Mass Balances

Question 0

Mass balance general equation

Answer 0

Accumulation rate = input rate - output rate +- reaction rate

Question 1

Law of conservation of mass

Answer 1

Mass can neither be created nor destroyed

Question 2

Calculate input rate (mg/s) of triclosan into a lake, WWTP 0.5μg/L, flow rate 6 m^3/s

Answer 2

Input rate= Q x C
= (6 m^3/s x 1000 L/m^3) x (0.5 μg/L x mg/1000μg)
= 3 mg/s

Question 3

Steady state system

Answer 3

• System has reached equilibrium, no accumulation of mass with time
• concentration in control volume is constant
• accumulation rate is set to zero

Question 4

Conservative system

Answer 4

• no reaction

* reaction rate is set to zero

Question 5

Steady state conservative system equation:

Answer 5

Input rate = output rate (bc accumulation and reaction rates are zero)

Question 6

Concentration of stream: Cs: 20 mg/L
Flow rate of stream: Qs: 10 m^3/s
Concentration in WWTP: Cw: 40 mg/L
Flow rate of WWTP: Qw: 5 m^3/s

Answer 6

(Cs x Qs) + (Cw x Qw) = Cm x Qm, Qm = Qs + Qw

Cm = (CsQs + CwQw) / (Qs + Qw)
= ([20x10] + [40x5])/ (10 + 5)
= 26.7 mg/L