Temperature dependent removal of sodium chloride (NaCl) from synthetic nitrified urine

Urine is the source of the major part of plant nutrients in municipal wastewater. Therefore, full nutrient recovery from source-separated urine is an attractive option for both treating wastewater and gaining a valuable fertilizer product. Full nutrient recovery can be achieved by first stabilizing collected urine by nitrification and then concentrating the urine by distillation. Since concentrations of all salts in urine increase with increasing removal of water also the sodium chloride (NaCl) content is high in the end. There are two problems related to NaCl, the first being the synergistic decomposition of ammonium nitrate by chloride and the second being soil salinity and sodicity related problems when applying the product as fertilizer. Solubility experiments using synthetic nitrified urine were carried out in the temperature range between 40 and 90°C. The synthetic urine solution contained seven inorganic ions at constant composition (NH4+ , Na+, K+ // NO3- , SO4-- , PO4--- , Cl- - H2O) and different water contents in order to determine the achievable extend of NaCl removal. The aim was to find the conditions, at which a maximal amount of sodium chloride can be removed with minimal loss of other nutrients, especially nitrogen. The underlying hypothesis was, that the solubility of ammonium chloride (NH4Cl) shows a much stronger temperature dependence compared to NaCl and therefore selective NaCl removal can be achieved at elevated temperatures. The analysis of the solids showed, that mainly Cl-, SO4-- , Na- and to a lower extend NH4+ were present. At higher temperatures, more Cl- relative to NH+4 was present in the solids. At 40°C, only NH4Cl was found while for all temperatures above 60°C no NH4+ was observed at similar water contents. The maximal removal of sodium chloride was achieved at a water content of 7.1 % and was around 50 %. Na+ removal was as high as 33 %. Nitrogen losses as NH4Cl precipitate were around 11 %, while basically no potassium or phosphate was lost. It was concluded, that selective removal of NaCl is possible at elevated temperatures. The maximal removal of Cl-, however, might not be sufficient to affect ammonium nitrate decomposition significantly. With regard to soil salinization, the achieved NaCl removal might well have a relevant impact. In order to rationalize results, two numerical models for electrolyte solutions, Pitzer and extended Uniquac, were applied. Both models could only give rough estimates for concentrations, at which crystallization started, as well as for the identities of solid phases. This result might be due to various reason, one being the very high ionic strengths in the nitrified urine system. Some preliminary results for full nitrification using sodium hydroxide (NaOH) were obtained, and this process alternative circumventing the problem of ammonium nitrate decomposition is discussed. Future investigations should focus on the following topics: the applicability of the results for synthetic urine on real urine; the effect of varying nitrogen contents in urine on NaCl removal; the mechanisms responsible for the loss of nitrogen during collection and storage of urine; full nitrification as an alternative process; further development of numerical models for electrolyte solutions at high ionic strengths.