THE BASICS
WHAT IS WASTEWATER TREATMENT?
We as human beings use a lot of water, not just directly (drinking, bathing, washing, cooking, etc.), but also indirectly through the goods we consume. These goods, be they the clothes you wear, the processed foods you eat, or the car you drive, were manufactured in a factory that uses clean water for various processes.
The Clean water we use for our personal needs as well as the Clean water that is used to process all the "stuff" we as a society use, ends up picking up various pollutants along the way becoming Dirty water—turning into sewage (when it's used by people), and industrial effluent (when it's used in industrial processes like garment washing, heavy metals processing, etc.). This Dirty water should not be directly discharged into storm water drains or into water bodies, or be used to recharge our groundwater, as this untreated water contains all sorts of bad things that pollute our soil and water bodies, killing fish, and potentially making the water in our aquifers (groundwater) and lakes unfit for human consumption.
Thus, this dirty "waste" water needs to be cleaned up or treated first. Residential complexes, office buildings, hospitals, hotels, malls, etc. generate sewage (primarily human excreta and organic waste), while dairies, textile mills, refineries, etc. generate industrial effluent (any mix of organic and inorganic wastes). Thus, there are two kinds of wastewater treatment plants—Sewage Treatment Plants (STPs), and Effluent Treatment Plants (ETPs).
DO I NEED AN STP OR AN ETP?
Yes! Firstly, because it's the law. Secondly because water is a precious resource, and treated wastewater from an STP or ETP can be reused onsite for various secondary applications such as car wash, flushing toilets, gardening (domestic), for cooling towers, washing processes that can do with grey water (industrial), etc. This reuse brings down your overall water consumption, and results in significant cost savings. A typical 200-flat residential complex with a properly functioning STP under Ecotech's AMC (annual maintenance contract) sees cost savings on the order of Rs. 1.5 lakhs per month. This is after accounting for operation costs of the STP. Not to mention the relief of not having to deal with foul odours, water anxiety, and having to tanker in clean water.
Lastly, because you are a good citizen, and it is the right thing to do for the environment
SO HOW DO STPS AND ETPS WORK?
Both STPs and ETPs work on pretty much the same overarching principle—perform a set of biological, chemical, and physical processes on the wastewater to clean it up.
This is nothing new really, nature does all these. Wastewater discharged into rivers eventually gets cleaned up by undergoing various natural physical, biological, and chemical processes. The catch is that these natural processes occur over much longer time- and length-scales, and nature cannot keep up with the rate at which we are generating wastewater. A treatment plant (STP or ETP) is a human-engineered solution to artificially accelerate the cleaning process by compressing the time- and length-scales of the treatment.
ETPs are the more challenging than STPs. Why? Because every industry has a different kind of effluent (wastewater) based on what goes on in the industry. For example, a brewery or distillery would have effluent that has a high concentration of sugars, while a textile mill with a garment washing unit might have high concentration of chemical dyes and detergents. The effluent from a potato chip factory would have a high concentration of—you guessed it—starches. STPs on the other hand deal with wastewater having pretty much the same characteristics—human waste, oils from cooking, soaps and detergents from cleaning/washing.
WHAT DO YOU MEAN BY PHYSICAL, BIOLOGICAL AND CHEMICAL TREATMENT?
Before we answer this, let's try to better define the metrics that determine how polluted the wastewater is. The most common metrics are turbidity (how murky the wastewater is), pH (the acidity or basicity—you want a pH of 7.0 to be neutral), BOD (biological oxygen demand), and COD (chemical oxygen demand). Limits for each of these metrics are prescribed by the local pollution control board. Additionally, treated water should be colour-, odour, and pathogen-free, and free of any heavy metals and other such contaminants.