Should wastewater treatment plants' operational mode radically change to minimize GHG emissions?

The operation of municipal wastewater treatment plants (WWTPs) invariably results in significant emission of greenhouse gases (i.e., CH4, N2O, and CO2) into the atmosphere. We propose to consider a radical change in the way municipal WWTPs are operated, with the aim of minimizing GHG emissions while recycling most of the nutrient mass. The means to this end are to reduce the WWTP energy demand while maximizing the recovery of resources (phosphorus, ammonia, methane). The suggested concept involves operating the activated sludge process at a low sludge retention time (SRT < 2 d), i.e., under conditions that maximize the heterotrophic mass yield and eliminate nitrification. The ammonia concentration that remains in the water (considering N in the excess sludge and struvite production in the sludge-dewatering supernatant line) would be separated from the WWTP effluents using a unique ion-exchange material (ZnHCF), which would be regenerated using a low-volume 4 M NaCl solution. The ammonia would be then stripped at high pH and re-adsorbed by an acidic solution for reuse as fertilizer. The high bacterial yield and lack of nitrification in the aerobic step are expected to boost methane yield 3-4-fold, induce lower oxygen consumption, and most importantly, yield much lower N2O release. An approximate energy mass balance shows the concept to merit further consideration, owing to the potential significant reduction in N2O(g) emissions and recovery of resources. Empirical work followed by LCA is required to corroborate the hypothesis presented herein.

A Prussian-blue analogue (PBA) ion-chromatography-based technique for selective separation of Rb+ (as RbCl) from brines.

A new general method is presented for separating pure RbCl(s) from solutions rich in Na+ and K+. The method relies on Rb+ adsorption via ion exchange performed by self-synthesized PES coated Zn-Hexa-Cyanoferrate material. The procedure starts by passing the wastewater through an ion exchange column, which is thereafter regenerated with 1 M NH4Cl. If the Rb+ absorbed on the column does not reach a minimal predetermined value (e.g., 8%, eq-based), the ammonia is removed by sublimation and the remaining salts are passed again through a Na+-preadsorbed column. Once the adsorbed Rb+ is substantial (>8%), a chromatography-based separation between Rb+ and Na+/K+ is performed, using a 2nd column, fully pre-adsorbed with NH4+. First, 0.05M NH4+-solution is used to extract Na+ and K+ out of the first column, along with a small Rb+ mass, which is thereafter partly re-adsorbed on the second column, while Na+/K+ ions are not. Once the exiting eluent solution is devoid of the competing ions, 1M NH4+-solution is used to extract all the remaining Rb+ into the regeneration solution, which is thereafter subjected to water evaporation followed by NH3/HCl sublimation to result in pure RbCl(s) product. We used theoretical simulations corroborated by empirical results to present proof of concept for the suggested approach. A detailed cost analysis (Capex and Opex) reveals that the RbCl(s) production cost does not exceed ∼25% of the current salt price.

Synthesis and characterization of zinc-hexacyanoferrate composite beads for controlling the ammonia concentration in low-temperature live seafood transports.

A new water treatment technology is presented for extending the longevity and increasing the maximal bio-load of container-bound, lucrative live seafood transportations. The technology is designed for removing ammonia and minimizing the bacterial concentration that develop in the water during the transport. This paper focuses on the characteristics of self-synthesized polyether-sulfone (PES) coated Zn-HCF composite beads, which have a high adsorbing capacity for NH4+ in seawater and constitute the heart of the developed technology. Adsorption isotherms show that the operational capacity of the composite material (PES = 20% w/w) at NH4+ concentration of 10 mgN/L at 3.5 °C is ∼3 mgN/g Zn-HCF. The kinetics of the PES-coated beads were shown to be considerably slower than the bare Zn-HCF, but since the retention time in the transport is long (many days), this does not detract from the effectiveness of the adsorption. Simulation experiments with and without live fish showed that the adsorbing material behaved as expected during a 21-d trip and that it did not have any effect on the fish. Repeated adsorption/regeneration (3 and 6 M NaCl) tests proved the composite material's stability and ion-exchange robustness. Electrooxidation of the ammonia in the exhausted regeneration solution was carried out with high efficiency and the treated solution could be used effectively in the following chemical regeneration step. The cost of a treatment unit installed in a 40-foot container was estimated at $40,000 and the ROI at 6 to 12 months.