Heterogeneity of zeolite combined with biochar properties as a function of sewage sludge composting and production of nutrient-rich compost

In the present study, biochar combined with a higher dosage of zeolite (Z) and biochar (B) alone were applied as additives for dewatered fresh sewage sludge (DFSS) composting using 130-L working volume lab-scale reactors. We first observed that the addition of a mixture of B and Z to DFSS equivalent to 12%B + 10% (Z-1), 15% (Z-2) and 30% (Z-3) zeolite (dry weight basis) worked synergistically as an amendment and increased the composting efficiency compared with a treatment of 12%B alone amended and a control without any amendment. In a composting reactor, the addition of B + Z may serve as a novel approach for improving DFSS composting and the quality of the end product in terms of the temperature, water-holding capacity, CO2 emissions, electrical conductivity, water-soluble and total macro-nutrient content and phytotoxicity. The results indicated that during the thermophilic phase, dissolved organic carbon, NH4+-N and NO3−-N increased drastically in all biochar amended treatments, whereas considerably low water-soluble nutrients were observed in the control treatment throughout and at the end of the composting. Furthermore, the maturity parameters and dissolved organic carbon (DOC) indicated that compost with 12%B + 15%Z became more mature and humified within 35 days of DFSS composting, with the maturity parameters, such as CO2 evolution and the concentration of NH4+-N in the compost, being within the permissible limits of organic farming in contrast to the control. Furthermore, at the end of composting, the addition of higher dosage of biochar (12%) alone and 12% B + Z lowered the pH by 7.15 to 7.86 and the electrical conductivity by 2.65 to 2.95 mS cm−1 as compared to the control, while increased the concentrations of water-soluble nutrients (g kg−1) including available phosphorus, sodium and potassium. In addition, greenhouse experiments demonstrated that the treatment of 150 kg ha−1 biochar combined with zeolite and that of 12%B alone improved the yield of Chinese cabbage (Brassica rapa chinensis L.). The highest dry weight biomass (1.41 ± 0.12 g/pot) was obtained with 12%B + 15%Z amended compost. Therefore, 12%B + 15%Z can be potentially applied as an amendment to improve DFSS composting.