Partial mix lagoons are commonly used to treat municipal and industrial wastewater. This technology has been widely used in the United States for at least 40 years. Aeration is provided by either mechanical surface aerators or submerged diffused aeration systems. The submerged systems can include perforated tubing or piping, with a variety of diffusers attached. In aerated lagoons, oxygen is supplied mainly through mechanical or diffused aeration. Aerated lagoons are usually classified by the amount of mixing provided. A partial mix system provides only enough aeration to satisfy the oxygen requirements of the system and does not provide energy to keep all total suspended solids (TSS) in suspension.