8.0 TYPES OF PRETREATMENT SYSTEMS

8.1 General Considerations

There are many different types of wastewater pretreatment systems that can treat specific and multiple contaminant problems. The complexity and installed cost of a complete pretreatment system will depend on many factors, including:

• the volume and nature of the wastewater

• the allowable contaminant levels in the final discharge

• the desired degree of pretreatment system automation

• the difficulty of system installation within an existing facility.

For example, one might imagine that a laundry facility would require a relatively simple pretreatment system for its wastewater. A local industrial laundry, however, recently installed a wastewater pretreatment system that consisted of seven different unit operations to remove regulated contaminants from its wastewater. The unit operations were equalization, solids removal (screening), pH adjustment, oil skimming, dissolved air flotation with chemical treatment, sand filtration, and activated carbon filtration.

A facility that must remove heavy metals from a wastewater stream might use pH adjustment to precipitate the metals from solution. Most heavy metals will precipitate out of solution as hydroxide salts at higher (alkaline) pH levels when sodium hydroxide is used as a pH adjustment reagent. Hexavalent chromium, on the other hand, is very soluble in wastewater and must be chemically reduced at lower (acidic) pH levels to the trivalent form before precipitation is attempted. Thus, the need for two different pH conditions for the chromium reduction and precipitation steps means that a pretreatment system for hexavalent chromium must have two separate reaction tanks. Often, a third tank is needed for final neutralization before discharge.

In addition, heavy metal removal to low levels (in the µg/L or ppb range) can be achieved by reverse osmosis (RO) or ion exchange pretreatment processes. If there are organic compounds or bacterial activity in the wastestream, however, the RO membranes or ion exchange resins may become fouled, thus reducing flow through the system or allowing the metals to pass through the system, respectively. Therefore, the offending organic compounds or bacterial activity should be controlled prior to these pretreatment processes.

The levels of organic compounds can be reduced or eliminated by implementing a source reduction program or by an initial pretreatment step such as carbon adsorption. A source reduction approach is generally preferred since it can lower the capital and operating costs of the pretreatment system. Bacterial activity can be controlled by chemical additions (i.e., oxidizers such as bleach or peroxides), high intensity ultraviolet light, high temperature exposure, or even filtration.

At times, a wastewater stream may contain chemical agents that can interfere with the heavy metal removal process. These chemical agents are called chelators or complexing agents. Source reduction steps may be needed to reduce or eliminate these agents. Alternately, the specific wastewater streams that contain these agents may have to be segregated from the main wastewater stream. The segregated streams could then either be piped into a separate specialized treatment unit within the pretreatment system or be collected and shipped to a licensed disposal facility.

Some form of pretreatment system for mercury removal may be needed if source reduction efforts alone fail to solve the problem adequately. Since mercury is a complex wastewater contaminant, multiple pretreatment unit processes may be necessary to reduce mercury levels sufficiently to reach compliance with discharge limits. For example, filtration would be an excellent candidate for an initial pretreatment unit process because mercury readily binds with particulate matter in wastewater. Depending upon the performance of the filter system in reducing mercury concentrations, subsequent pretreatment system unit operations may be smaller, have lower operating costs, or may be eliminated altogether.

For more detailed information on the characteristics of mercury in wastewater and on the performance of several pretreatment technologies in bench-scale feasibility tests on a clinical laboratory wastewater, please see the MWRA/MASCO Mercury Work Group, Technology Identification Subgroup Report, which is a companion to this Manual.

8.2 Types of Pretreatment Unit Operations

Usually, processes used in a wastewater pretreatment system can be placed into four categories as follows:

Biological Processes - Processes where living microbial organisms are used to metabolize organic wastes into carbon dioxide, water, methane gas, simple organic acids, and microbial matter. Aerobic microbial organisms require oxygen for their metabolisms. Anaerobic microbial organisms live in oxygen-limited environments. For municipal wastewater treatment, POTW’s use both aerobic and anaerobic biological processes.

Chemical Processes - Processes that alter the chemical structure of the constituents of the wastewater so they can be removed from the wastewater stream before discharge. An example is heavy metal precipitation by pH adjustment.

Physical Processes - Processes that separate components of wastewater without altering the chemical structure of the constituent materials. Examples are dissolved air flotation (DAF), reverse osmosis, and filtration.

Thermal Processes - Processes that operate at high temperature to reduce the volume of wastes and breakdown the toxic components into simpler less toxic forms. These processes are typically expensive to operate because of high energy costs, but they can be very efficient for certain types of pollutants.

The following are specific types of unit operations used in wastewater pretreatment systems:

1. Aerobic or Anaerobic Pretreatment - Operations that use aerobic bacteria or anaerobic bacteria to reduce organic wastes in wastewater. Levels of the organic wastes are usually measured in terms of wastewater biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Aerobic pretreatment requires a source of oxygen and can produce significant quantities of biomass (sludge). At times, high levels of organic wastes can more economically be treated by an anaerobic process. Many inorganic contaminants, such as heavy metals, can be adsorbed onto the biosolids produced during the treatment process. Because of sensitivity of the bacteria to sudden changes in conditions, protection of the bacteria by various initial physical or chemical process operations may be needed.

2. Disinfection (chemical, thermal, or UV sterilization) - Used to reduce or eliminate bacterial or viral activity in a waste stream. Chemical methods usually involve the use of oxidizers such as hypochlorite (bleach), permanganate, and peroxides. Some unit operations - such as ion exchange and membrane filtration (see below for descriptions) - can be adversely affected by oxidizers. Thermal disinfection is highly effective, but is usually impractical for large streams because of cost considerations. Ultraviolet light (UV) sterilization is especially useful and economical for smaller flows, but may be ineffective on cysts and spores.

Depending upon the specific pretreatment technology, disinfection may be used before a mercury removal step. In some systems, disinfection by oxidizers can serve a dual purpose: prevention of biological growth in the adsorbent media and oxidation of complexed mercury species to more readily removed ionic forms.

3. Clarification - Used to remove settleable solids from a wastewater stream. At times, this gravity separation process is chemically enhanced by adding polymers under controlled conditions to cause agglomeration of the solids into larger particles for faster and more efficient settling.

4. Simple Filtration - Used to remove particulate matter (usually greater than 5 microns in size) from a wastewater stream. Filtration systems in this category would include bag type, depth or fiber wound cartridges, and graded sand and diatomaceous earth filter media. Filtration is often used for wastestreams high in particulate matter that could disturb subsequent unit operations. Since mercury has a high tendency to bind to particulate matter, coarse and fine filtration may be routine as initial unit operations in a mercury pretreatment system.

5. Membrane Filtration (micro or nano) - Used to remove smaller particulate matter, down to the 0.1 micron range. These systems can employ organic membranes (cellulose-based) or synthetic membranes. The organic membranes can be adversely affected by organic solvents, chlorine, and other oxidants. The membrane pore size can be compared with 0.45 microns that is typically defined in laboratory analyses of wastewater samples as the differential point between dissolved matter and suspended solids. Membrane filtration has been successful in applications of precipitated metals and free oil removal from wastewater.

6. Reverse Osmosis (RO) - Used to remove sub-micron particulate and high molecular weight ions. Hospitals often use RO units for desalinization of incoming city water (often used with ion exchange processes). In wastewater pretreatment applications, RO membranes can readily be fouled by oil and grease and suspended solids. The membranes can also be adversely affected by organic solvents and chlorine or other oxidants.

7. Ion Exchange - Used to remove dissolved ionic compounds. Ion exchange resins usually require specific pH ranges for good operation and tend to be expensive, require regeneration, and are susceptible to degradation by oxidizers and to fouling by suspended solids, oil and grease, and organic compounds.

8. Dissolved Air Flotation (DAF) - Used to remove light particulate from a waste stream by infusing fine air bubbles into a holding tank. The air bubbles attach to the particulate and lift them to the wastewater surface where they can be skimmed off. DAF treatment is often used for fats, oil, and grease (FOG) removal after de-emulsification.

9. Adsorption - Used to remove high molecular weight compounds from air and wastewater streams. The process uses a surface-active medium, the most common of which is activated carbon. Often used for removing low concentrations of volatile and non-volatile organic contaminants (solvents, pesticides, PCB, and phenols ) and inorganic contaminants (such as mercury and other heavy metals) from wastewater.

10. Chemical Precipitation/Redox Reactions - Conversion of a dissolved pollutant to an insoluble form. Typical of these reactions is hydroxide precipitation of heavy metals from metal finishing wastewater. The reactions are usually followed by a particulate separation process, such as gravity clarification or filtration, to remove the formed solid particulate from the waste stream.