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Integration of a gas-permeable hydrophobic membrane in the cathode compartment of a bio-electrochemical system enables efficient ammonia recovery from wastewater.

BACKGROUND: The treatment of source separated urine allows for a more sustainable approach to nutrients recovery in actual wastewater treatment plants. Struvite precipitation from urine yields a slow-release fertilizer (struvite) with a high marketable value for agricultural use. Extensive research on struvite precipitation has been carried out at laboratory scale, but there is still a lack of specific knowledge about the operation and key parameters at larger scale and over the long term. The effectiveness of the struvite reactor was assessed, as well as the quality of the harvested struvite in terms of size and composition. RESULTS: The efficiency of recovery of P was maintained at 85-99% for 1 year of continuous operation. Phosphorus was recovered in the form of struvite with purity over 90%. Struvite was harvested as round pellets between 0.3 and 6 mm in diameter, with heavy metals concentration well below the legal limits for fertilizers in The Netherlands. The post-treatment of the reactor effluent yielded almost complete removal (>95%) of calcium and magnesium. A loss of 26 ± 5% of the influent chemical oxygen demand and 16 ± 1% of the NH 4 + -N occurred during the magnesium ammonium phosphate (MAP) precipitation and the post-treatment. CONCLUSION: This work shows how the treatment of source-separated urine by struvite precipitation in a fluidized bed reactor allows for efficient recovery of phosphorus in the form of a potential fertilizer.

Microbial Desalination Cell (MDC) represents an innovative technology which accomplishes simultaneous desalination and wastewater treatment without external energy input. MDC technology could be employed to provide freshwater with low-energy input, for example, in remote areas where organic wastes (i.e., urban or industrial) are available. In addition, MDC technology has been proposed as pre-treatment in conventional reverse osmosis plants, with the aim of saving energy and avoiding greenhouse gases related to conventional desalination processes. The use of oxygen reduction (i.e. O 2 + 2H 2 O + 4e − → 4 OH − , E 0 ′ = 0.815 V, pH = 7) was usually implemented as cathodic reaction in most of the MDCs reported in literature, whereas other strategies based on liquid catholytes have been also proposed, for example, ferroferricyanide redox couple (i.e. Fe(CN) 3− 6 + 1e − → Fe(CN) 4− 6 , E 0 = 0.37 V). As the MDC designs in the literature and operation modes (i.e., batch, continuous, semi-continuous, etc.) are quite different, the available MDC studies are not directly comparable. For this reason, the main objective of this work was to have a proper comparison of two similar MDCs operating with two different catholyte strategies, and compare performance and desalination efficiencies. In this sense, this study compares the desalination performance of two laboratory-scale MDCs located in two different locations for brackish water and sea water using two different strategies. The first strategy consisted of an air cathode for efficient oxygen reduction, while the second strategy was based on a liquid catholyte with Fe 3+ /Fe 2+ solution (i.e., ferro-ferricyanide complex). Both strategies achieved desalination efficiency above 90% for brackish water. Nominal desalination rates (NDR) were in the range of 0.17-0.14 L•m −2 •h −1 for brackish and seawater with air diffusion cathode MDC, respectively, and 1.5-0.7 L•m −2 •h −1 when using ferro-ferricyanide redox MDC. Organic matter present in wastewater was effectively removed at 0.9 and 1.1 kg COD•m −3 •day −1 using the air diffusion cathode MDC for brackish and Ramírez-Moreno et al.Comparative Performance of Microbial Desalination Cells sea water, respectively, and 7.1 and 19.7 kg COD•m −3 •day −1 with a ferro-ferricyanide redox MDC. Both approaches used a laboratory MDC prototype without any energy supply (excluding pumping energy). Pros and cons of both strategies are discussed for subsequent upscaling of MDC technology.