What is the detention time range for the contact tank in the contact stabilization process?

• A. 0.25 to 1
• B. 0.5 to 1.5
• C. 0.75 to 2
• D. 2 to 3.5

Answer: (C)

In a contact stabilization process, the purpose of the contact tank is to give the incoming wastewater a short but sufficient period to mix with the solids and allow initial biological contact to start, without letting the material proceed into digestion or clarification for too long. This requires a middle-ground detention time, typically around 0.75 to 2 hours (about 45 minutes to 2 hours). This range lets microbes begin stabilizing soluble organics and start forming flocs, while keeping the system from behaving like a full aeration or digestion stage, which could slow downstream processes or reduce overall efficiency. The exact time is determined by the tank’s volume and the flow rate (hydraulic retention time), but the standard practice for the contact tank sits within this 0.75–2 hour window.