Introduction:
Tannery wastewater is widely recognized as one of the most challenging industrial effluents due to its extremely high organic load, heavy metals (particularly chromium) suspended solids, and substantial salinity. The high salt content originates mainly from the extensive use of sodium chloride during hide preservation and tanning processes. Such salinity poses major environmental threats, including soil salinization, groundwater contamination, and ecological disruption in aquatic systems. Moreover, salinity is a critical inhibitory factor in biological wastewater treatment systems. Elevated salt concentrations impose osmotic stress on microbial cells, leading to reduced metabolic activity, enzyme inhibition, and ultimately lower COD and BOD removal efficiencies. Conventional activated sludge systems function properly at salinity levels below approximately 10 g/L TDS, while real tannery wastewater often ranges between 10 and 40 g/L. This discrepancy highlights the difficulty of biological treatment under real industrial conditions. Although previous studies have examined saline stress in biological systems, comprehensive evaluations based on real tannery wastewater (particularly in hybrid treatment configurations) are still limited. Therefore, this study aims to investigate the performance and salinity tolerance of an integrated chemical-biological-chemical treatment system when exposed to real high-strength tannery wastewater.

Materials and Methods:
A semi‑industrial pilot plant was developed in Varamin (2025) to evaluate the treatment performance of a hybrid system designed for real tannery wastewater with high salinity. The system consisted of three sequential units. First, a primary chemical pretreatment stage involving coagulation and flocculation was applied to reduce suspended solids, colloids, chromium, and part of the biodegradable organic load. This step was essential for minimizing shock load and ensuring more stable biological activity in the subsequent reactor. The second unit was a Moving Bed Biofilm Reactor (MBBR), selected for its ability to support robust biofilms and maintain microbial stability under fluctuating environmental conditions. The reactor was seeded with halotolerant activated sludge sourced from an industrial facility already treating saline wastewater. The initial MLSS concentration was maintained at approximately 1500 mg/L. Nutrient supplementation followed a COD:N:P ratio of 100:5:1 to ensure appropriate microbial growth. Before continuous operation, an acclimation phase of 3 to 6 days was implemented, during which the microbial community was gradually exposed to real tannery wastewater with salinity levels ranging from 19 to 24 g/L TDS. The third treatment stage provided secondary chemical polishing, aimed at improving effluent clarity and removing remaining suspended solids and residual organic fractions. The pilot operated continuously over three months. Operational parameters such as hydraulic retention time, aeration rate, MLSS content, and dosing of coagulants and flocculants were optimized as needed. Sampling was performed at the influent, after the primary chemical unit, at the MBBR outlet, and at the final effluent point. The main analyzed parameters included COD, TSS, TDS, EC, and pH. The naturally fluctuating salinity levels during the monitoring period enabled a realistic assessment of microbial tolerance and process stability under high‑salinity conditions.


Results and Discussions:
The hybrid treatment system demonstrated a notable reduction in organic pollution, achieving an overall COD removal efficiency of approximately 70%. The MBBR contributed the largest portion of this removal, accounting for about 35.5%, while the secondary chemical polishing stage added roughly 15.1%, confirming the need for a combined chemical–biological approach for complex saline wastewater. The primary chemical stage effectively removed suspended solids and colloidal material, thereby stabilizing downstream performance. The biological treatment performance was strongly influenced by salinity. The MBBR maintained stable COD removal when TDS levels remained at or below 22 g/L. Under these conditions, the halotolerant microbial community remained active, and biofilm carriers provided structural protection against osmotic stresses. Biofilms generally offer higher resilience than suspended‑growth systems due to slower mass transfer rates and gradual salt exposure within the biofilm matrix. However, when salinity exceeded the threshold of 22 g/L, noticeable decreases in COD removal efficiency occurred. Temporary spikes in salinity led to an immediate decline in metabolic activity, manifested as increased effluent COD concentrations. These observations suggest osmotic shock, which may cause cellular dehydration, membrane dysfunction, and in severe cases, microbial lysis. Although the system maintained partial functionality, the reduced performance reflected a diminished capacity of the biomass to process organic load under fluctuating saline stress. Throughout the entire study, the system exhibited no significant reduction in TDS. Final effluent salinity remained around 22 g/L, indicating that neither biological oxidation nor chemical precipitation effectively removes dissolved salts. This aligns with literature that highlights the inherent limitation of conventional biological and chemical processes for treating saline industrial effluents. Operational observations further emphasized the importance of stable loading conditions, gradual acclimation, and the use of halotolerant sludge. Despite these measures, managing salinity requires more advanced or complementary strategies. Technologies such as reverse osmosis, nanofiltration, or hybrid desalination approaches can significantly reduce TDS but require substantial investment and energy. Source reduction measures within tanning operations, including improved salt‑curing methods, recycling of brine solutions, or adoption of low‑salt tanning technologies, may also reduce the salinity load entering treatment systems. Overall, the results indicate that while the hybrid system effectively addresses organic pollution, salinity remains the dominant limiting factor governing biological performance. Therefore, long‑term wastewater management strategies must integrate both end‑of‑pipe treatment improvements and upstream process optimization.


Conclusion:
This study showed that the hybrid chemical–biological–chemical system effectively reduced organic pollutants in tannery wastewater, achieving about 70% COD removal. However, biological performance was constrained by salinity, remaining stable only up to 22 g/L TDS, with higher levels causing microbial inhibition and reduced efficiency. Since TDS remained virtually unchanged during treatment, the effluent still exceeded allowable discharge limits. Effective long‑term management therefore requires combining optimized biological treatment with desalination technologies or implementing upstream modifications to reduce salt input at the industrial source.