Sun Valley Water & Wastewater Project
Evaluation of Alternatives
The team created a decision matrix with water and wastewater alternatives to determine the most suitable systems for the Sun Valley Ranch. The team also created a preliminary cost analysis that could be used to compare the prices of the alternatives. This was used to determine a more appropriate ranking for the cost constraint of each alternative.
Table 2 below shows the constraints and criteria that were used to evaluate which alternative would be the most suitable for the site. The first column shows the weight of importance that was assigned to each criteria and constraint based on the client’s preferences. For each alternative, the ranking and score columns indicated the ranking that was calculated for each criteria and constraint. For the ranking system, each alternative was evaluated based on how well it met the constraint or criteria. A ranking of 1 indicated that the alternative met the criteria/constraint poorly, and a ranking of 3 indicated that it met the criteria/constraint very well. The score is the weight multiplied by the ranking. The overall scores for each alternative are shown at the bottom of the table.
Table 2: Water Supply Decision Matrix
Table 2 shows, the most suitable water supply alternative is importation. This was primarily due to the fact that the well’s installation and drilling needs would be so costly, particularly because several wells might need to be drilled with no certainty of success. The importation alternative was a substantially less complex system with less operation and maintenance needs. Also, the construction, startup and shutdown, and expansion would be easier for this alternative because of its simplicity.
Table 3 used the same ranking and scoring system as table 2 Effectiveness was added to the list of criteria and constraints to provide further clarification on which wastewater system would fit the client’s needs. The effectiveness represents how well the system will treat the wastewater supply. The stabilization ponds require large amounts of land and are also more suitable for large populations. And although septic tanks are typically a safe and reliable wastewater system alternative, the limitation that the depth to bedrock for the site would cause complications and be more expensive than a typical septic system. The composting toilets system is unique, effective, and can be designed for a lower cost if non-manufactured toilets are incorporated. This approach also allows for a graywater reuse system to be easily incorporated, which is advantageous to the client’s preferences for minimizing water usage. As table 3 shows, the most suitable wastewater system alternative for the site is composting toilets.
Table 3: Wastewater System Decision Matrix
The team determined that rainwater harvesting and gray water reuse systems could also be incorporated into the design to help minimize water usage. The Sun Valley, Arizona region receives approximately 9 inches of rainfall per year and the plants native to the region would not have a high water demand [1]. So, the team determined that rainwater harvesting would be a reliable source of water used for irrigation. Rainwater harvesting was selected as an additional water supply alternative for Sun Valley Ranch. The incorporation of a gray water reuse system would help divert this wastewater stream from the composting toilet system, where it would disrupt the mixture’s necessary composition. This water could be reused for irrigation purposes as well.
SELECTED PREFERRED ALTERNATIVES
After the evaluation of the feasible alternatives, the following systems were determined as the most suitable for Sun Valley Ranch, LLC:
- Wastewater System: Composting Toilets & Graywater Reuse
- Water Supply System: Importation & Rainwater Harvesting
References:
[1]Monthly Averages for Sun Valley [Online]. Available FTP:
http://average-rainfall.findthebest.com/l/25183/Sun-Valley-Arizona
