Objective   Ability of water hyacinth (Eichhornia crassipes) substrate in adsorbing contaminants in the biogas slurry from pig farms was studied and spent material utilization for fertilization explored.

  Method   Fresh E. crassipes substrate was blended into the biogas slurry to determine the optimal conditions for a maximized adsorption of heavy metals. The spent material was subsequently composted and analyzed for nutrient conversion and contaminant retention.

  Result   (1) The maximal adsorption on heavy metals was achieved by using 1.0–2.0 cm substratum to treat a liquid-solid ratio of 50∶1 mix for 3 h. The removal rates on total suspended solid (TSS), chemical oxygen demand (COD_Cr), total nitrogen (TN), ammonia nitrogen (NH₃-N), and total phosphorus (TP) from the slurry were 86.3%, 72.5%, 41.6%, 57.2%, and 69.6%, respectively. (2) After composting, 58.8% of N and 42.0% of P of the slurry were converted into organic nutrients in the spent material. (3) Aside from As, heavy metals like Cd, Pb, Cr, and Hg were not significantly retained in the spent material to be used as an organic fertilizer.

  Conclusion  The waste treatment of pig farm biogas slurry was seen superior in efficiency and resource utilization by combining the conventional water hyacinth wetland operation with the contaminant adoption by the E. crassipes substrate followed by the spent material composting for organic fertilization as demonstrated by this study.