Fertility assessment and nutrient conversion of hydrochars derived from co-hydrothermal carbonization between livestock manure and corn cob

As an effective and environmentally-friendly technology, the hydrothermal carbonization (HTC) can realize value-added utilization of livestock wastes. However, there are some problems such as high ash content and low nutrient recovery of products. Since the HTC of corn cob (CC) showed excellent N-fixation effect, the co-hydrothermal carbonization (co-HTC) of two typical agroforestry and livestock wastes, namely swine manure (SM) and CC, to prepare organic fertilizers was proposed. The effects of reaction temperature, residence time, and raw materials composition on hydrochar yield, total nutrient content (TNC) and nutrient elements distribution in co-HTC were investigated. Results showed that compared to SM and SM-derived hydrochars, the Co-hydrochars derived from co-HTC of SM and CC exhibited better fertilizer potentials due to the synergistic effects such as Maillard reaction between raw materials components. Under the optimal hydrothermal conditions of 240 °C, 120 min, and mixing ratio of 1:1, the Co-hydrochar showed the best fertility with TNC of 6.341 %, nitrogen (N) recovery of 47.61 % and phosphorus (P) recovery of 86.41 %. Meanwhile, the co-HTC treatment could reduce the phytotoxicity and inhibitory effect for seed germination. Moreover, an appropriate extension of residence time (from 60 min to 120 min) was conducive to the migration of N into the hydrochar. The hydrochars derived from co-HTC were also rich in medium-microelements such as calcium (Ca), magnesium (Mg), iron (Fe) etc., which played good roles in fixing P in hydrochars.