Carlsbad's Recycled Water System Expansion – What is Feasible Once the Low Hanging Fruit has been Picked?
Abstract:Carlsbad Municipal Water District (CMWD) has successfully developed a recycled water system that now serves about 4,000 acre-feet per year (AFY) of demand, which is about 20 percent of its entire water demand. The system is supplied from three different water reclamation plants (WRPs) and includes about 80 miles of pipeline, three storage tanks, three booster pumping stations, and five pressure zones. This extensive system was developed in two phases. Phase I started in 1993 and serves about 2,000 AFY, while Phase II currently serves about 2,000 AFY, added between 2004 and 2009.
CMWD's objectives for this project were to update the system layout under build-out demand conditions and to define the Phase III expansions that could be implemented in the near-term. However, with nearly 20 percent of CMWD's water demand already served, the low hanging fruit had already been picked, making it a challenge to identify system expansions that would be cost-effective and implementable.
The challenges of developing an implementable Phase III plan were overcome through a systematic approach that included: 1) identification of the right water supply mix; 2) identification of new large customers; 3) identification of new service areas outside of existing CMWD boundaries; 4) identification of new small customers, and 5) development of an affordable capital improvement program (CIP).
Identifying the Right Supply Mix CMWD currently obtains recycled water from three existing WRPs and two new supply sources were identified at the start of the project. These five potential sources represented numerous permutations and combinations for the supply system. To pare down the supply configurations, the project team focused on the right mix of sources to minimize capital and operational cost while addressing the uncertainties inherent in future system expansions. The advantages and constraints of the five different sources of supply are summarized in Table 1.
Table 1. Advantages and Disadvantages of CMWD's Supply Sources.
|Carlsbad WRF (existing)|
Largest expansion potential from 4 to 16 mgd.
Least Expensive unit cost (508/af).
Located at sea level, requires the most energy for distribution pumping.
|Gafner WRP (existing)|
Strategically located next to large existing and future customers.
Located at mid-range elevation, reducing pumping requirements.
This 1-mgd plant is owned and operated by another agency.
Space constraints require conversion to membranes for expansion to 3 mgd.
Current and future unit cost are high (988/af).
|Meadowlark WRF (existing)|
Desirably located at high elevation, allowing gravity supply.
Use of this WRP also provides access to seasonal surface storage in Mahr Reservoir (52 MG).
This 5-mgd plant is owned and operated by another agency.
No expansion potential due to lack of new wastewater flows.
Long detention times in Mahr cause water quality issues.
Moderate Unit Cost (561/af).
|Calavera Reservoir SWTF (potential)|
Desirably located at high elevation, allowing gravity supply.
Development of this facility allows utilization of excess flows from Lake Calavera.
Remote location requires long transmission mains and is difficult to access for O&M.
Limited flow availability restricts capacity to 1 mgd.
Small-scale treatment and substantial conveyance make this a costly supply source (1,058/af).
|Shadowridge WRP (potential)|
Strategically located next to large customers.
Allows wholesale service to neighboring agency.
An unused pipeline may allow inexpensive conveyance from this remotely located source.
This rehabilitated plant would be owned and operated by another agency.
The available capacity to CMWD is limited to 0.7 mgd under average day and 0.3 mgd under summer demands.
Use of the abandoned outfall pipeline does require distribution pumping.
Small scale rehabilitation make this the most costly supply source (1,520/af).
|WRF = Water Reclamation Facility; WRP = Water Reclamation Plant; SWTF = Stormwater Treatment Facility;|
As shown, the unit cost of each supply source in /acre-foot depends on the size of expansion potential, site constraints, WRP ground elevation, flow availability, and water quality characteristics. Based on the detailed supply alternatives cost analysis, it was recommended that CMWD continue to use Meadowlark WRP, abandon Gafner WRP, and only expand Carlsbad WRP to accommodate demand growth in the future.
Identifying New Large Customers: The current system already serves most of the large water users, which typically function as anchors in a recycled water system. The customer database was updated and over 400 new customers were identified. The 10 largest potential customers alone contribute to nearly 50 percent of the total potential demand. And connecting these new large customers is critical to keep substantial system expansions affordable. However, this becomes more challenging once the low hanging fruit has been picked. For example, the largest potential industrial customer, the new Carlsbad Power Plant, is part of an ongoing political and environmental debate. This has led to uncertainty for a new pipeline alignment along the coast that could also serve many other pickup customers. Also, the economic slowdown has delayed many new developments and thereby also impacting the timing of connecting smaller pickup customers.
Identifying New Small Customers: The connection of small customers is typically more costly on a unit cost ($/AF) basis because these customers often require longer and smaller diameter pipelines than larger customers. Small “pickup” customers are therefore typically only cost effective when located in close proximity of existing or planned pipelines. Through the update of the customer database and field work, nearly 400 new small customers were identified. These customers contribute to nearly 50 percent of the future demands. The hydraulic model was used to evaluate various alignments alternatives to connect as many small customers as cost-effectively feasible.
Identify New Customers Outside of Existing District Boundary: This study also identified more than 2,500 AFY of potential recycled water demand outside CMWD's boundary that cannot easily be served by the recycled water distribution systems of neighboring districts. As this demand represents more than 40 percent of the future demand potential, serving these customers is crucial for further system expansion beyond the current 20 percent. CMWD is therefore seeking partnerships with four of its adjacent districts (City of Oceanside, Vista Irrigation District, Vallecitos Water District (VWD), and Olivenhain Municipal Water District) to literally reach “outside-the-box.”
Development of an Affordable Phasing Strategy: Various alignment configurations were explored to identify the most cost-effective system layout. Expansion projects were divided into segments that each serves a combination of customers. These segments were then prioritized based on unit cost ($/AF) and the probability that the associated “anchor” customers will convert to recycled water. To differentiate between “Phase III” and “Build-Out” segments, CMWD staff reached out to key potential customers to gauge their interest in recycled water. The segments with committed anchor customers were prioritized and included in the Phase III implementation plan, which moves the selected projects into the design phase and get them shovel-ready. With this detailed Phase III implementation plan, CMWD can apply for grant funding to keep expansions beyond the low hanging fruit affordable.
Document Type: Research Article
Publication date: January 1, 2011
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