Solid inorganic NPK fertilizer (artificial fertilizer) was bought from the open market and used without any further treatment. Solid digestate (Safisana) was obtained from a batch of a commercially operated mesophilic anaerobic biodigester in Ghana, which produces biogas from the co-digestion of faecal matter and food waste for the generation of electricity. Pectomec variety of the tomato (Solanum lycopersicum) seeds, common in Ghana, was purchased and used for the study. The seeds were nursed in trays at the CSIR-IIR.

To avert the potential risk of introducing pathogens during irrigation (Hong & Moorman, 2005) or uptake of pathogens by plants from soils contaminated by poorly treated wastewater (Oluwadara et al., 2018) in the nutrient application, the microbial content of the digestates was assessed. Total coliform bacteria are a group of microorganisms found in the environment, and their presence may be indicative of the presence of harmful pathogens (Medeiros et al., 2024). Faecal coliforms are the thermotolerant subset of total coliforms and E. coli of faecal origin and constitute more than 95% of the coliform genera in human faeces. Therefore, it is suggested in literature that measuring E. coli in effluents is the most useful indicator in assessing the efficiency of a treatment process, providing greater public health protection (Elmund et al., 1999). Microbial activity also influences the quality of the fertilizer (Martin-Sanz-Garrido et al., 2025). The results, as shown in Table 1, indicated about 97% reduction in common pathogens such as faecal coliform after anaerobic digestion and further reduction in pathogens after modifying the solid digestate with incinerator ash (ca 42% CaO) to 99.9%. The CaO reacts with water to produce Ca(OH)₂, which has been known to have antibacterial activity because of its destructive effect on bacterial cell membranes and protein structures (Unknown Author, 1996). Treating digestate with 20% wt/wt. Incinerator ash gives a pH regime of a solid nutrient 8.83±0.08, which inhibits acidophilic and neutrophilic bacteria, including E. coli, successfully reducing the risk associated with contamination of fresh produce from nutrient application. The helminth eggs (Strongyloides stercoralis) were found in the untreated sewage at the inlet of the digester (6 helminth eggs/L). This presents a major health hazard and therefore necessitates treatment to prevent recontamination. No helminth eggs were observed in the liquid digestate, which suggests that the treatment was effective in the removal of S. stercoralis. 2 helminth eggs/L of S. stercoralis were found in the solid digestate initially. The population was, however, halved (1 helminth eggs/L) as a result of treatment with the incinerator ash. Earlier work by (Saqer et al., 2007) suggested that storing wastewater for more than 17 days or a pH modification of wastewater to high alkalinity levels (pH 10.2) can inactivate the larvae of S. stercoralis. The prevalence of S. stercoralis as a soil-transmitted helminth contamination makes further treatment by storage for longer than 17 days after formulation a prerequisite, and also the necessary precautions need to be observed during application in order not to activate an autoinfection cycle (White et al., 2019).

Plants generally require about 17 essential elements for growth and development (Brown et al., 2022). Three elements comprising, carbon, hydrogen, and oxygen, are often extracted from the atmosphere and soil water by the plant. However, the remaining 14 (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, zinc, manganese, copper, boron, molybdenum, chlorine, and nickel) are derived from sequestration from soil minerals and organic matter in the soil (Brown et al., 2022). The quality of the digestate as a nutrient source was thus assessed by the concentrations of essential nutrients, N, K, and P, as well as levels of secondary level micronutrients such as Ca, which improves the sellable part of the plant. The primary nutrients in the liquid digestate (supernatant liquid effluent) from the CSIR-IIR anaerobic digester show very low concentrations of N, P, and K (Table 2). This is because one of the major features of mesophilic anaerobic digester designs is the discharge of effluents with low nutrient levels (Gebreeyessus & Jenicek, 2016) to prevent eutrophication, which can cause algae bloom. Hence, a settling tank feature ensures the entrapment of slurry (solid digestate), which has richer nutrient concentrations. A comparison of the nutrient levels of a solid digestate produced in the two mesophilic anaerobic digester systems shows that N and K levels of the digestate from the commercially operated digester (Safisana) were higher than those from the CSIR solid digestate (SD), whilst CSIR SD had higher levels of P. This could be typical of the raw material, substrate, fed into the digester. This confirms that co-digestion of waste whilst boosting biogas production (Bayitse et al., 2014) can also improve the nutrient quality of the digestate for agricultural purposes. Also, the fortification of the digestate with CaO from the incinerator ash can increase N (NO₃^-), K, and P absorption from the soil as well as increase the size of sellable plant parts. The addition of nutrients from the treatment of organic waste has long-term benefits to soil health, as the humic matter helps in the retention of moisture and also prevents easy leaching of nutrients from the soil during rain runoffs. The materials' circularity also has long-term benefits to the global environment.

Toxic heavy metals (Pb, Hg, As, Cd) that could bioaccumulate with hazardous human health consequences were below the detection limit of the X-ray Fluorescence (XRF) spectrometer. This shows that segregation is an important process in waste treatment in order to valorize waste without exposure to greater health risks. The presence of other micronutrients like Cu, Fe, Zn, and S in the solid nutrient (Figure 4) could help in plant growth and development. However, there were also traces of Mn and Al, which have been shown to have toxicity to plants (Brown et al., 2022).

The physiological responses and crop yield for the various nutrients used in the field trial were assessed to determine whether the source of nitrogen had any significant effect on plant growth and fruit development. The data was analyzed using the Student’s t-test for unpaired variance. A p-value less than α=0.05 was considered statistically different. Figure 4 and Figure 5 show the response of the tomato plant to the different treatment sources administered at the same nitrogen content, with the control experiment having no nitrogen treatment. The results are represented as mean ± SD for 3 replicate plots with different alphabets indicating a significant statistical difference with p-value < α (0.05).

The chlorophyll content of the apical leaves on the apical meristem, responsible for primary growth, is indicative of absorption of nitrogen from the soil (Goncalves et al., 2020). The results (Figure 5) depict that in Week 1, the absorption of nitrogen from the soil in the control plot was significantly higher than the SDA (p=0.0129) and the artificial fertilizer (p=0.0084), though similar to the Safisana treatment plot (p=0.1091). Though both SDA and Safisana plots were treated a month before transplant, the chlorophyll content in the Safisana-treated plants was significantly higher than that of the SDA (p =0.0049). After Week 4, the plants responded very well to the uptake of the nitrogen application; thus, there was no significant difference in the chlorophyll content of the apical leaves in plants on all the experimental plots (Figure 5). However, after Week 5, the chlorophyll content in the apical leaves of SDA-treated plants becomes significantly higher than the control (p=0.0037). The nitrogen absorbed by the plants in the control experiment begins to lag behind the other treated plots by week 8 to week 12. A similar trend has been observed in the literature, attributing increased nitrogen requirement for the flowering stage of the plant for the synthesis of essential biomolecules (Zhao et al., 2025). This indicates that the tomato plant responded very well to the nitrogen application. The increase in chlorophyll content for the Artificial Fert and Ash treatment was 63% and 70%, respectively, higher than that of the control experiment, suggesting that the nitrogen source released the nutrients readily.

Physiological responses and crop yield of the tomato plants were studied, and the results are reported in Figure 5, showing averages ± standard deviation of 3 replicate plots with different alphabets indicating a significant statistical difference with p-value < α (0.05). The use of NPK biofertilizers and biostimulants can significantly influence seed germination, seedling survival, and mortality rates (BharathKumar et al., 2025). Solid humic substances have also been reported to be beneficial in enhancing transplant quality and post-transplant yield performance (Qin & Leskovar, 2020). The different nitrogen nutrient sources may have different concentrations of humic substances because of the different starting materials and their process methods. Thus, the survival rate of the plants in the field trials is evaluated. The survival rate of the transplanted seedlings in the artificial fertilizer application was not significantly different from SDA and the control (see Figure 5A). The SDA treatment, however, had about 18% increase in survival rate compared to the untreated solid digestate (Safisana) treatment plots (p-value = 0.0243). There was no significant difference between the survival rate in the Safisana treatment plot and the control (p-value = 0.4986). This shows that the plants adapted well to the SDA nutrient source and that the form of the nitrogen source did not negatively impact the plant survival.

Flowers and fruits have a close correlation in flowering plants like tomatoes. The number of flowers on a plant is an indication of the fruiting probability of the plant. It is reported that taller plants, which tend to have longer inflorescences and more flowers, produce more fruits (Taura & Gudzinskas, 2024). Though nitrogen is essential for this step of the plant development, the type of nitrogen nutrient has been reported to impact both on the quality and quantity of the yield. An optimum balance in nitrogen in the form of nitrates and ammonium is therefore required (Wang et al., 2024). The results depicted in (Figure 5B) show that whilst there is no significant difference between the three different nitrogen sources, all the treatment methods performed better than the control experiment SDA (p-value = 0.0239), Safisana (p-value = 0.0.0339), and Artificial fertilizer (p-value = 0.0499). The results confirm the importance of nitrogen as a nutrient in triggering fruit formation and that the nitrogen in the form of SDA is releasable for plant growth and fruit development (Wang et al., 2024).

Though a number of environmental conditions, like inadequate pollination, extreme temperature, and water stress, may cause a tomato to flower but not fruit, the effect of nutrient source on fruit development was investigated. The results in Figure 5C show the number of fruits per flowered plant. There were no significant differences between the different nutrient sources in fruit development, though all the different nutrient sources were significantly higher than the control. 58% more flowers on the plants in the SDA treatment plot developed fruits than the control. This shows clearly that whilst nitrogen is an important nutrient to plant health and its fruiting probability, the source of nitrogen cannot be discriminated clearly in terms of flowering and fruit formation. The quality of the produce in terms of weight of the fruit was assessed (Figure 5D) as it has been reported in the literature to be affected by the type of nitrogen and the ratios of the different nitrogen forms in the nutrient formulation (Wang et al., 2024). The results (Figure 5D) show whilst there was a clear difference between crop yield in the different treatment plots (SDA, Safisana and Artificial fertilizer) and the control, there was a significant difference in the yield (weight of fruits) between SDA and Safisana (p-value = 0.0191) which may be due to the nitrate, ammonium ratio (Wang et al., 2024). The conversion of ammonium nitrogen (NH₄⁺) to nitrate nitrogen (NO₃^-) in the presence of oxygen produces acidity (H⁺) as depicted in equation 2NH₄⁺ + 4O₂→2NO₃^- + 4H⁺ +2H₂O. Thus, the treatment of the digestate with alkaline ash is likely to shift the reaction towards the conversion of ammonium to nitrate, leading to a better nitrate ammonium ratio compared to just the solid digestate (Safisana treatment). The tomato yields in the field trial were estimated as ash-treated digestate, SDA (4.8 tonnes/ha), artificial inorganic fertilizer (4.3 tonnes/ha), untreated solid digestate, Safisana (3.26 tonnes/ha), and the control (0.90 tonnes/ha). Though the yield was significantly lower than the Ghana national average of 7.82 tonnes/ha and the global average of between 40 -50 tonnes per hectare (Food & United Nations, 2025), the yield was not a result of a nitrogen deficiency and could be optimized with better agronomic practices.

Analysis on the elemental composition in the fruit formation of a sample of tomatoes harvested from the plants was conducted to examine the level of uptake of heavy metals from the soil because of the health risk it poses through potential bio-concentration and accumulation. The results, as indicated in Table 3, show the presence of Pb (4 mg/kg) in only the fruit from the Artificial Fertilizer-treated plot. Also, the presence of Pb in the soil after application of artificial fertilizer, despite no detection in the soil of the control, as shown in Table 3. The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) permissible limits of Pb in fruits is between 0.01-3.0 mg/kg and 30-50 mg/kg in soil whilst the maximum permissible limits in fruits for As, Cd and Hg are 0.2 mg/kg, 2.0 mg/kg and 1.0 mg/kg respectively (Codex Alimentarius: International Food Standards, 1995 (Codex Alimentarius: International Food Standards, 1995)). This suggests a possible higher risk of heavy metal contamination during the nitrogen application, which may have led to uptake by the plant, as is seen in the case of the artificial fertilizer treatment plot, than in the other organic fertilizers. This suggests that the XRF with a detection limit of 1.0 mg/kg may be adequately sensitive to evaluate the upper limits of possible contamination of fruits from Pb, Hg, and Cd, but not adequately sensitive for As. From the results, it can be posited that the risk associated with the use of the artificial fertilizer was higher than that of using the ash-amended digestate. This highlights the risk of introducing toxic metals through fertilizer application and hence the need to continuously monitor the levels of these metals in food crops cultivated with nutrients from waste precursors.

NB^a represents the International standard, b represents the European Union standard, and BDL represents below detection limit.

The results from Table 3 show a slow uptake of heavy metals, particularly Fe, by the tomato plant from the soil when the digestate was treated with incinerator ash (Ash) as compared to the other treatment plots and even the control plot. This could be because of the alkalinity of the soil as a result of the higher pH from the incinerator ash. Thus, confirms earlier observations that alkaline soils reduce the bioavailability of heavy metals compared to acidic soils because the heavy metals tend to be immobilized in high pH soils (Seleiman & Kheir, 2018).

Since antiquity, the benefits of using ash to improve plant growth have been known. The practice of slash and burn, though harmful, particularly to microorganisms in the soil, remains prevalent in some communities in developing countries. The use of animal droppings as manure as a source of nitrogen and plant-based residue like food scraps, yard trimmings, and crop residues as mulching materials for soil enrichment is also extensively practiced for plant cultivation. However, these have negative implications through the introduction of pathogens (particularly from manure), the release of global warming gases, and the release of offensive odour. Compost fertilizer production from organic waste is thus seen as a sustainable practice for soil amendment, as the aerobic biodegradation largely reduces the odour (Bremaghani, 2024). Nevertheless, there are concerns about contamination with pathogens, particularly when using human excreta (Antwi-Agyei et al., 2016), and also the risk of excessive heavy metals introduction into the soil (Bremaghani, 2024). Though some pathogens (helminths eggs) persisted and can even multiply in anaerobic digester systems as suggested in the literature (Lin et al., 2022), this study shows that treating the anaerobic digestate derived from faecal wastewater and amending with ash reduces the risk considerably and makes it a viable source of plant nutrient for sustainable agriculture in agreement with the literature (Martin-Sanz-Garrido et al., 2025). The alkaline-amended digestate, apart from inactivating residual pathogens in potentially hazardous organic waste like human excreta, also reduces the bioavailability of heavy metals in the soil for plant uptake.

Not applicable.

The author(s) declare(s) that there is no competing interest.