Objective The efficiency of membrane separation technology in concentrating kitchen waste biogas slurry was evaluated, and the fertilization efficacy and economic performance of the compounded humic acid water-soluble fertilizer derived from the concentrated slurry was assessed.

Methods Ultrafiltration (UF) membrane technology was employed to concentrate kitchen waste biogas slurry at 2×, 4×, 6×, and 8×folds, respectively. The concentrations of N, P, K, and humic acid (HA) in the concentrates were analyzed. Based on the analysis results, concentration at 6 folds was chosen in the following research. Potassium humate, monopotassium phosphate, and urea were added to the 6-fold concentrate of kitchen waste biogas slurry to formulate a humic acid water-soluble fertilizer. A 1 L sample of this fertilizer was stored at 4±2°C and 30±2°C for 63 days. Samples were collected at 7-day intervals to determine humic acid, total N, total P, total K, pH, and EC. The fertilizer was diluted at 100×, 200×, and 300× for pot experiments using cherry radish as test crop material, with distilled water as a control. The seedling height, stem diameter, fresh and dry biomass, and leaf SPAD values were measured, and the contents of soluble sugars and vitamin C were analyzed. At the same time, soil pH, organic matter, and available N, P, and K levels were analyzed. Subsequently, the 100× dilution of the biogas slurry humic acid fertilizer was selected for comparison with humic acid, chemical fertilizer, and commercial humic acid fertilizer in another pot trial to evaluate their effects on plant growth and soil improvement. Economic cost analysis and sensitivity analysis were conducted to further assess the economic viability and cost-sensitive factors of the fertilizer.

Results As the concentration factor increased, the contents of nitrogen and potassium in the concentrate remained relatively stable, while humic acid and TP concentrations significantly increased. At 6× concentration, the humic acid content rose from 2.33 g/L to 13.96 g/L. The cost of concentration decreased initially and reached its lowest at 6× concentration before increasing again. After 63 days of storage at both 4±2°C and 30±2°C, the humic acid fertilizer exhibited stable nutrient content, pH, and EC. Pot experiment results indicated that higher dilution led to reduced plant growth-promoting effects and lower soil organic matter, alkaline hydrolyzable nitrogen, available phosphorus, and available potassium levels. Application of the biogas slurry humic acid fertilizer significantly enhanced cherry radish growth, biomass, and SPAD values compared to those treated with humic acid alone, chemical fertilizer, or commercial humic acid fertilizer. Economic sensitivity analysis revealed that, at a 6× concentration rate, a 20% increase in the prices of humic acid, NPK fertilizers, and concentration energy consumption resulted in cost increases of 2.57 CNY/L, 0.79 CNY/L, and 6.54 CNY/L, respectively.

Conclusion UF membrane technology effectively concentrates humic acid and P in kitchen waste biogas slurry effectively, with 100% humic acid and 80% P retention rate at 6× concentration, while the N and K retention is low, ranged from 10% to 50%. The resulting concentrate, with a humic acid content of 13.96 g/L, is suitable for formulating humic acid-based fertilizers. The prepared humic acid water-soluble fertilizer meets national standards, and maintains excellent stability under storage conditions of 4-30°C. When applied to soil at a 100× dilution, it demonstrates superior plant growth promotion and soil improvement effects compared to chemical and commercial humic acid fertilizers. The costs of humic acid addition and energy consumption during concentration are the primary factors influencing the overall production cost of the fertilizer.