Source Water and Purification

Salt water is made by mixing salt into fresh water (Module 5). Every week, more fresh water goes in to replace what evaporates. Whatever that fresh water carries goes into the system with it.

Evaporation makes this worse. When water evaporates, only pure water leaves. Everything dissolved in it stays behind (Module 5). So anything that comes in with top-off water builds up, week after week, unless water changes take it out.

That's why reef systems are filled and topped off with water that's been purified first. The goal is fresh water that adds nothing: the salt mix supplies what the animals need, and the source water adds nothing on top.

Water companies disinfect tap water so it's safe to drink. In Japan the law requires tap water to keep at least 0.1 mg/L of chlorine all the way to the tap. Some places use chloramine instead, a mix of chlorine and ammonia. Chloramine can release ammonia, which is deadly to aquatic animals.

Depending on where it comes from, tap water can also carry phosphate, nitrate and silicate, which feed nuisance algae and diatoms, and metals like copper, which is toxic to many invertebrates. Calcium and alkalinity vary too. Four tap water samples, all from within about 30 miles of each other, were tested: every one raised phosphate and alkalinity, and two raised alkalinity enough to threaten a reef.

None of this is a problem for people. It's a problem in a closed system where it builds up and where some animals react to very small amounts. And you can't know what your tap water carries today without testing it.

Well water isn't treated by a water company. What it carries depends on the ground it comes from, and you won't know until it's tested.

Natural seawater is collected from the ocean. It carries whatever is living and dissolved in the water at that spot and that day: plankton, larvae of pests, disease organisms, and anything washed in from the land. Using it means treating it and accepting that every batch is different.

Purified water, made with an RO/DI unit, starts as tap water and comes out with almost nothing dissolved in it. Every batch is the same. That's what makes it the safe starting point: when something goes wrong, the source water is one thing you can rule out.

Water passes through the stages in order. The sediment filter traps particles like sand, rust and dirt so they don't clog what comes after. The carbon block removes chlorine and chloramine. The RO membrane (reverse osmosis) does most of the work, removing most of what's dissolved in the water. The DI resin (deionization) catches what the membrane let through, so the water comes out at 0 TDS.

The membrane doesn't just hold back what it removes. It sends it away in a stream of waste water, which carries the rejected substances down the drain. That waste water is far dirtier than the tap water, and it never goes into a system.

Because each stage protects the next, a problem in one stage shows up later, in another. A used-up carbon block lets chlorine damage the membrane. A damaged membrane uses up the DI resin faster. Knowing the order is how you trace a problem back to where it started.

As the sediment filter fills with particles, it slows the water. A pressure gauge between the carbon block and the membrane shows this as a pressure drop. The cartridge also darkens as it clogs.

The carbon block removes chlorine and chloramine. This matters because the membranes used in these units can't tolerate chlorine: even very low levels damage them, and the damage can't be undone. Not every carbon block removes chloramine, so where chloramine is used, the carbon has to be rated for it.

To check the carbon, run the unit for about 30 minutes and test the waste water for chlorine with a test strip. Change the carbon block right away if total chlorine in the waste water is above 0.5 ppm. Sediment and carbon are often changed together on a schedule, with the tests as a backup.

Rejection is the share of dissolved solids the membrane removes. To work it out, measure the TDS of the tap water and the TDS of the water coming out of the membrane, before the DI resin. If tap water reads 100 ppm and membrane water reads 2 ppm, the membrane removed 98 parts in 100: 98% rejection.

Membranes are often rated between 96% and 99%. As a membrane ages or gets damaged, rejection falls and the TDS after the membrane creeps up. When it drops below its rating, it's time to replace it. With the pre-filters changed on time, a membrane can last two years or more.

How much water the unit makes depends on conditions. Colder water passes through the membrane more slowly: about 3% less water for every 1 C colder. Lower water pressure also means less water. So the same unit can make noticeably less in winter. Slower isn't the same as broken: the TDS reading tells you whether the water is still clean.

DI resin is made of tiny beads that hold onto dissolved substances as water flows past. Every bead can only hold so much. When the resin is full, substances start passing straight through, and TDS after the DI rises above 0.

Color-changing resin shows how much is used up: the beads change color as they fill, so the change moves through the canister. A maker's guide says to replace it when about 75% has changed color, and never to wait until it's all changed. Once the color has gone all the way through, the water coming out is no longer pure.

Refillable resin has to be packed tightly. If it's loose, water finds channels through it and skips the beads, and TDS rises even though fresh resin is still in there.

TDS means total dissolved solids, measured in ppm (parts per million). A TDS meter works by measuring how well the water carries electricity. Pure water barely carries any. Dissolved salts and minerals let it carry more. So the higher the reading, the more is dissolved.

A TDS meter tells you how much, not what. It can't tell chlorine from copper or phosphate. It's a quick check that the unit is doing its job, not a full test of the water.

Readings at different points tell you different things. Tap water TDS is the starting point, and it changes with place and season. After the membrane shows how well the membrane is working. After the DI resin is the water you'll use, and it should read 0. Many units have meters built in at these points. A handheld meter works too.

Start with what you noticed: TDS after the DI is above 0. That could be the resin or the membrane. Don't guess. Measure TDS after the membrane and compare it with what it usually reads.

If the membrane water reads its usual low number, the membrane is doing its job, and the DI resin is used up. Check the resin color to confirm. If the membrane water reads higher than usual, the membrane is letting more through. Before replacing it, check the carbon block: if chlorine got past the carbon, a new membrane will be damaged the same way.

Low production is a different problem. Look at the pressure gauge: a pressure drop points to a clogged sediment filter or carbon block. Then think about the season: colder water means less water made, with nothing broken.

This is the thinking loop from Module 1: observe, check the facts, list the causes, and rule them out one reading at a time. Write down what you found and what you changed.

Keep purified water in a container used for nothing else. A bucket that once held soap, cleaning products or chemicals can put them straight back into the water. Keep it covered so dust, splashes and anything else in the room can't get in, and label it so nobody fills it from the tap or uses it for something else.

Don't be alarmed if stored purified water gives a low pH reading. Pure water takes in carbon dioxide from the air, and that lowers its pH: even normal rainwater is slightly acidic for the same reason. It doesn't mean anything got in. TDS is the check that tells you whether the water is still clean.

Evaporation takes out pure water and leaves the salt behind, so salinity rises unless fresh water replaces it (Module 5). Top-off water is always fresh, never salt water. Topping off with salt water adds salt every time, and salinity climbs.

Many systems use an automatic top-off (ATO) that adds fresh water whenever the water level drops (Module 20). Some are fed from a container of purified water. Some are plumbed to the water supply through a filter, and can keep refilling without limit.

An automatic top-off can't tell evaporation from anything else that lowers the level. If you take water out for a task, it refills with fresh water and salinity drops. The fix is to add the same amount of matched salt water before you take water out, so the level never drops. If it's fed through a filter, that filter has to be kept up too: a used-up filter means unfiltered water going straight into the system.

Before using purified water, check that TDS after the DI reads 0, that the resin is well short of fully changed, and that the container is clean and covered. It takes a minute.

Write the readings down, along with every filter, membrane and resin change. A single reading tells you about today. A log tells you about the trend: TDS after the membrane slowly rising, or resin lasting less time than it used to, shows a problem coming before it reaches the tanks.