Upgrading Solid Digestate from Anaerobic Digestion of Agricultural Waste as Performance Enhancer for Starch-Based Mulching Biofilm

Developing a green and sustainable method to upgrade biogas wastes into high value-added products is attracting more and more public attention. The application of solid residues as a performance enhancer in the manufacture of biofilms is a prospective way to replace conventional plastic based on fossil fuel. In this work, solid digestates from the anaerobic digestion of agricultural wastes, such as straw, cattle and chicken manures, were pretreated by an ultrasonic thermo-alkaline treatment to remove the nonfunctional compositions and then incorporated in plasticized starch paste to prepare mulching biofilms by the solution casting method. The results indicated that solid digestate particles dispersed homogenously in the starch matrix and gradually aggregated under the action of a hydrogen bond, leading to a transformation of the composites to a high crystalline structure. Consequently, the composite biofilm showed a higher tensile strength, elastic modulus, glass transition temperature and degradation temperature compared to the pure starch-based film. The light, water and GHG (greenhouse gas) barrier properties of the biofilm were also reinforced by the addition of solid digestates, performing well in sustaining the soil quality and minimizing N2O or CH4 emissions. As such, recycling solid digestates into a biodegradable plastic substitute not only creates a new business opportunity by producing high-performance biofilms but also reduces the environmental risk caused by biogas waste and plastics pollution.


Introduction
The plastic film technology is widely employed in social development due to its economic suitability and easy processing [1,2]. A plastic mulching film is often used to protect agricultural soil, aid establishment and accelerate the growth of crops in the seeding stage and reduce GHG (greenhouse gas) emissions from farmlands [3,4]. The successful practice has been used in agricultural fields in the past decades, and crops planted in mulching surroundings can achieve high quality and productivity. However, the popularity of plastic film also causes a high risk of white pollution, because there are

Composition Analysis
The contents of cellulose, hemicellulose and lignin in solid digestates were quantified by using two-step acid hydrolysis according to the National Renewable Energy Laboratory method [21]. Ash content was determined by a muffle furnace at 550 • C according to the incineration method [22]. Humic acid content was measured by examining the visible spectrum of humic acid extracted in the alkaline solution according to the spectrophotometric method [23].

Pretreatment
Solid digestates from the biogas plant were washed repeatedly to remove watersoluble impurities and dried at 60 • C for 24 h in a drying oven (Yiheng Scientific Laboratory Instrument Co., Ltd., Shanghai, China). The dried samples were mixed with 2% sodium hydroxide solution under the action of ultrasonic (60 • C) for 30 min in an ultrasonic cleaner (KQ5200DE, Changfeng Instrument and Meter Co., Ltd., Beijing, China) to remove humic acid and degrade hemicellulose, lignin and ash [24,25]. Precipitates after filtration were washed and dried for further use.

Biofilm Preparation
Preparation protocols of the composite biofilm are shown in Figure 1. Pretreated solid digestates were mixed with starch by the ratios of 1:5, 1:10 and 1:15. The mixtures of starch and solid digestate were then mixed with plasticizers (composed with glycerol and xylitol by a ratio of 1:1) by a ratio of 7:3 and dissolved in deionized water to configure 10% (w/v) mixing solution. High-pressure homogenizer (AH100D, ATS Engineering Inc., Shanghai, China) was used to homogenize this solution three times, and the homogenized solution was heated in a 99 • C water bath (HHS-4S, Yulong Instrument Co., Ltd., Beijing, China) with a constant stirring of 300 r/min by an electronic stirrer (OS20-Pro, Dragon Laboratory Instruments Co., Ltd., Beijing, China) for 1 h to promote starch gelatinization. Evaporation was minimized by sealing the opening of the container [26].
The fully gelatinized solution was retired and sheared by using a high-speed disperser (T25, Truelab Lab-sci Co., Ltd., Shanghai, China) for 10 min at 6000 r/min, ultrasonically vibrated for 30 min at 60 • C and placed in a vacuum oven (DZ-3, Taisite Instrument Co. Ltd., Tianjin, China) at 45 • C for 10 min to remove air bubbles inside the solution. Five milliliters of solution were poured and spread uniformly onto a plastic dish (9 cm in diameter) and dried at 45 • C for 5 h (solution casting method). Biofilms were peeled carefully after conditioning in a constant temperature and humidity incubator (JYH-152, Jiayu Scientific Instrument Co., Ltd., Shanghai, China) at 20 • C and 43% relative humidity for 24 h to reach the equilibrium moisture content. Film thickness was measured by an electronic digital caliper (PRO-MAX, Fred V. Fowler Co. Inc., Auburndale, MA, USA) at 5 random positions, and the moisture content was determined by a halogen moisture balance (HB43-S, Mettler-Toledo Instruments (Shanghai) Co., Ltd., Shanghai, China). As shown in Table 1, the thickness and moisture contents of all biofilms had no significant differences at the 95% confidence level. This means that the influences of the thickness and moisture content on the biofilm characteristics can be ignored in this study. Film thickness was measured by an electronic digital caliper (PRO-MAX, Fred V. Fowler Co. Inc., Auburndale, MA, USA) at 5 random positions, and the moisture content was determined by a halogen moisture balance (HB43-S, Mettler-Toledo Instruments (Shanghai) Co., Ltd., Shanghai, China). As shown in Table 1, the thickness and moisture contents of all biofilms had no significant differences at the 95% confidence level. This means that the influences of the thickness and moisture content on the biofilm characteristics can be ignored in this study.

Morphology
The morphology of the biofilm (surface and cross-section) was visualized by scanning electron microscopy (JSM-6700F, JEOL Ltd., Akishima, Japan). Biofilm was cut, fixed on a stub with double-sided adhesive tape and coated with a thin layer of gold. Each biofilm was examined at 500× magnification for the surface and 2000× magnification for the cross-section, with an accelerating voltage of 10 kV.

Crystallinity
Biofilm was crushed and shifted by an 80-mesh sieve. The diffraction pattern of the biofilm powder was determined using an X-ray diffractometer (XD-2, Purkinje General Instrument Co., Ltd., Beijing, China) with a Cu kα radiation at a scanning angular range from 5 • to 80 • . The sampling interval was 0.02 • , and the scanning rate was 4 • /min. The voltage and current were set as 36 kV and 20 mA. The relative crystallinity was calculated by using Equation (1) [27].

Fourier-Transform Infrared Spectroscopy (FTIR)
FTIR characterization analysis of the biofilm was performed on a Fourier-transform infrared spectrometer (Affinity-1S, Shimadzu Co. Ltd., Kyoto, Japan) at a scanning range from 4000 to 400 cm −1 [28]. FTIR spectra were achieved in a transmittance mode with a 4 cm −1 resolution at an accumulation of 100 scans.

Mechanical Property
The main mechanical properties, including strength, elongation at break and elastic modulus, were evaluated by a dynamic mechanical analyzer (Q800, TA Instruments Co. Ltd., Newcastle, DE, USA) in tensile mode [29]. Biofilm was cut into a 30 × 10 mm rectangular strip, clamped and applied by a 0.01 N preload to avoid buckling. Then, the strip was loaded with changing stress at a rate of 5 MPa/s until a fracture occurred. The working temperature and frequency were set as 20 • C and 1 Hz.

Thermal Stability
The thermal stability was analyzed by a differential scanning calorimeter (DSC-Q10, TA Instruments Co. Ltd., Newcastle, DE, USA) [27]. Biofilm was crushed, and about 3.5-mg powder was sealed in an aluminum crucible with an empty one as the control. Biofilm powders were heated from 20 • C to 300 • C at a heating rate of 10 • C/min. Glass transition temperature and melting temperature were determined from DSC thermographs.

Transparency
Transparency measurements were carried out by an automatic microplate spectrophotometer (SpectraMax M2 e , Molecular Devices Instrument Co., Ltd., Shanghai, China) to determine the opacity of the biofilm. Biofilm was cut into a circular shape with a diameter of 20 mm and placed in a 12-well cell culture cluster. The absorbance spectrum was recorded at a range from 300 to 780 nm, with a spectral bandwidth of 2 nm [30]. The absorbance was then converted into transmittance using Equation (2).
Light transmittance of the biofilm was determined in the UV-B region (315 mm), UV-A region (380 nm) and visible region (700 nm), respectively, in this study.

Water Vapor Permeability
A fifty-milliliter beaker with a 50.27 cm 2 circular opening was sealed by the biofilm and stored in a desiccator at 20 • C. Anhydrous calcium chloride (relative humidity (RH) = 0%) was placed inside the beaker, and a saturated potassium carbonate solution (RH = 43%) was poured into the desiccator to maintain the RH gradient across the biofilm [31]. The weight of the beaker was measured every 12 h until the weight change was less than 0.01 g. The water vapor permeability (WVP) was calculated by the following equations [32]: where P is set as 2.3388 × 10 3 Pa (20 • C) in this study, and R 1 and R 2 are expressed in fractions.

Gas Transmission Rate
The N 2 O and CH 4 transmission permeabilities were determined by a gas permeability tester (N500Z, GBPI Co., Ltd., Guangzhou, China). Biofilm was cut into a circular shape with a diameter of 4 cm and placed in a closed chamber with two cells on both sides of the biofilm. Working temperature and relative humidity were set as 20 • C and 43%. N 2 O or CH 4 was supplied with an inlet pressure of 2 kPa from one cell, and the other was vacuumed. The concentration of gas was measured using a multiple gas analyzer (LGR 907-0011, Yiwin Instrument and Equipment Co., Ltd., Shanghai, China).

Statistical Analysis
Each test was repeated at least three times, and the values were determined as the mean ± standard deviation. Duncan's test from SPSS 20.0 software (SPSS Inc., Chicago, IL, USA) was used to check the significance of all results when necessary at a 95% confidence level. X-ray diffraction results were smoothed by using the FFT filter of Origin Pro 8.0 software (OriginLab Co., Northampton, MA, USA).

Pretreatment of Solid Digestates
As shown in Table 2, hemicellulose, lignin, ash and humic acid in the solid digestates were degraded by thermo-alkaline with an ultrasonic-assisted pretreatment while the relative content of the cellulose increased. The highest removal efficiency of humic acid was 58.85% for the solid digestate of cattle manure, the highest removal efficiencies of lignin and ash were 43.19% and 44.11% for straw and the highest removal efficiency of hemicellulose was 36.66% for chicken manure. Moreover, the growth rate of cellulose in the solid digestate of cattle manure (63.88%) was highest, followed by straw (46.94%) and chicken manure (5.10%). This indicated that the pretreatment method was more suitable for solid digestates of straw and cattle manure than chicken manure, because there were more complex compositions in the solid digestate of chicken manure, such as crude protein, which inhibited the removal effect of the pretreatment method [33].

Morphology
The morphology structures of the surface and cross-sections of the biofilms are shown in Figure 2. The SEM images of PSF (pure starch film) presented a uniform and compact surface microstructure with a dense arrangement of gelatinized starch granules. There was no protuberance observed in PSF, because the presence of the plasticizer disrupted completely the intermolecular and intramolecular hydrogen bonds of the native corn starch during biofilm formation [31,34]. Comparatively, the surfaces of the biofilms with the addition of a solid digestate showed a relatively rough microstructure with a homogeneous dispersion of solid digestate particles in the starchy matrix. There were also some aggregations detected on the composite biofilm surface, leading to a direct transformation of the biofilm structure to a more rigid state than PSF [13]. As compared with the SEM morphology of the solid digestate without any pretreatment, the ultrasonic thermo-alkaline method decreased its particle size significantly in this work [35]. Previous research has indicated that the small particle size of solid digestates can make them easy to incorporate into a bio-based matrix [36].

Crystallinity
As shown in the X-ray diffraction (XRD) pattern of PSF, there was an overlapping peak at 16.08° (2θ value) and two dual-peaks at 18.24° and 19.60° (Figure 3), similar with corn starch (A-type structure). However, the overlapping peak of PSF was an amorphous halo peak, which was different from net starch because of the addition of a plasticizer, which disorganized the starch granules [34]. Compared with PSF, the composite biofilms had a new sharp peak at 15.44° for the solid digestates of straw and cattle and chicken manures, the intensity of which increased with the increase of the addition of the solid digestates. Additionally, there were also obvious peaks at 25.42° for the straw digestate and 28.16° for the chicken digestate. These new peaks were related to the polymorphs structures of cellulose І and ІІ [37]. Moreover, there was also a transformation of the peak at 16.08° from an amorphous form to a crystalline one after the incorporation of a solid digestate in PSF. There was a clear scaly structure shown in the cross-section of all biofilms. This is a typical multilaminar structure, the advantage of which can generally help improve the performance of a biofilm. As shown in the cross-section of the composite biofilm, solid digestate particles were well-incrusted in the scaly structure under the action of strong adhesion between starch and cellulose in the solid digestates with the addition of plasticizers, directly stabilizing the whole biofilm structure [26].

Crystallinity
As shown in the X-ray diffraction (XRD) pattern of PSF, there was an overlapping peak at 16.08 • (2θ value) and two dual-peaks at 18.24 • and 19.60 • (Figure 3), similar with corn starch (A-type structure). However, the overlapping peak of PSF was an amorphous halo peak, which was different from net starch because of the addition of a plasticizer, which disorganized the starch granules [34]. Compared with PSF, the composite biofilms had a new sharp peak at 15.44 • for the solid digestates of straw and cattle and chicken manures, the intensity of which increased with the increase of the addition of the solid digestates. Additionally, there were also obvious peaks at 25.42 • for the straw digestate and 28.16 • for the chicken digestate. These new peaks were related to the polymorphs structures of cellulose I and II [37]. Moreover, there was also a transformation of the peak at 16.08 • from an amorphous form to a crystalline one after the incorporation of a solid digestate in PSF.
The relative crystalline degrees of all the biofilms are listed in Table 3. The crystallinity of the composite biofilm was significantly higher than that of PSF due to the high crystalline nature of cellulose in the solid digestates [38]. As reported, anaerobic digestion can efficiently remove the amorphous region of cellulose until only the crystalline one remains [39]. This means that the addition of a solid digestate increased the crystalline degree of the biofilm more significantly than natural cellulose. The highest crystalline degree was found in the biofilm enhanced by the solid digestate of straw, followed by the cattle and chicken manures.  The relative crystalline degrees of all the biofilms are listed in Table 3. The crystallinity of the composite biofilm was significantly higher than that of PSF due to the high crystalline nature of cellulose in the solid digestates [38]. As reported, anaerobic digestion can efficiently remove the amorphous region of cellulose until only the crystalline one remains [39]. This means that the addition of a solid digestate increased the crystalline degree of the biofilm more significantly than natural cellulose. The highest crystalline degree was found in the biofilm enhanced by the solid digestate of straw, followed by the cattle and chicken manures.

FTIR Analysis
The physical blends and chemical interactions of the composite materials were, in general, affected by changes in the characteristics of the spectral bands [36]. The FTIR spectra of the composite biofilm was similar with that of PSF ( Figure 4) because of the chemical similarity between starch and cellulose in the solid digestates [40]. As shown in PSF, the absorption bands at 996, 1336, 1412, 1647, 2926 and 3284 cm −1 were related to the stretching vibrations of the -C-O groups and the asymmetric modes of the C=O groups, plasticizer, bound water, the -CH groups and the -OH groups [41]. There were also some weak bands observed at low wavenumbers, which were related to the skeletal vibrations of the glucose rings [38]. All the absorption bands of the biofilm were shifted to relatively high wavenumbers (except for 1647 cm −1 ) after the addition of a solid digestate because of the formation of hydrogen bonding between the hydroxyl groups in starch and carboxyl groups in the solid digestates [42]. Moreover, the band intensity of the composite film was higher than that of PSF. The changes in the FTIR spectra indicated an obvious improvement in the compatibility and structural stability of the biofilm enhanced by the solid digestates.

FTIR Analysis
The physical blends and chemical interactions of the composite materials were, in general, affected by changes in the characteristics of the spectral bands [36]. The FTIR spectra of the composite biofilm was similar with that of PSF (Figure 4) because of the chemical similarity between starch and cellulose in the solid digestates [40]. As shown in PSF, the absorption bands at 996, 1336, 1412, 1647, 2926 and 3284 cm −1 were related to the stretching vibrations of the -C-O groups and the asymmetric modes of the C=O groups, plasticizer, bound water, the -CH groups and the -OH groups [41]. There were also some weak bands observed at low wavenumbers, which were related to the skeletal vibrations of the glucose rings [38]. All the absorption bands of the biofilm were shifted to relatively high wavenumbers (except for 1647 cm −1 ) after the addition of a solid digestate because of the formation of hydrogen bonding between the hydroxyl groups in starch and carboxyl groups in the solid digestates [42]. Moreover, the band intensity of the composite film was higher than that of PSF. The changes in the FTIR spectra indicated an obvious improvement in the compatibility and structural stability of the biofilm enhanced by the solid digestates.  Table 3 showed that the addition of a solid digestate improved the strength and elastic modulus of the biofilm but at the expense of elongation. The mechanical strength of the composite film in this work was nearly twice higher than that of the biofilm based on thermoplastic starch (only 3.3 MPa) [43] and even greater than the biofilm composite of starch and cellulose nanofiber (6.9 MPa) [44]. The increase in rigidity was attributed to the similarity of the polysaccharide structures between cellulose and starch [40], while the decrease in plasticity was because the rigid nature of cellulose in the solid digestates increased the starch viscosity and inhibited the motion of the polymer matrix [45]. Moreover, the addition of a solid digestate on the starchy matrix stabilized the skeleton structure of the biofilm and reduced the molecular mobility, also explaining the high elasticity and low plasticity in the composite biofilm [46]. In this study, mi-  Table 3 showed that the addition of a solid digestate improved the strength and elastic modulus of the biofilm but at the expense of elongation. The mechanical strength of the composite film in this work was nearly twice higher than that of the biofilm based on thermoplastic starch (only 3.3 MPa) [43] and even greater than the biofilm composite of starch and cellulose nanofiber (6.9 MPa) [44]. The increase in rigidity was attributed to the similarity of the polysaccharide structures between cellulose and starch [40], while the decrease in plasticity was because the rigid nature of cellulose in the solid digestates increased the starch viscosity and inhibited the motion of the polymer matrix [45]. Moreover, the addition of a solid digestate on the starchy matrix stabilized the skeleton structure of the biofilm and reduced the molecular mobility, also explaining the high elasticity and low plasticity in the composite biofilm [46]. In this study, micro-interactions between the starch and solid digestate and crystalline degree correlated positively with the mechanical performances of the biofilm. The composite biofilms enhanced by a solid digestate of straw or cattle manure exhibited higher strength and elastic modulus than chicken manure, because the solid digestates of straw and cattle manure contained higher cellulose contents, which improved the elastic mechanical performance of the biofilm more significantly. Lignin is often considered as a highly reactive thermoplastic polymer, interacting with glycerol, xylitol and starch and acting as an interfacial compatibilizer between cellulose and starch [46]. This means that lignin can also improve the mechanical performances of biofilms to a certain extent, although the content of which is limited in a solid digestate. Additional contents of the solid digestates in the composite biofilms enhanced the mechanical performances more obviously, including increasing the elastic modulus and decreasing the elongation. However, there was also a slight reduction in the strength of the composite biofilms when adding more solid digestates, such as solid digestates of straw and chicken manure.

Thermal Stability
The thermal characteristics of PSF and the composite biofilms measured by DSC are depicted in Table 3 and Figure 5. There were two clear endothermic transitions in the DSC curve of each biofilm, where the first one was a slight step change associated with the phase transition temperature, and the second one was a sharp galley over a broader temperature range related to the degradation temperature [47]. The values of the glass transition temperature (T g ) and degradation temperature (T m ) of the composite biofilms were higher than those of PSF, indicating an excellent improvement in the thermal stability of the biofilms caused by the incorporation of solid digestates in the starch matrix. This was because the addition of a solid digestate increased the regularity, compaction and crystalline degree of the biofilm, which correlated positively with the thermal performance of the biofilm. Ash in a solid digestate is considered a high-temperature-resistant material, which can improve the thermal performance of a biofilm by complicating the association, aggregation and helix structure of the starch-cellulose matrix [48]. The highest T g value was observed in the composite biofilm enhanced by the solid digestate of straw, followed by cattle manure and chicken manure, because the solid digestate of straw contained more ash. Moreover, the highest T m value was found in the composite biofilm enhanced by the solid digestate of cattle manure, followed by chicken manure and straw. cro-interactions between the starch and solid digestate and crystalline degree correlated positively with the mechanical performances of the biofilm. The composite biofilms enhanced by a solid digestate of straw or cattle manure exhibited higher strength and elastic modulus than chicken manure, because the solid digestates of straw and cattle manure contained higher cellulose contents, which improved the elastic mechanical performance of the biofilm more significantly. Lignin is often considered as a highly reactive thermoplastic polymer, interacting with glycerol, xylitol and starch and acting as an interfacial compatibilizer between cellulose and starch [46]. This means that lignin can also improve the mechanical performances of biofilms to a certain extent, although the content of which is limited in a solid digestate. Additional contents of the solid digestates in the composite biofilms enhanced the mechanical performances more obviously, including increasing the elastic modulus and decreasing the elongation. However, there was also a slight reduction in the strength of the composite biofilms when adding more solid digestates, such as solid digestates of straw and chicken manure.

Thermal Stability
The thermal characteristics of PSF and the composite biofilms measured by DSC are depicted in Table 3 and Figure 5. There were two clear endothermic transitions in the DSC curve of each biofilm, where the first one was a slight step change associated with the phase transition temperature, and the second one was a sharp galley over a broader temperature range related to the degradation temperature [47]. The values of the glass transition temperature (Tg) and degradation temperature (Tm) of the composite biofilms were higher than those of PSF, indicating an excellent improvement in the thermal stability of the biofilms caused by the incorporation of solid digestates in the starch matrix. This was because the addition of a solid digestate increased the regularity, compaction and crystalline degree of the biofilm, which correlated positively with the thermal performance of the biofilm. Ash in a solid digestate is considered a high-temperature-resistant material, which can improve the thermal performance of a biofilm by complicating the association, aggregation and helix structure of the starch-cellulose matrix [48]. The highest Tg value was observed in the composite biofilm enhanced by the solid digestate of straw, followed by cattle manure and chicken manure, because the solid digestate of straw contained more ash. Moreover, the highest Tm value was found in the composite biofilm enhanced by the solid digestate of cattle manure, followed by chicken manure and straw.

Transparency
The detailed transmittances at the UV-B, UV-A and Visible regions are presented in Table 3, and the results showed a lower transparency in the composite biofilm than PSF. A solid digestate is a good light-absorbing agent, the fine particles of which can be incorporated well into a starch matrix and inhibit light transmitted through a biofilm [49]. This indicated that the addition of solid digestates improved the film-barrier property against the UV and visible light radiation of the biofilms, which was also further enhanced with more additions of the solid digestates. Moreover, the light barriers of the composite biofilms were more significant for UV light than visible light. The best light barrier performance of a biofilm was achieved after adding the solid digestate of straw, followed by cattle and chicken manures, because of the relatively less impurity composition and highest cellulose content in straw, which contributed to improving the light barrier of the biofilm [9]. There are lots of advantages when using a mulching film with excellent light barrier performance in farmlands, such as inhibiting weed growth, keeping the soil temperature stable and increasing the crop yields.

WVP and GTR
As shown in Table 3, the composite biofilm had a lower WVP value compared to PSF. This meant that the addition of a solid digestate improved the water barrier property of the biofilm because of the strong interfacial adhesion between starch and cellulose in a solid digestate directly reduced water the vapor permeability of the biofilms [50]. The hydrogen bond formed in a mixture of starch and cellulose limited the formation of a void on the biofilm surfaces where water molecules could not pass [49]. Moreover, the water barrier performance of the composite biofilm enhanced by the solid digestate of cattle manure was better than the other composite biofilms. There are some benefits of applying mulching films with excellent water barrier performances to agricultural crops, including conserving the soil moisture and improving the crops' growth.
The GTRs of N 2 O and CH 4 were also reduced significantly by incorporating a solid digestate in a biofilm as an enhancer, because the solid digestate increases the density and crystalline degree of a biofilm, which relates negatively to the permeability of GHG. Moreover, the effect of a tortuous path caused by the addition of a solid digestate in a starch matrix lengthened the gas diffusive process when migrated through a biofilm, which also reduced the GTR of GHG [9]. N 2 O and CH 4 are, in general, the main GHGs emitted from farmlands due to the application of nitrogen fertilizers. The decrease in the permeability of N 2 O and CH 4 from farmlands by using mulching films can reduce GHG emissions to atmosphere, so as to lessen the risk of global warming.

Conclusions
The solid digestates of straw and cattle and chicken manures were pretreated by thermo-alkaline with the ultrasonic-assisted method to remove the nonfunctional compositions and then recycled as a performance enhancer in a starch-based biofilm. The addition of solid digestates stabilized the structure of the starch matrix and increased the crystalline degree of the biofilm, therefore improving the tensile strength and elastic modulus of the biofilm but at the expense of elongation. The values of T g and T m of the composite biofilm were much higher than PSF, exhibiting an excellent thermal stability. There was also a great potential of improving the light, water and gas barrier performances when incorporating a solid digestate into biofilm manufacturing. Particularly a composite biofilm with low GTRs of N 2 O and CH 4 can reduce the emission of GHG from farmlands when using it as the mulching film. Moreover, the different additional contents and types of solid digestates also affect the biofilm performances. Upgrading solid digestates as performance enhancers for starch-based biofilms can provide a high-value use of biogas waste, and composite biofilms will become a promising alternative for mulching materials used on farmlands to improve the soil quality and protect the agricultural ecosystem.

Data Availability Statement:
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest:
The authors declare no competing financial or nonfinancial conflicts of interest.
Sample Availability: Samples are available from the corresponding author upon request.  4 Methane XRD X-ray diffraction DSC Differential scanning calorimeter