Article pubs.acs.org/EF
High-Solid Anaerobic Codigestion of Horse Manure and Grass in Batch and Semi-continuous Systems Wangliang Li* NUS Environmental Research Institute, National University of Singapore, 5A Engineering Drive 1, Singapore 117411, Singapore ABSTRACT: This study investigated high-solid codigestion of horse manure (HM) and grass in batch and semi-continuous systems. After 28 days, the cumulative methane yield of HM was 298.7 mL/g of volatile solids (VS). When the ratio of HM/grass was 80:20, on a percent VS basis, with the C/N ratio of 20.42, the highest methane yield of 382.3 mL/g of VS was achieved. The methane concentration of fresh HM was higher than that of codigestion and that of grass as a result of the high volatile fatty acid concentration. The semi-continuous codigestion of HM and grass was carried out in a high-solid system with an organic loading rate (OLR) of 1.25, 1.88, and 2.50 g of VS L−1 day−1, which resulted in an average methane productivity of 152.9, 149.6, and 141.1 mL g−1 of VS day−1, respectively. The optimal OLR of semi-continuous codigestion was 1.88 g of VS L−1 day−1, with the hydraulic retention time (HRT) of about 14 days. Thermogravimetric and differential thermal analysis (TG/DTA) revealed that more than 80% of the fractions of digestion residues were volatile matter.
1. INTRODUCTION Anaerobic digestion (AD) is an attractive waste-disposal technique, in which both organic stabilization and energy recovery are achieved.1 There are numerous abundant sources of biodegradable organic waste, including horticultural waste and animal manure.2 It is reported that a horse produces 10−14 tons of waste manure (including dung, urine, and bedding) annually. About 27.6 million tons of used bedding materials are generated every year in the United States.3 Dry grass is one of the most commonly used bedding materials for horses. After the mixture with solid horse manure (HM), the dry grass becomes spent bedding materials, which should be disposed of properly for environmental and hygiene concerns. AD of HM and spent bedding materials can be used to mitigate environmental concerns and produce biogas.4 AD of HM in conventional biogas plants is already practiced but could cause problems in terms of mixing and conveying as a result of its fibrous characteristics, which can be solved in a high-solid anaerobic digestion (HSAD) system.5,6 In comparison to the liquid AD process, HSAD has the advantages of higher volumetric methane productivity, lower energy requirements for heating, producing less wastewater, and easier handling.7 The C/N ratio is an essential factor for AD, which should be in the range of 20−30.7,8 For the HSAD of HM, the low C/N ratio produces high concentrations of ammonia and leads to a low biogas yield. The volatile fatty acids (VFAs) accumulated during HSAD of horticultural waste or agricultural crop inhibition and even cause the failure of digestion. Codigestion of horticultural waste and animal manure can be operated by avoiding inhibition caused by high levels of ammonium and VFAs because the buffer systems formed by NH4+ and VFAs can maintain a high tolerance to NH4+ and VFAs in the system.7−10 Dependent upon the operating conditions and the characteristics, codigestion can significantly increase the methane yield of the animal manure.11 The better performance of codigestion was attributed to the improvement of nutrient balance, enhancement of the buffer capacity, or positive synergisms.7 © 2016 American Chemical Society
HSAD performance of HM was studied, and its biochemical methane potential (BMP) tests were tested in upflow anaerobic solid state (UASS). Dry grass is often used as bedding materials and mixed together with HM. The effects of the mechanical disintegration and pretreatment on the enhancement of methane production were investigated.12 To the best of our knowledge, few studies have investigated the C/N ratio on HSAD performance of HM and grass in batch and semicontinuous processes. This study investigated how the different VS mixing ratios of HM and grass affected the biogas production efficiency in a batch system with initial total solids (TS) of 20%. The substrates were evaluated separately and in mixtures with different proportions of each substrate. The main strategy was to determine the optimal volatile solids (VS) ratio of HM and grass, characterize their semi-continuous codigestion, and evaluate the organic loading rate (OLR) of the substrate on methane productivity. Furthermore, the feasibility of digestate used as feedstock for gasification was investigated by proximate analysis.
2. MATERIALS AND METHODS 2.1. HM, Inoculum, and Grass. The fresh HM was collected from Singapore Turf Club, Singapore, and sealed in airtight plastic bags. The TS and VS of HM was 21.25 and 18.97%, respectively. Grass was collected from yard waste of the National University of Singapore. The grass was shredded and homogenized to small pieces (approximately 2 cm in length for grass). The anaerobic sludge from the PUB Ulu Pandan Water Reclamation Plant had a pH of 7.62, TS of 2.93%, and VS of 2.14%. The sludge was centrifuged, and sludge cake was used as inoculum for HSAD. 2.2. HSAD in a Batch System. In the batch system, HM, grass, or the mixture of them were used as substrate. The HSAD experiments were performed in 250 mL digesters containing a substrate of 5.00 g of VS at the temperature of 35 °C. After inoculation or sampling, all Received: March 10, 2016 Revised: June 25, 2016 Published: July 5, 2016 6419
DOI: 10.1021/acs.energyfuels.6b00551 Energy Fuels 2016, 30, 6419−6424
Article
Energy & Fuels batch reactors were purged with nitrogen gas to create an anaerobic condition. 2.3. High-Solid Codigestion of HM and Grass in a Semicontinuous System. The high-solid semi-continuous codigestion was carried out with HM and grass (with the ratio of 80:20, % VS) in a 1000 mL bioreactor with 250 mL working volume at 35 °C with TS of 22%. The initial total VS was kept as 25.0 g. Deionized water was added to the digesters to maintain a constant TS of 20.0%. The startup period of the semi-continuous experiment was selected as 7 days. After 15 days of semi-continuous operation, the experiment reached stable operation and then samples of biogas and solid residues were taken. An equivalent volume of the digester content was discharged prior to feeding. Then, the bioreactor was purged using nitrogen gas to create an anaerobic condition. The reactor was gently mixed manually prior to sampling and then feeding with OLR of 1.25, 1.88, and 2.50 g of VS L−1 day−1. 2.4. Proximate Analysis (Dry Basis) of Degestate. After batch experiments, the digestate was dried and the gasification performance was tested on the thermogravimetric and differential thermal analysis (TG/DTA). The proximate analysis of the digested sample was conducted by thermogravimetric analysis (TGA, Shimadzu, DTG60A). The digestate was loaded in a platinum crucible, first heated in N2, and held at 105 °C for 15 min to remove its moisture. The sample was then further heated to 950 °C in N2 at 20 °C/min. After the temperature was steady, N2 was switched to air at 950 °C for another 10 min. The reaction lasted until the weight loss of the sample was no longer distinctive. From the TG curve presented in TGA, the volatile matter and ash content were easy to determine and the fixed carbon content could be calculated by difference. 2.5. Analysis Methods. The grass and HM samples were analyzed for TS and VS contents according to the standard methods of the American Public Health Association (APHA).13 Rapid simultaneous determination of the carbon, hydrogen, nitrogen, and sulfur (CHNS) contents in grass and HM were carried out with the instrument Elementar Vario Micro Cube. The cellulose, hemicelluloses, and lignin contents were analyzed according to the procedure of Van Soest et al.14 The soluble chemical oxygen demand (SCOD), ammonia content, and biogas analysis method were described in our previous research.15 The trace elements were investigated with PerkinElmer SCIEX DRC-e inductively coupled plasma mass spectrometry (ICP− MS). The argon gas flow was 0.80 L/min. The digested samples were further diluted and analyzed using high-performance ICP−MS. The instrument was calibrated using certified standards, and a suitable internal standard was used to compensate for the possible drift in instrument measurements.
Table 1. Physical Properties of Grass and HM feed C (wt %) N (wt %) C/N ratio S (wt %) TS (%) VS (%) composition NDS (%) cellulose (%) hemicelluloses (%) lignin (%) ash (%) element Ca (mg/g of TS) Mg (mg/g of TS) K (mg/g of TS) Na (mg/g of TS) Fe (mg/g of TS) Zn (mg/g of TS) Ni (mg/kg of TS) Co (mg/kg of TS) Mn (mg/kg of TS) organic acid (mg/g of VS) acetic propionic isobutyric butyric isovaleric valeric caproic
grass
HM
41.08 1.02 40.27
