Turning Seawater into Drinking Water
The desalination process is a multistep procedure. Follow along with the numbered schematic.
First, water collects in an intake well (1) and then is pumped (2) from the Brownsville ship channel into a pipeline that feeds to one of four pretreatment units. These units remove particles suspended in the water: silt, solids, and contaminants. This pretreatment reduces the work required of the critical reverse osmosis (RO) unit that removes salts, significantly reducing costs.
The four pretreatment units differ from one another:
- two ultrafiltration membrane units (3 & 4) from different suppliers using different technologies;
- a microfiltration unit (5); and
- a conventional treatment unit (6) that uses clarification, sedimentation, and filtration to remove contaminants.
The pilot is testing the various technologies to determine which works most efficiently under the specific conditions existing at the site. For this reason, it also features two parallel trains of holding tanks (7) and RO units (8):
- The water produced from the filtration processes (the "filtrate") is pumped to a holding tank designated only for the UF and MF processes. From there, the filtrate is pumped to an RO unit.
- The water produced from the conventional treatment unit is pumped to a separate holding tank and from there to a separate RO unit.
The RO unit is the key to the desalination process. Here, the water is forced by means of high pressure through fine membranes that keep salt molecules out while allowing water molecules through.
The RO process produces two streams of water: the "product water," cleaned of impurities and salts, and the "concentrate" or "brine," which is the wastewater from the process. The product water is pumped into holding tanks (9). In an operational plant, the product water would be further treated to ensure good taste and then pumped to homes and businesses. Because this pilot is for testing only, the holding tank is not connected to pipes that carry water to users. Instead, water from this tank is pumped along with concentrate and other water used in operating the facility to a mixing tank (10).
From there, the waters flow to a holding lagoon (11). The water is analyzed to determine its pH (i.e., whether it is acidic or alkaline). The pH is balanced (or "neutralized") (12) and then discharged into a receiving ditch (13), which flows back into the ship channel.
