Hybrid process for urine treatment and valorization

Sustainable processes for the valorization and reuse of a liquid waste : urine
Aurélien Triger^a, Jean-Stéphane Pic^a, Corinne Cabassud<sup>a

a</sup> Université de Toulouse; INSA, LISBP, 135, avenue de Rangueil, 31077

Toulouse, France

^aINRA, UMR792 Ingénierie des Systèmes Biologiques et des Procédés, F-31400 Toulouse

^aCNRS, UMR5504, F-31400 Toulouse, France

Authors Email: [e-mail], [e-mail]

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Humans and animal urines are often considered as wastes and low valuable liquids. The production of urine represents high volumes. About 550 liters of urine are rejected every year per person, in the world the total production represents about 7 million m³/day. Urine is mainly made of water (about 95%) but also contains an important concentration of elements (nitrogen, phosphorus, potassium...). Depending of health and sanitary conditions, urines can also be contaminated by bacteria and viruses. The worldly phosphorus diminishing reserves (Gilbert, 2009) might incite to consider urine as a renewable source of phosphorus and the water shortage makes water reuse an important issue.

This study aims to provide some elements about the feasibility of considering urine as both a resource of reusable water for specific uses (for example irrigation) and of nutrients production. The possible treatment of micropollutants or drugs in urine, that mainly concerns developed (or over-medicated) countries will be considered in a further study.

The treatment line must include a step of precipitation or crystallization of valuable species (phosphorus) and an efficient separation process to remove bacteria and viruses. The most efficient way to recover phosphorus from urine appears as the precipitation of struvite (MgNH₄PO₄,6H₂O) (Maurer et al., 2006). Ultrafiltration is an efficient technology to remove bacteria and viruses without adding chemicals. But today no experimental results are available to give some information on possible performances or limitations in the case of urine. A key question concerns the way to integrate these processes so as to obtain the most sustainable system. So, this study aims to provide some experimental-based results on these two processes and to propose a strategy of coupling.

Experimentations have been performed at lab scale with both synthetic and real urines.

Struvite crystallization was studied in a 2L batch reactor, with the objective to optimize and to control the final crystal size, and to determine struvite crystallization mechanisms (nucleation rate and crystal growths). Results show that the kinetics is very rapid and that more than 95 % of the total phosphorus could be recovered in less than 30 s. Due to the fast kinetics an indirect and cheap measurement method was developed to monitor crystallization process. Operating conditions as mixing and magnesium supply showed a great influence on the nucleation rate and on the final crystal size distribution.

Ultrafiltration of urines was studied at constant and low pressure with different membranes in order to select the most appropriate material and structure to minimize fouling. Different solutions were used: hydrolyzed/non hydrolyzed urines, after/before crystallization. Filtration performances were determined in terms of reversible and irreversible (adsorption) fouling and of possible permeate flux. The nature of deposit was characterized by SEM-EDS and this allows to conclude on the elementary composition of fouling.

On the basis of these results, the different possible treatment lines (coupling or hybridation of crystallization and ultrafiltration) will be introduced and discussed.
Gilbert, N. (2009) The disappearing nutrient. Nature 46.

Maurer, M., Pronk, W. and Larsen, T.A. (2006) Treatment processes for source-separated urine. Water Research 40(17), 3151.