Urine stabilisation and treatment using lactic acid from fruit and vegetable peel fermentation: a potential source of nitrogen fertiliser (Kenya)

The study explored stabilization and treatment of human urine using lactic acid produced from the anaerobic fermentation of fruit and vegetable peels, aiming to reduce nitrogen loss and provide a sustainable nitrogen fertiliser. Lactic acid was produced after three days of fermenting a mix of mango, pineapple, banana, orange, and cabbage waste at 1:1 and 1:2 substrate-to-water ratios at 34°C, 37°C, and 40°C. pH was measured with a pH meter, and lactic acid was quantified spectrophotometrically with maximum absorption at 410 nm. The lactic acid extract’s pH was between 3.2 and 3.7. The standard calibration curve equation was y = 1.9051x + 0.0267 with an R² of 0.9804. The highest absorbance (1.201) and concentration (1304.7 mg/L) were observed at 37°C. Fresh urine samples treated with lactic acid for four, seven, and ten days showed a pH reduction, with the lowest pH of 3.403 at 37°C. The urine pH ranged between 3.5 and 4.2, down from an initial pH of 6-6.3 before stabilisation. The nitrogen content in urine was determined using the Kjeldahl method, and MacConkey Agar tested for E. coli growth. Lactic acid inhibited E. coli growth, especially at higher temperatures. Nitrogen analysis revealed that samples treated at 37°C in a 1:1 ratio for 10 days had the highest nitrogen concentration (2450 mg/L), indicating effective urease inhibition and ammonia volatilization prevention. This demonstrates lactic acid’s potential for sustainable urine management and agricultural applications.