For the purposes of simplicity, the assumption will be made that the carrier fluid is liquid water throughout this treatise. Total suspended solids or TSS is to be differentiated from total solids, settleable solids and dissolved solids. As inferred, total solids is the sum of suspended, settleable and dissolved solids. Total solids is all the residue left after a volume of liquid sample has been totally evaporated away in an oven held between 103 and 105ºC (217 and 221ºF). This is expressed as a concentration usually in milligrams per liter (mg/L) or for water this is equivalent to parts per million (ppm).
Settleable solids are those materials that settle out of a suspension within a specified period of time. Floating material may also be included in this definition. The results are expressed as milligrams of solids per liter of water (mg/L or ppm). These solids may or may not be a problematic issue in water usage since they primarily remain in the bottom of a sump somewhere and are seldom entrained in the water flow.