# Solution-Conveys effluent the entire length of the trench

EHST 3370 Lab Assignment

1) Drinking water wells must be located at least 50′ from a septic system. The well casing is 1 ft in diameter. How much area (Ft2, acres) does this setback occupy?

2) A septic tank has a rated volume of 12,000 gallons and is 4′ wide and 8′ long. What is the liquid depth of the tank (Ft, inches)?

3) A pump tank is connected to the septic tank in question 2. The tanks have the same dimensions. When the pump activates, the water level in the tank drops 5″ in 5 minutes. What is the pump delivery rate (gallons/minute)?

4) If the pump in question 3 was designed to pump at a rate of 22 g/min, what is the pump delivery rate efficiency?

5) What is the BOD5 for this sample:
a. BOD bottle has a 300 mL volume
b. Tank sample was 5 mL, 295 mL of oxygenated, purified water
c. Initial DO was 8 mg/L

d. Final DO after 5 days was 3.5 mg/L

6) Septic system A has 3 – (100′ x 3′) trenches, system B has 5-(90′ x 2′) trenches and system C has a bed 10′ x 90′ (they all have the same area of trench bottom). Each system has 12″ of gravel. If the trenches are completely full of water, what are total areas (ft2) of soil/wastewater interface for each system? Which system provides the most treatment?

7) There is a septic system with 3-100′ trenches, each 3′ wide with 12″ washed stone. If the gravel has a porosity of 0.3, what is the wastewater storage capacity of the trenches (ft3, gallons) if the trenches are completely filled with gravel (no pipe in trench)?

8) Most trenches have a 4″ inside diameter pipe that conveys effluent the entire length of the trench. What is the total storage capacity for a 100′ x 4″ pipe (ft3, gallons)? If the 4″ pipe was in the gravel for each trench in question 7, what would the total pipe storage capacity be for the three trenches (ft3, gallons)? What would the total storage capacity be for the three trenches with 12″ gravel and 4″ conveyance pipe in the center of each trench?

9) What percentage of the total trench capacity does the 4″ pipe provide (question 8 data)?

10) Which of the systems in question 6 would have the most storage capacity (assume that the bed system has 3-pipes)?

11) If the on-site system used a chamber (semi-circle shape) for wastewater storage in each trench, and each chamber had a radius of 1 ft, and was 100 ft long, what would wastewater storage capacity (ft3 and gallons) be for the chambers (3 trenches, 100 ft long).

12) Septic tank effluent has a TN concentration of 95 mg/L and groundwater beneath the septic drainfield trenches has a TN concentration of 25 mg/L. What is the treatment efficiency of the septic system in reducing TN concentrations before effluent reaches groundwater?

Area of a circle = 3.14 x (radius)2
Volume of a cylinder = 3.14 x (radius)2 x length

Pump delivery rate efficiency =
(actual pumping rate/ design pumping rate) x 100

1 ft3 = 7.48 gallons 1 gallon = 3.78 liters
1 meter = 3.28 ft 1 inch = 2.54 centimeters
1 ft = 0.3048 meters 1 meter = 100 centimeters

1 ft = 12 inches

1 acre = 43,560 ft2
Treatment efficiency [ (influent – effluent) / influent] x 100%

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