Storage tanks and flow rates: two countries, two machines

There are thirteen water purifiers here: six sold only in India, seven sold only in China, and none sold in both. Storage runs from 0 L to 18 L with a median of 4 L. Flow runs from 3 L/h to 160 L/h with a median of 20 L/h. Those two spans sound like one catalogue with a wide range in it. They are not — they are two clusters with almost nothing between them.

All six Indian machines have a storage tank, and the tanks run from 7 L to 18 L. Six of the seven Chinese machines hold nothing at all: zero litres of storage, and flow rates of 90, 100, 120, 120, 150 and 160 L/h. The seventh Chinese row, Xiaomi’s countertop hot-and-cold dispenser, holds 4 L in a reservoir you fill by hand, which is what lets it work with no plumbing at all. So the median storage figure of 4 L belongs to a single unusual machine sitting in the gap, and the median flow of 20 L/h describes nothing in the Chinese half of the list.

It is tempting to read that as a quality gap — one market got the modern machines and the other did not. That reading is wrong, and it will lead you to buy badly if you carry it between countries. The tank is not a compromise. It is an answer to a specific question about infrastructure, and where that question is asked, the tankless machine is the worse product.

Two facts about Indian domestic supply drive the entire design. The first is that piped water in a great many Indian cities is intermittent: the mains is pressurised for a few hours a day on a schedule, and households store what arrives in overhead or underground tanks and draw from those the rest of the time. The second is that power cuts are a normal event rather than an emergency. Neither is true of the Chinese apartments the tankless machines were designed for.

Both facts hit reverse osmosis in the same place, because RO needs pressure to work at all. The membrane only passes water when the applied pressure exceeds the feed’s own osmotic pressure with enough margin left to drive flux — in domestic machines, roughly 3 to 5 bar at the membrane, which is why every RO purifier here contains a booster pump. A pump needs electricity and it needs water at its inlet. Take away either and an RO machine makes nothing.

That is what the tank is for. It is not a capacity feature; it is a buffer against the machine being unable to run. A purifier that filled its tank at four in the morning while the mains was pressurised, and holds 9 L of finished water at eight in the evening during a power cut, is a purifier that works. The same machine without a tank is a wall ornament for the duration.

The tank does a second, quieter job: it decouples flow rate from demand. The Livpure Glo makes 12 litres an hour, which as an on-demand figure would be miserable — a fifth of a litre a minute. Nobody experiences it that way, because the tank refilled while nobody was watching. This is why Indian flow figures look so low next to Chinese ones, and why the comparison means nothing without the storage column beside it.

Chinese urban supply is continuous and pressurised, and outages of water or power are rare enough not to design around. Remove the need to buffer and the tank becomes pure cost: cupboard space, a reservoir of unchlorinated water at room temperature, and a part to clean. So the market moved to machines that make water only at the moment you ask — which needs a much larger membrane, which is what the numbers on the box are about.

The G in 600G, 800G and 1000G is US gallons per day of permeate — the rated output of the membrane, not the size of anything. One US gallon is 3.785 litres, so converting a rating to the litres-per-hour figure this catalogue uses is one multiplication and one division: gallons per day × 3.785 ÷ 24.

Every catalogued figure sits a few per cent below the arithmetic, which is the honest direction for a specification to err in. The rated number is what the membrane does at the test condition; the number at your tap is what survives the post-carbon stage, the tubing and the faucet.

Two cautions before you shop on the label. The first is temperature. Gallon-per-day ratings are quoted at a standard inlet temperature, conventionally 25 °C, and RO flux falls as water gets colder — cold water is more viscous and diffuses through the membrane more slowly. Winter tap water at 10 °C will take a noticeable bite out of an 800G machine’s real output. This is normal, it is physics rather than a fault, and it is why machines feel slower in January.

The second is that not every number in a model name is a gallon rating. A. O. Smith’s R400 Max RO is catalogued at 100 L/h, which works backwards to about 634 gallons a day — the 400 is a model designation, not a rating. Going the other way, Angel’s A7 Lite Master is the fastest machine here at 160 L/h, which is about 1,014 gallons a day, making it a 1000G-class machine whose name does not say so. Read the L/h figure; it is the one that means something.

Drinking water demand is smaller than people expect and peakier than they plan for. Two to three litres per person per day covers drinking; add cooking and a family of five lands somewhere around 15 to 25 litres a day. Even the slowest plumbed machine here has ample daily capacity — the Livpure Glo’s 12 L/h is 288 litres in a day it will never be asked for. What bites is the burst: three water bottles and a cooking pot filled in the same ten minutes before everyone leaves in the morning. That is where a tank either covers you or does not, and where a 90 L/h tankless machine simply shrugs.

For the tanked machines, the figure worth working out is refill time from empty — tank volume divided by flow rate:

• Kent Grand Plus, 9 L at 20 L/h — about 27 minutes.
• Pureit Copper+ Eco, 8 L at 18 L/h — about 27 minutes.
• Aquaguard Aura, 7 L at 15 L/h — about 28 minutes.
• A. O. Smith Z8 Green, 10 L at 15 L/h — about 40 minutes.
• Livpure Glo, 7 L at 12 L/h — about 35 minutes.
• Tata Swach Cristella Advance, 18 L at 3 L/h — six hours, and it is not plumbed in, so you are pouring it.

None of those is a long wait, and in normal use you never see it, because the machine refills continuously as you draw. It matters in exactly one situation: you drained the tank and you want more now. About half an hour is the honest answer for every Indian machine here.

For the six tankless machines the arithmetic is instant rather than cumulative. 90 L/h is 1.5 litres a minute; 160 L/h is 2.67. A 250 ml glass therefore takes about ten seconds on the Qinyuan and under six on the Angel, and a one-litre kettle takes forty seconds and twenty-two respectively. Worth noting that model blurbs round generously in this department — 150 L/h is 2.5 litres a minute, so a glass is nearer six seconds than three.

One row does not belong to either cluster. The Tata Swach Cristella Advance holds 18 litres, the most storage in the catalogue, and produces 3 litres an hour, the least flow — and it is the only one of the thirteen that is not reverse osmosis. It is a gravity filter: you pour raw water into an upper chamber, it works its way down through a cartridge under nothing but its own weight, and it collects in a lower chamber with a tap on it. No pump, no electricity, no plumbing, no drain line, ₹2,449.

Three litres an hour is not a defect to be engineered away; it is simply what a gravity head can push through a filter medium fine enough to be worth having. And the 18-litre chamber is not generosity either — it is a necessity, because at that rate the storage is the only thing making the machine usable. For a couple, 18 litres is two or three days. The constraint is not capacity, it is remembering to refill it.

It is also the one row in the catalogue not rated for high-TDS water; the other twelve all are. A gravity cartridge removes particulates and pathogens, which is a genuine and important job, but it does nothing about dissolved salts because nothing is pushing water through a semi-permeable membrane. If your kettle scales, this machine will not change that. Understanding TDS and hard water and do you need reverse osmosis? between them settle whether that matters where you live.

Here is the cost of the tank that nobody mentions at the point of sale. The carbon pre-filter in an RO machine removes chlorine before the membrane, and it does so deliberately, because chlorine destroys the membrane. The consequence is that everything downstream — including the tank — holds water with essentially no residual disinfectant in it. A storage tank is therefore several litres of unchlorinated water sitting at room temperature, sometimes for days.

That is a hospitable environment, and over months a biofilm can establish on the tank wall. It is not a reason to avoid tanks; it is a reason to treat the tank as a serviceable part rather than furniture. Three habits cover it: let the annual service include a tank clean, which is one of the things a comprehensive maintenance contract genuinely earns; drain and discard the first fill after a week or more away; and where a UV lamp is fitted, keep it on schedule rather than treating it as optional — four of the six Indian rows name UV in the model for exactly this reason.

The six machines holding 0 L do not have this problem in any meaningful form. Between the membrane and the faucet there are a few tens of millilitres of water in tubing, flushed out by the first second of the next glass. That is a real hygiene advantage of tankless design and it is one of the reasons the Chinese market moved that way, alongside the space saving. It is also the counterweight to everything the previous sections said in the tank’s favour: the buffer that saves you during a power cut is the same volume that needs cleaning.

The Xiaomi countertop dispenser sits between the two with its 4 L reservoir, which is the tank you fill by hand and the reason it needs no plumbing. Whichever side of the membrane you consider it to be on, four litres standing at room temperature deserves the same attention as an Indian tank at a smaller scale — empty it if you are away, and wash it out on a schedule you will keep.

Storage and flow decide the internals; the form factor decides whether the machine can live in your kitchen at all. Four shapes are represented here, and the installation requirements differ more than the specifications do.

The under-sink row is the one that surprises people. A tankless machine is compact for its output, but it needs a permanent hole in the worktop for its dedicated faucet, a cupboard tall enough to stand in, and a drain connection for the concentrate. In a rented flat that is frequently the decision-maker — and it is precisely the gap the Xiaomi countertop dispenser fills in the Chinese half of this list. It is the renter’s machine: it holds its own 4 litres, plugs into a socket, and can be carried to the next flat, at the cost of filling it by hand and of holding standing water that the six under-sink rows do not. The gravity filter occupies the same niche in India for the same reason.

This is the whole argument compressed into one table, and it is the table to look at if you are buying across markets — importing a Chinese tankless machine into a city with load-shedding, or wondering why the Indian machines all carry tanks.

Read down the gravity row and you can see why an ₹2,449 non-electric filter still makes sense in an Indian kitchen that already owns an RO machine. It is not a competitor to the RO; it is the thing that still works on the evening when neither the mains nor the mains electricity does. For a household on soft municipal supply it can be the only machine. For one on borewell water it cannot, because it does not touch dissolved solids.

And read down the tankless row before you assume the higher-specification machine is the better one. 160 L/h is a genuinely superior experience — no waiting, no tank, no standing water, no counter space — in a flat where the water and the power are simply always there. Where they are not, the specification that matters is the one measured in litres held, not litres per hour. How to choose a water purifier puts this decision alongside the others, and what a water purifier costs to run covers what the tank adds to the service bill.