Maria-Teresa Borges

Phytoremediation of salt-affected soils: a review of processes, applicability, and the impact of climate change

Soil salinization affects 1–10 billion ha worldwide, threatening the agricultural production needed to feed the ever increasing world population. Phytoremediation may be a cost-effective option for the remediation of these soils. This review analyzes the viability of using phytoremediation for salt-affected soils and explores the remedial mechanisms involved. In addition, it specifically addresses the debate over plant indirect (via soil cation exchange enhancement) or direct (via uptake) role in salt remediation. Analysis of experimental data for electrical conductivity (ECe) + sodium adsorption ratio (SAR) reduction and plant salt uptake showed a similar removal efficiency between salt phytoremediation and other treatment options, with the added potential for phytoextraction under non-leaching conditions. A focus is also given on recent studies that indicate potential pathways for increased salt phytoextraction, co-treatment with other contaminants, and phytoremediation applicability for salt flow control. Finally, this work also details the predicted effects of climate change on soil salinization and on treatment options. The synergetic effects of extreme climate events and salinization are a challenging obstacle for future phytoremediation applications, which will require additional and multi-disciplinary research efforts.

Integration of consumer-targeted microalgal production with marine fish effluent biofiltration – a strategy for mariculture sustainability

EU regulations recommend effluent treatment and nutrient recycling for aquaculture sustainability, so a study was undertaken to provide base-line data for the integration of commercial fish-farm effluents with the production of microalgae. The project relates to a specific bivalve consumer (Tapes decussatus) and biofiltration. Effluent inorganic nutrient composition was assessed and evaluated as culture media for Phaeodactylum tricornutum, Tetraselmis suecica and Tetraselmis sp. Optimization of the microalgal compartment included studies on preparation of a simple medium, nutrient or dilution rate manipulation and nutrient removal. Cell harvest was increased chiefly by N correction (6-fold for Tetraselmis sp.) and semi-continuous or continuous operation (by a factor of 3 to 11). Nutrient removal efficiency was high for ammonium and nitrite-nitrogen (80–100%), depending on species, nutrient ratio (Si correction for P. tricornutum) and culture regime for nitrate (41–100%) or phosphorus (21–99%). Data obtained under cyclostat cultivation (yields of 1.38 and 0.50×106P. tricornutum or Tetraselmis sp. cells mL−1 d−1 and nutrient uptake rates of 2.32 mg N L−1 d−1 and 0.96 mg P L−1 d−1) were used to show clam production and simultaneous wastewater treatment possibilities through the proposed fish-microalgae-clam integrated aquaculture system.

Performance of outdoor seawater treatment systems for recirculation in an intensive turbot (Scophthalmus maximus) farm

Water treatment systems are mandatory in recirculating aquaculture facilities facing existing regulations, but data on system efficiency, especially for marine species, are scarce. The present work aimed at contributing to the evaluation of the effluent characteristics and the performance of a combined outdoor biological and non-biological treatment system in an intensive turbot (Scophthalmus maximus) farm, operating under different hydraulic regimes. A preliminary study on the biofilter bacterial populations was also undertaken. Changes in effluent characteristics with pumping, season of the year and fish biomass were observed. The treatment system showed performance instability under the conditions assayed (outdoors, changeable recycle rates). Maximum removal of solids was observed in winter, with microscreen or biological filtration (up to 60%) and nitrite removal (40–98%) was achieved with ozonation. Reduction in ammonium levels was higher in summer, either mechanically (74%) or biologically (33%). Phosphate removal was higher in winter with both systems (37 and 60%, respectively). Compliance with Portuguese discharge standards was achieved. For improvements in the treatment loop, further studies on biofilter bacteria under outdoor conditions are needed, and biological denitrification is encouraged.

Feasibility of typha latifolia for high salinity effluent treatment in constructed wetlands for integration in resource management systems

High salinity wastewaters have limited treatment options due to the occurrence of salt inhibition in conventional biological treatments. Using recirculating marine aquaculture effluents as a case study, this work explored the use of Constructed Wetlands as a treatment option for nutrient and salt loads reduction. Three different substrates were tested for nutrient adsorption, of which expanded clay performed better. This substrate adsorbed 0.31 mg kg−1 of NH4 +−N and 5.60 mg kg−1 of PO4 3−−P and 6.9 mg kg−1 dissolved salts after 7 days of contact. Microcosms with Typha latifolia planted in expanded clay and irrigated with aquaculture wastewater (salinity 2.4%, 7 days hydraulic retention time, for 4 weeks), were able to remove 94% NH4 +−N (inlet 0.25 ± 0.13 mg L−1), 78% NO2 −−N (inlet 0.78 ± 0.62 mg L−1), 46% NO3 −−N (inlet 18.83 ± 8.93 mg L−1) whereas PO4 3−−P was not detected (inlet 1.41 ± 0.21 mg L−1). Maximum salinity reductions of 52% were observed. Despite some growth inhibition, plants remained viable, with 94% survival rate. Daily treatment dynamics studies revealed rapid PO4 3−−P adsorption, unbalancing the N:P ratio and possibly affecting plant development. An integrated treatment approach, coupled with biomass valorization, is suggested to provide optimal resource management possibilities.

Role of three different plants on simultaneous salt and nutrient reduction from saline synthetic wastewater in lab-scale constructed wetlands.

Constructed Wetlands (CWs) can be a valuable technology to treat high salinity wastewaters but it is not known their potential for removal of both nutrients and salt, and the type of plants to use. This study evaluated the effect of three plants on salt reduction and simultaneous nutrient removal in CWs microcosms with expanded clay and in hydroponic conditions. Initial values of the synthetic wastewater tested were EC=15dSm-1, SAR=151; NH4+-N=24mgL-1; PO43--P=30mgL-1 and NO3--N=34mgL-1. With expanded clay CW removal efficiency for NH4+-N was 21, 88 and 85%, while for NO3--N, it was 4, 56 and 68% for Spartina maritima, Juncus maritimus and Arundo donax, respectively. PO43--P was adsorbed completely in the expanded clay. However, in hydroponic system, removal efficiencies for NH4+-N were 53 and 50%, while PO43--P removal was 89 and -14% for Spartina maritima and Juncus maritimus, respectively. Nutrient removal in planted microcosms was statistically higher than unplanted controls for NH4+-N and PO43--P. However, salt removal was apparent in the hydroponic system only after 23days of HRT, despite clear salt excretion visible in both Spartina maritima and Juncus maritimus. This study demonstrates the potential of two halophytic plants for saline wastewater treatment. However, salt removal in such a scenario could not be well documented and might prove to be impractical in future work.

Characterization of a novel dissolved CO2 sensor for utilization in environmental monitoring and aquaculture industry

A novel optical fiber sensor is presented for measuring dissolved CO2 for water quality monitoring applications, where the optical signal is based either on refractive index changes or on color change. The sensing chemistry is based on the acid-basic equilibrium of 4-nitrophenol, that is converted into the anionic form by addition quaternary ammonium hydroxide. The CO2 sensitive layer was characterized and tested by using simple absorbance/reflectance measurement setups where the sensor was connected to a fiber optic CCD spectrometer. A prototype simulating a real shallow raceway aquaculture system was developed and its hydraulic behavior characterized. A commercially available partial-pressure- NDIR sensor was used as a reference for dissolved CO2 tests with the new optical fiber sensor under development. Preliminary tests allowed verifying the suitability of the new optical sensor for accurately tracking the dissolved carbon dioxide concentration in a suitable operation range. Direct comparison of the new sensor and the reference sensor system allowed to demonstrate the suitability of the new technology but also to identify some fragilities there are presently being addressed.

Effect of plants in constructed wetlands for organic carbon and nutrient removal: a review of experimental factors contributing to higher impact and suggestions for future guidelines

Constructed wetland is a proven technology for water pollution removal, but process mechanisms and their respective contribution are not fully understood. The present review details the effect of plants on removal efficiency of constructed wetlands by focusing on literature that includes experiments with unplanted controls for organic carbon and nutrient (N and P) removal. The contribution of plant direct uptake is also assessed. Although it was found that several studies, mostly at laboratory or pilot scales, showed no statistical differences between planted and unplanted controls, some factors were found that help maximize the effect of plants. This study intends to contribute to a better understanding of the significance of the effect of plants in a constructed wetland, as well as to suggest a set of experimental guidelines in this field.