Plain Water

The vocabulary of water and its treatment, one term at a time: what it means and why it matters.

The water industry has a vocabulary, and most of it is never explained to the people who drink the water. TDS, BOD, COD, pH, reverse osmosis, brine, sludge, coagulation, disinfection: each is a word that decides what a plant does and what a river gets. These articles take the terms one at a time.

Every piece starts with what the thing is, in plain words, and then with why it matters: what the measurement tells an operator, what the process does inside the tank, what goes wrong when it is skipped. Where a term has a number attached, the number is explained: why the arsenic limit is ten micrograms a litre, why the PFAS limit is four parts per trillion, why a kettle furs up in hard water.

Together they are a working knowledge of how water is treated, cleaned and reused, the kind an engineer carries and a citizen can use to read a water bill, a news story or a label.

Plain Water: Ballast Water Every large ship carries seawater in its hull for stability, pumps it in at one port and out at another, and in doing so moves about five billion tonnes of water and everything living in it around the world each year. What ballast water is and why ships need it, what it carried and where, how a mussel from the Caspian closed the intakes of the Great Lakes and a jellyfish from America emptied the Black Sea, what the treaty of 2004 required and why it took thirteen years to come into force, and how a ship now treats its water before it lets it go. 26 September 2026

Plain Water: Rainwater Harvesting A house with a hundred square metres of roof in a place with six hundred millimetres of rain sheds about fifty thousand litres a year, which is most of what a careful household uses, and almost all of it runs down a pipe into a drain. What rainwater harvesting is and what it can supply, why a roof is the cleanest catchment there is and what still needs to come out, what a tank costs and how big it should be, why Bermuda and Chennai made it the law and Australia made it a habit, and why the water that falls on a city is the one source it has not been charged for. 26 September 2026

Plain Water: Private Wells About forty three million Americans, and a share of the population of every country, drink from a well in the garden that no utility treats and no regulator tests, and when the national survey looked, one well in five had something in it above the limit. What a private well is and why it sits outside the rules, what the surveys found in the water, why the owner is the only inspector, what a test costs and how rarely it is done, and why the water that people trust most is the water that nobody checks. 25 September 2026

Plain Water: Water Meters A household that gets a water meter uses about fifteen percent less water than it did before, without being asked to, and half the households in England still have none. What a meter does and why the number falls, what a bill without a meter is based on, how the tariff can be shaped so that the first litres are cheap and the pool is not, what a smart meter finds that a plain one cannot, why a country abolished water charges after protests, and why the cheapest water a utility can find is the water a meter saves. 25 September 2026

Plain Water: Chlorine Byproducts The chlorine that has kept drinking water safe for a century reacts with the traces of leaves and soil in it to make a family of compounds that were discovered in tap water in 1974 and have been regulated since, at a limit of a hundred micrograms a litre. What trihalomethanes are, how a water works makes them without meaning to, what the studies of bladder cancer found, why the answer is to take out the organic matter and not the chlorine, and why the trade between a small risk and cholera has only one right side. 24 September 2026

Plain Water: Groundwater Almost all of the fresh water on Earth that is liquid and not in a lake is underground, and the world pumps about a thousand cubic kilometres of it a year, a third of what it uses, and puts back less. What an aquifer is and how it fills, why a well is a loan and how much of the world is living on one, what the satellites saw, where the ground is sinking, and why the water under a field is the hardest water on this site to govern. 24 September 2026

Plain Water: Storm Overflows The sewers of most old cities carry rain and sewage in the same pipe, and when it rains hard enough the pipe cannot hold both, so the mixture is let out into the nearest river or sea through an overflow built for the purpose. England's overflows spilled for 3.6 million hours in 2023. What a combined sewer is, why it was built that way, what an overflow releases, how the monitoring turned a hidden practice into a public one, what London's twenty five kilometre tunnel does about it, and why the fix is slow. 23 September 2026

Plain Water: Cholera Cholera is the disease that taught the world what a water supply is for. It killed by the hundred thousand in the cities of the nineteenth century until a doctor traced an outbreak to one pump, and it has been absent from every city with a treated supply and a sewer since. It still kills tens of thousands a year, in the places that have neither. What the bacterium does, how a pump handle changed medicine, why it is a water problem and not a medical one, and where it is now. 22 September 2026

Plain Water: Lead The water that leaves a treatment plant has no lead in it. The lead comes from the pipe between the street and the kitchen, laid a century ago because lead was soft and cheap, and it comes off in amounts that depend on the chemistry of the water running through it. What Flint got wrong, why there is no safe level, why the fix is a pipe and not a plant, and how to tell whether your own tap has one. 22 September 2026

Plain Water: Nitrate Nitrate is the fertiliser that grew the food and the contaminant that followed it into the well. It is in the groundwater of every farming region on Earth, it takes decades to arrive and decades to leave, and it is the reason a large share of Europe's and America's rural water fails its standard. What nitrate does in a body and a river, where it comes from, why a well can take thirty years to catch up with a field, and what the treatment costs. 22 September 2026

Plain Water: Tailings Behind every mine on this site is a dam holding a lake of wet ground rock, and the dam is built out of the rock it holds. There are several thousand of them, some among the largest structures on Earth, and when one fails a valley is buried in minutes. What tailings are, why the water in them is the problem, how the dams are built and why they fall, what Brumadinho changed, and what dry stacking would cost. 22 September 2026

Plain Water: Antiscalants A reverse osmosis plant concentrates everything it rejects, and the salts of hardness come out of solution on the membrane as scale, where they block it. A few milligrams per litre of a chemical that persuades crystals not to grow is what keeps the plant alive. What scale is, why the membrane is where it forms, and how the dose is decided. 21 September 2026

Plain Water: Arsenic In the 1970s, millions of hand pumped wells were sunk across Bangladesh to give villages water free of the germs that killed their children. The water was free of germs. It carried arsenic, from the rock, at levels that poison slowly over decades. What happened, why nobody tested for it, what the numbers mean, and what a village can do about a poison it cannot see or taste. 21 September 2026

Plain Water: BOD The water industry's second number for dirt is measured by sealing a bottle in the dark for five days and seeing how much oxygen disappears. Why five days, why the dark, what the number says that COD cannot, and how it decides whether biology can clean a water. 21 September 2026

Plain Water: Brine Every membrane plant, every softener and every reuse project produces a second stream that carries everything the clean water left behind. The industry calls it brine or concentrate, it is the leftover nobody puts on the brochure, and what happens to it is the question every water recovery project ends up in a meeting about. 21 September 2026

Plain Water: CETP When a district holds two hundred small factories and none of them can afford a treatment plant, the answer is one plant for all of them. What a common effluent treatment plant is, why India built hundreds, why they are harder to run than any plant that serves one owner, and what decides whether the shared kidney works. 21 September 2026

Plain Water: Coagulation The oldest chemical trick in water treatment is to add metal to the water on purpose. What coagulation does to the particles a filter cannot catch, why aluminium and iron work, how a row of six beakers on a bench decides the dose for a whole city, and where the dirt goes afterwards. 21 September 2026

Plain Water: Day Zero The phrase entered the language in 2018 as the date on which a major city's taps would be switched off. What it actually meant, how a city gets there, what the last weeks before it look like, and why the number that mattered was not the date but the litres per person per day. 21 September 2026

Plain Water: Desalination About one percent of the world's people drink the sea, and in the Gulf it is most of them. How seawater becomes drinking water, why the process moved from boiling to membranes, what it costs in money and energy, and why a plant on a coast is easier to build than to explain. 21 September 2026

Plain Water: Disinfection The step that made cities survivable is a small dose of chlorine, a lamp, or a gas made from air, applied to water that already looks clean. How each one kills, why a plant chooses one over another, the parasite that chlorine cannot touch, and what the smell at the tap is for. 21 September 2026

Plain Water: Fluoride Fluoride is the substance that water utilities in some countries add on purpose and utilities in others spend fortunes to remove. At a milligram per litre it protects teeth. At three or four, in the groundwater of the East African Rift, the plains of India and the villages of northern China, it stains teeth and bends bones. What fluoride does, where it comes from, why the line is so narrow, and what it takes to get it out. 21 September 2026

Plain Water: Greywater Half to two thirds of the water that leaves a home was only lightly used, in the shower, the basin and the washing machine. The industry calls it greywater, it could flush every toilet in the house, and the reasons it usually does not are worth understanding before anyone fits a tank. 21 September 2026

Plain Water: Hard water The scale in a kettle, the film on a shower screen and the reason soap will not lather in some towns are all the same two minerals. What hardness is, why it is the most harmless of water's complaints and the most expensive for industry, and what a softener actually does with it. 21 September 2026

Plain Water: Leaks About a third of the water that the world's utilities treat and pump never reaches a customer. It leaks from cracked mains, runs from broken pipes under the street, and is stolen or unmetered. The industry calls it non revenue water, and it is the largest source of new water in almost every city, already treated, already pumped, and lost within the city's own pipes. 21 September 2026

Plain Water: Legionella The bacterium that killed twenty nine people at a convention in Philadelphia in 1976 lives in warm water, in the pipes of buildings, in cooling towers, hot tubs and showers, and it kills by being breathed rather than drunk. What Legionella is, why it is the water disease of the rich world, why the answer is a temperature, and what Flint's outbreak had to do with the lead. 21 September 2026

Plain Water: Membranes The most important piece of equipment in modern water treatment is a sheet of plastic with holes in it too small to imagine. What a membrane is, the four sizes it comes in, what each one catches, and why a plant built around them looks nothing like the tanks of the last century. 21 September 2026

Plain Water: Microplastics Every litre of bottled water tested with the newest instruments holds around a quarter of a million pieces of plastic too small to see, and tap water holds fewer, because a treatment plant removes most of them. What microplastics are, how they got into water, what a litre actually contains, what the science does and does not know about the harm, and why the answer is a filter that already exists. 21 September 2026

Plain Water: Nitrogen The element that feeds the world's crops also arrives at every sewage works, as ammonia, and does three kinds of harm to a river if it is let through. What nitrogen does in water, how a plant removes it with two sets of bacteria that want opposite things, and why the dead zones at the mouths of rivers are made of fertiliser. 21 September 2026

Plain Water: PFAS A family of several thousand chemicals that repel water and grease, that are in nearly every person's blood, and that do not break down. What PFAS are, how they got into drinking water, why the new limits are measured in parts per trillion, and what it takes to get them out. 21 September 2026

Plain Water: pH The number everyone half remembers from school is the one a water operator checks first and adjusts most often. What pH measures, why the scale is stranger than it looks, what it does to fish, pipes and bacteria, and how one city's failure to manage it poisoned its own water supply. 21 September 2026

Plain Water: Phosphorus The other fertiliser arrives at every sewage works too, and unlike nitrogen it cannot be sent back to the air, because it never came from there. It came from rock, in a handful of countries, and it is not unlimited. What phosphorus does in a lake, how a plant takes it out, and why the pile at the back of the plant is starting to look like a mine. 21 September 2026

Plain Water: Reverse osmosis The process that turns seawater into drinking water and used water into new water is a plant's worth of pumps pushing against a force that nature runs the other way. What osmosis is, why reversing it takes so much pressure, what the membrane actually does, and why the last thirty years cut its energy in half. 21 September 2026

Plain Water: Ultrapure water There is a kind of water so clean that it would leach the minerals from a person who drank it and dissolve the pipe it was carried in. Chip fabs, pharmaceutical plants and power stations make it by the tonne. What ultrapure water is, why anything at all in it is a problem, and how the purest substance in industry is made from a tap. 21 September 2026

Plain Water: ZLD Zero liquid discharge is the factory that sends nothing out of its pipe at all, because it has no pipe. Every drop is recovered and every gram of salt comes out as a solid. What the process is, what it costs, why a court in India made it law for a whole town, and when it is the right answer. 21 September 2026

Plain Water: TDS Total dissolved solids is the single most useful measurement in the water business. It explains why water tastes of anything, why limescale forms in a kettle, and why the hardest part of cleaning water is the part nobody can see. 24 August 2026

Plain Water: Sludge A treatment plant does not make dirt disappear. It gathers everything it took out of the water into one pile, and that pile is almost entirely water, eats up to half the running costs, and is at once a fertiliser, a fuel and a problem. Where the dirt actually goes. 12 August 2026

Plain Water: After you flush The flush is the last most people see of their water. From there it starts a journey under every street in the city, powered mostly by gravity, to a plant that runs on bacteria and partly on its own gas. The trip takes about a day, and most of the world's water never makes it. 10 August 2026

Plain Water: COD The water industry has one number for how dirty water really is, and by that number a glass of milk is hundreds of times worse than raw sewage. What chemical oxygen demand measures, how it is measured, and why every discharge permit in the world is written around it. 10 August 2026

Plain Water: ETP Beside almost every serious factory in the world stands a building that most people never notice. It decides whether the river next door stays alive, it runs on bacteria, and its biggest bill is air. What an effluent treatment plant is and how it works. 10 August 2026

Plain Water: Wastewater The word covers a shower drain, a dye house outfall and the rain off a car park, which is why it tells you almost nothing on its own. What wastewater actually is, what it carries, and why an industry exists for the last one percent of it. 6 August 2026