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MMWD
MMWD Water Supply Update
On March 19, the Municipal Water District (MMWD) conducted a Board meeting where it shared updated information regarding long-term water supply options including:
- Reservoir expansion
- Spillway flashboards
- Water recycling
- Potable reuse
This is a quick review of these options including their respective economic feasibility when cost information is available.
Reservoir expansion
The staff presented 7 different reservoir expansion options to add 20,000 to 23,000 acre-feet (AF) to the MMWD water storage capacity. Thus, water storage would increase from the current 80,000 AF up to 103,000 AF.
One acre-feet = 326,000 gallons
The three new reservoir options are not competitive. They would call for far higher dam walls and soil embankment ranging from 3.6 to 10.4 million cubic yards.
Among the 4 existing reservoir options, the Nicasio expansion entails inundating the town of Nicasio. That's a non-starter.
Among the remaining 3 existing reservoirs, economic feasibility will depend on the costs of raising the dam walls vs the cost of moving embankment soil. Alpine's 75-foot rise is about twice as high as the other two. Meanwhile, Kent's soil embankment is nearly 3 times greater than Soulajule's and 6.5 times greater than Alpine.
In the absence of more information, the preliminary leader appears to be Soulajule. It will be interesting to reevaluate the reservoir options when cost information is available.
Spillway flashboards
The staff presented a Nicasio spillway flashboards option. With boards' height of only 3.5 feet, MMWD could increase water storage by 3,000 AF. The graph below suggests that MMWD could consider higher spillway flashboards options. Raising them to 6 feet could hold 5,500 AF. That is quite a bit of water at most probably the lowest costs compared to other options. Actual cost not available at this time.
Water Recycling
The staff presented several water recycling options. These are most expensive. They are associated with huge upfront capital costs and operating costs, and very low water yield (acre-feet per year or AFY).
CCF = 748 gallons. The MMWD uses this unit to charge its customers.
To figure the yearly debt service, I assumed the bond financing would have a 30-year term and an interest rate of 4.00% (current Muni Aa rates). The total cost per AFY is equal to the sum of the yearly debt service per AFY and the operating cost per AFY.
Even if MMWD could entirely finance the water recycling capital costs with grants, the recycling options are not economically feasible as their operating costs are already a multiple higher than simply purchasing water from Sonoma (about $2,000 per AF or $4.59 per CCF).
Remember, MMWD has historically purchased as little water as possible from Sonoma, as it deemed it too expensive. It would not make sense for the MMWD to implement water recycling that costs 5 to 10 times more than purchasing water from Sonoma.
If we add the sum of the recycling options (which are aggregated into the combinations), we get less than 1,000 AF. That is less than 5% of the water we need for MMWD customers.
Potable Reuse
The staff presented several water potable reuse options. Similarly to water recycling, potable reuse options are associated with huge capital and operating costs.
The table below details the cost per AFY (326,000 gallons) or per CCF (748 gallons).
Just like for the water recycling options, the potable reuse options are a multiple more expensive than purchasing water from Sonoma.
Conclusion
Based on economics feasibility, water recycling and potable reuse are both way too expensive.
When cost information is available, it will be interesting to evaluate the economic feasibility of reservoir expansion and spillway gates. They are likely to be far more economical than water recycling and potable reuse.