Development of an innovative thermal-vacuum stripping assisted thermophilic anaerobic digestion process and system for complete utilization of liquid swine manure

Abstract

Liquid Swine Manure (LSM) is a valuable resource only if appropriately managed. Swine wastewater treatment, especially complete utilization of its nutrients and making it fully reusable or dischargeable, has received increasing attention in recent years in the U.S. due to more stringent wastewater regulations. Using the innovative Thermal-Vaccum Stripping (TVS) assisted thermophilic Anaerobic Digestion (AD), the thermophilic AD of LSM can overcome low methane productivity and inefficient substrate utilization due to ammonia inhibition and sulfide inhibition. In this dissertation, a lab-scale Thermal-Vaccum Stripping (TVS) process and system was developed for removing ammonia and sulfide content from LSM. The Corn Stover (CS) was evaluated as the co-digested substrate to enhance the fermentation quality of the LSM and thereby improved methane productivity. The developed TVS was implemented to reinforce the activities of hydrolysis, acidogenesis, acetogenesis, and methanogenesis in the thermophilic AD of LSM and CS (LSM+CS). Besides, the comprehensive nutrient assay was used to study the behavior of bacteria in the TVS assisted thermophilic AD process without the inhibitions of ammonia and sulfide. Two models, consisting of pretreatments and in-line intermittent treatment, were studied to explore the applications of TVS assisted thermophilic AD for the management of LSM.In Chapter 3, A lab-scale Thermal-Vacuum Stripping (TVS) apparatus was developed to remove the ammonia and hydrogen sulfide of Liquid Swine Manure (LSM). The developed TVS apparatus could remove 97% of ammonia and 99% of sulfide from LSM within 30mins at 87.39mmHg, initial pH=10, and 55ºC. The vacuum gradient was the most critical factor of TVS rather than pH and temperature. However, the total ammonia removal efficiency would be impaired during scale-up due to the decreases in the overall liquid-phase mass-transfer coefficients (kL) and the interfacial area (a). Pretreating LSM by TVS could result in a 52% methanogenesis improvement of the thermophilic Anaerobic Digestion (AD) through 1) reducing ammonia inhibition risk during thermophilic AD of LSM by removing the organic nitrogen, and 2) reducing inhibitions of thermophilic AD by removing ammonia and sulfide from LSM at the beginning. In chapter 4, the developed Thermal-Vacuum Stripping (TVS) was used to improve the utilization efficiency of co-digested Corn Stover (CS) and thereby to improve the methane production of thermophilic Anaerobic Digestion (AD) of Liquid Swine Manure (LSM). Adding CS as a supplement to LSM could significantly improve the performance of thermophilic AD of LSM. The Organic Loading Rate (OLR) of LSM+CS should have been initially controlled below 1.0 g-VS/ kg/ day in order to achieve the best substrate utilization efficiency. If the primary purpose of thermophilic AD is to generate as much CH4 as possible, increasing the OLR should have been a great option. The TVS pretreatment made less of a contribution to improving the hydrolysis of CS than the thermal-chemical treatment, without vacuum. However, the application of TVS did help to remove lignin components from the CS and therefore released more cellulose for thermophilic AD. Also, a shorter TVS pretreatment time, less alkali usage, and different alkali type should have a synergistic effect in improving the methane production of the CS portion. The results of both batch and Continuous Stirring Temperature-controlled Reactor (CSTR) experiments showed that the thermophilic AD co-digestion process could not fully utilize the benefits of enhanced methane production of TVS pretreated CS (pCS) without removing ammonia inhibition of the LSM portion. The results indicated that the acidogenesis of thermophilic AD was more sensitive to the change of OLR than the inhibition of ammonia. In chapter 5, various inhibitors and process variables were investigated to evaluate the feasibility of Thermal-Vacuum Stripping (TVS) pretreatment assisted thermophilic anaerobic co-digestion of Liquid Swine Manure (LSM) with Corn Stover (CS). High salinity was a more severe issue than high TAN, low C:N ratio, or high organic loading rate in the thermophilic AD of LSM+CS. The salts that accumulated during TVS pretreatment significantly increased the salinity of the substrate. The performance of thermophilic AD was worse after applying TVS pretreatment, even while eliminating the ammonia inhibition. The high concentration of ammonia had less impact on acetogenesis than methanogenesis. The inhibition of methanogenesis by ammonia would start from 500 mg/L, and present different inhibition patterns beyond 1000 mg/L. The maximum biogas loss by ammonia inhibition might be less than 50% for thermophilic AD of LSM. By contrast, salinity presented a critical inhibition for methanogenesis. The salinity increase from 20‰ to 44‰ resulted in a 59.3% reduction of VS utilization, and a 100% complete loss of methane production, during the 10-day thermophilic AD process. The metabolism of butyric acid might be the index for evaluating the inhibition of thermophilic AD due to salinity. In chapter 6, the in-line Intermittent Thermal-Vacuum Stripping (ITVS) assisted thermophilic Anaerobic Digestion (AD) apparatus system was developed. The application of ITVS significantly improved methane production and the utilization rate of LSM+CS for thermophilic AD. The activities of anaerobes were supposed to be temporarily limited during the ITVS treating period but would recover right after the end of ITVS. Overall, the ITVS was a better approach than TVS pretreatment for preventing the inhibition of salinity in the thermophilic AD of LSM+CS. Besides, the application of ITVS is more adapted to prevent ammonia issues coming from the thermophilic processing than lowering the high ammonia concentration of LSM (~5000 mg/L) to a non-inhibition level. In summary, the dissertation aims to provide the scientific reference, and the technical support for the research and further development of complete utilization of LSM via the Thermal-Vacuum Stripping (TVS) assisted thermophilic Anaerobic Digestion (AD). The implement of TVS can improve the methane productivity and the substrate utilization efficiency of thermophilic AD by removing contents of ammonia and sulfide in the LSM portion and reinforcing the disintegration of the Corn Stover portion, respectively.