biomass and bioenergy 45 (2012) 187e194

Biogas fermentation of swine slurry based on the separation of concentrated liquid and low content liquid

Liangwei Deng*, Ziai Chen, Hao Yang, Juanyu Zhu, Yi Liu, Yan Long, Dan Zheng

Biogas Institute of Ministry of Agriculture, 13 Section 4, South Renmin Rd., Chengdu, PR China

article info abstract

Article history:

Received 5 January 2011

Received in revised form 18 May

2012 Accepted 3 June 2012

Available online 2 July 2012

Keywords: Swine slurry Sedimentation Anaerobic digestion Biogas Excess heat Digestate

This paper aims to demonstrate the bene.ts of using gravitational sedimentation to separate raw swine slurry into two substances: one thick and highly-concentrated, the other a low-concentration liquid. The volume of the concentrated liquid, accounting for 18.3% of the raw slurry, contained about 61.0% of the chemical oxygen demand or biochemical oxygen demand and could produce about 60% of the total biogas production potential.

The digestion temperature could be raised by approximately 22 C by heating the concentrated liquid with the excess heat of biogas generation but only by about 6.43 Cby heating the raw slurry. The size of digester with the new process could be reduced by two-thirds that of the digester with the conventional process. After separation, the digestate of the concentrated liquid, with nearly three times total nutrient content as that of raw slurry, can readily be used as fertilizer and the low content liquid can easily meet discharge standards after aerobic treatment.

ª 2012 Elsevier Ltd. All rights reserved.

1. Introduction

For swine slurry treatment, anaerobic digestion throughout biogas production is a notable option. A number of studies on anaerobic digestion of swine slurry have been carried out in different reactor con.gurations [1e5]. Furthermore, many full-scale biogas plants fermenting swine slurry have been built in China, Germany, Austria, Sweden, and other countries [6e9].

The dominating factor in.uencing the ef.ciency of biogas production is temperature. The growth rate of methanogenic bacteria is higher at thermophilic than at mesophilic and psychrophilic temperatures [10]. At thermophilic temperatures, due to the higher degradation rate, the hydraulic retention time (HRT) is shorter and the reactor volume is smaller compared with those at mesophilic and psychrophilic temperatures. But the thermophilic process requires a larger energy input and holds a higher risk for ammonia inhibition caused by ammonia toxicity increasing with rising

* Corresponding author. Tel./fax: þ86 (0) 28 85236376. E-mail address: dlwbrtc@yahoo.com.cn (L. Deng).

temperature [11]. Consequently, in Germany, about 90% of all biogas plants operate at mesophilic temperatures [12]. Maintenance of mesophilic temperature for anaerobic digestion is generally by means of heating the reactor which is accomplished by routing excess heat from the gas engines of a combined heat and power station (CHP) to heat exchangers within the reactor [12,13]. Obtaining an optimal temperature for anaerobic digestion depends on ambient temperature, yield of biogas and quantity of feedstock. Among these three factors, it is impossible to change ambient temperature but feasible to control the other two. Higher temperatures of anaerobic digestion can be obtained by reducing the quantity of feedstock and increasing the biogas yield. To reach this goal, the usual method is to reduce the quantity of feedstock by enhancing its concentration of total solids (TS), and to add other substrates such as energy crops (maize silage, grass silage and cereal residues) or organic waste (industrial organic waste, source sorted household waste, and sewage sludge)

0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.06.004

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into the slurry if its total solids are low [14,15]. In Germany, the typical TS content of the digester feedstock usually ranges from 8% to 10% in biogas plants [15]. In order to reach mesophilic anaerobic digestion using this feedstock, up to one-third of the total biogas energy produced is used to heat the digester [16]. However, the total solids content of swine slurry is in the range of 1e2% [17], and methods to enhance the concentration of total solids, such as adding organic matters, could not be applied because it is more dif.cult for the .nal ef.uent with higher concentration to meet discharge standards in China. Consequently, the digestion temperature of swine slurry could not reach mesophilic conditions using conventional treatment methods, resulting in large volume of digester and high investment. Therefore, a new strategy for the biogas fermentation of swine slurry needs to be developed.

A new biogas fermentation system for swine slurry with TS content of about 1% was developed as a way to separate the raw swine slurry into concentrated liquid and low content liquid by sedimentation. The present research is to determine the separation ef.ciency of sedimentation, to assess the potential of biogas production from concentrated liquid and low content liquid, to calculate the potential increase in temperatures for concentrated liquid digestion and raw slurry that could be achieved. 2. Materials and methods

2.1. Swine slurry and inoculum

Swine slurry for the study was taken from a pig farm of Jinli Agricultural and Livestock Co. Ltd in Chengdu, China, 30 km away from the laboratory, which was a propagation and fattening pig farm with animal population of about 7000 heads bred on partially slotted .oor. The characteristics of the raw swine slurry are listed in Table 1. Sampling was carried out in the morning time. When the swine slurry was sent to the laboratory, the sedimentation separation experiment was started at once.

Anaerobic sludge obtained from a full-scale digester treating swine slurry of the same pig farm. 2.2. Sedimentation separation experiment The settling column (Fig. 1) comprised a cylinder made of transparent rigid, Plexiglas tubing with a 15 cm internal diameter and a 2.45 m height. Sampling ports

were uniformly spaced along the length of the column at the heights of 45, 85, 125, 165, 205 and 245 cm from the bottom of the column. After the raw swine slurry was pumped in the settling column, samples were collected from each sampling point at 10, 20, 30, 60, 90, 120 and 180 min. Analysis of the samples collected throughout the experiment showed the change in concentration with time and height difference.

Fig.1eSchematicdiagramofthesettlingcolumn.

After .nishing above sedimentation experiment, all liquid in the settling column was replaced with raw swine slurry. After 3 h’ settling, the supernatant above port 5 (at 45 cm height) was collected, referred to as low content liquid. Samples collected below port 5 were referred to as

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concentrated liquid. Low content liquid and concentrated liquid were stored at 5 C until being used for the experiment of biogas production potential. 2.3. Biogas production potential assessment of the concentrated liquid and the low content liquid The biogas production potential of the concentrated liquid and the low content liquid was measured with totally 9 sets of biogas fermentation devices (digesters). Each digester consisted of a 1000 mL glass .ask with a working volume of 900 mL. The digesters were sealed with rubber stoppers containing an in.uent/ef.uent port to allow injection of feedstock. A 500 mL glass .ask was used as a gasholder. The gasholders were also sealed with rubber stoppers containing an in.uent/ef.uent port to allow the entrance of biogas and the discharge of water. The digesters were connected by rubber tubes to the gasholder. Biogas was collected and measured by the displacement of water from the gasholders (Fig. 2). Initially, the reactors were inoculated with 500 mL of anaerobic sludge. The reactors operated in a draw-and-.ll mode twice a day. Draw-and-.ll mode means that a de.nite volume of supernatant of digester was decanted .rst, and then the same volume of feedstock (the concentrated liquid or the low content liquid) was .lled into digester. In the .rst 50 days, the volume of one draw-and-.ll was 50 mL for the concentrated liquid and 100 mL for the low content liquid. In the next 25 days, to ensure the same organic loading rate (OLR) in the experiment, the volume of one draw-and-.ll was changed to 50 mL for the concentrated liquid and 200 mL for the low content liquid. In order to distinguish the amount of gas produced by the inoculum itself, the control experiments were performed with digesters which contain the same volume of anaerobic sludge and 400 mL tap water. Each experiment was done in triple replication. A water bath was used to maintain the temperature of the digesters at 35 . 1 C. The biogas fermentation systems were operated for 75 days.

2.4. Analytical methods

The analysis on total solids (TS), suspended solids (SS), chemical oxygen demand (COD), ammonia nitrogen (NH3eN), total nitrogen (TN) and total phosphorus (TP) was carried out

according to standard methods [18]. pH was determined using a pH meter (inoLab. pH 7200, WTW-Wissenschaftlich Technische Werkstatten GmbH, Germany). Biological oxygen demand (BOD5) was determined using a BOD meter (Oxide. Control A12, WTW-Wissenschaftlich Technische Werkstatten GmbH, Germany). Total potassium (TK) was analyzed in HFeHClO4 extract. The potassium in all extracts was deter-mined using a .ame photometer (Systolic model no. 121).

Fig.2eSchematicdiagramofabiogasfermentationdeviceforassessmentofbiogasproductionpotential.

3. Results and discussion

3.1. Separation of swine slurry by gravity sedimentation Tables 2 and 3 show the variation of SS and COD in different heights of the settling column. During the .rst 30 min of settling, the concentrations of SS and COD increased with decreasing distance to the bottom of the settling column. After 30 min of settling, the SS and COD contents at each of the .ve sampling points ranged narrowly from 1.1 g L1 to 1.3gL1 and4.3gL1

to 4.6gL1, respectively. Comparing this with raw slurry, the overall reduced rate varied between 78.1% (bottom sampling port of the column) and 78.3% (top sampling port of the column) for SS, and from 52.3% (bottom sampling port of the column) to 56.0% (top sampling port of the column) for COD after 30 min of settling.

The mixed liquid above sampling port 5 (45 cm) could be referred to as supernatant (the low content liquid). The characteristics of the raw slurry and supernatant after 3 h’s settling are illustrated in Table 1. For supernatant, gravity sedimentation reduced COD, BOD5, TN and TP by 52.7%, 52.8%, 42.4% and 52.8%, respectively. The sedimentation ef.ciency of SS found in this study are comparable to those found in previous studies [19]. The sedimentation ef.ciency of COD may be comparable with the results obtained using a polyacrylamide (PAM)-aided solids .occulation treatment [20].

Through gravity sedimentation, raw swine slurry was separated into the concentrated liquid collected below port 5 (45 cm) and the low content liquid collected above port 5. The distribution of organic matters and nutrients in the concentrated liquid and the low content liquid is shown in Table 4. Table 4 indicates that although the volume of the concentrated liquid accounted for 18.3% of the total volume, the quantity of COD or BOD5 in the concentrated liquid both comprised about 61%, and that of TN, TP and TK comprised 53.0%, 61.4% and 50.0%, respectively, of the total content in the settling chamber. This indicated that most of the organic matters and nutrients were condensed in the concentrated liquid occupying a small portion of the total volume of the chamber. In the meantime, the concentration of COD, BOD5, TN and TP of the low content solid liquid was decreased remarkably compared with the raw slurry, resulting in that it can easily meet discharge standards after aerobic treatment.

3.2. The biogas production potential of the concentrated liquid and low content liquid

At a fermentation temperature of 35 . 1 C, the performance of biogas production of the concentrated liquid and low content liquid is shown in Fig.3. The biogas production potential re.ects the potential biogas production of the feedstock. For the

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a The distance from the bottom of settling column to the sampling port.

a The distance from the bottom of settling column to the sampling port.

concentrated liquid, 1 L of feedstock produced about 10e20 L of biogas, with an average volume of 15.9 L (Table 5). For the low content liquid, 1 L of feedstock produced only about 2e3Lof biogas, with an average value of 2.37 L (Table 5). The biogas production potential of the concentrated liquid was about 6.70 times that of the low content liquid. Moreover, the volumetric biogas production rate, which re.ects the production ef.ciency of the digester also showed the digestion ef.ciency of concentrated liquid, was superior to that of the low content liquid. At the same fermentation temperature and organic loading rate, the digesters using concentrated liquid as feedstock could produce about 1.5e2.0 L of biogas per liter of digester per day, with an average of 1.60 L L1d1. However, the digesters using

low content liquid could only produce about 0.70e1.0LL1d1, with an average of 0.946 L L1d1. The higher volumetric biogas production rates of concentrated liquid may be due to a longer hydraulic retention time (HRT) and high concentration of the substrate. These higher volumetric biogas production rates of digesters using concentrated liquidasfeedstock are comparable with the results obtained in a bench scale anaerobic digester with pig dung as feed material [21]. The biogas production potential and volumetric biogas production rate both illustrated that the performance of biogas production of concentrated liquid was much better than that of low content liquid.

From the biogas production potential, it can be calculated that 35.3 L of low content liquid, accounting for

81.7% of the total volume of raw slurry, could only produce 81.7 L of biogas, accounting for 40.0% of the total biogas yield. Although the volume of concentrated liquid was only 7.9 L, comprising 18.3% of the total volume of raw slurry, the biogas yield reached 125.6 L, accounting for about 60% of the total biogas yield (Table 5). These results were consistent with the distribution of COD and BOD5 in the concentrated liquid and low content liquid (Table 4).

3.3. Energy balances and estimates of the required digester volumes

Taking a 50,000-head pig farm as an example, the discharge of slurry was estimated at about 20 L per pig per day [22],adding up 1000 m3 d1 for the whole livestock farm. A schematic diagram composed of the new process and a conventional process for biogas fermentation of the swine slurry was shown in Fig.4.

Energy balances and estimates of the required digester volume in Table 6 were calculated on the base of the following criteria.

(1) The datum in Table 1 was taken as the concentration of raw swine slurry. (2) According to Table 4, in the new process reported in this paper, the separation ratio based on the volume of the

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a 20 0.7 18

0.6 Biogas yield per unit of feedstock

(L L-

1)

16 0.

514 12

0.4 1

0 0.3

Biogas yield per unit of COD-1

)

(L g

8 6 0.

2 4 0.1

2 0 0

1 3 5 7 9 11 13 15 17 19 21 23 25 Time (d)

Fig. 3 e The potential and ef.ciency of biogas production of the concentrated liquid and low content liquid (a) Biogas yield per unit of feedstock; (b) Volumetric biogas production rate.

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Fig. 4 e Schematic diagram of the new process and a conventional process.

concentrated liquid to low content liquid was assumed to be 2:8. Only the concentrated liquid was anaerobically digested, while the low content liquid was aerobically treated directly. According to Table 5, biogas yield per unit of feedstock for the raw slurry and the concentrated liquid were 4.84 L L1 and 15.9 L L1 of liquid respectively.

(3) In the process of conventional combined anaerobic-aerobic treatment of swine slurry, about one-third of the raw slurry must be added to the digested ef.uent to ensure steady running of the aerobic post-treatment process [23]. (4) Digestion temperature which could be increased was calculated according to the literature [24], using biogas lower heating value of approximately 21 MJ m 3. The biogas was used to generate electricity; then excess heat from the gas engine was used to heat the digester [16]. The lowest temperature of swine slurry assumed to be 5 C. The digester and feedstock were heated by excess heat which accounted for about one-third of the total energy of the biogas produced [16].

(5) Through calculation of energy balances, the digestion temperature could be increased 6.43 C for raw slurry, and 22.2 C for concentrated liquid (Table 6). Plus

the lowest temperature of swine slurry (5 C), the digestion temperature could reach 11 C for raw slurry, and 27 C for concentrated liquid. According to authors’ experience in designing and operating biogas plant fermenting swine

slurry [22], the volumetric biogas production rate of

d1digester was about 1.00 m3 m 3 at 25e30 C, and

0.33 m3 m 3d1 at 10e15 C.

In order to satisfy assumption (3) above, 667 m3

of water per day would be added to the digester in the process of conventional combined anaerobic-aerobic treatment, and the d

1

other 333 m3 would be added to the digested ef.uent [22,23]. For the new process reported in this paper, however, the amount of slurry (concentrated liquid) fed into the digester was about 200 m3 d1 (Fig. 4). The estimated biogas yield of the two processes was all about 3200 m3

d1. The biogas yield was almost the same, so was the excess heat from the gas engine. The more the amount of feedstock was fed into digester, the lower digestion temperature turned. As Table 6 shows, the digestion temperature could be increased by approximately 22 C using the excess heat of biogas generation for heating the concentrated liquid but only by about 6.43 C for heating the raw slurry. This indicates that normal operations of the biogas plant heated only by excess heat from the CHP in winter could not be maintained in the conventional process, but could do by the digester treating the concentrated liquid with the new process. To reach the same removal result and biogas yield, the required digester volume would be about 9700 m3 for the conventional process, and about 3200 m3 for the new process. The new process would therefore require

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a About one-third of the raw slurry was added to digested ef.uent to ensure steady running of aerobic post-treatment.

a digester only one-third the size of a conventional digester. The above facts indicate that the new process could remarkably reduce the cost of the digester and ensure steady running of the biogas plant during winter. 3.4. Comparison of total nutrient content and transportation costs

The best disposal use for digestate is to fertilize cropland. The nutrients within the swine slurry are largely retained during the process of digestion. Therefore, the calculations of nutrient concentrations in Table 7 were based on the nutrient content of the raw slurry. From Table 7, it can be seen that the total nutrient content of the concentrated liquid was nearly three times that of the raw slurry, and about .ve times that of the low content liquid. The distance traveled to haul and spread the manure is the most important variable affecting the cost of animal manure application on cropland [25]. The total nutrient content is an important factor in.uencing potential transportation distances of swine slurry or its digestate. The transportation cost per unit of nutrient is much higher for swine slurry or its digestate than for inorganic fertilizer. The 68.5-fold higher nutrient density in inorganic fertilizer is the reason that the unit transportation costs are

68.5 times higher for the digestate than for the inorganic fertilizer. When the transportation cost of slurry or its digestate is higher than the cost of purchasing inorganic fertilizer, farmers would not apply it, even if given for free. In our

analysis, the maximum transportation distance was de.ned as the distance at which the transportation cost of slurry or its digestate, if given for free, is equal to the cost of purchasing inorganic fertilizer. As Table 7 shown, the maximum transportation distance of concentrated liquid (13.1 km) was much longer than that of raw slurry (4.48 km). This indicates that the separation of raw slurry would facilitate its utilization.

4. Conclusions

The method shown here indicates that swine slurry could be separated into the concentrated liquid and the low content liquid by gravity sedimentation. The ef.ciency of biogas production using the concentrated liquid is superior to that using the low content liquid. Higher temperatures can be maintained for the digestion of the concentrated liquid heated by the excess heat of biogas generation, resulting in a reduced cost on the construction of digester and a stable operation of biogas plant through winter. After separation, the concentrated liquid canreadilybeusedasfertilizer, and the low content liquid can easily meet discharge standards after aerobic treatment.

Acknowledgments

This study work was supported by China Agriculture Research System (CARS-36-10B). The authors wish to express their

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special gratitude to Mr. Cyimana Mulinda for his English language review and correction.

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