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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
6.2.4 Metals Removal
6.2.4.1 Pretreatment
6.2.4.2 Precipitation
6.2.4.3 Flocculation
6.2.4.4 Clarification
6.2.4.5 Metals Removal Process Limitations
Hydroxide precipitation is the standard method of removing heavy metals from wastewater. This is achieved by adjusting the pH of the wastewater with an alkaline reagent to reduce the solubility of the dissolved metals and settling and removing the resultant metal hydroxide precipitates. Seventy-five percent of the respondents to the Users Survey employ this process. The metals removal process is often divided into four steps: (1) pretreatment; (2) precipitation; (3) flocculation; and (4) settling. Each of these steps will be discussed in this subsection, with reference to the methods and chemicals employed by survey respondents.
Prior to metals precipitation, many shops use "pretreatment" processes to affect substances that require special treatment or interfere with subsequent treatments. Common metal complexing compounds which will cause interference are listed in Exhibit 6-8. Some shops combine this step with the precipitation and/or flocculation steps. Exhibit 6-9 (PS 064, PS 116, PS 287) shows several configurations for pretreatment or co-treatment using various chemicals. Approximately 45% of the survey respondents use some form of precipitation aid (not including polymer, which is employed by nearly every shop to aid in flocculation). Chemicals used by survey respondents prior to or during metals precipitation include ferrous sulfate, sodium hydrosulfite, aluminum sulfate, soda ash and sodium dithiocarbamate (DTC). Ferrous sulfate is sometimes added to function as a reducing agent, but more often to provide co-precipitation for the removal of metals from chelated wastewaters. With the latter application, complexed metals are co-precipitated with ferric hydroxides. While being an effective metals removal method, this process generates approximately four times as much sludge as sodium hydroxide alone (ref. 144). Calcium chloride and aluminum sulfate are typically used during a pretreatment step (they are often used together1 because the combined results are better than for either one used alone) for the removal of fluoride, phosphate, silicates and/or emulsified oil (ref. 38).
Precipitation is the process by which dissolved metals are made insoluble, usually as metal hydroxides. With metal finishing wastewaters this is brought about by the addition of alkali treatment reagents. Two alkalis are most commonly used for hydroxide precipitation: sodium hydroxide (i.e., NaOH, or, as it is commonly referred to, caustic soda or simply caustic) and lime (i.e., calcium hydroxide or hydrated lime, Ca(OH)2). Of the survey respondents that have metals precipitation processes, 84% use caustic only, 5% use lime only and 9% use a combination of caustic and lime. Several other treatment reagents are used as either the sole alkali ingredient or as an adjunct to caustic or lime. The other primary alkalis are magnesium hydroxide (used by 4.3% of all respondents that perform metals precipitation, but not as the sole source of alkali) and calcium chloride (used by 11.9% of respondents that perform metals precipitation, but only 2% use it as their sole source of alkali). Other alkalis used by metal finishers are sodium carbonate and sodium bicarbonate (ref. 38). When magnesium hydroxide and calcium chloride are used in conjunction with either caustic or lime, this is sometimes done as a pretreatment step prior to metals precipitation, as discussed previously.
Each alkali has advantages and disadvantages. For example, of the two most common alkalis, hydrated lime has the advantage over caustic soda of lower cost per unit of neutralizing capacity. Also, the metal hydroxide precipitants produced with the use of lime have much faster settling rates because of co-precipitation of calcium solids. Further, the settled sludge from lime treatment is higher in solids content and much more amenable to dewatering. On the other hand, lime takes longer to react in the neutralizer than caustic soda, has a more complicated feed system and, most significantly, it generates a considerably higher mass of sludge solids (ref. 39). Sulfide precipitation, which precipitates metals as sulfides instead of hydroxides, has been found capable of achieving low levels of metal solubility in highly chelated wastestreams. However, sulfide precipitation is not used by any of the respondents to the Users Survey. This process has been proven as an alternative to hydroxide precipitation or as a method for further reducing the dissolved metal concentration in the effluent from a hydroxide precipitation system. Two processes are used for sulfide precipitation: the soluble sulfide process uses sodium sulfide as the treatment reagent, and the insoluble sulfide process uses ferrous sulfide. Both processes generate metal sulfide sludge and the sludge may be less amenable to off-site recovery than hydroxide sludge. An additional drawback of the insoluble sulfide precipitation process is that it generates a significantly larger volume of sludge compared with conventional hydroxide treatment. The large sludge volume is caused by the liberation of ferrous ions from the treatment reagent. These ions are converted to ferrous hydroxide and add to the sludge volume (ref. 39). Additional information and data on treatment reagents can be found in the literature (e.g., ref. 38, 39, 348, and 409).
The metals precipitation process begins with pH adjustment, as previously shown in Exhibit 6-1. The pH is usually adjusted to between 8.5 and 10.0, with 9.2 being the most frequent target. The optimal pH for precipitating metals from a wastestream will depend mostly on the species of regulated metals present. Each metal hydroxide has a characteristic solubility that is dependent on pH. Exhibit 6-10 shows the solubility curves for the hydroxides of commonly regulated metals (metal sulfides have different curves with minimum solubilities at much lower concentrations than hydroxides, ref. 348). The lowest point of each curve corresponds to the pH at which that metal species will be removed to its minimum solubility point. For example, copper will be removed to its minimum concentration if the pH of the precipitation step is held at 9.0. Since the low points of the curves occur at different pHs, it is necessary to make a compromise when selecting a target pH (pH control set-point). Often, the pH control set-point selection process is based on the need to reach a low concentration for a particular metal species (e.g., cadmium) that is regulated to a lower concentration than other species.
Exhibit 6-11 shows the process schematic of a single-stage neutralizer with provisions for adding both acid and base. Since most combined waste streams are acidic when entering the metals removal step, many facilities do not provide for the addition of acid. However, it is useful in instances when caustic has been inadvertently added in excess and therefore is present in most system designs. With a single-stage system, proportional control for reagent addition is required to maintain a reasonably constant effluent pH. The elevation of the reagent storage and gravity feeding through a proportioning valve is one mode of control. Use of variable speed pumps will achieve the same level of control. If incoming wastes are subjected to wide swings in reagent demand, a two-stage neutralization system should be used. The first stage would control at a pH of approximately 6 to 7 (ref. 39).
Required residence volume in the neutralizer depends on the reagent used. Assuming good mixing, a minimum of 15 minutes residence volume is required for sodium hydroxide; with lime as the alkali, a minimum of 30 minutes is required (ref. 39).
The level of residual dissolved metals after pH adjustment depends on the pH control set-point, the mixture of metals in the wastewater, and whether any compounds are present that interfere with metal hydroxide precipitation.
Nearly every metals removal system uses chemical aids to foster particle growth before the wastewater enters the clarifier. By adding coagulating/flocculating agents in slow-mix reactors, the solids in a wastewater can be agglomerated into sturdy, fast-settling particles easily separated in the clarifier. Coagulants/flocculants in broad commercial use include inorganic chemicals such as alum and ferrous sulfate (previously discussed as pretreatments and co-precipitants) and a highly diverse range of organic polyelectrolytes with varying characteristics suitable for different wastewaters. Organic polyelectrolytes are superior to inorganic compounds because both the charge density and valence have been synthetically introduced to the large polymer molecule. The length of the polymer also allows the particles to "knit" together. Normally the appropriate polyelectrolyte is selected by testing a range of different polymers and observing settling behavior after mixing.
Removal of solids by gravity settling (clarification) is the most common method of separating insoluble particles from a waste stream before discharge. Clarification is a relatively simple process that relies on a density difference between the particles and water and the presence of gravity. However, it is often the unit operation of the waste treatment process most subject to upsets. With effluent limits placing strict control on the level of suspended solids in the wastewater, many modifications to the original circular clarifier have resulted from research and development activities. The two most successful approaches include the sludge blanket clarifier and the plate settler. In the sludge blanket unit, the clarifer inlet first passes through a sludge blanket of agglomerated particles. The mixing tends to promote particle growth and reduce the concentration of slow-settling particle fines. The plate settler (Exhibit 6-12) relies on a series of inclined plates between which the wastewater flows in an upward direction. In essence, the particles must only settle a few inches before impinging on the plate surface. The particles then slide down the plate surface to the base of the separator. In a chamber of equal size, plate settlers can provide considerably greater effective settling volume than a conventional clarifier.
Performance of clarifiers varies significantly depending on the type of waste, the design of upstream components, and the design of the clarifier itself. According to EPA, a "properly operating system" has a total suspended solids (TSS) concentration of 50 mg/l or less and the mean concentration of such systems is 16.8 mg/l TSS (ref. 386). A well-operated clarifier will have, at best, 5 to 10 mg/l of suspended solids in the overflow (ref. 477). Frequently, turbidity in the overflow contributes to the metals content of the discharge and makes strict pollutant guidelines difficult to meet. Consequently, many advanced waste treatment systems employ a polishing filter that uses a sand bed or mixed media filter to remove suspended solids not effectively removed by clarification (ref. 39). Another option is the use of membrane filtration, which either supplements or replaces the clarification step (see Section 6.4.4).
Granular media polishing filters are used for the removal of suspended solids in the 5 to 50 mg/l range where an effluent of less than 1 Jackson Turbidity Unit2 (JTU) is desired (ref. 362). These units typically contain graded sand or multimedia such as garnet, ground anthracite and silica sand. The principal solids removal mechanism of these units is straining. When a single medium such as sand is employed, it will classify in the filtration tank according to size, with the smallest grains at the top. When water flows downward through the sand, which was common with older designs, the solids would form a mat on the surface and filtration only occurred in the top few inches. The sand bed was then cleaned by an upward washing of the bed with water or with water and air (ref. 362). A patented alternative design permits upward feed flow and a continuous backwash of the sand bed as described in Exhibit 6-13.
Multimedia filters employ two or more filter media with different grain size and densities. The media are selected such that the smaller particles are the most dense (e.g., garnet with a specific gravity of 4.5 and particle size of 0.2 to 0.4 mm), the medium-sized particles have an average density (e.g., sand grains with a specific gravity of 2.65 and a grain size of 0.5 mm) and the largest particles are the least dense (e.g., anthracite grains with a specific gravity of 1.6 and a grain size of 1.0 mm). When mixed and permitted to settle, the multimedia bed will grade itself according to the density of the material. Therefore, the smallest particles will locate at the bottom and the largest particles at the top. When the feed stream flows from top to bottom, the courser suspended solids will be removed in the upper layers of the filter and the smaller suspended solids near the bottom. Backwashing is performed in the opposite direction.
The results of the Users Survey indicate that 58 respondents (or 19.3%) employ polishing filters.
Some respondents to the Users Survey use alternative methods of removing metal hydroxides rather than using standard clarification. PS 036 and PS 168 pump the entire volume of treated wastewater containing precipitated metals through a filter press (use of the filter press is normally reserved for settled sludge). Both of these shops have wastewater flow rates below 3,000 gpd, which is the reason they are able to use this method of separation. As indicated by Roy (ref. 38), when direct filtration is used, the gelatinous character and the shear bulk of solids results in premature blinding or blockage of the filter, causing minimal hydraulic throughput. One respondent to the survey uses dissolved air flotation (DAF) rather than settling (PS 240). A DAF unit generates fine air bubbles that attach to the hydroxide particles and cause them to rise to the surface of a tank, where they are removed by skimming. It is a treatment technology commonly used by industries with oily wastestreams (e.g., petroleum refining and industrial laundries). Several shops use membrane filtration to separate precipitated metals. This method of treatment is a viable alternative to conventional clarification for plating shops, but currently is more common to the printed circuit board industry. Microfiltration is discussed in Section 6.5.3.
6.2.4.5 Metals Removal Process Limitations
As a summary for the conventional metals removal subsection, the following limitations of the hydroxide precipitation/clarification process are presented:
The process cannot precipitate metals to low levels of solubility in the presence of chelating compounds unless additional treatment reagents are employed. The use of additional treatment reagents may significantly add to the volume of sludge generated by the process.
The precipitation/clarification process cannot remove suspended solids below approximately 5 to 10 mg/l TSS and TSS can range up to 50 mg/l for a well operated system unless polishing filtration is employed. Because the suspended solids remaining in the effluent contain metals, this factor limits the attainable effluent concentration for metals.
Certain metal hydroxides are amphoteric; that is, solubility is at a minimum at a specified pH and increases if pH is higher or lower. The solubility minimum for different metals occurs at different pH levels (ref. 39). Therefore, a compromise is necessary with mixed metal wastes that limits attainable effluent concentrations.
The metal hydroxide sludge resulting from treatment of electroplating wastewater has been classified as a hazardous waste (F006) and must be managed, handled, transported, and recovered or disposed of under the provisions of RCRA.