Guidance Manua pdf
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50
Measuring Consumer Satisfaction
A metric called “consumer days below threshold” (CDBT) has been developed to quantify the quality of water delivered to customers. The threshold is a concentration of MIB and geosmin deemed acceptable. For example, 10 ng/L is considered a reasonable taste threshold for both MIB and geosmin. Ten ng/L could therefore be considered a “primary threshold,” and would be abbreviated “CDBT-10.” Because consumer complaints generally do not start until MIB and geosmin levels exceed 20 ng/L (W. Alsmadi, per. comm.), 20 ng/L could be used as a “secondary” threshold for program evaluation (“CDBT-20”).
For a given time period, the CDBT is calculated as: CDBT-XX = service population x number of days with product water below threshold
Where XX is a specified numeric threshold (e.g., 10 or 20 ng/L) Equation 6.1
Consumer days below threshold can be calculated for a given treatment plant. Because water production at a given treatment plant changes over time, the service population for a given month time can be estimated from monthly water production and the average per capita water consumption rate for Phoenix (Table 6-1).
Table 6-1. Average municipal water production for Phoenix, by month, from 1996-99.
Month Average water consumption, gallons/capita-day January 158
February 138
March 174
April 197
May 264
June 291
July 288
August 285
September 238
October 233
November 192
December 164
Average 219
The monthly measured MIB concentration of the product water can be used to determine whether the water delivered is above or below a given threshold.
51 Example: Val Vista, November 2001
MIB concentration in the source water = 17.1 ng/L (measured) MIB concentration at the end of the sedimentation basin = 7.4 ng/L (measured)
Monthly water production = 2,526 million gallons (from production records) Average per capita water production (from Table 6-1) = 285 gallons/day
Total consumer days = 2,526 x 10 6 gallons/(285 gallons/person/day x 31 days/month) = 8,854,678 consumer days
CDBT-10s: Since the MIB in production water was < 10, CDBT-10 = 8,854,678 days
CDBT-20s: Since MIB < 20 ng/L, CDBT-20 = 8,854,678 days
For technical evaluation of the T&O program, CDBT-10s and CDBT-20s should be computed for each plant by month, then aggregated by year and for the entire Phoenix service population. The concept is useful for system-wide program evaluation, by comparing CDBTs among years.
The reduction in CDBTs can be used as a metric of in-plant treatment. This is done by comparing the CDBTs of source water and production water. In the example of the Val Vista WTP, above:
Inlet: CDBT-10 = 0 (since MIB = 17.1 ng/L) Outlet: MIB < 10, therefore CDBT-10 = 8,854,678 days
In-plant treatment therefore increased the CDBT-10s by 8,854,678 days. Treatment would not always increase the number of CDBTs. For example, in May 1999, the following data apply to the Val Vista WTP:
MIB in = 9.7 ng/L MIB out = 8.8 ng/L Monthly water production = 3,055 x 10 6 gallons Average per capita use in May = 264 gallons/day
Note that MIB concentrations in both the inlet and outlet of the plant were < 10 ng/L. 52 Inlet CDBT-10s = 3,055 x 10 6 gallons/(264 gallons/person-day x 30 days/month) = 11,568,005 days Outlet CDBT-10s = 11,585,005 days
Increase in CDBT-10s due to treatment = 11,585,005 – 11,585,005 = 0 days Over 11 million CDBT-10s accrued during May, but this happened because the inflow water was of high quality (MIB < 10 ng/L), not because of treatment within the plant.
For annual evaluations, both concepts, CDBTs and reduction in CDBTs due to treatment, are useful. The concept of reduction in CDBTs could also be used to evaluate cost-effectiveness of T&O mitigation efforts when the costs are clearly defined. Ultimately, most measures can be evaluated on the basis of dollars spent per CDBT ($/CDBT). The CDBT concept was used to evaluate trends in T&O reduction from 1999 to 2001 and to evaluate causes of T&O reduction in 2001 (see Final Report, July 2002).
Review of Operational Issues
A second step in program evaluation is a review of operational issues. Typical operational issues might include: • delays in collecting or processing water samples, leaving operators without adequate information to make adjustments in treatment processes, • unavailability of equipment needed to brush canals due to mechanical or scheduling problems, • inadequate PAC storage and dosing facilities at the Deer Valley and Squaw Peak WTPs. • delays in PAC deliveries, and • poor communication among operational and administrative personnel.
These and other problems can greatly reduce the effectiveness of the T&O management program. Addressing these operational problems generally can improve implementation of the T&O management strategy for the following year.
Institutional and Economic Evaluation
Finally, institutional and economic issues should be examined during each T&O season. The total cost of the T&O program should be evaluated each year. Fairly definitive costs will include the following: • PAC purchases, • chemical/copper purchases, • reimbursement to SRP/CAWCD for canal management for water quality purposes, • water quality sampling, • lab analysis, Yüklə 0,58 Mb. Dostları ilə paylaş:
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