Edited 2/2/4/98 by Ronald Lee Still.

Introduction

As advancements in membrane filtration technology become available, many facilities are finding it profitable to recover chemicals, product, and even water from wastewater that had been previously discharged to the sewer. Improvements in the design and decreasing costs of these systems have made possible a multitude of applications for this technology. Table 4 provides a summary of some industrial applications of membrane separation systems.

As facilities investigate new applications, it is important that they understand the limitations and requirements of the system to ensure proper application and operation. This fact sheet provides general information on types of membrane filtration systems and tips on purchasing and operating them.

Types of Filtration Systems

This fact sheet targets membrane filtration systems used to filter contaminants ranging from 100,000 to 1 Å. Table 1 provides an overview of filtration terminology, the size of contaminants filtered, and required pressures.

Many of these filtration systems utilize a crossflow design that allows fluid to flow parallel to the membrane surface and helps to reduce contaminant build-up and fouling of the membrane.

In addition to the crossflow design, a variety of membrane filter configurations and media is used to improve filtration capabilities. Table 2 provides a listing of these configurations along with types of filter media and operating limitations and requirements.

Purchasing a Filtration System

As Table 2 shows, a variety of filter systems is available to recover resources, and each has advantages and disadvantages. Thus, it is important to identify the specific system that is most economical and appropriate for the project in question. Purchasers should follow the steps listed below to identify the system best suited for the project.

1. Require the vendor to supply a statement of qualifications along with a list of recent clients - preferably those with similar projects - and contact information.
2. Send a sample of the wastewater to the vendor for testing on a variety of filtration media.
3. Have an in-plant pilot scale system set up for on-line demonstration even if this system has been used previously on similar effluents. Often effluents treated at the source react differently from those tested in the laboratory after a holding period and from other similar effluents treated in previous projects.
4. Request that vendors evaluate your operations for potential problems and/or reduction activities. If water and chemical conservation and reuse programs are put in place before filtration, the size of the filtration system can be reduced considerably.
5. Require detailed operating costs in addition to capital expenses. Some costs that should be included and compared in bids are listed below:

   • Energy requirements: recirculating pumps, sensors, controllers, flux enhancement devices, etc.
   • Prefiltration: frequency and costs of waste disposal and equipment.
   • Filtration: frequency and costs of waste disposal, filter cleaning, and filter replacement.
   • Labor and maintenance: fully or partially automated or totally manual.
   • Product: value and volume of filtered material for reuse or resale.
   • Efficiency: capability to maintain high flux rates with little care.

6. Require future interval maintenance and support by the vendor or manufacturer.
7. Have vendor train necessary employees in proper procedures.

*Coating is typically titanium dioxide, zirconium dioxide, and/or an acrylic surface layer. **Other materials not listed include polypropylene, polyethersulfone, polyvinylpyrrolidone, & PTFE. *** Plate & Frame configurations were not included because of limited use on wastewater.

The vendor may charge a higher fee to provide these items, but the investment will usually pay for itself.

Operating a Filtration System

The greatest disadvantage to a filtration system is the problem associated with flux reduction (flux is the rate of filtration) and fouling. These factors can significantly increase the cost of operating a membrane filtration system unless preventative measures are taken.

Figure 1 provides an example of the progress of the flux during the operation of a new membrane system.1 Initially, the system will have a relatively high flux that will quickly diminish as the membrane is fouled. Fouling occurs from colloidial flocculation or inorganic scaling above the membrane and adsorption of organics to the membrane.2 When the flux reaches a predetermined minimum rate, cleaning is required. The following agents are typically used for the foulants listed:

As shown in Figure 1, cleaning does not return the flux to the initial rate but to a value considerably lower. more frequent the cleaning, the lower the expected membrane life. Also, plugs of trace contaminants in the system can dramatically reduce membrane life.

Proper pretreatment is an economical and effective way to reduce cleaning frequency and membrane replacement. The following steps and technologies can assist in effective pretreatment:

1. Operators should frequently monitor the pretreatment system.
2. Employees responsible for processes that produce wastewater to be treated by the system should understand the characteristics of the membrane and the effects of different kinds of trace contaminants on the system; e.g., trace oils and solvents can quickly ruin some membranes.
3. The following controls should be added to reduce fouling and damage to the system:
   • Permeate and concentrate flow meter.
   • Feed temperature indicator.
   • In-line conductivity meter for feed and concentrate.
   • In-line feed water pH indicator/controller.
   • Shut-off or adverse feed water, e.g., low-flow, high-temperature, high/low pH, high pressure.
4. Only top-quality control equipment should be used. Frequent membrane replacement will be required if inferior control equipment, i.e., a pH monitor, is used.

While pretreatment can help maintain flux and reduce fouling, other techniques can improve the flux of a membrane system. Figure 2, which shows the effects of pressure, flow, concentration of solution, and temperature on flux, indicates that increased flow, temperature, and pressure or decreased concentration can improve flux for a particular solution. It should be noted that increasing temperature reduces viscosity and that increased pressure effectively improves flux only to a certain point. Currently other technologies are being investigated that also improve and maintain flux bettr than conventional systems. Table 3 provides an overview of these technologies.

Figure 2. Factors Affecting Flux

The Division of Pollution Prevention and Environmental Assistance (DPPEa) can provide a list of membrane filtration vendors and manufacturers as well as technical articles on the topics discussed in the Fact Sheet. Detailed versions of the Case Studies mentioned in Table 4 are also available from the DPPEA.

Table 4. Case Summaries of Filtration for Chemical and Water Recovery

Table 5 lists vendors of membrane filtration systems.