Metal finishing is yet another industry that uses filter presses to treat the wastewater produced by its processes. Wastewater from metal finishing contains contaminants like heavy metals, organic substances, and cyanides. If released untreated, the wastewater can be hazardous to the environment and pose potential health risks to humans.
As a result, metal finishing wastewater is heavily regulated by government under policies like, the Clean Air Act, Clean Water Act and RCRA. Wastewater must be treated to meet standards outlined in these policies, and filter presses play a role in doing that.
What constitutes metal finishing?
Metal finishing is the process where a metal is treated (or is used to treat) a substrate and a thin metallic or non-metallic coating is applied. The coating is meant to enhance the appearance, function or performance of a product.
According to the EPA, the following processes are considered metal finishing, and as a result fall under its regulations:
coating (phosphating, chromating, and coloring)
chemical etching and milling
printed circuit board manufacturing
All of these processes produce large amounts of wastewater that must be treated.
How is wastewater treated?
Most of the waste produced by metal finishing is a slurry of metals dissolved in liquid, and the two must be separated. Typically the separation is done with a pH adjustment and a precipitation procedure where chemicals such as sodium hydroxide, calcium oxide and magnesium hydroxide are added.
During the metal finishing process there are two means of treating wastewater:
Point source - Also called point of use. In this method, wastewater is treated at the source of generation rather than at the final collection point.
End of pipe - In this method, all wastewater is collected in a centralized location and treated.
How are filter presses used?
Filter presses are typically used at the end of the "end of pipe" method (as shown in the illustration above). Once the wastewater has been collected in a sludge holding tank and thickened, it is pumped to the filter press for dewatering.
For filter presses used in dewatering, a slurry produced by metal finishing processes we recommend using filter plates that are made of polypropylene, because this material will hold up best against the corrosive metallic slurry. We also recommend using membrane (diaphragm) or recessed chamber filter plates. And when it comes to filter cloths, gasketed cloths are the best type to use in this process. Gasketed filter cloths are less likely to leak the potentially hazardous metallic slurry.
Once the slurry has gone through the filter press it is either recycled back to the equalization tank or discharged as filter cakes (as shown in the illustration). These filter cakes are generally 25-35% TSS (total suspended solids) by weight.
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