Soil application of treated sewage sludge is considered a beneficial management option for agriculture and environment. Sludge treated with clay minerals or biochar was evaluated as soil amendment, in comparison with limed or untreated sludge. Bentonite, vermiculite, biochar or lime was added to dewatered sewage sludge at 0 and 15% rates, in three replications and air-dried. Then, the treated and untreated sludge were added to two soils, one acid and one alkaline, at 0 (control), 1 and 3% rates, in three replications. The results showed that the pH of acid soil significantly increased compared to control upon addition of treated and untreated sludge at both rates, but remained acidic (<6.0) in all cases except for limed sludge. Upon addition of 3% treated or untreated sludge to both soils, organic carbon (OC) significantly increased compared to control. The same stands for soil available ammonium nitrogen (NH4-N), phosphorus (P), potassium (K), copper (Cu), zinc (Zn) and boron (B) for both soils and rates. Soil total heavy metals, which regulate sludge's agronomic use, were below the legislative limits. However, the 3% addition rate of all kinds of sludge, especially of untreated, to both soils increased the electrical conductivity of saturation extract (ECse >2 dS m(-1)) at unacceptable levels for sensitive crops and available P, B and Zn 2-3, 2-4 and 6-12 times above the initial, respectively. Consequently, sewage sludge treated with bentonite, vermiculite or biochar could be used as a soil amendment but at rates lower than 3% (approximate to 120 Mg ha(-1)).

The objectives of this study were to evaluate sewage sludge’s stabilization with untested until now materials, such as selective clay minerals or biochar in comparison with liming, for enhancing sludge’s fertilization capacity. Dewatered sewage sludge was mixed with bentonite, vermiculite, zeolite, biochar or lime at rates of 0, 15 and 30%, air-dried and analyzed for pathogens and chemical properties. Almost all fecal indicators of treated sludge with 15% bentonite, vermiculite or biochar were reduced by at least one-logarithmic unit (log 10 ) (indicative value of sludge’s stabilization), whereas those of limed sludge were undetectable. Electrical conductivity of all treatments significantly increased, and the highest values were obtained for untreated (6.1 dS m −1 ) and limed sludge (above 7.0 dS m −1 for both addition rates). The untreated sludge had the significantly highest water-soluble ammonium-nitrogen (2817 mg kg −1 ) and phosphorus (263 mg kg −1 ) concentrations followed by sludge treated with bentonite, vermiculite or biochar, whereas limed sludge had the lowest content. Boron concentration of the untreated sludge was similar to the treated sludge. Total concentrations of heavy metals were far below the legislative permissible levels for sludge’s agronomic use. Nutrients’ total content of treated sludge ranged at levels of similar magnitude to the untreated sludge, except for certain cases where they were increased because of the materials’ composition. Consequently, sewage sludge treated with 15% bentonite, vermiculite or biochar seems to be stabilized, retain bioavailable nitrogen and serve as a fertilizer of macro- and micronutrients. However, potential risks of agronomic use, i.e., soil salinization and boron phytotoxicity, should be considered.

Sewage sludge treated with 15% bentonite, vermiculite or biochar was evaluated as a soil amendment in comparison to limed and untreated sludge. Seven treatments were established to two soils, an acid and an alkaline, in three replications, i.e. 2% addition of sludge treated with bentonite, vermiculite, biochar and lime and application of 2% untreated sludge, inorganic fertilization and no sludge or inorganic fertilizers (control). Then, the soil treatments were used in a pot experiment with perennial ryegrass (Lolium perenne L.) as a test plant. Sludge treated with the clay minerals or biochar improved pH of the acid soil and significantly increased organic matter and available nutrients of both soils compared to control. Although no salinity or sodicity hazard was evidenced, the initial salinity of acid and alkaline soil increased by four-eight and two-three times, respectively, upon addition of all sludge treatments, especially that of untreated sludge. Moreover, soil available zinc (Zn) increased by four-eight times. Soil application of sludge treated with the clay minerals or biochar increased the total aboveground biomass yield of ryegrass in the acid and alkaline soil by 133%-171% and 72%-88%, respectively, compared to control and enhanced nutrient uptake by plants. Furthermore the microbial metabolic quotient indicated lack of low pH and heavy metal stress with addition of sludge to the acid soil. After three harvests of ryegrass, the residual effect of sludge on pH of acid soil and salinity, available phosphorus (P), Zn and boron (B) of both soils still persisted. Thus sewage sludge treated with 15% bentonite, vermiculite or biochar could be applied to soils at a rate of 2% (similar to 80 Mg ha(-1)) to serve as soil amendment and fertilizer for grasses and pasture species; however, caution is needed regarding possible P build-up, Zn phytotoxicity and salinization risks.

Under the framework of Cyclic Economy and EU Green Deal, sewage sludge represents an ideal soil amendment with a potential to increase soil OM, provide nutrients and reduce chemical fertilization, which otherwise would be disposed in landfills. Nonetheless, its agronomic use comes with an uncertainty of its potential to release ample plant-available N and trace-metals in a wide range of soils.This study investigated the N dynamics of municipal sewage sludge applied in two contrasting soils; an acidic (pH 5) and an alkaline (pH 8). Stabilized sewage sludge, limed (LM) or air-dried (AD), was applied (2% dw) in soil mesocosms (1500 g) that were incubated for 90 days (25oC; 12% soil moisture). A fertilized treatment (F: 100 mg/Kg NH4NO3), and a non-amended treatment (control) were also included. During the incubation soil NO3-, NH4+, N2O and CO2 were regularly monitored. Anaerobic mineralizable N (AMN) was determined at 15 days. At the end of the incubation, trace-metals, organic C and total Kjeldahl N were determined using standard methods.The acidic soil receiving LM and AD sewage sludge had 4x and 5x greater (p=0.004) AMN rates than the control. Whereas the alkaline soil receiving air-dried sewage sludge had 2x greater (p=0.01) AMN rates than the control. Soil organic C was on average(±SE) 10.4±0.6 g/Kg and no significant differences were found in the acidic soil. In the alkaline soil, organic C was on average 16.1±0.4 g/Kg, and LM and AD treatments had significantly more org. C than the control (p=0.01). Total N was on average 1.5±0.3 g/Kg and no significant differences were found in both soils. During the incubation, soil NH4+ decreased in LM and AD treatments, and slightly increased in F and C treatments in the acidic soil. Soil NH4+ in the alkaline soil slightly increased for all treatments. A sharp increase in soil NO3- in the acidic soil was observed in all treatments except the control at approx. 60 d. In the alkaline soil, soil NO3- remained at similar levels as initially. It appears that in acidic soils receiving sewage sludge, the relative low soil pH inhibits NH4+ oxidation, whereas in alkaline soils the relative high pH inhibits NO3- reduction. Cumulative CO2 emissions were ~1.3x greater in LM and AD than F and control treatments, and cumulative N2O emissions were ~1.5x greater in AD only than F and control treatments for both soils. Interestingly, N2O emissions for LM were at similar levels to the control treatment for both soils. At the end of the incubation, trace-metal concentration increased in all treatments, yet, it remained below legislative critical levels. The above effects varied slightly between LM and AD sewage sludge, therefore further experimentation is required to understand the effects of sewage sludge type and quality on soil fertility and crop productivity. Our preliminary results show that stabilized sewage sludge has the potential to be a safe soil conditioner and fertilizer under the framework of Cyclic Economy and EU Green Deal.Funding: The research work was supported in part by the Hellenic Foundation for Research and Innovation

From the perspective of finding new, more environmental-friendly methods for the stabilization of sewage sludge to be used in agriculture, sludge treated with 15% bentonite, vermiculite or biochar was investigated as a soil amendment for white clover (Trifolium repens L.) growth, by means of a pot experiment. The sludge treatments, which were applied to two soils (an acid and an alkaline soil), in three replications, were the addition of 2% (≈80 Mg ha−1) treated sludge with the clay minerals or biochar, as well as limed or untreated (air-dried) sludge (for comparison reasons). Additional treatments with inorganic fertilization or neither organic nor inorganic fertilization (control) were also included. The application of 2% sludge to both soils significantly increased salinity compared to the control, which remained below harmful levels for sensitive crops, except for the case of untreated sludge. Furthermore, it significantly increased the soil-available macronutrients N, P and K, and micronutrients Cu, Zn (several times, especially in the case of untreated sludge) and B (up to three times) compared with the control. Moreover, the pH of the acid soil was improved, except for the case of limed sludge, where an undesirable pH increase close to 8.5 was observed. Addition of the treated sludge with the clay minerals or biochar and untreated sludge to the acid and alkaline soil significantly increased the aboveground biomass yield of white clover by 117–233% and 114–153%, respectively, compared to the control, whereas limed sludge had no effect. Plant nutrient uptake increased as well. In general, the effect of sludge on soil microbiological properties and arbuscular mycorrhizal fungal root colonization was ambiguous. It was concluded that 2% soil addition of sewage sludge treated with 15% bentonite, vermiculite or biochar could improve soil fertility and enhance plant growth; however, caution is needed with respect to potential risks of soil salinization or Zn and B phytotoxicities.

5 Purpose 6 Steelmaking slag, a by-product of the steel-refining process, could be used for removing B excess from 7 irrigation natural and waste waters, due to its strongly alkaline reaction. The objectives of this study 8 were to: a) establish the optimum external solution / adsorbent ratio and equilibration time of B 9 adsorption by the slag and b) assess the slag's capacity to adsorb B. 10 Methods 11 Two preliminary B adsorption experiments were conducted to determine the optimum external solution 12 / adsorbent ratio and equilibration time. The optimum conditions were employed for the main B 13 adsorption experiment and the non-linear Langmuir and Freundlich isotherms were fitted to the B 14 adsorption data. 15 Results 16 Boron adsorption increased with the increase of the external solution / adsorbent ratio up to the ratio of 17 200:1. Although, almost 40 % of B was adsorbed within the first hour of equilibration period, the 18 adsorption gradually increased until the 72 h. The Langmuir B adsorption maximum was almost 150 19 mg g, considerably higher than other adsorbents, like fly ash, calcite and magnesia. At B initial 20 concentrations lower than 4 mg L, slag removed 60 % of B and reduced it below the permissible 21 levels for irrigation waters (< 3 mg L) for most crops. The pH of the equilibrium solution was 10.3 ± 22 0.8 and dropped to acceptable levels for irrigation waters (< 8.5), after contact with atmosphere for one 23

Increasing sewage sludge production requires efficient treatment and disposal routes. Agronomic use of sewage sludge promotes nutrient recycling and thus is considered a more desirable option than landfill disposal. With the appropriate treatment, removal of available heavy metals and reduction of microbial load, stabilized sewage sludge bio-fertilizers could be ideal for agronomic use. Nonetheless, we still lack treatment approaches that consider an integrated methodology towards microbial load reduction and heavy metals load control, as well as efficient nutrient reuse. The addition of clay minerals and biochars during sewage sludge stabilization, as strong ion adsorbents and bactericides respectively (Agyarko-Mintah et al, 2017; Alshameri et al, 2018; Mohanty et al, 2014; Williams, 2019), could be a promising alternative treatment. The objective of this study was to evaluate sewage sludge stabilization with clay minerals (bentonite, vermiculite, zeolite) and biochar in comparison to lime (Ca(OH)2) stabilization in the perspective of agronomic use of the treated sludge.

Sewage sludge production from wastewater treatment plants (WWTP) progressively exceeds 60 Million m3 p.a. in the EU. Although it is rich in organic matter (OM) and essential nutrients for crop production, sewage sludge is mainly disposed in landfills. Under the framework of Cyclic Economy and EU Green Deal, sewage sludge represents an ideal soil amendment and fertilizer with a potential to increase soil OM, provide nutrients and reduce chemical fertilization. Nonetheless, its agronomic use comes with limitations due to the presence of heavy metals and pathogenic microorganisms. Several stabilization technologies, including composting, thermal treatment and liming, aim to produce safe sewage sludge products suitable for agronomic use. This incubation study investigated the effects of municipal sewage sludge (stabilized by alternative and common methods) on nutrient and microbial dynamics in two soils; an acidic (pH 5) and an alkaline (pH 8). Stabilized sewage sludge (Thessaloniki WWTP, Greece) with clay minerals (bentonite and vermiculite), biochar (pine residues), Ca(OH)2 and air-drying, was applied at 1% and 3% dw, in soil mesocosms (300 g). Non-amended soils were also included as control. Soils were incubated (15 days; 25oC) and equilibrated with periodic wetting and air-drying. Then, chemical soil properties, heavy metal concentrations and microbial abundance were determined using standard methods. Treated sewage sludge addition in the acidic soil, noticeably increased soil pH (pH 5.2 – 8.5), compared to the control treatment (pH 5.0). In the alkaline soil, pH remained at similar levels (pH 8.1 – 8.6). Interestingly, EC increased from 0.42 up to 4.10 and 0.80 up to 3.08 dS m-1 for the acidic and alkaline soils, respectively. The C/N ratio was approx. 10 for all treatments, except biochar (C/N=16). Higher NO3- concentrations were observed for (CaOH)2, biochar and vermiculite stabilized sewage sludge treatments, and higher NH4+ concentrations were observed for air-dried, bentonite and vermiculite stabilized sewage sludge treatments, in both soils, when compared to the control. Heavy metal concentration increased in all treatments, yet, it remained below legislative critical levels. Sewage sludge amendment increased total heterotroph abundance in all treatments (5.4 – 7.5 log10 CFU g-1) compared to the control. Antibiotic resistant prokaryote abundance ranged between 3.9 – 7.0 log10 CFU g-1 and no persistent pattern was found. Pathogens remained below legislative critical levels in all treatments. Our preliminary results show that stabilized sewage sludge has the potential to be a safe soil conditioner and fertilizer under the framework of Cyclic Economy and EU Green Deal. A desirable increase in soil fertility and organic C was observed for both soils, and an advantageous pH increase for acidic soil. Though, care should be taken not to exceed EC>2 dS m-1 when amending agricultural soils with sewage sludge products. Also, further experimentation is required to understand the effects of soil amendments on plant nutrition and productivity. Funding Acknowledgement: The research work was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “First Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant” (Project Number: HFRI-FM17-1907).

Steelmaking slag, a by-product of the steel-refining process, could be used for removing boron excess from irrigation natural and waste waters, due to its strongly alkaline reaction. The objectives of this study were to: (a) establish the optimum conditions (external solution/adsorbent ratio, equilibration time) of boron adsorption by the slag, (b) assess the slag’s capacity to adsorb boron, and (c) study boron desorption from the slag with time. Boron adsorption increased with the increase of the external solution/adsorbent ratio up to the ratio of 200:1. Although, almost 40% of boron was adsorbed within the first hour of equilibration period, the adsorption gradually increased until the 72 h. The Langmuir adsorption maximum for boron was 145 mg g −1 , considerably higher than other adsorbents, like fly ash, calcite, and magnesia. At boron initial concentrations lower than 6 mg L −1 , slag removed 55% of boron and reduced it below the permissible levels for irrigation waters (< 4 mg L −1 ) for most crops. The pH of the equilibrium solution was 10.3 ± 0.8 and dropped to acceptable levels for irrigation waters (< 8.5), after contact with atmosphere for 1 week. Almost 25% of boron was released from samples of boron-laden slag during the first hour of desorption. Consequently, steelmaking slag can be used effectively for removing boron excess from irrigation waters. However, attention should be given to the pH of the slag-treated waters. Furthermore, the disposal of boron-laden slag to soils should be practiced with caution to avoid possible boron phytotoxicity risk.

The effect of sewage sludge, stabilized with steelmaking slag, on soil chemical properties and fertility and on wheat (Triticum aestivum L) growth was evaluated. Dewatered sewage sludge [75% (wet weight basis)] stabilized with steelmaking slag (25%) and three soils with different pH values were used in a pot experiment with winter wheat. The following treatments were applied: (i) sludge addition of 30 g kg(-1) (approximate to 120 Mg ha(-1), rate equivalent to the common inorganic N fertilization for wheat, based on sludge's water soluble NO3-N), (ii) sludge addition of 10 g kg(-1) (approximate to 140 Mg ha(-1), rate equivalent to the common inorganic N fertilization for wheat, based on sludge's Kjeldahl-N), (iii) addition of the common inorganic N fertilization for wheat (120 kg N ha(-1)) as NH4NO3, (iv) control (no fertilizer, no sludge). Sludge application at both rates to all soils resulted in a significant increase of pH, electrical conductivity of the saturation extract (ECse) and soil available NO3-N and P, in comparison to the other two treatments and this increase remained constant till the end of the pot experiment. In sludge treatments pH did not exceed the critical value of 8.5, whereas ECse, although it did not reach the limit of 4 dS m(-1), exceeded the value of 2 dS m(-1) at the rate of 30 g kg(-1). Concentrations of heavy metals, which regulate the agronomic use of sewage sludge according to the established legislation, ranged from not detectable to lower than the respective permissible levels. Both rates of sludge's addition in all soils improved wheat's growth, as judged by the significant increase of the aboveground biomass yield and the total plant uptake of almost all nutrients, compared to the other two treatments. It was concluded that sewage sludge stabilized with steelmaking slag could be used in agriculture, applied at rates based on sludge's Kjeldahl-N content and crop's demand for N. However, potential environmental impacts must also be considered. (C) 2017 Elsevier Ltd. All rights reserved.