The purpose of primary treatment is to remove suspended solids and floating material. In some countries, primary treatment that results removing of approximately 40% to 60% of the suspended solids and 25% to 35% of BOD5, together with the removal of material from the wastewater, is adequate to meet the requirement of the receiving water body. But not in other countries that follow higher standards. There, secondary treatments are done for further treatment of effluents.
Secondary treatment using a biological process is generally used when the primary treatment is insufficient to meet the regulatory effluent standards. It is mainly used as further treatment due to its greater removal efficiency and lower cost than chemical coagulation.
Secondary treatment removes the soluble and colloidal organics (BOD) that remain after primary treatment. This process further removes suspended solids and, in some cases, removing nutrients such as phosphorus and nitrogen.
Biological treatment processes provide similar biological activities to waste digestion, which would take place in the receiving waters, but within a reasonably shorter time.
Secondary treatment may remove more than 85% of BOD5 and suspended matter. But, it is not effective in removing non-biodegradable organics, heavy metals, and microorganisms.
Biological treatment systems are designed to maintain a large active mass and variety of microorganisms (bacteria and fungi, protozoa, rotifers, algae, etc.), within the confined system under favorable environmental conditions, such as dissolved oxygen, nutrient, etc.
Biological treatment processes are generally classified mainly as suspended growth processes (activated sludge), attached (film) growth processes (trickling filter and rotating biological contactor, RBC), and dual-process systems (combined).
Other biological wastewater treatment processes include the stabilization pond, aerated lagoon, contaminant pond, oxidation ditch, high-pure oxygen-activated sludge, biological nitrification, de-nitrification, and phosphorus removal units.
In the suspended biological treatment process, under a continuous supply of air or oxygen, living aerobic microorganisms are mixed thoroughly with the organics in the wastewater. They use the organics as food for their growth. As they grow, they clump or flocculate to form an active mass of microbes. This is termed as the so-called activated sludge.