MBR System Helps Meet Low Nutrient Discharge Limits

March 1, 2008
Rising real estate prices in Maryland and Virginia have forced home buyers to seek more affordable housing outside the Washington, D.

By Jason Allen

Rising real estate prices in Maryland and Virginia have forced home buyers to seek more affordable housing outside the Washington, D.C., metropolitan area. As a result, two nearby counties in West Virginia, Jefferson and Berkley, are experiencing unprecedented growth. Light rail, limited access highways, and motivated housing developers have attracted D.C. commuters, and the population explosion in West Virginia’s Eastern Panhandle has brought a sudden increase in demand for wastewater treatment.

The Eastern Panhandle lies within the 64,000-square-mile Chesapeake Bay Watershed. Currently listed by the EPA as an impaired waterway, the Chesapeake Bay has suffered from excess nutrient loadings from new point source nutrient generators (wastewater discharge) in the watershed. Left unchecked, uncontrolled wastewater discharge will cause severe decay of the area ecosystem via an increase in chemical nutrients, a process known as cultural eutrophication. In order to protect the Chesapeake Bay from further decay, the Chesapeake Bay Program, formed in 1982, has implemented a strategic plan to stave off eutrophication.

The Chesapeake Bay Program researched and instituted regulatory statutes with the goal of controlling nutrient and sediment related problems affecting the Chesapeake Bay Watershed. As a result, wastewater treatment providers must meet strict discharge requirements within the watershed to drive initiatives and promote changes geared to restoring the quality of the watershed and bay.

Eastern Panhandle developers are responsible for including wastewater facility planning, design, and permitting into their projects as specified by the Chesapeake Bay Program’s regulations. This responsibility means understanding wastewater technologies as they relate to the regulations, and that means understanding membrane systems. Membrane systems currently offered for municipal wastewater treatment include tubular, hollow fiber, and flat plate units. These systems are considered Best Available Technologies (BAT) and are very well suited for the demanding performance requirements set forth by the latest Chesapeake Bay Standards.

One such BAT is the Enviroquip® Membrane Bioreactor (MBR) using the Kubota membrane. The Enviroquip® system can meet or exceed the new low nutrient discharge allocations promulgated by the regional permitting bodies in addition to providing regional developers a low cost alternative for wastewater treatment. Specifically, Total Nitrogen discharges, required by permit, are generally in the range of TN< 5 – 3 ppm dependent upon plant throughput.

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Enviroquip®/Kubota has taken a proactive role in the West Virginia community of Harpers Ferry in educating the local permitting agencies and developers in the use of their technology. As a result, several of the company’s MBR wastewater applications are in the design, permitting or construction phase in the area. Old Standard WWTP is one particular 125,000 gallon per day (GPD) Enviroquip plant under construction in Harpers Ferry.

The influent/effluent characteristic design data is as follows:

The design average daily flow (ADF) for the Old Standard WWTP is 125,000 GPD with provision to double the capacity in the future. Hydraulically and biologically the plant is designed for the full build-out average daily flow; however, the initial capacity will be limited to 50,000 GPD by the number of submerged membrane units (SMUs) installed. As the subdivision continues to build out, additional SMUs will be installed to accommodate the additional flow, allowing the developer to defer the additional required capital expenditure. Inherently, the Enviroquip® systems can sustain 2.0-2.5 times the ADF for extended periods.

The Kubota flat-plate membrane was specifically developed for wastewater treatment. The Enviroquip MBR flat-plate units are comprised of multiple poly-chlorinated polyethylene cartridges vertically situated in cassettes which can be vertically stacked dependent upon the treatment throughput required. The plates have a 0.4 micron pore size dry and a less then 0.1 micron pore size when in service through the utilization of an additional bio-film filter layer.

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The system employs one of two kinetic process design standards in conjunction with the membrane filter. Applications which require a less than 10 mg/l total nitrogen (TN) discharge use the Modified Ludzack-Ettinger (MLE) process design approach. Alternatively, for applications which require a nutrient removal rate of less than 3-5 mg/l total Nitrogen (TN), as demanded by the new Chesapeake Bay Standards, the five-stage Bardenpho® Process design approach is adopted.

Raw wastewater is generally pumped from a regional collection system into the plant headworks. MBR design and implementation requires fine screening placement upstream of any process basins in order to remove all solids which could damage the membranes. The fine screens required are generally 3 mm punched-hole perforated plate or 2 mm bar screen units. Furthermore, dependent upon the characteristic waste stream data, a grit removal and fat, oil, and grease removal system may also need to be constructed upstream of the process basins.

Screened raw wastewater from the treatment plant headworks is then directed into the lead anaerobic process basin. Within the anaerobic basin the raw wastewater is completely mixed with a recycle mixed liquor stream from the aeration tanks. In this anaerobic environment the activated sludge promotes the organic release of phosphorus and the uptake of volatile fatty acids. The recycle stream from the aeration tanks provides a carbon source for the biomass and conditions are developed for the continued formation of phosphorus accumulating organisms.

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In creating an environment conducive to “luxury” uptake of phosphorus, the anaerobic basins allow for a substantial reduction in plant operation costs by reducing or eliminating the need for alum or ferric chemical doses. Without the BIO-P stage the phosphorus concentration in the waste activated sludge (WAS) will be 1.5%. However, with the BIO-P stage WAS phosphorus concentrations can range to 4%.

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A companion alum or ferric dosing system is always provided for applications which require very low phosphorus discharges; <0.05 mg/l TP levels are attainable. Alum or ferric is generally added to the pre-anoxic basin. The coagulant reacts and forms a sludge precipitant that will then be wasted in a WAS discharge. Typical dosing rates for MBR systems are much less than conventional dosing requirements due to the fact that large floc formation for gravity settling is not a requirement in the MBR system since the membrane units can filter small flocs.

The pre-anoxic basins are fed by gravity from the anaerobic basins. Within the pre-anoxic basins denitrification occurs. Denitrification reactions involve heterotrophic bacteria and the conversion of nitrates to nitrogen gas through cellular respiration. In short, the five day Biochemical Oxygen Demand (BOD5) carbon substrate is consumed and produces a synthesized cell mass utilizing nitrate as an energy source. This BOD5 consumption reduces process aeration requirements. Denitrification also helps to recover alkalinity lost in the nitrification stages and promotes pH stabilization.

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The pre-anoxic mixed liquor is generally pumped into the pre-aeration basin where air is supplied to provide carbonaceous BOD5 removal and the nitrification process occurs. Nitrification involves the conversion of ammonia to nitrates and can result in alkalinity consumption and pH reduction. A nitrified mixed liquor recycle stream from the pre-aeration basin back to the anaerobic basin provides a low DO carbon source for the BIO-P process.

The Enviroquip® flat plate system in conjunction with a biomonitoring system is employed in BIO-P applications. The Enviroquip® process generally allows for very low DO operation within the Pre-Aeration basins (0.2-0.8 mg/l). In a low DO aeration basin simultaneous nitrification/denitrification (SNdN) occurs. The SNdN process enables the reduction of total nitrogen while still providing biomass oxygen.

Enviroquip® utilizes a nicotinamide adenine dinucleotide (NADH) probe to monitor the amount of the coenzyme in the biomass. The amount of NADH present can be monitored and utilized as a control loop for aeration supply to the biomass within the basin. The speed of the aeration blowers is based upon a P&ID control algorithm executed by the MBR PLC. Essentially, a BPA parameter indicates how the changing floc conditions are affecting metabolic rates. The BPA parameter gives indication of anaerobic, aerobic or anoxic sludge conditions.

Operationally, air will be cycled in the pre-aeration basins between 0.2-0.8 mg/l DO. The 0.8 mg/l range promotes nitrification and the 0.2 mg/l range promotes denitrification. Monitoring the BPA value allows the control system to terminate the denitrification cycle if the BPA parameter exceeds a set point which will in turn initiate an aeration/nitrification cycle. Blower speed will strictly be a function of DO set point but the phase of operation will be indicated by the BPA parameter.

Mixed liquor from the pre-aeration basins is then directed into the post-anoxic basins. For applications with very low total nitrogen discharge requirements a post anoxic basin is utilized in order to further denitrify the wastewater. In this basin, nitrates created by nitrification in the pre-aeration basin will be converted to nitrogen gas through denitrification. The mixed liquor from the post-anoxic basin will be delivered to the MBR basins.

Partially stabilized mixed liquor entering the MBR basins is aerated with supplemental scour air provided by the membrane diffusers. The scour air mechanically controls the biofilm thickness on the plate surface and maintains a circular roll pattern up through each membrane unit. The primary purpose of the MBR basin is to provide a high quality permeate discharge. Permeate production can be driven by hydraulic head (gravity) or induced by low pressure vacuum (pumped). In either case the discharge flow is regulated with the use of modulating control valves or inverter duty driven pumps.

The number of MBR applications is growing exponentially worldwide. Kubota has more global MBR wastewater installations than any other company at over 2,100, including over 100 U.S. installations. The first Enviroquip MBR constructed in the Chesapeake Basin was built in the Eastern Panhandle of West Virginia. Due to the rapid growth in recent years and a shortfall of services for centralized wastewater collection, transportation and treatment facilities, real estate developers have been forced to construct small community wastewater treatment plants at their own expense. Since these counties are located in the Chesapeake Bay watershed all new WWTP’s are subject to the State of West Virginia nutrient removal standards for discharge to tributaries of the Bay. Many developers are selecting MBRs as the preferred technology for wastewater treatment.

Developers have shown a preference for this technology due to its ability to meet the low nutrient criteria as required by their discharge permits and the small facility footprint. The high quality effluent from these plants, with the extremely low total suspended solids (TSS) and BOD5, qualifies the effluent water as suitable for reclamation and reuse.

Another MBR characteristic that appeals to developers relates to the build-out schedule. Subdivisions are constructed over an extended period of time, sometimes five to ten years due to the variability of the housing market and various financial factors. As a result, the plants may not be required to treat the full design flow for many years. The Enviroquip MBR is capable of treating the initial low flow rates with limited operational impacts. The developer can purchase modular components as the subdivision is built out and the throughput requirement increases.

About the Author:

Jason Allen, BSCE is a Regional MBR Applications Manager for Enviroquip and has been working for the MBR Group for five years. His MBR Applications duties have entailed: business development, project management and product support. These core disciplines require knowledge of kinetic, hydraulic, controls and mechanical & civil design for field implementation. Allen may be contacted at [email protected].

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