The sampling of the influent, effluent, and solid materials of the solid-liquid separation (SLS) was conducted after pre-operation of each device for 5 to 30 minutes, in accordance with the characteristics of the respective equipment. The sampling method adhered to specific regulations and guidelines regarding the turbidity sampling criteria and testing methods for the liquid fraction. For the sampling of the influent, effluent, and solid materials of the SLS, sampling was performed after a pre-operation period ranging from 5 to 30 minutes, depending on the characteristics of each device. In the case of liquid samples, such as the influent and effluent, a T-valve was installed at the inlet and outlet points to facilitate sample collection upon request (Please note that in pig farms where the installation of a T-valve was physically impractical, sampling was conducted directly from the inlet and outlet pipes). The liquid samples were collected by the operator after an arbitrary time determined based on on-site conditions. The influent and effluent samples were collected at the midpoint of the predetermined time interval. Regarding the collection of solid samples, the sampler designated an arbitrary time frame (ranging from 1 to 30 minutes) and employed an appropriate container to collect the entire discharge from the solid outlet of the SLS. The weight of the collected sample was measured on-site, followed by the extraction of a portion of the solid sample from the collection container.

The samples were collected from 11 pig farms that used seven different kinds of SLS processes. Table 1 shows the sampling site, pig population (head) and the technologies they used while this study was being conducted. The average livestock population was 3,233 heads in surveyed farms and the finishing pig slurry was used for this study. Six out of 12 farms in this study paired screw press with other SLS processes and three farms used coagulation agents. The samples were collected after 5 to 30 minutes of pre-operation based on the characteristics of each solid-liquid separator from individual pig farms. The separated solids were sampled from the solid containers after 30 minutes of solid-liquid separator operation, and then collected solids were mixed uniformly and weighed on site. The influents and separated liquids were collected by installing T-shaped valves on the inlet and outlet. On farms where installing valves was not possible, the influents and treated liquid were collected from the inlet and discharge pipes. The influents and separated liquids were collected when the treated solids were past their half-time. The chemical analysis of the influents, separated liquids, and separated solids consisted of pH, EC, TN, TP, K, TS, SS, NaCl, and heavy metals. The COD_Mn, BOD₅, were analyzed for influent and separated liquid, and the moisture content was only analyzed for separated solids. These analyses were conducted according to the standard method for the examination of water and wastewater. The pH and EC were measured by YSI meter (multilab IDS 4010-2, xylem Inc. US). The heavy metal content (Cu, and Zn) was measured using Spectroblue IPS.OES (FMX36, Germany) based on the US EPA method 7476.

The removal efficiency was calculated using the formula below.

Where, η : removal rate (%)

A. A : Concentration before treatment

B. B : Concentration after treatment

The different screen mesh sizes from different SLS methods create differences between operation and removal efficiencies of SLS systems and manure characteristics 19). The pH and EC are two important manure characteristics, especially when manure is used for liquid or compost fertilizer. The changes of pH and EC by SLS processes show similarities with the finding of Jørgensen and Jensen (2009) 19), where separated liquids from the influents showed stable pH changes but variable EC changes. Our values show that the pH units remained at 6.61–8.63, which were near neutral, for 44 samples, while EC varied from 2.57 to 4.47 mS/cm. They also show that the Centrifuge (+coagulation agent) had the highest reduction of EC. The near-neutral pH (6.7 to 7.2 units) and widely varied EC (4.9–17.0 mS/cm) are also reflected in studies by Kumaragamage et al. (2016) 21) and Vanotti et al. (2018)14). However, the samples that used coagulation agents showed a little increment in pH that because of the chemical compositions of the coagulant agents but stayed near neutral 11,12,20). In this study, the pH range of influents and separated liquids did not change much by any of the SLS processes (Table 2). The pH changed from 6.90 162 to 6.65, 7.08 to 7.23, 7.20 to 7.23, 7.00 to 6.88, 7.28 to 7.25, 7.40 to 7.53, and 7.15 to 7.20 units for the V/S, D/S, I/S, S/P, Cf, Cf (+Cog), and B/P (+Cog) processes, respectively. The electrical conductivity (EC) reduction by Cf (+Cog) of 40.5% was the highest when the influent EC was 29.8 mS/cm and the separated liquid contained 17.8 mS/cm EC. For the V/S, the influents had the highest EC value, 50.1 mS/cm, but it was reduced 15.0% to 42.6 mS/cm after the treatment. The lowest EC removal was observed for the Cf, where EC reduction was only 1.5%, from 28.1 mS/cm to 27.7 mS/cm. On the contrary, B/P (+Cog) with a coagulant agent did not show much EC reduction (2.3%). The NaCl reduction by Cf (+Cog) and B/P (+Cog) with a coagulation agent showed the highest removal efficiency at 74.6% and 72.9%, respectively (Table 2). Among the three screening separator processes, the V/S added up to 2.3% NaCl, I/S reduced 32.0%, and D/S reduced 173 only 0.5%.

Figure 1 shows the changes in TS and SS by different SLS processes. The B/P (+Cog) and the Cf (+Cog) had 78.76% and 67.94% TS removal efficiency, respectively, the highest. Excepting these two coagulations agents–supported SLS processes, the rest of the SLS processes showed 12.8~23.0% TS removal efficiency. The coagulant-supported SLS processes had the highest SS reduction: 96.9% and 67.94% by B/P (+Cog) and Cf (+Cog), respectively. The Cf process had SS removal efficiency of 32.2%. The rest of SLS processes showed 8.1~24.1% TS removal efficiency. Meanwhile, the moisture in the separated solids was 73.2%, 68.6%, 74.7%, 65.7%, 74.0%, and 76.4% for the V/S, D/S, I/S, S/P, Cf (+Cog), and B/P (+Cog) processes, respectively. However, the Cf process contained the lowest moisture content at 47.41%.

Comparing the screw press (S/P) with a centrifuge (Cf), several authors found that the centrifuge achieves higher performance 8,10,22). According to Aguirre-Villegas et al (2019) 14), Cf processes are more effective for the removal of TS, SS, TN, TP, K, BOD₅, and COD_Mn than S/P processes. This finding is also reflected in this study (Table 1). Figure 2 shows the changes in nutrient content (TN, TP, and K), BOD₅, and COD_Mn by different SLS processes. When comparing Cf process with V/S, D/S, V/S shows better removal of TS, TN, and BOD₅. The Cf (+Cog) process had a better EC, NaCl, TS, and TP removal 9 efficiency than the B/P process. On the other side, the B/P (+Cog) process had a comparatively better performance for TN, K, BOD₅, and COD_Mn removal than the Cf (+Cog) process. The relatively low removal efficiency of TN, TP, and K in separated liquids by mechanical processes without coagulation agents could be because of their higher suable or dissolved form 12,14,23,24). The higher TP concentrations in separated liquids and the removal efficiency for B/P (+Cog) are similar to those in a study conducted by Møller et al. (2002) 25). However, the higher removal efficiency of TN, TP, and TK by Cf (+Cog) and B/P (+Cog) is due to the chemical treatment coupled with mechanical (centrifuge and belt press) treatment, which screened out smaller sized particles 20,26). The B/P (+Cog) showed the highest TN reduction of 41.0%, where the influents had 7,857 mg/l, and the separated liquid had 4,631 mg/l of TN. The Cf (+Cog) had the second highest TN removal of 36.4%, where the TN reduced from 5,197mg/l to 3,304mg/l. However, V/S, D/S, I/S, S/P, and Cf showed 2.9~13.1% TN removal efficiency. The removal of TP had similar results as that of TN. The Cf (+Cog) and B/P (+Cog) processes showed 94.2% and 91.5% of TP removal efficiency, respectively, in separated liquids. However, V/S, D/S, I/S, S/P, and Cf showed 8.0~29.8% TN removal efficiency. The B/P (+Cog) and Cf (+Cog) showed 59.4% and 25.6% K removal efficiency, respectively. The I/S, S/P, and Cf had only 0.4~3.5% removal efficiency for K. Moreover, in the case of V/S and D/S, the concentration of K somewhat increased somewhat in separated liquids. The B/P (+Cog) and Cf (+Cog) showed 59.4% and 25.6% K removal efficiency, respectively. The I/S, S/P, and Cf had only 0.4~3.5% removal efficiency for K. Moreover, in the case of V/S and D/S, the concentration of K somewhat increased in separated liquids. The concentration of COD_Mn and BOD₅ indicates the presence of oxygen-demanding substances in wastewater and are often used as pollution indicators. In this study, the chemical coagulant agent–supported centrifugal and belt press SLS were able to remove the highest amount of BOD₅ and COD_Mn. The B/P (+Cog) reduced 66.0% of BOD₅, and the Cf (+Cog) reduced 53.9% of it, and the concentration of BOD₅ in influents was 29,580 mg/l and 22,825 mg/l, and after treatment in separated liquid that reduced to 10,050 mg/l and 10,525 mg/l, respectively. The removal of COD_Mn had similar results as that of BOD₅, comparatively. The Cf (+Cog) and B/P (+Cog) processes showed 75.6% and 77.5% of COD_Mn removal efficiency, respectively, in separated liquids. However, other SLS processes showed low removal efficiency for BOD₅ (4.7~14.6%) and COD_Mn (5.3~27.8%). About 40% of BOD₅ was found in a solid fraction of manure; therefore, after performing SLS, about 60% of the BOD₅ was supposed to remain in the separated liquid 27). Therefore, the TS and BOD₅ 10 concentration in separated liquids remained in the same sequential order of V/S, S/P, D/S, Cf, I/S. For the coagulation agent–assisted SLS processes, the respective TS and BOD₅ removal was 67.9% and 53.9% for Cf (+Cog) and 78.8% and 66.0% for B/P (+Cog). Similar properties were noticed in changes in COD_Mn. Each sample that contained higher TS concentrations in both the influent and separated liquids showed higher COD_Mn concentrations as 28).

Separating of heavy metals such as Zn and Cu from liquid manure before land application reduces the risk of soil contamination 14,29). Among all the analyzed heavy metal contents, Zn and Cu were found in a higher concentration in the influents. The probable cause is that the pigs get the Zn and Cu from their feeding operation 30). Figure 3 shows the changes in nutrient content Zn and Cu by different SLS processes. The coagulant assist centrifuge (Cf (+Cog)) and belt press (B/P (+Cog)) were very effective at reducing the heavy metals such as Zn (100% and 98.6%) and Cu (99.1% and 98.1%). These findings are similar to those of Vanotti et al. (2018)11), which found that a PAM-based mechanical pressing SLS system could remove 88% of Zn and Cu. However, contrastingly, other SLS processes showed low removal efficiency for Cu and Zn. The V/S, D/S, I/S, and Cf showed 5.2~12.4% Zn removal efficiency and 0.4~33.7% Cu removal efficiency, respectively. Moreover, in the case of the I/P process, the concentration of Zn and Cu somewhat increased in separated liquids.