The process for undertaking a life cycle costing analysis for infiltration systems is the same as described in Life-Cycle Costing - Constructed Wetlands and Life-Cycle Costing - Bioretention Systems.
The origin of all of the ‘expected’ values and algorithms in MUSIC’s costing module, as well as the statistical operations used to generate ‘upper’ and ‘lower’ estimates for infiltration systems are explained in Table 1.
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Table 1. Summary of cost-related relationships for infiltration systems.
Element of Life Cycle Costing Model | Default Option for Estimation in music | Alternative(s) | Notes |
|---|---|---|---|
Life cycle | 50 years. (Expert judgement) | 20 years. (From collected survey data, n = 2) | One could convincingly argue the life cycle is infinite for well-maintained infiltration systems, but we need to set the life cycle to a finite number to calculate a life cycle cost. Expected, upper and lower estimates based on expert judgement. |
Total acquisition cost (TAC) | TAC ($2004) = 3230 |
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• (A)0.5772 R2 = 0.66; p < 0.01; n = 31. Where: A = surface area of the infiltration system in m2. | TAC ($2004) = 1410 |
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• (A) + 47,960 R2 = 0.74; p = 0.34; n = 3. Where: A = surface area of the infiltration system in m2. For literature values, see Taylor (2005b) |
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– Included in Appendix H.* |
Upper and lower estimates derived using a 68% (or 1 standard deviation) prediction interval for the regression. Note the high (>0.05) p value for the alternative cost / size relationship. | ||||
Typical annual maintenance (TAM) cost | TAM ($2004) = 30.15 x (TAC)0.4741 R2 = 0.80; p = 0.04; n = 5. | TAM ($2004) = 1,102 x (A)0.2364 R2 = 0.48; p = 0.08; n = 7. For literature values, see Taylor (2005b).* |
Upper and lower estimates derived using a 68% (or 1 standard deviation) prediction interval for the regression. Note the high (>0.05) p value for the alternative cost / size relationship. This relationship is also derived from a combined CRCCH data set of bioretention systems, swales and infiltration systems. | ||
Annualised renewal / adaptation cost (RC) | RC ($2004) = 4.1% of TAC p.a. n = 2 | No alternative size / cost relationships in music. For literature values, see Taylor (2005b). | Upper and lower estimates derived using a 84th and 16th percentile, respectively. | ||
Renewal period | 1 year (Default position due to lack of high quality data supporting an alternative period) | No alternative in music. | CRCCH data indicate that the period of time before infiltration media needs replacement varies greatly (e.g. from 0.6 to 25 years). | ||
Decommissioning cost (DC) | DC ($2004) = 35% of TAC n = 2 | No alternative size / cost relationships in music. |
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General caveats / notes for this type of device | * There are several estimates of capital and maintenance costs reported in the literature for infiltration systems (see Taylor, 2005b or Appendix H for a summary). | ||||