text: Water scarcity has prompted efforts to reuse waste water once it has been properly treated, known as "water reclamation" (also called wastewater reuse, water reuse, or water recycling). Among the treatment technologies available to reclaim wastewater, membrane processes stand out for their capacity to retain solids and salts and even to disinfect water, producing water suitable for reuse in irrigation and other applications.
  A semipermeable membrane is a material that allows the selective flow of certain substances.
  In the case of water purification or regeneration, the aim is to allow the water to flow through the membrane whilst retaining undesirable particles on the originating side. By varying the type of membrane, it is possible to get better pollutant retention of different kinds. Some of the required characteristics in a membrane for wastewater treatment are chemical and mechanical resistance for five years of operation and capacity to operate stably over a wide pH range.
  There are two main types of membrane materials available on the market: organic-based polymeric membranes and ceramic membranes. Polymeric membranes are the most commonly used materials in water and wastewater treatment. In particular, polyvinylidene difluoride (PVDF) is the most prevalent material due to its long lifetime and chemical and mechanical resistance.
  When used with domestic wastewater, membrane bioreactor processes can produce effluent of high enough quality for discharge into the oceans, surfaces, brackish bodies, or urban irrigation waterways. Other advantages of membrane bioreactors over conventional processes include reduced footprints and simpler retrofitting.
  It is possible to operate membrane bioreactor processes at higher mixed liquor suspended solids concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate.
  Recent technical innovation and significant membrane cost reduction have enabled membrane bioreactors to become an established process option to treat wastewater. Membrane bioreactors have become an attractive option for the treatment and reuse of industrial and municipal wastewater, as evidenced by their consistently rising numbers and capacity. The current membrane bioreactor market was estimated to be worth around US $216 million in 2006 and US$838.2 million in 2011, grounding projections that the market for membrane bioreactors was growing at an average rate of 22.4% and would reach a market size of US $3.44 billion in 2018.
  The global membrane bioreactor market is expected to grow in the near future due to various driving forces, for instance increasing scarcity of water worldwide which makes wastewater reclamation more profitable; this will likely be further aggravated by continuing climate change. Growing environmental concerns over industrial wastewater disposal along with declining freshwater resources across developing economies also account for increasing demand for membrane bioreactor technology. Population growth, urbanization, and industrialization will further complicate the business outlook.
  However, high initial investments and operational expenditure may hamper the global membrane bioreactor market. In addition, technological limitations, particularly the recurrent costs of membrane fouling, are likely to hinder production adoption. Ongoing research and development progress toward increasing output and minimizing sludge formation are anticipated to fuel industry growth.
  Membrane bioreactors can be used to reduce the footprint of an activated sludge sewage treatment system by removing some of the liquid components of the mixed liquor. This leaves a concentrated waste product that is then treated using the activated sludge process.
  Recent studies show the opportunity to use nanomaterials for the realization of more efficient and sustainable membrane bioreactors for wastewater treatment.