Project Description
The recycled water project that is currently underway is different than the City’s earlier study prepared in 2008 because: (a) this study focuses on recycled water use for cooling tower makeup water, which requires a lower quality of recycled water, (b) this study includes delivery of recycled water to potable water customers (primarily irrigation customers) within the City’s service area, and (c) this program is eligible for state and federal grants. The current Water Reuse Feasibility Study includes the following key components:
- Development of Recycled Water Quality Objectives,
- Evaluation of recycled water conveyance alternatives and selection of the most cost-effective alternative,
- Evaluation of recycled water treatment alternatives needed to produce recycled water, and selection of the most cost-effective alternative,
- Preparation of a feasibility study that will be eligible for state and federal implementation grants, and
- Assessment of the environmental impacts of the project and development of an initial study under the California Environmental Quality Act.
Water Quality Objectives:
Treatment upgrades at the City’s wastewater treatment plant are needed to address two sets of water quality objectives:
- The State of California’s Division of Drinking Water’s (DDW) Title 22 regulations for disinfection and filtration (solids/turbidity removal). Recycled water meeting these requirements can safely be used for any non-potable use.
- Protection of the Valero Refinery’s cooling tower systems from corrosion, scaling, or biofouling. This will require complete removal of ammonia and reduction of phosphate levels. Chloride in the recycled water is slightly higher than desired and can cause corrosion. Therefore, the City is carrying out a source control investigation to determine the source of chlorides and potential controls to reduce the problem at the source and to avoid costly treatment upgrades. Blending of recycled water with a lower chloride water has been determined to be acceptable as an alternative way of meeting the chloride goal.
Recycled Water Conveyance:
The objective of the recycled water conveyance system is to store and pump the recycled water and deliver it to the Refinery’s point of use and to other City customers’ point of use. Figure 3 illustrates the two primary alternatives that are being analyzed: (a) Below-ground recycled water conveyance through City-owned rights of ways, and (b) Above-ground recycled water conveyance along Refinery owned easements and pipe racks.
The recycled water conveyance alternatives were reviewed and analyzed against a set of evaluation criteria that include economic (e.g., life cycle costs) and non-economic factors (e.g., ease of operations and maintenance, constructability, energy efficiency, construction impacts to the community, ease of recycled water delivery to other City customers, and aesthetic impacts). Alternative 2 was identified as the preferred alternative because it provides a cost effective solution with construction as well as operations and maintenance benefits.
Figure 3. Alternative Recycled Water Pipeline Alignments. Click for larger image.
After identifying the preferred alignment, additional conveyance alternatives were reviewed to refine recycled water storage and pumping details. The conveyance alternatives currently being reviewed include locating a recycled water storage tank at the City’s wastewater treatment facility, at the City’s Corporation Yard, and at the Refinery, as shown in Figure 4.
Figure 4. Alternative Recycled Water Storage Tank Locations. Click for larger image.
Recycled Water Treatment:
Recycled water treatment alternatives were reviewed to identify technologies to reduce ammonia, phosphate, turbidity and suspended solids, and to provide disinfection that meets DDW’s requirements. If the recycled water project is implemented, it will minimize and potentially eliminate the discharge of wastewater effluent to the Carquinez Strait during the dry, summer months.
During wet weather months, the City’s wastewater treatment plant treats higher flows that exceed the recycled water demands. For this reason, the City will still need to maintain the ability to discharge treated wastewater to the Bay and treatment upgrades will need to enable flexibility to meet current as well as future discharge regulations.
Sixteen technologies were reviewed and evaluated against economic and non-economic criteria, similar to the recycled water conveyance alternatives. Non-economic considerations for the treatment alternatives include factors such as ease of operations and maintenance, ability to meet recycled water quality objectives, compatibility with future discharge regulations, constructability, ability to minimize new construction/reuse existing structures, energy efficiency, construction impacts to the community, and aesthetic impacts.
Nitrifying activated sludge followed by cloth media filtration and chlorination were identified as the preferred technologies for production of recycled water that meets the recycled water quality objectives. These technologies were selected to carry forward and be further developed because they are cost effective, reliable, proven, and provide operational flexibility to meet current and future discharge regulations.