San Quentin desalter an option for Marin MWD

A 1 MGD (3,785 m³/d) desalination plant to supply San Quentin prison is one of a number of options being considered by the Marin Municipal Water District, north of San Francisco, USA, which is facing an annual current supply deficit of 3,300 acre-feet (just over 4 million m³).

Other options include a 5-10 MGD (19,000-38,000 m³/d) desalination plant (see D&WR feature August/September 2008); more conservation than the already expensive and ambitious program planned and budgeted; and a small 300 acre-foot (370,000 m³) expansion to the district’s existing water recycling system, barely sufficient to meet the current shortfall and also very expensive.

One factor working against the larger desalination options is that Marin’s water distribution system cannot accommodate a new supply from a new location without significant increase in distribution system improvements. These have been estimated to be $ 22.6 million for a 5 MGD plant and $ 42 million for a 10 MGD plant.

For its own security reasons, San Quentin Prison has its own tank that holds several days supply for the prison. Because it is in close proximity to the proposed desalination plant, a 1 MGD plant would only require a minimum of distribution investment. However, the economics are not good and do not allow for any future expansion.

A major argument against more conservation is that it reduces the MMWD’s already stretched income, and recycling is of decreasing value in a drought. If the drought of record (two consecutive dry years) recurs, 65% rationing would be necessary in the district, even at present demand levels. Demand, however will also increase by 1,400 acre-feet (1.7 million m³) by 2025.

This is why the 5-person MMWD board is currently struggling to reach a decision on the way ahead.