Fluids flow through a filter due to a difference in pressure—fluid flows from the high-pressure side to the low-pressure side of the filter, leaving some material behind. The simplest method to achieve this is by gravity and can be seen in the coffeemaker example. In the laboratory, pressure in the form of compressed air on the feed side (or vacuum on the filtrate side) may be applied to make the filtration process faster, though this may lead to clogging or the passage of fine particles. Alternatively, the liquid may flow through the filter by the force exerted by a pump , a method commonly used in industry when a reduced filtration time is important. In this case, the filter need not be mounted vertically.
Regardless of the waste stream, chemical treatment requires adherence to a multi-step procedure. For oily water application , the procedure often referred to as chemical splitting, involves lowering the pH of the wastewater with acidic chemistry (H2SO4, HCl) to a range of 3-4, which causes the oil to separate from the water. The oil is then decanted from the water surface. After decanting, the pH is elevated to an acceptable range for discharge (typically -). Prior to discharging to the sewer or tributary, an analysis of the treated wastewater must be completed to confirm compliance.