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
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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.