Water: Where does it come from?

Even though I make my living as a cartographer and GIS analyst, my collegiate and post-graduate education are in urban planing. I've always been fascinated by cities and how they work--where does the water come from? Where does the sewage go? How is the electricity generated and transmitted? What happens to the garbage? I know the answers to all of these questions when it comes to Seattle, but not with respect to our temporary home here in San Pedro. So I jumped at the chance when Laurie sent Amy and me an invitation to tour the Consolidated Water plant.

Our tour guide was Dee Dillon, the general manager of Consolidated Water Belize Limited (CWBL). Dee is a mechanical engineer and worked on water production and distribution systems in Southern California for 27 years before retiring to Belize. He came out of retirement to manage the CWBL plant. Although CWBL produces the drinking water, it is distributed by the Belize Water Service.

Dee giving us an overview of the plant before the tour begins

The town of San Pedro is located on the island of Ambergris Caye. The island is long, narrow, and low-lying, with very little in the way of freshwater. To produce drinking water, CWBL extracts seawater from two underground wells, one on the south side of the plant, and one on the north side. The wells are about 40 feet deep.

Dee explains how a device very much like an Archimedes screw brings the water up from the well (Laurie and Steve in the background)

The water exits the well at 60 psi

The north well

Where the water from the north and south wells is mixed and enters the plant

The water from the two wells is combined just outside the plant, where it is pushed through a pump/turbine combination that jacks up the pressure to 980 psi! This is extremely high-pressure--the way Dee described it, if that amount of pressure were to be placed against the steel entrance door to the plant, it would blast through anything in its path and end up on the reef, about 3/4 of a mile away (maybe we could get Mythbusters to test this?!) The high-pressure water is pushed into the reverse-osmosis filters, which form the heart of the plant. These plastic filters are formed in a spiral pattern, and although I didn't quite follow all of the chemistry and physics, essentially the high pressure in conjunction with the structure of the filters literally rips the salt molecules from the water molecules. They also remove all particles larger than 5 microns, which eliminates almost all silt, bacteria, protozoans, and any other undesirable components of the water.

Cranking up the pressure

The reverse osmosis portion of the process--the water entering the white tubes is at 980 psi!!

At the other end of the filters, fresh water emerges at 10 psi

The injection well where the briny sludge is pumped underground

The salty sludge ends up in the center of the filter, and is routed back outside the plant where it is injected back into the ground, about 90 feet below the wells that are pumping out the seawater.

Although the water is now fresh, it still contains hydrogen sulfide gas (which smells a bit like rotten eggs). The water is also too basic at this point, so citric acid is added in a mixing valve, then the water is sent into a tall cylinder filled with mostly hollow spheres that use a catalytic reaction to remove the hydrogen sulfide gas, which is vented to the air. Interestingly, over time the spheres form a snot-like substance Dee calls "jelly" that must be cleaned off about once a year.

Citric acid is added here to lower the pH level to somewhere between 6.5 - 7.5

The pH-balanced fresh water flows out of the building in the blue pipes to the top of the tower...

...where it is filtered through thousands of these spheres to remove the hydrogen sulfide gas

After the gas is removed, chlorine is added to ensure the water stays free of bacteria as it makes its way through the Belize Water Service (BWS) distribution network.

The de-gassified water is passed over a series of weirs, where the chlorine is mixed in

The water is regularly tested for bacterial contamination, and Dee said they have never found fecal coliform bacteria in the water--which is a good thing! The fecal coliform bacteria is very common and is not harmful in and of itself, but its presence indicates that conditions are right for the nastier forms of bacteria to reproduce.

Culturing samples

The whole system is powered by a pair of diesel generators. Dee explained that the plant was originally powered by an electric motor, but the electricity supply to the island is so unreliable that the motor would frequently shut down, wreaking all kinds of havoc on the equipment in the plant.

Hard to convey how LOUD these suckers are

The plant is currently producing about 600,000 gallons of fresh water a day (that's about 7 gallons every second), 420,000 of which is being consumed. The balance is being routed to the million-gallon CWBL tank, to be used during high season in December and January.

One MILLION gallons

Dee said the plant is running at or above capacity every day, and he is a vocal proponent of conservation. I asked him how San Pedro's water usage compares to the US--he said that even compared to California, one of the most water-conscious states, residents here use about four times LESS water. However, until additional capacity is added, we'll all have to make do with a little less as more residents and visitors come to the island.

It was a fascinating tour, and quite obvious that Dee is really good at what he does and loves his job. We are lucky to have him in charge of producing our water.