In many cities in the southwestern part of the US, water is being withdrawn from
ID: 2716207 • Letter: I
Question
In many cities in the southwestern part of the US, water is being withdrawn from subsurface aquifers faster than it is being replaced. The groundwater depletion has forced some cities to take action to encourage water conservation. Beginning in the mid1990’s, a city undertook a project to encourage installation of ultralowflush toilets in existing houses. The toilet replacement program involved a rebate of 75% of the cost of the fixture (up to $100 per unit), providing the toilet used no more than 1.6 gallons of water per flush. In the twelve months before and after installation, the water consumption was monitored during the months of December, January and February. A total of 268 residences were monitored. Monthly consumption before and after installation was found to be 11.2 and 9.1 CCF respectively, resulting in a water savings rate of 18.8%. During the first 2 years of the program, the average cost of a toilet was $115.83 and the average rebate was $76.12. a. Determine the simple payback, and the payback period assuming an interest rate of 5%. Assume installation costs $50, the water charge is $0.76 per CCF and the sewer surcharge is $0.62 per CCF. b. If the toilet has a “productive” life of 5 years, is the toilet replacement program cost effective for the city?
Explanation / Answer
Answer:a Monthly consumptions before and after installations was found to be 11.2 and 9.1 CCF respectively, for an average reduction of 18.8%. When only the months of jan and feb are used in the before and after calculations, the respective values were 11.0 and 8.7 CCF, resulting in a water savings rate of 20.9%.
Payback period=Net cost of toilets+Installation cost/Net annual savings for water and sewer charges
=(115.83-76.12)+50/(2.1CCF/month)*12 months*(0.76+0.62)/CCF
=2.6 years
Answer:b Yes, the toilet replacement program was clearly very cost effective for the city.
c=$76.12/(2.1+2.1CCF/month)*(12 months*5years)
=$0.302/CCF or $0.40/1000 gallons
Thus unless the city can deliver water and treat the rsulting wastewater for less than $0.40 per 1000 gallons, the toilet replacement program considered economically attractive.For the city the operating cost alone,i.e without the capital expense for water and wastewater services that were not expensed about $1.10 per 1000 gallons.which far exceeds $0.40 per 1000 gallons.Therefore the toilet replacement program was clearly very cost effective for the city.
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