while simply adding to it rather then building all new tanks or unit processes. An IFAS system is a process using kinetics from a suspended growth treatment system (namely conventional activated sludge) and adding to it kinetics from a fixed film treatment system. Fixed film technologies are well known for biological treatment of ammonia in cold temperatures as the bacteria / biofilm grows on a substrate / media and are kept within the system rather than being washed out during colder temperatures as in a conventional activated sludge system would typically see happen. Thus, during the winter time operation, the majority of the ammonia degrading nitrification bacteria are found on the media and retained within the system rather than washed out due to low aerobic SRT’s. Factors that affect the overall design of an upgrade are: media type, aeration system pattern & type (complete floor coverage vs. spiral roll & fine bubble vs. course bubble), operational dissolved oxygen concentration, effluent NH3-N concentration, basin configuration and hydraulic profiles through the basin. All are necessary design factors which are required to help create a well working IFAS treatment system. Should one of these factors not be considered or designed properly, the overall treatment system could be adversely affected. Currently there are two (2) types of IFAS media categories in the market place, fixed media and free floating media. Fixed media can be similar to trickling filter type plastic media fixed in place and submerged in the reactor, or rope type media placed in a web or cage configuration and also submerged in the reactor. For both of these types of fixed media systems, the media is static and does not move around. These types of systems are placed above the activated sludge systems aeration system and flow makes its way through the basin and though the fixed media allowing contact of the wastewater constituents with the bacteria housed inside the fixed media. Free floating media are generally small plastic buoyant media or sponges which are placed in a reactor and move freely throughout the entire aeration basin volume. Since this media is moving around freely in the reactor, screens are required to retain them inside the aeration basin so they don’t escape. THE BROOMFIELD WWTP

Founded in the latter quarter of the Nineteenth Century, Broomfield, Colorado began as an agrarian community. Hard-working, community-oriented families located there on the heels of adventurous gold-seekers seeking their fortunes and hoping to strike gold in Colorado’s wilderness. In 1961, the city incorporated with a population of 6,000. Its growth continued at a moderate pace throughout the 70’s and 80’s. In the 1990’s with the development of one of Colorado’s premier employment centers, Interlocken Business Park, home to Sun Microsystems, Level 3 and the Flatiron Crossing regional shopping area, Broomfield’s population nearly doubled from 24,638 in 1990 to over 50,000 today.

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Broomfield’s original wastewater treatment facility was constructed in 1954 and consisted of a single treatment train of primary clarification, a trickling filter and secondary clarification. Over time as flows and loads increased, expansions took place in 1962, 1974, and 1988. As housing developments approached the treatment facility’s boundaries, odor control improvements were constructed in 1996 and 1998.

A Wastewater Utility Plan was completed in 1999 and contained the necessary information and direction to meet demands of providing wastewater collection and treatment service to the residents of Broomfield until the year 2020. The proposed wastewater system upgrades and expansions coordinated with water system upgrades and expansions.

Based on flow projections according to population, and industrial growth estimates to the year 2020, the WWTP needed to be expanded to a capacity of 12 MGD. It was proposed that the expansion take place in two phases with the first phase expanding the plant to 8 MGD, including upgrades to meet the new stream standards and to control odors. FULL SCALE PLANT RESULTS The full scale design criteria for the system were based on winter wastewater temperatures and the maximum month primary effluent concentrations. Specific design criteria and a flow diagram of the treatment facility are shown in Table 2 – Broomfield WWTP Design Specifications and Figure 2 – Full Scale Flow Diagram, respectively. Table 2. Broomfield WWTP Design Specifications

Flow Average Month 6.7 MGD Maximum Month 8.0 MGD – Summer 6.7 MGD – Winter

TSS Maximum Month 6,504 lbs/day (97.5 mg/L)

BOD5 Maximum Month 9,725 lbs/day (145.8 mg/L)

Soluble BOD5 Maximum Month 6,005 lbs/day (90 mg/L)

NH3-N Maximum Month 2,480 lbs/day (37.2 mg/L)

TKN Maximum Month 2,724 lbs/day (40.8 mg/L)

NO3-N Maximum Month 350 lbs/day (5.2 mg/L)

MLSS Concentration 3,500 mg /L Solids Retention Time Suspended Growth 4.7 days

Wastewater Temperature 13 - 25 C

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Figure 2 - Full Scale Flow Diagram

Figure 3 & Table 3 show in graphic form and table form, the monthly averages for the main influent characteristics of the facility. The overall flow to the facility over the past 3 years has been fairly consistent in the 4 - 5 MGD range, while the Figure 3 shows good variability in the concentrations for BOD, TSS & ammonia throughout the years. It should be noted that the table and graphs are only showing the influent ammonia while Organic N is not included as the facility doesn’t perform TKN analysis and as such the overall nitrification load is higher.

Figure 3 – Monthly Average Influent Data (Flow, BOD, TSS, Ammonia, Temp)

Broomfield WWTP Monthly Average Influent Data

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Process Schematic ofBroomfield BNR/IFAS Facilities

Pre-Anoxic –12,846 ft3 (364 m3)

Anaerobic –21,313 ft3 (604 m3)

Anoxic –46,590 ft3 (1,319 m3)

Aerobic –160,526 ft3 (4,546 m3)

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Process Schematic ofBroomfield BNR/IFAS Facilities

Pre-Anoxic –12,846 ft3 (364 m3)

Anaerobic –21,313 ft3 (604 m3)

Anoxic –46,590 ft3 (1,319 m3)

Aerobic –160,526 ft3 (4,546 m3)

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Table 3 – Broomfield Monthly Average Influent Data

Raw Influent CharacteristicsMonth, Year Influent Total Alkalinity

Flow TSS BOD5 NH3-N NO3-N NO2-N Total P PO4-P (as CaCO3) Temp (MGD) mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L ( C)

July-03 4.73 349.70 185.30 30.71 2.57 2.17 7.02 1.39 246.00 20.63August-03 4.85 321.81 195.86 32.15 2.74 2.42 6.62 1.66 238.00 21.61

September-03 4.76 337.59 202.32 33.61 2.81 2.17 7.38 1.39 245.20 21.11October-03 4.48 464.50 221.38 35.16 3.44 2.75 8.38 1.78 244.25 19.98

November-03 4.35 337.33 218.90 33.85 3.81 2.80 7.32 1.27 230.75 17.80December-03 4.09 322.13 216.83 33.76 3.80 2.77 7.12 1.79 229.40 15.93

January-04 4.28 308.10 206.86 34.88 4.18 2.88 7.36 2.64 204.25 14.62February-04 4.34 286.71 202.38 35.88 4.36 3.18 7.35 1.57 214.50 14.21

March-04 4.25 376.22 217.35 37.00 4.09 3.03 7.72 2.27 225.40 15.32April-04 4.52 406.29 223.05 34.83 3.44 2.71 8.56 1.81 234.00 16.18May-04 4.59 422.77 221.23 36.39 2.99 2.31 8.39 0.93 241.00 17.91

June-04 4.58 421.05 187.73 32.92 2.23 2.10 7.70 1.10 245.00 19.50July-04 4.90 420.32 188.24 28.41 1.96 2.19 7.12 0.82 242.00 20.77

August-04 5.06 370.22 192.05 29.66 1.72 1.81 6.91 0.92 244.25 21.51September-04 4.93 368.82 224.00 32.27 1.72 2.04 6.95 1.18 243.33 21.02

October-04 4.93 299.29 226.62 34.48 2.21 1.84 7.66 1.37 249.00 19.80November-04 4.56 309.82 200.45 36.48 2.57 3.01 7.67 1.86 242.20 17.11December-04 4.49 315.23 217.14 36.52 2.97 3.45 7.29 1.98 254.50 15.25

January-05 4.37 374.32 201.27 40.43 3.17 3.45 8.24 3.87 297.00 14.27February-05 4.38 368.21 214.53 40.59 3.25 2.63 9.66 5.05 283.00 14.13

March-05 4.41 380.12 199.60 39.07 3.25 2.88 8.61 3.67 284.00 14.87April-05 4.75 323.20 169.05 44.30 2.00 1.72 7.90 4.29 274.50 15.85May-05 4.89 375.57 189.57 34.29 1.87 2.38 9.97 4.67 301.00 17.03

June-05 5.18 372.09 203.32 39.46 0.06 0.12 9.86 4.75 303.40 18.59July-05 4.83 379.29 197.57 31.08 0.11 0.15 9.50 3.79 254.50 20.81

August-05 5.25 428.22 204.57 29.46 0.06 0.08 8.74 3.10 246.60 21.58September-05 5.07 419.41 210.82 30.10 7.58 3.08 251.25 21.26

October-05 5.53 344.74 185.65 31.12 7.00 2.89 262.50 19.75November-05 5.10 292.09 213.36 34.89 9.04 4.93 296.80 18.06December-05 4.93 285.26 220.43 37.88 10.56 6.77 273.50 15.68

January-06 4.75 375.76 232.29 38.27 12.38 7.22 255.75 15.16February-06 4.49 373.37 218.10 35.50 8.69 3.51 250.75 14.09

March-06 4.31 313.82 221.91 41.88 9.06 4.25 256.60 14.35April-06 4.47 375.76 229.17 39.02 9.98 5.11 258.50 15.83

Figure 4 – Monthly Average Influent Data (Flow, BOD, TSS, Ammonia, Temp)

Broomfield WWTP Monthly Average Effluent Data

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The IFAS system at Broomfield has performed & operated very consistently over the past 3 years and average monthly effluent data is presented in Figure 4 & Table 4 for BOD, TSS, NH3-N, NOx-N & Total P.

Table 4 – Broomfield Average Monthly Effluent

Final Effluent CharacteristicsMonth, Year Total

TSS BOD5 NH3-N NOX-N Total P PO4-P Temp mg/L mg/L mg/L mg/L mg/L mg/L ( C)

July-03 2.37 2.58 0.25 7.16 0.15 0.01 20.63August-03 2.74 1.60 0.17 6.80 0.74 0.48 21.61

September-03 2.55 2.23 0.18 5.78 1.69 0.99 21.11October-03 2.96 2.66 0.25 6.33 1.53 0.90 19.98

November-03 2.63 1.62 0.22 7.70 1.12 0.56 17.80December-03 4.11 1.88 0.25 8.62 1.42 1.01 15.93

January-04 5.73 2.75 0.77 10.48 1.37 1.15 14.62February-04 4.96 2.33 0.95 9.28 1.17 0.55 14.21

March-04 3.25 2.24 1.02 9.33 2.15 1.45 15.32April-04 5.58 2.36 0.32 6.78 1.25 0.80 16.18May-04 4.00 1.85 0.23 7.20 0.18 0.01 17.91

June-04 3.64 1.72 0.16 7.20 0.29 0.08 19.50July-04 3.70 1.78 0.26 6.15 0.24 0.03 20.77

August-04 2.85 1.91 0.16 5.79 0.23 0.05 21.51September-04 3.12 1.62 0.17 5.77 0.16 0.03 21.02

October-04 2.22 1.68 0.14 5.93 0.58 0.38 19.80November-04 3.33 1.67 0.16 7.55 0.48 0.29 17.11December-04 4.48 2.13 0.17 7.65 0.78 0.46 15.25

January-05 4.93 2.19 0.17 8.21 1.04 0.71 14.27February-05 5.91 2.66 0.25 8.26 2.63 2.03 14.13

March-05 4.85 2.21 0.25 7.20 1.18 0.87 14.87April-05 4.92 2.34 0.41 8.03 0.79 0.46 15.85May-05 4.08 2.51 0.26 8.37 1.48 1.19 17.03

June-05 2.57 2.43 0.14 11.56 1.33 1.11 18.59July-05 2.54 2.10 0.18 10.89 1.54 1.50 20.81

August-05 3.32 2.10 0.16 11.70 1.17 0.92 21.58September-05 3.09 1.88 0.11 10.78 0.44 0.38 21.26

October-05 3.76 1.93 0.10 11.42 1.31 1.34 19.75November-05 4.87 3.33 0.19 12.85 2.43 2.06 18.06December-05 6.20 3.57 0.21 10.80 1.81 1.18 15.68

January-06 5.87 3.94 1.21 1.83 1.31 15.16February-06 5.36 3.58 0.04 11.20 1.79 1.42 14.09

March-06 8.17 4.33 0.52 11.78 3.12 2.38 14.35April-06 7.30 3.00 0.18 12.28 2.05 1.68 15.83

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Table 3 & 4 along with Figures 3 & 4 show a facility operating very consistently. The data provided in Figure 5 & Table 5 show how the facility operated on an average monthly basis with respect to aerobic suspended MLSS SRT, Total aerobic SRT (suspended + fixed), Total Suspended SRT (Anoxic + Aerobic), Sludge Volume Index (SVI) and the operating MLSS & MLVSS concentrations. The Broomfield WWTP has been operating in typical carbonaceous removal SRT ranges, in the 3 – 4 day range, having the average temperature of the wastewater range between 22C during the summer time and 14C during winter operation, while still meeting baseline effluent ammonia concentrations.

Figure 5 – Broomfield Average Monthly Operational Information

Broomfield WWTP Monthly Operational Data

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Table 5 – Broomfield Average Monthly Operational Information

Operational CharacteristicsMonth, Year Total SRT Aerobic SRT Aerobic SRT TSS VSS

(susp.) (susp.) (susp.+ fixed) SVI (MLSS) (MLVSS) Temp d d d mL/g mg/L mg/L ( C)

July-03 9.99 6.62 98.87 2,510 1,887 20.63August-03 8.40 5.56 102.71 2,181 1,667 21.61

September-03 7.23 4.79 125.72 1,860 1,389 21.11October-03 6.76 4.48 138.51 1,864 1,534 19.98

November-03 6.15 4.07 5.16 154.91 1,906 1,540 17.80December-03 5.59 3.70 5.99 150.53 1,738 1,408 15.93

January-04 5.31 3.52 6.72 135.73 1,627 1,303 14.62February-04 5.78 3.83 7.65 117.22 1,944 1,450 14.21

March-04 6.35 4.21 8.44 105.88 1,917 1,511 15.32April-04 5.43 3.60 6.13 112.47 1,974 1,673 16.18May-04 6.29 4.17 6.46 114.01 2,056 1,637 17.91

June-04 6.05 4.01 6.92 116.10 1,745 1,323 19.50July-04 6.69 4.43 6.47 112.02 1,641 1,251 20.77

August-04 6.19 4.10 5.20 95.86 1,730 1,310 21.51September-04 5.50 3.65 4.76 103.50 1,893 1,462 21.02

October-04 5.73 3.79 4.99 106.33 1,780 1,377 19.80November-04 5.76 3.82 5.56 115.21 1,648 1,272 17.11December-04 5.19 3.44 6.08 119.47 1,633 1,313 15.25


March-05 5.31 3.52 6.63 130.15 1,708 1,352 14.87April-05 5.14 3.41 6.49 121.45 1,792 1,417 15.85May-05 5.70 3.78 7.20 120.20 1,691 1,333 17.03

June-05 5.18 3.43 6.21 107.96 1,748 1,542 18.59July-05 5.19 3.44 6.32 115.56 1,490 1,146 20.81

August-05 4.69 3.11 5.53 120.78 1,528 1,201 21.58September-05 5.22 3.46 5.94 120.88 1,488 1,261 21.26

October-05 6.97 4.62 7.74 109.67 1,665 1,387 19.75November-05 4.82 3.19 7.02 112.43 1,293 1,039 18.06December-05 4.46 2.96 6.51 111.28 1,401 1,098 15.68


March-06 10.13 6.71 18.39 99.21 1,357 1,217 14.35April-06 7.83 5.19 14.58 112.96 1,473 1,220 15.83

The following set of figures show all the data in either 7 day average or daily data points to illustrate how consistent the overall treatment plant really operates. Figure 6 shows the stability of the IFAS process with plastic media with respect to the Solids Retention Time (SRT) of the aerobic MLSS expressed as both suspended MLSS SRT and suspended MLSS + Fixed SRT. The effluent NH3-N concentration from the secondary clarifiers was consistently below 1 mg/L even though the temperature of the system was as low as 14C and the aerobic MLSS SRT were between 3 – 4 days. The media provides the additional biomass and surface area for growing of the nitrification bacteria and retains them in the system rather than being washed out.

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Figure 6 – Effluent NH3-N, Aerobic MLSS SRT & Temperature Profile vs. Time

Kaldnes HYBAS plant, Broomfield, CO

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Figure 7 shows the daily results of overall nutrient removal capability of the system with influent inorganic nitrogen concentrations increasing over time from 40 - 45 mg/L in 2004 to 45 - 50 mg/L in 2005 while still providing a consistently low effluent inorganic nitrogen concentration of less then 10 mg/L during winter time and less then 7 mg/L during summer time. All the denitrification was occurring in the pre-denitrification zone and utilized the influent carbon source for meeting these effluent limits. Based on mass balance of total nitrogen and the RAS and internal recycle rates, there is some type of simultaneous nitrification / denitrification occurring in the facility. The data indicates that the simultaneous nitrification / denitrification is not consistently year round and more research is being conducted to further explain these results.

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Figure 7 – Total Inorganic Nitrogen Profile with Temperature vs. Time


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Figures 8 & 9 show the daily BOD and daily TSS profile and demonstrates a very stable secondary clarified effluent concentrations with BOD’s less then 5 mg/L and TSS less then 10 mg/L during winter time operation and less then 5 mg/L during summer time operation. The reasoning for the increase in effluent BOD and TSS during 2006 is due to the lower MLSS concentrations of 1400 – 1500 mg/L that the facility is currently operating under. During 2004 & most of 2005, the system was operating at 1,800 – 1,900 mg/L. This loss of suspended biomass affected the aerobic SRT and as the SRT dropped, the effluent TSS increased, which likely is a reason for the increase in the effluent BOD. Effluent NH3-N during this period of time still meets < 0.5 mg/L.

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Figure 8 – Influent & Effluent BOD Profile with Temperature vs. Time


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Figure 9 – Influent & Effluent TSS Profile with Temperature vs. Time


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Figure 10 shows the biofilm characteristics over time and how the biofilm on the media in each of the reactors grows during winter time operation and decreases during summer time operation. This effect of the biofilm growing and sloughing is just part of the normal operation. When the MLSS concentration started a steady decline in March 2005 to April 2006, the overall biofilm thickness didn’t show the dramatic winter vs. summer effects as the overall MLSS system was being stressed and allowed more biofilm to grow on the fixed media. During the winter of 2006, the biofilm in reactor 1 showed similar g TS/m2 of biomass as was seen during the winter of 2004. From a design standpoint, one can see that it would be very difficult to say the biomass on the media is consistent; rather the only consistent value in the IFAS system is the amount of surface area. Thus, full scale designs shouldn’t count MLSS suspended biomass + biomass on the media. It should be MLSS suspended biomass + surface area kinetics which control the overall design of IFAS systems.

Figure 10 – MLSS Conc., Media TSS Reactor 1, Media TSS Reactor 2 & Temp. vs. Time


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Figure 11 shows the potential of the effect of simultaneous nitrification and denitrification (SND) occurring within the aerobic zone and the biofilm on the plastic media. In this figure the red line is the actual denitrification occurring in the system by monitoring the influent and effluent inorganic nitrogen. The blue line is a theoretical % removal of nitrogen based on the internal recycle rate and return activated sludge rate (i.e. 100% RAS + IR = 50% Nitrogen Removal). As can be seen in the graph, during periods of warmer weather the blue line is above the red line, indicating the potential for SND. The data from May 2005 to current is missing influent NOx-N data and thus the evaluation during this period of time is incomplete.

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Figure 11 – Potential Simultaneous nitrification and denitrification (SND)


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CONCLUSION

The full scale IFAS system operating at the City & County of Broomfield’s WWTP has demonstrated over the past three (3) years a very consistent operating and stable effluent system, which operates at low solids retention times and that the Hybrid MBBR process is effective in removing NH3-N to below the effluent requirements of 1 mg/L on a consistent basis. Based on the results obtained from this pilot study it is recommended that the full scale treatment system look at running a two stage aerobic system with media to help increase the nitrification capacity of the existing treatment system.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the dedication of the City & County of Broomfield’s operational staff at the wastewater treatment plant. Without their efforts the information presented would not be available. They also wish to thank the personnel at the Broomfield WWTP for all their help in gathering all the data for this report.

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REFERENCES

Hem, L. (1999) Oxygen transfer rates in Kaldnes Moving Bed™ biofilm reactors at two Norwegian wastewater treatment plants. Aquateam report (99-058).

Hem, L. (2002) Measurement of oxygen transfer rates in clean water and domestic wastewater. Aquateam report (02-009).

Hem, L., Rusten, B., Ødegaard, H. (1994) Nitrification in a moving bed biofilm reactor. Water Research, Vol. 28, No 6, pp. 1425-1433.

Johnson, T.L., Shaw, A.R., McQuarrie, J.P. (2004): “Integrated Fixed-Film Activated Sludge (IFAS): The New Choice for Nitrogen Removal Upgrades in the United Sates”. Proceedings, WEFTEC, New Orleans, LA, USA, October 2-6.

Ødegaard, H., Rusten, B., Wessman , F.(2004) State of the art in Europe Of the Moving Bed™ biofilm reactor (MBBRTM) Process, WEFTEC 2004

Ødegaard, H., Rusten, B., Siljudalen, J. (1999) The development of the moving bed biofilm process–from idea to commercial product. European Water Management, Vol.2, No. 2.

Ødegaard, H., Rusten, B., Westrum, T. (1994) A new moving bed biofilm reactor- Applications and results. Wat. Sci. Tech. Vol. 29, No 10-11, pp 157-165.

Rusten, B., Hem, L., Ødegaard, H. (1995b). Nitrogen removal from dilute wastewater in cold climate using moving bed biofilm reactors. Water Environm. Res., Vol. 67, No1, pp 65-74,1995.

Rutt, K.; Seda, J.; Johnson, C. (2005): ”A Successful Conversion – A Colorado facility adopts an integrated fixed film activated sludge system to great effect”. Water Environment & Technology, (62-66), March 2005

Shaw, A.R., Johnson, T.L., Johnson, C. (2003): “Intricacies of Modeling the Emerging Integrated Fixed Film Activated Sludge (IFAS) Process”. Proceedings, WEFTEC, Los Angeles, CA, USA, October 11-15.

Wessman F, Nielsen I, Bungum S. (2005) Nitrogen Removal in Moving BedTM Biofilm Reactor Plants IWA specialized conference proceedings

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