8.3. Microbes in Sewage Treatment

We know that large quantities of waste water are generated everyday in cities and towns. A major component of this waste water is human excreta. This municipal waste-water is also called sewage. It contains large amounts of organic matter and microbes, many of which are pathogenic. Have you ever wondered where this huge quantity of sewage or urban waste water is disposed off daily? This cannot be discharged into natural water bodies like rivers and streams directly – you can understand why.

Sewage is the waste water generated from households, which primarily contains human waste and other organic materials. When cities and towns produce a significant amount of sewage, it is vital to treat this waste before it enters rivers or streams to prevent pollution. Dumping untreated sewage can harm aquatic life and contaminate drinking water sources.

Think of sewage like the leftover food after a party. Just as you wouldn't dump your leftovers on the sidewalk, municipalities must treat sewage properly before releasing it into nature. Treating it ensures the environment stays clean and healthy.

Before disposal, hence, sewage is treated in sewage treatment plants (STPs) to make it less polluting. Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage. This treatment is carried out in two stages:

Primary treatment: These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent.

Secondary treatment or Biological treatment: The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent.

Sewage treatment is mainly conducted in two phases. In primary treatment, physical processes like filtration and sedimentation help to remove larger particles. The remaining liquid, known as effluent, is then treated biologically in the secondary treatment phase, where aerobic microbes consume organic matter, reducing pollutants. This process is crucial to prepare sewage for safe disposal into the environment.

Imagine cleaning a messy room. First, you would pick up the big pieces of garbage (primary treatment), and then you'd organize and clean the surfaces (secondary treatment). This ensures the room is clean and healthy, just like how treated sewage is ready to be released into water sources.

BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. The sewage water is treated till the BOD is reduced significantly.

BOD is a measure of the amount of oxygen bacteria will consume to break down organic matter in water. A higher BOD indicates more pollution, as it means more oxygen is needed for decomposition. Sewage treatment aims to lower BOD levels, making the water safer and cleaner. Monitoring BOD helps assess the effectiveness of sewage treatment.

Think of BOD like the appetite of bacteria for 'food' in the sewage. If there's a lot of food (organic matter), the bacteria will need more oxygen to digest it. By reducing how much 'food' is available through treatment, we ensure they're full and healthy, reducing the impact on the environment.

Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as source of energy as it is inflammable.

After reducing BOD, the remaining sludge, rich in microbes, is allowed to settle. This activated sludge helps replenish the next cycle of treatment. The digested sludge undergoes anaerobic digestion, producing biogas, which can be harvested and used as clean energy for cooking and heating. This process not only treats sewage but also recycles waste into useful energy.

Consider a compost bin where food scraps decay. The resulting compost is beneficial for gardens. Similarly, in sewage treatment, the decayed sludge releases gases that can be captured and used as clean energy, just like turning waste into compost nourishes soil.

You can appreciate how microbes play a major role in treating millions of gallons of waste water every day across the globe. This methodology has been practiced for more than a century now, in almost all parts of the world. Till date, no man-made technology has been able to rival the microbial treatment of sewage.

Microbes are essential in removing pollutants from sewage, making them a natural solution for waste treatment. Their ability to break down harmful substances efficiently has stood the test of time, showing that nature's methods can be effectively utilized for environmental preservation.

Microbes in sewage treatment are like nature's own recycling team, turning waste into clean water just as a recycling plant processes garbage into fresh products. They are essential to maintaining our ecological balance and health.

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