Diurnal variations of urine concentrations and loads with novel insights for nutrient recovery

The composition of human urine has been investigated in detail in medical research and grab samples, but information relevant to nutrient recovery on changes in urine composition during the day is still lacking. This study characterizes the diurnal dynamics of fresh urine, analyzing organic and inorganic composition alongside void volumes during morning, daytime, and evening. Concentrations of both inorganic and organic parameters were on average 1.9 times higher in morning than in daytime samples. Nutrient excretion patterns differ depending on the compounds: phosphate and calcium loads peak in the morning, while the potassium load peaks during daytime. This dictates the nutrient recovery potential: residential systems (collecting both morning and evening urine) capture P-rich morning voids that office systems largely miss, altering nutrient profiles: N:P:K mass ratios shift from 19:1:6 in office urine to 13:1:2 in household streams. As a result, homes capture 120% more phosphate and 44% more nitrogen than office installations. 24-h collection increases nutrient recovery, yielding 220% more phosphate and 144% more nitrogen relative to the office baseline. Sex-specific effects were also pronounced: male-derived datasets overestimate nitrogen capture by up to 34% when applied to mixed cohorts. A novel approach that normalizes the organic fraction measured by 1 H NMR to the independently measured chemical oxygen demand (COD), revealed that creatinine and hippurate each contributed approximately 10%–12% to the measured COD, values notably lower than those reported in literature. Benchmarking the concentrations of organic and inorganic parameters against standard literature indicates that historical reference values predominantly represent morning grab samples. Without essential metadata, such as void volumes, these values can introduce substantial design errors by overestimating nutrient loads. These findings underscore the importance of considering the time of day at which urine is collected and support the use of volume-weighted concentrations as more reliable baselines for designing nutrient recovery systems.