civil engineering WWTP question please answer

Question

A an activated sludge wastewater treat plant is designed to handle a design flow capacity of 10.0 MGD. Consider the following operational conditions: Influent BOD5 = 275 mg/L Influent TSS = 290 mg/L a. Calculate the amount of primary solids produced if the TSS removal efficiency is 70 %. b. Calculate the primary sludge volumetric flow rate if its solids concentration is 3.8 %. The plant in Problem 3, is operated at the following conditions for the activated sludge part of the plant: Hydraulic retention time in the aeration basins = 5.0 hours The MLSS concentration = 2400 mg/L (72% volatile) The mean cell residence time = 10 days Design effluent BOD5 = 8 mg/L Biomass yield coefficient = Y = 0.52 mg-VSS/mg-BOD5 Biomass decay coefficient = kd = 0.06 d-1 a. If the BOD5 removal efficiency in the primary clarifiers is 28 %, calculate the amount of total solids that must be wasted everyday from the activated sludge process. Wasting is from the return sludge line. b. If the waste activated total solids concentration is 11,200 mg/L, calculate its volumetric flow rate. c. What is amount of oxygen required by this plant per day. d. If the waste activated sludge is thickened using a rotary drum screen thickener to a solids content of 5.5 %, calculate the volumetric flow rate of the thickened sludge. Assume that thickener has a solids capture rate (efficiency) of 94 %.

Answer 1

a. To calculate the amount of primary solids produced, we need to determine the influent TSS (Total Suspended Solids) and the TSS removal efficiency.

Influent TSS = 290 mg/L TSS removal efficiency = 70%

The TSS removed in the primary treatment is equal to the influent TSS multiplied by the removal efficiency:

TSS removed = Influent TSS x Removal Efficiency TSS removed = 290 mg/L x 0.70 = 203 mg/L

Now, we can calculate the primary solids produced by subtracting the TSS removed from the influent TSS:

Primary Solids Produced = Influent TSS - TSS removed Primary Solids Produced = 290 mg/L - 203 mg/L = 87 mg/L

b. To calculate the primary sludge volumetric flow rate, we'll need to know the design flow capacity and the primary sludge solids concentration.

Design flow capacity = 10.0 MGD Primary sludge solids concentration = 3.8%

First, convert the design flow capacity from MGD to gallons per day (GPD):

1 MGD = 1,000,000 gallons Design flow capacity in GPD = 10.0 MGD x 1,000,000 GPD/MGD = 10,000,000 GPD

Now, calculate the primary sludge volumetric flow rate by multiplying the design flow capacity by the primary sludge solids concentration:

Primary Sludge Flow Rate = Design Flow Capacity x Primary Sludge Solids Concentration Primary Sludge Flow Rate = 10,000,000 GPD x 0.038 = 380,000 GPD

c. To calculate the amount of oxygen required by the plant per day, we'll use the information provided for the activated sludge process and the design effluent BOD5.

Design effluent BOD5 = 8 mg/L Biomass Yield Coefficient (Y) = 0.52 mg-VSS/mg-BOD5

The formula for oxygen required (O2) is:

O2 = [Influent BOD5 - Effluent BOD5] x Flow Rate x Y

First, convert the design flow capacity from MGD to GPD (1 MGD = 1,000,000 GPD):

Design flow capacity in GPD = 10.0 MGD x 1,000,000 GPD/MGD = 10,000,000 GPD

Now, calculate the oxygen required:

O2 = [275 mg/L - 8 mg/L] x 10,000,000 GPD x 0.52 mg-VSS/mg-BOD5 O2 = 267 mg/L x 10,000,000 GPD x 0.52 mg-VSS/mg-BOD5 = 1,387,600 mg/day

Convert milligrams to kilograms:

O2 = 1,387,600 mg/day ÷ 1000 = 1,387.6 g/day

Now, convert grams to kilograms:

O2 = 1,387.6 g/day ÷ 1000 = 1.3876 kg/day

So, the plant requires approximately 1.3876 kilograms of oxygen per day.

d. To calculate the volumetric flow rate of the thickened sludge, we'll need to consider the waste activated sludge concentration, the solids capture rate, and the primary sludge flow rate calculated earlier.

Waste Activated Sludge (WAS) Concentration = 11,200 mg/L Solids Capture Rate (Efficiency) = 94% Primary Sludge Flow Rate = 380,000 GPD

First, calculate the solids captured by the thickener:

Solids Captured = WAS Concentration x Solids Capture Rate Solids Captured = 11,200 mg/L x 0.94 = 10,528 mg/L

Now, calculate the thickened sludge flow rate:

Thickened Sludge Flow Rate = Primary Sludge Flow Rate + Solids Captured Flow Rate Thickened Sludge Flow Rate = 380,000 GPD + (10,528 mg/L x 10,000,000 GPD) Thickened Sludge Flow Rate = 380,000 GPD + 105,280,000 GPD = 105,660,000 GPD

Convert the thickened sludge flow rate to MGD:

Thickened Sludge Flow Rate in MGD = 105,660,000 GPD / 1,000,000 GPD/MGD = 105.66 MGD

So, the volumetric flow rate of the thickened sludge is approximately 105.66 MGD.