Circular Perspective for Utilization of Industrial Wastewaters via Phytoremediation

Abstract

Wastewater generated in municipal rendering facilities requires multi-step treatment, but it may also serve as a source of nutrients and water and thus may be valorized before or instead of the necessary wastewater treatment operations. In this work, wastewaters from a composting plant were utilized to support the growth of Miscanthus x giganteus, known as both a remediation plant and an energy biomass source. A pot experiment was established to compare the effects of different wastewater doses (0, 50, 100, and 200 mL per pot per week) on the miscanthus biomass yield, phytoextraction of heavy metals, biomass heat of combustion, and plant condition. The increase in the wastewater dose resulted in increases in both biomass yield (from about 44 to 139%) and biomass heat of combustion (from 7 to 17%) when compared to the control sample, with no adverse effects on plant physiological parameters. The highest concentrations of metals were found in miscanthus grown with the highest dose of wastewaters. It was found that higher wastewater dose correlates to both higher phytoextraction and phytorecovery of metals from plant substrate and wastewaters. The highest metal uptake was identified for Fe (431 mg·pot⁻¹), followed by Al, Zn, Mn, Cu, Ni, Cr. The lowest metal uptake was noted for Pb, Co and Cd (0.88, 0.11, and 0.95 mg·pot⁻¹, respectively). The results indicate that miscanthus can be recommended for industrial wastewater treatment. In addition, due to high absorption efficiency of the substrate components, miscanthus can be used as a remediation tool, e.g., for the ecological stabilization of remediation of metal-polluted soils, especially in municipal facilities like rendering plants. This presents a circular perspective for the valorization of post-fermentation wastewaters with subsequent growth of energy crops, with other potential benefits for the environment, such as soil treatment, absorption of CO₂, and air purification.

1. Introduction

Municipal waste management affects the local environment in many aspects (e.g., emission of odors, noise, land occupation) and produces secondary waste like wastewaters that need to be properly managed. Additionally, rendering facilities are usually designed to operate for tens of years; thus, their territories (especially soil) can be historically polluted and require remediation actions. Typical rendering facilities operate a set of installations including waste segregation, a hermetic composting plant, a point for selective collection of household waste, a landfill, and a wastewater treatment plant for preliminary treatment of landfill, leachates, and technological wastewaters before directing these streams to the municipal wastewater treatment plant. The rendering plant’s installation is designed to recover materials for further reuse according to the BAT (Best Available Technology) approach or safely landfill the waste materials if no recycling is possible [1,2].

Municipal wastes are selectively collected and sorted to separate recyclable materials (such as plastics, metals, glass, and paper) from compostable ones. This biodegradable fraction is directed to a hermetic composting plant. As a result of anaerobic decomposition of biodegradable waste, a solid product of commercial value is produced, i.e., a soil improver. The soil improver must fulfill defined physicochemical requirements for organic and mineral fertilizers and amendments, thereby improving soil quality, as stipulated by legal regulations [3,4].

During the composting of biodegradable waste, a large volume of post-process leachate is collected and further mixed and treated with the technological wastewater in the treatment plant. Such leachates and wastewaters contain—in addition to harmful chemicals such as heavy metals, chlorides, petroleum substance and polycyclic aromatic hydrocarbons [5,6]—valuable components in the form of potential biogenic compounds (e.g., nitrogen, phosphorus), organic matter, and microelements (

Table A1. Values of selected parameters of wastewaters (based on Annual Environmental Monitoring Report for Landfill in municipal rendering plant in Gdańsk, 2023).

Parameter	Unit	Value
pH		6.7 ± 0.2
Electrical conductivity	μS∙cm−1	38,000 ± 120
Turbidity	NTU	895 ± 10
Odor	TON	>1000 ± 10
Suspensions	mg∙L−1	1790 ± 36
Dissolved substances	mg∙L−1	37,630 ± 40
Dry residue	mg∙L−1	39,400 ± 80
BOD5	mg∙L−1	17,050 ± 40
CODCr	mg∙L−1	34,800 ± 56
Permanganate index	mg∙L−1	2360 ± 45
Ammonium nitrogen	mg∙L−1	3120 ± 64
Nitrate nitrogen	mg∙L−1	1.81 ± 0.10
Total nitrogen	mg∙L−1	3355 ± 30
Chlorides	mg∙L−1	8340 ± 105
Sulfates	mg∙L−1	11,500 ± 235
Fluorides	mg∙L−1	1630 ± 20
Phosphates	mg∙L−1	329 ± 24
Sulfides	mg∙L−1	34.6 ± 4.8
Phenol index	mg∙L−1	3.35 ± 0.05
Substance extractable with petroleum ether	mg∙L−1	290 ± 8
Copper (Cu)	mg∙L−1	0.184 ± 0.02
Zinc (Zn)	mg∙L−1	6.11 ±0.05
Lead (Pb)	mg∙L−1	<0.010
Cadmium (Cd)	mg∙L−1	<0.005
Chromium (VI)	mg∙L−1	<0.010
Chromium—Total (Cr)	mg∙L−1	1.56 ± 0.05
Nickel (Ni)	mg∙L−1	0.87 ± 0.04
Iron–Total (Fe)	mg∙L−1	41.2 ± 0.4
Manganese (Mn)	mg∙L−1	13.8 ± 1.1
Mercury (Hg)	mg∙L−1	<0.0005
Potassium (K)	mg∙L−1	2960 ± 36
Sodium (Na)	mg∙L−1	2654 ± 16
Calcium (Ca)	mg∙L−1	1790 ± 24
Magnesium (Mg)	mg∙L−1	357 ± 1
General hardness	mmol∙L−1	65.4 ± 4.5
PAHs	μg∙L−1	0.059 ± 0.005
TOC	mg∙L−1	10,750 ± 50

), and thus may be regarded as a material for possible environmental application.

Currently, much attention in the EU countries is paid to promoting and contributing to sustainable development through the cyclical use of materials and resources [7,8]. This approach is known as the circular economy, and it is based on reducing the demand for source materials by reusing undesirable products, while simultaneously minimizing waste generation to sustain economic growth through the use of natural resources [9]. These can be achieved, for example, by retaining the value of renewable resources and increasing circularity in material cycles [10]. Taking into consideration the ideas of circular economy and the assumptions of green deal policy, wastewaters from a composting plant can be recycled for the recovery of biogenic substances (nitrogen, phosphorus) as well as other components [11].

Physicochemical and biological processes are the most popular technologies for wastewater treatment. However, high costs and increasingly stringent limits of nutrients emissions to wastewaters resulted in the development of new biotechnologies for wastewater treatment. For example, autotrophic denitrification enables the recovery of nutrients from wastewaters for potential further reuse [12]. In addition, recovery of biogenic compounds with simultaneous treatment and environmental-cleaning from micro-pollutants may be realized using plants, e.g., via phytoremediation [13,14]. This approach appears to be of importance, especially in the context of the need to degrade toxic and bioavailable components of post-composting leachates into non-hazardous and stable forms, which can be achieved through plant-based remediation technologies during the process of phytostabilization [15,16]. Furthermore, proper management of harvested biomass—which may contain elements of economic value after phytoremediation, such as metals—as well as the energetic valorization of biomass, can further enhance the benefits of the circular economy approach [17,18].

The utilization of plants for treating post-composting wastewater was investigated by Justin et al. [19]. It was found that using a composting leachate can be a promising approach for irrigating poplar clones and willows, allowing for enhanced biomass production compared to the control water treatments. Phragmites australis and Ceratophyllum demersum have been applied to treat the landfill leachate in four municipal waste treatment facilities in Poland [20]. Recently, Bhagwat and co-workers investigated the relation between the leachate composition and effects on phytoremediation with a focus on heavy metal uptake from soil [21]. Phytoremediation of heavy metal-polluted wastewaters by using a Miscanthus sp. Goedae-Uksae 1 was studied by Bang et al., showing the potential of this plant towards its utilization for the remediation of marginally contaminated ecosystems [22]. Treatment of landfill soil contaminated with trace elements using miscanthus was investigated by Pidlisnyuk and co-workers, showing enhancement of phytoremediation efficacy by applying synergistic plant growth-promoting bacteria [23]. Recently, Andrić et al. suggested Miscanthus as a suitable plant to treat landfill leachate [24].

The environmental utilization of post-process wastewaters generated in rendering plants can be beneficial, not only due to their potential use in supporting vegetation and biological remediation of closed landfill sites, but also as a source of water during drought episodes [25]. Additionally, if energy plants are utilized for the remediation of the landfill sites, the biomass growth can be supported and enhanced, increasing the possible energy recovery. What is more, another product of biodegradable waste composting, i.e., soil improver, can be utilized for enhanced plant growth [26,27,28]. Such an approach of beneficial use of products and by-products of waste processing lies within the ideas of circular economy and sustainable development, as well as answers the current challenges related to climate change issues.

There is evidence in the literature on utilizing plants for wastewater treatment, including post-composting or landfill leachates, but few papers can be found that investigate such an approach for real substrate samples with additional use of a soil improver from biodegradable waste composting. In this work, an attempt was made to combine nutrient recovery from post-processing wastewaters with the simultaneous environmental and energetic valorization of Miscanthus x giganteus. Effects of using the wastewaters on the energetic potential of generated biomass, as well as the plant’s physiology and composition of both biomass and plant substrates, are discussed. In addition, the proposed circular perspective of valorizing waste or waste-originating products for enhanced plant growth offers other environmental benefits, including uptake of CO₂, green barriers for emission of odors, and air phytoremediation potential.

2. Materials and Methods

2.1. Experimental Conditions

The research on the effects of applying wastewaters to plants (giant miscanthus) and substrates was conducted in summer 2022 as pot experiments in the vegetation greenhouse conditions. The greenhouse was located on the territory of Waste Utilization Facility Ltd. in Gdańsk, Poland (54°19′12.107″ N, 18°32′23.62″ E).

2.2. Plant Characterization

Giant miscanthus seedlings were purchased from the Nursery of Ornamental Plants (Starogard Gdański, Poland, company traceability code: PW/6/01/2020/12736). One-year giant miscanthus seedlings with the RUOP registration code PL-22/13/77 were used. The country of origin was Poland.

2.3. Substrates for Plant Cultivation

The following substrates were used in the pot experiments: construction sand (Sa) and soil improver (SK-9). Construction sand used in the experiments was yellow in color and was sieved through a sieve with a mesh diameter of about 1–3 mm, with a dominant fraction of sand particles with a diameter of 2 mm. The construction sand is classified as very light soil according to the Polish Soil Classification [29]. No other mineral fraction or organic matter was identified in the sand. The following values of pH of the sand were determined: pH_H2O = 7.0 ± 0.2 and pH_KCl = 6.9 ± 0.2.

Soil improver (SK-9) is a product of Waste Utilization Facility Ltd. in Gdańsk, obtained during processing of biodegradable waste in the hermetic composting tunnel and further stabilization and aeration operations in the open field. The SK-9 product is legally allowed to be used as a soil-enriching agent, improving the physicochemical properties of all types of soil, including the cultivation of ornamental plants, as well as in the plant-based restoration of degraded areas [30]. SK-9 soil improver fulfills the requirements for organic fertilizers according to regulations [3,31,32]. Selected physicochemical parameters of the soil improver are given in

Table A2. Selected physicochemical parameters of SK-9 soil improver.

Parameter	Units	Value
pHH2O	-	7.3 ± 0.2
pHKCl	-	7.2 ± 0.2
Electrical conductivity,	μS∙cm−1	1740 ± 35
Carbon (organic),	g∙kg−1 D.M.	221.1 ± 6.0
Nitrogen (N)	g∙kg−1 D.M.	5.30 ± 0.50
Phosphorus (P)	g∙kg−1 D.M.	1.42 ± 0.35
Potassium (K)	g∙kg−1 D.M.	4.82 ± 0.24
Calcium (Ca)	g∙kg−1 D.M.	26.26 ± 1.50
Magnesium (Mg)	g∙kg−1 D.M.	1.81 ± 0.05
Sodium (Na)	g∙kg−1 D.M.	0.71 ± 0.05
Aluminum (Al)	mg∙kg−1 D.M.	3539 ± 607
Iron (Fe)	mg∙kg−1 D.M.	1458 ± 85
Zinc (Zn)	mg∙kg−1 D.M.	134 ± 14
Cadmium (Cd)	mg∙kg−1 D.M.	1.8 ± 0.3
Chrome (Cr)	mg∙kg−1 D.M.	54.2 ± 8.5
Cobalt (Co)	mg∙kg−1 D.M.	5.2 ± 1.1
Copper (Cu)	mg∙kg−1 D.M.	85.2 ± 7.0
Lead (Pb)	mg∙kg−1 D.M.	115.3 ± 8.7
Manganese (Mn)	mg∙kg−1 D.M.	58.4 ± 4.0
Nickel (Ni)	mg∙kg−1 D.M.	29.2 ± 3.0

.

2.4. Wastewater for Plant Irrigation

Wastewaters used for irrigation of plants were taken from the collection point for mixed technological wastewaters and landfill leachates as a concentrate after the reverse osmosis treatment. According to the law regulations, no exceedances were found for the permissible values of pollutant indicators introduced into the industrial sewage systems [33]. Selected parameters of the wastewaters are given in Table A1.

2.5. The Scheme and Conditions of the Experiment

The one-factor experiment was carried out in triplicate in 8 kg polyethylene pots filled with a construction sand and a soil improver SK-9, which were thoroughly mixed in a 1:1 weight ratio. The experiment design included five objects, i.e., only sand, only soil improver, and a mixture of sand and soil improver, as well as differing in the irrigation pattern using industrial wastewaters, dosed according to the experimental scheme (

Table 1. Scheme of the experiments.

Object No.	Substrate *	Dose of Water/Wastewater, mL∙pot−1∙Week−1	pHH2O (Substrate)
1	Control: Sa	500 H2O	7.8 ± 0.2
2	Sa: SK-9 (1/1 v/v)	500 H2O	7.5 ± 0.2
3	Sa: SK-9 (1/1 v/v)	1 May—15: 500 H2O 16 May—30 September: 50 W + 450 H2O	8.2 ± 0.2
4	Sa: SK-9 (1/1 v/v)	1 May—15: 500 H2O 16 May—30 September: 100 W + 400 H2O	7.7 ± 0.2
5	Sa: SK-9 (1/1 v/v)	1 May—15: 500 H2O 16 May—30 September: 200 W + 300 H2O	7.7 ± 0.2

* Sa—Sand; SK-9—Soil improver; W—wastewaters.

). Following doses of wastewaters per pot per week were used: 50, 100 and 200 mL. The wastewater doses were selected due to high nutrients requirement of miscanthus due to its high biomass growth. Due to high salinity, wastewaters were diluted with distilled water on order to limit the problem of low nutrient uptake in highly saline conditions.

Total volume of liquid, i.e., either pure water (H₂O) or wastewaters (W), diluted with water, was always 500 mL. The test plant, i.e., Miscanthus x giganteus, was selected due to its high resistance and high tolerance to pollutants present in the industrial wastewaters. In addition, miscanthus, as a monocotyledonous plant, is characterized by a large increase in biomass in a relatively short time [34]. Miscanthus was planted in pots in the form of seedlings in mid-April 2022. The miscanthus vegetation period lasted from 1 May 2022, to 30 September 2022.

For the first two weeks after starting the pot experiments, the seedlings were watered only with distilled water in order to acclimate the plant to the greenhouse conditions and the substrate type. After two weeks, during the growing season, the plants were watered according to the experimental scheme (Table 1), once a week with distilled water and industrial wastewaters, maintaining soil moisture at 60% of the maximum water capacity. When necessary, water losses were replenished by distilled water to a constant mass of the substrate. The doses of the wastewaters are related to the nutritional requirements of the miscanthus. This plant requires high doses of nutrients due to its high biomass production. In addition, the applied wastewater streams were diluted with distilled water in order to limit the high salinity of the waste streams, which can hinder the nutrient uptake from the solutions used for miscanthus irrigation. In the pot experiments, mineral fertilization (NPK) was not used, as the applied soil improver and industrial wastewater were assumed to be sources of nutrients and pollutants that could potentially impact the growth and development of the test plant. The average temperature during the experiment was 28 ± 5 °C.

2.6. Analytical Procedures

After a vegetation period, the aboveground parts of miscanthus were collected from all pots. The plant material was dried in a laboratory dryer at 105 °C to a constant weight (SLN 32 SMART, PRO-EKO APARATURA, Wodzisław Śląski, Poland). The dried plant material was weighed (laboratory balance AS220.R2, RADWAG, Radom, Poland). Biomass yield for dry samples was determined for each experimental pot. Dry plant material was ground on an ultra-centrifugal mill (ZM 200, RETSCH, Haan, Germany) to obtain particles with a diameter less than 1 mm.

Plant materials were further processed for analysis. One part of the collected biomass was dry-mineralized in a muffle furnace at 550 °C [35]. The obtained ash was dissolved in 5 mL of an aqueous solution of 35–38% HCl (1:1 v/v), then the liquid was evaporated and the residue was treated with 5 mL of HNO₃ (1:2 v/v) and evaporated over a sand bath. Again, the residue was treated with 5 mL of HNO₃ (1:2 v/v) and heated under the watch glass. Then, the content of the evaporator was filtered through a filter paper to a 50 mL measuring flask. The evaporator and the residue on the filter was washed multiple times with hot distilled water, to the volume of 50 mL. Upon cooling, the flask content was filled up to 50 mL with distilled water and thoroughly mixed. Such prepared samples were subjected to further analyses, i.e., selected elemental composition (P, K, Ca, Mg, Na, C, N, S).

The other part of plant material was subjected to microwave plasma-atomic emission spectrometry analyses (4210 MP-AES, Agilent Technologies, Santa Clara, CA, USA) for the determination of selected elements, i.e., Al, Fe, Mn, Co, Zn, Cd, Cr, Cu, Ni, Pb. The mineralization of plant samples was carried out using 65% HNO₃ solution (Avantor Performance Materials S.A., Gliwice, Poland) and a microwave-assisted system (Magnum II, Ertec, Wrocław, Poland). More details of this analytical procedure are available in another paper [27].

Substrate materials, i.e., sand and soil improver, after sieving through a sieve with a mesh diameter of 2 mm, were subjected to determination of dry matter content [36], pH [37], electrical conductivity, total organic carbon (TOC) content by the Tiurin method, and total nitrogen by Kjeldhal method [38]. Mineralization of soil samples, excluding the samples subjected to MP-AES, was performed using a 3:2 v/v mixture of 70% HClO₄ and 65% HNO₃ (POCH, Avantor Performance Materials S.A., Gliwice, Poland) [39]. The procedure for substrate sample preparation for MP-AES determination of Al, Fe, Mn, Co, Zn, Cd, Cr, Cu, Ni, Pb is similar to that given above for the plant material [27].

Determination of elements in mineralized samples of plant material and substrates was performed using atomic emission spectrometry, ICP-OES technique (OPTIMA 7300 DV, Perkin Elmer, Waltham, MA, USA) [40,41]. Quality assurance protocols included analysis of certified reference materials and processing blanks in parallel with samples. Method validation showed recovery rates exceeding 95%, with method detection limits established at 0.1 μg·kg⁻¹ for all analyzed elements. The limit of quantification was three times higher than the limit of detection.

Determination of the heat of combustion for the collected plant materials was performed using an automatic calorimeter with a calorimetric bomb (KL-14, PRECYZJA-BIT, Bydgoszcz, Poland). A total of 1 g of pelletized plant material sample was subjected to analysis. Three runs were performed for each analyzed sample, and average values of the heat of combustion are given.

All of the chemicals were of analytical grade.

2.7. Evaluation of Plant Physiological Activity

The physiological activity, growth, and chemical properties of plants were assessed according to methodologies developed based on national standards and previous studies conducted at the University of Lodz and State Institute of Horticulture, Poland [42,43,44,45].

During the pot experiments in greenhouse conditions, the plants were subjected to various agrotechnical treatments, and their evaluation was based on the physiological activity observed during the month of August. As part of the evaluation, gas exchange (net photosynthesis, transpiration, intercellular CO₂ concentration, stomatal conductance), chlorophyll content index, and chlorophyll fluorescence were analyzed. These measurements were carried out on fully expanded leaves located in the upper part of the plant. Ten plant specimens were randomly selected from each experimental variant, and one leaf was taken from each plant to be assessed for physiological activity.

The gas exchange in leaves, including net photosynthesis, transpiration, stomatal conductance, and the concentration of intercellular CO₂, was determined using the TPS-2 Portable Photosynthesis System (PP Systems; Amesbury, MA, USA; Operation Manual Software Version 2.00) [46]. The chlorophyll content index in leaves was determined utilizing the Minolta SPAD-502 chlorophyll meter (Konica Minolta, Tokyo, Japan) [44,45,46].

Analysis of chlorophyll fluorescence was conducted using a Handy PEA fluorometer from Hansatech Instruments Ltd. (Pentney, United Kingdom; software PEA+—Version 1.13). Comparing the fluorescence of the sample in darkness to its maximum fluorescence (maximum PSII photosystem efficiency, Fv/Fm) enabled the establishment of the correlation between the structure and functionality of the photosynthetic apparatus and estimated the vitality of the plants. Leaves of plants were acclimatized in darkness to ensure that all reaction centers in photosystem II were opened (oxidized) and prepared to receive electrons before measurement. This analysis of the physicochemical parameter suggests the potential utilization of chlorophyll fluorescence as an indicator of plant response to various environmental factors [44,45].

The results of each physiological characteristic were averaged and input into the ClustVis tool (Shiny, version 0.10.2.1.; pheatmap version 0.7.7.) The program utilizes several R language packages and borrows code from BoxPoltR, analyzing and comparing the provided data to output the effect in the form of a table of colored squares. The color is directly correlated with the deviation from a variable calculated by the program, such as the median or average. This graphical representation of differences in each physiological characteristic helps to visualize correlations between each experimental variant [47]. Moreover, to further visualize the comparison of physiological states observed in the studied objects, a radar chart was created using Apache ECharts, (Apache ECharts, version 5.4.2) which provided sufficient adjustability for this task.

2.8. Quality Control

The determinations for each of the analyzed samples were performed in triplicate. The accuracy of the analytical methods was verified based on certified reference materials: CRM IAEA/V/V-10 Hay (International Atomic Energy Agency, Vienna, Austria), CRM-CD281-Rey Grass (Institute for Reference Materials and Measurements, Geel, Belgium), CRM023-050-Trace Metals-Sandy Loam 7 (RT Corporation, Tokyo, Japan). Pelletized benzoic acid (POCH, Gliwice, Poland) was used as a reference during the determination of the heat of combustion.

2.9. Calculations, Statistical Analysis of the Results, and Graphics

In order to determine the phytorecovery of elements from industrial wastewaters, the following parameters were selected:

• Biomass dry matter yield (BY, g∙pot⁻¹ D.M.);
• Yield tolerance index (TI, -) defined as the ratio of the yield of plants (i.e., biomass dry yield) grown on substrates watered with industrial wastewaters to the yield of plants from control objects (i.e., grown without watering with wastewaters) [48];
• Concentration of elements in collected biomass (C, mg∙kg⁻¹ D.M.);
• Concentration index (Cin, -) of elements in biomass, calculated as the ratio of metal concentration in plants grown on substrates watered with industrial wastewaters to the concentration of metals in plants from control objects;
• Bioconcentration factor (BCF, -) defined as the ratio of the metal concentration in the plant to the metal concentration in the substrate on which the plant was grown [48];
• Metal uptake (U, mg∙pot⁻¹), calculated as the product of biomass dry matter yield and metal concentration.

Statistical analysis of the research results was carried out using Microsoft Office Excel 2013 spreadsheet and Statistica 13 PL package. One-way analysis of variance was used to assess the statistical significance of the analyzed sources of variability. The significance of differences between the mean values was verified based on Tukey’s HSD test, with the significance level α ≤ 0.01. In this work, the dispersion between the measurements in the chemical analysis was assumed to be a maximum of 5%.

Principal component analysis was performed using RStudio (v. 2023.06.0 + 421) software [49] using R [50].

3. Results and Discussion

3.1. Physico-Chemical Properties of Materials Used in Experiments

In the pot experiment, construction sand with a mesh diameter of 1–3 mm was used as the substrate component. The sand used in the experiment was washed with a solution of hydrochloric acid (10% concentration) and then with tap water to wash out chlorides. A compost (SK-9) was used in the substrates as well, which was a source primarily of organic matter and nutrients, as well as toxic substances, including heavy metals (Table A2). The content of heavy metals in the compost was below the permissible values for organic fertilizers, organic-mineral fertilizers and soil improvement agents [31,32].

The necessary condition for the natural management of a landfill is to create a reclamation layer, which eliminates or reduces the impact of waste from the landfill on the surrounding environment and creates conditions for plant adaptation to the landfill under reclamation [51,52]. European Union, as well as the national legislation, allows the use and composition of remediation layers from various waste materials, provided that they do not pose an ecological risk to the environment [53,54]. The use of sand as a substrate component was intended to increase permeability and, at the same time, to improve the infiltration conditions of the substrate in the context of industrial wastewater management. The compost used was a source of organic matter and nutrients for the plants being tested. Composing such a substrate (reclamation layer) from materials with different physicochemical properties (sand and compost) allowed for assessing the possibility of using industrial wastewaters and their impact on plants in the perspective of natural management of such wastewaters at a municipal rendering plant.

In landfill remediation, it is possible to use wastewaters to irrigate and fertilize remediation plants. However, significant limitations may be the high salinity of the soil and the accumulation of toxic substances, which prevent the proper growth and development of remediation plants [51,52]. Therefore, future field testing at a landfill is recommended and the studies will allow for the assessment of the effectiveness of the above remediation treatments.

3.2. Giant Miscanthus Yield in the Pot Experiment

The yield of miscanthus grown in the conditions of the pot experiment in the greenhouse depended on the type of substrate and the dose of the industrial sewage (

Table 2. Biomass yield (BY), tolerance index (TI) and heat of combustion (HC) for giant miscanthus.

Object No.	Dose of Industrial Sewage, mL∙pot−1	Biomass Yield g∙pot−1 D.M.	Tolerance Index	Heat of Combustion MJ∙kg−1 D.M. *
1	0	249 ± 7 **	-	14.5 ± 0.4
2	0	276 ± 11	1.1	15.4 ± 0.2 S
3	50	358 ± 11 S	1.4	15.6 ± 0.2 S
4	100	419 ± 12 S	1.7	16.3 ± 0.2 S
5	200	594 ± 13 S	2.4	17.0 ± 0.3 S
CV % ***		36	32.7	1.0
LSD α ≤ 0.01 ****		29	-	0.2

* D.M.—Dry Matter; ** SD—Standard deviation; *** CV—Variability Coefficient; **** LSD—Least Significant Differences; S—Significant.

). The yield analysis shows that the only mixture of quartz sand with compost used (object 2) significantly increased the yield, which amounted to over 11% compared to the control. The increase in yield in this object is explained by the improvement of the physicochemical properties of the substrate and the increased availability of nutrients from the applied compost [26,54].

The application of industrial sewage in doses of 50, 100, and 200 mL per week per pot to sand and compost mixtures (objects 3–5) increased the yield of miscanthus. The increase in the yield in these objects was over 44, 68, and 139%, respectively, compared to the control (Table 2). From the point of view of phytoextraction potential, the highest yield of miscanthus biomass was obtained in the object where the highest doses of industrial sewage were applied (object 5). Own studies confirm that industrial sewage, despite containing a high concentration of chemical pollutants, is a potential source of nutrients that promote the growth and development of plants [15,55]. In addition, the increase in miscanthus biomass yield with increased doses of wastewater is likely related to the plant’s high ability to overcome osmotic stress caused by high salinity [23]. The effect was, however, positive within the studied range of concentrations, but became toxic at higher concentrations. Studies on the fertilizing usefulness of industrial sewage indicate that nutrients also occur in easily soluble, bioavailable forms that can decrease or eliminate the toxic effects of heavy metals on plants [5,7].

3.3. Yield Tolerance Index

In this study, an attempt was made to assess the toxicity of industrial sewage based on the yield tolerance index (TI), which was estimated as the ratio of the yield obtained in the objects with sewage doses to the yield obtained in the control object (Table 2). The yield tolerance index (TI) has been recognized in recent years as the most reliable factor in determining the toxicity of a stress factor to plants [48,56]. The yield tolerance index can take the values TI < 1, TI = 1, and T > 1. If the index is lower than one, it means inhibition of plant growth and sometimes their complete death, equal to one—indicates no effect of pollution on yield, and higher than 1 indicates the ‘stimulating’ (positive) effect of pollution on plant growth and development. The values of the yield tolerance index (TI) show that only the mixture of sand and compost used (object 2) had a stimulating effect on the yield of miscanthus (Table 2). On the other hand, the introduction of industrial sewage to this mixture in the amounts of 50, 100, and 200 mL per week per pot (objects 3–5) resulted in greater stimulation of yield, and the value of the tolerance index was 1.4, 1.7, and 2.4, respectively. The obtained values of the yield tolerance index, i.e., above one, confirm that industrial sewage containing nutrients stimulates the growth and development of plants [57].

3.4. Biomass Heat of Combustion

The study attempted to assess the impact of industrial sewage on the calorific value of miscanthus biomass. The applied mixture of sand and compost (object 2) significantly increased the heat of combustion of the tested miscanthus biomass (Table 2). The increase in the heat of combustion of the miscanthus biomass can be explained by the improvement of the physicochemical properties of the substrate, which affects the lower moisture content of the tested plant. Industrial sewage applied in doses of 50, 100, and 200 mL per week per pot to sand and compost mixtures (objects 3–5) had an even greater impact on the increase in the heat of combustion of miscanthus biomass, compared to the results from object 2. The conducted studies showed that the highest value of the biomass heat of combustion was obtained in the object with the highest dose of industrial sewage, and the increase in the heat of combustion was over 17% compared to the control (object 1), (Table 2). This study indicates that the applied industrial sewage affects the production of a crop with less moisture content and a fast–drying tendency [58,59,60]. The research results indicate not only the possibility of using industrial wastewater for fertigation (i.e., irrigation and fertilization) of miscanthus, but also the possibility of obtaining more energy from the combustion of this biomass. Additionally, when considering both the increase in biomass yield and the increase in the calorific value of giant miscanthus samples, there is a significant rise in the overall biomass energy potential. For example, compare objects 2 and 5, which have biomass yields of 276 and 594 g per pot, and heats of combustion of 15.4 and 17.0 MJ·kg⁻¹, respectively (Table 2).

However, when considering the practical application of energy recovery from post-remediation miscanthus, one must be aware of the technical and environmental drawbacks. Incineration of the metal-rich biomass results on the formation of alkaline ash, which leads to the corrosion of the combustion boiler. In addition, metal emission to atmosphere is possible [61].

3.5. Concentrations of Elements in Substrates and Plants

The content of elements in the substrates and miscanthus was varied and depended on the type of substrate and the dose of industrial sewage. In the substrates in which miscanthus was grown, the content of elements was significantly higher than the content in plants. The higher content of elements in the substrates resulted mainly from the chemical composition of the substrate components used, i.e., compost obtained from the processing of organic-mineral waste and the applied doses of industrial sewage (Table 1 and Table A1). Among the analyzed elements, the highest concentrations of elements in the substrates were found in the case of Fe, then Al, Zn, Mn, Pb, Cu, Cr, Ni, Co, and the lowest in the case of Cd. In the experiment, the lowest content of the analyzed elements in the substrates was found in the control object (object 1), in which only sand was used, and only distilled water was used to maintain the humidity of the plants. On the other hand, the highest content of elements was found in the object in which the maximum dose of industrial sewage was used (object 5,

Table 3. Content of elements in substrates and miscanthus (mg∙kg⁻¹ D.M.) and bioconcentration factor (BCF).

Object No. *	Dose of Industrial Sewage, mL∙pot−1	Al			Fe
		Substrate	Plant	BCF	Substrate	Plant	BCF
1	0	2268 ± 37	24 ± 1	0.01	2835 ± 212	399 ± 22	0.14
2	0	2936 ± 106 S	32 ± 5	0.01	3879 ± 211 S	478 ± 31 S	0.12
3	50	3062 ± 102 S	44 ± 4	0.01	4078 ± 216 S	529 ± 31 S	0.13
4	100	3087 ± 46 S	72 ± 10 S	0.02	4734 ± 213 S	551 ± 31 S	0.12
5	200	3134 ± 94 S	86 ± 10 S	0.03	5118 ± 278 S	725 ± 33 S	0.14
LSD α ≤ 0.01		213	18	-	589	78	-
		Mn			Co
1	0	81.0 ± 0.8	22.9 ± 0.0	0.28	0.08 ± 0.01	0.07 ± 0.01	0.88
2	0	88.8 ± 0.9 S	27.8 ± 0.2 S	0.31	2.18 ± 0.11 S	0.18 ± 0.01 S	0.08
3	50	89.2 ± 0.5 S	29.1 ± 0.3 S	0.33	2.21 ± 0.07 S	0.18 ± 0.01 S	0.08
4	100	91.1 ± 1.4 S	32.8 ± 0.1 S	0.36	2.21 ± 0.04 S	0.18 ± 0.02 S	0.08
5	200	103.1 ± 0.8 S	35.5 ± 0.1 S	0.34	2.22 ± 0.03 S	0.19 ± 0.02 S	0.08
LSD α ≤ 0.01		2.4	0.4		0.16	0.03
		Ni			Zn
1	0	3.9 ± 0.6	2.3 ± 0.3	0.60	78 ± 1	27.0 ± 0.3	0.34
2	0	12.0 ± 0.5 S	4.8 ± 0.3 S	0.40	111 ± 5 S	30.7 ± 0.2 S	0.28
3	50	12.2 ± 0.1 S	5.2 ± 0.2 S	0.43	116 ± 3 S	32.6 ± 0.5 S	0.28
4	100	13.0 ± 0.5 S	6.1 ± 0.4 S	0.47	120 ± 6 S	36.7 ± 0.3 S	0.31
5	200	13.4 ± 0.4 S	6.5 ± 0.3 S	0.49	121 ± 3 S	41.3 ± 0.4 S	0.34
LSD α ≤ 0.01		1.1	0.8		10	1
		Cd			Pb
1	0	0.01 ± 0.000	0.01 ± 0.01	0.67	4.8 ± 0.3	0.3 ± 0.0	0.06
2	0	0.75 ± 0.09 S	0.06 ± 0.02 S	0.08	32.9 ± 2.5 S	1.4 ± 0.0 S	0.04
3	50	0.75 ± 0.03 S	0.07 ± 0.01 S	0.09	33.2 ± 0.6 S	1.4 ± 0.0 S	0.04
4	100	0.77 ± 0.01 S	0.07 ± 0.01 S	0.10	33.4 ± 0.1 S	1.4 ± 0.1 S	0.04
5	200	0.79 ± 0.02 S	0.08 ± 0.01 S	0.10	33.7 ± 0.5 S	1.5 ± 0.1 S	0.04
LSD α ≤ 0.01		0.11	0.03		3.0	0.1
		Cr			Cu
1	0	7.0 ± 0.3	1.6 ± 0.2	0.23	9.6 ± 0.4	2.5 ± 0.4	0.26
2	0	21.7 ± 0.1 S	3.6 ± 0.2 S	0.16	30.4 ± 0.9 S	6.6 ± 0.5 S	0.22
3	50	21.9 ± 0.1 S	3.7 ± 0.1 S	0.17	30.7 ± 0.6 S	6.9 ± 0.2 S	0.23
4	100	22.1 ± 0.8 S	4.2 ± 0.9 S	0.19	30.9 ± 0.5 S	7.6 ± 0.2 S	0.25
5	200	22.5 ± 0.9 S	4.3 ± 0.7 S	0.19	31.0 ± 0.3 S	7.8 ± 0.3 S	0.25
LSD α ≤ 0.01		1.4	1.4		1.6	0.9

* See Table 1; S—Significant.

). The research shows that the compost and industrial sewage used were a potential source of elements for cultivated plants. Studies by other authors confirm that industrial sewage from waste processing is a potential source of not only organic pollutants, but also nutrients and trace elements, including microelements [14,15,18,62].

The content of the analyzed elements in the miscanthus biomass was also varied, and the highest contents were found in the object with the highest dose of industrial sewage (object 5), and the lowest in the control object, in which only sand was used (object 1). Among the analyzed elements, the highest concentrations in the miscanthus biomass were found for Fe (725 mg∙kg⁻¹ D.M.), then Al, Zn, Mn, Cu, Ni, Cr, Pb, Co, and the lowest for Cd (0.08 mg∙kg⁻¹ D.M.). The above series of elements in the miscanthus biomass was arranged similarly to the series of elements in the substrates, which indicates the possibility of their availability and uptake by the root system of the tested plant.

In the object with the highest dose of industrial sewage (object 5), the highest increase in the metal content in miscanthus was recorded for Cd and was over 10 times higher than the control, then Pb (4.4), Al (2.5), Cu (2.1), Ni (1.8), Co (1.7), Cr (1.6), Fe (0.8), Mn (0.6), and the lowest for Zn and was over 0.5 times higher than the control. Own studies and those of other authors confirm that industrial sewage is often characterized by high solubility of components and, at the same time, their high availability to plants [60,61]. Moreover, the miscanthus used in the experiment was characterized by a large, bundle-shaped root system capable of intensively absorbing harmful substances from the industrial sewage used [14,34].

In this work, bioaccumulation factor values (BCF) were calculated, which describe the potential of miscanthus to accumulate elements from the substrate. The higher the value of this factor, the greater the potential to accumulate the element in the plant biomass [61]. The highest BCF values were observed in the control object, indicating that the sand used in the experiment did not limit the uptake of elements by plants (Table 3). The sand used in the experiment was devoid of organic matter, so it could not absorb elements in the substrate, and thus made them available to the tested plants. Some of the elements were also likely absorbed from atmospheric dust settling on the plants [63,64].

Bioaccumulation factors (BCF) of elements varied significantly and depended on the object. The highest BCF value was found for Ni (0.40–0.60), then Mn (0.28–0.36), Zn (0.28–0.34), Cu (0.22–0.26), Cr (0.16–0.23), Fe (0.12–0.14), Co (0.08–0.88), Cd (0.08–0.67), Pb (0.04–0.06), and the lowest for Al (0.01–0.03). The studies show that aluminum, despite its high content in the substrate, is not intensively taken up by plants, which probably results from the fact that plants do not show a high demand for this element [65,66]. However, the BCF values for Ni, Mn, Zn, and Cu were much higher, which indicates that these elements play an important role in the plant’s metabolic processes and in the removal of reactive oxygen species (ROS) produced under stress conditions caused by chemical pollutants [59,67,68].

3.6. Metal Uptake by Plants

The uptake of elements by Miscanthus depended on the yield and the content of these elements in the biomass (

Table 4. Metal uptake by Miscanthus (mg∙pot⁻¹).

Object No.*	Dose of Industrial Sewage, mL∙pot−1	Al	Fe	Mn	Co	Ni
1	0	6.1 ± 0.1	99.3 ± 8.1	5.7 ± 0.2	0.017 ± 0.002	0.58 ± 0.08
2	0	7.1 ± 0.9	105.2 ± 7.6	6.1 ± 0.2	0.040 ± 0.003 S	1.06 ± 0.09 S
3	50	15.6 ± 0.8 S	189.7 ± 16.3 S	10.4 ± 0.4 S	0.064 ± 0.003 S	1.87 ± 0.06 S
4	100	30.2 ± 3.5 S	231.4 ± 19.7 S	13.7 ± 0.5 S	0.074 ± 0.008 S	2.55 ± 0.25 S
5	200	51.2 ± 5.2 S	431.0 ± 29.4 S	21.1 ± 0.5 S	0.111 ± 0.007 S	3.88 ± 0.12 S
LSD α ≤ 0.01		7.3	46.8	1.0	0.013	0.36
		Zn	Cd	Pb	Cr	Cu
1	0	6.7 ± 0.3	0.002 ± 0.001	0.07 ± 0.01	0.40 ± 0.06	0.63 ± 0.10
2	0	6.8 ± 0.2	0.014 ± 0.003 S	0.30 ± 0.01 S	0.78 ± 0.08	1.46 ± 0.07 S
3	50	11.7 ± 0.3 S	0.025 ± 0.004 S	0.49 ± 0.01 S	1.34 ± 0.03 S	2.49 ± 0.02 S
4	100	15.4 ± 0.4 S	0.031 ± 0.002 S	0.58 ± 0.04 S	1.75 ± 0.41 S	3.18 ± 0.10 S
5	200	24.5 ± 0.8 S	0.048 ± 0.007 S	0.88 ± 0.07 S	2.53 ± 0.42 S	4.62 ± 0.07 S
LSD α ≤ 0.01		1.2	0.01	0.09	0.69	0.20

* See Table 1; S—Significant.

). The lowest uptake of elements was found in the control object, where miscanthus was grown basically on “clean” sand, without the addition of compost and mineral fertilization (object 1). In the object, where sand was mixed with compost (object 2), an increase in the amount of element uptake was found. The source of these elements was the used compost (SK-9), originating from the processing of municipal waste [27,30]. Increasing doses of industrial sewage (objects 3–5) resulted in an increase in the amount of elements taken up by Miscanthus. The highest amounts of elements taken up were found in the object with the highest dose of industrial sewage (object 5). Among the elements analyzed, the highest uptake was achieved by Fe (431 mg∙pot⁻¹), then Al, Zn, Mn, Cu, Ni, and Cr. The lowest uptakes were obtained by Pb, Co, and Cd (respectively: 0.88, 0.11, 0.05 mg∙pot⁻¹).

In the object with the highest dose of industrial sewage (object 5), the highest increase in uptake was recorded for Cd and was over 27 times higher than the control. This is followed by Pb (11.9), Al (7.4), Cu (6.3), Ni (5.7), Co (5.4), Cr (5.3), Fe (3.3), and Mn (2.7). Conversely, lowest increase in uptake was recorded for Zn and was over 2.6 times higher than the control. Studies by other authors confirm that industrial sewage is a good solvent, especially for trace elements that may have a toxic effect on plants during fertigation and phytoremediation [15,62]. Among the pollutants analyzed in sewage, cadmium was found to be an element that is highly soluble in the soil environment, especially in acidic conditions [13,67]. Regardless of the pH of the soil, plants easily uptake this metal. The results of this study confirm that industrial wastewater, despite its specific chemical composition and salinity, affects the more intensive uptake of these components by plants, which results in higher yields and the use of nutrients and trace elements in the environment [15]. Higher uptake of elements from the substrate supplied with industrial sewage qualifies Miscanthus for the group of remediation plants, as well as for the group of energy plants, recommended for cultivation in landfills and post-industrial areas [14,34]. Studies show that miscanthus is a plant resistant to salinity and chemical pollution and can be used to recycle industrial wastewater, which is important in a circular economy [23,26].

3.7. Selected Physiological Parameters of Plants

The results of chlorophyll index analysis indicate an increase in chlorophyll concentration in two of the samples studied compared to the control object (Figure 1a). Plants grown in soil improver with quartz sand and watered with pure water (object 2) exhibited the highest chlorophyll concentration (16.1% rise) in the biomass, suggesting superior plant health. A smaller increase of 8.4% was also observed in plants watered with the wastewater solution of the highest concentration (object 5). Other variants (objects 3, 4) had no significant impact on the chlorophyll index of the studied specimens compared to the control (object 1). Moreover, the fluorescence values did not fluctuate from the control in a significant manner. Therefore, the results suggest that the photosystem II activity of giant miscanthus is not significantly affected by the conditions of the experiment. Murchie and coworkers report that the optimal chlorophyll fluorescence value for unstressed, healthy plants is approximately 0.83 [68]. Most studies in this respect are related to Miscanthus sp. [65,69] and did not achieve control values above 0.78. Considering that all the values acquired in the study were above 0.80 and sometimes reaching 0.85, very good health of the studied plants can be concluded, indicating no apparent stress reaction (Figure 1b).

Figure 1. The chlorophyll index (a) and chlorophyll fluorescence (b) of greenhouse cultivated Miscanthus x giganteus grown in objects 1–5. The vertical lines indicate the standard deviation of the measurements. In the gas exchange assay, the results of which are presented in Figure 2, all plants supplemented with the studied compost showed a significant change in physiological parameters. The net photosynthesis values increased in all experimental variants. Although all the tests have shown somewhat similar results, the highest increase from control values was observed in plants grown on a mixture of sand and soil improver without further supplementation (Figure 2a; object 2), where the increase was about 15.3%. This effect was somewhat reduced for objects 3, 4, and 5 (the increases in net photosynthesis were by 13.2%, 12.4%, and 13.8%, respectively). Transpiration values also increased in the fertilized samples, except for object 2, where the transpiration rate dropped by 13.4% compared to the control (Figure 2b). Plants treated with industrial wastewater solutions showed no trend of increase, but the highest concentration provided the best results. Both stomatal conductance (St) and intercellular CO₂ concentration (ICC) showed the best results in the plants watered with pure water (13.1% increase and 10.0% decrease, for St and ICC, respectively), while the addition of industrial sewage improved the values according to the concentration (Figure 2c,d). In their work on sorghum ash, Romanowska-Duda and co-workers achieved similar or higher values of net photosynthesis, depending on the experimental variant [44,45]. However, the average values of transpiration and stomatal conductance in this study were similar. From a physiological standpoint, the healthiest plants were the ones grown in a mixture of quartz and soil improver, untreated with wastewater, and those treated with the highest concentration (object 5). Moreover, plants treated with 100 mL of wastewater weekly (object 4) showed somewhat decreased physiological state values compared to other variants [70].

Figure 1. The chlorophyll index (a) and chlorophyll fluorescence (b) of greenhouse cultivated Miscanthus x giganteus grown in objects 1–5. The vertical lines indicate the standard deviation of the measurements. In the gas exchange assay, the results of which are presented in Figure 2, all plants supplemented with the studied compost showed a significant change in physiological parameters. The net photosynthesis values increased in all experimental variants. Although all the tests have shown somewhat similar results, the highest increase from control values was observed in plants grown on a mixture of sand and soil improver without further supplementation (Figure 2a; object 2), where the increase was about 15.3%. This effect was somewhat reduced for objects 3, 4, and 5 (the increases in net photosynthesis were by 13.2%, 12.4%, and 13.8%, respectively). Transpiration values also increased in the fertilized samples, except for object 2, where the transpiration rate dropped by 13.4% compared to the control (Figure 2b). Plants treated with industrial wastewater solutions showed no trend of increase, but the highest concentration provided the best results. Both stomatal conductance (St) and intercellular CO₂ concentration (ICC) showed the best results in the plants watered with pure water (13.1% increase and 10.0% decrease, for St and ICC, respectively), while the addition of industrial sewage improved the values according to the concentration (Figure 2c,d). In their work on sorghum ash, Romanowska-Duda and co-workers achieved similar or higher values of net photosynthesis, depending on the experimental variant [44,45]. However, the average values of transpiration and stomatal conductance in this study were similar. From a physiological standpoint, the healthiest plants were the ones grown in a mixture of quartz and soil improver, untreated with wastewater, and those treated with the highest concentration (object 5). Moreover, plants treated with 100 mL of wastewater weekly (object 4) showed somewhat decreased physiological state values compared to other variants [70].

A comparison of the physiological characteristics of each experimental variant is represented in Figure 3a. As the comparison shows, the effects on the physiology of plants grown in objects 4 and 5 were deemed similar. The same is true for the control object 1 and object 3. This suggests that a fraction of approximately 10% industrial sewage has minimal effect on the plants. Moreover, the heatmap indicates that plants grown in the mixture of sand and soil improver without wastewater supplementation showed results more akin to the heavily supplemented ones than to the control.

In the visualization of the overall physiological states presented in Figure 3b, each axis in the chart represents one of the parameters analyzed in the study. The health of the plants can be evaluated by comparing the areas enclosed by the respective figures for each group. An exception is the intercellular CO₂ concentration axis, where values further from the center indicate a greater retention of CO₂ that is not utilized in photosynthesis, reflecting lower photosynthetic efficiency. The chart clearly demonstrates that the control group (red) performed worse compared to the experimental groups in almost all cases. The best results were observed in plants watered with a 200 mL effluent solution (green) and those watered with pure water (orange), both grown in a mixture of sand and soil improver. However, the addition of 200 mL effluent solution significantly increased transpiration intensity, which was not observed in the group treated with pure water only.

3.8. Principal Component Analysis

Principal component analysis was performed for 15 selected parameters, including biomass yield (BY), heat of combustion (HC), physiological parameters: chlorophyll index (CI), chlorophyll fluorescence (CF), net photosynthesis (Pn), transpiration (Tr), stomatal conductance (St) and intercellular CO₂ concentration (ICC) as well as for metals concentration, including aluminum, iron, zinc, chromium, copper, manganese and nickel (Figure 4a). Based on the scree plot in Figure 4b, the first two components can be considered the most significant, as they explain 83.5% of the total data information. The biplot, based on the first two principal component axes (Figure 4c), suggests multiple dependencies between the investigated parameters, showing a strong positive correlation, for example, between stomatal conductance and heat of combustion for the 1st principal component, while a strong negative correlation with this component was found for intercellular CO₂ concentration.

The PCA distinguished (Figure 4d) five groups of combinations, according to the objects of investigation. Figure 4d presents a clear separation of the experimental groups of objects, which confirms that these groups are statistically different from each other in terms of the studied parameters. Analyzing the position of the groups in Figure 4d and the variable vectors (Figure 4c), it can be determined that object 1 is distinguished by a different content of metals (especially Ni, Cr, Fe, Mn, and Cu). The differences between objects 2–5 are visible mainly in the content of aluminum (Al) and in the biomass yield (BY). The values of aluminum concentration and biomass yield increase with the increase in the dose of the wastewaters. A similar relationship can be observed for the chlorophyll index (CI) value. For objects 2–5, an increase in the CI parameter with the increase in the wastewater dose coincides with an increase in the biomass yield. In addition to biomass yield and metal content, the PCA also revealed relationships among physiological parameters. The first principal component (PC1), which explained 50.2% of the total variance, was mainly associated with parameters related to gas exchange and photosynthetic activity. Stomatal conductance (St) and transpiration rate (Tr) showed a strong positive loading on PC1, indicating that these traits contribute significantly to the differentiation of the samples. At the same time, intercellular CO₂ concentration (ICC) exhibited a strong negative loading, suggesting that higher photosynthetic efficiency (higher Pn and CF) is accompanied by a lower ICC, typical for plants with more efficient CO₂ assimilation.

The second component (PC2; 33.3% of variance) was primarily influenced by chlorophyll-related parameters, including the chlorophyll index (CI) and chlorophyll fluorescence (CF), reflecting differences in the photosynthetic apparatus condition and pigment concentration between treatments. The positive correlation of CI and CF with biomass yield (BY) along PC2 supports the conclusion that physiological activity and pigment status directly contribute to growth efficiency.

The PCA demonstrated that the physiological parameters (Pn, Tr, St, CF, CI, and ICC) form coherent groups reflecting the plants’ adaptive responses to increasing wastewater doses. Enhanced chlorophyll content, photosynthetic rate, and stomatal activity in higher wastewater treatments likely indicate stimulated physiological metabolism, resulting in higher biomass yield.

3.9. Limitations of the Study

This work explores the potential for environmental management of post-industrial wastewater streams generated by composting plants within waste treatment facilities. These streams were valorized in the perspective of mainly (a) wastewater treatment and (b) its use for enhancing plant growth. The study presents the results of preliminary investigations of the effects of wastewater utilization on plant condition, biomass yield and chemical composition. The context of the work focuses on the potential future applications in waste treatment facilities, particularly for the reclamation of wastewater streams for landfill use or for watering purposes during drought episodes. One of the most important issues regarding the environmental management of landfills is the evaluation of the materials used for such purposes. For example, material evaluation may consider microbial activity tests, bioavailability of heavy metals and their leaching capacity. Both national and EU regulations require evaluation of the metal as well as other pollution leaching capacity of the material from the perspective of soil and water pollution prevention. It is recommended that the research presented in this work be supplemented with the evaluation of the biological and chemical toxicity of both the leachate solutions and biomass. This will enable the inclusion of the proposed approach to landfill reclamation within the circular economy. In addition, open-field experiments on a real landfill reclamation site should be performed on a larger scale, which can present the scalability of the idea, including real environmental conditions and their effects on plant growth and wastewater management potential. However, it must be stressed that there is a need for continuous monitoring of physicochemical parameters of both the plant substrates and the wastewaters. When the allowable limits are exceeded, the wastewaters must be directed to wastewater treatment plant in order to prevent secondary pollution of environment [1].

4. Conclusions

Application of diluted, post-composting wastewaters to giant miscanthus resulted in the increased plant growth, with the highest biomass yield for the highest dose of the wastewater solution (200 mL diluted with water to 500 mL per week per pot). The increase in the wastewater dose resulted in an increase in the heat of combustion of miscanthus biomass. The increase in the heat of combustion was about 17% when compared to the control sample, suggesting that the application of sewage is beneficial from an energetic biomass production perspective. Elemental analyses of both soil and biomass samples were performed. The highest content metals found in the biomass of miscanthus were Fe, followed by Al, Zn, Mn, Cu, Ni, Cr, Pb, and Co, with the lowest content being found for Cd. The above series of elements in the biomass of miscanthus was arranged similarly to the series of elements in the substrates, indicating the potential availability and uptake by the root system of plants.

The use of post-composting soil improver and the application of industrial sewage increased the uptake of elements by miscanthus, which indicates that this species can be a good extractant of pollutants from the substrate and can be recommended for recycling pollutants from the composting of biodegradable waste. The best physiological characteristics achieved in the greenhouse pot experiments are those for the plants watered with pure water and industrial sewage of the highest concentration (200 mL per week per pot). The watering with lower doses of sewage, i.e., 50 and 100 mL per week per pot, seems to have no significant effect on selected plant characteristics, although the overall effect of even low concentrations was beneficial.

The use of energy plants for the natural management of industrial wastewaters at the territories of waste treatment facilities can induce increased biomass yields, thus increasing the potential for energy recovery. The use of soil improver from composting organic waste can improve the growth conditions of energy plants, leading to enhanced soil purification and reclamation of degraded lands. This approach consisting of the beneficial use of waste treatment products and byproducts aligns with the concepts of a circular economy and sustainable development and also addresses the current challenges of climate change.