Integration of Organic Waste for Soil Stabilization through MICP

Abstract

Microbial-induced calcite precipitation (MICP) is an innovative technology in civil engineering. However, the high cost of components and the fragility of the treated soil limit its wide use. One of the possible solutions is organic waste incorporation at different stages of the technology. In the present study, we consider the use of spent brewer’s yeast (BSY) to produce bacterial inoculates and wastepaper, flax shives and sawdust as reinforcing additives into the soil. We showed that the replacement of expensive components of LB medium by BSY extract increased biomass growth characteristics of Bacillus subtilis K51, B. cereus 4b and Micrococcus luteus 6 strains by 1.4, 1.5 and 1.8 times, respectively, while for B. subtilis 168, they were comparable to LB medium. The urease activities of all strains were not reduced compared to the control. Among the three kinds of cellulose-containing waste, wastepaper incorporation into MICP-treated soil samples led to an increase in compressive strength by 2.1 times and precipitated calcite percentage by almost 1.5 times compared to a sample without additives. Thus, we showed the potential for soil stabilization through MICP using organic waste.

1. Introduction

In recent decades, the trend towards using environmentally friendly materials and technologies in geotechnical engineering has noticeably increased. Microbial-induced calcite precipitation (MICP) is an innovative technology among engineering developments. Its appeal is based mainly on simplicity and environmental friendliness [1,2]. Technologies are actively being developed to prevent the liquefaction of sandy soil and the processes of slope destruction, repair cracks in cement-concrete structures and reduce erosion [3,4].

Soil with specified geotechnical properties is a prerequisite at the initial stage of any infrastructure construction; therefore, a lot of research is devoted to improving its mechanical properties. Of particular interest is the stabilization of soils using biological methods. MICP technology uses certain strains of bacteria, widely present in nature, capable of causing calcium carbonate precipitation due to their metabolic activity. The mechanisms of microbial induction of mineralization are based on numerous enzymatic reactions, leading to the formation of biocement from carbonate. A number of publications describe in detail several ways of mineral precipitation [5,6,7]. The most common and widely studied process is the induction of calcium carbonate precipitation using bacteria capable of hydrolyzing urea. The process is based on several sequential chemical reactions [8]. First, urea hydrolyzes to ammonia and carbon dioxide, increasing the pH value of the environment. Carbon dioxide reacts with water to form bicarbonate and then carbonate ions, which, together with calcium ions, form precipitated calcium carbonate. The general process can be described using Equation (1):

$$\mathrm{C}\mathrm{O}(\mathrm{N}{\mathrm{H}}_2{)}_2+2{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{a}}^{2+}\to 2\mathrm{N}{\mathrm{H}}_4^{+}+\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3.$$

(1)

The resulting biocement demonstrates high strength and stability and is able to fill the gaps between particles, connecting them to each other [9]. Biological cementation makes it possible to strengthen and stabilize the soil by increasing the solid content, reducing the pore size and increasing the rigidity of the contact between particles [10]. Microbial soil strengthening is an environmentally friendly and cost-effective alternative to chemical cementation, which causes ecological problems due to mixing bentonite, silicates, or acrylic with the soil.

To implement biocementation in practice and meet the requirements of civil engineering, a number of problems must be solved. Soils treated with MICP are known to exhibit uneven calcium carbonate deposition and fragility [11]. For a long time, mechanical methods such as synthetic reinforcement have been used as a traditional approach to soil stabilization. However, due to the high energy costs of producing and introducing these materials into the soil, as well as the hazardous environmental impact, this approach does not seem attractive [12]. Soil reinforcement with natural fibers has potential due to its environmental friendliness, abundance of resources and cost-effectiveness compared to other materials [13].

With increasing levels of prosperity in industrialized countries, the volume of organic waste is also increasing significantly. The nutrient-rich composition of some industrial wastes provides a source of value-added products (biofuels, biopesticides, biohydrogen and bioplastics) produced through microbial fermentation [14,15]. Some types of waste can be used for soil restoration [16,17]. As part of the development of environmentally friendly valorization strategies, it seems advisable to use by-products or production waste to strengthen soils using MICP technology [18,19]. To stabilize large volumes of soil through biocementation, it is necessary to produce a significant amount of microbial biomass, retaining its biomineralizing properties. The costs of the growth medium used for microorganism cultivation constitute a significant part of the cost of MICP technology [20]. Consequently, there is a need to reduce the cost of the growth medium for the cultivation of calcite-precipitating bacteria. One solution may be to use inexpensive sources of nutrients from food processing waste [9]. For example, spent brewer’s yeast, a common by-product of the brewing industry, contains amino acids and vitamins necessary for the growth of microorganisms and can, therefore, be used to prepare a culture medium [21,22,23]. Other organic wastes, especially those containing cellulose, can be directly used in biocementation as native fibers for soil reinforcement.

In our recent work [24], screening ureolytic bacteria identified several strains capable of repairing microcracks on the surface of cement concrete samples. Subsequent work [25] in soil stabilization field trials demonstrated the benefits of using a microbial consortium over a single strain. The first objective of this work was to study the effect of replacing some components of a widely used growth medium with brewing waste on the urease activity of biomineralizing bacterial strains. The second objective of this study was to analyze the effect of the addition of shredded wastepaper, flax shives and sawdust on the mechanical properties of MICP-treated soil.

2. Materials and Methods

Urea, CaCl₂, NaCl and HCl were acquired from JSC “VEKTON” (Russia, St. Petersburg), and glucose was obtained from Wuhan Servicebio Technology Co., Ltd. (Wuhan, China). LB medium was sourced from VWR, Life science (Radnor, PA, USA), while other chemicals were obtained from Sigma-Aldrich Corp. (St. Louis, MO, USA) unless otherwise specified.

2.1. Microorganisms and Growth Condition

2.1.1. Bacterial Strains

The bacterial strains used in this study were screened for mineralizing activity [24] and were maintained and stored in the home collection of the Biotechnology laboratory of the NRC Kurchatov Institute—PNPI. From them, 4 previously characterized strains [25,26,27] were selected: Bacillus subtilis 168 (DSMZ 23778), B. cereus 4b (B-14265 from the National Bioresource Center “All-Russian Collection of Industrial Microorganisms”), strains B. subtilis K51 and Micrococcus luteus 6, which were undomesticated prototrophic strains from our laboratory collection.

2.1.2. Extract from Brewer’s Spent Yeast

Brewer’s spent yeast (BSY) was provided by “Baltika Breweries” LLC (Russia, St. Petersburg). The brewer’s spent yeast extract (BSYE) was produced through autolysis as described in [28] with modifications. Dry spent brewer’s yeast was poured with a 10-fold amount of distilled water and heated for 10 h at 50 °C with stirring. Then, the suspension was kept at 85 °C for 15 min to inactivate residual hydrolytic activities [29] and centrifuged at 3900 rpm for 50 min (Eppendorf centrifuge 5810R, Tokyo, Japan) at room temperature. The supernatant was freeze-dried using Martin Christ Alpha 1-2 LD plus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany), yielding 20.0 ± 1.5% extract by dry yeast weight.

The content of dry BSYE was quantified with gas chromatography of trimethylsilyl derivatives according to [30]. The samples were treated with 1.1.1.3.3.3-hexamethyldisilazane in a mixture of 1 mL of pyridine and 1 mL of acetonitrile in the presence of trifluoroacetic acid at 60 °C for 1 h. The resulting solution was placed into the sampler of the chromatograph. Analysis conditions: SBP5-25 column (25 m × 0.25 mm × 0.2 m); carrier gas: N₂ at 20 cm/s; temperature program: 1 min at 70 °C, rise 4 °C/min up to 320 °C, 5 min at 320 °C; sample injection temperature: 240 °C; flow divider: 1:20; sample volume: 2 µL; flame ionization detector temperature: 325 °C; gas supply rates: hydrogen at 40 mL/min, nitrogen at 25 mL/min, oxygen at 250 mL/min. Peaks were assigned according to retention times after a series of calibration analyses of model mixtures of a given composition.

2.1.3. Growth Media

To prepare the growth medium, we used a liquid extract of spent brewer’s yeast, avoiding the freeze-drying stage. To select the optimal balance of the composition of the growth medium, the content of BSYE and glucose was varied, and pH was adjusted to 6.5–7 with 1 M NaOH solution (

Table 1. Composition of the growth media.

Medium	Ingredients, g/L
Medium 1	BSYE, 5; NaCl, 10
Medium 2	BSYE, 10; NaCl, 10
Medium 3	BSYE, 5; Glucose, 5; NaCl, 10
Medium 4	BSYE, 5; Glucose, 10; NaCl, 10
LB *	Yeast extract, 5; peptone, 10; NaCl, 10

* LB medium was used as a control.

):

The bacterial strains, namely B. cereus 4b, B. subtilis 168, B. subtilis K51 and M. luteus 6, were individually cultured in 5 mL of each type of media for 24 h at 37 °C with constant aeration. Then, an aliquot (1 mL) was removed, and the precipitate was separated through centrifugation for 10 min at 10,000 rpm and resuspended in 1 mL of a 0.9% NaCl solution. Biomass changes during cultivation were monitored in all media using measurements of the culture liquid optical density at 600 nm using a JASCO V 560 spectrophotometer (JASCO Corporation, Tokyo, Japan).

2.2. Urease Assay

Urease activity was assayed according to the method described in [24]. Each bacterial strain was cultured separately in 5 mL of the BY medium (Medium 4, Table 1) for 24 h at 37 °C with aeration. LB medium was used as a control under the same conditions. The resulting biomass of each strain was separated through centrifugation for 10 min at 10,000 rpm (ELMI, multi-centrifuge CM 6M, Newbury Park, CA, USA) and added to the B4 medium without CaCl₂. Bacteria were cultivated for 72 h at 37 °C with aeration while removing aliquots (1 mL) after 3, 6, 24, 27, 30, 48, 53 and 72 h to measure conductivity using a conductometer (DFRobot DFR0300-H Gravity: Analog Electrical Conductivity Sensor/Meter, K = 10, Shanghai, China). The molar concentration of degraded urea was calculated using Equation (2) with the coefficient obtained from [31]:

$$\mathrm{U}\mathrm{r}\mathrm{e}\mathrm{a} \mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{e}\mathrm{d}\left(\mathrm{m}\mathrm{M}\right)=\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{m}\mathrm{S}\right)\times 11.11.$$

(2)

One unit of urease activity corresponded to the enzyme amount capable of catalyzing the conversion of 1 mM urea per minute under the standard assay conditions (pH 5.5, 37 °C, 20 min). The data points are presented as the means of at least three independent experiments.

2.3. Soil and Additives from Organic Waste

2.3.1. Soil Characteristics

The soil samples used for this study were collected in the Gatchina region of Russia (WSG84 N59.5007, E30.00477). The soil in this area was characterized as sod-podzolic surface-gleied (pH 6.8) according to [32]. The soil samples were collected from a depth of 0.25 m using a sterile drill (washed with 70% ethanol) and stored at 4 °C until further analysis. Granulometric analysis was carried out using the dry sieving method [33].

The results of the granulometric analysis of the soil are presented in

Table 2. Granulometric composition of the soil, %.

Sieve size, mm	>20	20–10	10–5	5–2	2–1	1–0.5	0.5–0.25	0.25–0.1	0.1–0.05	0.05–0.01	0.01–0.005	<0.005
Percentage passing	10.5	3.5	6.0	3.7	4.0	5.7	4.4	15.0	3.8	3.2	5.5	45.2

.

2.3.2. Cellulose-Containing Organic Waste Additives

Shredded Wastepaper

The shredded wastepaper, with a humidity of 12–14% (pH 7.1), was provided by the wastepaper recycling plant LLC “PTF Rustom”, Russia, Smolensk region, Safonovo (Figure 1A).

Flax Shives

Flax production waste was provided by Russian Flax LLC (Russia, Smolensk region, Safonovo). Flax shives (Figure 1B) are the woody part of spinning plant stems formed during the mechanical processing of the raw material. The specific gravity of flax shives was 50% of the raw material [34]. Flax shives (pH 6.1) consist of cellulose (45–50%), lignin (25%) and hemicellulose (20%) [35]. Some industries use flax shives as fuel for boiler houses or for the manufacture of fuel briquettes; however, a significant part remains unused, accumulating on the territories of enterprises and posing fire hazards and environmental pollution risks [36].

Sawdust

Sawdust from deciduous trees (pH 4.3) was provided by the furniture factory LLC “Amalthea” (Russia, St. Petersburg). This material constitutes the waste of wood sawing (Figure 1C). Deciduous wood mainly consists of cellulose (35–52%), lignin (17–22%) and pentosans (17–25%) [37].

2.4. Stabilization of the Soil Samples through MICP

The Experimental design is shown in Figure 2.

2.4.1. Sample Preparation

Experimental soil samples (50 g) were prepared by mechanically mixing soil with organic waste. Shredded wastepaper, flax shives and sawdust were added to the soil in an amount of 4 wt%. Tubes of disposable 50 mL syringes (SFM Hospital Products GmbH, Berlin, Germany) were used as molds for experiments. Before filling a mold with soil, a filter paper (Ø 3 cm) was placed at the bottom to prevent soil leaching during treatment with solutions.

Each bacterial strain was grown in 5 mL of Medium 4 (details are given in Table 1) for 48 h at 37 °C with aeration until optical density reached 8 at 600 nm. The biomass was separated through centrifugation at 10,000 rpm for 10 min and re-suspended in 0.9% NaCl solution, followed by mixing all four strain suspensions. Syringes with experimental samples were injected with 12 mL of the resulting bacterial consortium suspension, kept at room temperature for a day, and then treated with 8 mL of cementation solution (CS: 0.5 M CaCl₂ + 1 M urea). Treatments with CS were repeated on the fourth and eighth days. After the last CS injection, the samples were kept at room temperature for 21 days. Distilled water (20 mL) was added to the control samples instead of bacterial suspension and cementation solution. All samples were then dried at 50 °C for 3 days, removed from molds, dried at room temperature for 2 days, and aligned to the same size (Ø 3 cm, 6 ± 0.5 cm) for further tests.

2.4.2. Assessment of Unconfined Compression Strength and the Calcium Carbonate Content

The unconfined strength test was carried out according to [38] using the compression test KP-9 device. Pressure was applied gradually with 0.5 MPa/s until the structure of the samples was destroyed. Unconfined compressive strength was calculated following Equation (3):

$$S=\frac{P}{A}$$

(3)

where S is the calculated compression strength, MPa; P is the compressive force applied until a sample collapse, N; A is the area of the biocemented soil column, sm².

To estimate the amount of calcium carbonate formed, 1 g of each sample was crushed in a mortar to a homogeneous structure and placed in a carbonometer (KM-NT with an MTI manometer 2.5/0.5), followed by 10 mL of 10% HCl addition [39]. The percentage of carbonate formed in 1 g of the sample was calculated according to Equation (4):

$$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3=\frac{(P\times 100)}{(m\times a)}$$

(4)

where P is the pressure, bar; m is the sample mass, g; a is the correction coefficient equal to 1.4116. All experiments were carried out in 3 repetitions.

2.4.3. Microscopy Analysis

The microstructure of the samples was investigated through optical microscopy using a Bresser Advance ICD 10×–160× (Rhede, Germany) and scanning electron microscopy (SEM) at an accelerating voltage of 1 kV using a Tescan Amber GMH microscope (Brno, Czech Republic).

2.5. Statistical Processing

For statistical data processing and plotting charts and diagrams, Excel 2010 (Microsoft, Redmond, WA, USA) and OriginPro 16 (Microcalc, San Diego, CA, USA) were used. The graphs and tables show average values of at least 3 independent experiments; bars represent the standard errors. The statistical significance of the observed differences was assessed using Student’s t-test.

3. Results

In the present work, we used a bacterial consortium comprised of B. subtilis 168, B. subtilis K51, B. cereus 4b and M. luteus 6 strains, which were previously selected as the most effective in stabilizing soil characteristics [25]. The main selection criteria included high urease activity, the formation of a large amount of calcium carbonate in a liquid medium and the improvement of soil strength characteristics in laboratory conditions.

3.1. Selection of a Growth Medium for Ureolytic Bacteria

The production of biomass of an individual ureolytic strain in the used bacterial consortium was necessary due to mutual inhibition of the strain’s growth during preliminary co-cultivation in the LB medium. Thus, to obtain an inoculum for each ureolytic bacterial strain, the reduction of the growth medium cost was necessary. This challenge was solved by replacing expensive components of the LB medium (yeast extract and peptone) with an extract prepared from spent brewer’s yeast. The composition of the spent brewer’s yeast extract is presented in Table S1 (Supplementary Materials). The final component ratio was selected in such a way that the developed medium did not reduce the biomass yield and biomineralizing properties of bacteria (specific urease activity as a marker) compared to those observed for the control LB medium.

Figure 3 shows the biomass growth characteristics of bacterial strains during their cultivation in media with different component ratios (see details in Table 1). After a 24-h growth period of all bacterial strains in Medium 4 (BSYE, 5 g; glucose, 10 g; NaCl, 1 g, per liter of distilled water), the highest values of optical density were observed compared to those obtained for the control LB medium. The growth biomass characteristics for B. subtilis K51, B. cereus 4b and M. luteus 6 increased by 1.4, 1.5 and 1.8 times, respectively, while for B. subtilis 168, values were comparable to those of the control medium. Therefore, Medium 4 (hereinafter referred to as BY medium) was chosen for further experiments.

An analysis of the strains’ specific urease activity during cultivations in BY and control LB media revealed a sharp increase in the activity of B. subtilis 168 and B. subtilis K51 during the entire observation period in BY medium compared to that in the LB medium (Figure 4B,C). The urease activity of B. cereus 4b (Figure 4A) in the BY medium was slightly lower than in the LB medium. The specific urease activity of M. luteus 6 (Figure 4D) reached its maximum after 27 h of cultivation in the LB medium but then decreased within two days. This contrasts with the continuous slow increase observed after 27 h and subsequent three days of cultivation in the BY medium.

Thus, when ureolytic bacteria were cultivated on an inexpensive medium based on spent brewer’s yeast extract, biomass yields and urease activity values were generally comparable to those obtained on the standard commercial LB medium. For further experiments on biocementation, each strain of the resulting bacterial mixture was grown in the BY medium.

3.2. Properties of MICP-Treated Soil Samples with Waste Additives

In accordance with the experimental plan (Figure 2), prepared samples with additives from various organic wastes were treated with a bacterial mixture and a cementing solution (Figure 5), followed by a detailed characterization of the resulting samples.

3.2.1. Effect of MICP Treatment and Additives on Compressive Strength of Soil Samples

Treatment of soil samples with MICP led to a significant increase in the compressive strength in each case. The soil sample with the addition of shredded wastepaper showed the highest compressive strength value (Figure 6B), which was 2.1 times higher than that for the soil sample without additives (Figure 6A). The lowest mechanical compressive strength was detected in the soil sample with the addition of sawdust. Interestingly, treating soil samples only with a CS (urea + calcium chloride) without the addition of the bacterial mixture also led to an increase in compressive strength compared to control samples treated with water, except for the sample with sawdust additive (Figure 6D).

3.2.2. Calcium Carbonate Formation during MICP Treatment of Soil Samples with Additives

Soil samples destroyed after compressive strength testing were analyzed for precipitated calcite content (Figure 7). The highest percentage of precipitated CaCO₃ was found in the sample with shredded wastepaper (Figure 7B), which was almost 1.5 times higher than that for the soil sample without additives (Figure 7A). The lowest percentage of precipitated calcium carbonate was found in the soil sample with sawdust additive (Figure 7D). The introduction of only a CS into soil samples without the addition of a bacterial consortium also led to an increase in the calcium carbonate content in all samples.

Our results demonstrate that MICP treatment of soil led to a significant increase in the compressive strength and calcium carbonate content in all samples. Soil treatment with a CS only also increased the strength and CaCO₃ content of the samples, although to a lesser extent. Therefore, we can conclude that endogenous soil bacteria also have biocementation potential. However, analysis of the effect of additives used in this study clearly showed that only shredded wastepaper addition into MICP-treated soil samples increased strength and calcite content compared to samples without additives. On the contrary, the introduction of flax shives and sawdust into the MICP-treated soil noticeably reduced the strength characteristics of the samples.

3.2.3. Microscopic Observations of MICP-Treated Soil Samples with Shredded Wastepaper Additive

MICP-treated soil samples, with and without added shredded wastepaper, which exhibited the highest Rc values for compressive strength and CaCO₃ content, were analyzed with light microscopy and scanning electron microscopy (Figure 8).

Figure 8 shows a decrease in soil homogeneity and the appearance of large aggregates covered with white CaCO₃ precipitates after MICP treatment (Figure 8A,C). It was shown that the structure of the shredded wastepaper was porous and fibrous, providing good fixation in the soil and enabling penetration of bacteria throughout the sample (Figure 8B,D). SEM images show (inserts in Figure 8B,D) that calcium carbonate crystals envelop the soil particles evenly or form separate structures on the surface, thereby filling pore spaces.

4. Discussion

The improvement of soil geotechnical properties using MICP is a complex process that includes the interaction of various components: soil, microorganisms and cementation solution. For this technology to become widespread, its cost must be reduced, efficiency and productivity must be increased. Recently, methods have been actively developed to improve MICP technology using inorganic and organic additives, such as low-grade chemicals, biopolymers and others [9].

In large-scale geotechnical projects, to improve the mechanical properties of soils using MICP, standard growth media for microorganisms are not considered cost-effective. To reduce the cost of growing bacteria for soil biomineralization, various approaches are being explored. For example, in a few studies, ureolytic bacteria were cultured under nonsterile conditions [40,41,42]. Natural and waste materials have been used to prepare growth media, such as corn steep liquor [40,43], lactose mother liquor from dairy industry waste [20], whey, buttermilk [44] and molasse [41]. Spent brewer’s yeast is often investigated as a source of nutrients for the cultivation of microorganisms [23]. Gowthaman [45] showed that even at relatively low biomass concentrations, the urease activity of Psychrobacillus sp. was twice as high when grown on brewer’s yeast extract compared to standard yeast ammonia extract medium. In our work, to prepare the BY growth medium for bacterial inoculates, commercial yeast extract and peptone were replaced with spent brewer’s yeast extract. For the growth of B. cereus 4b and M. luteus 6 in the BY medium, significantly higher values of biomass characteristics and almost the same values of urease activity were observed compared to the LB medium. B. subtilis K51 and B. subtilis 168 demonstrated a relatively low increase in biomass and significantly higher urease activity in the BY medium compared to the LB medium (Figure 3 and Figure 4).

The fragility of soil treated with MICP is a major limit for the widespread application of this innovative technology. The introduction of reinforcing synthetic or natural fibers into MICP technology enables increasing the compressive strength characteristics of soil. Qiu et al. reported [46] that the addition of 0.2% carbon fiber in sand biocementation could significantly improve the strength and toughness of the sample. In a study conducted by Li et al. [47], the strength of MICP-treated sand with the addition of 0.2–0.3% homopolymer polypropylene fiber was increased more than twice compared to the sample without the fiber addition. Imran and coauthors showed that the addition of natural jute fiber increased the durability of MICP-treated samples by more than 50% [48]. Chen et al. reported that the addition of 1% wastepaper fiber increased the unconfined compressive strength of a sand sample, but the addition of more than 1% fiber led to a decrease in this parameter [49]. In our work, the addition of shredded wastepaper to the soil treated with MICP increased compressive strength by 2.1 times compared to the sample without additives. On the contrary, the addition of flax shives and sawdust reduced the strength characteristics.

Several studies reviewed in [46] have shown a positive correlation between unconfined compressive strength values and the calcium carbonate content of the soil. It was previously reported [9,50] that fibers with a large surface area can sorb bacteria and absorb the cementation solution, which leads to a limited distribution of bacteria throughout the sample volume and, accordingly, to an uneven distribution of calcium carbonate deposits. The optimal amount of fiber prevents bacteria from being washed out and settling in the lower part of the treated soil under the influence of gravity and the presence of large pore spaces between large particles, which together contribute to a greater yield of precipitated calcium carbonate [49]. Obviously, in our case, the porous structure of shredded wastepaper fibers did not prevent the spread of microorganisms and the cementation solution and provided the sample strength. On the other hand, the denser structure and larger surface area of sawdust and flax shives did not allow for the bacterial consortium and CS to be evenly distributed throughout the soil sample.

5. Conclusions

This work demonstrates the feasibility of integrating multiple types of organic waste into MICP technology.

Firstly, biomineralizing bacterial inocula were grown in the BY medium, in which expensive components (yeast extract and peptone) were replaced by spent brewer’s yeast extract. The biomass concentration of bacteria B. subtilis K51, B. cereus 4b and M. luteus 6 increased by 1.4, 1.5 and 1.8 times, respectively, while B. subtilis 168 showed a comparable value to the control medium. At the same time, a significant increase in urease activity was observed during the growth of B. subtilis K51 and B. subtilis 168 in the BY medium in contrast to the LB medium. Meanwhile, B. cereus 4b and M. luteus 6 showed comparable values of urease activity in both media, respectively.

Second, the addition of shredded wastepaper to soil treated with MICP increased the compressive strength of the experimental sample by 2.1 times and the content of precipitated CaCO₃ by almost 1.5 times compared to the control sample. Flax shives and sawdust addition to the soil reduced the sample strength characteristics.