COMPARISON OF PROCESS ALTERNATIVES FOR ENHANCED NUTRIENT REMOVAL: PERSPECTIVES ON ENERGY REQUIREMENTS AND COSTS Derya Dursun 1, Jose Jimenez 1, Aaron Briggs 2 1 Brown and Caldwell, 850 Trafalgar Court, Suite 300 Maitland, FL, 32814 3 Dewberry, 3106 Lord Baltimore Dr, Suite 110, Baltimore, MD, 21244 Corresponding author: e-mail: ddursun@brwncald.com ABSTRACT As part of the Chesapeake Bay Restoration Act s Enhanced Nutrient Removal (ENR), the Maryland Department of Environment (MDE) issued a new permit for the Marlay Taylor Water Reclamation Facility (MTWRF) to reduce the effluent nitrogen and phosphorus loads from the facility to ENR levels. MTWRF has explored cost and energy effective solutions to upgrade the facility to meet the future ENR requirements for total nitrogen (TN) and total phosphorus (TP) limits. Three process alternatives were evaluated in terms of process and energy requirements. A calibrated model was used to determine process requirements. The initial capital cost along with a 15-year present worth analysis was also conducted to compare overall aspects of alternative processes. The footprint of the BioMag process was found to be significantly smaller than other options since this process eliminates the need for adding a secondary clarifier and effluent filters. Hence, this alternative would require notably lower initial capital costs compared to conventional four stage Bardenpho and hybrid IFAS processes. On the other hand, BioMag process was shown to be an energy intensive process due to high mixing requirements and additional energy consumption of the process related equipments. KEY WORDS Process modeling, Energy requirements, Bardenpho, IFAS, BioMag, Cost analysis INTRODUCTION As the nutrient requirements are tightening in the United States, one of the biggest challenges in wastewater treatment has become to reliably meet effluent limits in a sustainable manner. The reliability requirement is driven by the need to meet strict effluent daily or weekly limits set in permits so as to protect the designated uses of the receiving water. Hence, facilities facing with more strict nutrient requirements have to consider a wide and possibly confounding array of treatment technologies to select from. In order to address this issue, EPA has recently published a technical document including process descriptions and operating factors for over 40 different treatment technologies for removing nitrogen, phosphorus, or both from municipal wastewater streams (US EPA, 2009). However, nutrient removal processes comes at a cost to municipal wastewater treatment facilities and their ratepayers. Although funding from various sources might be available, they are not generally sufficient to address all aspects of the necessary improvements for nutrient removal. Another important factor affecting the cost of nutrient removal at wastewater facilities is site limitations on physical expansion of wastewater treatment facilities. Some plants are located in urban areas and do not have any way to obtain the physical space necessary to expand. Space limitations can severely limit the type of processes that can be used to reduce nutrients (Naik and Strenstrom, 2011)
It is well known that nutrient removal treatment facilities have a larger environmental impact than secondary treatment processes as a result of the additional energy consumption originated from advanced wastewater treatment processes. It is reported that the addition of ENR technologies such as nitrification/denitrification increases the energy consumption by about an additional 1,500 kwh/mg (Woertz, 2009). Additional power is required for pumping and mixing the biological treatment process to remove nitrogen and phosphorus, and for chemical feed systems and filters. Furthermore, biological removal of nitrogen or phosphorus often requires addition of a supplemental carbon source, such as methanol, ethanol, or volatile fatty acids (VFAs) and also increases the quantity of biosolids requiring disposal. Both the extra energy and extra chemicals increase the carbon footprint of the wastewater treatment process, as well as raising operating costs for the plant in terms of energy costs, chemical costs, and sludge disposal costs (Kang et al, 2009) The Marlay Taylor Water Reclamation Facility (MTWRF) located in Maryland is a 6.0-mgd average daily flow (ADF) plant and is owned and operated by the St. Mary s County Metropolitan Commission (MetCom). The MTWRF currently meets biological nutrient removal (BNR) requirements for total nitrogen concentrations of less than 10 mg/l and discharges secondary effluent to a tributary of the Chesapeake Bay. However, as part of the Chesapeake Bay Restoration Act s Enhanced Nutrient Removal (ENR), the Maryland Department of Environment (MDE) issued a new permit effective on February 01, 2008 to reduce the effluent nitrogen and phosphorus from the facility to ENR levels by August 2012. The proposed effluent ENR limit for the MTWRF includes total nitrogen (TN) and total phosphorus (TP) goals of 73,093 lb/year and 5,483 lb/year (corresponds to 4 mg/l and 0.3 mg/l at ADF), respectively. In response to the new ENR effluent discharge permit limits, MetCom is looking for a cost and energy effective solutions to upgrade the MTWRF to meet the future ENR requirements for total nitrogen (TN) and total phosphorus (TP) limits by August 2012. Facility Description The MTWRF was originally constructed in 1964. The facility was then expended to 4.5 mgd in 1984 and to 6.0 mgd in 1998. The 1998 expansion included BNR facilities which consisted of new Schreiber Reactors for nitrification-denitrification. More recently, in 2005, new solids handling facilities were added which includes the sludge thickening facility. Figure 1 shows the current overall process flow schematic for the existing treatment facility. Currently, the liquid treatment of the MTWRF comprises preliminary, primary and secondary treatment, chlorination and dechlorination and reaeration. The effluent from the treatment facility is discharged through outfall to the Chesapeake Bay. The solids produced during treatment are handled onsite. The sludge processing facility at the MTWRF includes sludge holding tanks, sludge thickener, anaerobic digesters, dewatering processes and sludge drying beds.
Influent Preliminary Treatment WAS Primary Clarifiers Schreiber Reactors FeCl3 Secondary Clarifiers Cl2 Contact Tank Dechlor / Post - Aeration Effluent PS+WAS RAS Polymer GBT Anaer Dig Anaer Dig Dewatering Drying Beds Disposal Blend Tank Recycle from GBT Thickening Tank Recycle from Rotary Press Figure 1. MTWRF Existing Process Flow Diagram OBJECTIVES The objective of the project was to assess different process options (conventional four-stage Bardenpho, hybrid IFAS process, and an emerging process alternative BioMag) in terms of process, chemical, energy and air requirements to meet upcoming nutrient limits while minimizing energy usage at low cost. Capital and energy costs of the three process alternatives were compared to assess the most feasible option for reaching cost and energy efficient facility. METHODOLOGY BioWin version 3.1 (EnviroSim Associates Ltd., Canada) was used to evaluate the most effective process configurations to meet ENR requirements at the MTWRF. A detailed wastewater characterization study was performed to determine the specific wastewater fractions for model calibration. The process simulator was calibrated using plant s daily historical data from January 2009 through June 2009. The dynamic simulations were performed with calibrated process simulator to evaluate three different treatment alternatives to meet the future ENR requirements within the existing reactor volume. RESULTS AND DISCUSSIONS Three process alternatives were evaluated to achieve nutrient removal at the MTWRF. Unlike the processes for Biochemical Oxygen Demand (BOD 5 ) and Total Suspended Solids (TSS) removal, operation of biological processes for nitrogen and phosphorus removal requires advanced knowledge and process control for successful and consistent removal to low effluent levels. In addition, these processes are susceptible to wet weather, cold weather, and inhibitory substances entering the plants. Plant influent characteristics also play a major role on determining tank volumes and chemical requirements. As a result of initial process evaluations, a four-stage Bardenpho process enhanced with chemical precipitation with ferric chloride addition was selected for nitrogen and phosphorus removal. This process recommendation was based on maximizing the capabilities of the existing
reactors; hence, minimizing the overall capital cost of the upgrades. However, during initial evaluations, limited information on flows and loads to the facility were available. A detailed analysis of historical data as well as information obtained during the wastewater characterization study conducted at the MTWRF indicated significantly higher loads entering the facility. Furthermore, additional ammonia and phosphorus loads from the filtrate return of dewatering anaerobically digested sludge were characterized in the special sampling campaign. Due to increased wastewater loadings entering into the plant, the original recommendation on process configuration was re-evaluated. Additional alternatives to reduce the capital costs and energy requirements for the upgrade of the MTWRF were considered. It should be noted that the alternatives considered are all based on the original concept of converting the existing Schreiber reactors into four-stage Bardenpho process with supplemental carbon addition. Phosphorus removal would be accomplished by chemical precipitation with ferric chloride. The following three processes were selected for further evaluation; Alternative 1: Conventional Process - Four Stage Bardenpho Alternative 2: Hybrid Process - Four-Stage Bardenpho with Integrated Fixed Film Activated Sludge (IFAS) Carriers Alternative 3: Emerging Process - BioMag Alternative 1: Conventional Process - Four-Stage Bardenpho This alternative consists of the original concept for the upgrade of the MTWRF to meet future ENR requirements. Figure 2 presents a typical process diagram for a four stage Bardenpho process. IMLR Carbon Primary Effluent Pre- Anoxic Aerobic Post- Anoxic Post- Aerobic Sec. Clarifier Effluent RAS Figure 2. Four Stage Bardenpho Process WAS The four-stage Bardenpho process that uses a two-stage activated sludge system to reduce nitrogen to ENR levels is recommended as a conventional process. The first stage of the process comprises an anoxic zone for denitrification followed by an aerobic zone for nitrification. This process uses carbon provided by the raw wastewater for denitrification in the first anoxic zone. This process also incorporates an internal mixed liquor recycle (IMLR) from the first aerobic zone to the first anoxic zone to return nitrified mixed liquor at a regulated rate to ensure adequate nitrates for the heterotrophic denitrification population in the anoxic zone. The second stage of the process uses a second (post) anoxic zone for denitrification with supplemental carbon addition after the aerobic zone. A post aerobic stage is normally required to remove excess
carbon and to strip the nitrogen gas from the mixed liquor before the secondary clarifiers to prohibit floating solids. The Schreiber reactors at the MTWRF are a circular, single tank systems that provide both nitrification and denitrification by cycling the aeration on and off with a center-pivot providing for mixing. These reactors operate as completely mixed reactors. Therefore, the conversion of these into four-stage Bardenpho would require extensive modifications to the internals of the tanks. Figure 3 given below shows the possible reactor configurations that were developed for process evaluations. Aerobic (AER 1B) Aerobic (AER 1A) Pre-Anoxic (AX 1B) [PE + RAS] Pre-Anoxic (AX 1A) Post-Aerobic (Post-AER) To Clarifiers Aerobic (AER 1C) IMLR Post-Anoxic (Post-AX) Figure 3. Possible Reactor Configurations for Converting Schreiber Reactors to a BNR Process
Alternative 2: Hybrid Process - Four-Stage Bardenpho with Integrated Fixed Film Activated Sludge (IFAS) Carriers As an alternative to conventional four-stage Bardenpho process, the use of Integrated Fixed-Film Activated Sludge (IFAS) carriers is considered in order to supplement the four-stage Bardenpho suspended-growth activated sludge process. The basic principal behind IFAS is to expand treatment capacity or upgrade the level of treatment by supplementing the biomass in a suspended-growth activated sludge process by growing additional biomass on fixed-film media contained within the mixed liquor. The higher biomass constituency allows a more effective rate of treatment within the existing process tanks, therefore, reducing the necessary suspended biomass inventory necessary for treatment. Several advantages in using this type of process are; Higher rate treatment process allows greater treatment in a smaller footprint Additional biomass for treatment without increasing solids loadings on the final clarifiers Similar operation to conventional Four-Stage Bardenpho Minimal additional operating cost Improved resistance to toxic shock and washout This hybrid process uses a four-stage Bardenpho configuration with the main difference of having the carriers (IFAS media) in the aerobic section of the reactor to maximize nitrification at lower solids retention times. This would allow upgrading the MTWRF without the need of any additional ENR reactor and/or secondary clarifier capacity. For the purpose of process evaluations, an IFAS system provided by AnoxKaldnes was considered. Alternative 3: Emerging Process- BioMag This emerging technology can be applied to conventional process with the advantage of eliminating the need of any additional ENR reactor and/or clarification capacity. BioMag is a ballasted flocculation-aid wastewater treatment process that uses magnetite to increase the specific gravity of biological floc. Magnetite (Fe 3 O 4 ) is an inert iron ore, with a specific gravity of 5.2 and a strong affinity for biological solids. Magnetite substantially increases the settling rate of the biomass. This provides the opportunity to increase the active mixed liquor concentration in the biological system, while still maintaining adequate settling and thickening in the secondary clarifiers (Catlow and Woodard, 2009). For the MTWRF, the BioMag process would allow to operate at elevated active mixed liquor concentrations without affecting its overall capacity, eliminating the need for additional reactor capacity to meet ENR requirements. Virgin and recovered magnetite are blended with mixed liquor or RAS in the magnetite mix tank. The ballasted mixed liquor then flows to the aeration tank, and then on to the secondary clarifier, where the solids settle and thicken. The majority of the resultant sludge (with ballast) is returned to the aeration tank via the RAS line. Waste sludge (WAS) is pumped through a shear mixer and then to the magnetic recovery drum, where the ballast is recovered and sent for blending with the mixed liquor in the magnetite mix tank. The excess biological solids (minus the magnetite) are wasted to sludge processing. Magnetite deposition at the floor of the reactor in cases with poor or limited mixing is a concern of the BioMag process. It is noted that mixing is influenced by the shape of basins with the configuration being preferred.
Comparison of Process Requirements Detailed process design was carried out by using calibrated BioWin model to evaluate process requirements for the conversion of the existing secondary BNR process at the MTWRF to fourstage Bardenpho facility. Based on the process analysis, significant increase in secondary clarification capacity would be required to handle 6 mgd ADF if conventional four stage Bardenpho process is preferred.a hybrid process using a four stage Bardenpho configuration equipped with biofilm carriers in the aerobic section of the reactor allows maximizing nitrification at lower solids retention times. This alternative allows upgrading the MTWRF without the need of any additional ENR reactor and/or secondary clarifier capacity and treat 6 mgd ADF. Both conventional and hybrid processes require filtration facility after secondary treatment not to impair effluent requirements. BioMag process can be applied to conventional four stage Bardenpho process with the advantage of eliminating the need of any additional ENR reactor and/or clarification capacity; and the need for a new filtration facility. BioMag process could also treat up to 7.5 mgd ADF without impairing the effluent quality, whereas other processes could only reach 6 mgd ADF even with additional reactor, and secondary clarifier volumes. The effluent quality is compatible between three process options despite the BioMag process not having filters. Table 1 summarizes important process requirements for three alternatives at 6 mgd ADF. Figure 4 shows the total footprint of process alternatives. Parameter Table 1. Process Requirements for Alternatives Conventional Process Hybrid Process Four Stage Bardenpho IFAS Emerging Process BioMag Total Primary Clarifier Area, ft 2 11,458 11,458 11,458 Total Reactor Volume, MG 3.63 3.63 3.63 Anoxic Zone 0.80 0.73 0.8 Aerobic Zone 2.18 1.99 2.10 Post Anoxic Zone 0.55 0.73 0.63 Post Aerobic Zone 0.10 0.18 0.10 Aerated SRT, days 12 4 (suspended), 12 12 MLSS Concentration, mg/l 3,400 1,500 3,800 IMLR Flow, mgd 18 18 18 Total Secondary Clarifier Area, ft 2 21,225 14,860 14,860 RAS Capacity, mgd 8.5 6.0 6.0 SVI, ml/g 150 150 40-60 Required Filter Area, ft 2 800 800 Not required Average Sludge Production (Primary+WAS), lbs/d 14,500 17,000 13,500
Footprint, ft2/ft3/day 0.090 0.080 0.070 0.060 0.050 0.040 0.030 0.020 0.010 0.000 Four Stage Bardenpho IFAS BioMag Figure 4. Footprint of Alternative Processes Besides process modifications, all three processes would require chemical additions to ensure meeting the effluent requirements. Table 2 summarizes the annual average (and maximum average month) chemical addition requirements for all alternatives. However in addition to these chemicals, BioMag process might necessitate the addition of polymers and coagulants for magnetite sorption. Table 2. Chemical Requirements for Alternatives at Design Conditions Process Target Dosage Location Quantity Alkalinity [Sodium hydroxide, 50% solution] Denitrification [Micro C-Glycerin] Chemical P Removal [Ferric chloride, 33% Solution] Influent to the biological reactors (splitter box) Influent to the post anoxic zone with flexibility to add carbon to pre-anoxic zone Secondary clarifiers splitter box with flexibility to add coagulant to secondary influent 125 (150) 280 (350) 450 (550) Comparison of Air and Mixing Requirements Based on the process analysis, conventional four stage Bardenpho process would have the lowest air requirement. Since, higher Dissolved Oxygen (DO) concentration would be necessary for IFAS process, air requirement of the IFAS alternative would be significantly higher than the conventional option. Furthermore, in the zones where media is included into the process, medium bubble diffusers would be utilized. It is known that medium bubble diffusers requires more process air than a fine bubble diffused air system to provide the same level of treatment. BioMag process also requires higher air flow compared to conventional processes. The aeration requirements of the BioMag system for fine bubble diffusers are given in Table 3. As the table shows, ranges of aeration requirements were defined for fine bubble diffusers to cover the uncertainties and risks associated with the selection of the design alpha value. In order to minimize spaces where the magnetite could accumulate in the aerobic zone, a minimum diffuser density of at least 10 percent was adopted for the BioMag system. In cases where the aeration
demands did not meet the minimum mixing requirement of 0.12 scfm/ft 2, the airflow rate was driven by mixing. Based on these results, the range for average and peak hour airflow rates for the fine bubble diffuser system are approximately 5,300 to 6,550 standard cubic feet per minute (scfm) and 9,720 to 12,140 scfm (total airflow rate), respectively. Two blowers would be sufficient to provide process air requirements of the three alternatives. Mixing could be supplied by 8 mixers in anoxic zones for 4 stage Bardenpho and hybrid options whereas BioMag option would need additional mixing in aeration zones to keep the magnetite in suspension. A total of 12 mixers would be essential for BioMag option. Table 2 lists the details of the air and mixing requirements for all alternatives. Table 3. Comparison of Air and Mixing Requirements Parameter Conventional Process Four Stage Bardenpho Hybrid Process IFAS Aeration System Fine Bubble diffuser Medium Bubble in IFAS zone Emerging Process BioMag Fine Bubble diffuser Fine bubble in other zones Max Month Air Requirement, scfm 5,050 6,500 5,300-6,550 Peak Hour Air Requirement, scfm 9,400 11,000 9,720-12,140 Number of Blowers 2 2 2 Avg Aeration Power Requirement, hp 135 150 160 Total Number of Mixers 8 8 12 Avg Mixing Power Requirement, hp 6 6 8 Comparison of Annual Energy Requirements Daily energy requirement of three processes were compared in Table 4. Based on the analysis, conventional 4 stage Bardenpho process would have the lowest energy demand, followed by the hybrid-ifas process. BioMag process would require higher energy mainly due to the energy requirement of process related equipments and additional mixing in aeration zones. Table 4. Comparison of Average Energy Requirements Parameter Conventional Process Four Stage Bardenpho Hybrid Process IFAS Emerging Process BioMag Aeration Requirements, kwh/day 4,890 5,433 5,795 Mixing Requirements, kwh/day 860 860 3,223 Total Filter Related Demand, kwh/day 164 164 0 BioMag Equipment Related Demand (incl mixers, ballast feed air compressors, shear mills, separators and pumps) kwh/day 0 0 1,976 Figure 5 depicts the total annual energy consumption of the processes based on the information provided in Table 4. As indicated in Figure 5, BioMag process proves to be an energy intensive technology. Ballast mixers, ballast feed air compressors, shear mills, magnetic drum separators and pumps increase the energy demand of the BioMag process significantly. However, conventional and hybrid processes would need filters which also requires pumping of the entire flow through the filters. The energy consumption of pumping through the filters did not included in this analysis since this would highly vary depending on the location of the new filter facility.
Annual Energy Consumption, MW.h However, the difference in energy requirements of the options might reduce drastically if it requires significant pumping to pass the flow through the filters. 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 Four Stage Bardenpho IFAS BioMag Figure 5. Total Annual Energy Consumption of the Process Alternatives Capital Costs The initial capital costs were developed for each alternative at 6 mgd ADF. Capital and energy costs were then used in present worth analysis of each alternative. Table 5 summarizes the capital costs associated with each process. Based on this analysis, conventional process would require the highest capital cost since this process would need additional secondary clarifiers, filters, modifications in chlorine tank and also a diversion pond to protect shellfish population. Table 5. Initial Capital Costs for Alternative Processes Parameter Conventional Process Four Stage Bardenpho Hybrid Process IFAS Emerging Process BioMag Blowers $675,000 $675,000 $675,000 Proprietary Equipment $0 $1,500,000 $3,300,000 Building/Infrastructure $0 $0 $400,000 Filters $3,200,000 $3,200,000 $0 Floc Chamber $150,000 $150,000 $0 Chlorine Tank Modifications $11,650 $11,650 $0 Secondary Clarifiers $2,300,000 $0 $0 Shellfish Diversion Pond $400,000 $400,000 $0 Total Initial Capital Cost $6,736,650 $5,936,650 $4,450,000 On the other hand, BioMag process eliminates the need of filters, additional secondary clarifier, and other modifications necessary to meet permit requirements. Hence, the initial capital cost of this emerging process would be significantly lower than the conventional process.
Present Worth Analysis As a basis for comparing the various options, a present worth analysis was conducted. The capital costs were already inflated to 2011 dollars which represent the present worth. Energy and maintenance costs were multiplied by annual present worth factors that provide the present worth for a series of values for a 15-year period (Lowe et al, 2011). An interest rate of 4.67% was used in analysis. Figure 6 exhibits 15-year present worth value of each alternative. $12,000,000 $10,000,000 $8,000,000 $6,000,000 $4,000,000 $2,000,000 $0 Four Stage Bardenpho IFAS BioMag Figure 6. 15-year Present Worth Value Based on this analysis, present worth value of three alternatives was quite similar. Conventional Four stage Bardenpho process resulted slightly higher value compared to other two alternatives. CONCLUSIONS The footprint, energy, chemical and capital costs associated with three treatment alternatives were compared for the MTWRF in order to meet ENR requirements. Significant increase in total process footprint would be required if conventional four stage Bardenpho process is preferred. However, the energy consumption of this alternative would be notably lower, especially compared to BioMag process. IFAS process would allow upgrading the MTWRF without the need of any additional ENR reactor and/or secondary clarifier capacity. The energy requirement of this process would be slightly higher than the conventional process but relatively lower from BioMag alternative. The BioMag process would provide more capacity without building additional unit. On the other hand, the process is energy intensive compared to other options. Chemical consumption of IFAS and Bardenpho processes would be similar, whereas BioMag process might necessitate the addition of polymers and/or coagulant for magnetite sorption. Initial capital cost of BioMag process would be significantly lower than other alternatives. The 15-year present worth value of three alternatives was found comparable.
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