Secondary treatment. A biological waste treatment plant
How to meet water cleanup deadlines -
Basic processes for accomplishing this task have been successfully put into operation by three important industries
Richard K. Schmidt Industrial Waste Treatment Division
Ecodyne Corporation Union, N.J. 07083
The legislatively complex 1977 Environmental Protection Agency (EPA) wastewater discharge standards have inadvertently created more technical confusion than they have cleared up. While individual industry regulations provide precise standards, some view that very precision as creating a hodge-podge of additional regulations requiring literally thousands of different treatment processes. A closer look at the EPA's industry standards reveals some uniformity for many of the constituents that must be removed from most industrial discharges. Total suspended solids, oxygen demanding materials measured as biochemical oxygen demand (BOD) or chemical oxygen demand (COO), and pH are good cases in point. For example, in Class I, Phase I. there are 27 standards that set forth acceptable levels for total suspended solids and pH. Sixty percent of the standards also call for specific BOD andtor COO levels. The lone exceptions are guidelines for steam generation plants that concentrate on heat discharges. Because many parameters appear in virtually all the EPA Standards, the technological "how to" of meeting 1977 guidelines can now be examined from a simple unit process approach, rather than from the regulatory complexity of the various standards. Most waste treatment techniques conceived to meet 1977 standards can be separated into three distinct phases. These phases are commonly referred to as primary, secondary, and tertiary treatment. While abatement objectives differ within each phase, the equipment used is frequently similar in configuration. Figure 1 presents a summary of common treatment objectives and the associated equipment frequently used in each phase. As a general rule, progressing from primary to tertiary treatment involves more complex application technology and invariably higher expense. Primary treatment Typically, primary treatment, the first, and sometimes only step, utilizes a physical or chemical process to remove large quantities of a specific contaminant. Because of the normally low costlbenefit ratio, primary treatment is also the most economical. One of the most common primary treatment processes involves solidlliquid separation by gravity sedimentation. If the liquid waste is inorganic in nature, suspended solids removal may be the only step required. However, if soluble inorganics or organics are present, further treatment will be required. Sometimes, solid particles are colloidal, and therefore are not susceptible to gravity sedimentation. In this instance, the equilibrium of the solution must be changed by chemical addition to effect precipitation and ultimate removal. Other primary treatment devices bring about removal of large particles through screening, removal of oil and grease by dissolved air flotation, and dual-media filtration and temperature reduction by means of a cooling tower.
Industrialwastewater treatment processes
1
Scale to lagoons
Steel mill
wastewater Heavy milling wastewater
f
To service
A
1
Slipstream
A
Backwash
Pulp and paper mill
Biologicaltreatment Screening
Wood preparation
Solids dewatering
Food processingplant
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Manhole
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Excess flow
Return sludge
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Final
Food plant
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.
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7
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Primary treatment is normally adequate to reduce the contaminant level to that acceptable by a municipal collection system. However, if stream discharge is anticipated, the next step of treatment must be considered. Secondary treatment
Normally, secondary treatment takes the form of biological treatment that can be accomplished by several different procssses. These include activated sludge, extended aeration, contact stabilization, and rock or synthetic-media trickling filters. Biological treatment removes the soluble waste treatment fraction. The basic mechanism involves the application of microbes that utilize the soluble organic substrate as an energy source. In this manner, the organic material is converted into harmless by-products, such as carbon dioxide and water. Unfortunately, since matter cannot be created or destroyed, residue, commonly referred to as sludge, is generated as a result of this process. In earlier years, sludge disposal was frequently neglected in initial design. More often than
not, it was disposed of in inadequately designed landfills. Environmental pressure has recently focused on this area; thus, sludge dewatering and disposal has gained in design importance. Secondary treatment can also be applied to inorganic waste and may involve heavy metal precipitation and removal. The Reactivator is one of the units used in secondary treatment. Secondary treatment technology is normally adequate to achieve BATPA (Best Available Treatment Practically Achievable) abatement standards as legislated by the Federal Water Pollution Control Act Amendments of 1972, P.L. 92-500. Tertiary treatment
Because of its high costlbenefit ratio, the application of tertiary treatment to achieve legislated goals is currently under attack by a broad spectrum of industry. Common practice indicates that 90% contaminant reduction can be achieved by a combination of primary and secondary treatment. Addition of tertiary treatment will raise the efficiency to Volume IO, Number 2, February 1976
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95% or more. However, as pointed out in the initial section of this paper, as one progresses through the flow sheet, one notes that the applied technology become more sophisticated and, therefore, more expensive. It is not unreasonable to assume that the cost of tertiary treatment could approach the sum of the installed cost for combination primarylsecondary equipment. With this in mind, one can see why the battle rages on. Sometimes, however, it becomes necessary to employ tertiary treatment technology. This technology is basically a polishing step used to meet extremely stringent discharge standards. One common tertiary technique utilized is duai-media filtration for suspended solids removal. Most of the soluble organic material is removed in the secondary phase; if additionai removal is required, it is normally achieved by further suspended solids removal. Typically, the effluent from a secondary plant will have suspended solids in the range of 30-50 mgll. Dual-media filters are capable of reducing this level to less than 10 mgll. The Reactivator, for example, can also be used as a tertiary treatment device for further suspended solids precipitation, pH adjustment, and color removal. There will be variations in water treatment needs from industry to industry. An examination of the four heaviest industrial water users will demonstrate how one or more of the treatment regimes use proven technology to meet specific EPA standards. Steel mills One of the most pressing problems in the steel industry is the disposal of mill scale effluents. This is the liquidlsolid waste created when steel is washed clean of oxidized scale. One Southwestern rolling mill solved this problem by recycling 4000 gpm of scale pit effluent. The contaminated water carries off particles of scale through a flume to one of two scale pits. One three-cell pit serves heavy milling functions that leave effluent with a high solids concentration. From there, water flows to a two-cell pit where it joins somewhat cleaner effluent from the finishing end of the mill. Part of the water recirculates to the scale flume without further treatment. The major portion, about 4000 gpm, goes to dual-media filters. Each of the 12-ft diameter filters maintains an average flow rate of 8 gpmlft' representing 64% of the full system capacity. The influent water is filtered by a dual-media bed containing 4 ft of anthracite and 2 ft of sand. After the filtration cycle, the clean water is collected and held for service in a 500 000-gallon reservoir. On demand, the recycled water is drawn off to service such areas as roll cooling, descaling, washdown, and clean up. Plating waste Because of its large diversity in both scope and size of operation, the plating industry presents a particularly interesting waste abatement problem. Contaminants include heavy metals, cyanide, and other potentially toxic materials. Since these are not of organic origin, conventional secondary biological treatment is precluded. Typically, a physicallchemical approach or recycle is utilized. One of the largest plating mills in the country, located near Baltimore, Maryland, uses a three-stage neutralization, clarification, and filtration treatment plant to reduce acid-alkali wastes from pickling, prefinishing, and porcelain enamel operations, and also to remove chrome from its plating and paint lines. The acid-alkali wastes flow by gravity to a stationary prescreening process, then to an acid mixing tank where pH values are lowered to a uniform 3.3 for proper oil removal. Wastes flow from the acid mixing tank into an oillwater separator, and from there then to a holding basin for storage. Chrome wastes treated in a separate stream flow by gravi. ty to a separate bar screen into a holding basin. The waste is pumped into an acid mixing chamber for pH adjustment to 142
Environmental Science
Technology
Dual-media filter. Tertiary treatment technique 2.0-2.5 range. Sulfur dioxide gas is also added to promote the reduction from Cr" to the non-toxic Cr+3 state. At this point, the precipitated chrome stream mixes with the acidalkali stream in a head tank, and the combined streams are fed by gravity to a chemical mixing chamber where lime again adjusts pH for optimum dissolved metal precipitation. Powdered activated carbon removes color and organic traces, while coagulation aids are added to promote clarification in the mixing chamber of the Reactivator clarifier. Clarified effluent flows by gravity to a' four-cell dual-media sand filter for trace turbidity removal. Food processing The food'industry is comprised of a diversity of small processors ranging from dairy processors and meat packers to canned fruits and vegetables and frozen seafood processors, all of whom work on relatively small profit margins. Many food processing wastes are biodegradable. The degree of required treatment will depend heavily on ultimate liquid disposal. Some companies discharge wastes into municipal sewage systems, while others discharge into streams or impoundments. The former do not have to meet standards quite as strict as those set for plants discharging directly into streams or ponds. Standards for municipal discharge normally call for maximum discharge levels of 300 mgll of suspended solids, and 300 mgll of BOD, plus removal of any toxic materials. To meet these levels, food processors will install systems incorporating pre-screening to remove large particulate matter, and then use dissolved air flotation to remove oil and grease. The water is finally discharged into the city sewer for further treatment. Processors discharging into streams will have to expand this basic system further. These processors will require secondary treatment, including activated sludge followed by ciarification, and tertiary treatment such as sand filtration. An example of how one dairy processor in upstate New York plans to meet the new standards will provide an indepth look at the food processing industry's problems. The primary product produced at this plant is mozzarella cheese. A production by-product is a material called whey. Because of its high protein and acid content, up to 32 000 mgll of BOD, whey presents a most difficult waste disposal problem. This installation collects a 'major share of this by-product and dries it in a special process. This reprocessed by-product is sold as a food supplement, and what was a difficult disposal problem becomes a profitable venture. Production washdown water must still be treated prior to discharge. New York standards require that the company remove 90% BOD and stabilize nitrogenous oxygen demand (NOD) prior to discharge. Raw wastewater flows by gravity from the cheese plant through an inlet bar screen, and grit chamber where large particulates and grit are removed before discharge into a
25 000-gallon flow equalization tank. Next, the flow is pumped at a constant rate into an aeration tank for BOD removal, then discharged into the final clarifier for solids separation. From the clarifier, water flows to two dual-media tertiary sand filters, each with a surface filtration area of 40 ft2. Effluent is then. chlorinated in the filter backwash tanks and discharged into the stream. In another plant, this one a meat packing operation in Washington, D.C., a wastewater treatment facility was designed as an integral part of a new plant. As a consequence, in-plant reduction methods were incorporated into the plant's design. All blood is recovered and sold; paunch manure is removed as dry residue; large particlas are screened and used in the rendering facility; grease is recovered and also used in rendering. The resultant wastewater volume is about 372 000 gallons with a BOD of approximately 3900 Ibs. Wastewater flows from the packing plant through bar screens; gravity flotation and pressure flotation for grease removal take place in an aerated-flow equalization hold tank. Wastewater is then metered into an activated sludge tank for BOD removal. From here, water flows into a specially modified clarifier to provide additional skimming, ensuring the removal of residual floating grease. Effluent finally flows into a series of lagoons that are used as evaporation and percolation ponds, effectively creating a zero discharge system. Pulp and paper
Pulp and paper mills are collectively the largest industrial water users, with the typical integrated paper mill using up to 20-25 million gallons per day. The biggest wastewater-producing areas in the mills are the liquid vacuum and paper drying operations. Their "white water" contains residual particulate matter from paper fibers. In part, because it is the heaviest industrial water user, the paper industry has to date been the most progressive in solving wastewater discharge problems. The industry was the first to recognize the water pollution problem and the first to do something about it. Consequently, it has developed a great store of internal technology to meet the 1977 standards. The wastewater treatment technology presently in use at most paper mills includes primary clarification for suspended solids removal, followed by biological treatment, which for the most part consists of stabilization lagoons for BOD removal. The 1977 standards, however, require paper mills to go one step further and to filter the effluent from the ponds for trace suspended solids removal before discharge. Another problem facing the paper industry is color removal. Here, a physical/chemical process of chemical addition and precipitation in a clarifier is being used to meet the standards.
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While the implementation of basic equipment varies slightly from industry to industry, it is fairly obvious that the technical problems are not nearly as complicated as the legislative ones. Upgraded technological improvements may alter this basic equipment in the future, but its fundamental merit has withstood change to date. The basic technology for meeting 1977 standards is adequate to the task when it is put into the proper engineering perspective.
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Richard K. Schmidt is vice president and general manager at Ecodyne lndustrial Waste Treatment Division. A professional engineer, he received his Ph.D. in environmental engineering from the University of Texas. He has written more than a dozen technical papers on the treatment of industrial wastewater. Coordinated by JJ
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Environmental Instruments Division 101 First Avenue Waltham, Mass. 02154 Telephone 61 7/8%?8700 Telex 92-3323 CIRCLE 6 ON READER SERVICE CARD
Volume IO, Number 2, February 1976
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