A novel approach for stabilizing fresh urine by calcium hydroxide addition

Abstract

In this study, we investigated the prevention of enzymatic urea hydrolysis in fresh urine by increasing the pH with calcium hydroxide (Ca(OH)2) powder. The amount of Ca(OH)2 dissolving in fresh urine depends significantly on the composition of the urine. The different urine compositions used in our simulations showed that between 4.3 and 5.8 g Ca(OH)2 dissolved in 1 L of urine at 25 °C. At this temperature, the pH at saturation is 12.5 and is far above the pH of 11, which we identified as the upper limit for enzymatic urea hydrolysis. However, temperature has a strong effect on the saturation pH, with higher values being achieved at lower temperatures. Based on our results, we recommend a dosage of 10 g Ca(OH)2 L−1 of fresh urine to ensure solid Ca(OH)2 always remains in the urine reactor which ensures sufficiently high pH values. Besides providing sufficient Ca(OH)2, the temperature has to be kept in a certain range to prevent chemical urea hydrolysis. At temperatures below 14 °C, the saturation pH is higher than 13, which favors chemical urea hydrolysis. We chose a precautionary upper temperature of 40 °C because the rate of chemical urea hydrolysis increases at higher temperatures but this should be confirmed with kinetic studies. By considering the boundaries for pH and temperature developed in this study, urine can be stabilized effectively with Ca(OH)2 thereby simplifying later treatment processes or making direct use easier.

Realizing the Circular Economy for Sanitation: Assessing Enabling Conditions and Barriers to the Commercialization of Human Excreta Derived Fertilizer in Haiti and Kenya

“…Etter et al [38] for instance reported challenges in making a profit out of struvite, at present only recovering 13% of the potential fertilizer value of urine. New techniques that do not use magnesium sulphate are being developed and could provide promising future avenues for urine valorization [39].…”

Urinals for water savings and nutrient recovery: a feasibility study

“…Other ways of preventing odour include stabilizing urine. Urea hydrolysis is the process by which carbon dioxide and free volatile ammonia are formed from urea (Randall et al, 2016). Pungent odours are expelled as a result of the urea hydrolysis process (Zhang et al, 2013).…”

Electrochemical phosphorus removal and recovery

“…In this process, the addition of a Ca source is most likely not necessary, as Ca 2+ is an abundant ion in most waste streams [31,32]. It is worth mentioning that even when an external Ca source is required, the cost of dosing Ca is cheaper than dosing Mg [33,34]. Moreover, as calcium phosphate is the key component of mined phosphate rock, it can be used directly as a raw material for the production of phosphorus fertilizer in the existing facilities [22,31].…”

“…CaP solids can form without adding Ca 2+ since there is often already sufficient Ca 2+ in water bodies [57]. Therefore, P recovery via CaP formation and precipitation is a preferred method and has received a lot of attention [33,56].…”

“…Phosphate can be removed from the wastewater by precipitation as useful P products, such as struvite (MgNH4PO4·6H2O) and calcium phosphate [23,29,33,54]. The most stable phase of calcium phosphate is hydroxyapatite (Ca5(PO4)3OH, HAP), which owns the highest atomic Ca/P ratio (1.67) [35].…”