Complex mechanism of sulphane odour production in water heating and suitable remedial measures

Drinking or heated water can have a wide range of disturbing odours which have many various causes. A well known example is sulphane (hydrogen sulfide) odour, reminiscent of rotten eggs, which naturally occurs in some underground waters. Less known and hitherto unexplained is this odour arising from hot water in water heaters. As a response to complaints by consumers we investigated 14 sites and thanks to experimental work in one of these recognised a complex mechanism at work. For production of sulphane to occur in a water heater the following conditions must be met: presence of sulphate-reducing bacteria, presence of sulphates, reduction properties of water indicated by a very low nitrate content (highest value was 10 mg/L, mostly <2 mg/L), and presence of sacrificial magnesium anode. We identified four possible remedial measures, some of which are less effective or only short-term, or have an undesirable side-effect (corrosion). The most effective measure was replacement of the magnesium anode for an aluminium or zinc anode, which retains the anticorrosion protection of the heater.

Electrochemical Crystallization for Recovery of Phosphorus and Potassium from Urine as K-Struvite, with A Sacrificial Magnesium Anode

Declining earth resources, rising ore cost and pollution call for better recycling of wastewater in the context of the circular economy. In particular, urine is a potential huge source of phosphorus (P) and potassium (K) agricultural nutrients, yet the efficiency of actual methods for P and K recovery are limited. Here we designed a electrochemical crystallization system using sacrificial magnesium anodes to recover P and K in the form of K-struvite (MgKPO4·6H2O) from simulated urine at low (P/K=0.25) and high (P/K=0.6) phosphate levels, respectively. Results show optimal recoveries of 88.5% for P and 35.4% for K in the form of rod-shaped K-struvite at 3.5 mA/cm2, yet higher current density reduced recovery due to side reactions and pH increase. Adding phosphate to urine increased K recovery to 35.4% versus 15.0% without phosphate. Adding prefabricated struvite crystals at 1.6 g/L into urine enhanced the recovery of K by 14.7% and of P by 23.7% compared to the control group. Overall, our findings show that electrochemical crystallization is promising for the recovery of K-struvite fertilizers.

Intrinsic differences and realistic perspectives of lithium-sulfur and magnesium-sulfur batteries

Following in the footsteps of lithium-sulfur batteries, magnesium-sulfur batteries offer a high theoretical energy content and are composed of cheap and more environmentally-friendly electrode materials. In comparison to lithium-sulfur, however, current magnesium-sulfur batteries suffer from higher overpotentials at the magnesium anode and the sulfur cathode, lower material utilization and reversibility at the sulfur cathode, and an excessive demand of electrolyte. Here, a side-by-side comparison of the processes at the two metal anodes and at the sulfur cathode in Li+- or Mg2+-based electrolytes highlights how most of the challenges facing magnesium-sulfur batteries are intrinsically rooted in the nature of the magnesium species, requiring different research directions than lithium-sulfur batteries. An evaluation of the energy content and the corresponding costs on a practical cell stack level illustrates the importance of overcoming these challenges.