Sewage is being treated in two aerated lagoons in series. each lagoon has a surface area of 5 hectares (ha) and depth of 1 m, and each operates as a cstr at steadystate. the influent to the first lagoon contains 100 mg/l of biodegradable organic material in a flow of 8640 m3/d. at steady-state, the effluent from the second lagoon must contain no more than 20 mg/l of the organics. what first-order biodegradation rate constant must be reached to achieve the treatment goal?