The Impact of Electric Current for Sewage Sludge Characteristics from Anaerobic Sequencing Bio-Electrochemical Treatment of Sewage Generated During Soilless Tomato Cultivation

The aim of the present study was to determine: the impact of electric current density on the quantity and quality of sewage sludge produced in anaerobic sequencing bio-electrochemical reactor (AnSBBER) with an iron electrode during the treatment of drainage from soilless cultivation of tomatoes. Direct electric current (DC) effect was de - termined at its following densities (J): 0.63 A/m 2 (R1), 1.25 A/m 2 (R2), 2.5 A/m 2 (R3), and 5 A/m 2 (R4). Sodium acetate in (C:N) ratio of 1.0 was supplied to the reactors to ensure the proper biofilm development. Contents of elements (K, P, S, Na, Al, Cu, Fe, Mn, Mo, Zn, Mg, C, N and Ca) in the biofilm were determined. Additionally, the content of total suspended solids and the percentage share of volatile suspensions (VSS) in the sludge were determined. The study showed that the organic matter content in the sludge corresponded to the values typical of the stabilized sludge (up to 28.8% d.m. in R2).The increase in electric current density caused an increase in the concentration of phosphorus in the formed sludge (from 6.34 to 8.00% d.m. in 0.63 and 5.00 A/m 2 , respectively). The analyzed sludge, compared to municipal sludge from wastewater treatment plants with biological reactors and activated sludge chambers, is richer in such elements as phosphorus, nitrogen, calcium, magnesium, potassium, sodium, and iron.


INTRODUCTION
The industrialization of agriculture entails, among other things, growing plants under complete control.An example in this case can be the soilless cultivation of plants in greenhouses or film tents, where temperature, humidity, sunlight, water supply, and composition of nutrient medium fed to the root system are continuously controlled.
Two agrotechnical treatments -irrigation and fertilization -are used simultaneously in soilless cultivation systems.These combined treatments are called fertigation.Fertigation allows, above all, to reduce the consumption of water and fertilizers and to use solutions in doses and dates optimal for plants, which promotes higher yields [Breś et al. 2012].Breś et al. [2012] emphasized that nutrient medium doses applied in soilless cultivation systems exceed nutritional demands of plants.This is due to the need to stabilize the chemical composition of solutions in the rhizosphere as well as the pH value and electrolytic conductivity (EC) of nutrient media.
Crops cultivated in the soilless system require systematic irrigation coupled with the feeding of complete nutrient media, in which the amount of nutrients needs to be adjusted not only to cultivation system type but also to the developmental stage of the plants of a given variety and to weather conditions.The composition of the media should be precisely determined and they should be properly dosed in these cultivation systems [Dyśko et al. 2014].
Preparation of the nutrient medium requires knowledge of the chemical composition of water.According to recommendations in force, the total salt concentration in water should not exceed 160 mg/dm 3 for highly sensitive plants and 800 mg/dm 3 for plants tolerating high salinity.When establishing the composition of nutrient media, account should be taken of all water components, with particular attention paid to the concentrations of calcium, magnesium, sulfates, and chlorides.
The highest assimilability of most elements in hydroponic crops was recorded at pH 5.5-5.8.Therefore, the pH values of nutrient media are adjusted by acid addition (usually nitric, phosphoric and hydrochloric acids).The nutrient concentration in the medium is determined based on the specific electrical conductivity (EC) of the solution and expressed in MS/cm.The average EC values of hydroponic media range from 1.5 to 4.5 MS/cm [Dyśko et al. 2014].In addition to the macroelements shown in Table 1, microfertilizers containing 6 micronutrients are also used for media preparation (Table 2) [Dyśko et al. 2014].
The composition of wastewater discharged from facilities for soilless cultivation of plantsis a resultant of the qualitative and quantitative characteristics of the nutrient medium and the efficiency of uptake of macronutrients and micronutrients by plants.The need to treat this type of wastewater has been growing for years.It is also recommended to be recirculated and re-used in crop fertilization and irrigation.However, the owners of the facilities point to difficulties and threats associated with these processes.The main problem in the soilless cultivation with the recirculated nutrient system is the proper balancing of all nutrients and the risk of pathogens [Dyśko et al. 2014].Therefore, it is widely believed that wastewater should be treated regardless of its intended use, even though the Directive does not specify quality requirements for effluents of this type [Dz.U. L 135 z 30.5.1991].
Among the implemented technological solutions, increasing attention is paid to reactors with a biofilm.Research conducted by Rodziewicz et al. [2019] with aerobic biological, electrochemical and bio-electrochemical rotating contactors demonstrated that such reactors have a whole range of limitations and often fail to ensure effective wastewater treatment that would allow for its discharge to the natural environment [Dz.U.2019 poz.1311].But very promising results were obtained in the case of another biological membrane The volume and quality of sewage sludge is a resultant of the composition of treated wastewater, the technology used and the adopted technological parameters of processes.In the case of electrochemical and bio-electrochemical reactors, the characteristics of the sludge largely depend on the type and density of the electric current and, to some extent, on the materials the electrodes are made of.More sewage sludge is generated in the reactors with direct current (DC) flow than in those with alternative current (AC) flow [Rodziewicz et al. 2023].Due to the composition of the treated wastewater, sewage sludge generated during the treatment of drainage from the soilless plant cultivation is expected to be rich in macroelements and microelements and can, therefore, be used as soil improver or fertilizer.
The aim of the present study was to determine: the impact of electric current density on the quantity and quality of sewage sludge produced in anaerobic sequencing bio-electrochemical reactor (AnSBBER) with an iron electrode during the treatment of drainage from soilless cultivation of tomatoes.With regard to sludge quality -the scope of the study included determination of the contents of selected macroelements and microelements and organic matter in the sludge.

MATERIALS AND METHODS
The research was conducted using four bioelectrochemical AnSBBER reactors with a volume of 2L each.The reactors contained 5 stainless steel discs (0.12 m diameter) mounted on a vertical shaft.The discs, which served as a cathode, were rotating at the speed of 14 rpm.The anode (sizes: 15 and 23 cm) made of carbon steel was mounted inside the reactor's tank.Chemical composition of discs was as follows: (chemical composition C ≤0.030, Si ≤ 0.75, Mn ≤ 2.25, P ≤0.025, S ≤ 0.01, N ≤ 0.1, Cr 17.00-19.00,Ni 13.00-15.00,Mo ≤ 2.25, and Cu ≤0.5; and that of anode was as follows: C ≤0.12, MN ≤0.60, P ≤0.045, S ≤0.045).
Sodium acetate was supplied to the reactors to ensure the proper biofilm development.The ratio of carbon to nitrogen in the treated wastewater was C/N=1.0.The first four weeks were the period of reactors' adaptation.Activated sludge from the municipal sewage treatment plant in Olsztyn (PE 270,000) was fed into the reactors to ensure faster achievement of their target efficiency.The sludge was collected from the denitrification chamber.The actual experiment lasted two months after the end of the adaptation period.Analyses were conducted under anaerobic conditions at 20-22°C.Drainage was supplied daily, immediately after the entire reactor contents were discharged.Hydraulic retention time of drainage in the reactor was 24 h.
The samples were mineralized in Teflon vessels with the addition of 10 mL of 69-70% nitric acid (Baker Instra-Analyzed Reagent, Phillipsburg, USA), using a microwave mineralizer The percentage contents of nitrogen and carbon in dried biomass from AnSBBER reactors was determined using the Flash 2000 particle analyzer (Thermo Fisher Scientific, Waltham, MA, USA).
The sludge discharged from the reactors was also determined for the contents of: • dry residue with the gravimetric method (PN-EN 12880:2004), • dry mineral residue and dry organic residue with the gravimetric method (PN-EN 12879:2004).In the manuscript Bryszewski et al.
[2022] have been presented results of physicochemical analyses of wastewater inflowing to and outflowing from the reactors (R1-R4), as well as the efficiency of dephosphatation and denitrification processes.

Results and discussion
The content of the total suspended solids in the effluent ranged from 349.5±154.0mg d.m./L (R1) to 760.4±134.5 mg d.m./L (R4) and increased along with increasing current density (Fig. 2 A).The content of dry organic matter ranged from 23.7% (R2) to 28.8% (R3) -(Fig. 2 B).No specific trend was observed in changes of the organic matter content in the suspended solids; however, the organic matter content below 30% indicates sludge stabilization [ Turek, Wieczorek and Wolf [2019] reported that the organic matter content of sewage sludge from the municipal sewage treatment plant ranged from 73% to 75%.Sludge stabilization with lime allowed reducing organic fraction content by 30%, i.e., to the value of 43.6%.In the study by Bartkowska et al. [2019], the content of dry organic matter determined during autothermal thermophilic stabilization of sewage sludge from 8 municipal treatment plants ranged from 29% d.m. to 59.4% d.m.
The presented volumes of the generated sewage sludge (Fig. 2) are lower than those obtained in the study by Rodziewicz et al. [2023],where the treatment of synthetic greenhouse effluents in an aerobic reactor at the same current densities generated sewage sludge volumes ranging from 3,873 mg d.m./L to 4,450 mg d.m./L at current densities of 0.63 A/m 2 and 5.00 A/m 2 , respectively.In the cited study, the organic matter content in the sludge reached up to 19.88% at HRT 24h and the current density of 0.63 A/m 2 , which indicates good stabilization of bio-electrochemical sludge from aerobic rotating-bed reactors.In the study by Bryszewski et al. [2021], during anaerobic treatment of greenhouse effluent using sodium acetate at a dose of C/N=1, the volume of sludge produced increased along with an increasing density of the alternating current, i.e., from 416.0±267.3mg d.m./L at 4.4 A/m 2 to 1,079.4±251.3mg d.m./L at 13.3 A/m 2 , whereas the organic matter content reached 28.6% and 22.4%, respectively.
Qualitative analyses of sewage sludge (Fig. 3) showed certain trends in the contents of P, C, Ca and Mg, namely their increase in the sludge along with current density increase.The contents of phosphorus and magnesium in the dry matter of  In the other reactors, they were below 6.2; hence, it is likely that phosphorus removal in these reactors was largely due to electrocoagulation.
The percentage content of carbon was between 7.25 and 11.79% d.m.Its highest value was determined in reactor R1 and the lowest one in reactor R4.The carbon content is due to the growth of autotrophic and heterotrophic microorganisms as a result of providing an external source of carbon in the form of sodium acetate.A lower percentage carbon content determined at higher electric current densities may result, among other things, from a higher fraction of chemical sludge.
Trends observed in the changes of the contents of the above elements are consistent with the results of physicochemical analyses of effluent described by Bryszewski et al. [2022].Similar trends in the contents of phosphorus, carbon, calcium and magnesium have also been reported by Bryszewski et al. [2021].The maximal contents of magnesium and phosphorus were determined at the highest AC density tested (13.3 A/m 2 ), i.e., 3.70% d.m. and 2.10%, respectively.In the cited work, the percentage contents of carbon and calcium decreased with current intensity increase The mean contents of phosphorus and nitrogen reported in this study are higher than in typical municipal sewage treated using activated sludge and biological reactors, which on average range from 0.9-1.5% d.m. for phosphorus and from 2.0-6.Sulfur is removed upon the adsorption of SO 4 2-ions on metal oxides and hydroxides formed during anode dissolution due to electric current flow [Murugananthan et al. 2004].
Literature data indicates that sodium concentrations in municipal sludge are similar and equal to: 0.122% d.m. according to Antonkiewicz et al. [2019], 0.123±0.005%d.m. according to Antonkiewicz et al. [2020], and 0.14-0.73%d.m. according to Podedworna and Umiejewska [2008].The sodium content determined in the sewage sludge in the present research is higher than that recorded in municipal sewage (Fig. 3), which may be due to the high salinity of sewage used in hydroponic crops caused by high concentrations of sodium and chlorine ions [Rozema, Gordon and Zheng 2014].
The percentage contents of iron, manganese, molybdenum, and copper increased with the increase in electrical current density (Figures 4A  and B).The iron content of the sludge from the AnSBBER-type reactor ranged from 1.81% d.m. to 3.43% d.m. in the case of R1 and R4, respectively.The maximal manganese content of the sewage sludge was 0.35% d.m. (R4) and the minimal one was 0.10% d.m. (R1 and R2).In turn, the maximal copper concentration in the sludge reached 0.08% d.m. (R4).
Aluminum was absent in the sewage sludge regardless of the electric current density applied.With current density increasing, the molybdenum content in the sludge increased from barely 0.004% d.m. to 0.018% d.m.In contrast, the zinc content was observed to decrease from 0.20% d.m. to 0.09% d.m. with current density increase.
The increasing electric current density resulted also in increased contents of metal ions released into the solution as a result of anode dissolution.The released Fe 2+ and Fe 3+ ions form monomeric and polymeric insoluble complexes with the hydroxyl group that are capable of adsorbing contaminants [Boinpally et al. 2023].In the research by Bryszewski et al. [2021], where an aluminum electrode was applied for the bio-electrochemical treatment of greenhouse effluents, the Al 3+ content ranged from 0.35% d.m. to 4.5% d.m. at alternating current densities of 4.4 A/m 2 and 13.3 A/m 2 , respectively.It is worth noting that despite the higher efficiency of contaminant removal by aluminum electrodes, the use of an iron electrode is justified by its lower toxicity compared to Al [Gautam et al. 2019].
The presence of copper, manganese and molybdenum in the sewage sludge may result from partial adsorption of their ions from wastewater during electrocoagulation, as well as from cathode dissolution upon the action of salts, acids and bases [Wang et al. 2022].According to Picard et al. [2000], the dissolution of the aluminum electrode is due to the impact of hydroxyl ions on the cathode.
Zinc present in the sewage sludge was most likely removed by electrocoagulation, which resulted in its content decrease by 81% (R3 and R4) to 95% (R1 and R2) from the initial value of 1.65±0.26mg Zn 2+ /L [Bryszewski et al. 2022].As Chen et al. [2018] posited, the removal of zinc during electrocoagulation is likely to be due to: (I) precipitation of hydroxides, (II) adsorption on iron hydroxide flocs, (III) cathode reduction to the metallic form, and (IV) co-precipitation of counter-charged colloidal particles formed during the electrocoagulation process.In the cited study, Zn was completely removed from synthetic sewage within 20 minutes at electric current density of 8.3 mA/cm 2 and an initial Zn concentration of 50 mg/L.Therefore, the content of this element in the sludge was below 0.2% d.m.Bartkowska et al. [2019] reported zinc and copper contents in municipal sludge from autothermal thermophilic stabilization within the ranges from 0.075% d.m. to 0.15% d.m. and from 0.021% d.m. to 0.053% d.m., respectively It is noteworthy that the Cu and Zn concentrations determined in the sewage sludge meet the requirements for their use on arable lands in accordance with Polish regulations in force [Dz.U.2015 poz.257].

CONCLUSIONS
Analysis of the sludge discharged from an anaerobic bio-electrochemical reactor of AnSBBER type treating wastewater from soilless tomato cultivation allowed the following conclusions to be drawn: • the volume of sludge produced was lower than in the electrobiological disc reactor and the anaerobic SBBER reactor operating under the flow of direct current and alternating current, respectively, • the increase in electric current density resulted in an increased sludge production; with each current density tested the organic matter content in the sludge corresponded to the values typical of the stabilized sludge, • the increase in electric current density caused an increase in the concentration of phosphorus in the formed sludge, • the increase in electric current density caused iron anode dissolution, which contributed to the increase in the iron content in the sludge dry matter, • the analyzed sludge, compared to municipal sludge from wastewater treatment plants with biological reactors and activated sludge chambers, is richer in such elements as phosphorus, nitrogen, calcium, magnesium, potassium, sodium, and iron, • the contents of copper and zinc in the sludge did not exceed the values permitted for sludge to be used for agricultural purposes, moreover aluminum was absent in the sludge.

Fig. 1 .
Fig. 1.Applied variants of AnSBBR reactor operation in the conducted studies: (R1-R4 reactors, J -density direct current, HRT -hydraulic retention time, C/N -the ratio of carbon to nitrogen, where sodium acetate was the external source of carbon) Metcalf and Eddy 2014].The use of a biofilm-based reactor as well as wastewater treatment under anaerobic conditions lead to lesser sewage sludge production [Boavida-Dias et al. 2022].

Fig. 3 .
Fig. 3.The content of K, P, S, Na, Mg, Ca, C and N in the excess sludge of wastewater treatment from greenhouse

Fig. 2 .
Fig. 2. Total suspended solids (A) and percentage of volatile suspended solids (B) (4.4-13.3A/m 2 ) from 6.30% d.m. to 4.70% d.m. and from 31.00% d.m. to 22.55% d.m., respectively.Kłodowska et al. [2018] obtained a maximal phosphorus content of 0.17% d.m. in the sewage sludge produced during the treatment of synthetic municipal wastewater with a low C/N ratio (C/N=1.5)using citric acid as an external carbon source and electric current with a density of 210 mA/m 2 .

Fig. 4 .
Fig. 4. The content of Fe (A) and Mn, Mo, Al, Cu, Zn (B) in the excess sludge (PD-below the detection threshold)

Table 1 .
Basic macronutrient fertilizers used in the preparation of nutrient solutions

Table 2 .
Examples of the most common micronutrient fertilizers used in hydroponic nutrient solutions content of nitrogen in the sewage sludge was recorded in R2 and R3, where it reached 1.35% d.m. and 1.32% d.m., respectively.The lowest N content in the sludge (1.00% d.m.) was determined at the highest electric current density tested.The content of nitrogen in sewage sludge is due to the biomass growth.