Closing the Loop: Can Anaerobic Digestates from Food Waste Be Universal Source of Nutrients for Plant Growth?

Abstract

Reducing waste production and improving waste treatment are key objectives in the EU’s Circular Economy Action Plan. Anaerobic digestion of food waste is a promising method, but safely disposing of its by-products, which contain valuable nutrients like nitrogen, phosphorus, and organic matter, remains a challenge. These nutrients suggest potential use in agriculture to enhance sustainability, yet their effects on plant growth need thorough understanding. This study investigated the impact of liquid digestates from anaerobic digestion of food waste, combined with fish sludge (from recirculated aquaculture systems), on plant growth (Lepidium sativum and Triticum aestivum) through direct soil tests. The content of biogenic elements in the digestates did not differ and was 0.32% for N, <0.05% for P, and 0.15% for K. Two tests were conducted, both using artificial soil prepared to OECD 207 standards: the Phytotoxkit™ test (ISO 18763:2016) and a pot experiment (OECD 208). Results showed that digestates initially delayed germination and hindered early plant growth, an effect that diminished over time. This may be due to the high organic matter content of digestates, similar to standard fertilizers like manure or compost. Pre-incubating digestates in soil before application, similar to common agricultural practices with soil amendments, is suggested as a potential solution.

1. Introduction

One of the foremost challenges in sustainable development is mitigating overproduction and overconsumption, with food overproduction playing a crucial role. A United Nations report [1] reveals that globally, 17% of food is wasted across households, food service, and retail sectors, amounting to 1.05 billion tons or 79 kg per capita in 2022 [2]. Addressing food loss and waste is urgent, as it impacts the twelfth Sustainable Development Goal (responsible production and consumption) and, due to the significant water resources required for food production, also affects Goal 6 (clean water and sanitation). Therefore, reducing food overproduction requires data-informed policies, investments in technology, infrastructure, education, and monitoring [1].

On the other hand, waste from the food industry, including expired food, can efficiently contribute to existing or newly established biogas plants, thereby enabling the recovery of a portion of the energy used in its production. Anaerobic digestion (AD) has emerged as a widely adopted solution for treating various waste streams due to its efficacy in degrading organic solids and producing biogas, composed primarily of 60% methane and 40% carbon dioxide [3].

Food waste stands out as an ideal substrate for AD owing to its high water content, favorable biodegradability, and significant methane production potential [4]. Anaerobic digestion (AD) is widely used to recover resources. Recently, bio-based fermentation has gained attention for producing medium-chain carboxylic acids (MCCA) from various organic wastes using mixed microbial communities [5]. However, the rapid adoption of AD in food waste treatment presents challenges in the effective disposal and utilization of digestate [6]. The yield of digestate from food waste AD is estimated to range from 0.20 to 0.47 tons per ton of waste [7], with the separated solid fractions typically exhibiting moisture content levels between 70% and 80% [8].

Digestate has immense potential as an organic fertilizer, soil conditioner, and landfill cover soil, but its disposal and utilization pose significant challenges. Despite being nutrient-rich, digestate utilization remains a bottleneck for the biogas industry’s development. Researchers advocate for converting digestate into valuable products enriched with nutrients [9]. Abundant in nitrogen (N), phosphorus (P), and organic matter, digestate holds promise for various applications [10,11]. In Germany, biogas residue significantly contributes to national fertilizer production, highlighting its role in fostering a circular economy and sustainable development [12,13].

In fish farming, various types of fish sludge are generated, serving as potential substrates for biogas production. This sludge originates from freshwater hatcheries and land-based food fish farms, with an emerging interest in sea-based facilities. Due to stricter regulations, new land-based plants with a trend towards closed-system Recirculation Aquaculture Systems (RAS) are planned in the region of Norway. These will generate more fish sludge inland, requiring sustainable treatment and management. Waste resources from the fisheries, aquaculture, and fish processing sectors are significant for promoting biogas production and nutrient recovery in Norway. Aquaculture waste encompasses all materials generated during the production process that remain within the system, stemming from fish growth or runoff from flow-through systems. These wastes primarily include unused feed, feces, used chemicals and pharmaceuticals, and fish tissue. Bergheim et al. [14] detail the composition of sedimentable sludge in sea cages. Assuming a minimum feed conversion ratio of 1.05 as an average for produced food fish, this results in a feed consumption of 1.05 kg per ton of production. The emissions from this are as follows:

• 355 kg O₂ as BODl
• 35 kg N, of which 77% is dissolved N (27 kg);
• 7 kg P, of which 14% is dissolved (1 kg).
• If there is feed spillage, the emissions will increase relative to the amount of spillage.

Notably, sludge from sea-based fish farms contains saltwater, necessitating adjustments for its use in biogas production due to high levels of chlorides.

The super-intensive culture in RAS results in the production of considerable sludge volumes that necessitate treatment before disposal. The characteristic parameters of fish sludge from RAS compared to domestic wastewater parameters are presented in

Table 1. Comparison of parameters of RAS sludge vs. domestic wastewater [15].

Parameter	Unit	RAS Sludge	Domestic Wastewater
		Average (Range)	Average (Range)
Total suspended solids (TSS)	%	1.8 (1.4–2.6)	5 (2–8)
Biological oxygen demand BOD5	mgO2/L	2760 (1590–3870)	6000 (2000–3000)
Ammonia nitrate (N-NH3)	mg/L	18.3 (6.8–25.6)	400 (100–800)
Total phosphorus (TP)	mg/L	1.3 (no data)	0.7 (no data)
pH	---	6.7 (6.0–7.2)	6.0 (5.0–8.0)
Alkalinity CaCO3	mg/L	334 (284–415)	600 (500–1500)

.

It is estimated that in Norway, around 27,000 tons of nitrogen and 9000 tons of phosphorus are lost to the sea each year as fish sludge emissions from fish farming [16].

The amount of nutrients lost in connection with fish sludge is in the same order of magnitude as the amount lost with animal manure [17], as fish sludge is rich in phosphorous (P) (approx. 2–3% total solids (TS)) and nitrogen (N) (approx. 4–11% TS) [18].

Aquaculture farms are concerned with improving the handling of the generated waste and are looking into drying the fish sludge, as this method performed on site instantly reduces smell and makes storage and transportation easier. After drying, the sludge can be directly applied as fertilizer in local agricultural farms [17,19,20] or used in the production of organic fertilizer products [21].

To close the biorefinery cycle, the remaining effluent from the AD process can be repurposed as a valuable fertilizer for agricultural use. Therefore, the aim of this work was to evaluate the agronomic potential of digestates obtained through anaerobic fermentation of broadly defined food waste. In the current experiment, fish sludge (specifically coming from the RAS systems) was used as a co-substrate for food waste (residue from MCCA production). The second sample was the digestate derived solely from food waste. The study focused on the analysis of nitrogen (N), phosphorus (P), potassium (K), and heavy metals. Additionally, pot test experiments with two plant species were conducted to assess how the digestate obtained influences plant growth.

2. Materials and Methods

2.1. Anaerobic Digestion and Co-Digestion Process

The Automatic Methane Potential Test System (AMPTS II, BPC Instruments, Lund, Sweden) was used to conduct the digestion and co-digestion experiments. This is a specialized apparatus designed for assessing the biogas potential of specific substrates or combinations thereof. This instrument was used to produce digestate for the pot test experiments. Figure 1 presents a schematic diagram of the AMPTS. The experimental set-up consisted of reaction vessels, a wash bottle, and a flow cell array unit. The wash bottle was filled with a caustic solution—3M NaOH (Avantor, Gliwice, Poland) and was connected to the reaction vessels. Methane flowed from the wash bottles to the flow cell array unit to measure the volume of the produced biogas. The cumulative volume of methane was logged in a data file and automatically recalculated for the produced methane.

The following 2 types of samples were used for digestion for 30 days in AMPTS:

• Residue biomass coming from MCCA production (80%) was supplemented by fish sludge (20%) FWDFS; fish sludge was obtained from Recirculating Aquaculture Systems (RAS).
• Residue biomass coming from MCCA production—FWD.

The operational parameters were as follows:

Substrates for the anaerobic digestion process were inoculated in amounts resulting in an inoculum/substrate ratio of around 2 gVS/gVS, as suggested by Raposo et al. [22]. The inoculum was typical anaerobic sludge from a full-scale biogas plant (Gdańsk, Poland). Samples were then transferred into 500 mL bottles with 400 mL working volume. Then, the pH of the digesters was set up to 7.0 ± 0.2 by using a concentration of 0.5N of NaOH (Avantor, Poland) solution and, in some cases, 0.5N HCl solution (Avantor, Poland). The batch tests were performed in triplicate by using a volumetric gas production method (AMPTS II, Bioprocess Control Sweden AB, 2014) at 50 °C for 30 days with continuous stirring.

Samples were gathered over the time from a series of experiments during the production of MCCA. Collected samples were mixed (accordingly with and without FS) as two representative samples. In this way, two representative samples of two digestates (with and without fish sludge addition) were obtained for the purpose of the pot test.

Analysis

The pH values of digestate samples were measured by a Multi 3401 pH meter (WTW, Weilheim, Germany). Total organic carbon (TOC) was determined with a TOC-L Total Organic Carbon Analyzer (Shimadzu, Kyoto, Japan).

2.2. Plant Experiment

To evaluate the potential of digestates in promoting plant growth, two tests were conducted: (i) the Phytotoxkit™ test and (ii) a pot experiment.

The first test, the short-term Phytotoxkit™, was aimed at determining the appropriate doses of post-ferments (not causing inhibition of growth) for use in pot tests. The second test (the pot test) was aimed at assessing the actual impact of digestates on plant growth over an extended period of seedling incubation.

The Phytotoxkit™ test involved a 3-day bioassay assessing seed germination and root growth of higher plant species exposed to the test samples. This preliminary test covered a broad range of digestate concentrations and utilized garden cress (Lepidium sativum) as a bioindicator. The seeds used in the tests were purchased from W. Legutko Breeding and Seed Company Ltd. (Jutrosin, Poland).

The pot experiment was a standardized test conducted under controlled culture conditions for 28 days. Two plant species were used in this test: monocotyledonous wheat (Triticum aestivum) and dicotyledonous garden cress (Lepidium sativum). Before each experiment, seeds of the selected species were soaked in tap water for 24 h. The assays were conducted in triplicate for each concentration and control.

Since standard OECD soil contains 10% peat, making it quite rich in organic carbon, we decided to conduct a second experiment with a reduced peat content of 5%. This study was conducted for both garden cress and common wheat.

2.2.1. Soil Substrate

Both tests employed artificial soil prepared according to OECD/ISO standards [23,24]. The substrate consisted of 10% (or 5%) sphangum peat (fraction < 4 mm) (Biovita, Tenczynek, Poland), 20% kaolin clay (Surmin-Kaolin, Nowogrodziec, Poland), up to 70% quartz sand (Biovita, Poland), and 0.3–1% calcium carbonate (Avantor, Poland), maintaining a soil pH of 6.0 ± 0.5. The total water-holding capacity (WHC) was determined according to a previous study [25].

2.2.2. Tested Samples

Digestate concentrations were prepared by diluting them with distilled water to reach the desired volume. The total volume of irrigation mixtures added to test containers was based on the measured WHC. For the Phytotoxkit™ test, it was 100% WHC, and for the pot test, it was 60% WHC.

Table 2. Application rates of LD to soil in both plant experiments as percentages of WHC and dose per unit of soil mass.

% (v/v) of Irrigation Liquid Volume	Dose [L digestate/kg soil]
Phytotoxkit™
Mass of soil per plate: 0.1 kg; volume of irrigation liquid: 0.043 L
1	4.3 × 10−3
5	21.5 × 10−3
10	43 × 10−3
Pot Experiment
Mass of soil per pot: 0.35 kg; volume of irrigation liquid: 0.090 L
0.5	1.3 × 10−3
1	2.6 × 10−3
5	13 × 10−3

shows the calculated volumes of liquid added to the samples to achieve the desired WHC levels, as well as the doses of digestates added, expressed as % v/v of the irrigation liquid and per kg of soil. The samples were thoroughly mixed with the soil substrate and then transferred to the testing containers.

2.2.3. Phytotoxkit™

The Phytotoxkit™ test was conducted according to the procedure described by the test manufacturer, MicroBioTests Inc. (Gent, Belgium) [26]. The experiment was conducted in transparent plates. A total of 0.1 kg of soil substrate moistened with an appropriate volume of irrigation liquid mixture was placed in the bottom part of the plate and covered with filter paper. Then, 10 seeds were evenly distributed in the middle of the plate and covered with a lid. In Figure 2A, an example plate with sown plants is presented. Digestate was tested at doses of 2.6, 13, and 26 mL digestate/kg soil. After 72 h of incubation in darkness at 20 °C, the total number of germinated seeds was counted, and the lengths of roots and stems were measured to calculate growth inhibition or stimulation compared to the control.

2.2.4. Pot Experiment

The study assessing the impact on plants was conducted according to the methodology of OECD Test 208 [27]. A total of 0.350 kg of soil substrate per pot was thoroughly mixed with the appropriate volume of previously prepared digestate dilutions. The test encompassed doses of 1.3, 2.6, and 12.9 mL digestate per kg of soil. Subsequently, the mixes were transferred into pots (9 cm × 9 cm × 9.6 cm), with nine seeds planted in each pot at a depth of approximately 0.5 cm below the soil surface. In Figure 2B, an example of pots with sown plants is presented. The pots were then placed in a climate chamber set to a 16–8 h day-night photoperiod at 23 °C and 70% humidity. Periodic watering with distilled water was administered to maintain 60% WHC. After 28 days, the plants were trimmed at ground level, cleansed, and evaluated for shoot heights and dry mass (determined after drying at 105 °C), facilitating the assessment of growth inhibition and stimulation relative to the control.

The effect of digestates on plant growth was calculated according to the following formula:

$$\mathrm{E}\mathrm{f}\mathrm{f}\mathrm{e}\mathrm{c}\mathrm{t}={\displaystyle \frac{\mathrm{C}-\mathrm{D}}{\mathrm{C}}}\times 100 \left[\%\right],$$

(1)

where

• C—parameter value in the control sample (length or mass);
• D—parameter value in dilution sample (length or mass).

The effect can take on both positive and negative values. If Equation (1) calculates a positive value, it means the dilution samples have shorter lengths or less mass than the control, indicating inhibition. If the calculated effect according to Equation (1) takes a negative value, it will indicate stimulation.

2.2.5. Statistical Analysis

Assays were conducted in triplicate for each concentration and control. Effect values are given as mean ± SD. Statistical analysis was performed using Statistica 14 software. Data were tested for assumptions of normality (Shapiro–Wilk test, p > 0.05) and equal variance (Levene test, p < 0.05). ANOVA with a post hoc comparison between the means was conducted.

3. Results

3.1. Biomethane Production

The fermentation tests conducted allowed for the calculation of the biomethane potential (BMP) of both the biomass residue after the production of short-chain fatty acids (FWD) and FWD enriched with fish sludge (FWDFS). BMP is defined as the maximum amount of methane that can be produced by anaerobic digestion from 1 kg of substrate. It was observed that BMP for FWD was within the range of 310–325 LCH₄/kgVS. A 20% enrichment of FWD with fish sludge resulted in an increase in BMP to values ranging from 375 to 445 LCH₄/kgVS.

3.2. Characteristic of Anaerobic Digestates

The characterization of the physicochemical parameters of the digestates aimed primarily at assessing the content of biogenic elements and detecting contaminants such as heavy metals. These values have been presented in

Table 3. Comparison of parameters of food waste digestate with fish sludge (FWDFS) with food waste digestate (FWD).

Parameter	Unit	FWDFS	FWD	Reference Value According to Regulation 2019/1009 [28]
Cr	mg/kg	5.66	9.45	7.00
Zn	mg/kg	509.43	749.19	800.00
Cd	mg/kg	0.38	0.65	1.50
Cu	mg/kg	100.00	172.64	300.00
Ni	mg/kg	3.96	8.79	50.00
Pb	mg/kg	4.72	8.47	120.00
Hg	mg/kg	0.007	0.007	1.00
Dry matter	%	5.30	3.07	---
Total solids	g/kg	50.21	32.54	---
Volatile solids	g/kg	39.29	22.67	---
N (total)	%	0.32	0.32	1.00
P (total)	%	<0.05	<0.05	1.00
K	%	0.15	0.15	1.00
pH	---	8.79	8.06	---
TOC	%	0.52	0.33	5.00
TOC/TN	---	8.56	5.88	---

along with the reference values specified by new EU regulations, which came into force in 2019 [28].

It can be noticed that when it comes to pollutants, the levels of metals determined in post-ferments do not exceed the permissible values. However, if one considers the levels of both organic carbon and biogenic elements, here, the levels of these parameters are unexpectedly low. It can also be stated that the addition of fish sludge to food waste did not significantly alter the characteristics of the post-ferment.

Despite the low concentrations of nutrients, it was decided to conduct tests on plants. We were motivated by the desire to demonstrate whether post-ferments, despite their relatively low biogenic content, would still have a beneficial effect on plant growth due to, e.g., the abundance of microorganisms, enzymes, hormones, or other compounds present in the digestates. All those compounds may support soil microbiome development and thus may also have a growth-supporting action for plants.

3.3. Plant Experiments

3.3.1. Phytotoxkit

The results of this preliminary test are presented in Figure 3. The results were not entirely consistent with our expectations. Instead of stimulating plant growth, inhibition was observed in most cases. This was particularly evident in the plates using standard soil with 10% peat content (Figure 3A,C). These preliminary observations confirmed that sowing plants too early after applying organic fertilization, when a substantial dose of organic matter is introduced into the soil, can cause growth inhibition. Significant standard deviations were also observed for the different experimental variants. This is often associated with studies conducted on plants, especially in the early developmental stages, where the seed coat functions as a protective barrier against adverse factors and pathogens [29,30,31,32,33,34].

Despite observing an unfavorable impact on plant growth, these preliminary studies proved to be interesting and provided valuable insights for further research. Primarily, it was necessary to reduce the range of applied doses to prevent the suspected negative effects of excess organic compounds on the plants. Additionally, we decided to extend the pot tests to 28 days, as any potential negative impact of the digestives might be transient with longer exposure. Furthermore, we conducted the study in pots with two variants: the standard 10% peat content and a reduced 5% peat content in the substrate. Figure 3D shows that this strategy led to shoot growth stimulation at lower concentrations of both FWDFS and FWD.

3.3.2. Pot Tests

The results of the pot experiment with Lepidium sativum and Triticum aestivum are shown in Figure 4 and Figure 5, respectively.

It was observed that dicotyledonous plants (Lepidium sativum) were significantly more sensitive to the applied doses of digestates (Figure 4 and Figure 5). This suggests that these digestates would be better for cereal crops as soil supplements. Comparing both endpoints of this study, namely, shoot length and plant biomass, a wide range of results was observed in both cases. It is inconclusive which endpoint better characterizes the effect of digestates on plant growth. This variability underscores the significant influence of individual sensitivity during such tests. Increasing the number of endpoints allows for a more comprehensive understanding of the impact of a given factor on plant growth. The pot experiment confirmed the observations made in the preliminary test. Achieving growth stimulation effects in plants was possible when using a soil substrate with lower organic carbon content. When planning the application of organic fertilizers, it is important to follow the recommendations for agricultural practices, particularly those related to the waiting periods associated with the use of fertilizers.

4. Discussion

The addition of fish sludge to food waste residue after MCCA production enhanced the production of biomethane. These results are in line with previous studies concerning the co-digestion of fish sludge with other substrates. As described by Estevez et al. [35], both dried and wet fish sludge supplementation can improve the production of methane from municipal sewage sludge. In thermophilic conditions, the additions of 30% fish sludge inhibited methane production; however, the application up to 20% stimulated the production between 30 and 50%. In mesophilic conditions, the application of 30% increased the yield by 35%. In another study, Netshivhumbe et al. [36] investigated the co-digestion of fish sludge, food waste, and fruit and vegetable waste in various compositions under mesophilic conditions. The highest biomethane production was achieved with proportions of 67% fish sludge, 18% food waste, and 19% fruit and vegetable waste, and this was eight times better than in case of digestions of only fish sludge.

Physicochemical parameters obtained for FWD and FWDFS are characteristic of this type of material, i.e., in terms of low nutrient, pH, or metal content. Akhiar et al. [37] characterized the liquid fractions of anaerobic digestates obtained from various substrates under mesophilic or thermophilic conditions. pH varied from 7.6 to 8.4, TP from 0.08 to 3.02 g/L, and TN from 1.5 to 6.5 g/L. As most of the organic matter degrades during the digestion process, TOC values of samples ranged from 0.452 to 3.163 g/L. The LDs have also demonstrated low biodegradability, which contributed to the presence of complex compounds such as fulvic acid-like, glycolated protein-like, melanoidin-like, and humic acid-like. Tawfik et al. [38] summarized the heavy metal content of LD from different feedstocks. The highest concentrations of these toxic elements are found in products obtained from animal manures or industrial wastes. In turn, digestion of food waste or grasses leads to materials with the lowest heavy metal concentrations. The heavy metal contents of FWD and FWDFS produced in this study did not exceed the permissible values of Regulation 2019/1009 for fertilizer products; however, they also did not meet the required parameters for mineral content.

The impact of FWDFS and FWD on short- and long-term plant growth utilizing two types of soil substrates was assessed. For OECD soil with 10% peat, exemplifying fertile soil with high organic carbon content and nutrient availability, the negative impact of digestates is evident. Short-term plant growth of garden cress root was inhibited in all of the doses used, with 84% and 64% inhibition for FWD and FWDFS, respectively, at the dose of 43 mL of digestate/kg soil (Figure 2A). This was also the only noticeable and significant difference between the digests. In the Phytotoxkit™ test, the roots proved to be more sensitive than the stem, with the highest significant inhibition of stem growth of 38% for FWDFS and 52% for FWD (Figure 3C). With an extended pot experiment, no statistically significant negative effect on stem growth (Figure 4A) or biomass increase (Figure 4C) in Lepidium sativum was observed for either digest at any of the applied doses. No inhibition of stem growth was observed in wheat either (Figure 5A). However, FWDFS at 2.6 mL of digestate/kg of soil and both digestates at 13 mL of digestate/kg of soil inhibited biomass gain (Figure 5C).

The decrease in phytotoxicity in pot or field experiments after a short period of time is a common phenomenon [39] associated with maturity, defined as the rate of decomposition of organic matter as soil additives [40]. High C/N ratios in organic amendments can lead to immobilization of N and crop deficiency, whereas ratios of C/N > 1 appear beneficial for N mineralization in the soil and favorable for plant growth [41,42]. In particular, as stated by Alburquerque et al. [43], liquid anaerobic digestates with DOC < 1.5 g/L, BOD5 < 2.5 g/L, and DOC/TN < 1 are considered suitable for use as a fertilizer, whereas those with DOC < 5.5 g/L, BOD5 < 6.0 g/L, and DOC/TN < 1.5, i.e., those that are highly biodegradable, are seen as less adequate. Regarding the digests investigated in this study, the DOC/TN values for FWDFS and FWD were 8.56 and 5.88. TOC values for both samples reached well above the recommended ones; 5.1 g/L for FWDFS and 3.26 g/L for FWD. According to the parameters presented by Alburquerque et al. [43], the TOC concentration seems to be more important than the C/N ratio. For instance, in a study presented by O’Connor et al. [44], which examined the effect of LD on the growth of Cenchrus clandestinus at a digest dosage of 50 kg N/ha, despite a C/N = 2.57 ratio, dry shoot and dry root of the treatment group were significantly higher than those of the unamended control.

For experiments conducted on soil with peat content reduced to 5%, a significant reduction in the negative effect on cress growth was noted. Again, in the short-term test, the root was more susceptible than the stem. Inhibition of the root was 33% for FWDFS and 38% for FWD, respectively, with no statistically significant differences between the two (Figure 3B). Neither treatment group showed statistically significant stem inhibition (Figure 3D). The same was true for the pot experiment (Figure 4B), and in the case of the highest dose of post-fertilizer, there was even stimulation of biomass growth by 28% and 43% for FWDFS and FWD, respectively (Figure 4D). For wheat, no negative effects on stem growth (Figure 5C) or biomass increase (Figure 5D) were observed. Furthermore, at 13 mL of digestate/kg of soil, LD stimulated stem growth by 35% and 33% for FWDFS and FWD, respectively (Figure 5C).

Growing plants could use organic matter and digest micro- and macroelements in low-fertile soils. For example, a study by Cristina et al. [45] compared the effects of primary and secondary liquid digestate on tomato growth on poor sandy soil or peat substrate. With the addition of digestate at 170 kg N/ha, there was no effect on plant dry matter gain on peat substrate compared to the control. However, in the case of sandy soil, a significant increase in dry matter for the treatment group was observed in each of the three months of cultivation, e.g., in the second month, the values were as much as about 30 times higher than those of the control. This would further suggest that low-fertile soils may benefit most from the addition of digestates.

5. Conclusions

As shown in this study, simultaneous utilization of food waste and fish sludge during anaerobic digestion can lead to increased biogas production, which is beneficial and in accordance with a closed-loop economy. However, proper disposal of the produced digestates, especially the liquid fraction, remains an issue. LDs are considered potential soil additives and fertilizer; nonetheless, their effects on plants need to be studied. Based on the results, it can be concluded that both FWD and FWDFS demonstrated inhibition of plant growth during the initial period. In the Phytotoxkit™ test, the roots proved to be more sensitive than the stems of garden cress. All applied doses of the digestates inhibited root growth. In conclusion, the digestates delayed germination and growth in the first few days of plant growth, but their effect began to diminish over time. This phenomenon may be due to the organic matter content of digestates and can also be observed when standard organic fertilizers, such as manure or compost, are used. The use of post-ferments as a plant growth-promoting substance may be justified only on soils low in organic matter. It is also worth considering the application of appropriate separation techniques to concentrate or dry the post-ferments, thereby making the resulting product richer in nutrients. Equally important is adhering to the recommendations of agricultural practices and applying such fertilizers in a manner that allows for soil pre-incubation with the fertilizer (i.e., in autumn or early spring), thus enabling a more beneficial impact on plants.