Introduction

Nowadays, more consumers are searching for products having functional properties, containing nutrients coming from natural resources, but not being chemical synthesis products [1]. Phosphate rocks, which are non renewable raw materials, are the basic raw materials used in phosphorus fertilizers production. From the environmental point of view, we are dealing with phosphorus uptake from an ore (phosphate rocks, apatites) and its effective dispersion in the environment as a result of the farming [2]. There is no phosphorus substitute, but at the same time, phosphorus after the ‘use’ does not disappear and it can be subjected to recycling. Simultaneously, the growing demand for phosphorus fertilizers is observed (Table 1) [3]. Thus, the attemps to obtain phosphorus from the renewable resources such as different waste materials, i.e. bone wastes, fish bone, and ashes from biomass incineration from sewage treatment plants [4], have become very essential.

Table 1 The world’s fertilizers demand in the years of 2012–2016 (thousands tones of the component) [3]
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The application of fertilizers from sewage sludge ashes to agricultural land is generally considered to be the good way to recycling phosphorus because the sewage sludge ashes (SSA) have usually high content of P2O5. However, the SSA are not ideal source of phosphorus because they often contain significant amounts of undesirable metals and phosphorus compounds and they must be converted into available form [5, 6]. Previous studies of phosphorus recovery from SSA have looked at acid or base leaching. According to these studies, the acid leaching is more effective than base leaching [7, 8]. But still the use of sulfuric acid for phosphorus extraction is not always commercially reasonable. Because of this, it is interesting idea to conduct bioleaching process using microorganisms that produce organic or mineral acids [9, 10], especially taking into account the further course of solubilization in the soil after the application of fertilizer containing microorganisms [11].

One of the possibility is the use of a natural ability of some microorganisms (bacteria and fungi) for solubilization of phosphates [2]. The effectiveness of activation (solubilization) processes of phosphorus from mineral resources and different types of wastes, depends on numerous essential factors, i.e. the type of phosphorus raw material subjected to solubilization, types of the microorganisms, inorganic additives, as well as a method of preparing the products and conducting the process. Bacteria (Phosphorus Solubilizing Bacteria-PSB) and fungi (Phosphorus Solubilizing Fungi—PSF) can be used for the process. In total, both of the microorganism groups that activate phosphorus are defined by the term Phosphate Solubilizing Organizm- PSO [12, 13]. Two groups of processes are realized in practice:

  • composting of organic waste with an optional addition of phosphorus raw materials (activation of phosphorus contained in raw materials takes place during the composting processes) [14],

  • activation of phosphorus compounds contained in high quality raw materials and in waste raw materials using microorganisms (fungi and bacteria) and optionally elementary sulphur [2].

Bacillus megaterium and Acidithiobacillus ferrooxidans [15, 16] bacteria or Aspergillus niger [17] fungi are examples of bacteria that allow the microbiological solubilization process to happen. Bacillus megaterium, which were used in this study, produce the mixture of citric acid, lactic acid and propionic acid which react with phosphorus compounds from SSA and convert them to available form [2, 9, 10, 15] (Fig. 1).

Fig. 1
figure 1

B. megaterium, next to E. coli cell. The B. megaterium cell volume is 100 times bigger. The white line marks the length of 2 μm [18]

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The aim of the study was to manufacture of suspension fertilizers on the basis of phosphorus waste materials (ash and phosphate rock as controls) and B. megaterium for the field experiments conducted by University of Warmia and Mazury in Olsztyn.

Methodology

The Description of Raw Materials

Ashes coming from incineration of sewage sludge from Łyna sewage–treatment plant near Olsztyn were used as a phosphorus raw material. A suspension fertilizer obtained on the basis of Morocco phosphate rock was the control. The content of particular compounds in raw materials has been presented in Table 2.

Table 2 Content of particular compounds in SSA and Morocco phosphate rock
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Bacillus megaterium bacteria was delivered by Wroclaw University of Technology and their multiplication was conducted on a culture medium having the content described in Table 3.

Table 3 The content of culture medium for Bacillus megaterium multiplication
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Measurement Methods

Determination of total phosphates was conducted by a gravimetric method with the use of quinoline phosphomolybdate according to EC Regulation No 2003/2003.

Determination of assimilable phosphates, soluble in 2 % citric acid was also conducted by a quinoline-molybdate gravimetric method. The determinations were proceeded by an extraction conducted with 2 % citric acid. The extraction was proceeded according to EC Regulation No 2003/2003.

A water soluble phosphate was determined by a phosphomolybdate blue spectrophotometric method with the use of a Jenway 6300 spectrophotometer.

The pH measurement of the reaction mixture was conducted with the use of an Elmetron CX-501 multifunctional pH meter.

A sedimentation tests of the suspensions were conducted in the way that the fertilizer sample was placed in a transparent air-tight 1 dm3 bottle and the content was shaked to mix it well. Subsequently, the bottle was left until the mixture deposition started and then changes after 24 h were observed. Ash powdered to below 200 and 50 μm was used in the research. For each of the ashes, 3 samples of 7.5; 10 and 12.5 wt% of bentonite, were prepared.

Methods and Principles of the Conducted Processes

The laboratory tests of obtained suspension phosphate fertilizers were conducted on the IKA LR-2. ST Package 1 chemical reactor with a working volume of 2 dm3 equipped with a EUROSTAR stirrer. The reactor measured and registered the following parameters: temperature, number of rotations, torque.

Preparing a culture medium for the B. megaterium bacteria was the first stage of suspension fertilizer production. The trials were conducted with the concentration of phosphate raw material in mixture of c = 30 g/1 dm3. The particular components of the culture medium were introduced to proper amount of water—1.5 dm3. Afterwards, the obtained solution was brought to the boil (100 °C) in order to sterilize it. After conducting this process, the solution was left for cooling down to temperature of about 35 °C. After the cooling, the batch was inoculated by a bacteria solution in an amount equalling about 10 % of the reactor batch. The prepared bacterial culture was grown in the temperature of 35 °C for 7 days and in the case of laboratory tests with additional aeration—for 6 days and the numbers of rotations were 60 rpm and torque was 140 Ncm. During the process, on every 24 h, an additional portion of the cultural medium, was introduced. The introduced additional culture medium equalled 10 % of the reactor batch mass during the whole process. At the end of the process bentonite was added for the stabilization of suspension. Bentonite was added to reach a pH of 7 or, if it’s necessary, until stable suspension was obtained.

Tests were also carried out at pilot scale. The pilot scale tests were conducted in a reactor having operating volume of 100 dm3 (Fig. 2). The reactor was equipped with a mechanical mixer and electric heating. The procedure for testing was the same as in the case of laboratory tests. Pilot scale tests have not been conducted with additional aeration of the reaction mixture.

Fig. 2
figure 2

The experimental pilot plant for liquid fertilizer production

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Research Results

The research on production process of suspension fertilizers on the basis of B. megaterium bacteria began from conducting laboratory scale tests, aiming at determining the initial process parameters for further pilot scale research.

Laboratory Tests

According to conducted laboratory research, it is stated that B. megaterium bacteria could be used to obtain phosphorus fertilizer at which about 2/3 of P2O5 is in the form of available to plants. Moreover, it was observed that additional airing of mixture, accelerates the gaining of optimal phosphorus compounds conversion rate to plant available forms but not influence its end value.

Figure 3 shows the changes in ammonium citrate-soluble P2O5 content in relation to total P2O5 during the process of suspension fertilizers production on the basis of B. megaterium bacteria. Figure 4 presents changes in pH value in reaction solution during the laboratory trials of obtaining the suspension fertilizers.

Fig. 3
figure 3

Changes in ammonium citrate-soluble P2O5 content in relation to total P2O5 during the process of suspension fertilizers production on the basis of Bacillus megaterium bacteria

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Fig. 4
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Changes in pH value during the process of conducting the suspension fertilizer on the basis of Bacillus megaterium bacteria

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The initial dynamics of the increase of ammonium citrate-soluble P2O5 content is higher in fertilizer on the basis of phosphate rock than in the case of fertilizer on the basis of ash, however in both cases the end content of ammonium citrate-soluble P2O5 is similar.

In both cases changes in dynamics of pH values are similar. At the whole process for the fertilizer on the basis of phosphate rock, the lowest pH value is observed. Table 4 shows the final phosphorus contents at particular samples.

Table 4 P2O5 contents of particular forms in fertilizer products
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Pilot Plant Research

Pilot plant research has confirmed the results obtained on the basis of laboratory research, however in the case of pilot plant scale, the slightly decrease of dynamics of microbiological solubilization process was observed. Moreover, at the ½ technical scale research, the trials were conducted with the highest quantity of phosphate raw material and the concentration of mixture was c = 100 g/1 dm3.

Figure 5 shows changes in ammonium citrate-soluble P2O5 content in relation to total P2O5 content during process of preparing the suspension fertilizers using B. megaterium bacteria.

Fig. 5
figure 5

Changes in in ammonium citrate-soluble P2O5 content in relation to total P2O5 content in suspension fertilizers using Bacillus megaterium bacteria

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The final content of available phosphorus forms is about 47 %, both in case of an ash fertilizer, as well as a phosphate one. A faster increase of soluble P2O5 in ammonia citrate can be observed for phosphate fertilizers, in the first 5 days of the process.

Fig. 6
figure 6

Changes in pH value during the process of conducting the suspension fertilizer on the basis of Bacillus megaterium bacteria

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The pH value is lower during the preparation of phosphate-based fertilizer, but the changes of pH value are similar for both preparation processes of suspension fertilizers. Table 5 shows final contents of phosphorus forms in particular trials (Fig. 6).

Table 5 Particular forms of P2O5 contents in obtained products, %mass
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The pilot plant research has showed that the increased supply of phosphate raw material does have limited impact on contents of available phosphorus forms at conducted trials with mixtures containing more phosphate raw material. Only total P2O5 content was increased in proportion to the total P2O5 content from laboratory trials with smallest quantity of phosphate raw material.

In comparison to studies on solubilization by microbs [9, 10] the content of available P2O5 in the obtained suspension fertilizers is about 10 times greater. However, recovery of P2O5 in this tests was lower than in the previously mentioned studies [7, 8] of the extraction of phosphorus from the ash by mineral acids. Comparing to these results was difficult due to various kinds of obtained fertilizers. Most of the research results concern a clear liquid fertilizers while our study has been conducted to obtained suspension fertilizers.

Figure 7 illustrates the problem of maintaining stability of the obtained suspension fertilizers. Sedimentation tests of the suspensions showed that after approx. 2 h of obtaining the fertilizer all solids particles settle onto the bottom of the bottle. In order to identify the cause of suspension delamination, the experiment was conducted in which the influence of the ash fineness and the amount of added bentonite on sedimentation was determined. From this experiment (Table 6) we concluded that the reduction of ash particles had a positive effect on the stability of the suspension. However, the key factor which most strongly influences the stability of the fertilizer was the amount of stabilizing agent. We observed the greatest stability of the suspension with the addition of bentonite at the 12.5 % by weight (Fig. 8).

Fig. 7
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Images ilustrating delamination of fertilizer suspension based on ashes

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Table 6 Volume of suspension after 24 h
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Fig. 8
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Images ilustrating the level of suspension delamination depending on the level of ash crushing and the content of bentonite

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Conclusion

The pilot plant tests have confirmed that it is possible to produce phosphate fertilizer suspension of the waste materials, using the B. megaterium for solubilization of phosphorus, and that the process is not complicated. However, obtained fertilizers are characterized by low P2O5 content and even the increase of quantity of phosphate raw material does not benefit on contents of available P2O5 forms.

The studies revealed a problem of low stability of the obtained suspension. In order to reduce delamination, additional gridding of raw phosphoric material is needed and the addition of a sufficiently large amount of bentonite.

Compared to traditional suspension fertilizers, the potential advantage of those types of fertilizers is the content of B. megaterium bacteria which introduced to soil could activate the accumulated phosphorus compounds.