Agronomic Potential of Pyrochar and Hydrochar from Sewage Sludge: Effects of Carbonization Conditions

Abstract

Thermochemical treatments such as pyrolysis and hydrothermal carbonization (HTC) are increasingly used to convert municipal sewage sludge into solid products, offering benefits in contaminant reduction, pathogen sanitization, and nutrient recovery. This study assesses the agronomic potential of pyrochars and hydrochars produced under varying temperatures and residence times. Pyrolysis was performed at 250–520 °C for 20 and 60 min, while HTC was conducted at 180–300 °C for 30–120 min. Proximate and ultimate analyses revealed that pyrochars exhibit higher thermal stability and fixed carbon content, whereas hydrochars contain less condensed aromatic structures, indicating greater chemical reactivity but lower long-term stability. Surface area measurements showed meso- and macropore development in both materials, with hydrochars ranging from 14.7 to 86.0 m2·g−1 and pyrochars from 12.7 to 41.7 m2·g−1. Pyrochars tend to have a near-neutral pH, while hydrochars are slightly acidic. Hydrochars also retain higher levels of available nutrients (N, P, and S), particularly at lower temperatures, making them promising for agricultural applications. Agronomic evaluation confirmed greater N-NH4+ and phosphorus availability in hydrochars compared to pyrochars, suggesting their potential as soil amendments or fertilizer additives. However, the mobility of heavy metals requires further assessment to ensure environmental safety.