Assessment of the Agronomic Value of Organic Fertilizer Made of Composted Sludge

A concrete platform located on the territory of the Astana Su Arnasy Republican State Enterprise with the Right of Economic Management was used to form the heaps under evaluation. Dehydrated sludge and crushed wheat straw were used as raw materials.

We used the aerobic method of composting sludge, and the parameters of the compost mixture and the speed of the composting process were determined depending on the type of biological preparation.

With the help of special equipment, eight heaps were formed (1.5m wide, 1m high, and 10m long), the total dry weight of each heap was 12-15t, and the moisture content of the heap was 60%. According to the experiment design, wheat straw and biological preparations were added and mixed. A BELARUS-80.1 tractor and a trailed agitator were used to mix the heaps.

The experiment was conducted according to the following scheme:

Variant 1. The control variant (sludge without biological preparations);

Variant 2. Sludge+straw (10%) + biological preparation Trichodermin SS (1 l/t);

Variant 3. Sludge+straw (20%) + biological preparation Trichodermin SS (1.5 l/t);

Variant 4. Sludge+straw (10%) + biological preparation Agromix SS (2 l/t);

Variant 5. Sludge+straw (20%) + biological preparation Agromix SS (2 l/t);

Variant 6. Sludge+straw (10%) + Consortium B (1 l/t);

Variant 7. Sludge+biological preparation Micromix (1 l/t);

Variant 8. Sludge+straw (10%) + Consortium A (1 l/t).

The specific combinations and concentrations of sludge, straw, and biological preparations were chosen based on preliminary studies and literature reviews that indicated their potential effectiveness in enhancing compost quality (Tables 1 and 2).

Table 1. Chemical composition of compost raw materials before composting

The use of biological preparations in composting has been widely documented to enhance the breakdown of organic matter and improve nutrient retention [12].

Table 2. Composition of biological preparations

Studies have shown that adding carbon-rich materials, such as straw, can significantly reduce nitrogen losses and improve the stability of compost [13, 14].

The heaps were treated with several biopreparations with a titer of 10⁶ colony-forming units (CFU)/ml mixed with water in a ratio of 1:4. Biological preparation solutions were used in the amount of 5 l/1 t of sludge. A sprayer mounted on the agitator was used to apply the biological preparations. The temperature and pH of the heaps were measured daily, and the humidity and chemical composition of the composted mass were monitored once every ten days.

Temperature changes in compost heaps were monitored using a mercury thermometer. The pH of the compost sample was determined using a pH meter using a suspension in a sample-to-water ratio of 1:10. The compost samples were dried and sieved to a size of less than 0.25mm. The organic matter content was determined by measuring the dry matter mass loss in a muffle furnace at 550℃ for 6 hours. The total N content was determined using the Kjeldahl method. The total P content was determined using the colorimetric method with vanadomolybdophosphoric acid, and the total K content was determined by photometry with flame emission after wet digestion [15, 16]. Atomic absorption spectrophotometry using flame was used to determine the available fraction of heavy metals in the final compost. All experiments were carried out in a three-fold repetition. Humidity was determined by drying: the sample was kept at a temperature of 70±5℃ for 24 hours [17].

With biological preparations like Agromix SS, Trichodermin SS, Consortium A, and Consortium B, the temperature in the heaps increased rapidly in the mesophilic phase and reached its maximum value in the thermophilic phase that was formed as a result of the degradation of simple molecules due to the active action of microbes distributed in the compost mixture.

Temperatures below 60℃ in the thermophilic phase are unsuitable from the point of view of sanitary disinfection. The American scientist P.D. Millner proved that coliform bacteria, streptococci, and salmonella were destroyed at a temperature of 60℃ within 48 hours, and pathogenic microflora was preserved in variants with a composting temperature of less than 50℃ [19]. The study confirms the important role of constant temperature control during the thermophilic phase to reduce pathogenicity.

Standard-compliant high temperature (>60℃) was preserved for 7 days in variants with a straw content of 10%. A similar trend of temperature change was observed in the heaps containing 20% of straw. However, in these heaps, the maximum temperature during the thermophilic period reached 55℃. Moreover, the temperature increase lasted for 9 days, which was longer than in the heaps using straw in the amount of 10%. Perhaps this is because, in heaps containing 20% of wheat straw, the decomposition of organic matter takes longer. This situation can lead to a longer thermophilic phase. During the maturation period, the temperature in the heaps gradually decreased. At the end of the experiment, the temperature in all variants had reached ambient temperature, which indicates the maturity of the compost.

The findings of this study align with previous research on sludge composting. For instance, Ajmal et al. [20] reported that maintaining high temperatures during the thermophilic phase is crucial for effective pathogen reduction and organic matter decomposition. Also, Xu et al. [21] observed that the addition of organic materials like straw can extend the thermophilic phase, enhancing compost stability and nutrient content.

The increase in pH in the compost to alkaline values in the first 10 days of the experiment is explained by the release of ammonia and mineralization of organic nitrogen as a result of microbial activity. A decrease in pH by day 20 may be caused by the evaporation of ammonia N during the thermophilic period, the transition of mesophilic microflora to thermophilic, and aeration. The pH value of the final fertilizer (6-8.5), which includes the values of 6.5-6.7 obtained in the study, indicates good compost quality from an agricultural point of view. These values have been recommended in several studies [22].

Checking the moisture content of the stored material is also very important for obtaining high-quality compost material. For the development of microorganisms, a moist, steam-filled environment is necessary. That is why it is necessary to maintain low compost humidity, which slows down the decomposition processes carried out by microorganisms and prevents the temperature in the heaps from rising. Excess moisture in the compost displaces air, creates an anaerobic environment, and leaches nutrients.

However, during the thermophilic period, as a result of the evaporation of water during microbial heat generation, the humidity level decreases to values lower than the recommended ones (40-50%). Moisture loss also occurs during the aeration of compost mixtures. Keeping the heaps moist during the composting process prevents them from drying out and reduces N losses.

The main way of N loss from compost is the evaporation of ammonia (NH₃), which leads to the loss of up to 50% of nitrogen from the initial amount in the compost [23]. In the experiment performed by Tiquia et al. [24], a researcher from the USA, 37-60% of the initial N in the compost was lost as a result of volatilization of NH₃. Some studies have also reported significantly lower N content in composted sludge (1.30%) than in stored sludge (1.98%), indicating significant losses during the thermophilic phase [25, 26]. Goyal et al. [27] noted that the decrease in the total N content at the initial stages of decomposition of organic waste was explained by its loss in the form of ammonia, which, in turn, depends on the type of material and its C-to-N ratio. In addition, some studies have examined the effect of storage temperature on NH₃ consumption. Pagans et al. [28] confirmed that NH₃ emissions increased with increasing temperature.

N losses are reduced by adding materials rich in available C. In this study, variants with a straw content of 20% by weight had the lowest proportion of total N content in the first ten days, 42%-46% of the initial values. The greatest loss of N, equal to 56%, was found in the variants of the experiment without straw, and the lowest among variants with biological preparations was found in the ones using Agromix SS.

The findings align with existing literature on nitrogen management during composting. Hoang et al. [23] reviewed strategies for mitigating nitrogen loss and highlighted that adding carbon-rich materials like straw can effectively reduce nitrogen volatilization. This aligns with our results, where higher straw content led to lower nitrogen losses. Similarly, studies by Wang et al. [29] demonstrated that carbon amendments improve the carbon-to-nitrogen ratio, thus reducing nitrogen loss through ammonia volatilization and promoting better compost stability.

An increase in the amount of total N during storage may be due to the concentration of the element as a result of mineralization of organic nitrogenous compounds and loss of total weight. N-fixing bacteria, which are part of Consortium B and Agromix SS, contributed to an increase in N content during the maturation of the compost.

The reduction of total C occurs due to the oxidation of organic substances to CO₂ and H₂O during aerobic storage. Y. Inbar noted that approximately 50% of organic matter decomposed to CO₂ and H₂O during storage. The conversion of organic substances into compost suitable for use as fertilizer depends on the ability of the microflora of the compost to secrete special enzymes [30].

The P and K content in the composting process can be increased under the influence of concentration with weight loss. Lin [31] suggested that the effect of concentration in compost occurred when organic C had a higher decomposition rate compared to organic forms of N, P, and K. The fertilizer can concentrate or dilute P, K, and trace elements in organic waste. The loss of chemical elements is manifested during alkalinization, and the decomposition of organic substances leads to their concentration [32].

Some heavy metals-in small amounts-are necessary for plant growth; however, in higher concentrations, they are likely to have negative effects. One of the main problems associated with the use of sludge compost is associated with the presence of high concentrations of heavy metals, which can have dangerous consequences for soils and crops and pose an indirect risk to human health due to the properties of biological accumulation. Thus, Du et al. [33] noted that P, Cu, Zn, and Cd increased after composting, while volatile solids, water, pH, and N decreased. He et al. [34] reported that aerobic composting processes increased the formation of heavy metal complexes in organic waste residues, which limits their solubility and potential biological availability in the soil.

In the obtained results, the highest concentrations of trace elements are represented in the form of Zn and Cu. This result is comparable to the result obtained by some scientists [35, 36], where Cu and Zn were the metals with the highest percentage of compostable materials among the measured ones [37].

The addition of straw had a diluting effect on the metal content since the values for the same types of metals in mixtures with 20% straw content were lower than in mixtures with 10% or variants without straw. The content of heavy metals in the final product was much lower than the standard limit values for compost, and therefore compost products can be safely used as a fertilizer.