Treatment of Suspended Solids

Suspended solids are another special characteristic of CMP wastewater. Removal or concentration of the solids in the waste stream may or may not 

In slurry service, UF is used primarily as a concentration technology. The UF reject stream can then be recycled back to a tank containing the concentrated slurry that can then be dewatered with a filter press. The permeate will contain little, if any, suspended solids and can be sent for metals removal treatment or to recycle/reclaim if metals have already been removed. 

Schematic showing flow path difference between MF and UF 

Inclined plate clarification is a traditional separation technology that has been used for decades to remove suspended solids from a liquid stream in various types of systems including traditional precipitation [12]. In the semiconductor industry, the clarifier is commonly used in fluoride waste treatment systems where calcium fluoride precipitate is concentrated prior to dewatering in a press, or in assembly/test operations to separate silicon fines from backgrind operations. The clarifier will concentrate the solid phase of slurry like a UF, but unlike the UF or MF, the clarifier may require the addition of a chemical polymer to facilitate the agglomeration of the suspended solids so that they settle and concentrate. Polymer addition adds another level of complexity to the waste treatment system. The clarifier does not provide a physical barrier to prevent the transport of solids to downstream equipment, so it may be necessary to install a UF or MF downstream of the clarifier to capture extraneous particles or to protect the downstream equipment from clarifier upsets. 

The key challenges with solids removal will be determining the most cost-effective way to remove the solids. Each of the technologies discussed here— microfiltration, ultrafiltration, and clarification—have their benefits and limitations. The big challenge with solids removal, as will be discussed later in this chapter, will be to determine when in the treatment system solids removal should occur. 

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