Soil Remediation after Sewage Sludge or Sewage Sludge Char Application with Industrial Hemp and Its Potential for Bioenergy Production

Abstract

Sewage sludge reuse in agriculture is increasing and is highly encouraged; however, it may pose environmental risk. Therefore, an integrated approach combining soil phytoremediation and further plant usage for bioenergy production is needed. In this study, we have examined the potential of industrial hemp (Canabis sativa L.) to remediate sewage sludge (SS) and sewage sludge char (SSCh)-amended soil (25–200 Mg ha⁻¹) and improve soil quality. Additionally, hemp’s biomass and probable bioenergy yield was calculated for biomass and methane production. Heavy metal soil content increased with SS and SSCh dose, though hemp cultivation significantly reduced their soil concentrations. The heavy metals’ removal efficiency could be ranked Zn > Cu > Cr > Ni. There was an enrichment of micro- (Ca, Mg, Mn, S) and macro-nutrients (P) in SS and SSCh-amended soils. P and S removal by hemp was highly efficient, whereas other macronutrients did not show a substantial decrease in the soil. Only marginal removal was detected for Ba, Fe, Na, Ti and Al. The study showed that the optimal fertilization with SS or SSCh could be up to 25 Mg ha⁻¹, when the highest efficiency of contaminant removal from the soil and the highest plant biomass production and bioenergy production were observed.

1. Introduction

Wastewater treatment generates huge amounts of sewage sludge, and its production in the world is projected to increase in the future. Sustainable sewage sludge management and disposal is a growing problem worldwide [1,2]. The annual total amount of sewage sludge produced in the EU is estimated to be more than 10 million tons [3]. Landfilling, incineration and agricultural use are the main routes of municipal and industrial sewage sludge disposal. About 37% of produced sewage sludge is used in agriculture, 40% is landfilled, 11% is incinerated and the remaining amount is used for other purposes (such as compost, forestry, land reclamation, etc.) [4]. EU sewage sludge policy and legislation highly encourages the reuse of sewage sludge in agriculture; therefore, agricultural sewage sludge application is continuously increasing [5].

Sewage sludge use in agriculture and forestry might be beneficial because of its valuable agronomic properties, as it is rich in organic matter, and contains essential plant nutrients (N, P, K) and micronutrients (S, Mg, Ca, etc.). Usually, sewage sludge contains 30–70 % organic matter (OM), up to 4% N, 0.5–2.5% P, and up to 0.7% K [4,6]. On the other hand, sewage sludge contains various toxic contaminants, both inorganic (heavy metals and nonmetals) and organic (PAHs, PCBs, PCDD/F, antimicrobial products, etc.). In addition, the presence of pathogens (viruses, parasites, nematodes, bacteria, etc.) might also restrict it usage.

Assessment of sewage sludge suitability for land application principally is based on nutrients (N, P) and heavy metal contents; therefore, it cannot reflect its overall possible impact on the environment. Some studies have shown that soils amended with sewage sludge or its char are toxic to plants and soil invertebrates [7,8,9,10,11], and thus could pose a serious risk to biota. Therefore, the use of short-rotation forestry or cultivation of fast-growing plants for a reduction of possible contamination risk for biota may be a practical solution. Numerous studies have analyzed heavy metal uptake from sewage sludge-amended soils by trees such as willows (Salix sp.) or poplars (Populus sp.) [12,13], energy crops, and grasses [14,15,16]; however, these studies focused only on the heavy metal extraction by the plants, and did not consider soil quality changes during the plant cultivation and plant growth efficiency.

Energy forest plantations and energy crop cultivation is highly encouraged in the EU to increase the share of energy coming from renewable sources, to reduce GHG emissions, and to mitigate climate change. In recent years, EU countries have considerably increased industrial hemp (Canabis sativa L.) cultivation, producing almost 30% of the total worldwide production [17]. In addition, projections have shown that climate change in the 21st century will enhance hemp’s spread into the northern latitudes, and this may lead to higher hemp production in Northern Europe [18]. The total biomass of hemp per hectare is similar to other common energy crops, such as miscanthus, poplars, or willows [19]. Moreover, industrial hemps could be cultivated in the fields not suitable for food production, in degraded or contaminated areas. Energy plants’ cultivation is a sustainable way of remediating contaminated areas, as it provides renewable energy sources as an alternative to fossil fuels, extracts pollutants from the soil, and could improve soil quality and sequestrate carbon [20,21]. Hemp was shown to be rather effective in sequestering carbon from the atmosphere (up to 2.5 Mg ha⁻¹) [22].

Since sewage sludge usage in agriculture is restricted due to its pollutant contents, temporal cultivation of energy crops, such as industrial hemps, could be a suitable solution to remediate sewage sludge-amended soils. Several studies have shown that during several vegetation periods, plants significantly change the soil pH and heavy metal content [23,24]. However, the efficiency of energy plants in removing such metals as Ti, Ba, Al, Na has not been reported. Thus, an integrated approach combining soil phytoremediation with energy plants and further plant usage for bioenergy production is highly promising. The aim of this study was to investigate the potential of industrial hemp (Canabis sativa L.) to remediate sewage sludge and sewage sludge char-amended soil, improve soil characteristics, and to assess biomass and energy production.

2. Materials and Methods

Experimental Design

Anaerobically digested and thermally dried sewage sludge was collected from a mu-nicipal wastewater treatment plant receiving municipal and industrial wastewater with a population equivalent between 10,000 and 100,000. Sewage sludge char was produced by pyrolyzing the origin sewage sludge, and more detailed information on sewage sludge char production and a detailed chemical composition are presented in the work of Praspaliauskas et al. [25]. The main chemical characteristics of sewage sludge and sewage sludge char are presented in

Table 1. Main chemical characteristics of sewage sludge and sewage sludge char.

	Sewage Sludge	Sewage Sludge Char
Moisture, %	9.84 ± 0.02	4.34 ± 0.02
Ash, %	34.57 ± 0.04	71.41 ± 0.59
C, %	32.30 ± 1.26	25.97 ± 1.05
N, %	4.23 ± 0.42	1.30 ± 0.04
S, %	1.42 ± 0.04	0.08 ± 0.03
P, g kg−1	27.15 ± 1.00	59.47 ± 1.40
Cd, mg kg−1	6.17	<0.01
Cr, mg kg−1	52.07	85.06
Cu, mg kg−1	124.37	263.50
Ni, mg kg−1	17.39	38.17
Zn, mg kg−1	2610	5049
Pb, mg kg−1	73.77	165.70

.

A surface soil (0–20 cm) was collected from the Experimental Research Station in Kaunas district (Lithuania). The selected soil was classified as clay loam, with a pH_KCl of 7.20 ± 0.04 and available nitrogen 10.38 mg kg⁻¹. Sewage sludge and SS char were mixed thoroughly with a mixture of sieved field topsoil, perlite, and sand (5:3:2, by volume). The SS and SSCh doses for soil amendment were as follows: 25, 50, 100 and 200 Mg ha⁻¹. All the treatments and control were executed in three replicates. The experiments were run under strictly controlled laboratory conditions to prevent chemical leaching and minimize the influence of the highly varied meteorological conditions. Industrial hemp (Cannabis sativa L.) was grown for four months in closed-top growth chambers under the following conditions: a day/night air temperature 21/14 °C (photoperiod 14 h/10 h (day/night)), the relative air humidity (RH) of 50/60%, and a light level of ~270 µmol m⁻² s⁻¹ photosynthetically active radiation (PAR). Environmental conditions correspond well to the long-term temperature average for Lithuania. The volumetric soil water content in pots was kept at 30% on average (Delta-T Devices Ltd., Cambridge, UK), i.e., up to 60% field capacity.

After harvesting aboveground and belowground, the hemp parts were separated and dried at 60 °C until constant mass. Soil samples were collected for analysis twice: before plant sowing, and after plant harvesting. Air-dried, crushed, and sieved soil samples were kept in the dark prior to the chemical analysis. Soil pH_KCl was measured potentiometrically (inoLab 720, WTW). C, N, and S concentrations were measured using a Flash 2000 analyzer. The soil samples for metal and non-metal contents were dried for 48 h at 70 °C temperature. Dried soil and plant samples were digested with a mixture of nitric acid, hydrofluoric acid, hydrochloric acid, and boric acid at 800 W, 6 MPa (MultiWave 3000). The concentrations of elements such as Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Ti, and Zn were measured using ICP-OES (Perkin Elmer, Pataskala, OH, USA). The analytical methods’ accuracy was verified based on certified reference materials: CRM—BCR—129 (Institute for Reference Materials and Measurements).

The soil pollution level was calculated for heavy metals of great environmental concern (Cr, Cu, Ni and Zn). The soil contamination factor C_f was calculated as the ratio between metal concentration in the soil (c_i) and metal background concentration (c_{i background}) (according to Lithuanian legislation (LAND 20-2005, 2005)) (Equation (1)):

$$C_f=\frac{c_i}{c_{i background}}$$

(1)

The soil contamination levels were classified as follows: low degree—C_f < 1, moderate degree—1 < C_f < 3, high degree—3 < C_f < 6, and very high degree—C_f ≥ 6 [26].

Based on soil contamination factors, the polymetallic contamination index (IPD), i.e., the sum of C_f was calculated. The soil was classified as low-contaminated if IPD < 5, moderately contaminated if 5 ≤ IPD < 10, considerably contaminated if 10 ≤ IPD < 20, and highly contaminated if ≥20 [27].

Potential ecological risk (PERI) indices were calculated based on the C_f and toxic response (Tr) factors of heavy metals [28]:

$$PERI={\sum }_{i=1}^n(T_{ri}\times C_{fi})$$

(2)

The toxic response factors for Cr, Cu, Ni and Zn were 2, 5, 6 and 1, respectively. based on PERI indices, the soil was classified into four categories (<65—low risk, 65 ≤ PERI < 130—moderate risk, 130 ≤ PERI < 260—considerable risk, and PERI ≥ 260—very high risk).

To assess the elements’ accumulation in industrial hemp, the concentrations of elements were multiplied by the aboveground biomass (stems + leaves) of hemp.

The probable bioenergy yield was calculated for biomass and methane production from industrial hemp. Bioenergy yield from biomass was calculated by multiplying the above ground dry matter yield of hemp by the higher heating value (HHV); methane energy yield was calculated according to the methane yield from hemp determined by Kreuger et al. [29].

A one-way analysis of variance (ANOVA) was used to assess the sewage sludge and sewage sludge char application dose effect on the estimated parameters. Significant differences between treatments were determined using an LSD test. A significance level of 5% was used in the calculations. Regression and correlation analyses were used to assess the relationship between sewage sludge and sewage sludge char doses and the chemical concentrations in the soil. Regression analysis was used for the assessment of the relationship between elements’ accumulation by plants and the plant biomass. Cluster analysis was used to elucidate the difference between the elements’ concentrations in the SS and SSCh-amended soil. All the statistical analysis was carried out using Statistica 10.0 software.

3. Results and Discussion

Soil amendment with sewage sludge and sewage sludge char slightly affected soil pH. Soil pH after the treatment with SSCh varied only little around 7.20–7.40, whereas soil treated with SS became more acidic (pH 5.70–6.60). Some changes in soil pH after the sewage sludge or its char addition was also reported in other studies; both an increase and decrease in pH were observed. Soil fertilization with 30 and 60 t ha⁻¹ of sewage sludge has decreased only pH of the topsoil layer (up to 5 cm) and no impact was found in the deeper layers [30]. An increase in pH (up to 0.2 units) was found in Spain after sewage sludge biochar addition, while sewage sludge addition decreased pH [31]. An increase in soil pH was also reported by Khan et al. [32]. Some changes in soil pH were observed during the hemp growing and after the hemp harvesting the soil pH varied between 5.82 (in case of highest SS dose) to 7.19 (in the treatment with 100 Mg ha⁻¹ SSCh). These results indicate that soil pH was stabilized during the plants growing and soil pH approached control soil pH (7.20).

Cluster analysis of chemicals concentrations in the soil amended with SS and SSCh revealed that all chemicals were allocated in two distinct clusters and only minimal differences could be seen between the clusters in SS and SSCh-amended soils (Figure 1). Cluster I showed macro- and micronutrients together with Ti, Ba and Al, whereas heavy metals were allocated to cluster II. Similarly formed clusters with only minor differences might indicate that both SS and SSCh’s impact on soil chemicals content is of the same nature, and only differences in the concentrations and chemicals’ bioavailability could be seen.

In analyzing soil contamination with heavy metals after its amendment with sewage sludge and sewage sludge char, we focused on Cr, Cu, Ni, and Zn. Despite the fact that Cd and Pb were present in sewage sludge and sewage sludge char, these metals’ concentrations in the soil after fertilization with SS and SSCh were below the detection limit. Soil amended with different doses of SS and SSCh showed a moderate contamination level with Cr, Cu and Ni (1 < C_f < 3), and a moderate to very high level of contamination with Zn (C_f ≥ 6).

Sewage sludge and sewage sludge char application highly affected Cr, Cu, Ni and Zn concentrations in soil (ANOVA, F_SS > 21.80, F_SSCh = 35.20, p < 0.001) and led to increased metal concentrations (p < 0.05) (Figure 2). The highest increase after SS and SSCh application was observed in Cu concentrations, i.e., the sewage sludge and sewage sludge application resulted in up to 49 and 54 times, respectively, higher Cu concentrations than in the control soil. Zinc concentrations after soil amendment with sewage sludge and its char were increased in the range of 1.85–9.67 and 3.34–14.69, respectively. A significant increase in Ni soil concentration was recorded only in the case of SS doses of 50 Mg ha⁻¹ or higher, and the increase constituted only 15–40%. Sewage sludge char application increased soil Ni concentration by 20–50%. Cr soil concentrations after SS or SSCh addition increased up to 1.9 times, with the exception of the treatment with 25 of Mg ha⁻¹ sewage sludge. After SS or SSCh addition to soil heavy metals, permissible levels (according to Lithuanian legislation (LAND 20-2005, 2005) were not exceeded, with the exception of Zn at the highest 200 Mg ha⁻¹ dose.

The index of polymetallic contamination (IPD) indicated that soil amended with 25–100 Mg ha⁻¹ SS and 25 Mg ha⁻¹ SSCh was moderately (5 ≤ IPD < 10) contaminated with the studied heavy metals (

Table 2. Polymetallic contamination (IPD) and potential ecological risk (PERI) indices for soil amended with different sewage sludge and sewage sludge char doses.

	IPD		PERI
Amendment Dose, Mg ha−1	Sewage Sludge	Sewage Sludge Char	Sewage Sludge	Sewage Sludge Char
0	3.12	3.12	11.38	11.38
25	5.86	6.37	17.38	20.46
50	7.39	10.10	21.96	29.42
100	9.25	12.32	25.32	33.01
200	13.23	17.29	34.72	40.51

). Application of 200 Mg ha⁻¹ SS and 50–200 Mg ha⁻¹ SSCh induced considerable contamination (10 ≤ IPD < 20), with the highest value of IPD for 200 Mg ha⁻¹ SSCh treatment. Our results indicate that application of SSCh resulted in higher polymetallic soil contamination compared with the soil amended with the same SS dose. However, the indices of potential ecological risk PERI indicated that amendment with SS and SSCh could pose only low ecological risk (PERI < 65). This could be explained by the fact that the highest C_f was characteristic for those heavy metals of relatively low toxicity (Zn), and soil SS and SSCh application did not induce any changes in the concentrations of highly toxic Cd and Pb. Comparison of 11 sewage sludges from wastewater treatments plants revealed that highest PERI values were characteristic for those SS with highest Cd, Pb, and As concentrations [26]. Nevertheless, PERI indices increased with SS and SSCh dose (r_SS = 0.98, r_SSCh = 0.92, p < 0.05), indicating that long-term soil amendment with high SS or SSCh doses might pose ecological risk.

Cultivation of industrial hemp for 4 months significantly reduced heavy metals soil concentrations. Cr soil concentrations in sewage sludge-amended soil after hemp harvesting were by 6.7–26.6% lower than before plants sowing. We excluded the treatment of 25 Mg ha⁻¹ from further analysis, as Cr soil concentrations were extremely high, and we can not explain this phenomenon. The efficiency of Cr removal from SSCh-amended soil was rather similar (13.3–21.5%) though some slight increase in Cr removal in the treatment with 100–200 Mg ha⁻¹ was observed compared to SS-amended soil.

Ni soil concentrations after hemp harvesting was slightly reduced (2.1–12.8%) or even increased comparing with the concentrations immediately after SSCh and SS soil amendment. Soil concentrations of essential micronutrients Cu and Zn were highly affected and decreased during the plants growing period. Cu concentrations in the soil amended with SS after the hemp harvest were up to 52.3% lower than those before the plants’ growth. Whereas from the soil amended with SSCh Cu was removed at a lesser efficiency (9.4–22.3%). As during char production from sewage sludge heavy metals mobility usually is reduced [31], so Cu bioavailability in SSCh-amended soil was reduced inhibiting plant uptake. No difference between Zn removal efficiency from SS and SSCh-amended soil was found, though the efficiency was also good and Zn concentrations after the hemps harvest were by 18.7–63.0% lower than before the plant sowing. In case of Zn, an increase in Zn removal efficiency with SS or SSCh soil amendment dose was observed (SS: r_s = 0.59, SSCh: r_s = 0.69, p < 0.05). Highest Zn removal efficiency could be explained by the fact that Zn is relatively labile in the soil and easily extracted by the plants from the soil, i.e., is in bioavailable form [33,34,35,36]. Additionally since Zn is usually present in sewage sludge in larger concentrations compared to other studied metals (in our case as well), therefore Zn is identified as the element of high concern in relation to potential impacts on soil microbial activity, soil fertility and plant performance [37].

Potential Ni, Cu and Zn accumulation in the soil in willow plantations amended with sewage sludge was recorded in Sweden as plants heavy metals accumulation did not compensate metals input with sludge [24]. A significant increase in the heavy metal soil content along with increasing sewage sludge dose (10–60 Mg ha⁻¹) was recorded during the field study in south-eastern Poland and three year cultivation of sweet sorghum (Sorghum bicolor) had a positive effect on the soil quality with decreasing heavy metal content [38].

Calculated soil contamination factors C_f after the harvest of hemps, has shown that cultivation of industrial hemps did not change the soil contamination level for Cr and Ni and soil could be classified as moderately contaminated (1 < C_f < 3). Though in cases of essential metals Cu and Zn, some significant changes have occurred. Soil Cu contamination level amended with 25–100 Mg ha⁻¹ of sewage sludge or 25 Mg ha⁻¹ of sewage sludge char has changed from moderate to low degree. The contamination level with Cu of other treatments has not changed and could be classified as moderately contaminated. In case of contamination with Zn, the level of contamination after hemp cultivation in soils treated with 25–50 Mg ha⁻¹ of sewage sludge or its char moved to the moderate level. No changes in Zn contamination level in the treatment with 100 Mg ha⁻¹ of sewage sludge were observed. Neither soil treated with 200 Mg ha⁻¹ of sewage sludge or 100–200 Mg ha⁻¹ of sewage sludge char was classified as very highly contaminated any more, i.e., contamination level has changed to high contamination (3 < C_f < 6). Our data indicate that if SS or SSCh application rates are 50 Mg ha⁻¹ or higher could pose soil contamination risk and even after plant cultivation soil pollution remain rather high.

Polymetallic contamination indices have decreased after hemp cultivation in the range of 16.80–36.10% and in most treatments the soil contamination has changed to a better state (

Table 3. Polymetallic contamination (IPD) and potential ecological risk (PERI) indices for soil amended with different sewage sludge and sewage sludge char doses after remediation with industrial hemp (Cannabis sativa).

	IPD		PERI
Amendment Dose, Mg ha−1	Sewage Sludge	Sewage Sludge Char	Sewage Sludge	Sewage Sludge Char
0	2.99	2.99	11.26	11.26
25	4.18	5.30	15.48	18.45
50	4.92	6.52	18.71	23.68
100	6.28	8.62	20.23	26.82
200	9.61	11.04	28.65	31.89

). The IPD values of the treatments 25–50 Mg ha⁻¹ of sewage sludge (SS) have been changed from moderate to low contamination. Only in the treatment of 200 Mg ha⁻¹ SSCh contamination class was of considerable contamination (IPD > 10), the rest—of moderate contamination class. The potential ecological risk indices (PERI) after industrial hemp cultivation were also up to 21.30% lower than before plant cultivation. PERI indices showed increasing trend along with SS and SSCh dose and the values in the treatments with 200 Mg ha⁻¹ were 2.54 and 2.83-fold, respectively, higher than in control soil. A decline in PERI values were also recorded after solid waste composting and vermicomposting for 60 days [39].

Soil amendment with SS and SSCh resulted in 1.5–3.95- and 2.37–4.71-fold higher Co concentrations, respectively (Figure 3). Industrial hemp cultivation significantly reduced the Co level in the soil, and higher phytoremediation efficiency (up to 49%) was observed in the treatment with SS. SS or SSCh might be a reasonable source of Fe, an essential micronutrient for plant growth. Fe soil concentration after fertilization with SS or SSCh up to 20 Mg ha⁻¹ did not exceed the average concentration in cultivated soils [40], indicating that SS/SSCh application could ensure only partial plant demand for Fe for an optimal growth of plants. As SSCh application led to a pH increase, it could reduce Fe solubility and bioavailability to plants. After industrial hemp cultivation for four months, Fe soil concentration decreased up to 30% with a somewhat higher decrease in SS-amended soil.

Only marginal changes in Na, Ti, and Al concentrations in the soil were detected after soil amendments with SS and SSCh, though slightly higher levels of these elements were determined in SSCh-amended soil. Removal of these chemicals during the plant growth was also very negligible, and only Na showed a more prominent decrease. An increase of 13–41.5% after SS or SSCh application was detected for Ba, a nonessential and toxic element to plants and animals [41]. Plant removal of Ba from the soil was in the range of 12–20%, and our results agree with Norini et al. [42] and Barbosa et al. [43], who reported low Ba plant accumulation from the SS of biochar-amended soils.

Changes in soil concentrations of essential nutrients such as Ca, Mg, K, and Mn after addition of sewage sludge or sewage sludge char were less significant compared to those of heavy metals. The highest increase along with SS and SSCh concentrations was observed in Mn soil concentrations (R²_SS = 0.94 and R²_SSCh = 0.89, p < 0.01) (Figure 4). Mn soil concentrations increased up to 2.5 and 3.2 times after soil fertilization with SS and SSCh, respectively. SS and SSCh addition increased the Ca content in the range of 17–52% and 26–126%. The increase in Mg content in the soil after SS and SSCh application constituted only 30% and 47% in the case of the highest 200 Mg ha⁻¹ dose. A significant increase in soil Ca and Mg content was also recorded in other studies [44,45]. Soil fertilization had no effect on K, a major plant nutrient, with soil content indicating that soil fertilization with SS and SSCh may be insufficient to meet the requirement for K for plant growth, and additional K supplementation could be required. Similar results, indicating only slight or even no changes in K soil content were recorded in other studies [30,32].

Concerning micronutrient soil concentration changes during plant cultivation, it was revealed the most prominent changes were in Mn, followed by K, Ca, and Mg in the case of SS fertilization, and Ca, Mg, and K in the case of SSCh-fertilized soil. Mn removal efficiency from the soil during plant cultivation was around 30%. Ca soil concentrations decreased during plant cultivation, and the changes tended to increase with increasing SS or SSCh dose. Since sewage sludge is not usually rich in Mg and K, it is essential to ensure sufficient levels of these micronutrients for plant needs. In SS-fertilized soil, Mg concentrations slightly increased during the plant growing seasons, which may be explained by the fact that base cations are easily washed out from the plants [46]. Soil K concentrations decreased during plant growth, though the changes were not very prominent. In soil amended with SSCh, the level of K and Mg after the plant harvest remained higher at all doses, indicating that SSCh immobilizes these nutrients more efficiency in the soil and may be available for longer for plant needs. Slight decreases in K, Ca, and Mg soil content after two cropping seasons were also observed by Faria et al. [47] during a field experiment in Brazil.

A high phosphorus content is characteristic of sewage sludge, and may lead to overdosage or might be effective alternative for fertilizers [48,49]. In our study, SS and SSCh application also significantly increased P content in the soil (up to 26.9-fold) (Figure 5). Soil P concentrations increased along with the SS and SSCh doses added to the soil (R² = 0.95 and R² = 0.92, respectively, p < 0.0001). During hemp growth, P soil content was significantly reduced, and the highest efficiency was observed with a fertilization rate of 50 Mg ha⁻¹, when the P concentration in the SS and SSCh-amended soil was reduced by 66.2% and 37.9%, respectively. These data suggest that when soil is fertilized with a moderate rate of SS or its char, energetic plants’ cultivation could mitigate the risk of P runoff and leaching, and SS or SSCh might partially substitute mineral P fertilizers [50].

However, at higher SS or SSCh doses, the soil P level even after plant cultivation remained up to 10.8 and 14.6-fold higher than in control soil. Our results are in line with the data of a study conducted by Krogstad et al. [51], wherein it was shown that after the application of 20 t ha⁻¹ sewage sludge soil, up to 50% of phosphorous might be left in the soil after the growing season of rye grass (Lolium perenne). A significant surplus in P soil content was also observed in the field in Spain, wherein sewage sludge was applied for 10 years, and fields were planted with maize and potato [52]. After 12 years, SS application at a rate of 90 t ha⁻¹ y⁻¹ resulted in a 4–5 times higher P soil content than in the control soil, and the P supply exceeded plant uptake [53], similar to our study. Moreover, it was shown that soil contamination with heavy metals might reduce P uptake from the soil [54], possiblye reducing P remediation from SS-amended soils. SS and SSCh application significantly increased soil S content (p < 0.05) after SS and SSCh application (Figure 5). In the treatments with SS soil, S concentrations were 42–63% higher than in the soil treated with SSCh. S soil content was also very effectively reduced (17.6–69.3%) during the hemp cultivation period, and the removal efficiency did not differ among SS and SSCh-treated soils.

Hemp Biomass Yield and Energetic Evaluation

A positive effect of soil fertilization with SS and SSCh on hemp biomass was observed only in the treatment with 25 Mg ha⁻¹ of SS (p < 0.05) (Figure 6). In other treatments with SS and SSCh, the total hemp biomass reached only up to 56.54% of the control biomass. Some 87.8–94.2% of accumulated hemp biomass in the SS and SSCh treatments was allocated to the aboveground compartments’ (stems and leaves) production, compared to 85.38% in the control biomass.

Higher biomass allocation to the aerial plant parts of the SS and SSCh-amended soil may be explained by the toxic impact of SS and SSCh on root growth [55]. Such a pattern of biomass partitioning indicates that the major parts of hemp could be used for bioenergy production or other purposes (e.g., fiber production). Theoretical calculations of bioenergy production from hemp grown in SS and SSCh-amended soil indicated that the highest efficiency of energy yield was achieved at the rate of fertilization of 25 Mg ha⁻¹ (Figure 7). Comparative studies of the energy yield of several energy plants (such as maize, sugar beet, lucerne, wheat) have shown that hemp has a similar or even higher energy yield for solid fuel and biogas [56].

The accumulation of studied elements in industrial hemp aboveground tissues (stem and leaves) differed among sewage sludge and sewage sludge char and their amendment doses (

Table 4. Metal and macro nutrient accumulation in the aboveground parts of industrial hemp (Cannabis sativa) (µg plant⁻¹).

Amendment Dose, Mg ha−1
0 (Control)		25		50		100		200
		SS	SSCh	SS	SSCh	SS	SSCh	SSCh
Cu	3.40 ± 0.90	46.35 ± 3.15	4.01 ± 2.43	6.54 ± 3.50	5.13 ± 2.66	3.85 ± 2.46	2.23 ± 1.38	0.65 ± 0.21
Ni	2.03 ± 0.68	0.28 ± 0.04	<LD	0.13 ± 0.10	<LD	0.15 ± 0.10	<LD	<LD
Zn	49.76 ± 10.65	486.52 ± 26.94	26.17 ± 15.67	78.65 ± 42.01	36.42 ± 19.71	29.31 ± 20.26	20.15 ± 13.16	5.61 ± 1.71
Co	2.41 ± 0.80	5.72 ± 0.54	0.37 ± 0.14	0.83 ± 0.51	1.32 ± 0.86	0.24 ± 0.18	0.60 ± 0.38	0.25 ± 0.07
Ba	12.26 ± 2.79	20.91 ± 1.15	4.80 ± 2.97	1.27 ± 0.67	14.49 ± 9.03	0.98 ± 0.69	11.37 ± 7.62	3.50 ± 1.05
Ti	0.22 ± 0.06	5.94 ± 0.29	0.00 ± 0.00	1.46 ± 0.75	0.62 ± 0.29	0.02 ± 0.00	0.46 ± 0.29	0.50 ± 0.20
Na	73.35 ± 21.68	609.96 ± 57.19	99.43 ± 86.23	44.67 ± 23.53	176 ± 125.38	10.09 ± 6.38	26.75 ± 21.16	5.92 ± 1.42
Fe	63.20 ± 13.60	135.69 ± 8.82	60.30 ± 43.28	57.75 ± 30.91	62.08 ± 36.43	26.53 ± 18.56	37.02 ± 23.50	9.58 ± 3.16
Al	1.70 ± 0.36	<LD	6.85 ± 3.86	<LD	26.97 ± 13.59	2.66 ± 1.98	31.22 ± 19.62	53.36 ± 22.99
Ca	7808 ± 2207	61,404 ± 2569	18,658 ± 12,606	8395 ± 4481	22,230 ± 12,220	3063 ± 2043	13,287 ± 8347	3587 ± 1121
Mg	5151 ± 1111	31,104 ± 1538	2386 ± 1349	6394 ± 3372	6100 ± 3357	3015 ± 2152	3115 ± 1981	1284 ± 386.0
K	47,277 ± 10,588	117,144 ± 6090	21,314 ± 12,764	30,467 ± 16,263	44,525 ± 24,970	15,238 ± 10,485	20,551 ± 13,366	6026 ± 1881
Mn	53.22 ± 12.73	734 ± 44.47	28.39 ± 18.09	233.88 ± 124.82	89.88 ± 58.17	537.94 ± 370.40	41.67 ± 29.74	13.08 ± 4.29
P	1155.76 ± 324.76	18,661 ± 1037	2056 ± 1232	4923 ± 2605	2892 ± 1542	1764 ± 1242	1312 ± 816	398 ± 66
S	1021 ± 234	19,441 ± 767	4521 ± 2648	2335 ± 1351	10,128 ± 5468	4704 ± 3365	8074 ± 5145	3714 ± 1170

<LD—below the detection limit.

). The highest amounts of heavy and other metals and micro and macro nutrients were accumulated in hemps grown in the soil amended with 25 Mg ha⁻¹ of SS and 25–50 Mg ha⁻¹ of SSCh. In these treatments, industrial hemps gained the highest biomass (Figure 6), implying a trade-off between plant performance in the amended soil and possible soil remediation. The accumulated amounts of most of studied elements increased with hemp aboveground biomass (R²_SS > 0.77, p < 0.05, with the exception of Al, Ni and Mn; R²_SSCh > 0.35, p < 0.05, with the exception of Al, Ti and S). The detrimental effect of toxic elements in cases of high SS and SSCh amendment doses results in low phytoextraction by plants, and will lead to a low remediation efficiency. These results are in line with other studies showing low phytoremediation efficiency in highly contaminated soil [57].

As the 25 Mg ha⁻¹ fertilization rate was shown to be the most appropriate for achieving the highest hemp biomass and bioenergy production and bioaccumulation in different hemp parts (as discussed in [55]), we compared chemicals’ removal from SS and SSCh-amended soils at this fertilization dose (Figure 8).

At this fertilization rate, the removal efficiency of most of chemicals was below 25%. A higher removal efficiency was recorded for phosphorous from both SS and SSCh-amended soils, indicating substantial P uptake for plant nutrition needs [58]. High efficiency removal was detected for heavy metals such as Zn, Cr, and Co. Taking into account the relatively short hemp growth experiment (four months), we may presume that during the whole vegetation period, the removal efficiency of toxic chemicals may differ.

However, chemicals’ uptake by plants from soil is highly regulated by the soil temperature, as low temperatures suppress chemical uptake by reducing plant root activity and the chemicals’ diffusion rates [59,60]. Therefore, during the spring and autumn months, removal efficiency in the field might be lowered. Moreover, during summer months and dry conditions, chemicals’ removal might be also inhibited [61]. These issues are really important in the face of climate change, as climate extremes (droughts, heat waves) are projected to increase in frequency and intensity [62].

4. Conclusions

Industrial hemp was shown to be a highly promising plant, not only for soil clean up, but also as an option for soil quality improvement and bioenergy production. The study demonstrated that even sewage sludge and sewage sludge soil treatment did not exceed the threshold values provided by European legislation for heavy metals, though the other nutrients, such as P and S, could be overdosed. The plants’ cultivation efficiently reduced the P and S contents; however, four months’ growth was insufficient to reach background values, and the risk of P surplus and eutrophication remained. Our data have shown that hemps efficiently remove the essential metals Zn and Cu from the soil, while Cr and Ni removal was moderate or even negligible. The results of our study show that the cultivation of industrial hemp is highly promising in soils with low or moderate contamination, as it remediates the soil and improves its characteristics; moreover, the produced biomass may be efficiently converted to bioenergy.