FINAL REPORT Evaluation of Seawater Desalination Projects ...

EVALUATION OF SEAWATER DESALINATION PROJECTS
PROPOSED FOR THE MONTEREY PENINSULA
possibly dissolved–air flotation, prior to the filtration process would be optimum. While the
CDR does include possible coagulant addition, the feedwater quality may justify the
inclusion of a clarification process to optimize the membrane filtration system cost and
performance. The use of membrane filtration is considered an appropriate selection for this
open intake seawater supply. While existing full-scale implementation of this technology on
seawater is not extensive, the track record as RO pretreatment on other challenging source
waters (e.g., municipal wastewater) is substantial. Additionally, several long-term seawater
pilot studies have provided strong indication of successful application of membrane filtration
on seawater. The CDR states that pilot testing of the pretreatment will be required to make a
final determination of actual chemical requirements and dosages. There are also other
critical membrane filtration design criteria, some of which are not defined in the CDR, which
must be verified through pilot testing. These include the design flux, which defines the
filtrate hydraulic loading on the membrane, typically in units of gallons per square foot of
membrane area per day (gfd). The flux defines the membrane area needed for production of
design capacity. The omission of design flux prevents assessment of the level of
conservatism in the membrane filtration design. The CDR indicates the use of chlorination
of the feed water for biological control and subsequent dechlorination, an approach that has
been identified at other projects as problematic. 12 Long-term pilot testing is needed to
validate a chlorination/dechlorination biological control strategy.
Reverse Osmosis
The CDR describes a traditional approach to seawater RO design that has been successfully
implemented at other sites. However, the operating flux of the RO system, which is a
customary design value to be defined in a CDR, has not been identified. While the stated
characteristics of the CWP RO process are considered to be reasonably conservative and
conducive to an efficient, reliable process, the indicated RO operating pressure (900 psi) is
possibly low. The documents do not provide clear indication of the operating temperature
and flux assumed to arrive at this pressure value. Underestimating the operating pressure
would impact the operation and maintenance (O&M) expense estimates. The level of
redundancy in the treatment system design has not been stated. The RO design includes the
use of an energy recovery device, which recovers energy from the high pressure
(800-950 psi) concentrate stream being discharged. The use of the energy recovery device
reduces the power requirements for the RO feed pump, a substantial component of the cost of
desalination. Energy recovery technology has seen significant advancement in the past few
years and it is important that proposed projects reflect the latest developments. The energy
recovery device performance stated in the CDR is reasonable and appropriate.
12 Hamida, A. & Moch, I., Controlling Biological Fouling in Open Sea Intake RO Plants without Continuous
Chlorination, International Desalination and Water Reuse Quarterly Nov/Dec 1996.
Monterey Peninsula Water Management District 3-5