Removal of low concentration nutrients in hydroponic wetlands integrated with zeolite and calcium silicate hydrate functional substrates

• CSH achieved PO43−-P removal >90% as functional filtration substrate.
• Zeolite achieved NH4+-N removal of 77% as functional filtration substrate.
• Hydroponic plants played a major role in PO43−P and NH4+-N removal in wetlands.
• CSH and zeolite are critical to ensure stable overall nutrient removal in wetlands.

Nutrient pollution in surface and ground water is one of the world’s most widespread, costly and challenging environmental problems, resulting in serious environmental and health issues. This study investigated the removal of low concentration nutrients from synthetic solution simulating slightly nutrient polluted water. Continuous-flow filtration column and hydroponic wetland experiments were conducted to investigate the impact of operating conditions and using zeolite and calcium silicate hydrate (CSH) as functional filtration substrates. The interactions between phosphorus (PO43−-P) and nitrogen (NH4+-N and total nitrogen) with hydroponic plants, CSH and zeolite at different hydraulic loading rates (HLR) were found to have significant impacts on nutrient removal. During filtration column experiments, CSH was highly effective for PO43−-P removal and achieved an average removal efficiency greater than 90%. Zeolite removed more NH4+-N than CSH, achieving an average removal of 77%. Longer filtration time improved nutrient removal, and the optimal retention time was found to be 12 h. The wetland testing demonstrated nutrient uptake by hydroponic plants played a major role in removal of PO43−-P and NH4+-N during plant growth season. Installation of CSH and zeolite functional substrates was critical to ensure stable overall nutrient removal in hydroponic wetlands. The functional substrates provided complementary physical–chemical sorption capacity for PO43−-P and NH4+-N, especially when the plants uptake varied between growth and harvest seasons. Higher HLR reduced slightly the overall nutrient removal in wetlands. Throughout the testing period of 70 days, hydroponic wetlands showed no advantage in organic nitrogen removal compared to subsurface flow constructed wetlands. The total nitrogen percent removal was below 30% in both wetland units probably due to lack of microbiological activities in the wetlands resulting from low organic matter in synthetic influent water.

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