The condition of investigated swine farms
Breeding scale of 15 farms producing solid compost were averaging 4,362±5,689 swines/farm and solid amounts after solid and liquid separation of swine manure were averaging 7,918.5±11,155.4 kg/d and each swine produced 1.65±0.57 kg/head·d solid manure (
Table 1). Most of the farms were using saw dust (80%) and few used rice hull (20%) as bedding materials averaging 25.4±10.82 g/head·d by mixing with solid part of manure. All the farms used turning methods for air-supply and only 6 farms were using blower for aeration and turning methods together. The average amount of compost produced was 4,487.7±8,759 kg/d of which differences were occurred by breeding scale and the amount of bedding material used. The compost produced by each swine was 0.69± 0.5 kg/head·d.
A total of 14 farms investigated were producing liquid compost, with an average breeding scale of 3,198±2,849 swines/farm (
Table 1). The influent for liquid composter is generally a part of water after solid and liquid separation. Thus, a certain amount of solid is present in the influent depending on the performance of a separator. The amount of liquid part has seasonal variation and direct investigation was impossible. Therefore, using the excretion unit values reported from ME, liquid part (urine+cleaning wastewater) was extracted of 4.23 kg/head·d. The average liquid amount after solid-liquid separation was 13,527±12,051 kg/d. There was no use of bedding material but three types of aeration methods were used for liquid composting i.e. continuous, intermittent, and no aeration. The average liquid compost produced was 12,329± 11,892 kg/d while each swine produced 3.70±0.30 kg/head·d liquid compost.
Nutrients in solid and liquid manure
According to the investigation, the solid part of about 1.65 kg/swine·d was produced for composting except bulking agent, which is 2 fold higher than ME report of 0.87 kg/swine·d. The deviation of mixture produced may be occurred by the excreta collecting structure such as scrapers, workforce collection, and slurry types. The concentrations of N and P in solid phase via this study and ME reports were 12.2 and 14.7 g N/kg and 4.7 and 10.2 g P/kg, respectively, as an average value from this study. Calculating the annual nutrient amounts based on both values, the amounts of N and P were 7.6 and 3.0 kg/head·yr, respectively, while ME reported values of 4.7 and 3.3 kg/head·yr, respectively, which are approximately 1.6 fold lower for N (
Table 2). On the other hand, although the VS production of 136.6 kg/head·yr was assessed, the value cannot be compared with ME where the organic compounds have been presented in biochemical oxygen demand rather than VS.
The nutrient contents in the liquid component of the samples (
Table 3) were measured and the average concentration of VS, N, and P were 14.3, 3.6, and 0.5 g/L, respectively, of which N values were relatively similar to the values of 3.5 g N/L and 0.78 g P/L from ME. The annual nutrient production for VS, N, and P were 22.1, 5.55, and 0.76 kg/head·yr respectively, while the ME reported 5.4 and 1.2 kg/head N and P, respectively.
Nutrients produced in solid and liquid compost
Analyses of the compost samples (
Table 4) revealed the average concentrations of VS, N, and P were 411.9, 21.4, and 11.5 g/kg, respectively. The values for N and P were almost three and four times higher than the values before composting, probably due to the vaporization of water during the composting period [
12,
13]. Nutrients obtained for TA were 360.3, 16.4, 10.5 g/kg for VS, N, and P, respectively, while for values for T were 437.7, 23.8, 12.0 g/kg for VS, N, and P, respectively. Besides, the bulk density of T influenced by moisture content after composting was 35% lower than TA, which is contrary to our expectation; this could be a result of frequent turning. Use of bulking material was 9.9±3.1 kg/head·yr, which was similar to TA (9.6) and T (10.5).
Swine manure is first treated for solid and liquid separation, followed by the composting procedure performed independently. Before being applied to arable land as fertilizers, the solid part passes through the composting procedure with the addition of bulking agents such as sawdust, rice husk, or straw. For high quality compost, the addition of bulking agents is required for appropriate moisture, carbon to nitrogen ratio, and porosity. Thus, the VS content in a mixture of dewatered manure and bulking agents are increased when considering the dewatered manure itself. In general, most nutrients decrease due to microbial degradation, with the amount of nitrogen loss being the highest, mainly via ammonia volatilization [
14–
17].
The characteristics of liquid compost differed considerably, according to the aeration methods used. As shown in
Table 7, the average concentrations of VS, N, and P in liquid compost were 3.96, 1.55, and 0.08 g/L, respectively, which are about half the concentrations found in manure. The aeration intensity governed the concentrations of each nutrient. Under continuous aeration, the influent had the lowest concentrations of VS, N, and P, averaging at 2.4, 1.0, and 0.06 g/L, respectively. With no aeration and simple storage, highest concentrations of VS, N, and P were achieved, at 8.19, 2.63, and 0.19 g/L, respectively, which are 3.4, 2.6, and 3.2 folds higher than values from continuous aeration method. Consequently, continuous aeration led to the lowest annual amount of VS, N, and P produced at 3.0, 1.28, and 0.07 kg/head·yr, respectively, whereas no aeration resulted in higher amounts of 11.67, 3.69, and 0.27 kg/head·yr, respectively. On an average, 5.46, 2.15, and 0.11 kg/head·yr VS, N, and P were produced in the liquid compost, respectively. Reductions of nutrients in liquid compost when compared to influent were higher during continuous aeration than intermittent aeration. Simple storage (no-aeration) also had lowered nutrient concentrations compared to the influent, which might be due to precipitation and microbial degradation under anaerobic conditions. These results indicate that when manure is in liquid phase and is aerated or stored, there is a reduction in nutrients despite evaporation, as compared to the solid phase there is an increase in nutrients due to water evaporation.
Weight loss rate and evaporation rate of solid and liquid compost
Weight loss rates (%) for solid compost are presented in
Table 5. For scenario S1, similar weight losses of 62% and 63% were observed for TA and T, respectively. Using the reference study, scenario S2 showed resultant weight losses of 71% and 80% by TA and T, respectively [
10]. The high weight reduction in S2 might be due to the calculation with flat volume reduction from a reference. In S3, the theoretical phosphorus changes were believed to be zero (ΔP = 0) as P losses only occur via leachate or runoff. In this scenario, weight losses of 58% and 55% were calculated in TA and T, respectively.
Daily solid compost production (kg/head·d) presented in
Table 5 was greatly affected by the weight loss rates (
Eq. 6). The compost production for S1 was higher than Case I (0.78 TA; 0.64 T) compared to Case II (0.34 TA; 0.34 T). Compared to scenarios S1 and S2, scenario S3 had the lowest weight losses, which is why the compost production is also higher for both Case I (0.85 TA; 0.78 T) and Case II (0.37 TA; 0.40 T). The compost production for Case I was higher (0.78, 0.55, 0.85 kg/head·d) for TA compared to T only (0.64, 0.35, 0.78 kg/head·d) for all three scenarios, while in Case II the values for S1 were somewhat similar. However, in scenarios S2 and S3, T only (0.30, 0.40 kg/head·d) had higher compost production when compared to TA (0.26, 0.37 kg/head·d).
The final form of organic carbon is carbon dioxide and water under aerobic condition. However, the VS amount did not decrease much after composting since bulking agents were added to control proper moisture content of around 65%. Therefore, the actual nutrient loading on soil could be calculated by measuring the nutrient concentrations in compost and the amount of compost produced after considering the weight loss during the composting period. The weight of compost was lower when compared to the solid phase (i.e., fresh manure) probably due to moisture vaporization, leachate, and nutrient decomposition [
18,
19].
During composting of the liquid manure, depending on the intensity of aeration water evaporation occurs during storage and aeration. Hence, the evaporation rate of liquid compost for each aeration method was assessed on a farm having a 200 m
3 liquid composter. The evaporation rates of 0.44, 0.36, and 0.26 m
3/d were measured on a 50 m
2 of surface area for continuous, intermittent, and no aeration, respectively, which corresponds to an evaporation rate of 8.75, 7.27, and 5.14 L/m
2·d, respectively (
Table 7). Continuous aeration resulted in higher evaporation rate, and even mere storage (no aeration) resulted in evaporated water content.
The average annual liquid compost production was 1.35 m
3/head·yr, calculated by considering the evaporation rates of each composting method (
Table 7). Of the three methods employed, continuous aeration produced bare minimum (1.28 m
3/head·yr) and no aeration produced maximum (1.41 m
3/head·yr) liquid compost. Due to the high evaporation rate, the final liquid compost obtained was lower at high intensity aeration and comparatively more under conditions of no aeration.
Nutrient loading coefficients
The solid phase NLCs obtained for combination of scenario 1 and case 1 (S1×Case I) were (VS, 0.67; N, 0.48; and P, 0.96) for TA while (VS, 0.93; N, 0.69; and P, 0.94) for T; these values were comparatively higher compared to the NLCs obtained with ME values (
Table 6). When the nutrient amounts in compost were compared with the nutrients produced in fresh manure, the percentage of nutrient loss during the composting period were (VS, 33.0; N, 52.0; P, 4.0) for TA and (VS, 7.0; TN, 31.0; P, 6.0) for T.
The liquid phase NLCs are presented in
Table 7. Based on the evaporation rate, the values obtained were (VS, 0.16; TN, 0.26; P, 0.08) for continuous aeration, (VS, 0.32; N, 0.52; P, 0.15) for intermittent aeration, and (VS, 0.82; N, 0.67; P, 0.91) for no aeration (simple storage). For liquid compost, the percentage (%) loss of nutrients was (VS, 84; N, 74; P, 92) for continuous aeration, (VS, 68; N, 48; P, 85) for intermittent aeration, and (VS, 18; N, 33; P, 9) for no aeration.
Using nutrient loading values, the NLCs from one swine (solid+liquid) obtained from sum of nutrients from solid and liquid phases were (VS, 0.79; N, 0.53; P, 0.71), whereas the annual VS, N, and P amounts were decreased by 21%, 47%, and 29%, respectively. Using values from ME, the obtained NLC values for N and P were 0.45 and 0.31, with nutrient losses of 55% and 69% N and P; this was 1.2 and 2.4 fold higher, respectively, than the farm investigated values.
Nutrient loadings from solid and liquid compost
Based on the above cases and scenarios, the annual amounts of nutrient loading on arable land were extracted according to composting methods TA and T (
Table 6). Except N of S1×Case I, and both N and P of S3×Case I, all combinations with the TA composting method had increased nutrient loading amount compared to the values from T only. For Case II, all values for TA were lower than T for all scenarios. The difference is not much, except for the VS of S1×Case I. The average nutrient loading obtained from S1×Case I of solid compost for VS, N, and P were 112.1, 5.0, and 2.5 kg/head·d, respectively.
On an average, 5.46, 2.15, and 0.11 kg/head·yr VS, N, and P, respectively, were produced in the liquid compost (
Table 7). The highest annual nutrient loadings were generated by no-aeration method, while continuous aeration produced the least amount of nutrients. Simple storage (no-aeration) also had lower nutrient concentrations compared to the influent, which might be due to microbial degradation under anaerobic conditions and precipitation.
The annual productions of VS, N, and P per head were obtained with the combination of S1×Case I, and the average values for solid and liquid compost, respectively. Thus, one swine breeding annually produces approximately 117.6, 7.2, and 2.7 kg/head·yr VS, N, and P in compost, respectively. The nutrient loading obtained for S1×Case II using ME values were 58, 4.6, and 1.4 kg/head·yr VS, N, and P, respectively, which was almost half of the values obtained using farm investigation.
The NLCs from Hanwoo reported by Won et al [
20] were 0.96, 0.31, and 0.60, and the annual production derived were 213.4, 14.4, and 5.0 kg/head·yr, for VS, N and P, respectively. For dairy cattle, the calculated NLCs were 1.48, 0.60, and 0.66, with annual nutrient production of 423.4, 43.3, and 10.6 kg/head·yr, for VS, N, and P, respectively [
21]. Per head nutrients (VS, N, P) produced from Hanwoo were 1.8, 2.0, and 1.9 fold higher, while they were 3.6, 6.0, and 3.9 fold higher for dairy cattle than the amounts generated from swine, respectively.
In 2012, the entire amount of livestock manure produced in Korea was 45,293 kt/yr, of which the highest fraction was occupied by swine manure (39.2%) followed by beef cattle (33.8%), chicken (14.5%), and dairy cattle (12.5%). Among the whole livestock manure, 88.7% of livestock manure was recycled as compost, and 9.1% of liquid phase mainly from swine manure was processed to release. Considering only the values of amounts of composting from NLCs of Hanwoo from Won et al [
20], dairy cattle from Won et al [
21], and swine from this study, and multiplying the nutrient (VS, N, P) concentrations obtained with the respective amount of manure produced, the respective amounts of VS, TN, and TP on soil after composting or storage period were found to be 606.8, 38.4, and 15.6 kt/yr for Hanwoo, 209.4, 17.7, and 4.5 for dairy cattle, and 517.5, 36.4, and 11.7 kt/yr for swine. Different from our expectation, the nutrients from Hanwoo manure on arable land contributed the most than from swine and dairy cattle manure.
Among OECD countries, Korea was ranked first in terms of N and second for P accumulation in soil. This is because of the direct calculation of the amount of nutrients generated by the number of livestock animals, as done in other OECD countries. The real manure management practices in Korea require treatment of manure before it is applied to arable land as fertilizer. As we know, during storage or treatment nutrients are lost from the manure, which are not calculated by OECD. Since circumstances for livestock breeding are very different in each country, it is very important to estimate the practical amount of nutrients on arable land in order to pursue sustainable agriculture. Other than swine manure, the investigation of all the livestock categories is required for building a database of nutrients in agricultural sector facilitates livestock manure management and conservation of environment.
In order to achieve sustainable agriculture, the recycling of nutrients from livestock manure is inevitable, and composting plays a key role. This study acknowledges that the Korean swine manure is primarily separated into solid and liquid phases before composting. Nutrients in compost applied to soil using ME values do not give a complete picture of the composting practices in Korea. It was observed that the composting was heavily affected by the weight loss rate during solid composting and evaporation rate during liquid composting. Farm investigation provided the combined nutrient load (solid+liquid) of VS, N, and P of 117.6, 7.2, and 2.7 kg/head·yr, respectively, from a single swine, while the ME values of nutrient loads were 58, 4.6, and 1.4 kg/head·yr for VS, N, and P, respectively. The nutrient load obtained with farm investigation was higher than that of obtained using ME values, suggesting that the nutrient calculations using ME values were half the values calculated in this study. This may be the cause of nutrient accumulation as different composting methods reduce nutrients differently. Providing the solid NLCs through building up a database of nutrient production from livestock manure accumulated for a few years, the NLCs will simplify the complex calculations to obtain the amount of nutrient loading directly from the nutrients from livestock manure on arable land, and its value will roughly represent the status of livestock manure and nutrient management.