Environmental Protection

Immersed in Its Work

Installed within the bioreactor, a new membrane technology redefines sewage treatment and effluent reuse

Membrane bioreactor (MBR) has emerged as the wastewater treatment technology of choice for an increasing number of municipalities over the last five years. This growth is driven by the very high quality effluent produced by MBR -- exceeding strict standards or ideal for direct reuse -- and is fuelled by a steady reduction in treatment cost resulting from improvements in membrane productivity.

What is MBR?
An MBR is a suspended growth activated sludge treatment system that relies upon an ultrafiltration (UF) or microfiltration (MF) membrane for liquids/solids separation prior to discharge. The use of a membrane instead of a clarifier results in consistently high quality effluent, and permits very high mixed-liquor suspended solids concentration in the bioreactor. Mixed liquor means a mixture of activated sludge and water containing organic matter undergoing activated sludge treatment in an aeration tank. This leads to a smaller bioreactor volume and ability to handle large load fluctuations without a loss of effluent quality.

There are two types of membranes used in MBR:

  1. Pressurized, tubular membranes that are located outside the bioreactor and require mixed liquor circulation at high pressure with attendant energy cost;
  2. Immersed membranes that are designed for installation within the bioreactor.

Immersed membranes have become the technology of choice for large-scale municipal and industrial MBR applications because of the low energy cost and the ability to handle high solids mixed liquor.

ZeeWeed® is an outside-in, hollow-fiber immersed UF membrane developed by ZENON. The hollow-fiber membranes are reinforced to ensure a long and reliable operating life under high solids and aeration environment. These are made from a chlorine-resistant polymer, which permits periodic chemical cleaning. Suction is used to draw clean water to the inside of the membrane fiber through 0.04 nominal micron surface pores. The membrane is strong enough to withstand reverse flow (backwash) for physical and chemical cleaning.

The hollow fibers are mounted vertically between headers with some slack to allow movement, air penetration and water renewal within the bundle. Modules are assembled side by side into cassettes, leaving space for water circulation and air scouring. Cassettes have integrated headers for permeate collection and air distribution and are the building blocks of an immersed membrane system.

In summary, the ZeeWeed MBR process offers the following benefits:

  • High MLSS concentration resulting in smaller bioreactor volume and simple capacity and process upgrades without increasing tank volume.
  • Suspended solids and pathogen free effluent.
  • Superior total phosphorus removal.
  • Superior nitrification efficiency even under cold temperature conditions due to retention of nitrifying bacteria.
  • Easy adaptation to denitrification, which is the anaerobic biological reduction of nitrate nitrogen to nitrogen gas. .
  • Potential for sludge digestion in smaller plants by increasing MLSS.

Creemore Case Study
The Town of Creemore, Ontario and the surrounding area was using individual septic tank systems to handle wastewater, trucking away waste sludge for further treatment. The town's major industry, a brewery, also trucked away their wastewater for treatment and disposal. The community was concerned about cesspools and undersized leaching beds and holding tanks and wanted to protect the underground aquifer from contamination and add value to local properties. They decided it would be beneficial to build a wastewater treatment plant to handle the combined municipal sewage and industrial wastewater.

The local residents proposed a number of conditions for the new plant. They wanted it to be unobtrusive, affordable to construct and operate, and they did not want it to negatively impact the surrounding environment. Since the treated effluent was to be fed to a river running through the community, stringent effluent discharge parameters were targeted. After a thorough review of the treatment options available to the town, it was decided that an MBR system was the best available technology, producing effluent that could be discharged directly into the sensitive Mad River.

In order to blend the new plant with the rural scenery, the exterior of the building was designed to look like a diary barn.

The interior of the plant was developed in two phases. The first phase was designed to accommodate the present needs of the local community of 1,500; the second phase will allow for a higher future flow through the plant by adding additional membranes. A number of features were also included to allow expansion beyond the second phase, which will serve approximately 2,500 residents.

The average daily flow rate of the first phase accommodated 227,200 gallons per day (gpd) (860 cubic meters per day). The sewage treatment process consists of an influent pumping station complete with an automatic fine screening system, two-basin embrane process tanks, UV disinfection, effluent re-aeration chamber and outfall to the Mad River. Phosphorous removal is achieved by alum addition ahead of the aeration tanks. A single basin aerobic digester chamber and six-month sludge storage and hauling facilities are also provided.

Typical MBR Treated Water Results

Parameter

Effluent Quality

Biochemical oxygen demand (BOD) 5

<2 mg/L

Total suspended solids (TSS)

<2 mg/L

Ammonia

<1 mg/L

Total nitrogen (TN)

<3 mg/L*

Total phosphorous (TP)

< 0.1 mg/L**

Turbidity

<0.1 NTU

SDI

<3

*with anoxic (lacking oxygen) zone

** with coagulant addition

Looking to the Future
The MBR technology will continue to expand to larger and larger municipal wastewater treatment systems over the next 10 years. Key drivers will continue to be improvements to the technology and increasing demand for higher quality effluent and water recycling.

MBR for Wastewater Reuse
Water availability has become an insurmountable barrier for municipal development and growth in many arid regions of the world. Many of these areas are considering compensating for the lack of water by developing large seawater desalination projects, which are expensive to build and operate. Water reuse is the answer to reduce water costs by reusing municipal wastewater for industrial, irrigation, aquifer recharge or other non-potable uses. With conventional activated sludge process, the wastewater must be treated using multiple process steps before it can be reused. MBR systems produce a tertiary quality effluent suitable for direct reuse for industrial or agriculture applications, or for feeding a reverse osmosis unit where high degree of purification is required.

For additional information about water reuse, see "Water Reuse: Reclaiming a Finite Resource," by Paul Hersch, Environmental Protection, February 2002, under "Archives" at www.eponline.com.

Membrane Separation Processes
Reverse Osmosis (RO). RO essentially is a pressure-driven membrane-diffusion process for separating dissolved solutes. It eliminates dissolved solids, bacteria, viruses and other organisms from water and goes beyond filtration, which succeeds only at removing some suspended materials larger than one micron. Still, RO mostly finds use for producing potable water from salt-bearing (primarily brackish) water. It involves no phase change, and uses -- relative to distillation -- little energy.

The pore diameter 0.5 nanometer (nm) to 1.5 nm of RO membranes -- being the smallest of all membrane types -- allows only the most minute organic molecules and uncharged solutes, along with the water, to pass through. Ninety-five to 99 percent of inorganic salts and charged organics are rejected.

The bulk of RO membranes are polymeric -- cellulosic acetate (CA) and aromatic polyamides. Two types of RO membranes predominate: asymmetric, or skinned, and thin-film composite (TFC). The support material commonly constitutes polysulfones, with the film constituting polyamines, polyureas and other polymers.

Nanofiltration (NF). NF has been termed "loose RO" since its membrane pores are larger than RO's. It concentrates divalent salts, bacteria, particles and other constituents that have a molecular weight greater than 1000 and works in situations requiring high organic and moderate inorganic removals.

In separating different fluids or ions, NF typically affords higher recoveries and requires considerably lower operating pressure than does RO, but at the expense of allowing more salt passage.

Membranes for NF are of CA and aromatic polyamide offering salt rejections from 95 percent (for divalent salts) to 40 percent (for monovalent salts). They have an approximate molecular-weight cut-off (MWCO, the smallest molecular-weight species for which the membranes have more than 90 percent rejection) for organics of 300.

Ultrafiltration (UF). Successful use of UF (i.e., concentration of rejected solutes) depends primarily on particle size and to some extent, on particle charge. Typically, water-plant rejected species include bio-molecules, polymers and colloidal particles. The driving force for transport across the membrane is pressure differential -- characteristically two to 10 bars (atmospheres) but as high as 25 to 30 bars.

UF throughput depends on such physical properties as permeability and thickness of the membrane and such system variables as feed consumption, feed concentration, system pressure, velocity and temperature.

UF membranes entrain molecular weights in the range of 300 to 500,000, with pore sizes ranging from 0.001 to 0.1 micron. Their nominal molecular weight cutoff is 1,000 to100,000. When processing a suspension, the solids collect as a porous layer over the membrane's surface.

Polymeric materials -- including polysulfone, polypropylene, nylon 6, polytetrafluoroethylene (PTFE), polyvinylchloride (PVC) and acrylic copolymer -- have found successful use as UF membranes. Also, inorganic materials such as ceramics, carbon-based membranes and zirconia are commercially available. These have about the same chemical resistance of polysulfone but are more heat resistant.

Microfiltration (MF). By far the most widely used membrane process, MF essentially provides sterile filtration, restraining pass-through of microorganisms and material of colloidal size and larger. MF is being appraised as a replacement for diatomaceous earth, a rock known for it s absorptive capacity and chemical stability.

MF's pores are 0.1 to 10.0 microns -- two to five orders of magnitude larger than that for other membrane classes. (The smallest bacterium, pseudomonas diminuta has a size of 0.3 micron.)

MF membranes constitute natural or synthetic polymers, such as cellulose nitrate or acetate, polyvinylidene difluoride (PVDF), polyamides, polysulfone, polycarbonate, polypropylene and polytrifluoroethylene (PTFE). Membranes of inorganic materials, such as metal oxides (including alumina, glass and zirconia-coated carbon) also find use.

Material selection directly reflects end-application demands, such as mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and cost.

Dialysis. This membrane-transport process is driven primarily by concentration differences, rather than by pressure or electric-potential differences, across the thickness of a membrane.

Electrodialysis (ED). ED is useful for several general types of separations of salts, acids and bases from aqueous solutions. It also serves to separate and concentrate monovalent ions from multiple charged components or to separate ionic compounds from uncharged molecules.

An ED system constitutes both cation- and anion-selective membranes placed in an electric field. (The former passes only the cations; the latter only the anions.)

ED membranes usually consist of cross-linked sulfonated polystyrene. Anion membranes can contain quaternary ammonium groups. Usually, ED membranes are fabricated as flat sheets containing about 30 percent to 50 percent water. Membrane fabrication is done by applying the cation- and anion-selective polymer to a fabric material.

Electrodialysis reversal (EDR) makes use of ion-specific membranes arrayed between anodes and cathodes to drive salt ions in controlled migrations to the electrodes.

Although not as prevalent as RO, it continues to be commonly used by the water industry for making potable product from sea or brackish water. EDR is exploiting new areas of application, including pH control without adding acid or base and regeneration of ion-exchange resins.

References

  • William D. Ruckelshaus. McGraw-Hill Recycling Handbook, 2nd Ed. Herbert F. Lund, editor. New York. 1998
  • Chin, K.K. and Kamarasivam, K. Industrial Water Technology: Treatment, Reuse & Recycling. Elsevier Publications, Amsterdam, New York, London. May 1986.

This article originally appeared in the 09/01/2002 issue of Environmental Protection.

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