An activated sludge plant with an MCRT of over 10 days is typically classified as what type of plant?

An activated sludge plant with a Mean Cell Residence Time (MCRT) of over 10 days is classified as an extended aeration plant. This classification refers to the operational characteristics of the facility, particularly the retention time of microorganisms in the system.

Extended aeration plants are designed to maintain a longer contact time between the biomass and the wastewater, usually to allow for comprehensive treatment and stabilization of organic matter. The extended MCRT enhances the aerobic degradation of organic solids and helps in achieving lower effluent concentrations of BOD and TSS because the microorganisms have more time to metabolize organic pollutants.

In contrast, a conventional aeration plant typically operates with shorter MCRT values, usually under 10 days, focusing on more rapid treatment. Secondary treatment plants can encompass both conventional and extended aeration systems, so this option is broader and does not specify the length of MCRT. Batch treatment plants operate differently, often involving a complete treatment cycle in discrete batches rather than continuous flow, thus having different operational parameters.

The specific designation of an extended aeration plant reflects the emphasis on prolonged aeration processes for enhanced wastewater treatment efficiency, making it the correct classification in this context.