Stable, high-rate anaerobic digestion through vacuum stripping of digestate.

This study coupled anaerobic digestion with vacuum stripping to achieve stable digestion at higher organic loading rates. Besides mitigation of ammonia inhibition, vacuum stripping of digestate improves solids solubilization and dewaterability due to vacuum-enhanced low-temperature thermal and mild-alkaline treatment under the vacuum stripping conditions (65 °C, 25-27 kPa, and pH 9). Batch vacuum stripping for 8 h removed 97.4-99.4% of ammonia, increased the dissolved fraction of volatile solids (VS) by 72.5%, and improved dewaterability with 30% decreases in time-to-filter and viscosity. The digesters having 2.9% of digestate replaced daily by vacuum stripped digestate were stable up to organic loading rate of 4.3 g-VS/Lreactor/d with biogas production at 3.15 L/Lreactor/d, while the digesters without stripping attained biogas production of 1.90 L/Lreactor/d at its highest stable organic loading rate of 2.5 g-VS/Lreactor/d. Acetoclastic Methanosaeta were the dominant methanogens, which became more resistant to ammonia stress in the digesters with vacuum stripping.

Elucidating the kinetics of ammonia inhibition to anaerobic digestion through extended batch experiments and stimulation-inhibition modeling.

Ammonia can be accumulated to a level inhibitory to methanogenesis. There are large discrepancies in the reported inhibition thresholds. Through extended batch digestion experiments (up to 110 d) at 6 ammonia concentrations (0.70-13 g N/L), this study discovered sequential occurrence of adaptable and unadaptable inhibition that reveals the discrepancies. Lag phase length representing adaptable inhibition increased exponentially with ammonia concentration. The kinetics of specific biogas yield that reveals unadaptable inhibition was best simulated with the Han & Levenspiel model. The 50% unadaptable inhibition thresholds were 10.7 g N/L with active inoculum and 6.8 g/L with stressed inoculum. The digesters with stressed inoculum had faster adaptation to adaptable inhibition though less resistance to unadaptable inhibition. The inhibition sequence was evidenced by microbial population shifts and confirmed by earlier studies employing short (2-65 d) and long (80-198 d) batch experiments. Distinguishing adaptable from unadaptable inhibition provides precise guidance for mitigating ammonia inhibition.

Three-stage treatment for nitrogen and phosphorus recovery from human urine: Hydrolysis, precipitation and vacuum stripping.

Source separation of human urine has not been widely adopted because of scaling on urine collecting fixtures and lack of verified technologies for on-site utilization of waterless urine. This study investigated the effects of flushing liquid, temperature and urease amendment on hydrolysis of urea to ammonia, explored ammonia recovery via vacuum stripping in connection with phosphorus recovery via struvite precipitation in different sequences, and performed economic analysis of a proposed nutrient recovery strategy. It was found that acetic acid could be dosed at 0.05-0.07 N to flush urine-diverting toilets and urinals for hygiene and prevention of scaling. However, a high dosage of 0.56 N completely inhibited urea hydrolysis. Source-separated urine could be stored at 25 °C with ample urease for complete urea hydrolysis within approximately 20 h. Fully hydrolyzed waterless urine contained 9.0-11.6 g/L ammonia-N, 0.53-0.95 g/L phosphate-P and only 2.3-9.1 mg/L magnesium. When magnesium was supplemented to attain the optimum Mg2+: PO43- molar concentration ratio of 1.0 in hydrolyzed urine, batch operation of a pilot-scale air-lift crystallizer removed 93-95% of phosphate and produced 3.65-4.93 g/L struvite in 1-5 h. Batch operation of a pilot-scale vacuum stripping - acid absorption system for 12 h stripped 72-77% of ammonia and produced 37.6-39.7 g/L (NH4)2SO4. Compared with the ammonia → phosphorus recovery sequence, the struvite precipitation → vacuum stripping sequence produced more struvite and ammonium sulfate. The strategy of urea hydrolysis → struvite precipitation → vacuum stripping of ammonia is a sustainable alternative to the conventional phosphorus fertilizer production and ammonia synthesis processes.

Recovery of ammonia in anaerobic digestate using vacuum thermal stripping - acid absorption process: scale-up considerations.

A vacuum thermal stripping process coupled with acid absorption has been developed at laboratory scale to recover ammonia in anaerobic digestate. To make this ammonia recovery process scalable, this study investigated the effects of feed depth on vacuum thermal stripping in a pilot system, developed sodium hydroxide dosages required to raise feed pH for stripping, and simulated the dynamics of ammonia reduction in batch stripping tests. As feed depth was increased from 8.5 to 34.0 cm, the ammonia mass transfer coefficient and ammonia stripping efficiency decreased while the mass of stripped ammonia increased. Digested municipal sludge had a greater ammonia mass transfer coefficient than digested dairy manure at each feed depth, which could be attributed to the difference in suspended and dissolved solids concentrations. The optimum feed depth was 18 cm of the digested sludge and 14 cm of the digested manure. Sodium hydroxide dosage for the digested manure was higher than that for the digested sludge and co-digested foodwaste. The dosages were correlated to concentrations of total dissolved solids and ammonia. Total ammonia concentration decreased exponentially in batch stripping of the digested sludge at 25.5 cm deep, with a first-order stripping rate coefficient of 0.087-0.144 h-1.

Anaerobic co-digestion of food waste and dairy manure: effects of food waste particle size and organic loading rate.

This study was to comprehensively evaluate the effects of food waste particle size on co-digestion of food waste and dairy manure at organic loading rates increased stepwise from 0.67 to 3 g/L/d of volatile solids (VS). Three anaerobic digesters were fed semi-continuously with equal VS amounts of food waste and dairy manure. Food waste was ground to 2.5 mm (fine), 4 mm (medium), and 8 mm (coarse) for the three digesters, respectively. Methane production rate and specific methane yield were significantly higher in the digester with fine food waste. Digestate dewaterability was improved significantly by reducing food waste particle size. Specific methane yield was highest at the organic loading rate of 2g VS/L/d, being 0.63, 0.56, and 0.47 L CH4/g VS with fine, medium, and coarse food waste, respectively. Methane production rate was highest (1.40-1.53 L CH4/L/d) at the organic loading rate of 3 g VS/L/d. The energy used to grind food waste was minor compared with the heating value of the methane produced.