The concept of sewage treatment evolved in the late 19th and early 20th century, with the advent of the activated sludge process in 1913. The first aerobic sewage treatment plant was established in 1914, following a fill-and-draw process in treatment, which in the time to come would be named the sequencing batch reactor (SBR) process.

Before the establishment of the activated sludge process, sewage from households was let into drains in towns and cities that had sewer networks, and where there were no sewers, there were cesspools and soakage pits. The soak pit systems created health issues, while the sewer networks drained all the water to natural water bodies and rivers. The River Thames was one of the most polluted rivers then, and there was a greater need for a different approach when nature could not do its work in cleaning up the pollutants.

Scientists in the UK found that the stink in the river was due to the depletion of dissolved oxygen in the water, evidently due to fish deaths, and came up with the idea to inject air into the water to improve the dissolved oxygen levels. Subsequently, when water with sewage was aerated, microbes developed that used the oxygen and consumed the pollutants in the water. This was named the activated sludge process as these microbes got activated and proliferated in the presence of oxygen and pollutants as food.

In lower concentrations, nature would have dealt with consuming the pollutants as food for microbes in water and soil. However, with cities growing bigger due to industrial growth, the pollutant loads became so high that technological intervention became necessary. When the environment’s capacity to assimilate pollutants naturally is overwhelmed, there is a need to support nature by creating conditions for nature through scientific and engineered processes to accelerate the same process in a short period. The activated sludge process could be considered one of the first-ever biomimicry processes, and this was developed to control pollution of natural water bodies and rivers.

In the activated sludge process, naturally occurring microbial populations grew in very high numbers when water with pollutants was aerated. So, a methodology was developed using a basin where water is filled and aerated until the water becomes clear. This water was drained after it was clean enough, and the next batch of water was treated. Subsequently, ideas of continuous treatment process developed where hydraulic retention time was introduced to assess the basin dimensions of the pumping rate, and the concept of wastewater treatment started to develop. More and more research was carried out to understand the activated sludge process, and improvements were implemented to improve process effectiveness and efficiency, while the activated sludge process remained relevant even to the present day.

Although there are different methodologies such as suspended growth, attached growth, aerobic granular sludge processes, etc., they are all again a subset of the activated sludge process. The SBR process has become one of the time-tested and reliable sewage treatment technologies worldwide.