membrane bioreactor performance compared to conventional wastewater treatment

Transcription

1 Water Technologies & Solutions technical paper membrane bioreactor performance compared to conventional wastewater treatment Authors: Thomas C. Schwartz and Brent R. Herring, Woodard and Curran Incorporated Ricardo Bernal and Janet Persechino, SUEZ introduction Governor Dummer Academy (GDA) is the oldest private day and boarding school in the United States. Founded in 1763, it is located on a 350-acre campus in Byfield, Massachusetts. The wastewater treatment plant (WWTP) that served the Governor Dummer Academy campus was originally constructed in the 1960s, with upgrade work performed in the 1990s. Despite the upgrade efforts, the WWTP was unable to consistently meet the effluent quality required by the permit under which it operates. This school was faced with the major dilemma of having to increase the throughput capacity of their current conventional wastewater treatment system in order to consistently meet the required discharge quality. At the same time, the school was faced with building constraints due to the wetlands that surround the WWTP, and, therefore, could not increase the physical size of the plant. After a thorough review of the options available to the school including prefabricated package treatment units, sequencing batch reactors and moving bed bioreactors, it was decided that the submerged membrane bioreactor (MBR) system was the best alternative for this application. In August of 2000, the installation of a 380 m 3 /day (100,000 gpd) MBR system used to treat domestic wastewater was completed. The new MBR-based facility was renamed the David A. Gaouette Wastewater Treatment Facility. Steady state operations were achieved during the month of September 2000, and the facility has been successfully operating since. The MBR approach to upgrading this facility allowed the academy to actually slightly decrease the WWTP footprint by maintaining the use of the existing treatment tank and removing the former filtration system building and tankage. The MBR system has successfully increased the WWTP treatment capacity, enabling the academy to meet tough permit discharge limitations for BOD, TSS, (<10 mg/l) and Total and Fecal Coliform (<10 FCU/100 ml sample). The David A. Gaouette Wastewater Treatment Facility is now serving not only as the GDA treatment plant, but also as a showcase facility and a research site for MBR process optimization. Influent and effluent parameters are being continually examined to refine the optimum operating conditions, and permeate water samples are monitored every week to ensure that all environmental discharge limits are met. The objective of this paper is to report the first year of operation of the plant and to compare the MBR performance with the conventional plant that was previously operated at the school. the conventional process A typical treatment train for municipal and domestic wastewater treatment is generally broken into primary, secondary, and tertiary treatment levels. 1. Primary treatment is the removal of floating and settable solids through processes including screening and sedimentation. 2. Secondary treatment is typically the aerobic biological treatment process by which bacteria oxidize the organic matter in the wastewater, producing cell mass (sludge), and carbon dioxide. In suspended growth systems, the bacteria are maintained in an aeration basin and referred to as mixed liquor. Blowers supply air to the mixed liquor to supply the necessary oxygen. The Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. Jun-01

2 bacteria are usually separated from the purified wastewater in a clarifier. The purified water is discharged to the next step and the sludge is returned to the aeration basin for reuse and a small portion is removed for disposal (waste). 3. Tertiary, or advanced treatment, includes processes beyond secondary treatment, most often to remove specific constituents or improve the quality of the final effluent. It is most often a form of filtration. the MBR process There are two popular types of MBR processes. A submerged system consists of a microfiltration (MF) or ultrafiltration (UF) membrane with pore sizes ranging from microns. These membranes are submerged in the reaction tanks, with the permeate being drawn into the membranes using a vacuum capable pump. Tubular systems are also available. These systems will treat a side stream of the mixture in the aeration tank. This type of system requires a high amount of pumping power to keep the velocities high to prevent membrane fouling, and high pressure to force the water through the membrane. In addition, tubular systems have a larger footprint than submerged systems due to the external location of the membranes. For these reasons, the decision was made to install a submerged MBR system at Governor Dummer using a 0.4-microns pore size polyethylene hollow fiber membrane. The MBR replaces the secondary clarification. The MBR separates treated effluent from the mixed liquor solids utilizing a hollow fiber microfiltration membrane with a 0.4-microns pore size. The submerged membranes are typically placed directly into the existing aeration tank. The membranes allow the purified water to pass through the pores (permeate), while creating a complete barrier to the passage of any solid greater than 0.4-microns, which includes almost all bacteria (mixed liquor solids). The permeate is drawn through the membranes using a suction lift pump leaving the suspended biomass material in the aeration tank. Biomass (mixed liquor) is removed using a sludge pump on an as-required basis. Figure 1 gives a basic flow sheet of a typical MBR chamber. Figure 1: Typical MBR Process In a conventional wastewater treatment plant, the secondary clarifier limits the solids concentration in the aeration tank. Typical mixed liquor suspended solids (MLSS) concentrations are 1,500 mg/l to 5,000 mg/l. The larger the clarifier relative to hydraulic and solids loading of the facility, the higher the possible concentration in the aeration tank. Membranes create a solids barrier, and, therefore, are not subject to gravity settling solids limitations, as in conventional clarifiers. MBRs are limited instead by the fluid dynamics of high solids mixed liquor, the effects on the ability to get permeate through the membrane, premature fouling of the membranes, and the effect on oxygen transfer. Typical MLSS concentrations in MBR systems are 10,000 mg/l to 15,000 mg/l and have been reported to be as high as 20,000 mg/l in certain instances. Hydraulic retention times (HRT), the amount of time the wastewater spends in the system, for MBRs are typically 4-20 hours. On most domestic wastes, this is enough time to allow for the oxidation of organic material and ammonia (nitrification). The average sludge age, the time the biomass spends in the aeration tank, or sludge retention times (SRT) is days. The older sludge ages and the higher MLSS concentrations in the MBR process compared to conventional systems enable the MBR to produce significantly less sludge for disposal than conventional treatment systems. As mentioned previously, the limitations on MLSS concentration and HRT are based on the solids content of the mixed liquor and the effects on permeate flow, fouling, and oxygen transfer. In high concentrations of MLSS, the permeate flow can be limited by the physical presence of solids at or near the individual pores which restricts flow. In addition, higher MLSS concentrations have a higher propensity to foul the membranes and restrict flow of water and Page 2

3 air at the surface of the membrane rendering the air scouring ineffective at cleaning the membranes. When the solids concentration becomes too high, the flow of permeate will become restricted and require a greater vacuum to initiate flow. Because of this, the pressure differential across the membranes, or the trans-membrane pressure (TMP), is continuously monitored and used to gage the degree of fouling in a system that is operating within acceptable solids concentrations. Under normal operation, the rate of fouling of the MBR membranes is reduced by injecting the blower air directly under the membranes. The continual agitation caused by the flow of air and water over the membrane surface serves as a surface scour. In addition, the permeate pump runs for eight minutes and then is turned off for two minutes. This allows the membranes to relax, and with no vacuum on the membranes, the air scour has a greater impact. Despite this continuous cleaning, a gradual accumulation of organic substances such as glycoprotein will occur at the membrane surface. For this reason, in-situ chemical cleaning of the membranes is recommended. Cleanings consist of in situ reverse flow of a dilute (3,000 ppm, 0.3% solution) sodium hypochlorite solution through the membranes for two hours. The cleanings should occur roughly every three months to maintain operation, or any time the TMP across the membrane increases more than 30 kpa (4.4 psi) above the start-up pressure. In addition, out-of-tank cleanings will be occasionally performed by removing the membranes from the aeration tank and soaking the membranes in a dilute (5,000 ppm, 0.5% solution) sodium hypochlorite and 4% (40,000 ppm) sodium hydroxide solution for hours. This should be done when the in-situ chemical cleanings do not show significant reduction in the TMP. MBRs were developed in the late 1980s and are now being used on wastes ranging from livestock waste with influent BODs of 18,000 ppm, industrial wastes with BODs from 5,000-10,000 ppm, and domestic wastes with BOD ranges of ppm. This technology has served customers who need integration of water and wastewater management for total water management systems. These systems must be consistent in producing water with low BOD and TSS, need to minimize space usage, and need to minimize sludge disposal amounts. description of the conventional Governor Dummer Academy wastewater treatment plant The treatment plant as it was prior to the MBR upgrade was a small conventional suspended growth type treatment plant built in the early 1960s and is illustrated in Figure 2. As depicted in Figure 3, the plant consisted of the following operations and tankage: Grinder (bypassed) Influent flow measurement manhole flume insert Flow equalization tank Common-wall secondary tankage Aeration basin Secondary clarifier Chlorine contact tank (chlorine no longer used) Filtration building Filter wet well with two submersible lift pumps Two UV disinfection modules Effluent flow measurement manhole flume insert Sand drying beds Figure 2: Original Conventional Plant Figure 3: Governor Dummer Academy Conventional WWTP PFD Page 3

4 The plant was drastically undersized and provided poor performance and poor effluent quality. Frequent pump-outs of the clarifier and the aeration tank by tanker truck were necessary to keep the system functioning at even a minimal level. As part of the initial investigation into upgrade alternatives for this facility, a capacity evaluation was performed on the existing treatment units. It was determined the aeration tank was capable of treating roughly 11,000 gallons per day (gpd) (42 m 3 /day), while meeting the ammonia limit of 1.0 mg/l. The clarifier was estimated to be capable of roughly 30,000 gpd (114 m 3 /day) if improvements were made. The flow data from the preceding years demonstrated an overall yearly average flow of roughly 30,000 gpd (114 m 3 /day) with several months between 40,000 gpd (151 m 3 /day) and 60,000 gpd (227 m 3 /day), while school was in session. In addition, several continuous days had influent flows between 80,000 and 100,000 gallons per day (303 to 495 m 3 /day) with peaks during those days in excess of 140,000 gpd (530 m 3 /day). The existing system was clearly not up to the task at hand. The layout of this tankage and building is depicted in Figure 4. The site was built-out to the fullest extent possible, due to the presence of surrounding wetlands on three sides and a steep hill on the influent side of the site. The lack of space at the site made sighting conventional treatment system extremely difficult and expensive. It also would have been challenging to maintain the existing treatment system or any other temporary system during construction. Figure 4: Layout of Pre-Existing GDA WWTP It was determined that the best alternative for this facility was to retrofit it with a submerged MBR system. This allowed for the continued usage of all tankage in a reconfigured flow pattern. The reuse of tankage greatly simplified construction and the downtime necessary. description of the upgraded MBR system The MBR approach required no major new infrastructure or tankage. With the MBR, the clarifier was no longer needed and the new rating for the aeration tank was estimated to be approximately 100,000 gpd (379 m 3 /day). Further, the system was promised to produce an effluent superior to that from a tertiary sand filter. This allowed for the discontinuation of tertiary filtration at this facility and the removal of the associated building. Figure 5 illustrates the modifications to the aeration basin for the upgrade. Figure 5: Upgraded Aeration Basin with MBR Retrofit The MBR units formed the core modifications needed for the upgrade and established the envelope of work for the overall upgrade, which included the following activities: New headworks including efficient grit removal, solids grinding and screening, and proper influent flow metering Piping and general process modifications for the new configuration Page 4

5 An additional UV disinfection module and a positive flow splitting control structure to evenly divide flow to all three units operating in parallel New operations building to provide an office, laboratory, bathroom, storage room, and equipment room for the blowers, pumps, flow meter, flow splitter UV units, and other ancillary equipment New discharge flow metering Filter building removal Electrical new equipment, instrumentation, controls, relocation of electrical feed, and needed upgrades of existing services Improvements to site drainage, potable water supply, landscaping, and pavement A process flow diagram of the MBR system at Governor Dummer is shown in Figure 6. The incoming raw water first enters the system at the new headworks, which consists of a grinder with a 3.0- millimeter auger screen. The grinder breaks up any large solids. Any of the ground solids that do not pass through the fine screen are removed by the associated auger. Fine screening is important for the submerged MBR to any small fibers that could wrap around the membrane fibers and also removes any particulates that could adhere to the membranes. The chopper will automatically reverse direction if it gets jammed, and the screen is cleaned with a screw auger which deposits the solids in a catch bag. clarifier basin, now used in a reverse flow configuration as the aerated grit chamber. Heavy solids settle out of the wastewater while aeration from the blower system provides adequate turbulence to keep the more buoyant-neutral organic solids in suspension. The grit tank serves as a settling tank for the removal of grit and as an equalization tank. The tank is aerated to prevent the settling of suspended organic materials. The water flows into the aeration tank/mbr chamber via a weir between the grit tank and the aeration basin. Two racks of membrane modules were placed into the existing 97 m 3 (25,650 gallon) tank. The modules are on a dual header arrangement. Each membrane is isolated one at a time for in-situ chemical cleaning by the use of ball valves. A walkway across the top of the aeration tank was constructed for easy access to all connections on the MBR modules and piping. Figure 7 shows a picture of the walkway and connections on the MBR system. Figure 7: Connections and Walkway Figure 6: Governor Dummer Academy WWTP MBR Upgrade PFD Also in the new headworks, a Parshall flume measures the influent flowrate. From the flume, the wastewater is directed into the former secondary Aeration for the MBR is provided by two blowers, with one used for stand-by. Each membrane module has an individual connection to the aeration header located conveniently along the walkway. The air provides the necessary oxygen for the biological digestion process, and also scours the membranes to keep them from fouling. The air enters the basin from the bottom portion of the membrane cassette. There is a dissolved oxygen (DO) meter in the aeration basin to warn of low DO levels, and pressure gauges on the blower discharge lines help to indicate if the diffusers are becoming clogged. Separate lines divert a portion of the system air to the grit tank and sludge holding tank. Page 5

6 Each module also has its own connection to the permeate line and water recycle line. There are two permeate pumps included with the system, one is used for stand-by purposes only. Each pump is equipped with a variable frequency drive (VFD) to adjust the flow based on the liquid level in the MBR tank. The flow setpoint is automatically adjusted to match the number of MBR modules in service. Each permeate pump has local inlet and discharge pressure gauges. Check valves are installed on the permeate lines. When the permeate pump shuts off, the check valves close and prevent the permeate pipes from draining, which aides in maintaining vacuum pump prime. The flow of permeate is measured using a magnetic flow meter and transmitter placed on the final discharge piping. Figure 8 gives the layout of the entire WWTP at the site. interaction, other than daily checks, is administered through the operator interface on the SCADA system. data comparison between the conventional and MBR system Table 1 summarizes the plant operational data from ten continuous months of operations prior to the MBR upgrade and the eight months of continuous operations (up to June of this year) after the MBR upgrade and startup. Table 1: Plant Performance Conventional vs. MBR Upgrade A sludge pump was installed at the center of the aeration tank to remove sludge when the MLSS of the tank accumulates over the recommended 10,000 ppm for optimal operation. The sludge is pumped to a holding tank. Due to the high sludge age, the excess sludge produced by the MBR does not settle readily. In order to thicken the sludge, a polymer is added to the sludge holding tank which causes the sludge to rise. A submersible pump placed on the bottom of the sludge tank then pumps the water that accumulates at the bottom of the sludge tank back to the aeration basin. Once a month the sludge is trucked off site. The pumps, blowers, controls, and power panels have been installed inside an operations building. This building also contains the UV disinfection, which is the last step in the process before the water is discharged. The new electrical system includes a motor control center (MCC) that provides power to the control system (contained inside the control cabinet), pumps, blowers, and the wiring to convey power to pumps, blowers, instruments, the control system, and two VFDs. The control cabinet is located inside the electrical room in the operations building, and it contains a programmable logic controller (PLC), relays to buffer high voltage output devices, and an operator interface panel, as well as switches and indicators necessary for system operation. The system is monitored by a SCADA software package where data is logged twenty-four hours a day. It is set up to run on automatic control and requires very low labor hours and maintenance. Normal operator As illustrated in Table 1, the plant performance since the upgrade is greatly improved, and is consistently well within discharge permit limitations. The conventional plant operations produced roughly 12.3 dry kilograms (27 lbs.) per day of sludge and discharged 3.6 (8 lbs.) dry kilograms per day of solids in the effluent for a total sludge production of 15.9 (35 lbs.) dry kilograms per day. This is in contrast to the MBR system which produces roughly 11.8 (26 lbs.) dry Page 6

7 kilograms per day of sludge and discharges a mere 0.27 (0.6 lbs.) dry kilograms per day of solids in the effluent for a total sludge production of 12.2 (27 lbs.) dry kilograms per day or a 22% reduction in solids production. conclusion When the oldest boarding school in the United States was faced with the problem of having to increase the throughput capacity of their wastewater treatment system without increasing the physical size of the system, and still be able to meet tough discharge regulations, they opted to install an MBR system. The MBR technology replaces the sedimentation process that was used in their conventional wastewater treatment plant. The facility includes a chopper, 3.0 mm screen, grit tank, aeration tank where the membranes are located, blowers, permeate pumps, ultraviolet disinfection lamps, data logging system and instrumentation, PLC controller, sludge pump and holding tank, and a tank to mix the clean-in-place (CIP) solution. The product water is consistently below the discharge limits containing less than 2 mg/l of suspended solids, BOD and COD, also fecal coliform units less than 10 colonies/100 ml and ammonia less than 1 mg/l. The MBR plant has produced 22% less sludge than the conventional treatment plant. After being sent through ultraviolet disinfection, the academy can safely and confidently discharge the water to a local river. Since plant start up in August 2000, one hypochlorite CIP has been performed, and the optimum mixed liquor suspended solids concentration, flux rate, air diffuser cleanings, dissolved oxygen levels, and other minor operating parameters have been established. Data is logged 24 hours a day to a SCADA system. Influent and effluent parameters are being examined and permeate water samples are monitored every week. Page 7