All about TDS and Water Quality

What is TDS? 

Total dissolved solids (TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized ormicro-granular (colloidal sol) suspended form. A TDS meter will measure all the charged ions (both positively and negatively) that are found present in the water. These ions include calcium, magnesium, nitrates, iron to name just a few. Other pollutants like chlorine and bacteria could not be tested out. So it's hard to conclude simply from the TDS reading that the tested water is good or bad, qualified or not qualified if the water source is unknown.

Why is TDS test helpful?

Now that TDS covers both good and bad things and something harmful could not be covered, will it be meaningless to do the test? The answer might be No. In the real practice, the water source is confirmed instead of being unknown. For tap water or bottled water, they are from either lakes or rivers on the surface of the earth. Most of the solids inside tested out won't be healthy minerals. So we could generally tell from the reading if the water is clean or not and compare the tested samples. Typically for the filtered water, we could use the TDS test to tell the differences from tap water. The bigger the difference is, the better the effectiveness might be.
(For the natural bottled water, things will be a bit different. Since they are either from mountains, springs or from underground whose sources have been fully testified with good things and protected. Thus, when doing the test, it's hard to compare this to tap water or treated bottled water.) 

Are there standards over TDS?

Actually no theoretically and yes practically. Many counties set a maximum TDS level allowed for the drinking water but the level varies a lot from nation to nation.There is no known minimum for drinking water. But there are maximum TDS restrictions(500 for EPA, 1000 for WHO). Generally, the higher the reading for the TDS is for the tap water, the more potentially for the water to be with problems.

What is TDS reading for Filtered Water?

It all depends on which kind of filtration is used. In the market, there are 2 kinds of filtration. One is in-depth filtration like the reverse osmosis( distillationother type of in-depth treatment, but it's mainly used industrially.) It removes virtually everything. For water through the RO or distillation based systems, the reading for TDS will be less than 50. Bottled water from treated tap water is just of this type. Distilled water reads zero while remineralized purified water reads about 30. However, all the other non-RO filters will not change TDS that much.

Why simple/basic filter is unable to reduce the TDS reading?

Basic filters refer to almost all non-RO filters include carbon filter, KDF filter, ultra-filtration filter, etc, which are often found in whole house filtration syste, water softeners, shower filters, faucet filters and so on. The basic filtratio system mainly tareget at the big sized pollutants like rust, sands, chlorine and many other chemicals in the water, which means the smaller ones like metals and other charged particles can easily escape from the capture of the basic filters. That is to say, no matter how many stages of filters are used inside, the filtered water will be with almost no change for the reading of TDS. These basic filters can even increase the TDS reading when the surface of the filter is clogged up by excessive amount of pollutants if the filters fail to be changed in time. However, it does not necesssarily mean that these basic filters are not working if they are used for non-drinking purposes. 

 Tips to do the test  TDS

Temperature: Temperature will affect the testing results as it will affect the moving speed of the ions in the water. When doing a test, it's better to collect cold water or samples with same temperature.
Time: Before collecting a sample, to make sure the sample is representative, it‘s better to let the tap run a while ( 30 seconds) first as the water inside the pipes could possibly collected something inside over a night or some time ahead.
Quantity: In terms of the quantity of water sample, 1 fourth or 2 fifth of a cup will be enough.
Extra notes: TDS test is good to help tell if the bottled water（not natural mineral ) is clean and if the reverse osmosis drinking water purifieris working properly. 
If your bottled water (not natural mineral) or purified water from either your individual reverse osmosis filter or the centralized reverse osmosis system in your compound reads over 50, that will mean the water is fake, expired or polluted. You are not recommended to drink that water again.
TDS test is very basic. Lab tests will be still needed if you want to know what kinds of pollutants are inside and the content of each pollutant in the water.