Question

The city of Stay at Home counted 150,000 people in the 1970s, when its wastewater treatment plant was built. Because of a recent economic boom, the city has been growing at a fast rate and new developments will bring the total population to 350,000 people within 10 years from now. There is no more footprint available to expand the existing plant, which is land locked in the middle of an industrial zone. The city council, therefore, decided to build a new plant on the other side of the town. This new plant will be designed to treat the wastewater from the additional 200,000 people. You are the designer. Analysis of the last 5 years of operating data shows that the old plant received an average volume of 15 million gallon per day (MGD) with an average BOD5 concentration of 250 mgBOD5/L and an average TSS concentration of 270 mgTSS/L. This average BOD5 and TSS data will be used as the reference to calculate the wastewater concentration and mass expected to be supplied to the new system. We know that the new houses and buildings that will be connected to the new plant will be much more water efficient (through a mandate for the new homes to use high efficiency water fixture and a mandate for the new buildings to treat and recycle wastewater onsite). It is, therefore, expected that customers connected to the new wastewater treatment plant will use half the volume of water used by customers connected to the old system. In order to estimate the new plant BOD5 and TSS mass loading, you assumed that, although people connected the new system will use half the amount of water, the mass of waste contributed by each person connected to both the old and new plants will be the same.

1. How much water does each person connected to the old wastewater treatment plant consume? (answer will be given in the unit of gal/person*day)

2. How much water does each person connected to the new wastewater treatment plant is expected to use? (answer will be given in the unit of lbBOD5/day)

Answer 1

Conversions used -

(a) 1 gallon = 3.78541 liters

(b) 1 mg = 2.20462e-6 lb

I adopted a factor 0.8 to convert water supplied to water consumed. Usually that factor lies in the range (0.7-0.8). Use accordingly.

I hope the unit conversions and the solution is clear. Please do comment for further clarification.

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