Question

A dairy in Fresno County discharges its wastewater to a circular treatment pond. The dairy wants to expand its operations, which will increase the influent flow but not the influent concentration. What is the maximum influent flow that can be handled by the existing pond in the future without exceeding the legally allowed effluent concentration of 30 mg/L.

Data on the existing facilities and flows are shown below. The soil is sandy and there is considerable seepage. The wastewater pollutants are particulate in form and degrade with a rate constant of 0.25 d^-1.  

Current conditions

influent Flow = 3200 m³/d

Influent concentration = 220 g/m³

Pond effluent concentration = 22 mg/L

Pond volume = 120,000 m³

Evaporation negligible

Seepage = 1500 m³/d

Answer 1

Soln:-

Given:

influent Flow = 3200 m​​​​​​3/d

Influent concentration(L​​​​​​o)= 220 g/m​​​​​​3​ = 220mg/l

Pond effluent concentration = 22 mg/L

Pond volume = 120,000 m​​​​​​3

Seepage = 1500 m​​​​​​3/d

K​​​​​=0.25 /day

Now max allowed concentration of effluent (L​​​​​​t​​​​​)= 30mg/l

Now,

2.303log10(Lt/Lo)=-kt

t= 2.303log10(30/220)/-0.25

=7.97 days say 8 days

Let max allowed flow = x

Now net flow into pond = (x-1500) m3/day

Now volume of pond =120,000 m3

(x-1500)*8= 120,000

x= 120,000/8+1500

X= 16,500 m3/day

the maximum influent flow that can be handled by the existing pond in the future without exceeding the legally allowed effluent concentration of 30 mg/L is 16,500m3/day