xvEPA United States Environmental Protection Agency EPA/540/S-93>!504 September 1993 SUPERFUND INNOVATIVE TECHNOLOGY EVALUATION Emerging Technology Summary Electro-Pure Alternating Current Electrocoagulation Naomi P. Barkley, Clifton Farrell, and Trade Williams The Superfund Innovative Technol- ogy Evaluation (SITE) Program was au- thorized as part of the 1986 amend- ments to the Superfund legislation. It represents a joint effort between the U.S. Environmental Protection Agency's (EPA) Office of Research and Develop- ment and Office of Solid Waste and Emergency Response. The program is designed to assist and encourage the development of waste treatment tech- nologies that would contribute innova- tive solutions to our hazardous waste problems. Under the Emerging Tech- nology portion of the SITE Program, a 2-yr research effort was conducted by Electro-Pure Systems, Inc., to evaluate the technical and economic feasibility of alternating current electrocoagulation (ACE) for remediation of aqueous waste streams at Superfund sites. The ACE Technology introduces low concentrations of nontoxic aluminum hydroxide species into the aqueous media by the electrochemical dissolu- tion of aluminum-containing electrodes or pellets. The aluminum species that are produced neutralize the electrostatic charges on suspended material and/or prompt the coprecipitation of certain soluble ionic species, and thereby fa- cilitate their removal. Electrocoagulation has been demon- strated to enhance the filtration and de- watering rates for solids removed from an effluent; such enhancements are prompted by growth in the mean par- ticle size from typically <0.3 p.m in di- ameter to as much as 150 |xm, depend- ing on the degree of electrocoagulation. Significant reductions in the total sus- pended solids (TSS) loading of particu- late slurries and in the concentrations of metals (lead, copper, zinc, chromium), fluorides, and phosphates from aque- ous streams can be achieved under cer- tain pH conditions. Treatment does not entail adding chemicals (polymers, metal salts, polyelectrolytes) whose ac- cumulation might inhibit reuse of the effluent as process water. Rather, the insoluble aluminum hydroxide result- ing from electrocoagulation may be re- moved by standard filtration practices. This Summary was developed by EPA's Risk Reduction Engineering Laboratory, Cincinnati, OH, to announce key findings of the SITE Emerging Tech- nology Program that is fully docu- mented in a journal article (see order- ing information at back). Introduction Chemical coagulation has been used for decades to destabilize colloidal sus- pensions and to effect precipitation of soluble metal species as well as* other inorganic species from aqueous streams, Printed on Recycled Paper ------- thereby permitting their removal through sedimentation or filtration. Alum, lime, and/or polymers have been the chemical coagulants used. These processes, how- ever, tend to generate large vplumes of sludge with a high bound-water content that can be slow to filter and difficult to dewater. These treatment processes also tend to increase the total dissolved sol- ids content of the effluent, making it un- acceptable for reuse within industrial ap- plications. The ACE Technology was originally developed in the early 1980s to break stable aqueous suspensions of clays and coal lines produced in the mining indus- try. Traditionally, these effluents were treated with conventional techniques that made use of organic polymers and inor- ganic salts to agglomerate and enhance the removal of the suspended materials. This ACE Technology was developed to simplify effluent treatment, realize cost savings, and facilitate recovery of fine- grained coal. ACE Technology is based upon colloi- dal chemistry principles-principles using alternating electrical power and electro- phoretfc metal hydroxide coagulation. The basic mechanism for the technology is electroflocculation wherein small quanti- ties (generally <30 mg/L) of aluminum hydroxide species are introduced into so- lution to facilitate flocculation. Electrofloc- culation causes an effect similar to that produced by the addition of chemical co- agulants such as aluminum or ferric sul- fate. These cationic salts destabilize col- loidal suspensions by neutralizing nega- tive charges associated with these par- ticles at neutral or alkaline pHs. This enables the particles to come together closely enough to agglomerate under the influence of van der Waals attractive forces. See Figure 1 for the ACE basic process flow. Although the electroflocculation mecha- nism resembles chemical coagulation in that cationic species are responsible for the neutralization of surface charges, the characteristics of the electrocoagulated floe differ dramatically from those gener- ated by chemical coagulation. An electrocoagulated floe tends to contain less bound water, is more shear resis- tant, and is more readily filterable. Application of an AC electric field to the electrodes induces dissolution of the aluminum and formation of the polymeric hydroxide species. Charge neutralization and particle growth are initiated within the electrocoagulation cells and continue following discharge of the aqueous me- dium from the apparatus. (In this way, product separation into solids, water, and oils may be achieved.) The ACE Technology was tested us- ing two designs of the ACE Separator™: (1) a parallel electrode unit in which a series of vertically oriented aluminum electrodes form a series of monopolar electrolytic cells through which the efflu- ent passes and (2) a fluidized bed unit with nonconductive cylinders equipped with nonconsumable metal electrodes be- tween which a turbulent fluidized bed of aluminum alloy pellets is maintained. In the fluidized bed unit, introduction of com- pressed air into the electrocoagulation cells assists in maintaining the turbulent fluidized bed and in enhancing the alu- minum dissolution efficiency by increas- ing the anodic surface area. Typically, the fluidized bed unit dissolves alumi- num at least one order of magnitude more efficiently than does the parallel electrode unit. Gas Outlet Influent Liquid Figure 1. Schematic of an ACE Separator* used in alternating current electrocoagulation. ------- Electrocoagulation operating conditions are highly dependent on the chemistry of the aqueous medium; especially conduc- tivity. Other influent characteristics such as pH, particle size, and chemical con- stituent concentrations will influence op- erating conditions. Treatment generally requires application of low voltage (<150 VAC) to the electrocoagulation cell elec- trodes; current usage is typically 1 to 5 amp-min/L. The flow rate of the aqueous medium through an electrocoagulation cell depends on the solution chemistry (conductivity), the nature of the entrained suspension or emulsion, and the extent of electrocoagulation required to achieve the treatment objective. Retention times as short as 5 sec are sometimes sufficient to break a suspension. Electrocoagulation may be accomplished in a single pass or multiple passes (recycle mode). In the fluidized bed unit, a mechanical scrub- bing action is created within the electrocoagulation cell—an action that re- duces buildup of impermeable oxide coat- ings on the aluminum pellets and the inherent loss of efficiency that would re- sult. Depending on system configuration, maintenance of the apparatus is limited to periodic replenishment of the alumi- num fluidized bed material and/or elec- trodes. For most applications, pellets for the fluidized bed unit can be produced from recycled aluminum scrap or bever- age containers. Where sludge reclama- tion is the objective, however, the use of higher quality pellets is required to re- duce the introduction of impurities into the sludge. This summary describes the research effort associated with bench- and pilot- scale testing of various surrogate waste- waters for determining the optimum op- erating conditions, treatment effective- ness, and cost of treatment. Experimental EPA's SITE Program research entailed testing the ACE Technology (in both the parallel electrode and fluidized bed sys- tem configurations) on various surrogate wastes containing emulsified diesel fuel, metals, and clays. The wastes were pre- pared to resemble those of leaking, un- derground storage tanks and soil wash- ing operations. The primary testing ob- jective of such testing was to establish optimum operating conditions for the ACE Separator™ to break the oil/water emul- sion and achieve reductions in clay sus- pended solids and soluble metal pollut- ant loadings. Experiments were con- ducted on'surrogate wastes prepared by mixing 0.2 to 3.0 wt. % of the -230 mesh (clay and silt) fraction of the EPA's Syn- thetic Soil Matrix (SSM) with 0.5 to 1.5 wt. % Number 2 diesel fuel, with 0.05 to 0.10 wt. % of an emulsifier (Titon-100X* or Alconox soap), and with from 10 mg/L to 100 mg/L of one or more of the follow- ing contaminants: copper, nickel, zinc, orthophosphate, or fluoride. The pH of each surrogate mixture was adjusted with either sodium hydroxide or calcium ox- ide to the desired value (5, 7, or 9) and the conductivity raised to roughly 1200 microSiemens per centimeter (u.S/cm) to 1500 p,S/cm with sodium chloride to simu- late values expected in nature. Initially, bench-scale electroco- agulation experiments using the parallel electrode unit were conducted on five aqueous-based systems that included a metals mixture, a clay suspension, a die- sel fuel emulsion, a soluble organic solu- tion, and a diesel fuel/soluble organic emulsion. The operating conditions de- termined during the Year One SITE work effort were used as the conditions for these tests. Optimum treatment time was established by examining the trend in contaminant loadings as a function of treatment time. To compare the results with conventional treatment processes, aliquots of each surrogate stock solution were treated with alum. Sufficient alum was added to give the aluminum equivalent to that introduced in the electrocoagulation experiments. During the Year Two investigations, operating difficulties, persistent electrode coating and fouling, and low efficiencies of aluminum generation prompted includ- ing the alternative, fluidized-bed, electrocoagulation cell design in the re- maining bench- and pilot-scale testing program. Three phases of laboratory ex- periments were undertaken to evaluate both electrocoagulation units: (1) prelimi- nary screening experiments to demon- strate the feasibility of reducing the con- centration of each metal, (2) matrix ex- periments to define the most opportune retention time and current (or current den- sity), and (3) optimization experiments to define other ACE Separator™ operating parameters to achieve the most cost- effective removaj conditions. The pH was adjusted to 5, 7, or 9 and the conductiv- ity raised to approximately 1200 |j,S/cm with sodium chloride. The conductivity of some surrogate wastes was increased to approximately 3000 u.S/cm and subjected * Mention of trade names or commercial products does not constitute endorsement or recommendation for to electrocoagulation. Surrogate wastes subjected to these experiments included the five aqueous systems described above as well as surrogate wastes con- taining individual constituents such as nickel, zinc, copper, fluoride, and phos- phate. Pilot-scale tests were performed by us- ing both the parallel and fluidized bed configurations of the ACE Separator™. A 12-hr experiment using the ACE Fluid- ized Bed Separator™ was conducted on 208-L batches of surrogate waste solu- tion containing 0.2 wt % SSM fines, 0.5 wt % diesel fuel, 0.05 wt % Alconox surfactant, and 10 mg/L each of Cu2+, Zn2+, PO43~, F", and Ni2*, and whose con- ductivity and pH had been raised to 1200 |j.S/cm and 7, respectively. This surro- gate was recycled through a 4-in.- diam- eter, Schedule 80, PVC pipe, 24-in.-high pilot-scale ACE Separator™, which was equipped with two Type 316 stainless- steel electrodes (24 in. high, 2.5 in. wide) and whose interior was filled with 8 to +16 mesh aluminum pellets. The unit was powered at a constant 20 amp, and the voltage was allowed to vary as the electrocoagulation treatment progressed over the 12-hr period. In this experiment, the flow of the surrogate solution through the ACE Separator™ was varied from 1 to 6 gpm and the quantity of compressed air introduced into the solution feed line ranged up to a maximum of 10 psig. Samples of the surrogate solution were collected at various times throughout the experiment to document the rate of alu- minum ion generation and the reductions in /concentration of the metal contami- nants, chemical oxygen demand (COD), and TSS. In a similar pilot-scale test using the parallel plate unit, the surrogate waste was composed of essentially the same constituents as that for the fluidized bed experiment. The notable changes were that the conductivity of the solution was increased to approximately 3,000 |xS/cm and no fluoride salt was added. The other operating parameters were based on re- sults obtained from the bench-scale tests. The aluminum generation and consump- tion rates and the electrical power re- quired to effect acceptable phase sepa- ration as well as contaminant reductions were monitored. Throughout the various phases of the experimental program, samples of the treated effluent were collected and al- lowed to settle for 30 min. The supernate was removed and analyzed. The subnate, containing the settled floe, was filtered and the filtrate and filter cake analyzed. ------- Results and Discussion Optimum Operating Conditions Optimum operating conditions for the parallel electrode unit were developed from these studies (Table 1). These con- ditions served as the basis for the sub- sequent pilot-scale tests. Optimum operating conditions for par- allel electrode unit as a result of bench- scale tests were electrode spacing 0.5 in.; current, 4 amps; retention time, 3 to 5 m!n; frequency, 10 hertz; and fully sub- merged. Similarly, based on the bench-scale testing of the fluidized bed design, the optimum operating conditions established were electrode spacing, 1-in.; aluminum pellets size, 8 to +16 mesh; and, current, 20 amps. Treatment Effectiveness Based upon bench-scale experiments conducted on the EPA surrogate wastes, the following summarizes the findings: • When compared with alum treatment, etedrocoagulation provided approximately 83% less sludge volume arid a 76% improvement in filtration rate • For the fluidized bed configuration, aluminum or stainless steel may be used as electrode material in the system, with comparable results • With both increased frequency for the AC and increased retention time, the agglomerated particles tend to disaggregate. Pilot-scale tests were conducted with the use of both the parallel and fluidized bed configurations of the ACE Separator™ on a 3% soil slurry contain- ing roughly 50% clays, 1.5% diesel fuel, and 0.1% of a strong surfactant. Electrocoagulation reduced the TSS (222 mg/L to 4.5 mg/L) and total organic car- bon (TOO) (130 mg/L to 6.6 mg/L). Cop- per was reduced by 72%, cadmium by 70%, chromium by 92%, and lead by 88%. No appreciable change in total sol- ids (TS) loadings in the supernate re- sulted from electrocoagulation. Table 1. OptimumOperatingCondltionsforPar- a/fe/ Electrode Unit Based on Bench- Scale Tests Parameter Value Current Electrode Spacing Retention Time Frequency Submergence 4 Ampere 0.5 Inches 3 to 5 Minutes 10 Hertz Fully Submerged Particle size was enhanced by the clay fraction as a result of electrocoagulation. The mean particle diameters of the ACE Separator™ treated particulates, both in the supernate and the filtrate (188 u,m and 230 u.m, respectively), increased by a factor of approximately 85 and 105, respectively, over that in the original slurry (2.2 u.m). Data obtained from the 12-hr, pilot- scale, fluidized bed test showed that af- ter 30 min of treatment, more than 90% of the metals and phosphates were re- moved. Aluminum generation rates were highest when the throughput flow rate was <4 gpm. This upper flow limit may reflect compaction of the fluidized bed aluminum pellets against the upper screen of the electrocoagulation cell, thus placing them out of the range of the electrodes. As the emulsion is destabi- lized, the surrogate solution most likely becomes less resistive to ion mobility and, thereby, improves the operational efficiency of the ACE Separator™. Filtration time for solids coagulated from particulate suspensions and oily emulsions by electrocoagulation is much less than for solids formed by chemical coagulant addition. Slurries tested were treated with alum addition and with an ACE Separator™. Electrocoagulation im- proved the filtration rate of titanium ox- ide by 63%. Other examples (for an oily emulsion and for biological sludge) indi- cate highly enhanced filtration rates for ACE Separator™ treated waste streams when compared with those of either un- treated or alum-treated waste streams. Shear strength of an electroco- agulation floe is generally much greater than the shear strength of an alum floe. Both sonic treatments (used to evaluate the structural integrity of the floe) and actual filtration tests demonstrated higher shear strength of the electrocoagulation floes. Electrocoagulation of metal and phos- phate-bearing industrial solutions indi- cates excellent nickel, copper, and phos- phate concentration reductions. More than 90% (concentration basis) of phos- phate and copper can be removed from such solutions at low aluminum and elec- trical power requirements. Reduction in the nickel concentration varies between 75% and 85% (concentration basis). Electrocoagulation of synthetic labora- tory solutions and industrial wastewater also confirmed the feasibility of utilizing electrocoagulation for phosphate removal. Treatment of effluent from a commercial laundry reduced the phosphate concen- tration from 45 mg/L to 5.4 mg/L after low-intensity electrocoagulation (0.36 kW, 0.75-min retention time). Electrocoagulation of process water from a phosphate min- ing operation reduced the phosphate level by 91%, from 160 mg/L to 14 mg/L (3.3 kW, 0.17 min). Finally, treatment of di- lute phosphoric acid solutions with a nominal 100 mg/L total phosphate con- centration and a conductivity of approxi- mately 2000 u.S/cm resulted in >95% re- ductions in soluble phosphate over a range of acidities. Capital and Operating Costs As part of this research effort, pro- jected treatment cost estimates were de- veloped. Overall treatment operating costs (electricity, aluminum pellets, op- eration, and maintenance) will vary up- wards from $0.50/1000 gal, depending on emulsion strength, unwanted compo- nent concentration(s) (for example, emul- sifiers) in the effluent, and its TSS. (It should be noted that, at the outset, we established that operating conditions ex- ceeding $3.00/1000 gal would not be competitive with conventional treatment processes, and thus, they were elimi- nated from future testing consideration.) Estimates are based on bench- and pi- lot-scale testing, additional considerations may be involved in full scale operation. Operator supervision and maintenance would be limited to periodic replenish- ment of the aluminum pellets, chemical pretreatment systems (for example, salt addition for conductivity enhancement), and electrode replacement. Estimated operating costs are based on laboratory and limited pilot-scale testing of efflu- ents, and currently these costs exceed those for comparable traditional chemi- cal treatment (alum or polyelectrolytes). The lower maintenance and operator su- pervision required for ACE Separator™ operation and the capability to use ACE Separator™ treated water in closed-loop, zero-discharge applications adds to its attractiveness. Successful commercializa- tion of the technology requires further research to significantly improve alumi- num dissolution efficiency. If the ACE Separator™ can be engineered to regu- larly generate sufficiently high aluminum dissolution concentrations, the technol- ogy may be applicable to industrial efflu- ent treatment trains, as well as for some Superfund site remediation activities. Incorporation of an automated process control system based on influent conduc- tivity and solids loading, or on the dis- charge solution's supernate turbidity, needs ------- to be engineered. An automated system for addition of replacement aluminum pel- lets may be justified, a'tthough the frequency of pellet addition should be predictable after an initial period of operation. The capital cost for a standard ACE Separa- tor™ with nominal throughput capacity of 50 gpm is estimated at $80,000 and for a 250-gpm unit, $300,000. Conclusions ACE offers a promising alternative for treat- ing waste streams containing clays, certain metal constituents, and other soluble pollut- ants. As an alternative to chemical condition- ing, ACE Technology agglomerates the par- ticles without adding any extraneous soluble species (i.e., SO42'); the sludge it produces has a lower bound-water content that will filter more rapidly and has a lower susceptibility to filter shear of the coagulated particulates and emulsion droplets. As a result of the pilot-scale efforts, we compared the effectiveness of ACE Separator™ treatment, alum addition, and polymer coagulation. Conclusions drawn from this comparison for various contami- nant parameters were: • TSS: ACE Separator™ treatment and the polymer treatment yielded equivalent results for the reduction of TSS in the treated supernates. TSS values for alum treatment were four to five times greater than those for ACE Separator™ treatment or polymer treatment. • COD: ACE Separator™ treatment resulted in the highest COD reductions of the three methods. Removal efficiency for COD was from two to four times higher than removal efficiency for either alum treatment or polymer treatment. Lead: ACE Separator™ treatment achieved approximately 6(3% removal of lead in the high metals runs, whereas polymer treatment showed a slightly higher removal (71%). Because some difficulties were experienced with the alum treatment, these test results were invalidated. Further ACE Separator™ treatments of slurries with low concentrations of metals yielded the highest lead removal (96%). • Copper: Copper in the supernate achieved dramatic removal by electrocoagulation in both the high (90% reduction) and low (99% reduction) metals concentration experiments. In the former, however, polymer and alum addition achieved greater removal (approximately 100% reduction). • Chromium: ACE Separator™ treat-ment resulted in good removal for total chromium (87% and 94% reductions for the high and low concentrations). Alum and polymer addition accomplished similar removal. • Cadmium: Cadmium levels in the supernates dropped as a result of ACE Separator™ treatment: 14% in the high metals runs and 99% in the low metals tests. The inconsistency between these two sets of experiments, as well as the high concentrations remaining in the supernates and filtrates, raises questions about the accuracy of experimental results involving high concentrations of metals. For the low metals tests, the cadmium concentrations in both ACE Separator™ filtrates were much lower than were the concentrations for either alum or polymer treatment. The following generalization on the ef- fectiveness of the ACE treatment can be made: • ACE Separator™ treatment con- sistently reduced the TS and TSS loadings to an equivalent degree and to approximately one-quarter the level achieved through alum addition; and • Better reductions in soluble metal concentrations are achieved with electrocoagulation treatment than with alum treatment. In summary, ACE offers a promising technically simple method for achieving solids-liquid separations in aqueous- based waste streams. The majority of the nontoxic, aluminum ionic species in- troduced will be removed in the coagu- lated solids phase. The ACE Technology may be particularly suitable for zero-dis- charge applications in which the addition of chemicals and the buildup of residual concentrations (dissolved solids) would adversely affect effluent quality or inhibit effluent reuse. Other potential applica- tions of the ACE Separator™ include: (1) remediation of groundwater and leachates (metals, COD/BOD removal), (2) enhancement of clay separation from aqueous suspensions/emulsions result- ing from soil-washing operations, (3) breakage of oil/water emulsions produced in the pumping of hydrocarbon contami- nated groundwater, and (4) removal of TSS from stormwater runoff. Possible in- dustrial applications are fine-grained product recovery (pigments, PVC) and extraction of TSS from waste streams that contribute to high BOD and COD loadings. l.S. GOVERNMENT PRINTING OFFICE: 1993 - 750-071/80064 ------- ------- ------- Naomi P. Barkleyfalso the EPA Project Officer, see below) is with the Risk Reduction Engineering Laboratory, Cincinnati, OH 45268; Clifton Farrell is with Electro-Pure Systems, Inc., Amherst, NY 14228; and Trade Williams, a U.S. Environmental Protection Agency research apprentice, is with the University of Cincinnati, Cincinnati, OH 45221. Details of the completed SITE Emerging Technology project are given in a journal article published in Air and Waste, Vol. 43, No. 5, p. 784-789, May 1993. The journal article entitled "Alternating Current Electrocoagulation for Superfund Site Remediation," (Order No. PB93-20S 144; Cost: $12.50, subject to change) is also available from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Risk Reduction Engineering Laboratory U.S. Environmental Protection Agency Cincinnati, Ohio 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 Official Business Penalty for Private Use $300 EPA/540/S-93/504 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 ------- |