So what do you think happens after you flush the toilet?
Would you imagine that your waste is eventually disposed of by ravenous bacteria?
That is the method of disposal for about 20 percent of the wastewater in Cayman, which flows through the treatment plant that is operated by the Water Authority. The wastewater comes in sludgy and emerges on the other side as a clear, treated effluent that gets buried deep within a disposal well.
And since a small percentage of homes are connected to the local sewer system, there are different rules required for different types of residences arrayed around Grand Cayman. Twenty percent will be disposed of at the wastewater plant, but the vast majority of waste is disposed on site.
“If a development produces under 1,800 U.S. gallons of wastewater per day, then that can be a septic tank,” said the Water Authority’s Hannah Reid of Cayman’s sewer regulations. “That’s a small house or at most a few houses or apartments, but not a large multi-residential development. All commercial facilities have different requirements. Anything over 1,800 gallons of wastewater produced per day has to have on-site water treatment, which is basically like a smaller version of our plant. They’re biological treatment systems that are called aerobic treatment units. Most large condo complexes will have an ATU.”
Cayman’s treatment plant was originally brought into operation in 1988, but at that point, it was just a series of stabilization pools that used sunlight to treat the wastewater.
That was highly energy efficient but required a large land area and a long time to work, so Cayman switched to sequencing batch reactors in 2005.
Now, the treatment plant can go through 2.2 million gallons of wastewater per day, and the aerobic bacteria will eat the caloric equivalent of about 20,000 McDonald’s Big Mac sandwiches per day.
How exactly does that work?
The bacteria are already in the wastewater and the Water Authority does its best to make sure the organisms have the conditions necessary to grow and thus dispose of the waste.
“We need to make sure there are adequate levels,” said Ms. Reid of the aerobic bacteria. “We need to make sure enough oxygen is available for them. They need oxygen in order to be able to grow and thus digest the waste. If there’s too little oxygen, that’s a problem. And if there’s too much oxygen, we are wasting energy. They’re constantly maintaining the level of bacteria to make sure it’s correct.”
The wastewater is rendered clean and clear after going through the sequencing batch reactors, and it’s discharged into deep disposal wells never to be seen again. Those wells are kept far apart from the abstraction wells for the reverse osmosis plants, but in theory, the treated wastewater is clean enough to recycle.
“All water winds up getting recycled at some point,” said Ms. Reid when asked if wastewater eventually ends up getting reused and turned into drinking water. “The long-term answer is yes, because the amount of water that exists on Earth is the same amount of water that existed on Earth when dinosaurs were here. We’re using the same water over and over again.
“That’s something that’s becoming of greater interest to countries around the world because of the potential for the reuse of wastewater, especially in water-scarce countries. We don’t currently reuse the wastewater in Cayman, but it is something that’s drawing a lot of attention in other countries.”