Wastewater treatment is sometimes seen as a mystical process because we don’t normally get to see how it works. For all we can see, the water goes into our sink after washing up, and simply disappears, never to be seen again. But that is far from the truth. This article will explain to you exactly what the process of treating water is, step by step, using wastewater treatment plants (WWTPs).
What Exactly Is Wastewater?
Let’s start with a basic definition of wastewater. Wastewater is used water originating from domestic, industrial, agricultural, medical or transport activities. Used water becomes wastewater upon the change of its quality, composition and/or temperature. Wastewater can be divided into two major groups, namely sewage water (from domestic use, originating from toilets, showers, and sinks) and industrial wastewater (from industrial, production, and commercial activities, often containing chemicals).
Wastewater goes through a series of processes in the WWTPs in order to be treated properly. The first process is that the wastewater is drained to the WWTP by gravity through the main sewer system, which is the size of a car. With such a large size, all kinds of items reach the WWTPs, from objects as large as mattresses to small tree branches.
Once inside the WWTP, pre-treatment begins. The water is passed through a gravel chamber, which separates the heavy gravel and debris from the water (later to be disposed of at the dump). Next, the water goes through bar screens. The first level uses coarse screens to remove larger objects from the water, and then fine screens to take care of smaller objects like matches and cigarette butts. Finally, the water goes through a grit chamber, which works much like the gravel chamber in that it allows grit to settle at the bottom of the chamber, later to be disposed of. Neither gravel nor grit can be reused, as they are highly contaminated.
Primary Treatment: Sedimentation Stage
In the primary treatment process, wastewater is brought into “pre-settling basins”, better known as primary settling tanks. In this stage of sedimentation, water is driven towards the hopper in the base of the tank. The hopper arm moves around the edge of the tank at the velocity of 4cm per second. Due to the difference in velocity, the treated water moves towards the edges and the particulates of higher sedimentation velocity than the flow velocity settles at the bottom of the tank. By this stage of the treatment, the level of pollution of the wastewater has dropped to 60%.
Secondary Treatment: Biological Stage
This stage is based on natural processes, using bacteria to consume the bio-contaminants, in particular, biodegradable organics, carbon, and phosphorus. The dead bacteria and organic residues will become sludge. This sludge water is pumped into settling tanks, where the sludge settles and is transported to digestion tanks for further treatment. Here, sludge is heated and mixed, producing biogas which WWTPs can reuse as a source of electrical and thermal energy.
Second digestion takes place in storage tanks, where water is separated from the semi-solid sludge and transported back for further treatment, while the residual sludge has water mechanically removed. Sludge which is adequately digested and dehydrated after a month can be disposed, or even used for fertilization of industrial crops.
Finally, the treated water is given a deep inspection, to analyze the contamination levels and ensure the treated water complies with the highest standards. It is at this stage where the use of the treated water is determined.
To learn more about WWTPs, or if you have specific burning questions that need answering, write to Carter Pump, experts in the field of industrial and commercial pumps since 1897.