An innovative intermittent-vacuum assisted thermophilic anaerobic digestion process for effective animal manure utilization and treatment
Introduction
Over 70 million pigs from concentrated animal feeding operations (CAFOs) produced tons of liquid swine manure (LSM) discharge, which has become a serious concern and received increasing attention to completely utilize and treat due to the increasing clean water requirement in U.S. (Adair et al., 2016). Bioprocessing methods for LSM treatment have shown remarkable results, offering full nutrients utilization, environmental friendly processes, and renewable energy production benefits, which are specific to algae cultivation, hydroponic system and thermophilic anaerobic digestion (TAD), respectively. The methane production from TAD supports energy and nutrients recovery benefits, with higher particle degradation rate, less residual pathogens and less hydraulic retention time (HRT) compared to psychrophilic and mesophilic conditions (De Vrieze et al., 2016). In previous study, algae shows a significant nutrient utilization ability with 75.7–82.5% nitrogen, 62.5–74.7% phosphorus, and 27.4–38.4% chemical oxygen demand (COD) utilized and removed from dairy manure as biomass and profitable byproducts (Wang et al., 2010). And hydroponic system has been proved as a sustainable process with higher nutrients utilization efficiency than traditional farming without the discharge issues of nitrogen and phosphorus, which shows an outstanding efficiency for manure nutrients utilization (Capulín-Grande et al., 2000). However, none of an individual system shows the completely and efficiency utilization/treatment ability with LSM. Since the anaerobic metabolism is much lower than aerobic condition, most residual bioavailable nutrients as phosphorus, nitrogen, carbon and heavy metals are still in the TAD elution, compared to the elution of algae and hydroponic system. In the meanwhile, the chemical components strength of LSM is too high for algae and plants to directly grow in LSM without abundant water dilution. Combination of TAD, algae cultivation and hydroponic system is considered as a potential way to completely and efficiency recycle the nutrients in LSM. However, a main challenge for the sequential nutrients utilization of the combination process is the different COD, phosphorus and nitrogen concentration range for optimal conditions of TAD, algae cultivation and hydroponic system, in particular, inhibition of substantial total ammonia nitrogen (TAN) and hydrogen sulfide (H2S) in LSM. TAN and H2S can inhibit TAD, algae cultivation and hydroponic systems from 1000 mg/L, 200 mg/L, 1 mg/L of TAN and 65 ppm, 3.7–200 ppm and 320 ppb of H2S, respectively (Meyer-Jens et al., 1995, Miller and Bebout, 2004, Pastor et al., 2017, Rajagopal et al., 2013, Wang et al., 2010).
For complete nutrient utilization and less water assumption purposes of LSM treatment, an innovative process with three units was designed to improve the nutrients utilization of LSM treatment through eliminating the TAN and H2S inhibition with minimum water usage (Fig. 1a). (a): Intermittent-vacuum stripping (IVS) was designed to assist TAD for TAN recycle and H2S removal, which recycled TAN as ammonium sulfate for liquid nitrogen fertilizer application and absorbed H2S and CO2 with alkali solution to get the purified methane. Hydrolysis and methanogenesis efficiency of TAD were predicted to be improved by removing most TAN from TAD medium, where free ammonia nitrogen (FAN) is considered as the main inhibitor for AD through interference of membrane proton balance, enzyme activity and intracellular pH stability (Yenigün and Demirel, 2013). In addition, most particles degradation was designed to achieve in this unit. (b): Algae cultivation unit was designed to grow in IVS-TAD elution or directly from pretreated LSM (PLSM), which was aimed to absorb most nutrients with limited water dilution usage. Once TAN and H2S were removed in IVS-TAD/TVS unit, the algae growth rate was assumed to be improved in IVS-TAD elution. In addition, less water consumption was required for algae cultivation, since previous study showed over 10 times dilution was required to reduce FAN inhibition for algae growth in LSM/PLSM (Hu et al., 2012). (c): Hydroponic system was suspected to utilize the remaining nutrients in waste water for high-value plants growth. Meanwhile, no waste water was considered to discharge after process, since the input from algae cultivation was designed to matched the evaporation in the hydroponic system.
To reduce the huge TAN gap between TAD and algae cultivation with a significant hydrolysis and methanogenesis efficiency, several methods have been investigated, such as co-digestion with low TAN substrates and LSM (Zhang et al., 2015), coupling microbial electrolysis cell and anaerobic digestion (AD) to remove TAN as nitrogen gas (Cerrillo et al., 2016) and coupling ammonia air stripping with AD to recycle TAN as ammonium sulfurate (Huang et al., 2016). Ammonia stripping has obvious economic superiority to others, in which TAN is recovered as nitrogen fertilizer instead of converting to nitrogen gas with an electrochemistry method. Moreover, low TAN substrate sources limited the AD set location with expensive transportation costs for specific co-digestion substrates. However, the high energy costs associated with high air flow (up to 600 L per liter stripping liquid per hour) restricts application of ammonia air stripping (Bousek et al., 2016). Thermal-vacuum stripping (TVS) has been proved to completely recycle TAN from digested dairy manure elution at 65 °C, 25.1 kPa and pH 9.0, which shows a profitable production of ammonium sulfurate with TVS compared to chemical synthesis (Anwar and Tao, 2016, Ukwuani and Tao, 2016). Moreover, potassium hydroxide and steam explosion have shown the ability to improve the methanogenesis and hydrolysis stages of AD by hemicellulose degradation, which implies that IVS may contribute TAD by more degraded carbohydrates due to the similar thermal-alkaline condition (Li et al., 2015). Furthermore, TAD is suspected to be improved by total organic nitrogen (ONt) removal enhancing in TVS, since present results show TAN accumulation from ONt degradation is the main reason for less stability of long-term AD. For example, TAN will continuous be released by nitrogen-rich substrates during long-term operation, which increases from 1200 mg/L to over 4000 mg/L during 378 days operation with 45% methane yield decease (Garcia and Angenent, 2009).
The first objective of this study was to prove the possibility of IVS-TAD system through investigating the synergistic effect of temperature, pH and pressure on TAN removal efficiency in a short-term TVS treatment to simulate intermittent stripping process in IVS-TAD. Then, ONt degradation and nitrogen forms inter-conversion in TVS were monitored to evaluate the long-term TAN accumulation risk of IVS-TAD and the nitrogen source adaptability for algae cultivation. Furthermore, to investigate the hydrolysis and methanogenesis improvement of TAD though the short-term TVS, biogas composition analysis and nutrients variance between PLSM and LSM were inspected through biomethane potential testing under thermophilic condition (55 °C).
Section snippets
TVS set up and pH, pressure and temperature interaction investigation
The experiment assembly is shown as Fig. 1b. TAN was absorbed by 3-N sulfuric acid solution as ammonium sulfate. Carbon dioxide (CO2) and hydrogen sulfide (H2S) were absorbed by 3-N NaOH. A graham condenser was connected directly to the 500-mL flat-bottom flask to maintain reaction volume throughout condensing evaporated water during stripping. LSM was picked up from a local swine barn, provided by Holden Farms, Inc. Northfield, Minnesota, U.S., which was sieved by a metal sifter (100-mm sieve
Effect of initial pH
Significant TAN removal was observed in IVS with pH over 9, where a positive correlation between initial pH and TAN removal efficiency was observed. 85% and 95% of TAN was removed from LSM in pH 9.0 and pH 10.0 conditions after 60 min IVS, then TAN was further decreased from 3300 mg/L to 308 ± 60 mg/L and 38 ± 10 mg/L after 120 min, respectively (Fig. 2a). Nevertheless, no significant TAN removal efficiency variance was observed, when initial pH was over 10.0. Significant TAN removal efficiency improved
Conclusion
IVS assisting could improve methanogenesis and hydrolysis efficiency (50% methane yield increase and 40% hydrolysis time reduce) through preventing FAN and H2S inhibition from LSM in TAD. IVS-TAD also supported a higher stability than TAD for long-term operation with less TAN accumulation risk by recycling 38% ONt at 55 °C, 100.63 ± 3.79 mmHg and initial pH 10 in 60 min, while the bioavailability of IVS-TAD elution was improved for algae and plant growth with higher particles degradation and lower
Acknowledgements
This work is supported in part by grants from Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative Citizen Commission on Minnesota Resources (LCCMR), Grants number CON000000055335 and CON000000046824, and University of Minnesota Center for Biorefining.
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