Innovative Strategy for Nitrate Removal from Groundwater: Synergistic Interactions of Autotrophic Microalgae and Heterotrophic Denitrifiers in Sequential Batch Process

Abstract

Groundwater nitrate pollution threatens health and environment. Traditional biological methods face struggle with carbon shortages and byproducts. We developed a process where microalgae provide organic carbon as COD through CO₂ fixation, efficiently supporting heterotrophic denitrifiers (HTDs). This study evaluated batch interactions of microalgal-bacterial consortia at different mass ratios to determine the optimal combination for maximizing nitrate-nitrogen removal rates (NRR) and minimizing hydraulic retention times (HRT) to reduce NO₃⁻-N to 10 mg/L, suitable for sequential batch photobioreactor applications. Individually, Chlorella vulgaris and Scenedesmus sp. removed 50 mg NO₃⁻-N/L at rates of 10 and 12.7 mg/L/d over 96 and 76 h, respectively. A mixed microalgal culture improved performance, reducing HRT to 73 h and achieving a peak NRR of 13.2 mg/L/d, but generated 85 mg/L of COD. HTDs alone achieved a maximum NRR of 56 mg/L/day within 18 h but generated nearly 1 mg/L of nitrite and ammonia byproducts. Combining microalgae with HTDs at 1:1 and 2:1 mass ratios enhanced NRRs to 15.2 and 20 mg/L/d, respectively, with reduced HRTs of 63 and 48 h, while minimizing byproduct formation. The 1:2 ratio showed decreased NRR (11.4 mg/L/d) and longer HRT (84 h), likely due to reduced organic carbon availability as COD. Higher HTD proportions also improved biomass settling efficiency (up to 0.62). A 21-day sequential batch photobioreactor study showed that reducing HRT from 3 to 2 days promoted microbial adaptation and nitrate removal, with the 1:1 ratio achieving faster nitrate reduction than 2:1, making it applicable for large-scale application.

Reducing hydraulic retention from 3 to 2 days sped up nitrate removal in the sequential batch process.