How does dehydration change the chemical composition of urine and which compounds increase?

1. Dehydration is primarily a concentration effect — what increases and why

When water is removed from urine the absolute mass of most solutes stays the same while total volume drops, so measured concentrations and osmolarity climb: urea, creatinine, uric acid and the common electrolytes (Na+, K+, Cl–, Mg2+, Ca2+, PO43–, SO42–) become more concentrated as a direct physical consequence of water loss ^{[1] [2] [5]}. Foundational references report that over 98% of urinary solute mass is accounted for by a relatively small set of chemicals, meaning dehydration disproportionately raises those dominant constituents and the urine’s osmolarity, which normally spans a wide range depending on hydration ^{[6] [5]}.

2. Nitrogen species: urea and ammonia dynamics during dehydration

Urea is the principal nitrogenous solute in urine and will concentrate during dehydration, but chemical and enzymatic pathways can alter its fate: at elevated pH urea hydrolysis (to ammonia and CO2) is inhibited, enabling recovery of nitrogen in dried products, whereas uncontrolled hydrolysis or volatilization can transform urea into ammonia and allow nitrogen loss ^{[7] [3] [4]}. Experimental alkaline dehydration used for nutrient recovery deliberately raises pH (e.g., with MgO or Ca(OH)2) to suppress enzymatic urease activity and limit chemical hydrolysis, which affects whether nitrogen stays as urea in the concentrated product or shifts toward ammonium/ammonia ^{[7] [3]}.

3. Electrolytes, phosphate and nutrient-rich solids: dehydration concentrates fertilizers

Studies aimed at recovering nutrients from source‑separated urine show that dehydrating (and often alkalizing) urine can yield high‑value solids: concentrated end‑products contain measurable percentages of nitrogen, phosphorus and potassium—examples include reported dry‑matter values of ~10% N, ~1% P and ~4% K or, in acid dehydration variants, even higher N and defined P/K ranges ^{[3] [4]}. Those numbers illustrate that dehydration transforms dilute urinary nutrients into dense material with agricultural value, while the specific elemental distribution depends on treatment chemistry (alkaline vs acidic) and secondary reactions (e.g., precipitation of carbonates) ^{[4] [3]}.

4. pH, speciation and secondary chemistry that change compound identities

Dehydration is not purely a physical concentrating step; pH shifts and precipitation reactions during drying alter chemical speciation. Raising pH favors formation and potential volatilization of free ammonia from ammonium, while alkalization can also precipitate mineral phases or inhibit urease-mediated reactions—tools used in engineered dehydration to retain nitrogen ^{[7] [3]}. Conversely, acid or different drying media change which salts precipitate and thus the final composition of solids ^[4]. Available studies model speciation across pH, temperature and water content to predict how much of each nitrogen species or salt will be present after dehydration ^[7].

5. Clinical signs and limitations of the available reporting

Clinically, concentrated urine appears darker and smells stronger because pigments like urochrome and volatile metabolites are more concentrated, and osmolarity increases with dehydration ^{[8] [9]}. The reviewed sanitation and chemistry literature provides clear evidence that dehydration raises concentrations and can change speciation, and it gives representative fertilizer compositions from engineered processes ^{[3] [4]}. However, the sources do not provide a single, comprehensive table of exact fold‑increases for every urinary compound under all dehydration conditions, so precise numeric multipliers for urea, creatinine, each ion, or pigment under arbitrary dehydration scenarios cannot be asserted from these documents alone ^{[7] [1] [6]}.