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The objective of this study was to elucidate the effect of adding acid and alkali to hydrothermal liquefaction (HTL) of two waste biomass feedstocks: manure digestate and carbohydrate-rich food waste. HTL reactions were conducted at 300 °C for 60 min, with and without the addition of acid or base. We measured the quantity and characterized the quality of the three main HTL products: oil, aqueous and hydro-char. For both feedstocks, carbon recovery distributions had wide ranges among (1) biocrude oil (26–61 wt %), (2) aqueous product (9–49 wt %) and (3) hydro-char (1–36 wt %). The addition of acid affected HTL reactions for manure more than for food waste. For the aqueous phase, the addition of acid decreased the recovery of C1–4 carboxylic acids and increased the production of cyclic furan compounds. GC–MS analysis of the biocrude oil suggested that dehydration reactions were enhanced by adding acid to the HTL media. FTIR spectroscopy coupled with principal component analysis showed that hydro-char samples cluster according to acid-modified and base-modified reactions, based on distinct chemical structures. This study clarifies the role of pH during HTL and its effect on chemical pathways and carbon distribution among products.

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