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. Highlights <OrderedList> <ListItem> <ItemNumber>1.</ItemNumber> <ItemContent> Mixed microalgae removed 13.2 mg NO₃⁻-N/L/d in 73 h, outperforming monocultures. </ItemContent> </ListItem> <ListItem> <ItemNumber>2.</ItemNumber> <ItemContent> No nitrite or ammonia formed during nitrate removal by heterotrophic denitrifiers (HTD) and microalgae. </ItemContent> </ListItem> <ListItem> <ItemNumber>3.</ItemNumber> <ItemContent> Ratios of 2:1 and 1:1 (microalgae:HTD) achieved max nitrate removal of 20 and 15.23 mg/L/d in 48 and 63 h. </ItemContent> </ListItem> <ListItem> <ItemNumber>4.</ItemNumber> <ItemContent> Increasing HTD proportion improved settling efficiency, reaching 0.62 at a 1:2 ratio. </ItemContent> </ListItem> </OrderedList> Reducing hydraulic retention from 3 to 2 days sped up nitrate removal in the sequential batch process.

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

期刊论文
发表日期：

2025年10月2日

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Fariba Rezvani,
Mohammad-Hossein Sarrafzadeh

摘要

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.

Highlights

Mixed microalgae removed 13.2 mg NO₃⁻-N/L/d in 73 h, outperforming monocultures.

No nitrite or ammonia formed during nitrate removal by heterotrophic denitrifiers (HTD) and microalgae.

Ratios of 2:1 and 1:1 (microalgae:HTD) achieved max nitrate removal of 20 and 15.23 mg/L/d in 48 and 63 h.

Increasing HTD proportion improved settling efficiency, reaching 0.62 at a 1:2 ratio.

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