Question

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Question 4 (20 points) A lagoon with volume 1.200 m' has been receiving a steady flow of conservative waste at a rale of 100 m'iday for a long enough time to assume steady state conditions apply. The waste entering the lagoon has a concentration of 10 mg/L. Assuming completely mixed conditions. a) What would be the concentration of pollutant in the effluent leaving the lagoon? (10 points) b) If the input waste concentration suddenly increased to 100 myl.. what would be the concentration in the cluent be 7 days later (10 points).

Answer 1

Solution:- Givendata Volume of lagoon = 1200m Rate of blow= loomal day waste concentration=10 mglL la) since the blaw us Steady, o the sata ef mass accumulation ] (Rata of mass accumulahon Incoming rati- outgeing hatt +hata ef genesation Since the tYpe of waste s conservative, therebore there uill be zero rati of generaton Generatoncta = o 1- e Letthe discharge be q incomingoutgloowlday Nouo equateng Incoming ratt and outgeing rate = outgeing rate geing rati .Inoming Cineming imamingougeingng . CinCout 1o mglL Incoming hate-outgeing ratt (b) Ratr of rmass accumulation waste genuration natë Volume of the lagoon,v= 120om aiven If the tlous es not stealy mass accumulahon rata = vdczo ct Since the polutast us consesved, there usil be mosata etgoneehon 1-e, Tatz ofgenerahon= o .Rati ef mass accumulation = Incoming Frat ugoing 2ato

deCinin Couttout ince the lagoon behaves luke a Complete Mix Fles Reactos and there is a Conditon ef complete mixnhich meam that the Concentrahon of waste (c)nthu lagoonCouinthe ettluent Leaving the lagcon The equation tollousc as Vdcout Cinin Cout%out dt Cout,7 dcout dcout dt = Co Cin-Lour Cin-Cout Luhere CoValus of outgoing concentration(hera input wo Constaut) we have, 1o malL et time t oday . Co= concuntration io already From above a. velume and ncoming Knewn nein-cour)-n(n-Cour(-0) cin- Cout, е (4-) Cin-Co Cout,tCin -Cin-Co) e (loawldayas) 100Mdaydays 1200 m3 CoutloomglL (1oomglL- lomgL) e Solving bor Courgoing 7rdays LOut days9.7 mg lL Therehore the concentrahon ofwaste in ettluent atter 7dlays will be 49.781 Mgl L