UPGRADING FOR TOTAL NITROGEN REMOVAL WITH A POROUS MEDIA IFAS SYSTEM

Transcription

1 UPGRADING FOR TOTAL NITROGEN REMOVAL WITH A POROUS MEDIA IFAS SYSTEM T. Masterson, J. Federico, G. Hedman, S. Duerr BETA Group, Inc. 6 Blackstone Valley Place Lincoln, Rhode Island ABSTRACT The Westerly, Rhode Island wastewater treatment facility (WWTF) is an activated sludge plant with a monthly design average daily flow of 3.3 million gallons per day (MGD). Construction and commissioning of an integrated fixed-film activated sludge (IFAS) process completed in September 2003, upgraded the biological process at the facility from carbonaceous BOD 5 removal only, to include total nitrogen removal. The IFAS system met the treatment performance criteria soon after commissioning and startup and has performed well to date with total effluent nitrogen concentrations averaging approximately 5 mg/l. INTRODUCTION The modifications were completed to comply with revisions of the facility s discharge permit issued by the State of Rhode Island Department of Environmental Management (RIDEM). The revised discharge permit includes seasonal limits for ammonia and total nitrogen, in addition to the previous discharge permit limits for BOD 5, total suspended solids, fecal coliform and total residual chlorine. A summary of the current permit limits that the process modifications were designed to achieve is below: Table 1 Discharge Permit Limit Summary Parameter BOD 5 TSS Total Nitrogen Ammonia Fecal Coliform Total Residual Chlorine Discharge Limit (Average Monthly) 30 mg/l 30 mg/l 15 mg/l, June - October 5.5 mg/l, June October 30.9 mg/l, November May 200 MPN 65 µg/l Prior to the construction of the process modifications, the facility consisted of a manual influent bar rack, grit removal, primary clarification, mechanical aeration, secondary clarification, and sodium hypochlorite disinfection. Improvements were made to the preliminary treatment, primary treatment and disinfection processes as part of the overall facility improvements as well as to the biological treatment and secondary clarification processes. The focus of this paper will be on the improvements made to the biological treatment process to achieve total nitrogen

2 removal. This paper will present the construction sequence issues addressed in order to construct the improvements while maintaining compliance with the facility s previous discharge permit, discuss the installation and commissioning of an IFAS System in the existing aeration basins for total nitrogen removal, and present process performance data that confirms the process capability of the system. KEYWORDS Integrated fixed-film, IFAS, total nitrogen removal, porous support media CONSTRUCTION SEQUENCE The primary focus of the upgrade of the biological treatment system at the facility was the modification of aeration basins to accommodate the IFAS system while the facility maintained compliance with its discharge permit limits of 30 mg/l for BOD 5 and TSS. The aeration tanks are two tanks in parallel with each tank divided into 3 cells. Each cell is approximately 34 feet square with a side water depth of 14 feet. Aeration was provided by mechanical surface aerator equipment that was beyond its useful operating life and had difficulty maintaining adequate dissolved oxygen (DO) for BOD 5 oxidation. Two centrifugal blowers and aeration piping, part of the permanent process improvements, were installed while the surface aeration equipment remained in service. Temporary coarse bubble diffusers were installed in each aeration tank to supplement the function of the mechanical aeration equipment. The dissolved oxygen (DO) concentrations in the aeration tanks improved quickly and the periods of low DO and the related process upsets were eliminated. Interim permit discharge limits for BOD 5 and TSS were negotiated with RIDEM for the construction period that would have allowed less restrictive discharge concentrations for both BOD 5 and TSS without violation. The construction sequence was coordinated such that supplemental coarse bubble aeration and the construction of a third secondary clarifier allowed the aeration tanks to be taken out of service one at a time for completion of the IFAS system upgrade work. One of the two aeration tanks was in service at all times along with three secondary clarifiers during construction of the IFAS improvements. As a result, the facility maintained compliance with its BOD 5 and TSS permit limits of 30 mg/l for BOD 5 and TSS during construction. The porous support media was not introduced into the reactors until the mechanical process improvements to both aeration tank s were complete. The anoxic zones were operated as aerobic zones (with temporary coarse bubble aeration) during the period prior to the introduction of the porous media in order to maximize the aerobic HRT in the system for BOD 5 removal. The mechanical aeration equipment was decommissioned as each aeration tank was removed from service. Septage receiving was suspended during the construction to reduce the pollutant loads on the biological treatment process while one aeration tank was out of service.

3 IFAS SYSTEM The IFAS system installed in each aeration tank is a LINPOR -CN System supplied by Mixing & Mass Transfer Technologies (m 2 t). The LINPOR -CN System operates with a suspended porous support media, in combination with a freely suspended biomass. This combination results in total mixed liquor biomass concentrations in the aerobic biological reactor that are substantially higher than conventional activated sludge systems. The higher total effective biomass concentration allows higher reactor volumetric pollutant loadings at biomass loadings similar to a conventional air activated sludge process. The porous support media is sponge-like, 15 millimeter polyurethane cubes. Structural and process equipment modifications were made to the two existing aeration tanks in order to provide a nitrogen removal configuration. The upstream cell of each aeration tank was converted to a baffled anoxic zone with submersible mixers. A single fiberglass reinforced plastic baffle and support structure was installed in each anoxic zone to minimize flow shortcircuiting and improve mixer performance. Each anoxic zone is mixed by two, stainless steel submersible mixers.

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5 The other two cells of each aeration tank were converted to the aerobic LINPOR -CN reactors. The reinforced concrete wall separating the second and third cells was demolished to provide single aerobic reactors 34 feet wide by approximately 70 feet long. Steel beams were installed in the removed wall s former location for supporting the tank exterior walls. These beams were installed above the liquid level to allow free surface flow. Tubular membrane-type fine pore air diffuser grids were installed in the aerobic reactors. The diffusers were designed in a tapered configuration with the highest diffuser densities being installed at the feed (highest demand) end of the reactors. Photo 1 Tubular Membrane Diffusers The porous support media is retained in the aerobic reactors by a perforated, stainless steel screen installed at the effluent weirs. The media retention screen prevents media overflow to the final clarifiers. The screen extends the full depth of each reactor and has circular perforations 8 millimeters in diameter spaced at 11 millimeters on center. Media retained by the screen is kept from impinging on the screen by a coarse-bubble air knife installed at the base of the screens. This air knife provides air scouring of the screen and maintains free passage of the mixed liquor through the screen to secondary clarification. Uniform support media distribution is maintained in the aerobic reactors by recycling the media within the reactors with air-lift pumps. Air for this service is supplied by the process aeration blowers. No dedicated air supply source is required. The air-lift pumps return media which migrates from the feed end to the effluent end of the plug-flow aerobic reactors.

6 Photo 2 Air-Lift Pump Inlet and Media Retention Screen The air-lift pumps discharge the media and mixed liquor above the water surface through a stainless steel pipeline that is perforated to allow mixed liquor to drain back into the reactor. The discharge is also equipped with a flat plate fixed across the opening to cause excess biomass to be separated from the support media and reenter the tank as suspended, rather than fixed, biomass. This action controls the amount of fixed biomass in the system. The flowrate of the media recycle is controlled manually by operating a butterfly valve on the air drop legs to the media air-lift hoods. Photo 3 - Air-Lift Pump Media Recycle Discharge

7 Nitrified mixed liquor is pumped from the end of the aerobic reactor to the anoxic zone by submersible, mixer-type, axial pumps. There is one nitrate recycle pump in each aerobic reactor and each delivers approximately 735 gpm. The pumps are installed downstream of the media retention screen and do not contact the support media. The flowrate of these pumps is controlled by variable frequency electric drives that operate based on a manually programmed speed setpoint. The pumps are interlocked with a point level sensor installed in each reactor. The nitrate recycle pumps will shut down based on a high level in the reactors. This condition may result from high flows or the unlikely blinding of the media retention screen. Process air is supplied to the IFAS system by two, multi-stage, centrifugal air blowers. Two blowers are installed with one duty unit and one backup as 100 percent redundancy. The system operates under average design load conditions with one blower in operation at approximately 4,300 scfm and 7.2 psig. The output of the blowers is controlled by variable frequency electric drives that operate in automatic control by a programmable logic controller (PLC) based dissolved oxygen (DO) control system. The blowers are protected locally by a vibration and surge alarm system that will report trouble to the system s main PLC and send alarms to operators via local telemetry and dialer systems. Photo 4 Centrifugal Blowers and Vibration/Surge Panel Each aerobic reactor has two DO sensors and analyzers that report DO and temperature information to the control system PLC. The output of the blowers and the modulation of airflow control valves at the reactors are controlled automatically by this system. The output of process air to the reactors is adjusted automatically as the system loads vary with time. The DO control setpoints in the system are approximately 2.0 mg/l.

8 Photo 5 DO and ph Sensors and Analyzers

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10 IFAS SYSTEM PERFORMANCE AND DISCUSSION The LINPOR - CN IFAS system was initially commissioned in July 2003 and began to demonstrate nitrification, total nitrogen removal and steady process operation within 4 weeks. Table 2 presents performance data for the initial period of operation showing compliance with the requirements of the current discharge permit limits soon after startup. This data was collected during the system performance test conducted in September and October 2003 as required by the construction contract. Influent flows during this performance test period averaged approximately 2.5 mgd. Table 2 - LINPOR Performance Test Data Influent 9/22/03 9/23/03 9/24/03 9/25/03 BOD 5 (mg/l) Ammonia (mg/l) TKN (mg/l) Effluent BOD 5 (mg/l) 19 7 <3 <3 Ammonia (mg/l) Nitrite (mg/l) <0.01 <0.01 <0.01 * Nitrate (mg/l) * TKN (mg/l) * Total N (mg/l) * Influent 9/26/03 9/27/03 9/28/03 10/1/03 BOD 5 (mg/l) Ammonia (mg/l) TKN (mg/l) Effluent BOD 5 (mg/l) 30 < Ammonia (mg/l) Nitrite (mg/l) <0.01 <0.01 <0.01 <0.01 Nitrate (mg/l) TKN (mg/l) Total N (mg/l)

11 Influent 10/2/03 10/3/03 10/8/03 10/9/03 BOD 5 (mg/l) Ammonia (mg/l) TKN (mg/l) Effluent BOD 5 (mg/l) 6 <3 <3 3 Ammonia (mg/l) Nitrite (mg/l) <0.01 * <0.01 <0.01 Nitrate (mg/l) 2.8 * TKN (mg/l) 3.2 * Total N (mg/l) 6 * Influent 10/15/03 10/16/03 10/22/03 10/23/03 BOD 5 (mg/l) Ammonia (mg/l) * * TKN (mg/l) * * Effluent BOD 5 (mg/l) 4 < Ammonia (mg/l) Nitrite (mg/l) <0.01 <0.01 <0.01 <0.01 Nitrate (mg/l) TKN (mg/l) Total N (mg/l) * - Indicates Parameter not Analyzed Additional system operating and performance data is presented in Table 3. This group of data shows consistent total nitrogen removal performance through the cold weather months. This full-scale data validates the cold-weather data collected during the system piloting in 2002.

12 Table 3 - IFAS System Performance and Operating Data Summary Oct. '03 Nov. '03 Dec. '03 Jan. '04 Feb. '04 Mar. '04 Apr. '04 Plant Influent Flow (Q) mgd BOD5 mg/l TSS mg/l Ammonia (1) mg/l TKN (1) mg/l Temperature (Avg.) C Temperature (Min.) C Plant Effluent BOD5 mg/l TSS mg/l Ammonia mg/l TKN mg/l TN mg/l Reactors RAS Flow mgd Nitrate Recycle (NR) Flow mgd Aerobic HRT (Q) hrs Aerobic HRT (Q+RAS+NR) hrs Anoxic HRT (Q) hrs Anoxic HRT (Q+RAS+NR) hrs MLSS free mg/l MLSS fixed mg/l SVI ml/g WAS lbs/day Effluent Suspended Solids lbs/day SRT - free biomass days SRT - free + fixed biomass days (1) Influent ammonia and TKN values from Nov. '03 through April '04 are estimates based on historical ratios to BOD5. Some process upsets were encountered during this time where excessive foaming was experienced. This foaming was attributed to fats, oils and grease introduced through septage addition at the facility headworks. Process modifications were subsequently made at the facility headworks to control the flow of septage. The foaming has subsided to this date as a result. The system has also shown excellent performance during wet weather events and the resulting high flows. Wash-out of the fixed biomass cannot occur in the system due to the media retention

13 screens and therefore the impact of the high flows to the process was shown to be negligible. This was shown in April 2004 when record wet weather caused the influent flowrate to the system to rise above the design peak hour flow of the facility. The system maintained compliance with the pollutant discharge limits even though the nitrate recycle pumps had shut down briefly due to high water elevations in the aerobic reactors. The system did not require reseeding or excessive return sludge pumping to restore the loss of biomass since the fixed biomass of approximately 10,000 mg/l was maintained in the reactors. It is noteworthy that the system provides nitrogen removal to below 5 mg/l with nitrate recycle rates significantly less than the literature values of 2 to 4 times the reactor influent rates (2Q to 4Q). This system operates at a fixed nitrate recycle rate of 2.12 mgd total (1.06 mgd per reactor). This equates to a nitrate recycle rate of approximately 1Q, or less than half of the rate and pumping energy requirements of some conventional nitrogen removal systems such as a typical Modified Ludzack-Ettinger (MLE) process. This reduction in the nitrate recycle rate is due to simultaneous nitrification and denitrification that occurs in the aerobic reactors. The simultaneous reactions are possible since the interior of the support media cubes can act as small anoxic environments where nitrate in the aerobic reactor is denitrified without being recycled to the anoxic zone by the nitrate recycle pumps. Photo 6 - LINPOR Aerobic Reactor Prior to IFAS Media Addition

14 Photo 7 - Sponge Media Addition to Aerobic Zone and Wetting with RAS Photo 8 - Media Recycle Discharge During Media Wetting at Startup

15 WEFTEC 2004 Photo 9 IFAS Media on Surface Prior to Assimilation with Mixed Liquor Photo 10 - LINPOR Aerobic Reactor Following IFAS Media Addition Copyright 2004 Water Environment Federation. All Rights Reserved.

16 Photo 11 - LINPOR Aerobic Reactor with Completely Assimilated Media CONCLUSIONS This project has shown that existing conventional activated sludge facilities can be retrofitted for total nitrogen removal without the costly construction of additional reactor tankage. Although additional influent hydraulic capacity was not required as part of this upgrade design, the Westerly, Rhode Island facility was converted from carbonaceous removal only to nitrification/denitrification within the existing tankage by the incorporation of the LINPOR - CN IFAS process. The LINPOR - CN IFAS system has shown that effluent ammonia and total nitrogen concentrations below the discharge limits can be consistently met even in wet and winter weather. The system has proven to require no more attention than the conventional activated sludge process previously operated at the facility, and has been relatively simple to troubleshoot and adjust. The operations staff also report that since the system was placed in service, a significant improvement in the sludge volume index (SVI) values has occurred, thereby allowing more consistent solids removal and processing at the facility.

17 ACKNOWLEDGMENTS The authors extend their deep appreciation to the following groups for their contributions to this project and to this paper: The Town of Westerly, Rhode Island Engineering Department Facility Owner Aquarion Operating Services Facility Operator BETA Group, Inc. Consulting Engineer Hart Engineering Corporation Constructor and Construction Manager Mixing & Mass Transfer Technologies (m 2 t) LINPOR System Engineer and Supplier REFERENCES Water Environment Federation (1992) Design of Municipal Wastewater Treatment Plants, Manual of Practice No. 8; Alexandria, Virginia. U.S. Environmental Protection Agency (1993) Nitrogen Control; EPA/625/R-93/010; Washington, D.C. Sedlak, Richard I. Phosphorus and Nitrogen Removal from Municipal Wastewater. New York: Lewis Publishers, 1991.