By Boaz Keinan
In December 2015, IDE Technologies opened the largest desalination plant in the Western Hemisphere. The Claude “Bud” Lewis Carlsbad Desalination Plant is in California, a region which has been threatened by extreme drought in recent years. Developed and owned by Poseidon Water, the plant overcame significant practical, regulatory and economic hurdles to deliver a cost-effective and environmentally-friendly water supply to 300,000 residents and businesses in San Diego County.
The Carlsbad plant taps into the largest reservoir in the world – the Pacific Ocean. It uses the seawater reverse osmosis (SWRO) technique to produce more than 204 million litres of drinking water per day. In reverse osmosis, pumping energy moves seawater through a series of filtering membranes with pores that let water molecules permeate but retain salt and debris. Utilizing a proprietary design, the plant has implemented a minimal number of independent trains fed by both feed pumping centers.
The plant is located adjacent to the Encina Power Station, so project financing relies on a true partnership model. It shares the existing intake and outflow systems with the Encina Power Station and takes up to 420,000 m3 per day of cooling water from the power plant. The water is then filtered through gravel and sand to reduce particulates, before going through reverse osmosis filtration.
Half of the saltwater taken into the plant is converted into pure potable water and the rest is discharged as concentrated brine. The outflow of the plant is put into the discharge from the power plant for dilution, for a final salt concentration about 20% higher than seawater. Desalination plants primarily discharge water with about 50% extra salt. This leads to dead spots in the ocean as the super-saline brine does not mix well with seawater.
Seawater from the Encina Power Station discharge channel flows through the intake vault and common inlet line and is distributed to the intake pumps. The intake vault is adjacent to the power station discharge channel and is equipped with a stop log for isolating the intake pump station from the channel during heat treatment of the power station cooling system, or for maintenance purposes. The seawater pumping station includes three vertical intake pumps: two operating and one stand by. Each intake pump provides up to 216 million litres/day. The intake station includes seawater quality monitors that allows online monitoring of the raw seawater quality.
Pre-treatment and post-treatment processes
Pretreatment is composed of a flocculation stage and a gravitational dual media filter stage. Pretreatment feed flow is controlled by the four flocculation chamber flow control valves. Coagulant and flocculant are added to the water at the static mixer, upstream of the flocculation chambers.
After coagulation and flocculation, the water enters the common feed channel and is distributed to 18 dual media filters. Each filter contains two filtration layers: coarse coal (anthracite) and fine silica sand.
The flocculation basin facilitates the process to separate suspended solids and the remaining impurities are removed through dual media gravity filtration. Filtered seawater is then pumped by the low pressure feed booster pumps to the reverse osmosis section for desalination. Post-treatment at Carlsbad involves re-mineralization of the desalinated water, followed by final disinfection.
SWRO pressure center
IDE designed the Carlsbad plant based on its proprietary multi-media filtration (MMF) and pressure center design, which has shown increased availability and reliability, higher efficiencies and greater flexibility under variable operational modes, and lower capital expenditure/operating expense costs. It utilizes horizontal centrifugal axially split high pressure pumps, with an optimized size in order to achieve the highest efficiency. Optimization is based on the pumps specific speed (Ns), pump flow rate, total dynamic head, etc.
The pressure center offers economy of scale and simplified erection, and allows feed pressure to the RO trains to be increased or decreased. This means that all RO trains remain operational during periods of reduced production, thereby decreasing system recovery, without increasing the total feed to the plant.
The Carlsbad plant produces 8,517 m3/hr at its peak. The operating pressure of the seawater reverse osmosis section varies from 60 bar to 65 bar, according to the seawater characteristics and the operating regime.
Carlsbad is the first major California infrastructure project to eliminate its carbon footprint. The plant has a system to reuse energy that is otherwise lost in the desalination process. This makes it possible to reduce the total energy consumption of the plant by 46%.
At all stages of the process, IDE adopted mitigation measures to preserve the region’s valuable resources. The increased salinity of the brine discharged to the sea does not have an adverse effect on marine organisms in the vicinity of the discharge channel. After the brine is returned to the discharge channel, and prior to its discharge to the Pacific Ocean, the brine stream is diluted with the return flow from the power plant’s cooling water system.
The Carlsbad Desalination Plant has already produced more than 55 billion litres of high-quality water, and will generate over $50 million annually for the regional economy.
Boaz Keinan is with IDE Technologies. This article appears in ES&E Magazine’s December 2016 issue.