Reusing water and nutrients from municipal wastewater can conserve resources and reduce wastewater treatment costs. In this study, the suitability of different qualities of treated wastewater for plant production in a hydroponic flow-through system was investigated. Lettuce (Lactuca sativa L.) was grown in hydroponic lines fed with treated wastewater, i.e., conventional effluent from aerobic wastewater treatment by the activated sludge process (CE), ozonised CE (CEO), anaerobically pre-treated and nitrified wastewater (AN) and biological activated carbon filtered AN (ANC) in comparison with a modified Hoagland nutrient solution. In CEO, AN and ANC, the lettuce reached a similar weight and elemental composition as that in HS. The low N and P concentrations in CE and CEO were quickly depleted, resulting in limited plant development at the ends of these lines. The lower water content in the CE shoots was probably related to hypoxia that occurred at higher temperatures. In the CEO line, this condition did not arise due to the constant decay of O₃. At lower temperatures, the CEO shoot dry weight was 90% higher than that in CE. This was possibly an effect of residual ozone and/or oxygen supersaturation. AN produced the highest yield, while carbon filtration lowered the content of cations in ANC, inducing deficiency of Fe > Mn > Cu > Zn > K. Coupling wastewater treatment with hydroponics allows for efficient nutrient recovery, and thus could reduce the energy and reactor volume needed for N and P elimination.

The Vietnam Mekong Delta (VMD) is threatened by increasing saltwater intrusion due to diminishing freshwater availability, land subsidence, and climate change induced sea level rise. Through irrigation, saltwater can accumulate in the rice fields and decrease rice production. The study aims at evaluating topsoil salinity and examining a potential link between topsoil salinity and rice production systems in a case study in the Tra Vinh province of the VMD. For this, we applied two geophysical methods, namely, 3D electrical resistivity tomography (ARES II) and electromagnetic induction (EM38‐MK2). 3D ARES II measurements with different electrode spacings were compared with EM38‐MK2 topsoil measurements to evaluate their respective potential for monitoring topsoil salinity on an agricultural scale and the relationship between land‐use types and topsoil salinity. Results show that EM38‐MK2 is a rapid and powerful tool for obtaining high‐resolution topsoil salinity maps for rice fields. With ARES II data, 3D maps up to 40 m depth can be created, but compared with EM38‐MK2 topsoil maps, topsoil salinity was underestimated due to limitations in resolution. Salt contamination of above 300 mS m⁻¹ was found in some double‐cropped rice fields, whereas in triple‐cropped rice fields salinity was below 200 mS m⁻¹. Results clearly show a relation between topsoil salinity and proximity to the saline water sources; however, a clear link between rice production and topsoil salinity could not be established. The study proved that geophysical methods are useful tools for assessing and monitoring topsoil salinity at agricultural fields scale in the VMD.

Organic residues, as a nutrient source suitable of producing solutions for hydroponic crop production, have the potential to reduce the dependence on mineral fertilizers. Especially in remote and resource-constrained regions, organic residues might be the only option to produce hydroponic nutrient solutions. However, nutrient solutions made from organic residues, called bioponic solutions, are usually unbalanced in their nutrient composition, which leads to deficiencies and poor plant growth. This study aimed to experimentally develop a low-tech approach to produce bioponic stock solutions rich in NO₃⁻, P, and K, to create a balanced bioponic solution. The mixed bioponic solution contained 58 mg L⁻¹ NH₄⁺-N, 43 mg L⁻¹ NO₃⁻-N, 50 mg L⁻¹ PO₄³⁻-P, and 246 mg L⁻¹ K⁺. This approach resulted in satisfactory levels of P, K and micronutrients. The solution was tested pure and spiked with Ca(NO₃)₂ on lettuce in comparison with a mineral Hoagland nutrient solution. Neither the bioponic nor the spiked bioponic solution achieved comparable lettuce yields to the Hoagland solution. The poor growth of the plants in the bioponic solution was attributed to an unfavorable NH₄⁺:NO₃⁻ ratio, high microorganism load, and elevated pH levels. However, the approach of preparing bioponic stock solutions could be promising for future research into the production of balanced bioponic nutrient solutions from organic residues.

Climate change, decreased river flow and land subsidence lead to saltwater intrusion posing a significant threat to rice production in Vietnam's Mekong Delta (VMD), one of the world's largest rice exporting regions. Soil salinity in the VMD can be caused by saltwater intrusion into lowland areas through the canal system, or by capillary rise of water from the near surface saline water table, both resulting in salt accumulation in the top soil. Developing appropriate management strategies for adapting rice production systems of the VMD to climate change, both in terms of water and salinity management, requires characterizing and subsequently monitoring of the spatial distribution and temporal dynamics of salinity in the near‐surface aquifers underneath the rice producing area. The distribution of subsurface salinity was investigated using electrical resistivity tomography in the VMD's province, Tra Vinh, as a case study area. Soil salinity was measured for profiles of approximately 300 m length at 44 locations along geological transects in a case study area. Results show that saline water appears at a shallow depth, particularly along the coast and the lower reaches of rivers. Double‐cropped rice fields seem to be more susceptible to salinization via the near‐surface aquifer than other rice cropping systems. The study suggests that temporal fluctuations of the near‐surface aquifer and the dynamics of the exchange between the river and the shallow aquifer need to be investigated in future research.