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New environmental standards require caustic soda plants using membrane cell technology to pass a fish-survival test for wastewater toxicity.
The rules say at least 90% of fish must survive after 96 hours in 100% wastewater during laboratory-based bioassay testing.
The test is meant to assess whether treated industrial effluent is toxic to living organisms, not just whether it meets individual chemical limits.
The standards also set limits for pH, chloride, suspended solids, dissolved solids, water use and wastewater generation.
Experts say enforcement will be key, as bioassay testing requires specialised laboratories, trained staff and regular independent monitoring.
Wastewater from caustic soda plants using membrane cell technology will now have to pass a laboratory-based fish survival test under new environmental standards notified by the Union government.
The Environment (Protection) Second Amendment Rules, 2025, state that at least 90 per cent of fish must survive after 96 hours in 100 per cent wastewater during biological assay (or bioassay) testing, which is a test that measures the potency or biological effect of a substance using living organisms, cells, or tissues.
The rule does not mean fish will be released into drains or industrial sewers. Instead, samples of factory wastewater will be tested under controlled laboratory conditions to assess their effect on fish survival.
The requirement marks an important shift in how industrial pollution is assessed. Regulators will not only measure the concentration of chemicals in wastewater, but also test whether the effluent is toxic enough to kill living organisms.
The notification, issued on March 26, 2025, sets new standards for water use, wastewater discharge and emissions from standalone caustic soda plants based on membrane cell technology.
Caustic soda, also known as sodium hydroxide, is one of India’s most widely used industrial chemicals. It is used in industries such as soap, detergents, paper, textiles, aluminium, petrochemicals and water purification.
India has about 32 to 37 caustic soda plants, with annual production of more than five million metric tonnes. But the industry can also pose environmental risks because its wastewater is often highly alkaline and saline. It may contain chlorine, hydrochloric acid and high levels of dissolved solids.
The new standards set a pH range of 6.5 to 8.5 for wastewater from these plants. They also set limits of 250 milligrams (mg) per litre for chloride, 100 mg per litre for total suspended solids and 2,100 mg per litre for total dissolved solids (TDS).
The notification also introduces limits on water consumption and wastewater generation. A maximum of 5 cubic metres of water will be allowed per tonne of caustic soda produced, according to the rules. Wastewater generation must not exceed 1 cubic metre per tonne of caustic soda produced.
The new standards apply to standalone caustic soda plants that use membrane cell technology. Older caustic soda manufacturing processes used mercury, which was later found to pose serious pollution risks. Mercury pollution can harm fish, soil and human health, including the nervous system.
Membrane cell technology uses a special membrane to control the chemical process and is considered less polluting than older mercury-based processes. The new rules therefore focus on plants using a cleaner technology, while still requiring stricter checks on wastewater toxicity.
The bioassay test is intended to measure the combined toxicity of wastewater. This is important because a factory may meet individual chemical limits but still produce wastewater that is harmful to living organisms because of the combined effect of several pollutants.
Scientists often refer to this as a whole effluent toxicity test, or WET test. India already uses bioassay testing in some other sectors, including the pesticide industry. The notification says bioassay testing for caustic soda wastewater must be carried out according to IS: 6582-1971 standards prescribed by the Bureau of Indian Standards.
Such testing is more complex than standard chemical analysis. It requires live fish, 96 hours of continuous observation, controlled water temperature and oxygen levels, and records of behaviour and mortality. That makes it a combined test of biology and environmental toxicology, rather than only a chemical test.
The new notification raises important questions about how the standards will be monitored and enforced. It does not clearly specify whether regular bioassay testing will be conducted by industries, private laboratories or pollution control boards.
The notification also introduces weight-based pollution standards. For example, the TDS limit has been set at 2.1kg per tonne of caustic soda produced. This is significant because industries can sometimes reduce pollutant concentration by diluting wastewater with more water.
By measuring pollution generated per tonne of production, regulators can better assess the total pollution load from a plant.
Under India’s current system, industries usually get samples tested through accredited laboratories and submit reports to state pollution control boards. Regulators may then conduct sample testing for verification.
This means implementation may depend heavily on self-monitoring by industries unless pollution control boards carry out regular independent checks. Environmental experts have previously raised concerns about relying too heavily on self-certification for pollution monitoring.
The National Green Tribunal has also stressed the need for regular monitoring, independent sample collection and online monitoring in several pollution-related cases. There is currently no clear public data on how many bioassay tests are conducted each year by the Central Pollution Control Board and state pollution control boards.
This has raised questions about whether the country has enough scientific, laboratory and monitoring capacity to enforce the new standards effectively.
