The activated-sludge process, developed in England in the early 1900s, is remarkably successful at removing soluble organics from wastewater. The process removes the microorganism sludge by settling, while digestion of adsorbed organics continues, which activates the sludge for recycling. The biological, fluidized-bed, wastewater treatment process is a new adaptation of the fixed-film, biological reactor— the trickling filter. The mixed microbial cultures associated with wastewater treatment have excellent adhesion characteristics. The fluidized biomass acts like an organics equalization tank by adsorbing organics and gradually digesting them. The ion-exchange reaction is the interchange of ions between a solid phase and a liquid surrounding the solid. Exchange sites exhibit an affinity for certain ions over others. This phenomenon is helpful in removing objectionable ionic materials from process streams. Modern ion-exchange materials are prepared from synthetic polymers such as styrene-divinyl-benzene copolymers that have been sulfonated to form strongly acidic cation exchangers or aminated to form strongly basic anion exchangers.