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Simultaneous ammonia stripping and anaerobic digestion for efficient thermophilic conversion of dairy manure at high solids concentration
Energy 141 (2017) 179e188
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Simultaneous ammonia stripping and anaerobic digestion for efficient thermophilic conversion of dairy manure at high solids concentration Yiqing Yao a, Liang Yu a, Rishikesh Ghogare a, Alexander Dunsmoor a, Maryam Davaritouchaee b, Shulin Chen a, * a b
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USA The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2017 Received in revised form 17 September 2017 Accepted 18 September 2017 Available online 19 September 2017
A major challenge to take high rate advantage of thermophilic anaerobic digestion (AD) is overcoming ammonia inhibition without adding any significant cost to the process. A concept of thermophilic AD with simultaneous ammonia stripping was tested for treating dairy manure at high total solids concentration (TS%) as an attempt to address this challenge. The results showed that ammonia inhibition occurred at 1.8e2.4 g/L total ammonia nitrogen (TAN) concentration which corresponded to 10% TS as a threshold concentration. Thermophilic AD of dairy manure efficiency at the threshold TS% was significantly improved by simultaneously stripping ammonia with the optimum stripping rate of 1 L min1. The required time for reaching stable state was 4 days shorter than control, and the highest methane content (56.5e75.5%) was obtained. The ammonia stripping strategy maintained TAN level below the inhibition limit of 1.5 g/L throughout AD process. The maximum cumulative methane production of 192.3 L/kgvolatile solids (VS) was obtained, which was 2.3-fold the control. Therefore, simultaneous ammonia stripping overcame the ammonia inhibition on thermophilic AD of dairy manure at the threshold TS%. The proposed concept simplified the system by combining ammonia stripping and thermophilic AD within the same digester. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Simultaneous ammonia stripping High-solids thermophilic anaerobic digestion Methane production from manure Ammonia inhibition
1. Introduction Anaerobic digestion (AD) has been recommended as a primary process for treating livestock manure, because waste reduction, energy (biogas) production and mitigation of pollutant emissions (odor, greenhouse gases, and animal pathogens) can be all accomplished [1]. The increasing concerns about the environmental impact of animal feeding operations and the growing needs for renewable energy make AD an attractive alternative for managing animal manure [2]. Enhancing AD process performance is desirable for dropping the cost of operation. Temperature is a key factor affecting AD process
Abbreviations: AD, anaerobic digestion; SS-AD, solid-state anaerobic digestion; SSFW, source sorted food waste; TKN, Kjeldahl nitrogen; TAN, total ammonia nitrogen; NH3, free ammonia; S/I, substrate-to-inoculum; sCOD, soluble chemical oxygen demand; CK, no urea treatment; VFAs, volatile fatty acids; TS, total solids; VS, volatile solids. * Corresponding author. E-mail addresses: [email protected] (Y. Yao), [email protected] (S. Chen). https://doi.org/10.1016/j.energy.2017.09.086 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
performance. There are mainly three different types of AD processes based on the operating temperature: thermophilic (>45 C), mesophilic (20e40 C) and psychrophilic (<25 C). Thermophilic AD has many inherent advantages over mesophilic and psychrophilic AD, including faster reaction rate, higher biogas production, less foaming occurrence, enhanced pathogen reduction, and facilitated the decomposition of coarse fibers in dairy manure [3,4]. However, as is well known, temperature affects the threshold of ammonia inhibition [5], and thermophilic AD tends to be unstable in processing the nitrogen-rich wastes such as manure mainly due to the fact that increasing temperature results in more accumulation of ammonia and then the severe ammonia toxicity to microbes [6]. The effects of ammonia inhibition on microbial consortia are reported to have a pronounced impact in later stages of AD, including the activity of acetate-utilizing (acetoclastic) methanogens or hydrogen/formate-utilizing (hydrogenotrophic) methanogens, and the acetoclastic methanogens (Methanosaeta sp. and certain Methanosarcina sp.) which are considered to be most sensitive to ammonia [7]. In addition, the elevated ammonia accumulation releases overload free ammonia (NH3), which can cause
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intercellular pH variation, proton imbalance, potassium deficiency, and even cease the growth of methanogens by permeating the cells [8]. Most evidence has confirmed the high sensitivity of methanogens to NH3 [9]. Protein-rich materials are common sources for sulfide formation, which is not only toxic for microbial populations but also forms complexes with metals, resulting in decreased bioavailability of trace elements for microbial activity [7,10]. Various attempts have been made to overcome ammonia inhibition to maximize the specific methane production and enhance the process stability of thermophilic AD of nitrogen-rich materials. The simplest operation is to dilute the organic waste with water to avoid ammonia inhibition. Apart from dilution, chemical precipitation like magnesium ammonium phosphate (MAP) process [11], zeolite and clay process [12,13], and the addition of phosphorite ore [14] have been used. Biological processes like nitrification/denitrification [15] and bipolar bioelectrodialysis [16] have been studied as a way to reduce the ammonia inhibition. Methanogenic consortia acclimation to elevated ammonia levels was effective to improve ammonia tolerance for enhancing methane production [17]. The process of acclimation usually takes a long time and the methane production remains relatively low [18]. Gas-permeable membrane technology is also used to recover NH3 from liquid manures [19]. Removing ammonia from thermophilic AD process through stripping is thought to be a feasible approach, especially for treating organic wastes with high ammonia content, such as source sorted food waste (SSFW) digestate [20], chicken manure [21], and poultry litter leachate [22]. During the process, free ammonia is stripped out from the organic wastes and entered the gas phase, which caused the reduction of ammonia content in the liquid phase. Ammonia stripping generates no extra sludge, and the reagents cost is relatively low. Ammonia stripping has been proved to be useful in recovering ammonia from liquid waste such as AD effluent and poultry litter leachate [22,23]. Some studies have also reported the application of ammonia stripping for dehydrated waste of activated sludge and dairy manure with high TS% [18,24]. Conventional ammonia stripping requires NaOH, CaOH, or KOH addition to increase pH within 10e11 [25]. However, after ammonia stripping, AD could be inhibited by the toxicity of residual Naþ, Caþ, and Kþ [26]. 1.5, 2.5 and 2.5 g/L of Naþ, Caþ, and Kþ concentrations, respectively, are known to begin inhibition effect on microorganisms [27]. In addition, the chemicals addition can enhance the sludge precipitates, cause mechanical problems, and form pipe scale [28]. For these studies, ammonia stripping is deployed as a separate stage followed by thermophilic AD. A feasible option to efficiently treat nitrogen-rich materials with low cost, may be thermophilic AD coupled with ammonia stripping. The related studies are limited and only focused on chicken manure, piggery wastewater and poultry litter leachate so far [21,22,26]. However, there was no work done on dairy manure, so mechanism of the response of process performance and methane production to the stripping rate and the ammonia inhibition remain unclear. The characteristics of different nitrogen-rich materials are different, which influence the effectiveness of ammonia stripping. Therefore, it is necessary to investigate thermophilic AD of dairy manure coupled with ammonia stripping in order to enhance the knowledge base in the field of simultaneous ammonia stripping. In the present study, a thermophilic AD system coupled with ammonia stripping was proposed to treat dairy manure. Compared with conventional ammonia stripping, there are some advantages: first, ammonia stripping and thermophilic AD were realized in a single stage; second, in order to eliminate the negative effect of chemicals addition on microbial activity as aforementioned, no chemicals were added to adjust pH, so the feedstock remains
native; third, this proposed approach simultaneously eliminated the inhibitory effect on microbial activity caused by ammonia accumulation along with the thermophilic AD process, so this method can improve the economic feasibility and the applicability. Based on the above points, the threshold total solids concentration (TS%) that led to ammonia inhibition on thermophilic AD of dairy manure was firstly identified to examine the feasibility of simultaneous ammonia stripping on the removal of ammonia inhibition, and then the improvement of methane production was verified based on the threshold TS%. Additionally, the effect of stripping rate on the removal of ammonia inhibition, the process performance, and the methane production was investigated. The AD process was finally optimized to maximize the methane production. 2. Materials and methods 2.1. Feedstock and inoculum Dairy manure was collected from Washington State University Dairy Center in Pullman, WA, USA and stored at 4 C prior to use. The inoculum was sampled from a mesophilic anaerobic digester at Wastewater Treatment facility in Pullman, WA, USA. The characteristics of dairy manure and inoculum are shown in Table 1. 2.2. Anaerobic digestion design 2.2.1. Thermophilic anaerobic digestion of dairy manure at different high solids concentrations The required amount of dairy manure and inoculum for each digester were 400 g and 100 g, respectively, based on the wet weight. The TS and VS for each digester were 51.93 g and 42 g, respectively. Deionized water was added to match the requirement of certain TS%. At 6% and 8% TS, 365.5 g and 140 g of deionized water were used, correspondingly. At 10% TS, no deionized water was added. This test was conducted in batch mode at laboratory scale and the volume for each digester was 1 L. The headspace of digesters was flushed with nitrogen gas for about 5 min to obtain anaerobic condition, after which digesters were capped tightly with rubber stoppers and incubated at 55 C and shaken at a speed of 120 rpm [29,30]. Digestion experiments were conducted in triplicate for each condition. 2.2.2. Simultaneous ammonia stripping for thermophilic anaerobic digestion of dairy manure at the bottleneck solids concentration Simultaneous ammonia stripping was conducted in a 3.0 L reactor. The selected TS% for stripping was the bottleneck TS% obtained from the previous experiments. Nitrogen gas, as the stripping gas, was introduced into the liquid phase from the bottom of the reactor via an aquatic air stone, and it helped to stir the mixed liquor in the reactor. The soft pipe connected with the aquatic air stone was introduced through the rubber cap and connected to nitrogen gas cylinder. The nitrogen flow rate was controlled at 1.0 L min1, 3.0 L min1, and 5.0 L min1 by a flow meter. The flow Table 1 Characteristics of dairy manure and inoculum. Parameter
Dairy manure
Inoculum
Total solids (%) Volatile solids (% of total solids) pH Total carbon (%) Total nitrogen (%) Total hydrogen (%) Total ammonia nitrogen (g N/l)
12.5 ± 0.4 80.9 ± 0.1 8.4 ± 0.0 40.7 ± 0.1 1.8 ± 0.0 5.7 ± 0.0 1.4 ± 0.0
1.9 ± 0.0 74.4 ± 0.1 7.6 ± 0.0 35.5 ± 0.0 5.5 ± 0.0 5.3 ± 0.0 0.6 ± 0.0
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rate method of this study was accorded to previous studies [23]. Simultaneous ammonia stripping was done for 2 h each time every two days. The pH value for all experiments before AD was 8.1 without adjustment. Experiment with no simultaneous stripping was set as a control. The exhaust gas was passed through solutions of 50% (w/w) H2SO4 and 20% (w/w) sodium hydroxide to prevent the release of ammonia and other volatile compounds into the atmosphere [26] (Fig. 1). Other specifications of experiment were the same as mentioned above. 2.3. Analytical methods 2.3.1. Chemical analyses TS, VS, soluble chemical oxygen demand (sCOD), and pH were determined according to standard methods (APHA, 2005) [31]. TAN was measured according to Standard Methods using a Tecator 2300 Kjeltec Analyzer (Eden Prairie, MN, USA; 4500-NorgB; 4500NH3BC) [31]. An elemental analyzer (LECO) was used to measure total carbon, total nitrogen, and total hydrogen. 2.3.2. Biogas analyses Biogas production was measured by the method of water displacement every 2 days, the total volume of biogas was calculated after AD. Gas samples were collected every 2 days and stored in 12 ml vacuumed borosilicate vials (Extainer, Labco Limited, Wycombe, England). A gas chromatograph (GC) (GC, CP-3800, Varian Inc. Palo Alto, CA, USA) equipped with a thermal conductivity detector was used to analyze the methane content of biogas sample [32]. 2.4. Statistical analysis The software SPSS 19.0 was applied to determine the standard deviations and whether the observed differences between two or more groups of experimental results were significant. Differences were compared with a p value of 0.05. 3. Results and discussion 3.1. Thermophilic anaerobic digestion of dairy manure at different high solids concentrations 3.1.1. Process performance TS% had a significant effect on daily methane production. As illustrated in Fig. 2A, all the TS% had distinct peaks at the variable time point. The 6%, 8% and 10% TS, peaked at 13.8 L/kg-VS/d, 24.3 L/
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kg-VS/d, and L/kg-VS/d on the 14th day, 16th day, and 18th day, respectively. The level of daily methane production at 8% TS was higher than that at 6% TS. In addition, AD at 6% and 8% TS lasted until day 40. At 10% TS, on the other hand, the daily methane production after 20 days showed a sharp decrease and ceased on the 28th day. A previous study investigated the maximum TS% at which methanogenic activity was observed. Cabbage, carrot, and meat had a maximum 20% TS while sludge cake even had a higher maximum 43.4% TS [33], which was much higher than that obtained in the present study. Another study investigated high-solids AD of sludge and found methanogenic activity dropped to 53% from 100% when TS% was increased from 4% to 10%. The rate of methane production was stable at 4e10% TS in a pH range of 6.6e7.8 (optimum at pH 6.8) [34]. This result indicates that the capacity of digester for treating dairy manure cannot be enhanced efficiently by increasing TS%. The results can be used as an upper limit of TS% for testing the effect of simultaneous ammonia stripping on methane production and dairy manure treatment. TS% also had a significant effect on the start-up of AD (Fig. 2B). Methane content exceeds 50% means the achievement of stable state [35]. At 6%, 8% and 10% TS, methane content exceeded 50% on the 10th day, 10th day and 14th day, respectively. Methane content at 6%, 8% and 10% TS during the stable state were 51.8e63.5%, 55.7e69.2% and 57.4e64.8%, respectively. The time for reaching stable status at 10% TS was 4 days more than that at 6% and 8% TS. This result indicates that the start-up of thermophilic AD of dairy manure was prolonged by increased TS%, therefore, the efficiency of thermophilic AD was reduced. Total ammonia nitrogen (TAN) levels associated with TS% are presented in Fig. 3. 6% TS had the lowest TAN level (<1.2 g/L) throughout AD process, while an increase in TS% from 6% to 8% and further to10%, led to TAN levels increase substantially. TAN levels at 8% and 10% TS increased along with AD process and maintained its level around 1.8 g/L. In an AD system, fermentative bacteria, acetogenic bacteria, and methanogenic bacteria constitute the microbial consortium [36]. As reported previously, acidification and methanogenic stages can be negatively affected by the accumulation of free ammonia [4]. In a long-term study on pilot scale thermophilic AD of organic fraction of municipal solid waste (OFMSW), it was reported that ammonia inhibition occurred at 1.2 g/L TAN and pH of 7.2 [34]. Previously, thermophilic AD of dairy manure at 50 C with TAN level above 3 g/ L resulted in inhibition of AD process or reduced methane production [37]. In a wide pH range of 6.5e8.5, it has been observed that methanogenic activity decreased with an increase in TAN, the activity decreased by 10% at 1.67e3.72 g/L, 50% at 4.09e5.55 g/L and
Fig. 1. Schematic diagram of thermophilic anaerobic digestion coupled with the simultaneous ammonia stripping.
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Fig. 2. Process performances at different TS%. A: Daily methane productions; B: Methane contents.
100% at 5.88e6.6 g/L [34]. In the present study, NH3, as a portion of TAN, had much lower level than 1.2 g/L TAN at 6% TS and even lower than previous reports. Therefore, there was likely no ammonia inhibition at 6% TS, while at 8% and 10% TS, TAN level was 1.8 g/L, which was in the range of 1.67e3.72 g/L [34], suggesting slight ammonia inhibition. That is why the peaks of daily methane productions at 8% and 10% TS appeared later than that at 6% TS. At 10% TS, TAN level was the highest within 1.8e2.4 g/L after 8 days, so the AD process was inhibited severely and even ceased on the 28th day; the strong ammonia inhibition also took more time to reach stable state compared to that at 6% and 8% TS (Fig. 2B). pH of AD system is usually used to indicate the process stability And the pH level is associated with VFAs level [38]. The acidic environment was caused by the accumulation of VFAs, which led to the pH drop [39]. Therefore, low pH was associated with high VFAs level and vice versa. As a result, pH can be used for indicating the VFAs level [38,40]. As shown in Fig. 4, in the initial period of AD, a decrease of pH can be attributed to the acidification in the digestion system [4]. The accumulation of acidic products usually occurs in the initial period, such as acetate [21], and leads to pH decrease. The decrease of pH can inhibit the efficiency of ammonia fermentation [21]. The pH increased later and maintained a high level especially at 10% TS, which was related with the high TAN level (Fig. 3), the
amount of free ammonia significantly depends on pH. Therefore, this result confirms the ammonia inhibition. The nitrogen content in the substrate is actually the governing factor instead of TS%. Effect of ammonia inhibition can be intensified at higher TS than 5% when chicken manure was used as a substrate of AD [21]. However, in the present study, the inhibitory effect did not occur at 6% TS, but at higher TS than 8%. This is because of the difference of TAN content between chicken manure and dairy manure. The TAN content in chicken manure (approximately 3.7 g/L) is higher than that of dairy manure (1.4 g/L-Table 1) [41,42]. 3.1.2. Cumulative methane production Cumulative methane production was significantly affected by TS % (Fig. 5). The maximum cumulative methane production of 161.7 L/ kg-VS was achieved at 8% TS and day 40, which was 1.7-fold (p < 0.05) and 2.0-fold (p < 0.05) those at 6% and 10% TS, respectively. Cumulative methane productions at 6% and 10% TS were 93.9 L/kg-VS and 80.4 L/kg-VS, respectively, which were obtained at day-40 and day-28, respectively. At 10% TS, cumulative methane production was even lower than that at 6% TS. This is due to the serious ammonia inhibition on the activity of methanogenesis. As mentioned above, there was no ammonia inhibition at 6% TS, but
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Fig. 2. (continued).
the cumulative methane production was less than that at 8% TS. This might be the lower content of organic matter at 6% TS than that at 8% TS. In summary, severe ammonia inhibition on thermophilic AD of dairy manure occurred at 10% TS, the total methane production under this condition was the least. This established the threshold TS % that can be used for testing simultaneous ammonia stripping.
3.2. Simultaneous ammonia stripping for thermophilic anaerobic digestion of dairy manure at the threshold solids concentration (10%) 3.2.1. Process performance at different stripping rates Simultaneous ammonia stripping at 1 L min1 was beneficial for the enhancement of AD process performance (Fig. 6A). Levels of daily methane productions at different gas flow rates can be ranked as 1 L min1> 3 L min1> control> 5 L min1, so the highest level of daily methane production was obtained at 1 L min1. Distinct peaks were observed at 1 L min1 and 3 L min1, with methane production of 23.6 L/kg-VS/d and 21.3 L/kg-VS/d, respectively. For control, the highest peak appeared on the 14th day, which was no more than 15.0 L/kg-VS/d and the time for the peak appeared was 2 days later than those at 1 L min1 and 3 L min1. The results indicate that simultaneous ammonia stripping was beneficial for improving
thermophilic AD. However, at 5 L min1, the level of daily methane production was the lowest. The stripping rate is important for thermophilic AD of substrates with high nitrogen content. High stripping rate has some disadvantages such as water evaporation, foaming, cooling of the substrate [43]. Lost of organic materials due to high stripping rate may also contribute to the low level of daily methane production [26]. The low level daily methane production resulted from the high stripping rate, may be also caused by the low efficiency of mass transfer. In methanogenic digester containing waste water, aggregates tend to be formed. First, aggregation can enhance the exchange of formate and hydrogen between syntrophic microbes [44]; second, aggregation is beneficial for cells to directly exchange electrons [45]. Therefore, the high stripping rate may disrupt energy and nutrient transfer between microbes. For the delay of appeared peak for control, as previously reported, the process of spontaneous acclimation of methanogenic consortia to a high level of ammonia took quite a long time; the methane production from chicken manure was low, and this phenomenon has been well demonstrated [18]. This result indicates that stripping rate at 1 L min1 improved the level of daily methane production. Trends of methane content for control, 1 L min1 and 3 L min1 were similar (Fig. 6B). For control, 1 L min1, 3 L min1 and 5 L min1, methane content reached 50% on the 10th day, 6th day, 6th day and 8th day, respectively. Therefore, at 1 L min1 and
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Fig. 3. TAN concentrations at different TS%.
3 L min1, time lasted for reaching the stable state was the shortest, which was 4 days shorter than the control. Similar to daily methane production, methane content at 1 L min1 was the maximum (56.5e75.5%) compared to other samples. Heavy fluctuation of methane content throughout AD process appeared at 5 L min1 and the level was the minimum (44.5e61%). As a result, high stripping rate had a negative effect on AD process performance. Methane contents for the control and 3 L min1 were 55.9e65.1% and 61.9e68.5%, respectively. Similar to daily methane production, the methane contents ranked as: 1 L min1> 3 L min1> control> 5 L min1. These results indicate that stripping rate at 1 L min1 reduced the reaching stable state time and improved the methane content. 3.2.2. sCOD (soluble chemical oxygen demand), pH and TAN changes at different stripping rates Simultaneous ammonia stripping at 1 L min1 was beneficial for the utilization of available substrate and the removal of ammonia inhibition. sCOD was used as an indicator to examine the available substrate level for microorganisms in the AD system. For all experiments, sCOD content experienced initial increase followed by a slow decrease (Fig. 7A). In the initial period of AD, the readily available COD for anaerobic microbes was limited. However, particulate matter or complex particulate compounds were increasingly converted into the soluble organic substrate along with the acclimation and breeding of hydrolytic bacterium [38], which resulted in the high level of sCOD content and then the high level of daily methane productions. This result is in good agreement with a previous study [46]. When most of the sCOD was utilized by
microbes, daily methane productions decreased accordingly. For control, the peak appeared on the 8th day, which was longer than those of the other experiments. According to Abouelenien et al.’s study, the microbes that responsible for decomposing substrate were inhibited by the high level of ammonia (Fig. 7B) [18]. However, after a long time of acclimation of microbes to the high level of ammonia, an increase of sCOD release occurred, and the amount of sCOD was really significant. This can be attributed to the ammonia inhibition on methanogenesis, which led to the accumulation of available substrate that cannot be utilized by methanogens for methane production. However, sCOD level at 1 L min1 had the smallest amount, considering the corresponding highest level of daily methane production, suggesting microbes at 1 L min1 consumed the available substrate throughout the process of AD in time, so there was no significant accumulation of available substrate. At 3 L min1 and 5 L min1, the mass transfer was lower than that at 1 L min1 as aforementioned. That is to say, at 3 L min1 and 5 L min1, the available substrate/sCOD content was higher than that at 1 L min1. As illustrated in Fig. 7C, trends of pH changes for control, 1 L min1, 3 L min1 and 5 L min1 were similar, which initially decreased followed by a slow increase along with the increase of ammonia release and then maintained a stable level. The pH values were 6.45e8.55, which was within the suitable pH range of 6.5e8.5 for AD [34]. In general, animal manure has higher moisture, nutrient, salt contents, and pH buffer capacity compared to food wastes, which is beneficial for maintaining pH constant in anaerobic digester [47]. As aforementioned, the decline in pH resulted from the accumulation of acidic products (VFAs) reduces the
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Fig. 4. pH values at different TS%.
Fig. 5. Cumulative methane productions at different TS%.
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Fig. 6. Process performances for control and experiments with simultaneous ammonia stripping. A: Daily methane productions; B: Methane contents.
efficiency of ammonia fermentation [21]. Therefore, the TAN level in the initial period increased slowly and maintained a relatively stable state (Fig. 7B). The initial low pH level resulted from the hydrolysis of dairy manure that functioned as a rate limiting step
contributed to the initial low methane production [38]. After a period of adaptation and breeding of methanogens, acidic products were further utilized for methane production, this was in agreement with a previous study [38], and the pH increased accordingly
Fig. 7. sCOD, TAN and pH changes for control and experiments with simultaneous ammonia stripping. A: sCOD concentrations; B: TAN concentrations; C: pH changes.
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(Fig. 7C). The increase of pH is beneficial for the increase of TAN [48]. As a result, TAN level for control increased relatively quickly from 10th day on (Fig. 7B). During days 14e20, the TAN level was the highest, which was in line with that of pH (Fig. 7C). Values of TAN for control was 1.5e2.1 g/L within the range of 1.5e3.0 g/L. However, for the experiments with simultaneous ammonia stripping, TAN levels were lower than 1.5e3.0 g/L, so there was unlikely ammonia inhibition occurred. However, TAN level at 1 L min1 was higher than those at 3 L min1 and 5 L min1. This could be attributed to the higher stripping rate (3 L min1 and 5 L min1) that tripped the produced ammonia out of reactor more quickly, so the TAN level was lower compared with that at 1 L min1. Efficiency of ammonia removal were 53.5%, 44.4% and 64.4% at 1 L min1, 3 L min1, and 5 L min1 respectively. Methane production at 1 L min1 was the uppermost, suggesting that microbes activity at 1 L min1 was intensified, so the microbes could ferment substrate efficiently for ammonia formation, the produced ammonia could be thereby stripped out in time. Therefore, the efficiency of ammonia removal was high. However, at 5 L min1, the mass loss was the too much in comparison with other samples as mentioned above, this may be the reason for the highest ammonia removal. Thus, the lowest ammonia removal happened at 3 L min1. Comparatively, the efficiency of ammonia removal for previous studies was higher than this study, 88.2% ammonia removal was obtained at fermentation conditions of ammonia stripping for 12 h and higher pH (9.0, 9.5, 10.0, and 11.0) [26]. 78% ammonia removal was obtained at pH 12 by Lei et al., who applied ammonia stripping for AD effluent [23]. 75e95% ammonia removal was reached by applying ammonia stripping for poultry leachate at 1e2% TS. There are some reasons responsible for the lower efficiency of ammonia removal obtained in present study (44.4e64.4%) compared to traditional ammonia stripping: first, higher pH is usually used for only ammonia stripping process without AD [26]; second, TS% is usually lower than what we used in this research, which allows better gas-liquid association to trap more ammonia due to higher fluidity [21]; third, higher temperature was used for the separate ammonia stripping process [24]. However, high pH (9.0), high temperature (>60 C), and low TS % are not economically favorable from the point of energy supply and cost, and only suitable for the ammonia stripping as a separate stage rather than the AD coupled with ammonia stripping. 3.2.3. Cumulative methane production at different stripping rates The maximum cumulative methane production was obtained at 1 L min1. Cumulative methane production for control, 1 L min1, 3 L min1 and 5 L min1 were 84.7 L/kg-VS, 192.3 L/kg-VS, 168.8 L/ kg-VS, and 68.5 L/kg-VS respectively. The maximum cumulative methane production was obtained at 1 L min1, which was 2.3-fold, 1.1-fold, and 2.8-fold those of control, 3 L min1, and 5 L min1, respectively (Fig. 8), so the improvement of cumulative methane production was significant (p < 0.05). The maximum cumulative methane production was much higher than that obtained by Zhang et al. (2012) (p < 0.05), who used the ammonia-stripped piggery wastewater for batch bio-methanolization and obtained the
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Fig. 8. Cumulative methane productions for control and experiments with simultaneous ammonia stripping.
maximum methane production (128 L/kg VS) at conditions of 4.0 L min1 air stripped piggery wastewater, pH 9.0, and 37 C of incubation [26]. Mixture of ammonia stripped chicken manure and raw chicken manure in the ratio of 1:1 was used for thermophilic AD coupled with ammonia stripping. The initial pH and temperature were 8 and 55 C, respectively, the stripping rate was 4.0 L min1, and 157 L/kg VS of methane production was obtained [21], which was lower than the maximum cumulative methane production in present study. Therefore, dairy manure could be used for efficient methane production via simultaneous ammonia stripping at an equivalent level to 1 L min1. 3.2.4. Substrate degradations at different stripping rates The highest level of substrate utilization was obtained at 1 L min1. TS and VS reductions were calculated to evaluate the biodegradability of the substrate and the efficiency of methane production at different stripping rates. As shown in Table 2, the highest reductions of TS and VS were obtained at 1 L min1, which were 34.4% and 41.3%, respectively, and were in line with the maximum cumulative methane production. Compared to control, enhancements of TS and VS reductions were 251.0% and 250.0%, respectively. In general, higher substrate degradation was associated with higher methane production. However, in the present study, cumulative methane production for control was higher than that at 5 L min1, while TS and VS reductions for control were lower than that at 5 L min1. This might be due to the high stripping rate that led to the loss of organic substrate as aforementioned. It can be inferred that stripping rate of 1 L min1 was beneficial for the removal of ammonia inhibition and the assimilation of organic matters via anaerobic flora, which directed to the high efficiency of substrate degradation. This result indicates stripping rate at 1 L min-11 was beneficial for thermophilic AD of dairy manure at high TS%.
Table 2 Total solids and volatile solids quantities before and after anaerobic digestion and degradations.
Before anaerobic digestion (g) After anaerobic digestion (g) Degradation (%)
Composition
Control
1 L min1
3 L min1
5 L min1
Total solids Volatile solids Total solids Volatile solids Total solids Volatile solids
52.1 42.0 46.9 ± 1.1 37.0 ± 5.7 9.8 ± 4.2 11.8 ± 2.8
52.1 42.0 34.1 24.7 34.4 41.3
52.1 42.0 36.9 27.3 29.1 34.9
52.1 42.0 46.2 36.5 11.1 13.1
± ± ± ±
2.5 1.8 1.1 1.9
± ± ± ±
1.1 4.7 3.9 4.7
± ± ± ±
6.3 4.2 7.3 2.4
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4. Conclusions Inhibition of thermophilic AD of dairy manure due to ammonia accumulation occurred at 10% TS as a threshold TS%. The simultaneous ammonia stripping helped to overcome the ammonia inhibition. The optimum stripping rate of 1 L min1 was identified and the maximum methane production of 192.3 L/kg-VS was achieved. Overall, this approach has potential to efficiently utilize dairy manure for methane production. The combination of different stages into one digester makes this process simpler and more cost effective than other reported approaches. In addition, simultaneous ammonia stripping at 55 C instead of higher temperature and with no pH adjustment further reduces the process cost. Therefore, dairy manure could be used for efficient thermophilic AD for methane production via simultaneous ammonia stripping at a low stripping rate of 1 L min1. Acknowledgments We are thankful to Washington State University (WSU) for the equipment support. References [1] Banks C, Salter A, Chesshire M. Potential of anaerobic digestion for mitigation of greenhouse gas emissions and production of renewable energy from agriculture: barriers and incentives to widespread adoption in Europe. Water Sci Technol 2007;55(10):165e73. [2] Salminen E, Rintala J. Anaerobic digestion of organic solid poultry slaughterhouse wasteea review. Bioresour Technol 2002;83(1):13e26. [3] Krishania M, Kumar V, Vijay VK, Malik A. Analysis of different techniques used for improvement of biomethanation process: a review. Fuel 2013;106:1e9. [4] Nasir IM, Mohd Ghazi TI, Omar R. Anaerobic digestion technology in livestock manure treatment for biogas production: a review. Eng Life Sci 2012;12(3): 258e69. DI, Rajagopal R, Singh G. Technical and operational feasibility of psy[5] Masse chrophilic anaerobic digestion biotechnology for processing ammonia-rich waste. Appl Energy 2014;120:49e55. [6] Liao X, Li H. Biogas production from low-organic-content sludge using a highsolids anaerobic digester with improved agitation. Appl Energy 2015;148: 252e9. [7] Westerholm M, Moestedt J, Schnürer A. Biogas production through syntrophic acetate oxidation and deliberate operating strategies for improved digester performance. Appl Energy 2016;179:124e35. [8] Warren KS. Ammonia toxicity and pH. Nature 1962;195(4836):47e9. ski J, Weatherley L. Anaerobic co-digestion of swine and [9] Borowski S, Doman poultry manure with municipal sewage sludge. Waste Manag 2014;34(2): 513e21. €rn A, Skyllberg U. Thermodynamic modeling of iron [10] Yekta SS, Svensson BH, Bjo and trace metal solubility and speciation under sulfidic and ferruginous conditions in full scale continuous stirred tank biogas reactors. Appl Geochem 2014;47:61e73. [11] Demeestere K, Smet E, Van Langenhove H, Galbacs Z. Optimalisation of magnesium ammonium phosphate precipitation and its applicability to the removal of ammonium. Environ Technol 2001;22(12):1419e28. n Z, Sa nchez E, Weiland P, Borja R, Martı A, Ilangovan K. Influence of [12] Mila different natural zeolite concentrations on the anaerobic digestion of piggery waste. Bioresour Technol 2001;80(1):37e43. [13] Tada C, Yang Y, Hanaoka T, Sonoda A, Ooi K, Sawayama S. Effect of natural zeolite on methane production for anaerobic digestion of ammonium rich organic sludge. Bioresour Technol 2005;96(4):459e64. [14] Krylova NI, Khabiboulline RE, Naumova RP, Nagel MA. The influence of ammonium and methods for removal during the anaerobic treatment of poultry manure. J Chem Technol Biotechnol 1997;70(1):99e105. [15] Dong X, Tollner EW. Evaluation of Anammox and denitrification during anaerobic digestion of poultry manure. Bioresour Technol 2003;86(2): 139e45. [16] Zhang Y, Angelidaki I. Recovery of ammonia and sulfate from waste streams and bioenergy production via bipolar bioelectrodialysis. Water Res 2015;85: 177e84. [17] Abouelenien F, Kitamura Y, Nishio N, Nakashimada Y. Dry anaerobic ammoniaemethane production from chicken manure. Appl Microbiol Biotechnol 2009;82(4):757e64. [18] Abouelenien F, Nakashimada Y, Nishio N. Dry mesophilic fermentation of chicken manure for production of methane by repeated batch culture. J Biosci Bioeng 2009;107(3):293e5.
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Pretreatment of anaerobic digestion effluent with ammonia stripping and biogas purification. J Hazard Mater 2007;145(3):391e7. [24] Nakashimada Y, Ohshima Y, Minami H, Yabu H, Namba Y, Nishio N. Ammoniaemethane two-stage anaerobic digestion of dehydrated waste-activated sludge. Appl Microbiol Biotechnol 2008;79(6):1061e9. [25] Park S, Kim M. Innovative ammonia stripping with an electrolyzed water system as pretreatment of thermally hydrolyzed wasted sludge for anaerobic digestion. Water Res 2015;68:580e8. [26] Zhang L, Lee Y-W, Jahng D. Ammonia stripping for enhanced biomethanization of piggery wastewater. J Hazard Mater 2012;199:36e42. [27] McCarty PL, Rittmann BE. Environmental biotechnology: principles and applications, vol. 2. McGraw Mil Cap; 2001. p. 19e55. [28] Kim K, Cui F, Yoon H, Kim M. Treatment of copper wastewater using optimal current electrochemicalecoagulation. Environ Technol 2013;34(3):343e50. [29] Lin J, Zuo J, Gan L, Li P, Liu F, Wang K, et al. Effects of mixture ratio on anaerobic co-digestion with fruit and vegetable waste and food waste of China. J Environ Sci 2011;23(8):1403e8. [30] Zheng M, Li X, Li L, Yang X, He Y. Enhancing anaerobic biogasification of corn stover through wet state NaOH pretreatment. Bioresour Technol 2009;100(21):5140e5. [31] Association APH, Association AWW, Federation WE. Standard methods for the examination of water and wastewater. 1998. [32] Ma J, Yu L, Frear C, Zhao Q, Li X, Chen S. Kinetics of psychrophilic anaerobic sequencing batch reactor treating flushed dairy manure. Bioresour Technol 2013;131:6e12. [33] Lay J, Li Y, Noike T, Endo J, Ishimoto S. Analysis of environmental factors affecting methane production from high-solids organic waste. Water Sci Technol 1997;36(6e7):493e500. [34] Mata-Alvarez J, Mace S, Llabres P. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresour Technol 2000;74(1):3e16. [35] Brown D, Li Y. Solid state anaerobic co-digestion of yard waste and food waste for biogas production. Bioresour Technol 2013;127:275e80. [36] Gong W-j, Liang H, Li W-z, Wang Z-z. Selection and evaluation of biofilm carrier in anaerobic digestion treatment of cattle manure. Energy 2011;36(5): 3572e8. [37] Hashimoto AG. Thermophilic and mesophilic anaerobic fermentation of swine manure. Agric Wastes 1983;6(3):175e91. [38] Yao Y, He M, Ren Y, Ma L, Luo Y, Sheng H, et al. Anaerobic digestion of poplar processing residues for methane production after alkaline treatment. Bioresour Technol 2013;134:347e52. [39] Liew LN, Shi J, Li Y. Enhancing the solid-state anaerobic digestion of fallen leaves through simultaneous alkaline treatment. Bioresour Technol 2011;102(19):8828e34. [40] Yao Y, Luo Y, Yang Y, Sheng H, Li X, Li T, et al. Water free anaerobic codigestion of vegetable processing waste with cattle slurry for methane production at high total solid content. Energy 2014;74:309e13. 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Simultaneous ammonia stripping and anaerobic digestion for efficient thermophilic conversion of dairy manure at high solids concentration Yiqing Yao a, Liang Yu a, Rishikesh Ghogare a, Alexander Dunsmoor a, Maryam Davaritouchaee b, Shulin Chen a, * a b
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USA The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2017 Received in revised form 17 September 2017 Accepted 18 September 2017 Available online 19 September 2017
A major challenge to take high rate advantage of thermophilic anaerobic digestion (AD) is overcoming ammonia inhibition without adding any significant cost to the process. A concept of thermophilic AD with simultaneous ammonia stripping was tested for treating dairy manure at high total solids concentration (TS%) as an attempt to address this challenge. The results showed that ammonia inhibition occurred at 1.8e2.4 g/L total ammonia nitrogen (TAN) concentration which corresponded to 10% TS as a threshold concentration. Thermophilic AD of dairy manure efficiency at the threshold TS% was significantly improved by simultaneously stripping ammonia with the optimum stripping rate of 1 L min1. The required time for reaching stable state was 4 days shorter than control, and the highest methane content (56.5e75.5%) was obtained. The ammonia stripping strategy maintained TAN level below the inhibition limit of 1.5 g/L throughout AD process. The maximum cumulative methane production of 192.3 L/kgvolatile solids (VS) was obtained, which was 2.3-fold the control. Therefore, simultaneous ammonia stripping overcame the ammonia inhibition on thermophilic AD of dairy manure at the threshold TS%. The proposed concept simplified the system by combining ammonia stripping and thermophilic AD within the same digester. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Simultaneous ammonia stripping High-solids thermophilic anaerobic digestion Methane production from manure Ammonia inhibition
1. Introduction Anaerobic digestion (AD) has been recommended as a primary process for treating livestock manure, because waste reduction, energy (biogas) production and mitigation of pollutant emissions (odor, greenhouse gases, and animal pathogens) can be all accomplished [1]. The increasing concerns about the environmental impact of animal feeding operations and the growing needs for renewable energy make AD an attractive alternative for managing animal manure [2]. Enhancing AD process performance is desirable for dropping the cost of operation. Temperature is a key factor affecting AD process
Abbreviations: AD, anaerobic digestion; SS-AD, solid-state anaerobic digestion; SSFW, source sorted food waste; TKN, Kjeldahl nitrogen; TAN, total ammonia nitrogen; NH3, free ammonia; S/I, substrate-to-inoculum; sCOD, soluble chemical oxygen demand; CK, no urea treatment; VFAs, volatile fatty acids; TS, total solids; VS, volatile solids. * Corresponding author. E-mail addresses: [email protected] (Y. Yao), [email protected] (S. Chen). https://doi.org/10.1016/j.energy.2017.09.086 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
performance. There are mainly three different types of AD processes based on the operating temperature: thermophilic (>45 C), mesophilic (20e40 C) and psychrophilic (<25 C). Thermophilic AD has many inherent advantages over mesophilic and psychrophilic AD, including faster reaction rate, higher biogas production, less foaming occurrence, enhanced pathogen reduction, and facilitated the decomposition of coarse fibers in dairy manure [3,4]. However, as is well known, temperature affects the threshold of ammonia inhibition [5], and thermophilic AD tends to be unstable in processing the nitrogen-rich wastes such as manure mainly due to the fact that increasing temperature results in more accumulation of ammonia and then the severe ammonia toxicity to microbes [6]. The effects of ammonia inhibition on microbial consortia are reported to have a pronounced impact in later stages of AD, including the activity of acetate-utilizing (acetoclastic) methanogens or hydrogen/formate-utilizing (hydrogenotrophic) methanogens, and the acetoclastic methanogens (Methanosaeta sp. and certain Methanosarcina sp.) which are considered to be most sensitive to ammonia [7]. In addition, the elevated ammonia accumulation releases overload free ammonia (NH3), which can cause
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intercellular pH variation, proton imbalance, potassium deficiency, and even cease the growth of methanogens by permeating the cells [8]. Most evidence has confirmed the high sensitivity of methanogens to NH3 [9]. Protein-rich materials are common sources for sulfide formation, which is not only toxic for microbial populations but also forms complexes with metals, resulting in decreased bioavailability of trace elements for microbial activity [7,10]. Various attempts have been made to overcome ammonia inhibition to maximize the specific methane production and enhance the process stability of thermophilic AD of nitrogen-rich materials. The simplest operation is to dilute the organic waste with water to avoid ammonia inhibition. Apart from dilution, chemical precipitation like magnesium ammonium phosphate (MAP) process [11], zeolite and clay process [12,13], and the addition of phosphorite ore [14] have been used. Biological processes like nitrification/denitrification [15] and bipolar bioelectrodialysis [16] have been studied as a way to reduce the ammonia inhibition. Methanogenic consortia acclimation to elevated ammonia levels was effective to improve ammonia tolerance for enhancing methane production [17]. The process of acclimation usually takes a long time and the methane production remains relatively low [18]. Gas-permeable membrane technology is also used to recover NH3 from liquid manures [19]. Removing ammonia from thermophilic AD process through stripping is thought to be a feasible approach, especially for treating organic wastes with high ammonia content, such as source sorted food waste (SSFW) digestate [20], chicken manure [21], and poultry litter leachate [22]. During the process, free ammonia is stripped out from the organic wastes and entered the gas phase, which caused the reduction of ammonia content in the liquid phase. Ammonia stripping generates no extra sludge, and the reagents cost is relatively low. Ammonia stripping has been proved to be useful in recovering ammonia from liquid waste such as AD effluent and poultry litter leachate [22,23]. Some studies have also reported the application of ammonia stripping for dehydrated waste of activated sludge and dairy manure with high TS% [18,24]. Conventional ammonia stripping requires NaOH, CaOH, or KOH addition to increase pH within 10e11 [25]. However, after ammonia stripping, AD could be inhibited by the toxicity of residual Naþ, Caþ, and Kþ [26]. 1.5, 2.5 and 2.5 g/L of Naþ, Caþ, and Kþ concentrations, respectively, are known to begin inhibition effect on microorganisms [27]. In addition, the chemicals addition can enhance the sludge precipitates, cause mechanical problems, and form pipe scale [28]. For these studies, ammonia stripping is deployed as a separate stage followed by thermophilic AD. A feasible option to efficiently treat nitrogen-rich materials with low cost, may be thermophilic AD coupled with ammonia stripping. The related studies are limited and only focused on chicken manure, piggery wastewater and poultry litter leachate so far [21,22,26]. However, there was no work done on dairy manure, so mechanism of the response of process performance and methane production to the stripping rate and the ammonia inhibition remain unclear. The characteristics of different nitrogen-rich materials are different, which influence the effectiveness of ammonia stripping. Therefore, it is necessary to investigate thermophilic AD of dairy manure coupled with ammonia stripping in order to enhance the knowledge base in the field of simultaneous ammonia stripping. In the present study, a thermophilic AD system coupled with ammonia stripping was proposed to treat dairy manure. Compared with conventional ammonia stripping, there are some advantages: first, ammonia stripping and thermophilic AD were realized in a single stage; second, in order to eliminate the negative effect of chemicals addition on microbial activity as aforementioned, no chemicals were added to adjust pH, so the feedstock remains
native; third, this proposed approach simultaneously eliminated the inhibitory effect on microbial activity caused by ammonia accumulation along with the thermophilic AD process, so this method can improve the economic feasibility and the applicability. Based on the above points, the threshold total solids concentration (TS%) that led to ammonia inhibition on thermophilic AD of dairy manure was firstly identified to examine the feasibility of simultaneous ammonia stripping on the removal of ammonia inhibition, and then the improvement of methane production was verified based on the threshold TS%. Additionally, the effect of stripping rate on the removal of ammonia inhibition, the process performance, and the methane production was investigated. The AD process was finally optimized to maximize the methane production. 2. Materials and methods 2.1. Feedstock and inoculum Dairy manure was collected from Washington State University Dairy Center in Pullman, WA, USA and stored at 4 C prior to use. The inoculum was sampled from a mesophilic anaerobic digester at Wastewater Treatment facility in Pullman, WA, USA. The characteristics of dairy manure and inoculum are shown in Table 1. 2.2. Anaerobic digestion design 2.2.1. Thermophilic anaerobic digestion of dairy manure at different high solids concentrations The required amount of dairy manure and inoculum for each digester were 400 g and 100 g, respectively, based on the wet weight. The TS and VS for each digester were 51.93 g and 42 g, respectively. Deionized water was added to match the requirement of certain TS%. At 6% and 8% TS, 365.5 g and 140 g of deionized water were used, correspondingly. At 10% TS, no deionized water was added. This test was conducted in batch mode at laboratory scale and the volume for each digester was 1 L. The headspace of digesters was flushed with nitrogen gas for about 5 min to obtain anaerobic condition, after which digesters were capped tightly with rubber stoppers and incubated at 55 C and shaken at a speed of 120 rpm [29,30]. Digestion experiments were conducted in triplicate for each condition. 2.2.2. Simultaneous ammonia stripping for thermophilic anaerobic digestion of dairy manure at the bottleneck solids concentration Simultaneous ammonia stripping was conducted in a 3.0 L reactor. The selected TS% for stripping was the bottleneck TS% obtained from the previous experiments. Nitrogen gas, as the stripping gas, was introduced into the liquid phase from the bottom of the reactor via an aquatic air stone, and it helped to stir the mixed liquor in the reactor. The soft pipe connected with the aquatic air stone was introduced through the rubber cap and connected to nitrogen gas cylinder. The nitrogen flow rate was controlled at 1.0 L min1, 3.0 L min1, and 5.0 L min1 by a flow meter. The flow Table 1 Characteristics of dairy manure and inoculum. Parameter
Dairy manure
Inoculum
Total solids (%) Volatile solids (% of total solids) pH Total carbon (%) Total nitrogen (%) Total hydrogen (%) Total ammonia nitrogen (g N/l)
12.5 ± 0.4 80.9 ± 0.1 8.4 ± 0.0 40.7 ± 0.1 1.8 ± 0.0 5.7 ± 0.0 1.4 ± 0.0
1.9 ± 0.0 74.4 ± 0.1 7.6 ± 0.0 35.5 ± 0.0 5.5 ± 0.0 5.3 ± 0.0 0.6 ± 0.0
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rate method of this study was accorded to previous studies [23]. Simultaneous ammonia stripping was done for 2 h each time every two days. The pH value for all experiments before AD was 8.1 without adjustment. Experiment with no simultaneous stripping was set as a control. The exhaust gas was passed through solutions of 50% (w/w) H2SO4 and 20% (w/w) sodium hydroxide to prevent the release of ammonia and other volatile compounds into the atmosphere [26] (Fig. 1). Other specifications of experiment were the same as mentioned above. 2.3. Analytical methods 2.3.1. Chemical analyses TS, VS, soluble chemical oxygen demand (sCOD), and pH were determined according to standard methods (APHA, 2005) [31]. TAN was measured according to Standard Methods using a Tecator 2300 Kjeltec Analyzer (Eden Prairie, MN, USA; 4500-NorgB; 4500NH3BC) [31]. An elemental analyzer (LECO) was used to measure total carbon, total nitrogen, and total hydrogen. 2.3.2. Biogas analyses Biogas production was measured by the method of water displacement every 2 days, the total volume of biogas was calculated after AD. Gas samples were collected every 2 days and stored in 12 ml vacuumed borosilicate vials (Extainer, Labco Limited, Wycombe, England). A gas chromatograph (GC) (GC, CP-3800, Varian Inc. Palo Alto, CA, USA) equipped with a thermal conductivity detector was used to analyze the methane content of biogas sample [32]. 2.4. Statistical analysis The software SPSS 19.0 was applied to determine the standard deviations and whether the observed differences between two or more groups of experimental results were significant. Differences were compared with a p value of 0.05. 3. Results and discussion 3.1. Thermophilic anaerobic digestion of dairy manure at different high solids concentrations 3.1.1. Process performance TS% had a significant effect on daily methane production. As illustrated in Fig. 2A, all the TS% had distinct peaks at the variable time point. The 6%, 8% and 10% TS, peaked at 13.8 L/kg-VS/d, 24.3 L/
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kg-VS/d, and L/kg-VS/d on the 14th day, 16th day, and 18th day, respectively. The level of daily methane production at 8% TS was higher than that at 6% TS. In addition, AD at 6% and 8% TS lasted until day 40. At 10% TS, on the other hand, the daily methane production after 20 days showed a sharp decrease and ceased on the 28th day. A previous study investigated the maximum TS% at which methanogenic activity was observed. Cabbage, carrot, and meat had a maximum 20% TS while sludge cake even had a higher maximum 43.4% TS [33], which was much higher than that obtained in the present study. Another study investigated high-solids AD of sludge and found methanogenic activity dropped to 53% from 100% when TS% was increased from 4% to 10%. The rate of methane production was stable at 4e10% TS in a pH range of 6.6e7.8 (optimum at pH 6.8) [34]. This result indicates that the capacity of digester for treating dairy manure cannot be enhanced efficiently by increasing TS%. The results can be used as an upper limit of TS% for testing the effect of simultaneous ammonia stripping on methane production and dairy manure treatment. TS% also had a significant effect on the start-up of AD (Fig. 2B). Methane content exceeds 50% means the achievement of stable state [35]. At 6%, 8% and 10% TS, methane content exceeded 50% on the 10th day, 10th day and 14th day, respectively. Methane content at 6%, 8% and 10% TS during the stable state were 51.8e63.5%, 55.7e69.2% and 57.4e64.8%, respectively. The time for reaching stable status at 10% TS was 4 days more than that at 6% and 8% TS. This result indicates that the start-up of thermophilic AD of dairy manure was prolonged by increased TS%, therefore, the efficiency of thermophilic AD was reduced. Total ammonia nitrogen (TAN) levels associated with TS% are presented in Fig. 3. 6% TS had the lowest TAN level (<1.2 g/L) throughout AD process, while an increase in TS% from 6% to 8% and further to10%, led to TAN levels increase substantially. TAN levels at 8% and 10% TS increased along with AD process and maintained its level around 1.8 g/L. In an AD system, fermentative bacteria, acetogenic bacteria, and methanogenic bacteria constitute the microbial consortium [36]. As reported previously, acidification and methanogenic stages can be negatively affected by the accumulation of free ammonia [4]. In a long-term study on pilot scale thermophilic AD of organic fraction of municipal solid waste (OFMSW), it was reported that ammonia inhibition occurred at 1.2 g/L TAN and pH of 7.2 [34]. Previously, thermophilic AD of dairy manure at 50 C with TAN level above 3 g/ L resulted in inhibition of AD process or reduced methane production [37]. In a wide pH range of 6.5e8.5, it has been observed that methanogenic activity decreased with an increase in TAN, the activity decreased by 10% at 1.67e3.72 g/L, 50% at 4.09e5.55 g/L and
Fig. 1. Schematic diagram of thermophilic anaerobic digestion coupled with the simultaneous ammonia stripping.
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Fig. 2. Process performances at different TS%. A: Daily methane productions; B: Methane contents.
100% at 5.88e6.6 g/L [34]. In the present study, NH3, as a portion of TAN, had much lower level than 1.2 g/L TAN at 6% TS and even lower than previous reports. Therefore, there was likely no ammonia inhibition at 6% TS, while at 8% and 10% TS, TAN level was 1.8 g/L, which was in the range of 1.67e3.72 g/L [34], suggesting slight ammonia inhibition. That is why the peaks of daily methane productions at 8% and 10% TS appeared later than that at 6% TS. At 10% TS, TAN level was the highest within 1.8e2.4 g/L after 8 days, so the AD process was inhibited severely and even ceased on the 28th day; the strong ammonia inhibition also took more time to reach stable state compared to that at 6% and 8% TS (Fig. 2B). pH of AD system is usually used to indicate the process stability And the pH level is associated with VFAs level [38]. The acidic environment was caused by the accumulation of VFAs, which led to the pH drop [39]. Therefore, low pH was associated with high VFAs level and vice versa. As a result, pH can be used for indicating the VFAs level [38,40]. As shown in Fig. 4, in the initial period of AD, a decrease of pH can be attributed to the acidification in the digestion system [4]. The accumulation of acidic products usually occurs in the initial period, such as acetate [21], and leads to pH decrease. The decrease of pH can inhibit the efficiency of ammonia fermentation [21]. The pH increased later and maintained a high level especially at 10% TS, which was related with the high TAN level (Fig. 3), the
amount of free ammonia significantly depends on pH. Therefore, this result confirms the ammonia inhibition. The nitrogen content in the substrate is actually the governing factor instead of TS%. Effect of ammonia inhibition can be intensified at higher TS than 5% when chicken manure was used as a substrate of AD [21]. However, in the present study, the inhibitory effect did not occur at 6% TS, but at higher TS than 8%. This is because of the difference of TAN content between chicken manure and dairy manure. The TAN content in chicken manure (approximately 3.7 g/L) is higher than that of dairy manure (1.4 g/L-Table 1) [41,42]. 3.1.2. Cumulative methane production Cumulative methane production was significantly affected by TS % (Fig. 5). The maximum cumulative methane production of 161.7 L/ kg-VS was achieved at 8% TS and day 40, which was 1.7-fold (p < 0.05) and 2.0-fold (p < 0.05) those at 6% and 10% TS, respectively. Cumulative methane productions at 6% and 10% TS were 93.9 L/kg-VS and 80.4 L/kg-VS, respectively, which were obtained at day-40 and day-28, respectively. At 10% TS, cumulative methane production was even lower than that at 6% TS. This is due to the serious ammonia inhibition on the activity of methanogenesis. As mentioned above, there was no ammonia inhibition at 6% TS, but
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Fig. 2. (continued).
the cumulative methane production was less than that at 8% TS. This might be the lower content of organic matter at 6% TS than that at 8% TS. In summary, severe ammonia inhibition on thermophilic AD of dairy manure occurred at 10% TS, the total methane production under this condition was the least. This established the threshold TS % that can be used for testing simultaneous ammonia stripping.
3.2. Simultaneous ammonia stripping for thermophilic anaerobic digestion of dairy manure at the threshold solids concentration (10%) 3.2.1. Process performance at different stripping rates Simultaneous ammonia stripping at 1 L min1 was beneficial for the enhancement of AD process performance (Fig. 6A). Levels of daily methane productions at different gas flow rates can be ranked as 1 L min1> 3 L min1> control> 5 L min1, so the highest level of daily methane production was obtained at 1 L min1. Distinct peaks were observed at 1 L min1 and 3 L min1, with methane production of 23.6 L/kg-VS/d and 21.3 L/kg-VS/d, respectively. For control, the highest peak appeared on the 14th day, which was no more than 15.0 L/kg-VS/d and the time for the peak appeared was 2 days later than those at 1 L min1 and 3 L min1. The results indicate that simultaneous ammonia stripping was beneficial for improving
thermophilic AD. However, at 5 L min1, the level of daily methane production was the lowest. The stripping rate is important for thermophilic AD of substrates with high nitrogen content. High stripping rate has some disadvantages such as water evaporation, foaming, cooling of the substrate [43]. Lost of organic materials due to high stripping rate may also contribute to the low level of daily methane production [26]. The low level daily methane production resulted from the high stripping rate, may be also caused by the low efficiency of mass transfer. In methanogenic digester containing waste water, aggregates tend to be formed. First, aggregation can enhance the exchange of formate and hydrogen between syntrophic microbes [44]; second, aggregation is beneficial for cells to directly exchange electrons [45]. Therefore, the high stripping rate may disrupt energy and nutrient transfer between microbes. For the delay of appeared peak for control, as previously reported, the process of spontaneous acclimation of methanogenic consortia to a high level of ammonia took quite a long time; the methane production from chicken manure was low, and this phenomenon has been well demonstrated [18]. This result indicates that stripping rate at 1 L min1 improved the level of daily methane production. Trends of methane content for control, 1 L min1 and 3 L min1 were similar (Fig. 6B). For control, 1 L min1, 3 L min1 and 5 L min1, methane content reached 50% on the 10th day, 6th day, 6th day and 8th day, respectively. Therefore, at 1 L min1 and
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Fig. 3. TAN concentrations at different TS%.
3 L min1, time lasted for reaching the stable state was the shortest, which was 4 days shorter than the control. Similar to daily methane production, methane content at 1 L min1 was the maximum (56.5e75.5%) compared to other samples. Heavy fluctuation of methane content throughout AD process appeared at 5 L min1 and the level was the minimum (44.5e61%). As a result, high stripping rate had a negative effect on AD process performance. Methane contents for the control and 3 L min1 were 55.9e65.1% and 61.9e68.5%, respectively. Similar to daily methane production, the methane contents ranked as: 1 L min1> 3 L min1> control> 5 L min1. These results indicate that stripping rate at 1 L min1 reduced the reaching stable state time and improved the methane content. 3.2.2. sCOD (soluble chemical oxygen demand), pH and TAN changes at different stripping rates Simultaneous ammonia stripping at 1 L min1 was beneficial for the utilization of available substrate and the removal of ammonia inhibition. sCOD was used as an indicator to examine the available substrate level for microorganisms in the AD system. For all experiments, sCOD content experienced initial increase followed by a slow decrease (Fig. 7A). In the initial period of AD, the readily available COD for anaerobic microbes was limited. However, particulate matter or complex particulate compounds were increasingly converted into the soluble organic substrate along with the acclimation and breeding of hydrolytic bacterium [38], which resulted in the high level of sCOD content and then the high level of daily methane productions. This result is in good agreement with a previous study [46]. When most of the sCOD was utilized by
microbes, daily methane productions decreased accordingly. For control, the peak appeared on the 8th day, which was longer than those of the other experiments. According to Abouelenien et al.’s study, the microbes that responsible for decomposing substrate were inhibited by the high level of ammonia (Fig. 7B) [18]. However, after a long time of acclimation of microbes to the high level of ammonia, an increase of sCOD release occurred, and the amount of sCOD was really significant. This can be attributed to the ammonia inhibition on methanogenesis, which led to the accumulation of available substrate that cannot be utilized by methanogens for methane production. However, sCOD level at 1 L min1 had the smallest amount, considering the corresponding highest level of daily methane production, suggesting microbes at 1 L min1 consumed the available substrate throughout the process of AD in time, so there was no significant accumulation of available substrate. At 3 L min1 and 5 L min1, the mass transfer was lower than that at 1 L min1 as aforementioned. That is to say, at 3 L min1 and 5 L min1, the available substrate/sCOD content was higher than that at 1 L min1. As illustrated in Fig. 7C, trends of pH changes for control, 1 L min1, 3 L min1 and 5 L min1 were similar, which initially decreased followed by a slow increase along with the increase of ammonia release and then maintained a stable level. The pH values were 6.45e8.55, which was within the suitable pH range of 6.5e8.5 for AD [34]. In general, animal manure has higher moisture, nutrient, salt contents, and pH buffer capacity compared to food wastes, which is beneficial for maintaining pH constant in anaerobic digester [47]. As aforementioned, the decline in pH resulted from the accumulation of acidic products (VFAs) reduces the
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Fig. 4. pH values at different TS%.
Fig. 5. Cumulative methane productions at different TS%.
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Fig. 6. Process performances for control and experiments with simultaneous ammonia stripping. A: Daily methane productions; B: Methane contents.
efficiency of ammonia fermentation [21]. Therefore, the TAN level in the initial period increased slowly and maintained a relatively stable state (Fig. 7B). The initial low pH level resulted from the hydrolysis of dairy manure that functioned as a rate limiting step
contributed to the initial low methane production [38]. After a period of adaptation and breeding of methanogens, acidic products were further utilized for methane production, this was in agreement with a previous study [38], and the pH increased accordingly
Fig. 7. sCOD, TAN and pH changes for control and experiments with simultaneous ammonia stripping. A: sCOD concentrations; B: TAN concentrations; C: pH changes.
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(Fig. 7C). The increase of pH is beneficial for the increase of TAN [48]. As a result, TAN level for control increased relatively quickly from 10th day on (Fig. 7B). During days 14e20, the TAN level was the highest, which was in line with that of pH (Fig. 7C). Values of TAN for control was 1.5e2.1 g/L within the range of 1.5e3.0 g/L. However, for the experiments with simultaneous ammonia stripping, TAN levels were lower than 1.5e3.0 g/L, so there was unlikely ammonia inhibition occurred. However, TAN level at 1 L min1 was higher than those at 3 L min1 and 5 L min1. This could be attributed to the higher stripping rate (3 L min1 and 5 L min1) that tripped the produced ammonia out of reactor more quickly, so the TAN level was lower compared with that at 1 L min1. Efficiency of ammonia removal were 53.5%, 44.4% and 64.4% at 1 L min1, 3 L min1, and 5 L min1 respectively. Methane production at 1 L min1 was the uppermost, suggesting that microbes activity at 1 L min1 was intensified, so the microbes could ferment substrate efficiently for ammonia formation, the produced ammonia could be thereby stripped out in time. Therefore, the efficiency of ammonia removal was high. However, at 5 L min1, the mass loss was the too much in comparison with other samples as mentioned above, this may be the reason for the highest ammonia removal. Thus, the lowest ammonia removal happened at 3 L min1. Comparatively, the efficiency of ammonia removal for previous studies was higher than this study, 88.2% ammonia removal was obtained at fermentation conditions of ammonia stripping for 12 h and higher pH (9.0, 9.5, 10.0, and 11.0) [26]. 78% ammonia removal was obtained at pH 12 by Lei et al., who applied ammonia stripping for AD effluent [23]. 75e95% ammonia removal was reached by applying ammonia stripping for poultry leachate at 1e2% TS. There are some reasons responsible for the lower efficiency of ammonia removal obtained in present study (44.4e64.4%) compared to traditional ammonia stripping: first, higher pH is usually used for only ammonia stripping process without AD [26]; second, TS% is usually lower than what we used in this research, which allows better gas-liquid association to trap more ammonia due to higher fluidity [21]; third, higher temperature was used for the separate ammonia stripping process [24]. However, high pH (9.0), high temperature (>60 C), and low TS % are not economically favorable from the point of energy supply and cost, and only suitable for the ammonia stripping as a separate stage rather than the AD coupled with ammonia stripping. 3.2.3. Cumulative methane production at different stripping rates The maximum cumulative methane production was obtained at 1 L min1. Cumulative methane production for control, 1 L min1, 3 L min1 and 5 L min1 were 84.7 L/kg-VS, 192.3 L/kg-VS, 168.8 L/ kg-VS, and 68.5 L/kg-VS respectively. The maximum cumulative methane production was obtained at 1 L min1, which was 2.3-fold, 1.1-fold, and 2.8-fold those of control, 3 L min1, and 5 L min1, respectively (Fig. 8), so the improvement of cumulative methane production was significant (p < 0.05). The maximum cumulative methane production was much higher than that obtained by Zhang et al. (2012) (p < 0.05), who used the ammonia-stripped piggery wastewater for batch bio-methanolization and obtained the
187
Fig. 8. Cumulative methane productions for control and experiments with simultaneous ammonia stripping.
maximum methane production (128 L/kg VS) at conditions of 4.0 L min1 air stripped piggery wastewater, pH 9.0, and 37 C of incubation [26]. Mixture of ammonia stripped chicken manure and raw chicken manure in the ratio of 1:1 was used for thermophilic AD coupled with ammonia stripping. The initial pH and temperature were 8 and 55 C, respectively, the stripping rate was 4.0 L min1, and 157 L/kg VS of methane production was obtained [21], which was lower than the maximum cumulative methane production in present study. Therefore, dairy manure could be used for efficient methane production via simultaneous ammonia stripping at an equivalent level to 1 L min1. 3.2.4. Substrate degradations at different stripping rates The highest level of substrate utilization was obtained at 1 L min1. TS and VS reductions were calculated to evaluate the biodegradability of the substrate and the efficiency of methane production at different stripping rates. As shown in Table 2, the highest reductions of TS and VS were obtained at 1 L min1, which were 34.4% and 41.3%, respectively, and were in line with the maximum cumulative methane production. Compared to control, enhancements of TS and VS reductions were 251.0% and 250.0%, respectively. In general, higher substrate degradation was associated with higher methane production. However, in the present study, cumulative methane production for control was higher than that at 5 L min1, while TS and VS reductions for control were lower than that at 5 L min1. This might be due to the high stripping rate that led to the loss of organic substrate as aforementioned. It can be inferred that stripping rate of 1 L min1 was beneficial for the removal of ammonia inhibition and the assimilation of organic matters via anaerobic flora, which directed to the high efficiency of substrate degradation. This result indicates stripping rate at 1 L min-11 was beneficial for thermophilic AD of dairy manure at high TS%.
Table 2 Total solids and volatile solids quantities before and after anaerobic digestion and degradations.
Before anaerobic digestion (g) After anaerobic digestion (g) Degradation (%)
Composition
Control
1 L min1
3 L min1
5 L min1
Total solids Volatile solids Total solids Volatile solids Total solids Volatile solids
52.1 42.0 46.9 ± 1.1 37.0 ± 5.7 9.8 ± 4.2 11.8 ± 2.8
52.1 42.0 34.1 24.7 34.4 41.3
52.1 42.0 36.9 27.3 29.1 34.9
52.1 42.0 46.2 36.5 11.1 13.1
± ± ± ±
2.5 1.8 1.1 1.9
± ± ± ±
1.1 4.7 3.9 4.7
± ± ± ±
6.3 4.2 7.3 2.4
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4. Conclusions Inhibition of thermophilic AD of dairy manure due to ammonia accumulation occurred at 10% TS as a threshold TS%. The simultaneous ammonia stripping helped to overcome the ammonia inhibition. The optimum stripping rate of 1 L min1 was identified and the maximum methane production of 192.3 L/kg-VS was achieved. Overall, this approach has potential to efficiently utilize dairy manure for methane production. The combination of different stages into one digester makes this process simpler and more cost effective than other reported approaches. In addition, simultaneous ammonia stripping at 55 C instead of higher temperature and with no pH adjustment further reduces the process cost. Therefore, dairy manure could be used for efficient thermophilic AD for methane production via simultaneous ammonia stripping at a low stripping rate of 1 L min1. Acknowledgments We are thankful to Washington State University (WSU) for the equipment support. References [1] Banks C, Salter A, Chesshire M. Potential of anaerobic digestion for mitigation of greenhouse gas emissions and production of renewable energy from agriculture: barriers and incentives to widespread adoption in Europe. Water Sci Technol 2007;55(10):165e73. [2] Salminen E, Rintala J. Anaerobic digestion of organic solid poultry slaughterhouse wasteea review. Bioresour Technol 2002;83(1):13e26. [3] Krishania M, Kumar V, Vijay VK, Malik A. Analysis of different techniques used for improvement of biomethanation process: a review. Fuel 2013;106:1e9. [4] Nasir IM, Mohd Ghazi TI, Omar R. Anaerobic digestion technology in livestock manure treatment for biogas production: a review. Eng Life Sci 2012;12(3): 258e69. DI, Rajagopal R, Singh G. Technical and operational feasibility of psy[5] Masse chrophilic anaerobic digestion biotechnology for processing ammonia-rich waste. Appl Energy 2014;120:49e55. [6] Liao X, Li H. Biogas production from low-organic-content sludge using a highsolids anaerobic digester with improved agitation. Appl Energy 2015;148: 252e9. [7] Westerholm M, Moestedt J, Schnürer A. Biogas production through syntrophic acetate oxidation and deliberate operating strategies for improved digester performance. Appl Energy 2016;179:124e35. [8] Warren KS. Ammonia toxicity and pH. Nature 1962;195(4836):47e9. ski J, Weatherley L. Anaerobic co-digestion of swine and [9] Borowski S, Doman poultry manure with municipal sewage sludge. Waste Manag 2014;34(2): 513e21. €rn A, Skyllberg U. Thermodynamic modeling of iron [10] Yekta SS, Svensson BH, Bjo and trace metal solubility and speciation under sulfidic and ferruginous conditions in full scale continuous stirred tank biogas reactors. Appl Geochem 2014;47:61e73. [11] Demeestere K, Smet E, Van Langenhove H, Galbacs Z. Optimalisation of magnesium ammonium phosphate precipitation and its applicability to the removal of ammonium. Environ Technol 2001;22(12):1419e28. n Z, Sa nchez E, Weiland P, Borja R, Martı A, Ilangovan K. Influence of [12] Mila different natural zeolite concentrations on the anaerobic digestion of piggery waste. Bioresour Technol 2001;80(1):37e43. [13] Tada C, Yang Y, Hanaoka T, Sonoda A, Ooi K, Sawayama S. Effect of natural zeolite on methane production for anaerobic digestion of ammonium rich organic sludge. Bioresour Technol 2005;96(4):459e64. [14] Krylova NI, Khabiboulline RE, Naumova RP, Nagel MA. The influence of ammonium and methods for removal during the anaerobic treatment of poultry manure. J Chem Technol Biotechnol 1997;70(1):99e105. [15] Dong X, Tollner EW. Evaluation of Anammox and denitrification during anaerobic digestion of poultry manure. Bioresour Technol 2003;86(2): 139e45. [16] Zhang Y, Angelidaki I. Recovery of ammonia and sulfate from waste streams and bioenergy production via bipolar bioelectrodialysis. Water Res 2015;85: 177e84. [17] Abouelenien F, Kitamura Y, Nishio N, Nakashimada Y. Dry anaerobic ammoniaemethane production from chicken manure. Appl Microbiol Biotechnol 2009;82(4):757e64. [18] Abouelenien F, Nakashimada Y, Nishio N. Dry mesophilic fermentation of chicken manure for production of methane by repeated batch culture. J Biosci Bioeng 2009;107(3):293e5.
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