The first stage of treatment at the reverse osmosis plant in Carlsbad, California, is to flow the water through rock, sand, and gravel as shown here. Would this step remove particulate matter? Would this step remove dissolved salts?

[Section 18.4]

Video transcript

All right. Hello everyone. So this question says that the first stage of treatment at a reverse osmosis plant is to pass the seawater through layers of anthracite sand and gravel. What would this step remove? Option? A says particulate matter only. Option B says dissolved salts only option C says both particulate matter and dissolve salts and option D says neither particulate matter nor dissolved salts. All right. So here recall first and foremost that osmosis is the net movement of solvent molecules through a semi permeable membrane. And so in the case of reverse osmosis, the solvent, in this case, water is going to pass through from a more concentrated area towards a less concentrated area or a more dilute area. And so the reason it's called reverse osmosis is because normally we would see the opposite movement. In other words, the solvent would go from a less concentrated area to a more concentrated area. And so during this reverse osmosis process, the semi permeable membrane being used here are called hollow fibers. And so under pressure, saline water which is essentially salt water is introduced through the fibers under pressure. It passes through the semi probable membrane and desalinated water is recovered. And so here, the step involving the hollow fibers is the true reverse osmosis step. But before that, the first step is essentially a physical filtration method. And when I refer to physical filtration, I'm mainly just referring to the removal of particulate matter. Before the actual reverse osmosis step recall that the main goal of water treatment is to convert our source water into usable water or cleaner water. And so here in the first step, particulate matter is filtered out through these layers of anthracite sand and gravel. However, after this first step, it's worth mentioning that salts or ionic compounds are much smaller, then these layers filter. And so because of this, the salts present in our source water are going to pass through after this first step, which is why hollow fibers or rather the part involving the hollow fibers is needed to filter further. But at the very least, this first step is only going to remove those larger or that larger particulate matter, which means that our answer is option a and the multiple choice and there you have it. So with that being said, thank you so very much for watching and I hope you found this helpful.