Performance Evaluation of a Hybrid Vertical-Horizontal Flow Constructed Wetland System for Domestic Wastewater Treatment Under Sub-Tropical Climatic Conditions in Rajasthan, India

Environmental Engineering

Abstract Constructed wetlands (CWs) offer a decentralized, low-energy alternative for domestic wastewater treatment, particularly suitable for water-scarce, sub-tropical regions like Rajasthan, India. However, standalone vertical flow (VF) or horizontal flow (HF) systems have limitations: VF excels at nitrification but requires intermittent feeding and has limited denitrification; HF provides good denitrification but requires large land area. This study evaluates a hybrid VF-HF system treating domestic wastewater (13.5 m³/day) from a labor colony of 300 persons in Ajmer, Rajasthan, over a 10-month operational period (February–November). The system comprised a 90 m² VF bed (0.7 m depth, 20 mm gravel, Phragmites karka) followed by a 280 m² HF bed (0.5 m depth, 15 mm gravel, Typha angustata). Influent and effluent samples were analyzed weekly for BOD₅, COD, TSS, NH₄-N, TN, and TP. Results showed mean removal efficiencies of 92.7% (BOD₅), 90.3% (COD), 93.8% (TSS), 81.3% (NH₄-N), 76.5% (TN), and 73.2% (TP), achieving final effluent concentrations of BOD₅ 16 mg/L, COD 42 mg/L, and TSS 12 mg/L—compliant with CPCB discharge standards for reuse in irrigation and flushing. Seasonal variation was observed: summer (28–42°C) achieved highest removals (BOD 96%), while winter (12–25°C) showed reduced performance (BOD 89%). First-order kinetic modeling yielded area-based removal rate constants (k) of 0.32 m/d for BOD in VF and 0.11 m/d for BOD in HF. The hybrid system required 370 m² (7.4% of site area) at a capital cost of ₹4.01 lakhs and O&M cost of ₹3,500/month. The study concludes that hybrid VF-HF constructed wetlands are technically feasible, cost-effective, and climate-resilient for decentralized domestic wastewater treatment in India's sub-tropical, semi-arid regions.

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