Parts Per Million (PPM) is a common metric for assessing drinking water quality. It quantifies the concentration of Total Dissolved Solids (TDS) present in a given volume of water. Understanding a “normal” PPM reading requires defining these dissolved solids and examining established guidelines for their presence.
Defining Total Dissolved Solids
Total Dissolved Solids (TDS) represent the combined weight of all inorganic and organic substances dissolved in water. These substances are small enough to pass through a filter with pores around two micrometers in size. The measurement is expressed in parts per million (PPM), which is numerically equivalent to milligrams per liter (mg/L) of water.
The composition of TDS primarily consists of inorganic salts. Common mineral components include cations like calcium, magnesium, and sodium, as well as anions such as chloride, sulfate, and bicarbonate. These minerals often originate from the natural weathering of rocks and soil as water travels through the environment.
TDS can also include trace amounts of organic matter, which may come from natural decay or from human activities like agricultural and residential runoff. Although the TDS reading itself does not identify specific contaminants, it serves as a broad indicator of the water’s overall mineral content and purity. Measuring the electrical conductivity of water is the common method used to estimate the TDS level, as dissolved ions increase the water’s ability to conduct an electric current.
Acceptable PPM Ranges for Drinking Water
The concept of a “normal” PPM range is defined by regulatory guidelines focused on aesthetic quality rather than direct health effects. In the United States, the Environmental Protection Agency (EPA) sets a Secondary Maximum Contaminant Level (SMCL) for Total Dissolved Solids at 500 ppm. This guideline is established to prevent undesirable effects like poor taste, odor, and staining.
Water with a TDS level above 500 ppm is generally considered to have poor aesthetic quality. High concentrations can cause the water to have a noticeably salty, metallic, or otherwise unpleasant taste and may lead to mineral scaling or deposits on plumbing fixtures and appliances. Consumers frequently find water with TDS levels between 50 and 300 ppm to be the most palatable, due to a balance of purity and pleasant flavor from naturally occurring minerals.
Extremely high TDS levels, particularly those approaching 1,000 ppm or more, can make the water nearly undrinkable for many people. Conversely, water with very low TDS, such as that produced by distillation, can taste flat because it lacks the dissolved minerals that contribute to flavor. While very low TDS water is not inherently unsafe, it can sometimes be slightly corrosive to plumbing systems and is not necessarily superior for health.
How Water Sources and Treatment Affect PPM
A water source’s PPM level is significantly influenced by its geological surroundings. Groundwater, such as that found in private wells, often exhibits naturally high TDS readings because the water has spent extensive time dissolving minerals from rocks and soil. Conversely, surface water supplies, like rivers and reservoirs, typically have lower TDS because their contact time with mineral deposits is shorter.
Human factors also heavily influence the TDS of municipal supplies. For instance, coastal areas may see elevated TDS due to the intrusion of saltwater into freshwater aquifers. Urban and agricultural runoff can introduce various salts and organic materials into both surface and groundwater, further elevating the PPM.
Standard municipal water treatment, which includes coagulation, filtration, and disinfection, often does not dramatically reduce the TDS level. These processes are primarily focused on removing suspended particles and harmful microorganisms.
Specific advanced filtration methods are highly effective at altering PPM. Reverse Osmosis (RO) systems use a semipermeable membrane to physically separate water molecules from nearly all dissolved solids, resulting in a significantly lower PPM reading. Similarly, distillation actively removes dissolved solids by boiling the water and collecting the resulting pure steam. Standard carbon filters, commonly found in pitcher and faucet-mount systems, are designed to remove specific contaminants like chlorine and some organic chemicals, and they typically do not cause a notable reduction in overall TDS.