Over the past decade, membrane technologies have revolutionized water and wastewater treatment. Microfiltration (MF) and ultrafiltration (UF) membranes typically are used in water, wastewater, recycled water and industrial treatment applications to separate solids (suspended and colloidal, organic and inorganic) from fluids. MF and UF systems provide a consistently high-quality, low-turbidity water (typically less than 0.1 ntu) independent of the source water turbidity or solids variations. The use of MF and UF treatment permits wastewater treatment plants (WWTPs) to more easily meet stringent effluent requirements for discharge or recycling.
In wastewater and recycled water treatment applications, nanofiltration (NF) and reverse osmosis (RO) membranes generally are used downstream of filtration treatment to remove dissolved species such as salts and microconstituents (i.e., pharmaceutically active components, endocrine disrupters and personal care products). NF and RO treatment permits WWTPs to produce recycled water that is more suitable for irrigation and recharge, or for possible indirect potable reuse.
For years, the focus of membrane technologies was on large water treatment systems, especially publicly owned water systems. The use of membranes for wastewater treatment did not find its place until the early 1990s. By 2010, however, the use of membranes in wastewater treatment is expected to increase by 15% annually. Much of this growth has been due to increasingly stringent discharge regulations, especially relating to salts in effluent applied to land.
Membrane treatment of wastewater and recycled water offers the advantage of higher effluent water quality, a more compact footprint and often simpler operations as compared to a conventional WWTP. With the industry’s acceptance of this technology and the rapid growth in the number of operating facilities, the costs of membrane systems are now approaching those of conventional systems. Sooner or later, membranes are likely to be in your future.
Membrane Technology Overview
Only 10 years ago, membranes were considered specialty items and varied widely from manufacturer to manufacturer. One manufacturer’s membranes would not fit into another’s process scheme, let alone its treatment vessel. Now membranes have become commodities in water and waste- water treatment and, especially in the case of RO membranes, are becoming interchangeable.
There are four major membranes categories, based on pore size, which are commercially used at the present time. These are listed below, from largest pore size to smallest:
MF. Pore size screens particles from 0.1 to 0.5 microns.
UF. Pore size screens particles from 0.005 to 0.05 microns.
NF. Pore size screens particles from 0.0005 to 0.001 microns.
RO. Pore size screens molecular size down to 10 molecular weight cutoff.
All four membrane categories can be found in wastewater treatment and water reuse. As competition in the membrane business has grown over the last decade, more membrane providers have come into the market. As a result, membrane prices have fallen substantially.
In addition to companies that manufacture membranes, there are companies that design entire membrane treatment systems, including upstream anoxic and aerobic treatment systems, aeration systems for the membranes and auxiliary equipment such as pumps and tanks. These companies are known as membrane system integrators. Some membrane manufacturers specialize in supplying membranes to different membrane system integrators, but the majority of the membrane suppliers also act as membrane integrators. The decision to move from supplier to system integrator involves the evaluation of many criteria, for instance market location and market shares.
There are two main applications for membrane treatment in wastewater and water reuse:
Wastewater Treatment. Membrane bioreactors (MBRs) typically are used for this application. The membranes are submerged in an aerated tank or encapsulated in the low-pressure vessels downstream of an anoxic/aerobic biological treatment system. The mixed liquor suspended solids (MLSS) are separated from the supernatant using immersed (vacuum) or pressurized MF or UF membranes.
Water Reuse/Recycling. Because a much cleaner effluent is required for this application, MBRs or some other biological treatment/filtration is followed by tertiary treatment such as MF or UF, followed by NF or RO. This process generally provides an effluent with biological oxygen demand and total suspended solids less than 1 mg/L, as well as total dissolved solids (TDS) approaching very low numbers. Because the discharge TDS requirements may not require very low TDS, the flow upstream of the NF/RO is often split, partially bypassed around the NF/RO and blended with the NF/RO effluent downstream. This allows for the use of a smaller NF/RO unit, which reduces capital and operational costs.
Membrane technologies can be used in a stand-alone process or combined with conventional technologies. The best approach for process selection is based on the wastewater chemistry, requirements for the treated effluent and system capacity, as well as operational requirements. The MBR utilizes MF or UF membranes as a separation barrier to retain activated sludge in the process. MF or UF membranes can also be submerged or located on the side streams, separate from the activated sludge process. Figure 1 schematically illustrates the differences between submerged membranes and side- stream membranes.
MBR technology offers unique features such as a smaller facility footprint, superior effluent quality and a number of other benefits compared to conventional processes for treating wastewater; however, one major feature that still needs to be improved in the MBR process is energy demand. Statistically, the MBR process may require an additional 15% to 25% energy or even more to treat wastewater versus the conventional activated sludge process. The more intensive energy demand is explained by the way in which the MBR process operates.
Most MBR processes for municipal applications are designed with the MLSS in the range of 8,000 to 12,000 mg/L, which is higher than in the conventional activated sludge process in which the MLSS ranges from 3,000 to 3,500 mg/L. As a result, the Alpha factor in the MBR process is lower than it is in the conventional system and the efficiency of the oxygen transfer is lower, so more energy is required to achieve the same biological decomposition in the MBR process. Also, membranes need air-scouring to keep clean and maintain liquor uniformly around membrane fibers or plates. Because MLSS in the MBR process is much higher than in the conventional process, the MBR tankage is much smaller, reducing construction costs; however, this is offset by the cost for the energy needed to operate the MBR system.
Currently, the capital cost of MBR plants is competitive with those of conventional processes, while operational costs are higher due to higher energy demand. The lifetime cost of the project can favor the MBR technology.
Membranes for water reuse typically involve the use of MF or UF as a tertiary treatment downstream of the biological treatment. In cases where the dissolved species need to be removed from the MF or UF effluent, the process can be followed by RO for the entire or partial stream, depending on the effluent quality requirements. In the past, RO has been used as the only process, but this often leads to excessive plugging and wear on the membranes.
Recent applications employ a biological treatment system upstream of the RO process (i.e., an MBR, activated sludge with dual media filtration or anaerobic/aerobic treatment followed by clarification). RO is then used to remove TDS, salts, microconstituents and other contaminants not removed by the upstream system. A second RO system often is used to concentrate the primary RO concentrate and reduce the volume of RO concentrate being disposed; this is especially important where salts are being removed because increasingly stringent regulations limit the disposal of salty concentrate.
Membrane vs. Conventional Treatment
Membranes offer many advantages over conventional technologies including:
Due to the number of significant benefits that they provide, membranes have become cost-effective for many applications. Recent developments have increased the likelihood that membranes will be one of the most desired selections for upgrading existing facilities and meeting new wastewater treatment needs.