New Delhi, June 9: The Union government has introduced stricter environmental standards for caustic soda manufacturing units, making it mandatory for wastewater from plants using membrane cell technology to pass a fish survival test before being considered environmentally safe.
Under the new standards notified through the Environment (Protection) Second Amendment Rules, 2025, at least 90 percent of fish must survive after being exposed to 100 percent wastewater for 96 hours during laboratory-based bioassay testing. The requirement aims to determine whether industrial effluent remains toxic to living organisms even when it complies with individual chemical limits.
The new rule represents a shift in pollution monitoring by focusing not only on chemical concentrations but also on the overall biological impact of wastewater. Experts say this approach can reveal hidden toxicity caused by the combined effects of multiple pollutants.
Caustic soda, or sodium hydroxide is a key industrial chemical used in sectors including textiles, paper, detergents, aluminium, petrochemicals and water treatment. India currently has more than 30 caustic soda manufacturing plants producing over five million tonnes annually. However, wastewater from these facilities can contain high levels of alkalinity, salts and other contaminants that may harm aquatic ecosystems.
The revised standards prescribe a pH range of 6.5 to 8.5 for discharged wastewater. They also cap chloride levels at 250 milligrams per litre, total suspended solids at 100 milligrams per litre and total dissolved solids at 2,100 milligrams per litre.
In addition, the government has imposed limits on resource consumption. Plants will be allowed to use a maximum of five cubic metres of water and generate no more than one cubic metre of wastewater for every tonne of caustic soda produced.
The bioassay requirement specifically applies to facilities operating with membrane cell technology, which replaced older mercury based production methods known for causing severe environmental contamination. While membrane technology is considered cleaner, regulators have decided to strengthen monitoring by evaluating the actual biological effects of treated wastewater.
Environmental scientists note that whole effluent toxicity testing can identify ecological risks that traditional chemical analysis may overlook. The testing process requires live fish, controlled laboratory conditions and continuous monitoring over four days, making it significantly more complex than routine wastewater analysis.
The notification has also introduced weight based pollution standards, including limits on total dissolved solids generated per tonne of production. Such measures are intended to prevent industries from simply diluting pollutants with additional water to meet concentration based standards.
However, questions remain about implementation and enforcement. The rules do not clearly specify whether routine bioassay testing will be conducted by industries themselves, accredited private laboratories or pollution control authorities. Experts have pointed out that effective enforcement will require specialized laboratories, trained personnel and independent verification by regulatory agencies.
Environmental observers say the success of the new standards will largely depend on regular monitoring and stronger oversight by pollution control boards to ensure industries comply with the biological safety requirements as well as chemical discharge norms.