Case Study

Combining Treatment Technologies to Eliminate Wastewater Discharge

Complete wastewater recovery and reuse are possible when conventional and advanced treatment technologies are combined.by David Wolfe, Charles Suenkonis and Daniel Dellicker

Imagine a world where industries routinely recover and reuse all wastewater generated onsite and, at the same time, eliminate hazardous and residual waste generation. It is possible when engineering combines conventional treatment with advanced treatment technologies in the plant's design. And it has been shown to be possible at a large battery manufacturing complex. Process debris, oils, solids and metals can be treated and removed in an industrial wastewater treatment facility by enhancing removal efficiencies of conventional processes. The wastewater can be further polished using advanced treatment processes to remove residual organics and the remaining dissolved solids (salts). The treated and recycled wastewater approaches the purity of distilled water.

Conventional technologies

Conventional treatment technologies include screening, oil/water/solids separation, neutralization/iron coprecipitation, sedimentation and filtration. Screening removes from the influent wastewater debris that could cause excessive wear and maintenance of downstream process equipment - such as stringy material, metal chips and packaging materials. Rotary drum-type screens prove very effective because of their simple design and selfcleaning features. For cost-effective resizing or replacement of the screen fabric, rotary screens with a support drum design and a stretched screen skin are appropriate. Rotary screens constructed of 316 stainless steel or higher alloys are suited for nearly all process wastewaters.

Oil/water/solids separators remove from the screened wastewater free (non-emulsified) oil and settleable solids, which could cause operational and maintenance problems in other downstream treatment units. These types of separators are particularly effective in applications using machining fluids, such as in metals forming, battery manufacturing, automotive and other industries.

A coalescing oil/water/solids separator has approximately half the volume and as little as one-fourth the length of a standard American Petroleum Institute (API) design oil/water separator. The slant rib coalescers are installed in a rectangular tank containing special baffles and weirs designed to direct flow, settle solids, skim oil and control the liquid levels in the separator. The coalescer plates are made of a plastic media particularly effective because of its oleophilic (oilattracting) characteristics. Special materials are available for high-temperature process applications. The coalescing action of the plates allows the removal of smaller droplets than is possible with a traditional API design separator. In addition, the large solids collection hoppers facilitate the removal of heavy sediments from the process flow.

Iron coprecipitation is one of the most effective processes for removing metals from wastewater. This process centers on the solubility of iron rather than on the individual solubilities of various metal hydroxides. The enhanced effectiveness lies in associating the metals with the soluble iron, then rapidly precipitating the resulting complex.

Conventional metals removal typically involves the use of lime or sodium hydroxide to form an insoluble metal hydroxide precipitate. The solubility of metal hydroxides is a function of pH. Wastewaters containing metals such as lead and cadmium will reach their minimum solubility at a different pH value; therefore, hydroxide precipitation alone is insufficient for low-level metals removal.

To overcome the multiple-solubility obstacle encountered when several metals need to be removed from the wastewater, an iron coprecipitation process can be incorporated into the wastewater treatment process. In this process, all metals are precipitated at the optimum pH for the oxidation of ferrous iron and the precipitation of ferric iron, typically at a pH between 8.0 and 8.5. Another advantage of this process is that virtually all the iron reagent added in the process is removed as a result of the low residual solubility of iron, typically less than 10 parts per billion (ppb) to 20 ppb. This metals removal process typically is followed by sedimentation (clarification) and sand filtration, which capture the precipitated heavy metals as a metal hydroxide sludge. This sludge is gravity thickened before it is dewatered in a filter press.

Advanced technologies

Advanced treatment technologies further condition the pretreated process wastewater, converting it into a high-quality water suitable for reuse in manufacturing processes. These include carbon adsorption, evaporation/crystallization and reverse osmosis. Carbon adsorption is well suited for the removal of soluble organic compounds that may cause odor or color problems in the recycled water or in the residual salt. A physiochemical process in which carbon forms chemical and ionic bonds with organic molecules and certain inorganic compounds, it is most effective on soluble organic compounds or certain inorganic compounds such as lead hydroxide and lead sulfate.

Activated carbon has a small granular size and is itself a highly efficient filter. Carbon adsorption is most efficient on filtered feeds with a low concentration of suspended solids because of the plugging effect of such solids. The effluent from granular activated carbon adsorbers is a relatively pure brine suitable for process water and salt recovery.

Evaporation and crystallization - thermal processes used to concentrate dilute brines (salt solutions) to concentrated liquids and precipitated solids - are effective in producing distilled quality water and a solid crystalline salt product from a preconditioned brine.

Vertical-tube falling film evaporators are an efficient type of evaporator for dilute brines. These evaporators consist of a flood box on top of a vertical shell and a tube heat exchanger on top of a sump. The evaporation process is driven by the transfer of heat through the tube wall by steam applied to the outside - or shell side - of the tube.

Mechanical vapor recompression is the most efficient heat source for this type of evaporator when waste steam sources are not available, requiring about one-tenth the energy of a single-effect, steam-driven evaporator. The hot distillate from the condensed steam passes through a feed heat exchanger, where the feed is preheated and the distillate is cooled.

Concentrated brine from the evaporator sump transfers to the crystallizer system for further volume reduction. The crystallizer can effectively concentrate the salts in the brine beyond saturation to produce a precipitated salt crystal. Using a special pressure filter or centrifuge, these crystals can be harvested from the recirculated slurry of the crystallizer. All filtrate or centrate is returned to the crystallizer body for further concentration and precipitation of salt solids. These salt solids can be sold as a coproduct such as sodium sulfate, if they have market value, or disposed of in a landfill.

A forced-circulation crystallizer takes a concentrated brine to a solid. It is constructed of a submerged shell and a tube heater piped tangentially to a vapor body that is connected to a slurry recirculation pump. The crystallization process is driven by the transfer of heat through the tube wall by steam applied to the outside of the tube. Mechanical vapor recompression also is a cost-effective source of heat for the crystallizer.

To further purify the combined distilled water from the evaporator and crystallizer system for reuse in manufacturing, reverse osmosis can be used. This process also facilitates the incorporation of make-up water by blending potable water with the distillate from the evaporator and crystallizer system in the feed to the reverse osmosis system.

Reverse osmosis removes dissolved ions from water by pressuring the water through a semi-permeable membrane element, which will pass the water but reject most of the dissolved solids. This driving force - called osmotic pressure-can be measured, and the resulting flow of water from a low concentration to a high concentration, as is usual with the phenomenon of osmosis, can be halted by applying a pressure equal to the osmotic pressure on the more concentrated solution side. If this external pressure is further increased, the flow of water will be reversed from its natural direction toward the more dilute solution.

Many membrane elements have good qualities of rejection, or salt separation. The goal in the selection of the proper membrane element and array or configuration of elements is to select an element that yields the highest water flow or flux rate at the lowest driving pressure while still rejecting salts at an acceptable level. Today's thin-film composite membrane elements offer excellent flow and rejection characteristics. The water flux rate at a given pressure is directly proportional to the dissolved solids concentration of the solution. As this concentration increases, the driving pressure required to sustain a given flux rate also will increase. The most important factors in achieving sustained long-term performance of a reverse osmosis system are controlling membrane fouling and keeping membrane elements clean. Fouling can result from excessive amounts of fine suspended solids in the water plugging or blinding the membrane surface. In addition, it can be caused by scaling resulting from the precipitation of solids on the membrane surface (i.e., barium sulfate), or by uncontrolled bacteria growth on the membrane's surface. Fouling results in the loss of flux rate and an increased operating pressure. Uncontrolled bacteria growth-biological fouling - is typically the most difficult type of fouling to control and often requires the use of membrane-compatible biocides. To provide thorough cleaning and ensure reasonable membrane life, an adequate clean-inplace (CIP) system is essential.

Practical use

One of the first companies to combine these conventional and advanced technologies on a large-scale basis was the East Penn Manufacturing Co. East Penn has been producing lead acid batteries for more than 50 years at a large battery manufacturing complex in Lyon Station, Pa., U.S.A. Facilities at this 225-acre site include an industrial battery plant, three automotive battery plants, a specialty battery plant, a secondary lead smelter and refinery and an acid reclamation plant.

As a result of manufacturing operations, East Penn produced a number of residual and hazardous waste streams. Its industrial wastewater stream was of particular concern. The company's existing wastewater treatment plant, installed in 1976, no longer was capable of meeting recently mandated state and regional permit discharge standards. In addition, it was important to East Penn to be a good neighbor by protecting the environment and adding to the local economy.

Although East Penn's treatment facility effectively removed heavy metals, it was incapable of removing dissolved solids - mainly calcium sulfate and sodium sulfate. These salts were present in concentrations up to 20,000 parts per million (ppm), high enough to have a potential negative impact on aquatic organisms and drinking water quality. As a result of the neutralization and metals precipitation processes, the treatment plant also generated a significant volume of sludge, which was classified as hazardous because of its lead content.

To meet state, regional and federal regulatory requirements and to minimize the facility's environmental impact, East Penn and its consultant designed new facilities to better manage the wastewater stream. After investigation and research, it was determined the new facilities not only could answer the manufacturer's need to eliminate wastewater discharge, but also could go a step further and allow all the wastewater to be reused in the manufacturing operations.

The consultant also designed a wastewater reclamation system to recover and reuse onsite all hazardous byproducts from the treatment plant. The remaining residual material from the new treatment plant is a high-quality, 99.7 percent pure sodium sulfate salt East Penn now sells. This innovative design allowed the facility to go beyond government standards and essentially eliminate the discharge of process wastewater. And because a new source of clean water is available to the manufacturing process, East Penn can minimize the use of well water and water-conditioning units in its manufacturing processes.

The new facility has been operating for more than two years. East Penn's project attracted the attention of the commonwealth of Pennsylvania and last year won the Governor's Environmental Excellence Award. Through the elimination of off-site hazardous waste disposal and the sale of salt, the company also is expected to achieve a return on its multimillion dollar investment.

This multiple-technology solution is applicable to many other industries, particularly those that use acids or bases in a manufacturing process, creating a wastewater stream that contains excess metals and dissolved solids. These industries might include non-ferrous metals forming, metal finishing and textiles.

David Wolfe, P. E., is a division manager and Charles Suenkonis, PE., is a senior project manager for Rust Environment & Infrastructure, Harrisburg, Pa., U.S.A., +1 717 796 0336; Daniel Dellicker, PE., is environmental affairs director for East Penn Manufacturing Co., Lyon Station, Pa., U.S.A.

Reprinted from POLLUTION ENGINEERING INTERNATIONAL Winter 1998
©1998 by CAHNERS BUSINESS INFORMATION