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

The caged duck industry in China has developed rapidly because it can solve the pollution problem of the water environment caused by the free-range breeding of the duck to a large extent. However, the feces of caged ducks not only yield large volumes but also a high moisture content, which does not match the time required for fertilizer demanding of major crops such as paddy rice, and often requires storage. The traditional long-term open-storage method of high moisture content duck feces (HMCDF) leads to a great loss of ammonia nitrogen and a prominent problem of heavy metal (Cu²⁺ and Zn²⁺) residue, which leads to the loss of the value of HMCDF as a fertilizer. To promote the sustainable development of the caged duck industry, it is essential to improve traditional open storage methods for HMCDF. Traditionally, the long-term open-storage of HMCDF results in rapid nitrogen escape, leading to low nitrogen content and high residual of heavy metals, rendering it useless as fertilizer. In this study, the nitrogen, phosphorus, potassium, and carbon content, as well as the concentrations of Cu²⁺ and Zn²⁺ in the liquid phase of HMCDF, were continuously tracked and analyzed under maintained quasi-anaerobic storage conditions for a duration of 12 months. During this process, effective microorganisms (EM) and photosynthetic bacteria (PSB) were inoculated sequentially, with pH monitored every two weeks, and sulfuric acid was gradually added to maintain the liquid phase pH at the required experimental levels. After 12 months of quasi-anaerobic storage, phase separation occurred in the substances in HMCDF, with the formation of floating layer, liquid phase, and solid sediment. There were significant differences (P<0.05) among the different acidity treatments. In the acidic environment of pH 4, the total organic carbon (TOC) in the liquid phase of HMCDF accumulated significantly (P<0.05). At the end of storage, the TOC content reached 10366.67 mg/L; the ammonium nitrogen (NH₄⁺-N) content first increased and then stabilized, effectively preserving nitrogen. At the end of storage, the NH₄⁺-N content reached 2785.07 mg/L; the total phosphorus (TP) content first decreased and then gradually recovered to 99.14% of the initial value. At the end of storage, the TP content reached 249.50 mg/L. indicating that the retention effect of phosphorus in the liquid phase of HMCDF was significant; the removal rates of Cu²⁺ and Zn²⁺ in the liquid phase were 82.02% and 90.15%, respectively. At the end of storage, the Cu²⁺ and Zn²⁺ contents were 0.64 mg/mL and 0.16 mg/mL, respectively, significantly reducing the heavy metal pollution risk in the fertilizer utilization of HMCDF; while the K⁺ content increased by 11.02% compared to the initial value, at the end of storage, the K⁺ content reached 2430.67 mg/L, significantly enhancing the fertilizer efficacy of HMCDF used as a basal fertilizer after quasi-anaerobic storage. In the quasi-anaerobic storage of HMCDF, maintaining the pH of the liquid phase at 4 and sequentially inoculating the liquid with EM and PSB achieves the best preservation effect and the highest heavy metal removal efficiency. This is an effective technical approach to solving the HMCDF problem in large-scale caged duck farms.