Wastewater includes solid waste as well, and all of this collectively goes into the municipal wastewater treatment plant, also called the sewage treatment plant (STP)
Have you ever wondered what constitutes waste? Let us consider a scenario: You are having a sumptuous meal but realized you cannot finish your plate, as it is too much food. For lack of a better option at that point, let us say the food is discarded into a trashcan. After some point, the food starts decaying.
Just think about it, at what stage did the food become a waste? Was not the food still very much in an edible condition when it got transferred from your plate? But once it reaches the trashcan, it is considered a waste. Here the food is a waste just because it is moved from a desirable to an undesirable location. Subsequently, once the food had started decaying, the food was clearly no longer edible. Here the waste is due to the transformation of the food, which makes it unfit for our consumption.
Let us now move from waste to wastewater, more specifically to municipal wastewater, because this is directly generated from of our daily activities. Water is an essential component in our everyday life, where we use water alongside other products ranging from toothpaste, soaps, detergents, food, and medicines. The water generated after all these activities is labeled as wastewater, and it is thus a concoction of the chemicals from the products we use daily. In certain cases, like when we ingest medicines, large quantities of their ingredients can be released from our body without being consumed, i.e., they retain their chemical composition; like the food that had just gone from the plate to the trashcan. In other cases, the chemicals are transformed, or broken down into their subcomponents. This is what happens to the food which gets digested in our body and is excreted as solid waste in toilets along with the wastewater.
Wastewater thus includes solid waste as well, and all of this collectively goes into the municipal wastewater treatment plant, also called the sewage treatment plant (STP). If not for the STPs, the wastewater would be discharged into the environment untreated. This would pollute cleaner water bodies like lakes, rivers and even groundwater reserves, thereby causing serious health and environmental problems. A lot of water-borne diseases and water pollution like harmful algal blooms, are a result of untreated or inadequately treated wastewater.
However, if we examine the solids in the wastewater, the residual waste from digested food is still rich in nutrients like organic carbon, phosphorus, and nitrogen. These nutrients are all required in fertilizers that are in turn required to grow food crops. Thus, on one hand, while the wastewater needs to be treated, on the other hand, it is also a valuable pool from which resources can be recovered.
Now we are all set to make a distinction between a linear and circular economy in the context of wastewater. In a linear economy, most of the emphasis is on treating the wastewater and there is less emphasis if any, on recovering value from the waste. At best, the solids from the wastewater, which were rich in nutrients, would directly be spread as fertilizers on farmlands, but this was mostly done in an uncontrolled way. This would lead to overloading of nutrients in the soil, which can be washed off to nearby water bodies and result in pollution. Over the decades, we have however been transitioning to the circular economy, where there is a big emphasis not just on treating the wastewater, but also maximizing the resource recovery. These resources could be in terms of the nutrient, energy, or clean water. For instances, STPs often process the solid waste to generate biogas, resulting in significant electricity generation. Also, more technologies are being developed to selectively recover nutrients that can be used to fertilize farms in a sustainable way. Moreover, the treated wastewater is being considered for reuse in industrial, agricultural, and even domestic purposes. And to add to that, the chemical constituents in the wastewater can be transformed from a low value product to a high value product. The protagonists who help achieve this transformation are the micro-organisms which use our waste as their food. For e.g., microalgae like to feed on the organics in the wastewater and while they clean the water during this process, they also lead to formation of high value products. This could be biofuels that can be used for driving vehicles, bioplastics that can be used for packaging materials, or other biopolymers with applications in diverse fields.
A circular economy thus gives us an opportunity to recover and reuse resources from waste, in the form of energy, nutrients, high value products, and perhaps the most important commodity of them all, clean water. These concepts of a circular economy can apply to industrial wastewater as well, it is just that the constituents will be different. So, next time you look at wastewater, consider looking at it as VALUE water, one with a huge potential for resource recovery. At some point, water reuse from waste sources will become an immense necessity. When that time comes, we need to be prepared not just in terms of technology, but in terms of our mindset.
The author is Faculty, Plaksha University. Views are personal.
[This piece was written exclusively for First Post by Dr. Prashant Suresh Kumar, Associate Director, PhD program, Plaksha University on March 21, 2022]