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Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry
Journal of Cleaner Production xxx (xxxx) xxx
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Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry Panpan Wang a, b, Xin Zhang a, b, Shaban G. Gouda a, c, Qiaoxia Yuan a, b, * a
College of Engineering, Huazhong Agricultural University, Wuhan, 430070, China Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture and Rural Affairs, Wuhan, 430070, China c Agricultural and Biosystems Engineering Department, Faculty of Agriculture, Benha University, Benha, 13736, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2019 Received in revised form 14 October 2019 Accepted 2 November 2019 Available online xxx
Biogas slurry, the by-product of anaerobic digestion, represents 90%e95% of the input feed for digesters. Treatments of biogas slurry to reduce its volume and add value to it are difficult and costly. This study used the humidification-dehumidification process to concentrate biogas slurry and find a more economical and feasible method to provide appropriate conditions for sustainable utilization of biogas slurry. The effect of heating temperature, heating time, air flow rate and initial pH on the efficiency of water removal and nutrients (ammonia nitrogen, total phosphorus, chemical oxygen demand, soluble salt) recovery was investigated. The results showed that the humidification-dehumidification system could be effectively used to concentrate the biogas slurry. The recovery of ammonia nitrogen, total phosphorus, and soluble salt in the condensed phase reached 96% when the initial pH of biogas slurry was 6, the heating temperature was 70 C, the air flow rate was 10 L/min, and the heating time was 30 min. Under these conditions, the mass concentrations of ammonia nitrogen, total phosphorus, chemical oxygen demand, and electrical conductivity in the dilute phase were 157.49 mg/L, 0.66 mg/L, 8.70 mg/L, and 0.55 mS/cm, respectively, which was far below the mass concentrations of them in raw biogas slurry. Water removal efficiency of 34.12% and ammonia nitrogen recovery of 98.04% were obtained by the response surface methodology optimization when the heating temperature was 61.92 C, heating time was 48.54 min, air flow rate was 10 L/min, and initial pH was 4.80. Treatment of biogas slurry by the humidification-dehumidification system can result in a higher efficiency of water removal and nutrient recovery in a relatively short time. Moreover, the simple process, low operation cost and requirement for influent quality is conducive to engineering applications of the humidificationdehumidification process and the value-added utilization of biogas slurry. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Prof. S Alwi Keywords: Biogas slurry Concentration Humidification-dehumidification Water removal efficiency Nutrient recovery
1. Introduction The output of agricultural waste in rural China is increasing with an annual average rate of 5%e10%, which wastes resources and pollutes the environment if not effectively treated (Wu et al., 2017b). Anaerobic digestion, or biogas production, is an attractive approach for the utilization of agricultural waste resources (Wang et al., 2018). It can produce clean energy while dealing with agricultural waste, which is of great significance for alleviating the energy crisis and controlling environmental pollution (Ni et al., 2017). However, biogas plants produce a large amount of biogas
* Corresponding author. College of Engineering, Huazhong Agricultural University, Wuhan, 430070, China. E-mail address: [email protected] (Q. Yuan).
slurry and biogas residue. Biogas residue is mainly composed of microbial biomass and its residues as well as non-fermented feedstock leftovers; it can improve crop growth and replace other organic fertilizers (Coban et al., 2015). Biogas slurry is increasingly used as fertilizer in organic farming systems (Wentzel et al., 2015). However, farmland has limited carrying capacity and cannot continuously receive nutrient from the biogas slurry; excessive nutrient will run into water bodies and cause serious eutrophication issues (Knight et al., 2000; Wang et al., 2017). In some areas of China, the distance between the biogas plant and the farmland receiving the biogas slurry is too far to economically justify the transportation cost, which also limits the farmland utilization of biogas slurry (Han et al., 2015). It is necessary to reduce the volume of biogas slurry, and convert it into high-quality liquid fertilizer for easy transportation and effective utilization.
https://doi.org/10.1016/j.jclepro.2019.119142 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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There is a growing interest in membrane technology for biogas slurry concentration because of its simple process, lower nutrient loss, unchanged composition characteristics, and easy recovery and utilization of wastewater (Waeger et al., 2010; Xia and Murphy, 2016; Xu et al., 2016). Researchers have used various membrane peztechnologies to concentrate biogas slurry: ultrafiltration (Lo ndez et al., 2011), nanofiltration (Konieczny et al., 2011), Ferna reverse osmosis (Gong et al., 2013), and hybrid membrane processes (Ruan et al., 2015). However, some problems such as high energy consumption, high cost, and membrane fouling limit the application of membrane technology in the field of biogas slurry concentration (Li et al., 2018; Lu et al., 2017). Recently, the use of vacuum concentration technology in biogas slurry concentration has also been reported. Deng (2014) discovered a good concentration effect on chemical oxygen demand (COD) and total phosphor (TP) in biogas slurry when the evaporation absolute pressure was 0.01 MPa and evaporation temperature was 44 C (boiling point) with the evaporation time of 1 h and concentration rate of 20%. However, the poor concentration effect of ammonia nitrogen (NH3eN) (Huang et al., 2011) and the high requirement of vacuum (Song et al., 2017) make it difficult to apply vacuum concentration technology on a large scale to concentrate biogas slurry. It is necessary to choose an appropriate method to concentrate biogas slurry with low energy consumption and high efficiency of concentration and nutrient recovery. In recent years, the seawater desalination system based on humidification-dehumidification (HDH) technology has gradually attracted public attention due to its simple process, convenient operation, low investment and operating costs, and ease of use with low-grade heat sources such as solar energy, geothermal, wind, or their combinations to reduce energy consumption (Muthusamy and Srithar, 2017; Rahimi-Ahar et al., 2018; Wu et al., 2017a). The HDH system is a distillation technology that uses a carrier fluid such as air or water to obtain thermal energy from a heat source, which is then transferred to a humidifier for water evaporation from saline water and then to the dehumidifier for condensation of the evaporated water to fresh water, so that the saline water is concentrated (Giwa et al., 2016a; Gu, 2013). Yuan et al. (2011) investigated a 1000 L/d solar HDH system. The system was composed of a 100 m2 solar air heater field, a 12 m2 solar water collector, an HDH unit, pre-treatment and post-treatment systems and other sub-systems. They found that water production from the system reached 1200 L/d, when the average intensity of solar radiation was 550 W/m2 and the heating water temperature was below 50 C. Zhao et al. (2019) studied a novel four-stage cross flow HDH solar desalination system and established the heat and mass exchange models of the main parts. The results showed that when seawater was 83 C, spraying quantity was 1.1 t/h and air flow rate was 300 m3/h, the water yields of the HDH system with 560 m2 total heat transfer area was 63 kg/h. In this study, the HDH process was used to concentrate biogas slurry. The effects of heating temperature, heating time, and air flow rate on the efficiency of water removal and nutrient recovery of biogas slurry were studied. The migration of nutrient from the biogas slurry under different initial pH was also explored. Response surface methodology (RSM) was carried out to analyze the optimal combination of biogas slurry concentration. Based on the optimal combination, the operation cost was calculated. These results provide a basis for the application of the HDH process in biogas slurry concentration. 2. Materials and methods The biogas slurry was obtained from an anaerobic digester in Ezhou, Hubei, China. The capacity of the anaerobic digester was
500 m3. The digester used pig manure as the main raw material and a small amount of domestic sewage was added to ferment at a mesophilic temperature (35 C) in the continuous stirred tank reactor (CSTR). The hydraulic retention time (HRT) was 15 d, the organic load rate (OLR) was 2 kg/(m3$d), and the volume of biogas product was 400 m3/d. The collected biogas slurry was stored anaerobically at ambient temperature (20 ± 5 C) prior to experiments until no biogas was produced. The biogas slurry was centrifuged (TSZ5-WS, Hunan Xiangyi Centrifuge Instrument Co., Ltd.) at 4000 r/min for 10 min and then the supernatant was sealed and stored in a refrigerator at 4 C until it was used for testing. The physicochemical properties of the biogas slurry after centrifugation are shown in Table 1.
2.1. Experimental procedure Fig. 1 shows the schematic diagram of the HDH system to concentrate biogas slurry. Limited by the experiment condition, a rotary evaporator (RV 10 basic, IKA, Germany) was used as the main tool to simulate the process of HDH, and a vacuum pump was used as the driving force (Fig. 2). First, the biogas slurry (200 mL) was added to the rotary evaporation vessel (humidifier) with a volume of 1 L and heated in a water bath. The charge aperture and the vacuum pump open to start the test when the water bath temperature reaches the set value. The biogas slurry in the rotary evaporation vessel was heated to generate water vapor during the evaporation process; at the same time the dry cold air outside entered the rotary evaporation vessel through the charge aperture driven by the vacuum pump, and was humidified by the steam. The hot humid air with a large amount of steam was condensed into liquid water in the condensation tube (dehumidified) and passed to the balloon flask. The liquid in the balloon flask is the dilute phase, and the remaining solution in the rotary evaporation vessel is the condensed phase. The condensed phase and the dilute phase after evaporation were collected for analysis.
2.2. Experimental design In this study, single factor tests were carried out to study the effect of heating temperature, heating time, and air flow rate on the concentration efficiency of biogas slurry. In order to make the experiment results more obvious, the maximum air flow rate of 10 L/min and the longer heating time of 30 min (when the heating time exceeded 30 min, the concentration efficiency was too high and almost all of the biogas slurry was evaporated at 90 C) were selected to study the effect of heating temperatures (50, 60, 70, 80, and 90 C) on the concentration efficiency of biogas slurry. Similarly, the maximum air flow rate of 10 L/min was chosen to study the effect of heating time (10, 20, 30, 40, 60, 90, 120, 150, 180, and 240 min) on the concentration efficiency of biogas slurry at different heating temperatures (50, 60, 70, 80, and 90 C). The effect of air flow rate (6, 7, 8, 9, and 10 L/min) on the concentration efficiency of biogas slurry at different heating temperatures was also studied. Moreover, the changes of main nutrient in the condensed phase and dilute phase after the HDH process were investigated. Migration of the main nutrient of the biogas slurry under different initial pH (4, 5, 6, 7, and 8, adjusted by sulfuric acid) was investigated. RSM was used to comprehensively analyze the effect of the HDH process on the concentration efficiency and nutrient recovery of biogas slurry. Finally, cost analysis of the HDH process was carried out. The experiment was carried out at room temperature of 25 C, humidity of 50%, and absolute pressure of 0.1 MPa with three repetitions.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Table 1 The basic physicochemical properties of biogas slurry after centrifugation. Parameters
pH
NH3eN (mg/L)
TP (mg/L)
COD (mg/L)
EC (mS/cm)
TS (mg/L)
VS (mg/L)
Value
8.46 ± 0.06
1715.94 ± 11.16
35.17 ± 0.58
3208.17 ± 29.60
17.55 ± 0.18
4213.27 ± 58.45
2938.84 ± 32.16
Notes: NH3eN is ammonia-nitrogen, TP is total phosphorus, COD is chemical oxygen demand, EC is electrical conductivity, TS is total solid, VS is volatile solid.
Fig. 1. Schematic diagram of the HDH system.
Fig. 2. Schematic diagram of biogas slurry HDH concentration device.
2.3. Chemical analysis There are many indicators to determine the nutrient recovery efficiency, including NH3eN, TP, COD, and EC (electrical conductivity). NH3eN and TP were tested using a fully automatic intermittent chemical analyzer (SmartChem200, AMS, Italy). The COD was measured by CM-03 portable COD water quality meter (Beijing
Shuanghui Jingcheng Electronic Products Co., Ltd.). The mass concentration of total solids (TS) and volatile solid (VS) was measured by gravimetric analysis. The pH and EC were measured using a pH meter (FE28, Metler, Switzerland) and an electrical conductivity meter (DDS-307A, Shanghai Instrument and Electrical Science Instrument Co., Ltd.).
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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2.4. Data analysis The concentration efficiency and nutrient recovery efficiencies were the main test indicators. The concentration efficiency is reflected by the water removal (WR) efficiency of biogas slurry, which can be calculated as follows:
WR ¼ ðVo Vi Þ=Vo 100%
(1)
where WR is the water removal efficiency (%), Vi is the volume of biogas slurry at time ti (mL), and V0 is the initial volume of biogas slurry (mL). The nutrient recovery efficiency (NR) of NH3eN, TP, COD, and EC in the condensed phase can be calculated using Eq. (2):
NR ¼ Ci Vi =Co Vo 100%
(2)
where NR is the nutrient recovery efficiency (%), and Ci and C0 are the mass concentrations (mg/L) of NH3eN, TP, COD, and the value of EC (mS/cm) in the condensed phase and stock solution of biogas slurry. One-way ANOVA and Duncan’s multiple comparison tests (P < 0.05, IBM SPSS statistics 25) were used to test the statistical differences among the related parameters on nutrient recovery in the condensed phase and the mass concentration in the dilute phase. RSM of Box-Behnken (BBD) was used to optimize the process parameters with Design-Expert version V8.0.6. 3. Results and discussion The concentration efficiency of biogas slurry and nutrient changes in dilute and condensed phases at different heating temperatures, heating times, and air flow rates after processing with the HDH system have been presented in the following sections. 3.1. Effect of heating temperature, heating time and air flow rate on the concentration efficiency of biogas slurry Fig. 3 shows the effect of heating temperature, heating time, and air flow rate on the water removal (WR) efficiency. The results showed that under the conditions of the HDH process the WR efficiency increased with increased heating temperature. The WR efficiency increased from 12% to 53.49% when the temperature increased from 50 to 90 C. The volume of the dilute phase produced by the HDH was higher (19.98, 7.55, and 6.37 times) than that obtained under natural evaporation at the same temperature (50, 70, and 90 C). Under HDH conditions, the air humidity was increased to near saturation in the humidifier (Giwa et al., 2016b), and the saturated moist air was then fed to the dehumidifier where the moisture was partially condensed and collected. Increasing the heating temperature increased the saturated vapor pressure of the air in the humidifier, and the vapor parameters near the interface surface influenced the interface layers of the condensed phase (Shishkova et al., 2019). This resulted in more water vapor being carried into the dehumidifier by airflow and accelerated the evaporation process (Rajaseenivasan and Srithar, 2017). Compared with natural evaporation, the method of biogas slurry HDH concentration achieved better concentration efficiency at a lower temperature. The influence of heating time on the WR efficiency of biogas slurry at different heating temperatures is shown in Fig. 3 (2). The WR efficiency increased with the increase of heating time at different temperatures. It increased rapidly and reached the highest values in a short time, where the maximum WR efficiencies were achieved at times of 40, 60, and 90 min with corresponding heating
Fig. 3. The water removal efficiency of biogas slurry at different heating temperatures, heating time and air flow rates. (1) Natural evaporation (0 L/min) and HDH (10 L/min), heating temperature: 70 C, heating time: 30 min (2) Air flow rate: 10 L/min. (3) Heating time: 30 min.
temperatures of 90, 80, and 70 C, and maximum WR efficiencies of 79.17%, 84%, and 84.67%. In contrast, the maximum values of WR efficiency were obtained over longer time periods of 150 and 240 min with corresponding temperatures of 60 and 50 C, and the maximum WR efficiencies were 90.83% and 96.17%. Linear fitting was the suitable form to represent the relationship between the WR efficiency and heating time for different heating temperatures. The linear fitting equations and the correlation coefficients are listed in Table 2. These equations had good correlations, especially when the heating temperatures were 50, 60, and 70 C. Fig. 3 (3) shows the changes of WR efficiency of biogas slurry at different air flow rates. The air flow rates had a positive influence on the concentration of biogas slurry at different temperatures, which
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
P. Wang et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 2 The linear fitting equations and the correlation coefficients for the relationship between WR efficiency of biogas slurry and heating time processes at different heating temperatures. Temperature ( C)
Linear fitting equation
correlation coefficient (R2)
50 60 70 80 90
y ¼ 0.404 y ¼ 0.598 y ¼ 0.932 y ¼ 1.381 y ¼ 1.868
0.999 0.999 0.999 0.996 0.982
6t þ 0.403 7 1tþ 0.770 6 9t þ 0.516 2 8t þ 0.950 5 2t þ 2.083 3
4 3 1 4 0
Note: y is WR efficiency (%) and t is time (min).
was consistent with the results of (Dai and Zhang, 2000) and (Shafii et al., 2018). The main mechanism in HDH process was convective evaporation, larger air flow rate resulted in greater convective evaporation (Al-Enezi et al., 2006). The main reason for this result was due to the partial pressure of water vapor in the rotary evaporation vessel, which was higher than the pressure of outside air. The driving force for evaporation was the vapor pressure difference between the air and the biogas slurry, which led to the movement of water molecules from the region with high partial pressure of water vapor to the region with low partial pressure (Jain et al., 2011).
3.2. Nutrient changes in dilute and condensed phases at different temperatures, heating times, and air flow rates after processing with the HDH system The variation in nutrient in the two phases (condensed phase and dilute phase) at different temperatures and 8.46 initial pH after HDH is shown in Fig. 4. Fig. 4 (1) shows that the recovery of NH3eN in the condensed phase decreased with the increase of heating temperature, which reached 59.36% at 50 C, 51.37% at 60 C, and 45.19% at 70 C. However, increasing the heating temperature to 80 C sharply decreased NH3eN in the concentration of 425.37 mg/ L with a 15% recovery rate. This can be clearly seen at 90 C heating temperature, where the concentration of NH3eN was only 145.17 mg/L with a recovery rate of 3.93%. Higher temperature reduced evaporation time and allowed a higher WR efficiency, but was also causing a higher flowrate of NH3 emissions (Awiszus et al., 2018). This occurred because the nitrogen in the biogas slurry generally existed in the form of free ammonia (NH3) and ammonium (NHþ4), and the NHþ4 was more stable, while the NH3 escaped more easily with the evaporated water molecules. The higher the temperature, the higher the mass fraction of free ammonia in biogas slurry, and the more ammonia lost after heating (Liao et al., 1995). A large amount of the NH3eN diffused from the biogas slurry to the dilute phase causing the decreased of fertilizer value (Maurer and Muller, 2012). The mass concentration of NH3eN in the dilute phase was higher than 550 mg/L at different heating temperatures, which was far higher than the requirement of 80 mg/ L daily discharge standards of pollutants according to GB 185962001 "Discharge standard of pollutants for livestock and poultry breeding". There was no significant difference of NH3eN concentration in the dilute phase at different temperatures (P > 0.05). The value of EC was an indicator of the concentration of soluble salts in solution and the results of EC value at different temperatures are shown in Fig. 4 (1). For the condensed phase, the EC recovery rate decreased almost linearly with the increase of heating temperature. The value of EC in the dilute phase increased as a result of the release of NH3eN and CO2 from the biogas slurry, resulting in a decrease of soluble salt ion concentration in the condensed phase and an increase of the concentration of CO2-3, HCO-3, and NHþ4 in the dilute phase. (Gustin and Marinsek-Logar,
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2011; He et al., 2016). The recovery rate of TP in the concentrated phase was higher, at more than 90%. However, there was no significant difference in TP recovery at different heating temperatures (P > 0.05). This was largely because the phosphorus in biogas slurry existed in a steady state and hardly escaped during evaporation (Bai et al., 2015). The mass concentration of COD in the dilute phase was low, at less than 30 mg/L, and there was no significant difference between 50 C and 60 C (P > 0.05). However, there was a significant difference (P < 0.05) when the heating temperature increased to more than 60 C. This was due to the fact that the chemical bonds of the organic compounds were strong and not easy to break under the low temperature condition. It was difficult for the macromolecular organic compounds in the biogas slurry to decompose into small molecular organic compounds that would evaporate easily (Li et al., 2009), and most of the organic compounds remained in the concentrated phase. Table 3 lists the pH of condensed and dilute phases. The results indicated that the pH of the two phases was not affected by heating temperature and air flow rate, where it was about 8.72 and 9.76 in the condensed phase, and 8.79 and 9.94 in the dilute phase of the biogas slurry after HDH. COD and TP in the dilute phase met the emission requirements for the livestock and poultry breeding industry. The maximum allowable daily discharge standards of pollutants are not higher than 400 mg/L for COD and not higher than 8.0 mg/L for TP, according to GB 18596-2001. Fig. 4 (2) shows the nutrient recovery in the condensed phase and nutrient mass concentration in the dilute phase under different heating time. The recovery of NH3eN and EC decreased with the increase of heating time. In the early stage of evaporation, the loss of NH3eN and EC was faster (Wu et al., 2018). The NH3eN recovery decreased from 75.24% to 45.19%, and the EC recovery decreased from 72.68% to 42.78% at 30 min. The mass concentration of NH3eN and EC in the dilute phase also decreased with the increase of heating time. The NH3eN mass concentration of 320.48 mg/L and EC value of 1.26 mS/cm at 90 min was well below 1024.45 mg/L and 3.85 mS/cm at 10 min. This result occurred because a large amount of NH3eN and soluble salt was lost and entered into the dilute phase at the beginning of evaporation (Zhang et al., 2014), but the volume of the dilute phase was small, which caused high NH3eN concentration and EC value. As evaporation proceeded, the rate of NH3eN and soluble salt loss decreased, but the dilute phase volume increased continuously, resulting in a decrease of NH3eN concentration and EC value. The TP recovery had no significant difference under different heating times (P > 0.05), and the TP concentration was very low in the dilute phase (< 2 mg/L). The COD concentration in the dilute phase was low (40 mg/L) and decreased with the increase of heating time. The variation of nutrient concentration in the condensed and dilute phases of the biogas slurry after HDH at different air flow rates is shown in Fig. 4 (3). The NH3eN recovery decreased slightly with increasing air flow rate: it was 55.48% at 6 L/min air flow rate and decreased to 45.98% when the air flow rate increased to 10 L/ min. The soluble salt recovery rate also decreased as the air flow rate increased, and it decreased from 47.67% to 42.66% when the air flow rate increased from 6 to 10 L/min. The decrease in both NH3eN and soluble salts was due to the fact that the NH3eN and CO2 generated by evaporation were more easily entrained by air into the condensation tube, which increased the EC value and NH3eN concentration in the dilute phase. The EC value and NH3eN concentration were above 2.0 mS/cm and 550 mg/L, which were not conducive to the safe discharge of the dilute phase of the biogas slurry. There was less evaporation loss of TP during evaporation, and its recovery rate exceeded 95%. There was no significant difference in the TP recovery under different air flow rates (P > 0.05). The dilute
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Fig. 4. Effects of heating temperature, heating time and air flow rate on the nutrients change in the two phases: (A) condensed phase; (B) dilute phase. (1) Air flow rate: 10 L/min, heating time: 30 min. (2) Heating temperature: 70 C, air flow rate: 10 L/min. (3) Heating temperature: 70 C, heating time: 30 min.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
P. Wang et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 3 The pH in the two phases under different conditions. Parameters
Condensed phase
Dilute phase
Temperature Time Air flow rate Initial pH
8.72 ± 0.08 8.64 ± 012 8.79 ± 0.04 e
9.76 ± 0.05 9.83 ± 0.13 9.94 ± 0.18 9.47 ± 0.55
phase of biogas slurry contained small amounts of COD and TP, mainly due to the volatilization of volatile fatty acids (VFA), and the phosphorated compounds that closely combined with water molecules entrained by the bubbles during the evaporation into the dilute water (De la Rubia et al., 2010). The mass concentrations of COD and TP were under 20 mg/L and 1.6 mg/L, respectively, which met the emission requirements. The pH of the dilute phase was 9.94, slightly higher than that of the condensed phase. This was due to the higher mass concentration of NH3eN in the dilute phase, which made the dilute phase alkaline.
3.3. Nutrient changes in dilute and condensed phases at different initial pH values after HDH Fig. 5 (A) shows the recovery rate of NH3eN, TP, and EC in the condensed phase at a heating temperature of 70 C and air flow of 10 L/min. It can be seen that the recovery rate of NH3eN was more
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than 95% and the mass concentration of NH3eN in the dilute phase was lower than 157.49 mg/L when the initial pH was less than 6. Moreover, there was no significant difference in the NH3eN recovery when the initial pH was 4, 5, or 6 (P > 0.05). However, when the initial pH was more than 6, the recovery rate of NH3eN decreased rapidly. The NH3eN recovery decreased from 96.83% to 68.33% when the initial pH increased from 6 to 7. When the initial pH increased to 8, the NH3eN recovery rate was only 52.48%. At the same time, the mass concentration of NH3eN in the dilute phase increased significantly (P < 0.05), up to 500 mg/L. This can be explained be the fact that the ammonia in the biogas slurry mainly exists in the form of NHþ4, and was not easily evaporated. The lower the pH, the higher the proportion of NHþ4, and the lower the driving force of ammonia volatilization, so that the majority of NH3eN was retained in the condensed phase (Tao and Ukwuani, 2015). The trend of EC recovery was consistent with that of NH3eN. When the pH was below 6, the recovery rate of soluble salt was about 97%, and it decreased gradually with the increase of pH. The recovery rate of soluble salt fell to 56.16% when the pH was 8. The main reason was that the content of NHþ4 in the biogas slurry varied greatly during evaporation, while other salt ions such as SO2-4, Cl, Naþ, and Kþ were non-volatile substances, and the concentrations changed little, which had less influence on the EC value of the condensed phase (Guo et al., 2018; Wu et al., 2012). When the initial pH was less than 6, the EC value in the dilute phase was lower than 0.55 mS/cm, when the initial pH of biogas slurry increased from 7 to 8, the EC value increased from 1.56 mS/cm to 1.99 mS/cm. The recovery of TP was not affected by the value of pH, and the recovery was as high as 96%. The pH of the condensed phase increased with the increase of initial pH of biogas slurry. The pH increased from 4.82 to 8.66 when the initial pH increased from 4 to 8. The pH of the dilute phase had no significant change (P > 0.05), and the average pH was 9.47. When the initial pH of biogas slurry was 6, the recovery of NH3eN, TP, and soluble salt in the condensed phase reached 96%. The EC value was 0.55 mS/cm, and the mass concentrations of NH3eN, TP, and COD in the dilute phase were 157.49 mg/L, 0.66 mg/ L, and 8.70 mg/L, respectively. The lower content of each component was conducive to its safe reuse.
3.4. Response surface methodology (RSM) optimization According to previous experiments, the higher the air flow rate, the higher the water removal rate of biogas slurry. Air flow rate has little effect on nutrient recovery, and the NH3eN recovery decreased from 55.48% to 45.98% when the air flow rate increased from 6 L/min to 10 L/min. Therefore, the air flow rate of 10 L/min was chosen to optimize the parameters of heating temperature, heating time, and initial pH by RSM to achieve maximum WR efficiency and NH3eN recovery (nutrient recovery was mainly NH3eN recovery). The factors and levels are shown in Table 4. The quadratic model was used to analyze the WR efficiency. The Model
Table 4 The factors and levels of WR efficiency and NH3eN recovery. levels
1 0 1
Factors Temperature/ C [X1]
Time/min [X2]
pH [X3]
60 70 80
20 40 60
4 5 6
Fig. 5. Effects of the initial pH on the nutrients change in the two phases: (A) condensed phase; (B) dilute phase.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Fig. 6. Response surface for WR efficiency and NH3eN recovery under optimal parameters.
F-value of 387.38 implied that the model was significant. Values of "Prob > F" less than 0.05 indicated that the model was significant at the 0.05 significance level. In this case, the heating temperature and the heating time had significant effects (P < 0.05) on the WR efficiency of biogas slurry, and the significance order was heating time > heating temperature. The initial pH was not significant (P > 0.05). The final response surface regression model of actual factors was:
The quadratic model was used to analyze the NH3eN recovery. The Model F-value of 25.59 implied that the model was significant. Values of "Prob > F" less than 0.05 indicated that the model was significant at the 0.05 significance level. The heating temperature, heating time, and initial pH had significant effects (P < 0.05) on the NH3eN recovery. Under given levels, the significance order was heating temperature > heating time > initial pH. The response surface regression model of actual factors was:
WR ¼ 57:70 2:59X1 1:29X2 þ 9:89X3 þ 0:04X1 X2 0:0095X1 X3 0:036X2 X3 þ0:019X 21 0:0044X 22 0:76X 23
R2 ¼ 0:9980
(3)
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Table 5 Cost analysis of test process and practical application. Test process
Large-scale production
Units
Power/kW
Cost/yuan
Units
Power/kW
Cost/yuan
Rotary evaporator Vacuum pump Low temperature cooling liquid circulation pump Total
0.075 0.18 0.575
0.061 0.12 0.39 0.57
e Fan e
0.000 3
0.000 21
NH3 N
Recovery ¼ 97:84 5:35X1 2:13X2 4X3 0:54X1 X2 2:97X1 X3
1:44X2 X3 5:5X 21 1:32X 22 0:23X 23
R2 ¼ 0:9705
The 3D response surface and the 2D contour plots of the response are shown in Fig. 6. The report of RSM showed that the optimal parameters were: WR efficiency and NH3eN recovery by the models was a heating temperature of 61.92 C, heating time of 48.54 min, and initial pH of 4.80. Under these conditions, the WR efficiency was predicted to be 34.12% and the NH3eN recovery was predicted to be 98.04%. 3.5. Cost analysis The optimal result of heating temperature of 61.92 C, heating time of 48.54 min, and initial pH of 4.80 was used to estimate operating cost of 200 mL treatment capacity. In the study, the rotary evaporator was used to simulate the process of HDH. The biogas slurry was heated by the water bath and the air was introduced by the vacuum pump. However, the heat can be provided by solar energy or excess heat of heat and power cogeneration in a biogas plant, and the fan is an appropriate choice to meet the air volume requirement in large-scale production. Moreover, the low temperature cooling liquid circulation pump for the dilute phase can be substituted by raw biogas slurry to realize the reuse of waste heat. Cost analysis of the experimental process and large-scale production is shown in Table 5. It can be seen that the cost of the experimental process is 0.57 yuan, which is far higher than the practical application of 0.00021 yuan. The cost of the HDH process is about 1.01 yuan/t to concentrate biogas slurry. 4. Conclusions The humidificationedehumidification process was used to concentrate biogas slurry and the effects of operating parameters such as heating temperature, heating time, and air flow rate on the water removal efficiency, and the initial pH of biogas slurry on the nutrient concentration in two phases (dilute and condensed phases) were investigated. The results indicated that the HDH system can be used efficiently for this purpose where the water removal efficiency of biogas slurry increased with increasing heating temperature, heating time, and air flow rate. However, the recovery of NH3eN and soluble salt was low at higher temperatures. At a heating temperature of 70 C, heating time of 30 min, air flow rate of 10 L/min, and initial biogas slurry pH of 6, the recovery of NH3eN, TP, and soluble salt in the condensed phase reached 96%. The value of EC was 0.55 mS/cm, and the mass concentrations of NH3eN, TP, and COD were 157.49 mg/L, 0.66 mg/L, and 8.70 mg/L in the dilute phase, respectively, and the water removal efficiency of biogas slurry reached 30%. Water removal efficiency of 34.12% and NH3eN
0.000 21
(4)
recovery of 98.04% were obtained by the response surface methodology optimization when the heating temperature was 61.92 C, heating time was 48.54 min, and initial pH was 4.80. Under these conditions, the cost was about 1.01 yuan/t to concentrate biogas slurry. Finally, the HDH process used to treat the biogas slurry can increase the concentration efficiency, reduce energy consumption, allow reuse of wastewater, and has broad application prospects. Additionally, replacing the heating source with solar energy can provide a heating source for this system at low temperature should be considered in future work. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Key Research and Development Program, China (No. 2017YFD080080804-01). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.119142. References Al-Enezi, G., Ettouney, H., Fawzy, N., 2006. Low temperature humidification dehumidification desalination process. Energy Convers. Manag. 47 (4), 470e484. Awiszus, S., Meissner, K., Reyer, S., Mueller, J., 2018. Ammonia and methane emissions during drying of dewatered biogas digestate in a two-belt conveyor dryer. Bioresour. Technol. 247, 419e425. Bai, X., Li, Z., Yin, F., 2015. Evaporation treatment on biogas slurry from anaerobic fermentation. Trans. Chin. Soc. Agric. Mach. 46 (5), 164e170. €stner, M., 2015. The contribution Coban, H., Miltner, A., Elling, F.J., Hinrichs, K.U., Ka of biogas residues to soil organic matter formation and CO2 emissions in an arable soil. Soil Biol. Biochem. 86, 108e115. Dai, Y.J., Zhang, H.F., 2000. Experimental investigation of a solar desalination unit with humidification and dehumidification. Desalination 130 (2), 169e175. De la Rubia, M.A., Walker, M., Heaven, S., Banks, C.J., Borja, R., 2010. Preliminary trials of in situ ammonia stripping from source segregated domestic food waste digestate using biogas: effect of temperature and flow rate. Bioresour. Technol. 101 (24), 9486e9492. Deng, R., 2014. Vacuum Concentration and Nanofiltration Concentration of Livestock Biogas Slurry. Southwest University, Chongqing, China. Giwa, A., Akther, N., Housani, A.A., Haris, S., Hasan, S.W., 2016a. Recent advances in humidification dehumidification (HDH) desalination processes: improved designs and productivity. Renew. Sustain. Energy Rev. 57, 929e944. Giwa, A., Fath, H., Hasan, S.W., 2016b. Humidification-dehumidification desalination process driven by photovoltaic thermal energy recovery (PV-HDH) for small-
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry Panpan Wang a, b, Xin Zhang a, b, Shaban G. Gouda a, c, Qiaoxia Yuan a, b, * a
College of Engineering, Huazhong Agricultural University, Wuhan, 430070, China Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture and Rural Affairs, Wuhan, 430070, China c Agricultural and Biosystems Engineering Department, Faculty of Agriculture, Benha University, Benha, 13736, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2019 Received in revised form 14 October 2019 Accepted 2 November 2019 Available online xxx
Biogas slurry, the by-product of anaerobic digestion, represents 90%e95% of the input feed for digesters. Treatments of biogas slurry to reduce its volume and add value to it are difficult and costly. This study used the humidification-dehumidification process to concentrate biogas slurry and find a more economical and feasible method to provide appropriate conditions for sustainable utilization of biogas slurry. The effect of heating temperature, heating time, air flow rate and initial pH on the efficiency of water removal and nutrients (ammonia nitrogen, total phosphorus, chemical oxygen demand, soluble salt) recovery was investigated. The results showed that the humidification-dehumidification system could be effectively used to concentrate the biogas slurry. The recovery of ammonia nitrogen, total phosphorus, and soluble salt in the condensed phase reached 96% when the initial pH of biogas slurry was 6, the heating temperature was 70 C, the air flow rate was 10 L/min, and the heating time was 30 min. Under these conditions, the mass concentrations of ammonia nitrogen, total phosphorus, chemical oxygen demand, and electrical conductivity in the dilute phase were 157.49 mg/L, 0.66 mg/L, 8.70 mg/L, and 0.55 mS/cm, respectively, which was far below the mass concentrations of them in raw biogas slurry. Water removal efficiency of 34.12% and ammonia nitrogen recovery of 98.04% were obtained by the response surface methodology optimization when the heating temperature was 61.92 C, heating time was 48.54 min, air flow rate was 10 L/min, and initial pH was 4.80. Treatment of biogas slurry by the humidification-dehumidification system can result in a higher efficiency of water removal and nutrient recovery in a relatively short time. Moreover, the simple process, low operation cost and requirement for influent quality is conducive to engineering applications of the humidificationdehumidification process and the value-added utilization of biogas slurry. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Prof. S Alwi Keywords: Biogas slurry Concentration Humidification-dehumidification Water removal efficiency Nutrient recovery
1. Introduction The output of agricultural waste in rural China is increasing with an annual average rate of 5%e10%, which wastes resources and pollutes the environment if not effectively treated (Wu et al., 2017b). Anaerobic digestion, or biogas production, is an attractive approach for the utilization of agricultural waste resources (Wang et al., 2018). It can produce clean energy while dealing with agricultural waste, which is of great significance for alleviating the energy crisis and controlling environmental pollution (Ni et al., 2017). However, biogas plants produce a large amount of biogas
* Corresponding author. College of Engineering, Huazhong Agricultural University, Wuhan, 430070, China. E-mail address: [email protected] (Q. Yuan).
slurry and biogas residue. Biogas residue is mainly composed of microbial biomass and its residues as well as non-fermented feedstock leftovers; it can improve crop growth and replace other organic fertilizers (Coban et al., 2015). Biogas slurry is increasingly used as fertilizer in organic farming systems (Wentzel et al., 2015). However, farmland has limited carrying capacity and cannot continuously receive nutrient from the biogas slurry; excessive nutrient will run into water bodies and cause serious eutrophication issues (Knight et al., 2000; Wang et al., 2017). In some areas of China, the distance between the biogas plant and the farmland receiving the biogas slurry is too far to economically justify the transportation cost, which also limits the farmland utilization of biogas slurry (Han et al., 2015). It is necessary to reduce the volume of biogas slurry, and convert it into high-quality liquid fertilizer for easy transportation and effective utilization.
https://doi.org/10.1016/j.jclepro.2019.119142 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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There is a growing interest in membrane technology for biogas slurry concentration because of its simple process, lower nutrient loss, unchanged composition characteristics, and easy recovery and utilization of wastewater (Waeger et al., 2010; Xia and Murphy, 2016; Xu et al., 2016). Researchers have used various membrane peztechnologies to concentrate biogas slurry: ultrafiltration (Lo ndez et al., 2011), nanofiltration (Konieczny et al., 2011), Ferna reverse osmosis (Gong et al., 2013), and hybrid membrane processes (Ruan et al., 2015). However, some problems such as high energy consumption, high cost, and membrane fouling limit the application of membrane technology in the field of biogas slurry concentration (Li et al., 2018; Lu et al., 2017). Recently, the use of vacuum concentration technology in biogas slurry concentration has also been reported. Deng (2014) discovered a good concentration effect on chemical oxygen demand (COD) and total phosphor (TP) in biogas slurry when the evaporation absolute pressure was 0.01 MPa and evaporation temperature was 44 C (boiling point) with the evaporation time of 1 h and concentration rate of 20%. However, the poor concentration effect of ammonia nitrogen (NH3eN) (Huang et al., 2011) and the high requirement of vacuum (Song et al., 2017) make it difficult to apply vacuum concentration technology on a large scale to concentrate biogas slurry. It is necessary to choose an appropriate method to concentrate biogas slurry with low energy consumption and high efficiency of concentration and nutrient recovery. In recent years, the seawater desalination system based on humidification-dehumidification (HDH) technology has gradually attracted public attention due to its simple process, convenient operation, low investment and operating costs, and ease of use with low-grade heat sources such as solar energy, geothermal, wind, or their combinations to reduce energy consumption (Muthusamy and Srithar, 2017; Rahimi-Ahar et al., 2018; Wu et al., 2017a). The HDH system is a distillation technology that uses a carrier fluid such as air or water to obtain thermal energy from a heat source, which is then transferred to a humidifier for water evaporation from saline water and then to the dehumidifier for condensation of the evaporated water to fresh water, so that the saline water is concentrated (Giwa et al., 2016a; Gu, 2013). Yuan et al. (2011) investigated a 1000 L/d solar HDH system. The system was composed of a 100 m2 solar air heater field, a 12 m2 solar water collector, an HDH unit, pre-treatment and post-treatment systems and other sub-systems. They found that water production from the system reached 1200 L/d, when the average intensity of solar radiation was 550 W/m2 and the heating water temperature was below 50 C. Zhao et al. (2019) studied a novel four-stage cross flow HDH solar desalination system and established the heat and mass exchange models of the main parts. The results showed that when seawater was 83 C, spraying quantity was 1.1 t/h and air flow rate was 300 m3/h, the water yields of the HDH system with 560 m2 total heat transfer area was 63 kg/h. In this study, the HDH process was used to concentrate biogas slurry. The effects of heating temperature, heating time, and air flow rate on the efficiency of water removal and nutrient recovery of biogas slurry were studied. The migration of nutrient from the biogas slurry under different initial pH was also explored. Response surface methodology (RSM) was carried out to analyze the optimal combination of biogas slurry concentration. Based on the optimal combination, the operation cost was calculated. These results provide a basis for the application of the HDH process in biogas slurry concentration. 2. Materials and methods The biogas slurry was obtained from an anaerobic digester in Ezhou, Hubei, China. The capacity of the anaerobic digester was
500 m3. The digester used pig manure as the main raw material and a small amount of domestic sewage was added to ferment at a mesophilic temperature (35 C) in the continuous stirred tank reactor (CSTR). The hydraulic retention time (HRT) was 15 d, the organic load rate (OLR) was 2 kg/(m3$d), and the volume of biogas product was 400 m3/d. The collected biogas slurry was stored anaerobically at ambient temperature (20 ± 5 C) prior to experiments until no biogas was produced. The biogas slurry was centrifuged (TSZ5-WS, Hunan Xiangyi Centrifuge Instrument Co., Ltd.) at 4000 r/min for 10 min and then the supernatant was sealed and stored in a refrigerator at 4 C until it was used for testing. The physicochemical properties of the biogas slurry after centrifugation are shown in Table 1.
2.1. Experimental procedure Fig. 1 shows the schematic diagram of the HDH system to concentrate biogas slurry. Limited by the experiment condition, a rotary evaporator (RV 10 basic, IKA, Germany) was used as the main tool to simulate the process of HDH, and a vacuum pump was used as the driving force (Fig. 2). First, the biogas slurry (200 mL) was added to the rotary evaporation vessel (humidifier) with a volume of 1 L and heated in a water bath. The charge aperture and the vacuum pump open to start the test when the water bath temperature reaches the set value. The biogas slurry in the rotary evaporation vessel was heated to generate water vapor during the evaporation process; at the same time the dry cold air outside entered the rotary evaporation vessel through the charge aperture driven by the vacuum pump, and was humidified by the steam. The hot humid air with a large amount of steam was condensed into liquid water in the condensation tube (dehumidified) and passed to the balloon flask. The liquid in the balloon flask is the dilute phase, and the remaining solution in the rotary evaporation vessel is the condensed phase. The condensed phase and the dilute phase after evaporation were collected for analysis.
2.2. Experimental design In this study, single factor tests were carried out to study the effect of heating temperature, heating time, and air flow rate on the concentration efficiency of biogas slurry. In order to make the experiment results more obvious, the maximum air flow rate of 10 L/min and the longer heating time of 30 min (when the heating time exceeded 30 min, the concentration efficiency was too high and almost all of the biogas slurry was evaporated at 90 C) were selected to study the effect of heating temperatures (50, 60, 70, 80, and 90 C) on the concentration efficiency of biogas slurry. Similarly, the maximum air flow rate of 10 L/min was chosen to study the effect of heating time (10, 20, 30, 40, 60, 90, 120, 150, 180, and 240 min) on the concentration efficiency of biogas slurry at different heating temperatures (50, 60, 70, 80, and 90 C). The effect of air flow rate (6, 7, 8, 9, and 10 L/min) on the concentration efficiency of biogas slurry at different heating temperatures was also studied. Moreover, the changes of main nutrient in the condensed phase and dilute phase after the HDH process were investigated. Migration of the main nutrient of the biogas slurry under different initial pH (4, 5, 6, 7, and 8, adjusted by sulfuric acid) was investigated. RSM was used to comprehensively analyze the effect of the HDH process on the concentration efficiency and nutrient recovery of biogas slurry. Finally, cost analysis of the HDH process was carried out. The experiment was carried out at room temperature of 25 C, humidity of 50%, and absolute pressure of 0.1 MPa with three repetitions.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Table 1 The basic physicochemical properties of biogas slurry after centrifugation. Parameters
pH
NH3eN (mg/L)
TP (mg/L)
COD (mg/L)
EC (mS/cm)
TS (mg/L)
VS (mg/L)
Value
8.46 ± 0.06
1715.94 ± 11.16
35.17 ± 0.58
3208.17 ± 29.60
17.55 ± 0.18
4213.27 ± 58.45
2938.84 ± 32.16
Notes: NH3eN is ammonia-nitrogen, TP is total phosphorus, COD is chemical oxygen demand, EC is electrical conductivity, TS is total solid, VS is volatile solid.
Fig. 1. Schematic diagram of the HDH system.
Fig. 2. Schematic diagram of biogas slurry HDH concentration device.
2.3. Chemical analysis There are many indicators to determine the nutrient recovery efficiency, including NH3eN, TP, COD, and EC (electrical conductivity). NH3eN and TP were tested using a fully automatic intermittent chemical analyzer (SmartChem200, AMS, Italy). The COD was measured by CM-03 portable COD water quality meter (Beijing
Shuanghui Jingcheng Electronic Products Co., Ltd.). The mass concentration of total solids (TS) and volatile solid (VS) was measured by gravimetric analysis. The pH and EC were measured using a pH meter (FE28, Metler, Switzerland) and an electrical conductivity meter (DDS-307A, Shanghai Instrument and Electrical Science Instrument Co., Ltd.).
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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2.4. Data analysis The concentration efficiency and nutrient recovery efficiencies were the main test indicators. The concentration efficiency is reflected by the water removal (WR) efficiency of biogas slurry, which can be calculated as follows:
WR ¼ ðVo Vi Þ=Vo 100%
(1)
where WR is the water removal efficiency (%), Vi is the volume of biogas slurry at time ti (mL), and V0 is the initial volume of biogas slurry (mL). The nutrient recovery efficiency (NR) of NH3eN, TP, COD, and EC in the condensed phase can be calculated using Eq. (2):
NR ¼ Ci Vi =Co Vo 100%
(2)
where NR is the nutrient recovery efficiency (%), and Ci and C0 are the mass concentrations (mg/L) of NH3eN, TP, COD, and the value of EC (mS/cm) in the condensed phase and stock solution of biogas slurry. One-way ANOVA and Duncan’s multiple comparison tests (P < 0.05, IBM SPSS statistics 25) were used to test the statistical differences among the related parameters on nutrient recovery in the condensed phase and the mass concentration in the dilute phase. RSM of Box-Behnken (BBD) was used to optimize the process parameters with Design-Expert version V8.0.6. 3. Results and discussion The concentration efficiency of biogas slurry and nutrient changes in dilute and condensed phases at different heating temperatures, heating times, and air flow rates after processing with the HDH system have been presented in the following sections. 3.1. Effect of heating temperature, heating time and air flow rate on the concentration efficiency of biogas slurry Fig. 3 shows the effect of heating temperature, heating time, and air flow rate on the water removal (WR) efficiency. The results showed that under the conditions of the HDH process the WR efficiency increased with increased heating temperature. The WR efficiency increased from 12% to 53.49% when the temperature increased from 50 to 90 C. The volume of the dilute phase produced by the HDH was higher (19.98, 7.55, and 6.37 times) than that obtained under natural evaporation at the same temperature (50, 70, and 90 C). Under HDH conditions, the air humidity was increased to near saturation in the humidifier (Giwa et al., 2016b), and the saturated moist air was then fed to the dehumidifier where the moisture was partially condensed and collected. Increasing the heating temperature increased the saturated vapor pressure of the air in the humidifier, and the vapor parameters near the interface surface influenced the interface layers of the condensed phase (Shishkova et al., 2019). This resulted in more water vapor being carried into the dehumidifier by airflow and accelerated the evaporation process (Rajaseenivasan and Srithar, 2017). Compared with natural evaporation, the method of biogas slurry HDH concentration achieved better concentration efficiency at a lower temperature. The influence of heating time on the WR efficiency of biogas slurry at different heating temperatures is shown in Fig. 3 (2). The WR efficiency increased with the increase of heating time at different temperatures. It increased rapidly and reached the highest values in a short time, where the maximum WR efficiencies were achieved at times of 40, 60, and 90 min with corresponding heating
Fig. 3. The water removal efficiency of biogas slurry at different heating temperatures, heating time and air flow rates. (1) Natural evaporation (0 L/min) and HDH (10 L/min), heating temperature: 70 C, heating time: 30 min (2) Air flow rate: 10 L/min. (3) Heating time: 30 min.
temperatures of 90, 80, and 70 C, and maximum WR efficiencies of 79.17%, 84%, and 84.67%. In contrast, the maximum values of WR efficiency were obtained over longer time periods of 150 and 240 min with corresponding temperatures of 60 and 50 C, and the maximum WR efficiencies were 90.83% and 96.17%. Linear fitting was the suitable form to represent the relationship between the WR efficiency and heating time for different heating temperatures. The linear fitting equations and the correlation coefficients are listed in Table 2. These equations had good correlations, especially when the heating temperatures were 50, 60, and 70 C. Fig. 3 (3) shows the changes of WR efficiency of biogas slurry at different air flow rates. The air flow rates had a positive influence on the concentration of biogas slurry at different temperatures, which
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
P. Wang et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 2 The linear fitting equations and the correlation coefficients for the relationship between WR efficiency of biogas slurry and heating time processes at different heating temperatures. Temperature ( C)
Linear fitting equation
correlation coefficient (R2)
50 60 70 80 90
y ¼ 0.404 y ¼ 0.598 y ¼ 0.932 y ¼ 1.381 y ¼ 1.868
0.999 0.999 0.999 0.996 0.982
6t þ 0.403 7 1tþ 0.770 6 9t þ 0.516 2 8t þ 0.950 5 2t þ 2.083 3
4 3 1 4 0
Note: y is WR efficiency (%) and t is time (min).
was consistent with the results of (Dai and Zhang, 2000) and (Shafii et al., 2018). The main mechanism in HDH process was convective evaporation, larger air flow rate resulted in greater convective evaporation (Al-Enezi et al., 2006). The main reason for this result was due to the partial pressure of water vapor in the rotary evaporation vessel, which was higher than the pressure of outside air. The driving force for evaporation was the vapor pressure difference between the air and the biogas slurry, which led to the movement of water molecules from the region with high partial pressure of water vapor to the region with low partial pressure (Jain et al., 2011).
3.2. Nutrient changes in dilute and condensed phases at different temperatures, heating times, and air flow rates after processing with the HDH system The variation in nutrient in the two phases (condensed phase and dilute phase) at different temperatures and 8.46 initial pH after HDH is shown in Fig. 4. Fig. 4 (1) shows that the recovery of NH3eN in the condensed phase decreased with the increase of heating temperature, which reached 59.36% at 50 C, 51.37% at 60 C, and 45.19% at 70 C. However, increasing the heating temperature to 80 C sharply decreased NH3eN in the concentration of 425.37 mg/ L with a 15% recovery rate. This can be clearly seen at 90 C heating temperature, where the concentration of NH3eN was only 145.17 mg/L with a recovery rate of 3.93%. Higher temperature reduced evaporation time and allowed a higher WR efficiency, but was also causing a higher flowrate of NH3 emissions (Awiszus et al., 2018). This occurred because the nitrogen in the biogas slurry generally existed in the form of free ammonia (NH3) and ammonium (NHþ4), and the NHþ4 was more stable, while the NH3 escaped more easily with the evaporated water molecules. The higher the temperature, the higher the mass fraction of free ammonia in biogas slurry, and the more ammonia lost after heating (Liao et al., 1995). A large amount of the NH3eN diffused from the biogas slurry to the dilute phase causing the decreased of fertilizer value (Maurer and Muller, 2012). The mass concentration of NH3eN in the dilute phase was higher than 550 mg/L at different heating temperatures, which was far higher than the requirement of 80 mg/ L daily discharge standards of pollutants according to GB 185962001 "Discharge standard of pollutants for livestock and poultry breeding". There was no significant difference of NH3eN concentration in the dilute phase at different temperatures (P > 0.05). The value of EC was an indicator of the concentration of soluble salts in solution and the results of EC value at different temperatures are shown in Fig. 4 (1). For the condensed phase, the EC recovery rate decreased almost linearly with the increase of heating temperature. The value of EC in the dilute phase increased as a result of the release of NH3eN and CO2 from the biogas slurry, resulting in a decrease of soluble salt ion concentration in the condensed phase and an increase of the concentration of CO2-3, HCO-3, and NHþ4 in the dilute phase. (Gustin and Marinsek-Logar,
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2011; He et al., 2016). The recovery rate of TP in the concentrated phase was higher, at more than 90%. However, there was no significant difference in TP recovery at different heating temperatures (P > 0.05). This was largely because the phosphorus in biogas slurry existed in a steady state and hardly escaped during evaporation (Bai et al., 2015). The mass concentration of COD in the dilute phase was low, at less than 30 mg/L, and there was no significant difference between 50 C and 60 C (P > 0.05). However, there was a significant difference (P < 0.05) when the heating temperature increased to more than 60 C. This was due to the fact that the chemical bonds of the organic compounds were strong and not easy to break under the low temperature condition. It was difficult for the macromolecular organic compounds in the biogas slurry to decompose into small molecular organic compounds that would evaporate easily (Li et al., 2009), and most of the organic compounds remained in the concentrated phase. Table 3 lists the pH of condensed and dilute phases. The results indicated that the pH of the two phases was not affected by heating temperature and air flow rate, where it was about 8.72 and 9.76 in the condensed phase, and 8.79 and 9.94 in the dilute phase of the biogas slurry after HDH. COD and TP in the dilute phase met the emission requirements for the livestock and poultry breeding industry. The maximum allowable daily discharge standards of pollutants are not higher than 400 mg/L for COD and not higher than 8.0 mg/L for TP, according to GB 18596-2001. Fig. 4 (2) shows the nutrient recovery in the condensed phase and nutrient mass concentration in the dilute phase under different heating time. The recovery of NH3eN and EC decreased with the increase of heating time. In the early stage of evaporation, the loss of NH3eN and EC was faster (Wu et al., 2018). The NH3eN recovery decreased from 75.24% to 45.19%, and the EC recovery decreased from 72.68% to 42.78% at 30 min. The mass concentration of NH3eN and EC in the dilute phase also decreased with the increase of heating time. The NH3eN mass concentration of 320.48 mg/L and EC value of 1.26 mS/cm at 90 min was well below 1024.45 mg/L and 3.85 mS/cm at 10 min. This result occurred because a large amount of NH3eN and soluble salt was lost and entered into the dilute phase at the beginning of evaporation (Zhang et al., 2014), but the volume of the dilute phase was small, which caused high NH3eN concentration and EC value. As evaporation proceeded, the rate of NH3eN and soluble salt loss decreased, but the dilute phase volume increased continuously, resulting in a decrease of NH3eN concentration and EC value. The TP recovery had no significant difference under different heating times (P > 0.05), and the TP concentration was very low in the dilute phase (< 2 mg/L). The COD concentration in the dilute phase was low (40 mg/L) and decreased with the increase of heating time. The variation of nutrient concentration in the condensed and dilute phases of the biogas slurry after HDH at different air flow rates is shown in Fig. 4 (3). The NH3eN recovery decreased slightly with increasing air flow rate: it was 55.48% at 6 L/min air flow rate and decreased to 45.98% when the air flow rate increased to 10 L/ min. The soluble salt recovery rate also decreased as the air flow rate increased, and it decreased from 47.67% to 42.66% when the air flow rate increased from 6 to 10 L/min. The decrease in both NH3eN and soluble salts was due to the fact that the NH3eN and CO2 generated by evaporation were more easily entrained by air into the condensation tube, which increased the EC value and NH3eN concentration in the dilute phase. The EC value and NH3eN concentration were above 2.0 mS/cm and 550 mg/L, which were not conducive to the safe discharge of the dilute phase of the biogas slurry. There was less evaporation loss of TP during evaporation, and its recovery rate exceeded 95%. There was no significant difference in the TP recovery under different air flow rates (P > 0.05). The dilute
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Fig. 4. Effects of heating temperature, heating time and air flow rate on the nutrients change in the two phases: (A) condensed phase; (B) dilute phase. (1) Air flow rate: 10 L/min, heating time: 30 min. (2) Heating temperature: 70 C, air flow rate: 10 L/min. (3) Heating temperature: 70 C, heating time: 30 min.
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
P. Wang et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 3 The pH in the two phases under different conditions. Parameters
Condensed phase
Dilute phase
Temperature Time Air flow rate Initial pH
8.72 ± 0.08 8.64 ± 012 8.79 ± 0.04 e
9.76 ± 0.05 9.83 ± 0.13 9.94 ± 0.18 9.47 ± 0.55
phase of biogas slurry contained small amounts of COD and TP, mainly due to the volatilization of volatile fatty acids (VFA), and the phosphorated compounds that closely combined with water molecules entrained by the bubbles during the evaporation into the dilute water (De la Rubia et al., 2010). The mass concentrations of COD and TP were under 20 mg/L and 1.6 mg/L, respectively, which met the emission requirements. The pH of the dilute phase was 9.94, slightly higher than that of the condensed phase. This was due to the higher mass concentration of NH3eN in the dilute phase, which made the dilute phase alkaline.
3.3. Nutrient changes in dilute and condensed phases at different initial pH values after HDH Fig. 5 (A) shows the recovery rate of NH3eN, TP, and EC in the condensed phase at a heating temperature of 70 C and air flow of 10 L/min. It can be seen that the recovery rate of NH3eN was more
7
than 95% and the mass concentration of NH3eN in the dilute phase was lower than 157.49 mg/L when the initial pH was less than 6. Moreover, there was no significant difference in the NH3eN recovery when the initial pH was 4, 5, or 6 (P > 0.05). However, when the initial pH was more than 6, the recovery rate of NH3eN decreased rapidly. The NH3eN recovery decreased from 96.83% to 68.33% when the initial pH increased from 6 to 7. When the initial pH increased to 8, the NH3eN recovery rate was only 52.48%. At the same time, the mass concentration of NH3eN in the dilute phase increased significantly (P < 0.05), up to 500 mg/L. This can be explained be the fact that the ammonia in the biogas slurry mainly exists in the form of NHþ4, and was not easily evaporated. The lower the pH, the higher the proportion of NHþ4, and the lower the driving force of ammonia volatilization, so that the majority of NH3eN was retained in the condensed phase (Tao and Ukwuani, 2015). The trend of EC recovery was consistent with that of NH3eN. When the pH was below 6, the recovery rate of soluble salt was about 97%, and it decreased gradually with the increase of pH. The recovery rate of soluble salt fell to 56.16% when the pH was 8. The main reason was that the content of NHþ4 in the biogas slurry varied greatly during evaporation, while other salt ions such as SO2-4, Cl, Naþ, and Kþ were non-volatile substances, and the concentrations changed little, which had less influence on the EC value of the condensed phase (Guo et al., 2018; Wu et al., 2012). When the initial pH was less than 6, the EC value in the dilute phase was lower than 0.55 mS/cm, when the initial pH of biogas slurry increased from 7 to 8, the EC value increased from 1.56 mS/cm to 1.99 mS/cm. The recovery of TP was not affected by the value of pH, and the recovery was as high as 96%. The pH of the condensed phase increased with the increase of initial pH of biogas slurry. The pH increased from 4.82 to 8.66 when the initial pH increased from 4 to 8. The pH of the dilute phase had no significant change (P > 0.05), and the average pH was 9.47. When the initial pH of biogas slurry was 6, the recovery of NH3eN, TP, and soluble salt in the condensed phase reached 96%. The EC value was 0.55 mS/cm, and the mass concentrations of NH3eN, TP, and COD in the dilute phase were 157.49 mg/L, 0.66 mg/ L, and 8.70 mg/L, respectively. The lower content of each component was conducive to its safe reuse.
3.4. Response surface methodology (RSM) optimization According to previous experiments, the higher the air flow rate, the higher the water removal rate of biogas slurry. Air flow rate has little effect on nutrient recovery, and the NH3eN recovery decreased from 55.48% to 45.98% when the air flow rate increased from 6 L/min to 10 L/min. Therefore, the air flow rate of 10 L/min was chosen to optimize the parameters of heating temperature, heating time, and initial pH by RSM to achieve maximum WR efficiency and NH3eN recovery (nutrient recovery was mainly NH3eN recovery). The factors and levels are shown in Table 4. The quadratic model was used to analyze the WR efficiency. The Model
Table 4 The factors and levels of WR efficiency and NH3eN recovery. levels
1 0 1
Factors Temperature/ C [X1]
Time/min [X2]
pH [X3]
60 70 80
20 40 60
4 5 6
Fig. 5. Effects of the initial pH on the nutrients change in the two phases: (A) condensed phase; (B) dilute phase.
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Fig. 6. Response surface for WR efficiency and NH3eN recovery under optimal parameters.
F-value of 387.38 implied that the model was significant. Values of "Prob > F" less than 0.05 indicated that the model was significant at the 0.05 significance level. In this case, the heating temperature and the heating time had significant effects (P < 0.05) on the WR efficiency of biogas slurry, and the significance order was heating time > heating temperature. The initial pH was not significant (P > 0.05). The final response surface regression model of actual factors was:
The quadratic model was used to analyze the NH3eN recovery. The Model F-value of 25.59 implied that the model was significant. Values of "Prob > F" less than 0.05 indicated that the model was significant at the 0.05 significance level. The heating temperature, heating time, and initial pH had significant effects (P < 0.05) on the NH3eN recovery. Under given levels, the significance order was heating temperature > heating time > initial pH. The response surface regression model of actual factors was:
WR ¼ 57:70 2:59X1 1:29X2 þ 9:89X3 þ 0:04X1 X2 0:0095X1 X3 0:036X2 X3 þ0:019X 21 0:0044X 22 0:76X 23
R2 ¼ 0:9980
(3)
Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142
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Table 5 Cost analysis of test process and practical application. Test process
Large-scale production
Units
Power/kW
Cost/yuan
Units
Power/kW
Cost/yuan
Rotary evaporator Vacuum pump Low temperature cooling liquid circulation pump Total
0.075 0.18 0.575
0.061 0.12 0.39 0.57
e Fan e
0.000 3
0.000 21
NH3 N
Recovery ¼ 97:84 5:35X1 2:13X2 4X3 0:54X1 X2 2:97X1 X3
1:44X2 X3 5:5X 21 1:32X 22 0:23X 23
R2 ¼ 0:9705
The 3D response surface and the 2D contour plots of the response are shown in Fig. 6. The report of RSM showed that the optimal parameters were: WR efficiency and NH3eN recovery by the models was a heating temperature of 61.92 C, heating time of 48.54 min, and initial pH of 4.80. Under these conditions, the WR efficiency was predicted to be 34.12% and the NH3eN recovery was predicted to be 98.04%. 3.5. Cost analysis The optimal result of heating temperature of 61.92 C, heating time of 48.54 min, and initial pH of 4.80 was used to estimate operating cost of 200 mL treatment capacity. In the study, the rotary evaporator was used to simulate the process of HDH. The biogas slurry was heated by the water bath and the air was introduced by the vacuum pump. However, the heat can be provided by solar energy or excess heat of heat and power cogeneration in a biogas plant, and the fan is an appropriate choice to meet the air volume requirement in large-scale production. Moreover, the low temperature cooling liquid circulation pump for the dilute phase can be substituted by raw biogas slurry to realize the reuse of waste heat. Cost analysis of the experimental process and large-scale production is shown in Table 5. It can be seen that the cost of the experimental process is 0.57 yuan, which is far higher than the practical application of 0.00021 yuan. The cost of the HDH process is about 1.01 yuan/t to concentrate biogas slurry. 4. Conclusions The humidificationedehumidification process was used to concentrate biogas slurry and the effects of operating parameters such as heating temperature, heating time, and air flow rate on the water removal efficiency, and the initial pH of biogas slurry on the nutrient concentration in two phases (dilute and condensed phases) were investigated. The results indicated that the HDH system can be used efficiently for this purpose where the water removal efficiency of biogas slurry increased with increasing heating temperature, heating time, and air flow rate. However, the recovery of NH3eN and soluble salt was low at higher temperatures. At a heating temperature of 70 C, heating time of 30 min, air flow rate of 10 L/min, and initial biogas slurry pH of 6, the recovery of NH3eN, TP, and soluble salt in the condensed phase reached 96%. The value of EC was 0.55 mS/cm, and the mass concentrations of NH3eN, TP, and COD were 157.49 mg/L, 0.66 mg/L, and 8.70 mg/L in the dilute phase, respectively, and the water removal efficiency of biogas slurry reached 30%. Water removal efficiency of 34.12% and NH3eN
0.000 21
(4)
recovery of 98.04% were obtained by the response surface methodology optimization when the heating temperature was 61.92 C, heating time was 48.54 min, and initial pH was 4.80. Under these conditions, the cost was about 1.01 yuan/t to concentrate biogas slurry. Finally, the HDH process used to treat the biogas slurry can increase the concentration efficiency, reduce energy consumption, allow reuse of wastewater, and has broad application prospects. Additionally, replacing the heating source with solar energy can provide a heating source for this system at low temperature should be considered in future work. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Key Research and Development Program, China (No. 2017YFD080080804-01). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.119142. References Al-Enezi, G., Ettouney, H., Fawzy, N., 2006. Low temperature humidification dehumidification desalination process. Energy Convers. Manag. 47 (4), 470e484. Awiszus, S., Meissner, K., Reyer, S., Mueller, J., 2018. Ammonia and methane emissions during drying of dewatered biogas digestate in a two-belt conveyor dryer. Bioresour. Technol. 247, 419e425. Bai, X., Li, Z., Yin, F., 2015. Evaporation treatment on biogas slurry from anaerobic fermentation. Trans. Chin. Soc. Agric. Mach. 46 (5), 164e170. €stner, M., 2015. The contribution Coban, H., Miltner, A., Elling, F.J., Hinrichs, K.U., Ka of biogas residues to soil organic matter formation and CO2 emissions in an arable soil. Soil Biol. Biochem. 86, 108e115. Dai, Y.J., Zhang, H.F., 2000. Experimental investigation of a solar desalination unit with humidification and dehumidification. Desalination 130 (2), 169e175. De la Rubia, M.A., Walker, M., Heaven, S., Banks, C.J., Borja, R., 2010. Preliminary trials of in situ ammonia stripping from source segregated domestic food waste digestate using biogas: effect of temperature and flow rate. Bioresour. Technol. 101 (24), 9486e9492. Deng, R., 2014. Vacuum Concentration and Nanofiltration Concentration of Livestock Biogas Slurry. Southwest University, Chongqing, China. Giwa, A., Akther, N., Housani, A.A., Haris, S., Hasan, S.W., 2016a. Recent advances in humidification dehumidification (HDH) desalination processes: improved designs and productivity. Renew. Sustain. Energy Rev. 57, 929e944. Giwa, A., Fath, H., Hasan, S.W., 2016b. Humidification-dehumidification desalination process driven by photovoltaic thermal energy recovery (PV-HDH) for small-
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Please cite this article as: Wang, P et al., Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119142