There are several different types of wastewater treatment systems. The methods used generally come down to two factors: the technology level of the area and whether the water is gray or black. Gray water is water that is basically clean, but contains easily removable contaminants that make it unsafe; runoff in storm sewers is generally gray water. Black water contains harmful substances or bacteria, which is often industrial wastewater or sewage. Gray water requires significantly fewer steps to clean and generally undergoes fewer processes.
In less developed areas, the most common wastewater treatment systems use natural processes. This means pouring out wastewater far away from lakes or streams and allowing the water’s movement through the ground to clean it. In these areas, evaporation is also used to separate water from solids in order to reuse the solid material for fertilizer or fuel. These wastewater treatment systems work well in these areas since the overall population is lower; higher populations would contaminate the groundwater using these same methods.
In more developed areas, more scientific methods are often used. The first step in most wastewater treatment methods, regardless of the water's origin, is settling. The water is held in large tanks and kept still. Heavy solids will fall to the bottom of the tank and light materials will float to the top. Once settled, the extra materials are skimmed off the top or simply removed from the tank. Settling tanks allow a large portion of gross contaminants to come out of the water before treatment begins.
Depending on the contaminants present in the water, the rest of the processes differ. For instance, one of the early steps in black water treatment is the introduction of beneficial bacteria that will consume harmful chemicals and biological material. These bacteria will eat microscopic materials and then excrete them in larger waste packets that are easier to remove from the water. They will also convert harmful chemicals and petroleum products into other less dangerous chemicals.
Other methods, often used with gray water, involve adding a flocking substance to the water. The additive will bind with contaminants present in the water and with itself. This results in large balls full on contaminants that are then easily filtered out of the water. This method is especially useful when wastewater with consistent composition, such as wastewater from a specific factory, needs cleaning. The flocking agents can easily be tailored directly to the substances that are always present.
Regardless of the methods used, the final step in most wastewater treatment systems is evaporation. Just like in less developed areas, the water is pumped into evaporation lagoons where the basically clean water can separate completely from any remaining materials. If the wastewater treatment systems' previous step was done correctly, the water sitting in the ponds is completely safe.