The use of reactive dyes in textile manufacturing for coloring cotton products can contribute elevated concentrations of salinity and alkalinity in effluents from municipal waste-treatment facilities, thereby causing periodic failure of permit-compliance toxicity testing. The sources of salinity from the textile manufacturing process are primarily Na, Cl, and SO₄, and the predominant sources of alkalinity are HCO₃ and B₄O₇. The effects of these major ions, as well as hardness, on effluent toxicity were evaluated using reproduction (mean number of young/female) from the chronic three-brood Ceriodaphnia dubia test. Reproduction was significantly increased in NaCl solutions at hardness concentrations of 80 and 160 mg/L compared to exposures at 20 and 40 mg/L hardness. Reproduction was significantly lower in 160 mg/L carbonate alkalinity than in 20 mg/L alkalinity. Salinity produced by NaSO₄ resulted in lower reproduction than salinity produced by NaCl, under similar conditions of alkalinity and hardness. Borate toxicity tended to increase with a decrease in hardness, and carbonate appears to add to the toxicity. These data demonstrate that: 1) alkalinity and hardness can influence the results of chronic-toxicity testing used in compliance monitoring; and 2) adjustments to the alkalinity and hardness of effluents could reduce the chronic toxicity of salinity.

INTRODUCTION

Compliance to water quality standards in permitted effluents from the textile industry is often hampered by elevated concentrations of chemicals that facilitate the dyeing process. For example, sodium chloride (NaCl) is often used to enhance the binding of reactive dyes on cotton. In general, one kilogram of salt is used for each kilogram of cotton. As part of the dyeing process, pH is also altered to enhance chemical reactions and this altered pH must be chemically adjusted back to near neutral conditions prior to leaving the plant. Chemical adjustments of the effluent often result in elevated concentrations of alkalinity. The sources of salinity in textile effluents are primarily Na, Cl and SO₄, and the predominant sources of alkalinity are HCO₃, B₄O₇ and OH. The objectives of this study were to evaluate the effects of these major ions and hardness on the toxicity of salinity imparted by NaCl using the Ceriodaphnia dubia three-brood chronic-toxicity tests.

METHODS Reconstituted test water adjusted chemically         -  NaCl toxicity              * Hardness and alkalinity varied              * Ion ratios remained constant         -  Source of salinity toxicity              * NaCl vs NaSO4              * 100 mg/L hardness - 80 mg/L alkalinity         -  Boron toxicity              * B4O7 source of alkalinity              * B4O7 + HCO3 source of alkalinity Chronic three-brood test - Ceriodaphnia dubia         -  Static renewal         -  25• C         -  16 h light:8 h dark         -  30 mL container         -  15 mL test solution         -  <24-h old animal         -  1 per container         -  0.2 mL YCT-Selenastrum daily         -  Endpoint - survival & mean number of young         -  >80% survival in control & 60% with 3 broods

DISCUSSION

Water hardness and alkalinity influenced the chronic toxicity (measured as mean number of young neonates per female) of salinity produced by NaCl to Ceriodaphnia dubia. Reproduction was lowest in saline solutions of 20 and 40 mg/L hardness compared to 80 and 160 mg/L hardness, and reproduction was lower in 160 mg/L alkalinity than in 20 mg/L. These tests demonstrated that an increase in hardness ameliorated NaCl toxicity, but increasing alkalinity tended to negate this amelioratory effect. Alkalinity by itself resulted in reduced reproduction as concentrations increased from 20 mg/L to 160 mg/L. Salinity contributed by sulfate (Na₂SO₄) was more toxic than salinity produced by chloride (NaCl). Sodium does not appear to contribute to the toxicity of these solutions. Increasing hardness reduced borate toxicity, but the presence of HCO₃ as an equal source of alkalinity tended to reduce reproduction. These data suggest that adjustment of the basic chemistry contributed by alkalinity and hardness could reduce chronic toxicity of textile effluents associated with salinity.

SUMMARY AND CONCLUSIONS

• Reproduction was higher in NaCl solutions at 80 and 160 mg/L hardness compared to 20 and 40 mg/L hardness
  
• Reproduction was lower in 160 mg/L carbonate alkalinity than in 20 mg/L alkalinity
  
• Reproduction in NaSO₄ salinity was lower than in NaCl salinity
  
• Hardness decreased borate toxicity
  
• Alkalinity and hardness influence chronic toxicity of salinity
  
• Adjustments of alkalinity and hardness can reduce toxicity contributed by salinity in textile effluents