Unhairing effluents treated by an activated sludge system

Area of Science:

• Environmental Engineering
• Wastewater Treatment Technologies
• Bioremediation

Background:

• Leather tannery unhairing processes generate highly toxic, alkaline wastewater.
• This effluent contains significant organic matter, sulphides, suspended solids, and salts.
• Effective treatment is crucial to mitigate severe environmental impacts.

Purpose of the Study:

• To evaluate the biodegradability and toxicity of diluted tannery unhairing wastewater after activated sludge (AS) treatment.
• To assess the impact of varying organic load rates (OLR) and hydraulic retention times (HRT) on AS performance.
• To investigate the activity of the AS biomass and the characteristics of the treated effluent.

Main Methods:

• An activated sludge (AS) system was operated for 180 days with diluted unhairing effluent.
• Organic load rate (OLR) was increased from 0.23 to 2.98 g COD/l/day.
• Biomass activity was measured using specific oxygen uptake rate (SOUR) and endogenous oxygen coefficient.
• Toxicity was assessed using bioassays with Daphnia magna and Daphnia pulex.
• Ultrafiltration (UF) was used to characterize the recalcitrant COD fraction.

Main Results:

• At OLR < 2 g COD/l/day, BOD5 removal efficiency reached 99% and COD removal was around 80%.
• System efficiency decreased significantly (COD and BOD5 removal < 40%) at OLR > 2 g COD/l/day.
• Biomass activity (SOUR and endogenous coefficient) decreased substantially, potentially due to ammonia and chloride content.
• Toxicity removal was limited (24-31%) even after aerobic treatment, with recalcitrant COD fractions > 10,000 Da comprising ~55% of influent COD.

Conclusions:

• Activated sludge treatment is effective for tannery unhairing effluent at moderate organic loads.
• High organic loads and inorganic compounds in the effluent negatively impact AS performance and biomass activity.
• A significant portion of COD remains recalcitrant to aerobic treatment, necessitating further investigation for complete effluent detoxification.