Enhanced methane production and ammonia inhibition mitigation in intensified anaerobic digestion of high nitrogen biosolids: ex-situ vacuum stripping versus thermal hydrolysis

• Ex-situ vacuum-AD showed stable methane production compared to integrated thermal hydrolysis −AD. • Ammonia and propionate accumulation led to failure of the integrated thermal hydrolysis-AD. • Biokinetic activities were improved under ex-situ vacuum-AD compared to integrated thermal hydrolysis-AD. • Methanosarcinaceae dominated vacuum-AD, while Methanobacteriaceae dominated thermal hydrolysis-AD. • Ex-situ vacuum-AD showed higher methanogens resistance to ammonia inhibition compared to thermal hydrolysis-AD. This study compares integrated thermal hydrolysis-anaerobic digestion (THP-AD) with the IntensiCarb TM (IC) vacuum-enhanced AD (IC-AD) of mixed primary and secondary sludges under identical organic loading rates (OLR) of 8–8.7 kgCOD/m 3 ·d and solids retention times (SRT) of 18–20 d. IC-AD achieved a stable methane yield of 0.22 L-CH 4 /gCOD fed (55% COD destruction), while THP-AD produced < 0.1 L-CH 4 /gCOD fed and failed due to the toxicity of high ammonia (>3.3 gN/L) and propionate (>2.8 g/L) accumulation when using both acclimatized and unacclimatized THP biomass. The IC-AD reduced digester ammonia by 49%-56% via ex-situ vacuum application whereas in THP-AD, ammonia accumulated. Off-line batch tests showed acetate, butyrate and propionate degradation rates were 2.3–2.7 times higher in IC-AD than THP-AD. Ammonia inhibition batch tests showed methane production rate reductions of 27% in IC and 58% in THP at 2–4 g-ammonia/L, highlighting the higher inhibition threshold of the IC biomass. The microbial communities showed distinct differences: IC-AD was dominated by Firmicutes with enriched Petrimonas and Syntrophomonas , while THP-AD was dominated by Bacteroidota with enriched Corynebacterium and Syntrophomonas . Methanogen counts were 6.2 times higher in the IC-AD due to the presence of high-growth-rate acetoclastic Methanosarcinaceae while hydrogenotrophic Methanobacteriaceae were the most abundant methanogen in the THP-AD. In offline tests with acetate, the biomass-specific methane production rate in the IC-AD was 8.4 times higher, suggesting that the acetoclastic pathway associated with Methanosarcinaceae provided superior process performance to the hydrogenotrophic Methanobacteriaceae pathway. Overall, when operated at similar loadings IC-AD outperformed THP-AD, achieving stable methane production and enriched beneficial microbial communities while also recovering ammonia as an additional value-added product.