The Guild has been working on a solution to reduce disinfection byproduct (DBP) levels in the treated water since the Division of Drinking Water (DDW) issued a violation citation in April 2021. The creek water we treat contains dissolved organic carbon (DOC), which the existing microfiltration skid can’t remove. Chlorine injected to further disinfect the filtrate reacts with DOC to breed DBPs as the water ages in the storage tank and distribution system. Based on previous testing and analysis, there are two feasible options: 1) Chloramination, which reduces free chlorine residual level by injecting ammonia, or 2) Pass the filtrate thru Granular Activated Carbon (GAC) before chlorination, which reduces DOC concentration. 20-year service life cost estimates showed that chloramination, in addition to being unpopular, is more expensive in the long run due to the need for precise control and increased testing.
The board selected the GAC option.
The Granular Activated Carbon (GAC) project has four main components:
- Specify and order a two-vessel GAC skid The contract was awarded to Tetrasolve Filtration. The forecast shipping date is Aug. This is expected to be the critical path.
- Design and construct a concrete pad for the GAC skid outside the filter treatment plant.
- The pad has been designed based on load bearing and seismic parameters provided by a structural engineer, the site is being prepared, and pad construction will be awarded soon.
- Specify the plumbing between the filter treatment plant and GAC skid. The contract to detail the design and install the plumbing was awarded to Express Plumbing (EPS).
- Install, connect, backwash, and operate the new GAC processing stage. Hopefully complete in Sept.
Concerning regulatory requirements:
- San Mateo Co. requires no planning or building permits (this took seven weeks to determine).
- Progress Reports are being submitted to DDW quarterly.
- An Environmental Intake Form was submitted to DDW in accordance with CEQA requirements.
- A Technical Report describing the project was submitted to DDW as a condition to start construction and was accepted.
- An Operations Plan and 100% as-built drawings need to be submitted to DDW.
- If all goes well, DDW will issue a revision to our Domestic Water Supply Permit to approve and require the GAC stage upgrade.
