Wastewater treatment plant (WWTP) designers and decision-makers tasked with finding more cost-effective performance for challenging applications want new options. Here is how flat-sheet membrane aerated biofilm reactor (MABR) technology tweaks the chemical and biological functions of conventional activated sludge (CAS) processes to reduce energy consumption and operating expenses (OPEX) in demanding applications.
The Difference Is In The Design
Wastewater operations challenged by remote treatment locations, growing populations, or tightening effluent requirements are already reaping the potential of flat-sheet MABR in selfcontained treatment plants ranging from 4,000 GPD to more than 300,000 GPD. Based on that proof of concept, the opportunity for retrofitting the same core technology into existing CAS basins for even larger operations is now on the horizon.
Here are the key design principles behind flat-sheet MABR performance that differentiate it from
traditional CAS installations, in terms of process performance and operating costs:
Low-Cost Passive Aeration. Unlike open-basin CAS aeration systems that continuously pump a high volume of air through an open tank, the low-flow, low-pressure flat-sheet MABR approach prevents the wasteful loss of a large volume of air escaping from the surface of the mixed liquor being treated.
Simultaneous Nitrification-Denitrification. Oxygen permeating through the flatsheet
membrane supports a biofilm that produces nitrates by consuming ammonia in the mixed liquor (Figure 1). Those nitrates produced by the biofilm in turn support the reduction of biochemical oxygen demand (BOD). Anoxic conditions in other areas of the mixed liquor promote denitrification.
Figure 1. The essence of the flat-sheet MABR design is the establishment of an autotrophic
biofilm on the membrane to consume ammonia and generate nitrate that in turn oxidizes the
Programmed Recirculation. Periodic agitation of the mixed liquor with minimal aeration — about a 5-percent duty cycle — from diffusers at the bottom of the MABR reactor (Figure 2) keeps solids in suspension to promote even nitrification/denitrification throughout the process.
Figure 2. Short circulation cycles require very little air to keep suspended solids in the mixed
liquor evenly distributed across the face of the membrane.
Low Chemical Requirements. With about 90 percent of phosphorus reduction achieved through the biological process, the addition of chemicals required to meet effluent regulatory standards is minimized.
How Flat-Sheet MABR Works
Many water industry practitioners have been exposed to the concept of membranes for filtering applications — e.g., ultrafiltration (UF) or reverse osmosis (RO) — designed to let water pass through while particles are filtered out based on the membrane’s nominal pore diameter. Flatsheet MABR flips the focus of the membrane from filtering water to diffusing air. Oxygen passing through the self-respiring membrane envelope to the mixed liquor side of the MABR surface (Figure 3) helps to grow and sustain the biofilm of autotrophic organisms capable of using chemical energy to synthesize their own food from inorganic substances (i.e., ammonia).
When that multi-layered membrane envelope is spiral-coiled around a central core (think of a large roll of paper towels standing on end), it provides an extremely large oxygenated surface area to support biofilm growth in a compact anoxic chamber. The combination of converting ammonia into nitrates and using them to help oxidize BOD leads to a significant decrease in nitrogen and phosphorus levels by the time the flow leaves the anoxic chamber.
Figure 3A (left). Air and water spacers sandwiched between the membrane surfaces allow water and oxygen to circulate freely throughout the spiral-wound unit with maximum surface area for chemical interactions in a minimal volume of space.
Figure 3B (right). The resulting envelope controls oxygen diffusion to support the growth of an ammonia-consuming biofilm that will eventually cover the entire membrane surface.
Reaping The Practical Benefits OF MABR
The concept of establishing a biofilm on a physical surface has already been proved in moving bed biological reactor (MBBR) applications for open-aeration basins. Flat-sheet MABR takes that concept to a new level by creating full biofilm coverage across a relatively large membrane surface area in a compact chamber. As a result, MABR technology has gained recognition to the point that it is included as an option in some of the simulation tools for WWTP operations. Documented performance in a series of demanding applications, worldwide, has demonstrated the advantages of flat-sheet MABR across a variety of operating environments:
· Capacity. Around the globe, self-contained flat-sheet MABR installations are providing favorable results in systems serving towns of up to 5,000 in population. Evolving designs for adding flat-sheet MABR coils to existing open concrete basins also offer promise for upgrading the efficiency of current CAS facilities serving larger populations.
· Nutrient Removal. Typical effluent requirements of 10.0 mg/L for nitrogen and 3.0 mg/L for phosphorus are routinely met. Enhanced MABR applications have achieved nutrient removal levels down to 3.0 mg/L for nitrogen and 0.3 mg/L for phosphorus.
· OPEX Savings. Flat-sheet MABR technology consumes just 12.8 percent of the aeration energy required by CAS systems. Total energy consumption for process fans, mixing blowers, and pumps has been estimated at just 0.267 kWh/m of wastewater — about 40 percent of the overall energy consumption of a typical small nitrifying CAS secondary treatment plant. The ultra-efficient process reduces costs on multiple levels:
o Reduces Energy For Aeration. Small aeration pumps require only minimal energy — 10 percent of conventional aeration — to generate a sufficient volume of air to diffuse through the membrane at slightly higher than atmospheric pressure.
o Reduces Energy For Circulation. With a circulation duty cycle of only 5 percent or less for keeping solids evenly suspended throughout the mixed liquor, MABR uses just a fraction of the total air volume required by traditional CAS processes.
o Reduces Carbon Source Costs. Because the BOD in the water provides a carbon source for denitrification, it lowers the cost of adding carbon to the process.
o Reduces Footprint Requirements. By combining both nitrification and denitrification steps in the same compact vessel, MABR significantly reduces space requirements for the volume of wastewater treated.
o Simplified Once-Through Operation. The simple flow-through design of the spirally wound MABR units provides good nitrogen removal in a single pass, without typically needing to reprocess the same wastewater in order to achieve effluent limits. The selfsufficient process makes MABR a reliable solution for small, isolated facilities that are not manned around the clock by highly skilled WWTP operators.
o Decentralized Convenience. Treating wastewater at its source minimizes the need for expensive infrastructure to collect and transport wastewater from many small, scattered locations to one large regional WWTP facility. Situating multiple packaged MABR systems that can run independently at local sites, then networking their monitoring and control systems to a central location, enables a single trained wastewater professional to manage multiple facilities. A part-time local maintenance person can handle daily inspections and resupply, while the experienced professional visits every two to three weeks, or as needed.