Incorporating hydrothermal liquefaction into wastewater treatment-Part III: Aqueous phase characterization and evaluation of on-site treatment

For the incorporation of the hydrothermal liquefaction (HTL) process into wastewater treatment plants (WWTPs), the liquid by-product of the process, named HTL aqueous, creates the biggest bottleneck because of its high volume and contaminant levels. In this study, an extensive characterization of various HTL aqueous sam-ples, obtained from the liquefaction of mixed sludge cake at 290-360 degrees C and 0-30 min, was performed. From the most to least abundant, compounds found in HTL aqueous samples were N-heterocyclics, volatile fatty acids (VFAs), amides, ketones, amines, and phenolics. The characteristics that make HTL aqueous suitable for bio-logical treatment were pH, alkalinity, macro-and micro-nutrients. However, along with N-heterocyclic com-pounds, total ammonia levels of 3.9-7.1 g/L and total phenol levels of 1.5-2.3 g/L were identified as compounds that would potentially inhibit biological treatment processes. The chemical oxygen demand (COD) levels of HTL aqueous samples were 87-103 g/L. HTL aqueous obtained under 325 degrees C-0 min (no retention at 325 degrees C) condition was identified as the most suitable for biological treatment owing to its high VFA (17 g/L), carbohydrate (827 mg/L), and low phenol (1.6 g/L) concentrations. By increasing HTL reaction severity (temperature and retention time), COD, dissolved protein, total carbohydrate, total phenol, and total short-chain VFA levels decreased, whereas total ammonia, pH, alkalinity, valeric acid, and iso-valeric acid concentrations increased. Regression analysis was used to obtain equations that reveal patterns between HTL reaction parameters and aqueous phase characteristics. Overall, a pretreatment application was found necessary for the detoxification of HTL aqueous before biological treatment.

Automated summary

This study characterized HTL aqueous samples and identified their composition, including compounds such as N-heterocyclics, VFAs, amides, ketones, amines, and phenolics. The COD levels of HTL aqueous samples were 87-103 g/L. Pretreatment is necessary for detoxification of HTL aqueous before biological treatment.