Wastewater Treatment Technology and Applications in Industrial Facilities Treatment that industrial facilities give wastewater before discharging it to the local wastewater treatment facility is referred to as "pretreatment." Methods for providing pretreatment can be divided into four categories: physical, chemical, biological, and membrane. This briefing addresses the first three categories; membrane technology is addressed in a separate End-Use Briefing. A pretreatment system may use one or several pretreatment processes. In general, physical pretreatment requires the least energy but is least effective in reducing pollutant concentrations. Biological processes tend to have the greatest energy cost, but are highly effective in reducing conventional pollutants. Chemical processes tend to lie in between the other two both in energy use and effectiveness. This end-use briefing provides a quick tutorial of wastewater treatment technology and applications in industrial facilities. The following topics are addressed: Conceptual overview of industrial wastewater treatment. Industrial pretreatment processes. Typical pretreatment processes for selected industries. Items to look for in the field for efficient operations. A list of references to other documents that provide more detailed information on the topics in this briefing. Overview Most industrial facilities discharge their wastewater to a local treatment facility. Due to Federal regulations regarding pretreatment and fees charged by local wastewater treatment facilities, many industrial facilities provide pretreatment. Federal regulations prohibit the discharge of wastes incompatible with: 1) the conveyance of the wastewater to the local facility (such as highly odorous compounds), 2) the processes at the local facility (such as chemicals that would inhibit biological treatment) or 3) the use or disposal of the treated wastewater or resulting sludge (such as certain pesticides). The EPA has set pretreatment standards which apply to more than 40 specific industry categories. Pretreatment standards also apply to the discharge of more than 120 specific pollutants. Details of pretreatment requirements are in 40 CFR Part 403 "General Pretreatment Regulations for Existing and New Sources of Pollution".
Figure 1: Schematic of an Activated Sludge Wastewater Treatment Facility Most treatment facilities charge industrial users a fee based on the amount and types of pollutants discharged. Other considerations that affect the amount of pretreatment a particular industrial facility would provide include: Pollutant or pollutants to be removed. Site constraints, such as space availability for equipment. Amount and variation of wastewater flow and pollutant concentration, both hourto-hour basis and seasonal A schematic of a typical activated-sludge, secondary wastewater treatment facility is in Figure 1. The energy used by any unit process can very dramatically from one treatment facility to another. Variation in energy use can be a result of differences in flow, pollutant loading, process control methods or prior or subsequent unit processes used at different facilities. Typical electric energy use in such a facility could be distributed as follows: Influent, intermediate, or effluent 10-20% pumping Primary sedimentation 2-5% Activated sludge 30-70% Sludge processing 10-50% Lighting, monitoring, controls 1-3% Disinfection (by purchased 1-3% chemicals) Odor control 1-2% (The related topics of sludge processing and hazardous waste treatment are not discussed in this End-Use Briefing.)
Industrial Pretreatment Processes Following is a brief description of common physical, chemical and biological pretreatment processes: Physical Screening is removal of coarse solids by use of a straining device. Sedimentation is gravity settling of pollutants out of the wastewater. Flotation is the use of small gas bubbles injected into the wastewater which causes pollutant particles in the wastewater to rise to the surface for subsequent removal. Air stripping is removal of volatile and semi-volatile organic compounds from wastewater by use of air flow. Chemical Neutralization is adjustment of alkalinity and acidity to the same concentration (ph 7). Precipitation (ppt) is addition of chemicals to wastewater to change the chemical composition of pollutants so that the newly formed compounds settle out during sedimentation. Coagulation is use of chemicals to cause pollutants to agglomerate and subsequently settle out during sedimentation. Adsorption is use of a chemical which causes certain pollutants to adhere to the surface of that chemical. Disinfection is use of a chemical (or other method such as ultraviolet radiation) to selectively destroy disease-causing organisms. (Sterilization is the destruction of all organisms.) Breakpoint chlorination is the addition of chlorine to the level that chloramines will be oxidized to nitrous oxide and nitrogen, and chlorine will be reduced to chloride ions. Biological Air activated sludge is an aerobic process in which bacteria consume organic matter, nitrogen and oxygen from the wastewater and grow new bacteria. The bacteria are suspended in the aeration tank by the mixing action of the air blown into the wastewater. This is shown schematically in Figure 1. There are many derivations of the activated sludge process, several of which are described in this section.
High purity oxygen activated sludge is an aerobic process very similar to air activated sludge except that pure oxygen rather than air is injected into the wastewater. Aerated pond/lagoon is an aerobic process very similar to air activated sludge. Mechanical aerators are generally used to either inject air into the wastewater or to cause violent agitation of the wastewater and air in order to achieve oxygen transfer to the wastewater. As in air activated sludge, the bacteria grow while suspended in the wastewater. Trickling filter is a fixed film aerobic process. A tank containing media with a high surface to volume ratio is constructed. Wastewater is discharged at the top of the tank and percolates (trickles) down the media. Bacteria grow on the media utilizing organic matter and nitrogen from the wastewater. Rotating biological contactor (RBC) is a fixed film aerobic process similar to the trickling filter process except that the media is supported horizontally across a tank of wastewater. The media upon which the bacteria grow is continuously rotated so that it is alternately in the wastewater and the air. Oxidation ditch is an aerobic process similar to the activated sludge process. Physically, however, an oxidation ditch is ring-shaped and is equipped with mechanical aeration devices.
Pollutant Bio-Chemical Oxygen Demand (BOD) Total Suspended Solids (TSS) Nitrogen Phosphorus Heavy metals Fats, Oil and Grease (FOG) Volatile Organic Compounds Pathogens Pretreatment Processes Activated Sludge Trickling filter or RBC Aerated lagoon Oxidation ditch Sedimentation Screening Flotation Chemical precipitation Nitrification/denitrification Air stripping Breakpoint chlorination Chemical precipitation Biological treatment Air stripping Biological treatment TChemical precipitation Evaporation Membrane process Coagulation Flotation Biological treatment Membrane process Air stripping Biological treatment Carbon adsorption Chemical disinfection UV radiation Table 1: Selected Pollutants and Associated Pretreatment Processes Table 1 identifies the pretreatment processes most commonly used to treat specific industrial pollutants. Pretreatment often involves more than one process and the order of multiple processes is very important. With wastewater whose ph is high, neutralization would be needed prior to using a biological process, as high ph would adversely affect the growth of the organisms. The order of the treatment processes also affects operating costs. Primary sedimentation often precedes biological treatment, for several reasons: to remove some of the BOD by a low operational cost method; to remove waste matter that may adversely affect the biological process; and to provide some flow equalization prior to the wastewater entering the biological treatment process.
Typical Pretreatment Processes for Selected Industries Several industries, their associated wastewater pollutants and common pretreatment processes used to treat those pollutants are shown in Table 2. Industry Apparel Textiles Leather goods Laundry Detergents Associated Wastewater Polluants BOD, TSS, alkalinity BOD, TSS, Chromium Alkalinity, BOD, turbidity Pretreatment Process Neutralization, chemical precipitaion, biological treatment Sedimentation, biological treatment Screening, chemical precipitaion, adsorption BOD, suponified Floatation and skimming, chemical ppt soaps Food Brewed beverages BOD Centrifugation, biological treatment Meat & poultry BOD Screening, sedimentation, biological treatment Rice BOD, TSS Chemical precipitation Bakeries BOD, FOG, Biological treatment Soft drinks detergents BOD, TSS, alkilinity Pharmaceuticals BOD Pulp and paper Metal-plating Plastics and resin High or low ph, TSS, inorganic compounds Acidity, heavy metals Hign or low ph, volatile organic compounds Neutralization, screening, biological treatment Evaporation, drying Sedimentation, neutralization, biological treatment Neutralization, sedimentation, chemical precipitation Neutralization, biological treatment Table 2: Selected Industries, Associated Wastewater Pollutants, and Pretreatment Processes
What to Look for in the Field for Efficient Operation Energy demand varies significantly between facilities, depending on any of several parameters, including: variation in wastewater flow rate, pollutants removed, treatment processes utilized, physical characteristics of the pretreatment site, methods of process control, processes used for sludge treatment and amount and type of air emissions controls. For the above reasons it is difficult to provide accurate energy use data for a particular unit process. However, there are operational aspects of a unit process which relate to its efficiency that are relatively independent of the particular site characteristics. Table 3 suggests things to look for in the field to assess the efficiency of operation of various pretreatment processes. Pretreatment Process Physical Screening Sedimentation Centrifugation Air stripping Chemical Neutralization Precipitation Coagulation Adsorption Disinfection Biological Activated sludge Trickling filter Rotating biological contactor (RBC) Items to Look for in the Field for Efficient Operation No blinding or clogging of screens, no excessive build-up of material on the screen Low flow rate, no short circuiting of flow, no floating sludge, scum removal if appropriate Knowledgeable operations staff No scaling of packing and piping, or freezing problems at low temperatures ph monitoring, automated chemical feed, adequete mixing Automated chemical feed system, adequate mixing & contact timer Automated chemical feed system, adequate mixing & contact timer Efficient means of regeneration is key to preformance Automated chemical feed system, adequate mixing & contact timer Fine bubble aeration, even distribution of air and mixing, dissolved oxygen concentration monitoring, air flow turndown capability, no bulking/floating sludge Method for positive air circulation, even & periodic dousing of filter media Steady shaft rotation Table 3: Pretreatment Processes and Items to Look for in the Field for Efficient Operation
Tips for Efficient Operation and Maintenance The areas of largest energy use in wastewater treatment facilities are usually pumping the wastewater, providing biological treatment and solids processing. Pumping Implement process control improvements to optimize flow rates, for example: minimize trickling filter recirculation and under-pumping sludge from clarifiers. Consider using variable frequency drives when flow rates are variable. Biological Treatment Optimize primary treatment efficiency. BOD removed by primary treatment requires less energy than BOD removed by biological treatment, also surplus sludge should not accumulate in the primary clarifier to avoid dissolving and carry-over to the secondary process. Control amount of aeration to avoid excessive dissolved oxygen. Improve aeration system efficiency. For example, fine-bubble diffusers can increase oxygen transfer efficiency in comparison to coarse-bubble diffusers. More Information Call 1-800-468-4743 for more information about PG&E's energy efficiency programs and other services. References 1. Alberi, et al, "Pretreatment of Industrial Wastes," Manual of Practice No. FD-3, Water Environment Federation, 1994. 2. Edwards, "Industrial Wastewater Treatment," Lewis Publishers, 1995. 3. Metcalf, Eddy, "Wastewater Engineering," Third Edition, McGraw-Hill, Inc., 1991. 4. Nemerow, "Liquid Waste of Industry," Addison-Wesley Publishing Co., 1971. 5. Miorin, et al, "Wastewater Treatment Plant Design," WPCF Manual of Practice No. 8, Water Environment Federation, Second Printing, 1982. 6. Renzo (editor), "Pollution Control Technology for Industrial Wastewater," Noyes Data Corporation, 1981.