Effects of acidified storage conditions on nutrient retention and heavy metal removal in caged duck manure wastewater

Caged duck industry has developed rapidly to solve the pollution of water environment that caused by the free-range breeding. However, the large volumes and high moisture content can be found in the feces of caged ducks, leading to mismatch the time required for fertilizer demanding of major crops, such as paddy rice. It is often required for the storage. The conventional long-term open-storage of high moisture content duck feces (HMCDF) can lead to the rapid nitrogen escape and a prominent residue of heavy metal (Cu²⁺ and Zn²⁺). The low nitrogen content and high residual of heavy metals can cause the great loss in the value of HMCDF as a fertilizer. It is essential to promote the sustainable development of the caged duck industry. This study aims to continuously track the nitrogen, phosphorus, potassium, and carbon content, as well as the concentrations of Cu²⁺ and Zn²⁺ in the liquid phase of HMCDF under quasi-anaerobic storage conditions for a duration of 12 months. Effective microorganisms (EM) and photosynthetic bacteria (PSB) were inoculated sequentially, with pH monitored every two weeks. Sulfuric acid was gradually added to maintain the liquid phase pH at the required experimental levels. Phase separation occurred in the substances in HMCDF after 12 months of quasi-anaerobic storage, leading to the formation of floating layer, liquid phase, and solid sediment. There were the significant differences (P<0.05) among the different acidity treatments. The total organic carbon (TOC) in the liquid phase of HMCDF was accumulated significantly (P<0.05) in the acidic environment of pH 4. The TOC content reached 10366.67 mg/L at the end of storage; The ammonium nitrogen (NH₄⁺-N) content firstly increased and then stabilized, thus effectively preserving nitrogen. The NH4+-N content reached 2785.07 mg/L; The total phosphorus (TP) content firstly decreased and then gradually recovered to 99.14% of the initial value. The TP content reached 249.50 mg/L at the end of storage. There was the significant retention effect of phosphorus in the liquid phase of HMCDF; The removal rates of Cu²⁺ and Zn²⁺ in the liquid phase were 82.02% and 90.15%, respectively. Furthermore, the Cu²⁺ and Zn²⁺ contents were 0.64 and 0.16 mg/mL, respectively, at the end of storage. It was significantly reduced the heavy metal pollution risk in the fertilizer utilization of HMCDF; While the K+ content increased by 11.02% (reached 2430.67 mg/L) at the end of storage, compared with the initial value. The fertilizer efficacy of HMCDF was significantly enhanced to serve as a basal fertilizer after quasi-anaerobic storage. Therefore, the pH of the liquid phase at 4 was used to sequentially inoculate the liquid with EM and PSB. The best preservation and the highest efficiency of heavy metal removal were achieved in the quasi-anaerobic storage of HMCDF. This effective technical approach can be expected to solve the HMCDF issues in large-scale caged duck farms.