A Membrane for All Seasons
Flexible design allows membrane treatment systems to fit almost any wastewater application
- By Diane Rapaport
- Jul 01, 2006
The treatment, recovery, and recycling of wastewater is a well-established membrane separation application with several advantages: waste is treated to the point where resulting water quality is equivalent to fresh water, fresh water usage is decreased significantly, waste hauling and cost is minimized, heat energy is recovered, chemical usage is reduced, and waste treatment operation and maintenance is simplified. Designed as either a point source or end-of-pipe waste treatment method, membrane technology can even eliminate loading and associated costs on downstream treatment works and other effluent treatment processes.
Industrial waste process fluids contaminated with heavy metals, chemical oxygen demand (COD), biochemical oxygen demand (BOD), oils and greases, etc., are fed to a membrane system where the colloidal, suspended and dissolved solids, greases, and other contaminants are separated out. Treated water can be recycled in the plant as process water, as make-up to boilers or cooling towers, or for non-contact uses.
Membrane systems are designed in variations of two fundamental operating modes -- batch and continuous. Batch modes include simple batch (also known as straight batch) and modified batch (also known as topped-off batch), a semi-continuous mode of operation. Continuous operation can comprise a single membrane system or multiple staged-in-series membrane units. Selection of the appropriate mode of operation is based on process objectives and project economics.
In determining process objectives, a number of criteria are considered, including concentration factor, volume reduction requirements, final concentrate solids target, permeate quality, feed stability with time, process run time, cleaning frequency, plant location, and space limitations for the membrane system and its associated tanks. With the project defined, and in combination with pilot and feasibility testing and/or comparative commercial-scale experience, the membrane and membrane configurations are selected, and the membrane process mode is specified. An understanding of each of the modes available is required to properly select the most economical system that best meets application objectives.
The following summarizes those operating modes and presents guidelines in determining the best mode for a particular membrane system installation.
The simple batch mode requires the least membrane area but the largest process tank. The tank is first filled with the entire quantity of feed to be concentrated, then, as feed circulates through the membranes, concentrate flows back into the tank. The feed is concentrated by small amounts with each pass through the membranes, and the overall concentration in the feed tank increases as water is removed by permeation through the membrane. Batch systems are straightforward in operation and design. They are mainly used in smaller system wastewater treatment applications that typically process fewer than 1,000 gallons per day (gpd). Simple batch is also seen in the chemical process industries where batch operation is more commonplace.
Like simple batch, the modified batch mode offers the advantage of minimal membrane area, but with less process tank volume. As permeate is removed, an equal volume of fresh feed is added to the process tank to keep it topped off. After a predetermined concentration, flux, or time is reached, fresh feed is no longer transferred from the feed tank to the process tank, and the remaining fluid in the process tank is further concentrated as batch. When final concentration is reached, the system is shut down and cleaned, while the concentrate in the process tank is disposed of. The process tank is then refilled and processing begins again.
Compared to simple batch, modified batch allows larger volumes of feed to be processed before disposal of concentrate is required. The feed tank size should permit balancing of variable flows from the plant. Because all recirculation flow is returned to the process tank, the membrane system and process tank should be in relatively close proximity to each other to save on piping costs, especially for larger membrane systems. Modified batch is particularly useful and is usually the preferred mode of operation for industrial applications where maximum concentration is the objective.
In feed-and-bleed modified batch operation, the process fluid is pumped from the process tank and through the membrane system. At this point, the process fluid is divided into two streams: one that returns to the process tank, and the other that is recirculated through the membrane system. Permeate is directed to drain, the next processing step, or it is reused, depending on the application. A relatively small volume of bleed returns to the process tank, but most remains at membrane outlet pressure and returns to the suction of the recirculation pump.
The advantages of feed-and-bleed modified batch are many, such as lower energy costs than those for modified batch. In addition, the piping to and from the process tank is much smaller because of lower flow to and from the tank. Consequently, the turnover of the process tank occurs at a much slower rate. The smaller pipe size has the obvious advantage of being considerably less expensive and results in a more compact system.
The slower rate of turnover in the process
tank also has several advantages because of decreased agitation. First, the problem of foaming in the tank is greatly reduced or eliminated; second, the quiescent (still) condition of the tank allows for settling of undesirable solids; and third, any floatable solids are allowed to rise to the top of the tank, thus facilitating skimming operations. Like modified batch, the process tank is smaller than for simple batch. Because feed-and-bleed piping sizes and costs are relatively low, the process tank can be located farther away from the membranes, allowing flexibility in the layout and installation of membrane system and tanks.
Also, as in modified batch, fluid that does leave the process as permeate is replaced with fresh feed from the feed/supply tank, keeping the process tank topped off until it is time to further concentrate as batch. Used with microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes, feed-and-bleed modified batch mode design is standard on larger industrial- and waste-treatment membrane systems for water treatment recovery and recycling, and for maximum concentration of contaminants with low final volumes of concentrate as the goal.
The single-stage continuous operation mode requires the least tank space but requires more membrane area than the other modes. In this mode, the feed tank is always at initial concentration, while the recirculation loop is always at final concentration. Because fluxes are generally lower at higher concentrations, this membrane system runs at a consistent but lower flux, rather than over a range of fluxes, as in batch systems. Feed rate from the tank is dependent on the permeate rate and the desired final concentration. Flow ratio control is used to adjust the retentate (concentrate) flow out of the system. This design is suitable for dilute waste streams where achieving high solids concentration in the retentate is not a primary economic driver.
Multistage continuous operation combines advantages of both batch and continuous modes. In this design, each stage concentrates the feed by some increment. The more stages used, the higher the average flux for the entire system, approaching the efficiency of a batch operation. Each stage has a recirculation loop and pump to maintain cross flow velocity at the membrane surface. In addition, because feed does not return to the feed tank, the multistage continuous mode (like single-stage continuous) has minimal tank requirements.
Another advantage is the short residence time in the system for the fluid being concentrated. This is important when processing liquids susceptible to decay or biological degradation. Also, different membrane configurations can be specified in different stages to permit use of more economical membranes in the initial dilute stages and more robust membranes in the later stages where solids are more concentrated. Continuous systems are specified when equipment up and/or downstream is also continuous. This mode is suitable for most systems and can be expanded with the addition of more stages. For wastewater treatment, the multistage concept is used for relatively high flow rates where tank requirements for a modified batch operation would be prohibitively large.
The arrayed continuous operating mode is like multistage continuous, but without the recirculation loop and pump on each stage. This system is tapered -- each downstream stage/bank is smaller that the previous stage/bank. The progressively smaller banks compensate for permeate removed from the previous bank and ensure that adequate cross-flow velocity is maintained in each. This mode is used on systems processing dilute streams with very low or no suspended solids in the feed. Desalination and water purification operations commonly employ this operating mode. It is also used in recycling processes on RO system designs polishing permeate generated by waste treatment UF systems. Continuous designs are commonly applied in applications with high flows and relatively diluted solids where the primary objective is water recovery.
Submerged membranes operate like the single-stage continuous mode described above. Here, though, the membrane is submerged in the process tank itself. Concentrate is bled from the tank to control solids both in the tank and at the membrane surface. A vacuum is applied to the permeate side of the membrane to draw clarified permeate through the membrane and out of the system for discharge or recycling. Instead of pumped cross-flow along the membrane surface, these designs use aeration or other turbulence promotion, on the membrane's outside surface to minimize fouling and optimize flux. Combined with biological treatment, this design is known as a membrane bioreactor (MBR). Submerged membrane units are increasingly being specified to treat some of the toughest wastewaters in both municipal and industrial applications. They are well suited to the treatment, recovery, and recycling of water from waste.
The production and discharge of wastewater is subject to increasingly stringent regulation. Demand for water is increasing while its availability is becoming limited. Although regulations and supply may vary between areas and countries, the need for advanced waste treatment methods to conserve water and preserve the environment has become apparent.
Membranes are a proven, cost-effective method to close the loop, and sometimes even cap the sewer. They are reliable, rugged, and simple to operate, and have, on a large scale, been treating wastewater to recyclable quality for more than 35 years. Membranes are used in sewage treatment for municipalities; they treat wastewater in the food and beverage, textile, paper, and chemical industries; and they have been used for many years in the metal finishing, pharmaceutical, cosmetics, automotive, oil and gas, printing, mining, and power industries. By using membranes to recycle process water, industries cut wastewater disposal costs and reduce consumption of freshwater.
Membrane systems provide a positive barrier with immediate results and consistent, high-quality permeate. There are many choices in the design of a membrane system. An understanding of all the options available, whether it is membrane type (MF, UF, NF and RO), membrane configuration (tubular, hollow fiber, spiral wound, and plate and frame), or process mode (simple batch, modified batch, and continuous) is key. With the right design, a membrane-based system is the best available method to economically treat wastewater for recovery.
This article originally appeared in the 07/01/2006 issue of Environmental Protection.
Diane Rapaport is a freelance writer specializing in environmental issues.