Sewage treatment, or domestic waste water treatment, is the process of removing contaminants from sewage. It includes physical, chemical and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a wastestream (or treated effluent) and a solid waste or sludge also suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with toxic organic and inorganic compounds. Sewage is created by residences, institutions, and commercial and industrial establishments. It can be treated close to where it is created (in septic tanks or onsite package plants and other aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant (see Sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards (regulation and controls). Industrial sources of wastewater often require specialized treatment processes (see Industrial wastewater treatment). Typically, sewage treatment involves three stages, called primary, secondary and tertiary treatment. First, the solids are separated from the wastewater stream. Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne bacteria. Finally, the biological solids are neutralized then disposed of or re-used, and the treated water may be disinfected chemically or physically (for example by lagooning and micro-filtration). The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used for theirrigation of a golf course, greenway or park. If it is sufficiently clean, it can also be used for groundwater recharge. Sewage is the liquid waste from toiletsbathsshowerskitchens, etc. that is disposed of via sewers. In many areas sewage also includes some liquid waste from industry and commerce. In the UK, the waste from toilets is termed foul waste, the waste from items such as basins, baths and kitchens is termed sullage water, and the industrial and commercial waste is termed trade waste. The division of household water drains into greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. Much sewage also includes some surface water from roofs or hard-standing areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include stormwater runoff. Sewage system capable of handling stormwater is known as a combined system. Such systems are usually avoided since they complicate and thereby reduce the efficacy of sewage treatment plants owing to their seasonality. In addition, heavy storms may overwhelm the sewage treatment system, causing a spill or overflow. It is preferable to have a separatestorm drain system for stormwater. Sewerage systems that transport liquid waste discharges and stormwater together to a common treatment facility are called combined sewer systems. The construction of combined sewers is a less common practice in the United States and Canada than in the past and is no longer accepted within building regulations in the UK and otherEuropean countries. Instead, liquid waste and stormwater are collected and conveyed in separate sewer systems, referred to as sanitary sewers and storm sewers in the U.S. and as foul sewers and surface water sewers in the UK. Overflows from foul sewers designed to relieve pressure from heavy rainfall are termed storm sewers or combined sewer overflows. As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles, (sediment), heavy metalsorganic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include sedimentation basins, wetlands, and vortex separators (to remove coarse solids). The site where the process is conducted is called a sewage treatment plant. The flow scheme of a sewage treatment plant is generally the same for all countries:

  • Mechanical treatment; Influx (Influent) Removal of large objects Removal of sand and grit Pre-precipitation
  • Biological treatment; Oxidation bed (oxidizing bed) or aerated system Post precipitation Effluent
  • Chemical treatment

Activated sludge

The activated sludge process was invented around 1914 and is today still the most commonly used biological wastewater treatment process. This widespread use is due to the fact that activated sludge can be a rather easy process to implement and one that can attain high treatment efficiency. That is to say, if it works! Activated sludge is susceptible to process disturbances making it a very problematic technology for many of its users. Problems arise most when the wastewater to be treated varies significantly in composition and/or flow. A problem that often frustrates the performance of activated sludge is bulking sludge due to the growth of filamentous bacteria. Sludge bulking can often be solved by careful process modifications. However, different filamentous bacteria such as Microthrix, Sphaerotilus, Nostocoida, Thiothrix or ”Type 021N” and others cause bulking for very different reasons. Many filamentous species have not even been given a scientific name yet. Consequently, in order to make the right kind of process modification, knowledge to identify them and much experience with the process ecology are required.  The potential for instability with activated sludge is an acute problem when strict demands on treatment performance are in place.