Fish-survival test for wastewater toxicity-New Environmental Standards

Why in News?

The Union Government has notified the Environment (Protection) Second Amendment Rules, 2025, introducing stricter environmental standards for standalone caustic soda plants using membrane cell technology. A key provision mandates that wastewater from these plants must pass a laboratory-based fish survival (bioassay) test, with at least 90% fish survival after 96 hours in 100% wastewater samples. 

Fish Survival (Bioassay) Test

• Industrial wastewater will be tested under controlled laboratory conditions using fish.
• At least 90% of fish must survive for 96 hours in untreated wastewater samples.
• The test evaluates the overall biological toxicity of effluents rather than only measuring individual chemical concentrations.
• Marks a shift from chemical-based monitoring to ecological impact assessment.

Wastewater Quality Standards: The new rules prescribe:

Parameter	Standard
pH	6.5 – 8.5
Chloride	250 mg/L
Total Suspended Solids (TSS)	100 mg/L
Total Dissolved Solids (TDS)	2,100 mg/L

About Caustic Soda (Sodium Hydroxide)

• Chemical formula: NaOH
• Strong alkali widely used in industrial processes.
• Major Uses:

• Soap and detergent manufacturing
• Paper and pulp industry
• Textile processing
• Aluminium production
• Petrochemicals
• Water treatment and purification

• Around 32–37 caustic soda plants operate in India. Annual production exceeds 5 million metric tonnes.
• Environmental Concerns: Wastewater from caustic soda plants can contain:

• High alkalinity
• Excess salinity
• Chlorine residues
• Hydrochloric acid
• High dissolved solids

What is a Bioassay Test?

A bioassay (biological assay) is a test that measures the effect or toxicity of a substance using living organisms, tissues, or cells. 

• Significance
  • Assesses the actual biological impact of wastewater.
  • Detects harmful effects that chemical analysis alone may miss.
  • Provides a more realistic measure of environmental safety.

About Membrane Cell Technology 

• Membrane cell technology is an advanced electrolytic process used for producing caustic soda (NaOH) and chlorine.
• Working:
  • Anode and cathode are separated by a selective ion-exchange membrane.

• The membrane allows sodium ions (Na⁺) to pass through while preventing chlorine and hydrogen ions from mixing.
• Sodium hydroxide is produced directly at the cathode.

Advantages

• No need for a decomposer because sodium hydroxide or potassium hydroxide forms directly at the cathode without the mercury amalgam step.
• No use of toxic mercury.
• Eliminates asbestos-based diaphragms.
• Consumes significantly less electricity than traditional diaphragm or mercury cells
• Higher efficiency and product purity.
• Reduced environmental risks.

Sources: DTE & Others