Exercise problems

EXERCISE PROBLEMS

Design an aerated lagoon given the following data:

• Wastewater flow = 12,400 cu.m/day
• BOD5 =  300 mg/L
• Population = 70,000 people
• Detention period = 3 days
• k' = 0.015 d-1 at 20℃
• Y=0.5
• Kd = 0.07 d-1 [BOD5 basis]
• Use ideal  complete-mixing model
• Lagoon is proposed to be followed by another treatment unit

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Design a flow through aerated lagoon to treat a wastewater flow of 3800 cu.m per day. The treated liquid is to be held in a settling basin with a 2 day retention time before discharge. Total biological solids produced are equal to computed volatile solids divided by 0.8. Assume: 

• Influent suspended solids (non biologically degraded) = 200 mg/L; 
• Influent soluble BOD5 = 200 mg/L; 
• Effluent suspended solids after settling = 20 mg/L; 
• Kinetic coefficients:
•  Y = 0.65; 
• Ks = 100 mg/L; 
• 𝞵max = 6.0 d-1; 
• Kd = 6.07 d-1; 

• Lagoon depth = 3m; 
• Design mean cell retention time = 4 days

1. On the basis of mean cell retention time, determine the surface area of the lagoon. 
2. Estimate the soluble effluent BOD5 measured at the lagoon outlet
3. Estimate the concentration of biological solids produced
4. Estimate the suspended solids in the lagoon effluent before settling

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Design a facultative aerated lagoon  to serve 40,000 people given the following data:

• Sewage flow = 180 LPCD
• Raw BOD5 = 50 g/person/day
• BOD5 should not exceed 40 mg/L in winter
• Average temperature of air in winter = 18℃
• Average temperature of air in summer = 37℃
• KL = 0.7 per day and Kd = 1.2 per day at 20℃
• Coliforms in sewage - 10^7 per 100 ml

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Design a flow-through type aerated lagoon without recycling for the following data:

• Volume of wastewater to be treated = 3.0 MLD 
• Influent suspended solids (non-biodegradeable) = 150 mg/L
• Effluent suspended solids = 50 mg/L
• Influent soluble BOD5 = 150 mg/L
• Effluent soluble BOD5 = 150 mg/L
• Summer air temperature = 35℃
• Winter air temperature = 25℃
• Wastewater temperature = 27℃
• Mean cell residence time (ϴc) = 3 days
• Water depth in the lagoon = 3.5m
• Elevation of plant = 600m

Assumed data:

• Proportionality constant (f) = 0.5
• Oxygen concentration in aerated lagoon = 1.5 mg/L
• First order soluble BOD5 rate constant, Kbase e = 2.5 d-1 at 20℃

• Mixed Liquor Volatile Suspended Solids, X = 0.8 * MLSS (MLSS = Mixed Liquor Suspended Solids)

• Aeration constants α = 0.85 and β = 1.0

Temperature coefficient = 1.06

Kinetic coefficients:

                                Y = 0.6

                                K = 6.0 d-1

                                Ks = 90 mg/L

                                Kd = 0.06 d-1

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Determine the aerated lagoon size and power requirements to serve 50,000 people assuming sewage generation rate = 250 LPCD, influent BOD5 = 300 mg/L and effluent BOD = 20 mg/L.
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Design a facultative pond in a waste stabilization pond to treat 3.5MLD of wastewater which has a design loading of 440 kg BOD/ha-d. The design temperature is 30C.
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