Toxic chemicals, heavy metals, and harmful bacteria have seeped into drinking water sources and pose serious health risks. Citing national data that revealed that India recorded over 20.98 crore cases of waterborne diseases during 2005-2022, the National Green Tribunal (NGT) said that drinking water contamination amounts to water pollution and violates the constitutional guarantee of the right to life.This raises some pertinent questions, including how to ensure safe water for the growing population, especially when freshwater resources are so limited. Can wastewater, once regarded solely as a waste product, become a valuable resource through appropriate treatment? Let’s explore. Freshwater is among the planet’s most valuable natural resources. Of Earth’s total water supply, approximately 96.5% exists as highly saline water in seas and oceans, whereas freshwater in lakes accounts for only 0.007%, and rivers and streams together contribute merely 0.002%, note Gilbert M Masters and Wendell P Ela in Introduction to Environmental Engineering and Science (1991). Therefore, recycling and reusing wastewater is one of the most logical and feasible solutions to sustain industrial activity, growing human population, and agriculture. Wastewater treatment requires a coordinated effort by scientists, engineers, and technicians. Water contaminants span a wide range of biological and chemical agents. Biological contaminants include pathogens like viruses (e.g., enteroviruses), bacteria (e.g., salmonella, shigella), protozoa (e.g., entamoeba), and helminths (e.g., hookworms). Chemical pollutants include oxygen-demanding wastes (like sewage, agricultural waste, industrial effluent), excess nutrients (like nitrogen and phosphorus compounds). Oxygen-demanding wastes elevate biological oxygen demand (BOD), whereas excess nutrients cause algal blooms. Elevated nitrate concentrations in drinking water, another consequence of nutrient pollution, is associated with methemoglobinemia (‘blue baby syndrome’) in infants.Story continues below this adOther important classes of chemical pollutants include metals, pesticides, volatile organic compounds (VOCs), dissolved solids, and emerging water pollutants. Metals in water damage vital organs, pesticides cause widespread ecotoxicity and biomagnification, VOCs primarily contaminate groundwater, and high salt concentrations increase total dissolved solids (TDS). In addition, emerging water pollutants – particularly endocrine-disrupting chemicals (EDCs) such as ethinylestradiol used in oral contraceptives – can significantly affect fish and amphibian populations.Indicator organisms in water quality assessmentIt is not feasible to simultaneously determine the diversity and abundance of all microorganisms present in a water sample. Therefore, to establish water quality standards and evaluate the efficiency of water treatment processes, coliform bacteria, particularly Escherichia coli (E. Coli), are widely used as indicator organisms. This approach is based on several considerations: 1. Fecal contamination is a primary threat to public health because fecal matter can carry disease-causing organisms.Story continues below this ad2. Since the human gut contains a large amount of coliform, naturally the presence of disease-causing bacteria is not possible without the corresponding presence of coliform.3. The survival of diseases-causing pathogens outside the host is generally much lower than that of coliform bacteria.4. Relatively few diseased individuals in the general population contribute to the transmission of these pathogens.Although indicator organisms have proven useful for routine monitoring, this approach is not foolproof. Chemical inactivation of coliform bacteria does not necessarily ensure the inactivation of more resistant pathogens such as Giardia, Cryptosporidium, and enteric viruses. Story continues below this adResultantly, modern drinking water regulations specify stricter rules for treatment performance targets, commonly referred to as 4-log, 3-log, 2-log rules to ensure 99.99% inactivation of enteric viruses, 99.9% inactivation of Giardia, and 99% removal or inactivation of cryptosporidium. Furthermore, when distinguishing between human and animal sources of fecal contamination is necessary, the ratio of fecal coliforms to fecal streptococci is often determined as a supplementary indicator.Detection and wastewater treatment Determination of coliform bacteria in water is conventionally confirmed through a three-step procedure described in Standard Methods for the Examination of Water and Wastewater (APHA). The method consists of three stages:1. A presumptive test based on acid and gas production during lactose fermentation.2. A confirmed test involving gas production and characteristic growth in brilliant green lactose bile broth or eosin methylene blue agar.3. A completed test involving gas production and Gram staining.Story continues below this adOnce coliform contamination is confirmed, disinfection procedures become necessary. Overall wastewater treatment involves physical, chemical, and biological processes. Primary treatment includes screening to remove large particles, followed by rapid mixing, coagulation and flocculation to encourage suspended particles to adhere to form large particles.Flocculation or gentle mixing in water treatment makes light particles suspended in water come together into larger clumps called flocs, which are subsequently removed by sedimentation. The clarified water is then filtered in rapid sand bed filters before undergoing final disinfection. For municipal wastewater, activated sludge processing is performed with aeration so that microorganisms consume the organic matter and convert them to biomass that can be removed by sedimentation and filtration. Story continues below this adWater disinfection: Methods and challengesThe most widely used disinfection strategy is chlorination through the addition of chlorine gas or sodium/calcium hypochlorite. These generate hypochlorous acid (HOCL), the principal disinfecting species, while chlorine gas and hypochlorite ions contribute to a lesser extent. The secondary treatment involves the presence of leftover chlorine that prevents further growth of microorganisms. To increase the lifetime of leftover chlorine, ammonia is added sometimes that lead to the formation of chloramines that are more persistent but are somewhat less effective. A major disadvantage of chlorination is the generation of disinfectant by-products (DBPs), like trihalomethanes, chloroform and haloacetic acids, which are human carcinogens. Furthermore, chlorination below 0.5mg/L is ineffective against enteroviruses and protozoan parasites like Giardia. An alternative approach relies on the use of costly chlorine di-oxide, which is effective against viruses but generates chlorites, which have a safe concentration limit of only 1mg/L.Ozone is more effective than chlorination against cysts and viruses than chlorination, but it can generate bromate, whose safe concentration limit is 0.01mg/L. Ozone also has the advantage over chlorine with no leftover odour and taste. But it has no secondary disinfectant properties because it disappears quickly, therefore, it cannot keep protecting the water the way chlorine can after treatment.Story continues below this adWhen microbes fight backFrom a biological perspective, chemical disinfection of water can act as a selective pressure that favours microbial adaptations associated with survival under stress. Since the efficacy of water treatment is directly related to public health, it is essential to identify and limit adaptations that allow microorganisms to thrive in this nutrient-depleted condition under disinfectant stress. To investigate these adaptations, a recent study addressed these challenges by analysing metagenome and metatranscriptome both at the intracellular and extracellular levels. What are these techniques? What did the study find, and can technological solutions provide long-term water security? The second part of the article will address these questions. Post read questions1. Freshwater constitutes only a tiny fraction of Earth’s total water resources. Discuss the importance of wastewater treatment in ensuring sustainable water security.2. Discuss the major biological and chemical contaminants found in water and explain their impact on human health and aquatic ecosystems.Story continues below this ad3. What is Biological Oxygen Demand (BOD)? Explain its significance as an indicator of water quality. Explain the causes and consequences of eutrophication. How does nutrient pollution affect freshwater ecosystems?4. Discuss the advantages and limitations of chlorination as a method of water disinfection. Compare chlorination, chlorine dioxide treatment, and ozonation as methods of drinking water disinfection.5. Safe drinking water is not merely the absence of pathogens but the outcome of effective monitoring, treatment, and regulation. Comment.(Dr. Arunangshu Das is the Principal Project Scientist at the Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi.) Share your thoughts and ideas on UPSC Special articles with ashiya.parveen@indianexpress.com.Click Here to read the UPSC Essentials magazine for June 2026. 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