In order to safeguard the public health, environ­ment, public beaches, water quality, and economy of south San Diego County, California, and Tijuana, Mexico, federal agencies in the United States and Mexico developed four alternatives for treating wastewater prior to discharge into the ocean. The project will minimize untreated waste­water flows that have caused chronic and substan­tial pollution in the Tijuana River Valley, the Tijuana River National Estuarine Research Re­serve, coastal areas used for agriculture and public recreation, and areas designated as critical habitat for federal- and state-listed endangered species. For the costs and benefits estimated, which alter­native should be selected on the basis of a B/C analysis at 6% per year and a 40-year project pe­riod? All

Question

monetary amounts are in $ million units.

Answer 1

Calculate AW of total cost (in $ millions), then rank according to increasing AW value

AWPond = 58(A/P,6%,40) + 5.5
= 58(0.06646) + 5.5
= $9.35
AWExpand = 76(A/P,6%,40) + 5.3
= 76(0.06646) + 5.3
= $10.35
AWAdvanced = 2(A/P,6%,40) + 2.1

= 2(0.06646) + 2.1
= $2.23

AWPartial = 48(A/P,6%,40) + 4.4
= 48(0.06646) + 4.4
= $7.59

Benefits are directly estimated; ranking is: DN, Advanced, Partial, Pond, Expand

Advanced vs. DN: ?B/C = 2.7/2.23
= 1.21 eliminate DN

Partial vs. Advanced: ?B/C = (8.3 - 2.7)/(7.59 - 2.23)
= 1.04 eliminate Advanced

Pond to Partial: ?B/C = (11.1 - 8.3)/(9.35 - 7.59)
= 1.59 eliminate Partial

Expand to Pond: ?B/C = (12.0 - 11.1)/(10.35 - 9.35)
= 0.90 eliminate Expand

Select the Pond System