Any water which has been used can be considered wastewater. This is as true for the water left in the tub after a bath as it is for wastewater that is discharged from any manufacturing operation. The same holds true for water which has been used for agriculture and the process has made the water unfit for use by humans. Although much ends up in municipal treatment plants, in certain situations it is held for further processing in wastewater holding tanks.
The most common and perhaps the most voluminous waste water is that which is discharged from homes, offices, etc as liquid sewage. This water has been contaminated by human waste, dishwashing, clothes washing and any other contaminating materials which have entered the system. Fortunately in the US most municipalities have treatment facilities for this water, once it has gone through the process of purification, this wastewater can be released into streams, rivers and lakes.
There are many industries that use aggressive chemicals in the process of manufacturing; a good example is gold mining and processing. Processing the raw ore results in chemical slurry which must be contained in on-sire wastewater holding tanks until is sent on for further treatment. In some situations the wastewater processing is done in a similar fashion to a septic tank. In this case a number of tanks will be linked together, as the tank fills the sediment drops to the bottom and the liquid moves to the next tank, this may happen three or four times before the water is safe to be discharged.
Not all wastewater is sent for processing. Here is wastewater that has other uses as is, especially around the home. Bath water is often collected and used to water plants in the garden and flowers. Water which is used in the kitchen for food preparation, perhaps the water that was used to boil pasta is equally useful and does no harm to the environment when used on plants and flowers.
As wastewater covers a very broad spectrum, from water which is still usable as is for various applications around the home to wastewater which contains caustic chemicals which may be the result of petrochemical processing, what is important is to understand that it must be held in a certain manner. In many cases wastewater holding tanks are part of the treatment process.