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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.5 ALTERNATIVE TECHNOLOGIES FOR METALS REMOVAL
6.5.3 Membrane Filtration
6.5.3.1 Overview
6.5.3.2 Development and Commercialization
6.5.3.3 Applications and Restrictions
6.5.3.4 Technology/Equipment Description
6.5.3.5 Costs
6.5.3.5.1 Capital Costs
6.5.3.5.2 Operating Costs
6.5.3.6 Performance Experience
6.5.3.7 Operational and Maintenance Problems
6.5.3.8 Residuals Generation
s65316.5.3.1 Overview
Membrane filtration, which includes microfiltration and ultrafiltration (see definitions in Section 4) is applied as an alternative technology to conventional precipitation/clarification or as a polishing technology. When used as an alternative technology to conventional precipitation/clarification the membrane filter separates treated wastewater from precipitated metals and concentrates the precipitated metals to approximately 2 to 5% solids. When used as a polishing technology, it typically follows a conventional clarifier and it removes residual solids and organics from the wastewater. The polished effluent can be discharged and will usually meet strict limitations, or it can be reused as rinse water. The membrane filtration technology is used by four (or 1.3%) of the respondents to the Users Survey.
Membrane filtration equipment is typically more capital intensive than its conventional treatment equipment counterparts. However, membrane filtration has found use in several specific areas where the conventional technology lacks sufficient performance ability. These applications include: (1) where a shop is required to meet effluent limitations significantly below the Federal standards; (2) at printed circuit board shops where metal complexing agents are heavily used; and (3) for meeting organic parameter discharge limitations such as biochemical oxygen demand (BOD). The first two of these applications were identified from the Users Survey and are believed to be the most frequently used purposes for this technology by the metal finishing industry. The third application was identified from the literature which referenced a microfiltration installation at an industrial laundry facility (ref. 414). Presumably, similar applications are present within the metal finishing industry.
Of the four respondents using this technology (PS 007, PS 105, PS 233, and PS 257), three respondents used microfiltration as a replacement for conventional precipitation/clarification and one used it as part of a complex zero-discharge configuration involving conventional precipitation, followed by ultrafiltration and reverse osmosis, with vacuum evaporation applied to the concentrate discharged from the ultrafiltration unit (PS 233). Of the remaining three shops, two had discharge limitations below the Federal limits and one had CFR 433 standards, but indicated that they had difficulty meeting the nickel limitation (PS 105). One of the two shops with low limitations was a direct discharger (PS 007). Three of the shops were job shops performing common metals plating and one was a captive printed circuit board manufacturer (PS 257).
An end-of-pipe treatment system employing microfiltration is similar to a conventional hydroxide precipitation system with the exception that the clarifier/thickener is replaced by the microfiltration unit. Also, since microfiltration systems do not depend on the ability of the precipitated metals to settle, a polymer addition/flocculation step is not present. In fact, polymers cannot be used with membrane filtration because they will foul the membrane.
Membrane filtration is capable of producing an effluent with a much lower suspended solids concentration than conventional precipitation/clarification. This is an important consideration for shops trying to achieve low metals limitations. Data from the literature (Exhibit 6-30) indicate that the contribution of TSS to metal discharges can be significant and will exceed the soluble contribution at a TSS level as low as 1 mg/l. This indicates that, for plants needing to meet low limitations, the TSS metals fraction is a bigger and better target than the soluble fraction for treated wastewaters. When complexed metals are present and conventional hydroxide precipitation is employed, TSS will make-up a smaller fraction of the total metals concentration than that shown in these data. In such cases, or even in the absence of complexing chemicals, the effect of the membrane technology is enhanced by using additional treatment reagents. Reagents that are commonly used to enhance metals removal with membrane systems include: calcium chloride, sodium sulfide, ferrous sulfate and DTC.
6.5.3.2 Development and Commercialization
The development and commercialization of membrane filtration is reviewed in Section 4.3. The literature indicates that in 1988, more than 100 full-scale industrial end-of-pipe systems had been installed by Memtek, the leading manufacturing company of this technology. These systems ranged in capacity from 10 to 400 gpm and were installed in facilities in the following industries: electroplating, printed circuit board manufacturing, battery manufacturing and photographic processing. Projecting the results from the Users Survey to the entire electroplating population, the expected number of systems would be approximately 170 (based on 4 of 318 respondents with systems and a total U.S. plating population of 13,500 plants).
6.5.3.3 Applications and Restrictions
The general applications for this technology were discussed in Section 6.5.3.1. Exhibit 6-31 shows a generic EOP microfiltration system that could substitute for conventional treatment. For this application, the process chemistry will vary depending on the type and concentration of the constitutes in the feed stream and the target effluent levels. As discussed in Section 6.5.3.1, additional reagents may be used to enhance the precipitation of metals. Membrane system performance data from the literature, for this type of application, are presented in Exhibit 6-32.
The presence of oil and grease in the feed stream can cause fouling of the membrane. If present, additional pretreatment may be necessary. Most systems are installed with integral chemical cleaning systems that handle routine cleaning requirements. These systems are typically utilized once per week for a period of two hours (ref. 348).
6.5.3.4 Technology/Equipment Description
This subsection contains a description of commercially available microfiltration equipment that is manufactured and/or sold by vendor survey respondents. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.
Memtek Corporation manufactures complete end-of-pipe wastewater treatment systems that employ microfiltration for solids separation. These are modular systems to which various reactor skids (e.g., chromium reduction and cyanide oxidation) can be added, depending on the characteristics of the wastewater and the treatment requirements. For metals removal, precipitation is performed in a reactor/recirculation tank and the mixture of particles and water are pumped through the membrane filtration tubes at 40 to 50 psi and back to the reactor/recirculation tank. The filtrate passes through the pores of the filter to a final neutralization tank. Concentrated solids (2% to 5%) are periodically drawn from the reactor/recirculation tank to a sludge holding tank and subsequently dewatered.
The Memtek filtration membranes are contained in one-inch diameter porous plastic tubes. The membrane has uniform 0.1 micron pores that are funnel-shaped to minimize clogging. Filtration rates range up to 200 GFD. The membrane is chemically resistant to both acid and bleach cleaning solutions (ref. Memtek file). Every system includes a cleaning loop, consisting of a pump, tanks and the necessary piping and valving to permit in-place cleaning of the modules.
Capital costs for end-of-pipe membrane filtration systems are presented in Exhibit 6-33. These systems include a two-tank reactor skid for pretreatment, filtration reactor skid and final pH adjustment skid. Installation costs (38% over basic equipment costs) include: engineering (10%), shipping (5%), piping (8%) and electrical/instrumentation (15%).
Major operating costs for end-of-pipe systems that employ membrane filtration for solids separation are presented in Exhibit 6-33. The labor and chemical costs are based on the results from the Users Survey. Vendor data indicate that lower chemical costs may be experienced. Data presented by Memtek show chemical costs of $2.26 to $4.95 per 1,000 gal for two EOP applications (ref. Memtek file).
6.5.3.6 Performance Experience
Available information and data from the Users Survey is presented in Exhibit 6-34. Available diagrams of the respondentsí treatment systems are presented in Exhibits 6-35, 6-36 and 6-37.
Generally, plating shops using this technology were satisfied with its performance. The average satisfaction level of the four respondents is 3.75. Two of the shops indicated that failure of their treatment system resulted in a compliance excursion.
6.5.3.7 Operational and Maintenance Problems
Only one of the four respondents to the Users Survey that employ microfiltration for an EOP application reported any operational and maintenance problems. PS 105 reported that a rupture of their membrane occurred that resulted in a compliance excursion.
Membrane filtration, when used in an end-of-pipe treatment scheme, separates precipitated metals from the treated wastewater. The solids are subsequently dewatered and transported off-site as a hazardous waste (F006 or F019) for recovery or disposal. The mass (as dry solids) and characteristics of the sludge and its potential for recovery will depend on the characteristics of the original wastestream and the type of chemical reagents used in treatment. Membrane filtration, used as a substitute for clarification/thickening can be operated with conventional hydroxide precipitation reagents (e.g., caustic soda or lime). More often, due to the fact that membrane filtration is usually installed to meet stringent effluent limitations that cannot be attained with conventional clarification/thickening, additional treatment reagents are used to enhance the precipitation of metals, including the fraction that is normally soluble with the use of lime or caustic soda alone.
As shown in Exhibits 6-35 and 6-36, the two shops employing microfiltration as a substitute for clarification/thickening used additional reagents to supplement pre-cipitation. These additional reagents included ditho-carbamates (DTC) and lime. Ferrous sulfate and sodium sulfide, although not used by any of the respondents, are other reagents that are commonly used to enhance precipitation. The use of additional reagents, especially ferrous sulfate, will add to the mass of sludge generated by the treatment process. The use of sulfur based compounds will result in the generation of sulfide sludges and may reduce the potential of using off-site recycle as a disposal option.