Mixed-media filters are especially effective on oil refinery wastewater and also are employed in the pulp and paper industry. Walnut shell media filters work well on refinery wastewater, steel mill direct spray and caster water, ethylene plant quench water and other oily wastes. The system operates at twice the flux rate of conventional sand filters and removes more than three times the amount of solids before cleaning. After a filtering cycle, the shell media are pumped through a scrubbing system to clean, and are returned to the filter vessel to resume on-line filtration. Compounds adsorb to the particles of the carbon, which usually is derived from bituminous coal, anthracite coal or coconut shells.
Filtration takes place inside steel pressure vessels.
When the carbon is fully spent, it is removed for reactivation and reuse. Activated carbon systems commonly are used for pretreatment in reverse osmosis and other high-purity treatment systems. Rotating precoat filters can remove particles down to 0.5 micron if diatomaceous earth is used as a precoat material. These filters work well on wastewater with a high solids content. When the material being filtered has a consistent particle size, the solids can be used as the precoat and the filtered solids can be recovered.