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[Display omitted] • Biogas circulation at 5 L/min could not completely replace mechanical stirring. • Intermittent stripping mode proved applicable for in situ ammonia stripping. • Thermophilic digestion could be promoted to 90% of the methanogenic potential. • Exceeding the loading 4.2 g COD/L/d resulted in failure of the stripping mode. High-solid anaerobic digestion of hydrothermal sewage sludge has been developed. In order to upgrade the process by focusing on ammonia inhibition, a simply-equipped stripping system without additional alkali or heat supply was introduced by in situ biogas self-circulation. As the determined limit of total ammonia nitrogen at 1500 mg/L and 1000 mg/L for the mesophilic (MAD) and thermophilic anaerobic digestion (TAD) respectively and stripping rate at 5 L/min, continuous MAD and TAD was conducted in parallel. The stripping system successfully polished up the ammonia inhibition, and methanogenic capability of the TAD was promoted to approximately 90.0 % of the potential. Intermittent stripping mode proved usable. More frequent stripping was inevitable for the TAD as compared to the MAD. Hydraulic retention time below 20 d resulted in failure of the stripping mode due to rapid ammonia generation. Overall, this technology was practical in upgrading high-solid sludge digestion by effective ammonia control. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A newly acid-resistant strain JR1 was isolated from pharmaceutical wastewater. • A high nitrogen removal could be achieved by JR1 under the acidic condition. • Nitrate began to be utilized as substitute N-source after exhausting of ammonium. • Ammonium was utilized through assimilation along with heterotrophic nitrification. Abstract A new acid-resistant bacterium Acinetobacter sp. JR1 was isolated, and its feasibility in nitrogen removal was investigated under acidic condition. Results show that JR1 indicated excellent ammonium and nitrate removal abilities with no accumulation of intermediates, and the maximum ammonium and nitrate removal efficiencies were 98.5% and 91.1%, respectively. Further experiments demonstrated that JR1 preferred to use ammonium with ammonium and nitrate as the mixed N-sources. For JR1, ammonium was assimilated directly as nutrients into cells and also converted into N 2 through heterotrophic nitrification-aerobic denitrification. Under acidic condition, JR1 performed comparable nitrogen removal abilities to other strains under neutral or weak alkaline environment, and the efficient removal of ammonium occurred at pH 4.5–10, C/N 12–24, 20–40 °C, DO ≥4.72 mg/L, 0–1.5% of salinity, 10 mg/L Zn2+ or 20 mg/L Mn2+. All these make JR1 a promising candidate for treating acidic wastewater containing nitrogen. [ABSTRACT FROM AUTHOR]

• Recuperative thickening (RT) upgraded digestion of hydrothermal high-solid sludge. • Comparative processes with RT were operated at 35 ℃ (MD-R) and 55 ℃ (TD-R). • The MD-R and TD-R increased gas production by 22.8% and 11.0%, respectively. • RT increased hydrolysis rates by 11.1% and 7.4% in the MD-R and TD-R, respectively. • The improved numbers of archaea led to upgraded performances in the RT systems. Recuperative thickening (RT) process was introduced to further upgrade anaerobic digestion of hydrothermal high-solid sludge. Continuous mesophilic (MD-R) and thermophilic (TD-R) digestion with RT (MD-R) were operated synchronously, with corresponding single digestion without RT as controls, namely MD and TD. The MD-R and TD-R increased biogas production rates by 22.8% and 11.0%, and achieved 16.6% and 9.7% higher volatile solids reductions, respectively. The improved performance was partly attributed to increased hydrolysis rate, with 11.2% and 7.4% higher for the MD-R and TD-R than the controls, respectively. The RT increased the numbers of total archaea in the mesophilic and thermophilic systems by 844% and 108%, and the numbers of dominant archaea by 50.4% and 38.1%, respectively, which promoted the degradation of organic matter and the production of biogas. Thus, RT is applicable to further upgrade digesting high-solid sludge. [ABSTRACT FROM AUTHOR]

• Mesophilic (MAD) and thermophilic (TAD) digestion of 10–11% sludge were studied. • 160 °C for 210 min was the optimal hydrothermal pretreatment (HTP) condition. • TAD exhibited higher methane production than MAD under stable conditions. • 2800 mg/L of NH 4 +-N resulted in an inhibition of methane production in TAD. • The energy balance of MAD with HTP was the highest, 5.25 kJ/g VS. Hydrothermal pretreatment (HTP) conditions were optimized for continuous mesophilic (MAD) and thermophilic (TAD) anaerobic digestion of high-solid sludge (10–11% total solids). COD solubilization increased with prolonged HTP durations, and became not significant after 210 min. According to the methane production rate and energy consumption, the optimal HTP temperature was determined at 160 °C. Regarding continuous operation without HTP, TAD achieved higher methane yield and volatile solids (VS) reduction, at 0.12 L/g VS added and 23.9%, respectively. After HTP, methane yield and VS reduction in MAD and TAD were increased by 400% and 191% (MAD), 67% and 72% (TAD), respectively. TAD was limited due to the inhibition from about 2800 mg/L of NH 4 +-N concentration. The methanogenic activity of MAD was enhanced, whereas TAD displayed a reduced value owing to ammonia inhibition. Ultimately, MAD with HTP and TAD without HTP achieved the higher energy balance, 5.25 and 3.27 kJ/g VS, respectively. [ABSTRACT FROM AUTHOR]