Flat Plate Membrane Bioreactor Provides Higher Effluent Quality

Ashwini Khare & Matthew M. Seng

In 1997, the U.S. Environmental Protection Agency (EPA) issued a five-year Filtration Avoidance Determination (FAD). The determination required New York City to acquire environmentally sensitive land in the New York City watershed, adopt strong watershed rules and regulations and institute and maintain a comprehensive watershed protection program.

EPA issued another five-year FAD in November 2002 that included significant enhancements to the overall watershed protection program. In 2007, EPA proposed to continue to allow the city to not filter drinking water from the Catskill/Delaware system and filter the Surface water instead under the Surface Water Treatment Rule (SWTR). The 2002 FAD established a new successor program called the Community Wastewater Management Program, which is administered by the Catskill Watershed Corporation (CWC). The program funds the planning, design and construction of community septic systems, including related sewerage collection systems, and/or the creation of septic maintenance districts, including septic system replacement, rehabilitation and upgrades as well as operation and maintenance of the district.

If community septic systems or septic maintenance districts are not practicable due to site conditions, and there is a demonstrable water quality problem due to failing septic systems, the New York Department of Environmental Protection (DEP), in consultation with CWC and the participating community may elect to allocate program funds for the project to construct a new wastewater treatment plant, including related sewerage collection systems. Upgrading wastewater utilities had become an important part of Watershed management.

Waccabuc Country Club (WCC) qualified as one such state funded project. The facility was in need of an upgrade or replacement of a failing septic system. WCC is a century-old private golf, tennis and aquatic club, boasting a spectacular view of Lake Waccabuc. The location is within the New York Watershed area.

In interest of protecting the watershed, the new facility had to produce high quality effluent. Preserving the aesthetics of this historic club located in an upscale neighborhood was important as well. A membrane bioreactor (MBR) system was the right fit and is completely contained in a small building blending into the surroundings.

Design Criteria

The MBR was designed to be capable of treating flow variations from a peak day flow of 15,000 gal per day (gpd), to an average daily flow of 8,000 gpd and a low flow of 1,500 gpd. There was also a redundancy requirement that the plant be designed to meet the peak daily flow with one membrane unit online while the other unit is out of service.

Influent characteristics were:

• • 90 mg/L TSS at average design flow; 
• • 200 mg/L BOD5 at average design flow; 
• • 60 mg/L ammonia at average design flow; 
• • 2 to 9 mg/L phosphorus at average design flow; and
• • 6 to 9 pH.

Effluent requirements were:

• • 5 mg/L BOD5, daily maximum; 
• • 10 mg/L TSS, daily maximum; 
• • 1.5 mg/L (as NH3) ammonia, daily maximum from June 1 to October 31;
• • 2.5 mg/L (as NH3) from November 1 to May 31;
• • 1.0 mg/L phosphorus, 30-day average; and 
• • 6.5 to 8.5 pH.  

The MBR system was prefabricated and composed of an anoxic basin and MBR basin with two flat plate submerged membrane units capable of providing treatment of influent wastewater to the specified levels complete with a factory assembled, piped, wired and tested equipment skid containing all MBR system equipment, instrumentation and controls. The system was designed to control through a programmable logic controller (PLC) which continuously monitors and controls all components of the MBR system as well as the downstream microfiltration units and ultraviolet (UV) disinfection system to provide a complete integrated control system.

Initial Design Direction

The DEP previously required tertiary filtration for every wastewater treatment facility to comply with the watershed rules and regulations. Microfiltration units were required to have membranes with at least 0.2 µ of pore size and maximum flux rate of 23.3 gpd per sq ft.

Initially the design incorporated the effluent from the septic tank was pumped to the MBR anoxic after passing through a fine screen. Within the anoxic basin, incoming wastewater mixes with nitrified wastewater and mixed liquor recycled from the MBR basins. While operating at low dissolved oxygen (less than 1 ppm), denitrification occurs. From the anoxic basin, the wastewater pumps to the MBR basin, where two submerged membrane units are installed to treat the flow.

Within the MBR basin, the wastewater is mixed with active biomass and oxygen in a traditional activated sludge process. Diffusers on the membrane units and in the basins will supply air and mixing, as well as continuous scouring of the membrane surfaces. Treated wastewater will pass through the submerged membranes and will be pumped by the permeate pumps to the sand filters and then by microfiltration feed pumps to the Microfiltration units for tertiary treatment. Three UV units will be provided in series downstream of the microfiltration units for disinfection. The filtrate will then flow to a cascade aeration system leading to the discharge point.

Alum dosing system was provided for phosphorus removal. Alkalinity dosing system was provided to maintain a stable pH. Waste activated sludge will be periodically wasted by pumping mixed liquor from the MBR.

MBR Performance & Savings

Concurrently with the construction of the facility, the DEP was in process of evaluating MBRs for being capable of meeting the tertiary treatment requirement.

They determined that, even though the mean pore diameter of the submerged membrane units (SMUs) is 0.4 µ, the effective pore size was found to be less than 0.2 µ. The core principal of this is the biofilm, a beneficial molecular matrix that forms on all submerged membrane systems. Biofilm creates a secondary membrane that can allow for enhanced nutrient removal, degradation of refractory organics, and most importantly, prevent reversible and irreversible fouling. Additionally, the SMUs met their flux cap requirements. This satisfied their tertiary filtration requirements and they decided to approve using MBR systems for secondary as well as tertiary treatment.

The findings led to a decision by the DEP to completely eliminate tertiary treatment for the Waccabuc wastewater treatment plant. This decision caused substantial savings on capital and operational cost. The two microfiltration units came with their automated backwash system, automated clean in place system, backwash surge tanks, PLCs, compressed air system interconnecting piping and appurtenances.

Construction & Startup

The MBR system was shipped to the site, preassembled, wired and tested with an equipment skid and steel tank that required minimal installation and assembly time at the site.

Because the system was robust, flexible and pre-engineered, the start-up process was quick. Start-up consisted of pipe integrity testing and clean water testing. The plant was seeded with activated sludge from a nearby facility and operator training was conducted. The DEP witnessed the operation of the plant for 96 hours before granting acceptance. The plant is meeting the discharge permit limits.

Waccabuc Country Club wastewater treatment plant was the first MBR plant in the New York Watershed to comply with the tertiary filtration requirement without additional microfiltration. It sets a precedent for other wastewater treatment plant designers to follow. The DEP is now also looking at retrofitting MBRs to replace existing failing or aging secondary treatment system and getting rid of its existing tertiary treatment system in order to increase capacity, achieve higher effluent quality and reducing operating costs.

Ashwini Khare is project manager and Matthew M. Seng, P.E. is regional manager for Ovivo. Khare can be reached at [email protected] or 512.834.6036. Seng can be reached at [email protected] or 512.963.3421.