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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
6.2.5 Sludge Dewatering
Dewatering is primarily a physical operation that separates the liquid and solid portions of the dilute sludge generated during the precipitation/clarification process. The resulting material is a stiff fluid. Dewatering is usually performed in a series of steps utilizing two or more pieces of equipment, with each subsequent step reducing the percentage of liquid in the sludge as previously shown in Exhibit 6-1.
In the treatment of metal-bearing wastes, the dewatering process begins once the dilute sludge is removed from the clarifier. The sludge from the clarifier of most precipitation processes will have a solids concentration of 0.5 to 3 percent (ref. 39). Further sludge concentration is first accomplished by the use of thickening equipment which will increase the solids content to between 2 and 5 percent. Sludges are thickened primarily to decrease the capital and operating costs of subsequent sludge processing steps by substantially reducing the volume. There are three general methods of thickening:
- Gravity Thickening
- Flotation Thickening
- Centrifugal Thickening
Gravity thickening is by far the most commonly used method in the metal finishing industry. It can be achieved in a separate tank or within the clarifier, if it is so designed (ref. 331). Thickening within the clarifier is achieved at the lowest part of the clarifier within a sludge storage zone or hopper. Within the hopper, the sludge is slowly mixed with a motorized rack to enhance the release of water. Often, a secondary tank is used to supplement or replace the thickening zone of the clarifier, especially when extended thickening times are required. These units are typically designed to store the accumulated solids for at least 24 hours. Chemicals are sometimes added to aid the thickening process (e.g., iron and aluminum salts, polymers).
The main design variables to be considered in selecting a thickening process are:
- Solids concentration and volumetric flow rate of the feed system
- Chemical demand and cost if chemicals are employed
- Suspended and dissolved solids concentrations and volumetric flow rate of the clarified stream
- Solids concentration and volumetric flow rate of the thickened sludge.
Other variables that impact the selection of a thickening process are: subsequent processing steps; operation and maintenance (O/M) cost; and the reliability required to meet successful operational requirements. Detailed selection, design and operating parameters for thickening devices can be found in the literature (e.g., ref. 331).
After thickening, further dewatering of the sludge is achieved through the use of mechanical dewatering and thermal dehydration devices. The most widely used mechanical dewatering device is the filter press. Other technologies employed in the metal finishing industry include vacuum filters (used by only two respondents), centrifuges and belt presses (latter two not presently used by any of the respondents). Mechanical dewatering will produce a sludge with an approximately 10 to 60 percent solids content. Sludge dryers (Section 6.2.6) are used to further remove moisture from the sludge and are capable of producing a material with a 90 percent solids content. Increasing the solids content of the sludge greatly reduces its volume, which in turn reduces transportation costs and often disposal/recovery costs. The following discussion focuses on the filter press.
The filter press has a number of advantages over other filtration equipment such as vacuum filters and centrifuges. Filter presses can operate well at variable or low feed solids conditions. They can also produce a relatively dry cake because of the high pressure differential they can exert on the sludge. A typical filter press operating at 100 psig will produce a sludge with a solids content of 25 to 60 percent solids, depending on the chemicals used for precipitation. As a comparison, the basket centrifuge produces a sludge with a solids content of 10 to 25 percent and the vacuum filter produces a sludge with 15 to 40 percent solids (ref. 395).
The disadvantages of the filter press include its batch operating cycle, the labor associated with removing the cakes from the press, and the downtime associated with finding and replacing worn or damaged filter cloths.
The original filter press design consisted of alternating plates and frames and these types of units were referred to as the plate-and-frame filter press. The modern technology is the recessed plate filter press (see Exhibit 6-14). The plates (usually constructed of polypropylene) are recessed on each side to form cavities and they are covered with a filter cloth. The two types of presses work in basically the same manner. At the start of a cycle, a hydraulic pump clamps the plates tightly together and a feed pump forces a dilute sludge slurry into the cavities of the plates. The liquid (filtrate) escapes through the filter cloth and grooves molded into the plates and is transported by the pressure of the feed pump to a discharge port. The solids are retained by the cloth and remain in the cavities. This process continues until the cavities are packed with sludge solids. The hydraulic pressure is then released and the plates are separated. The sludge solids or cake is loosened from the cavities and falls into a hopper or drum.
Under normal to ideal conditions, a filter press subjected to a 100 psi pumping pressure will produce a cake with a dryness of approximately 25 to 40 percent solids for caustic soda precipitated metal hydroxides and 35 to 60 percent solids for lime precipitated hydroxides (ref. 410). The level of dryness attained depends heavily on the length of the drying cycle. Altmayer suggests that most shops only achieve 20 to 25% dry solids and that to reach higher dryness levels requires a sludge dehydration unit (see Section 6.2.6) (ref. 482).
Modern recessed plate filter presses can be specified with the following design enhancements (ref. JWI file):
- Lightweight polypropylene plates that exhibit good chemical resistance and provide a long service life.
- Gasketed plates that reduce leakage during the filtration cycle. These replace non-gasketed types where the filter cloth extends beyond the plate to form the seal between plates.
- An air blow-down manifold that is employed at the end of the filtration cycle to drain remaining liquid in the system, thereby improving sludge dryness and aiding in the release of the cake.
- Microprocessor control which permits unattended operation throughout the filtration cycle. Capable of automatically adjusting the feed pressure and deactivating the pump whenever hydraulic pressure falls below preset limits.
- Manual, semi-automatic or automatic plate shifters that are used to separate the plates prior to releasing the sludge cake.
Filter presses are available in a very wide range of capacities. Respondents to the Vendors Survey sell equipment with a capacity range of 0.6 ft3 to 200 ft3. A typical operating cycle is from 4 to 8 hr, depending on the dewatering characteristics of the sludge. Units are usually sized based on one or two cycles per day.