Microalgae as a Nature-Based Solution for Nitrate-Impacted Hard Groundwater Reuse in Cyprus: Performance, Constraints, and Scale-Up Pathways

Groundwater contamination by nitrates, together with high salinity, hardness and sulfates, increasingly constrains safe irrigation reuse in Mediterranean hotspots. Microalgae-based Nature-Based Solutions (NbS) can couple nutrient removal with biomass co-production; however, implementation in real groundwater matrices requires strategies that sustain phototrophic function under high Ca²⁺/Mg²⁺ and micronutrient limitation. Here we evaluate a naturally resilient Chlorella sp. strain characterized by an extensive extracellular matrix as an NbS-based treatment process for hard groundwater from Nicosia (Cyprus), targeting nitrate decontamination with resource recovery.

The groundwater exhibited a challenging ionic profile (45.2 mg·L^-1 NO₃-N; 1700 mg·L^-1 SO₄²⁻; 361 mg·L^-1 Na⁺; 148 mg·L^-1 Mg²⁺; 660 mg·L^-1 Ca²⁺; EC ~4.6 mS·cm^-1), together with low bioavailable phosphorus and trace metals. In 7-day batch tests, nitrate removal was consistently high (>98%), while biomass formation remained substantial despite the unfavorable substrate (VSS increased from 160±5 mg·L^-1 up to 1250 mg·L^-1 depending on supplementation). Trace-mineral supplementation supported the “trace-metals-as-enabler” principle, as cultures in untreated groundwater exhibited strong stress, whereas Hutner’s trace-metals amendment restored photophysiology and pigment recovery, demonstrating that Fe/Mn/Cu limitation—not nitrate supply—governs culture robustness.

Phosphorus management emerged as the main scale-up constraint in this hard groundwater. A phosphate-buffer addition (6.66 mM K₂HPO₄ + 3.34 mM KH₂PO₄) promoted rapid Ca–phosphate mineral formation, driving acidification and removing phosphate beyond what could be explained by biomass assimilation; consequently, changes in Ca/Mg could not be interpreted as biological uptake. Consistent with this, dissolved Ca²⁺ decreased by ≥61% immediately and reached approximately 73% by day 7, indicating predominantly abiotic removal during medium preparation and cultivation. Dissolved Mg²⁺ also decreased by ≥15% at day 0, consistent with co-precipitation or sorption onto the newly formed mineral phases, while subsequent Mg decreases likely reflect a combination of continued chemical association and biosorption to algal surfaces.

To translate the approach toward field feasibility, we implemented a lab-scale photobioreactor (800 mL) using a bioenergetic cultivation strategy: low, demand-matched P dosing (5 mg·L^-1 PO₄–P as KH₂PO₄) with Hutner’s trace metals, daily pH control at 7.2 (acid/base adjustment), and semi-continuous operation (10% daily exchange). Under these conditions, no precipitation occurred, PO₄–P remained near-depleted, and nitrate was fully removed by day 14 (>99.9%), alongside moderate co-reductions of Ca²⁺ (27%) and Mg²⁺ (21%). In the absence of phosphate-driven scaling, these co-removals are consistent with biosorption to the EPS-rich extracellular matrix and cell surfaces and removal with harvested biomass.

The validated combination of resilient strain selection, trace-mineral support, and low-dose P delivery with pH control provides a transferable design rule for cultivating microalgae in hard, nitrate-impacted groundwaters while achieving reliable decontamination and biomass co-production. This operating strategy is being validated for large-scale implementation in Nicosia within the CARDIMED demonstrator, including transfer to an outdoor tubular photobioreactor (1200 L) under real climatic conditions.

Acknowledgements: This research has been funded by the European Union’s Horizon Europe Innovation Programme under the CARDIMED project, Grant Agreement No. 101112731.