## 1 Introduction  
Climate change, an increase in population, and environmental pollution present the world with two challenges in the near future: providing a sufficient amount of adequate-quality food and sustainable use of natural resources [1]. An opportunity for facing these challenges is created by soilless (hydrophobic) cultivation, which allows for reducing soil contamination (by pesticides), gives a high-quality yield and reduces the amount of water consumed [2, 3]. The cultivation is conducted on a special substrate (for tomatoes, it is mainly mineral wool), which is permeated by an aqueous solution of fertilizers. The medium contains macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium and sulphur); trace elements (iron, manganese, boron, copper, zinc and molybdenum) and microelements: sodium, chlorine and aluminium [4]. The crops are mainly fertilized in open systems [5]. According to [PERSON] et al. [6], 70% of the medium supplied is used by plants, with the remaining 30% being used as overflow and discharged, untreated, into the environment. According to the regulations in Poland, horticultural effluent should be treated, and the nitrogen and phosphorus levels, before being discharged to the environment, should not exceed 30 mg N/L and 3 mg P/L [5, 7, 8]. Therefore, effluent treatment and nutrient recovery are still research objects worldwide. The effluent treatment methods currently under development can be divided into: methods based on denitrification, systems using constructed wetlands (CWs), systems based on microalgae cultivation, and the use of activated carbon methods [9]. Nitrogen removal can be performed with the use of heterotrophic or autotrophic denitrification or by electrochemical nitrogen reduction. Autotrophic denitrification uses inorganic carbon (and gaseous hydrogen) as an electron donor. Cathode water electrolysis is a source of hydrogen in the bio-electrochemical reactor [5; 10; 11]. The use of an electric current also allows for cathode nitrate reduction [12; 13]. Monomeric and polymeric forms of aluminium or iron hydroxides are formed when a current flows through the electrodes. This is followed by agglomeration of the hydroxides with destabilised contaminations and the formation of floccules [14; 15]. Applying electric current and suitable pH helps to retrieve phosphorus by electrochemical means without adding chemical substances [16]. Due to lower sludge formation and lower energy demand, an iron electrode is a better solution than an aluminum one [17]. [PERSON] et al. [18] noted that a combination of electrooagulation with biological treatment methods helps to remove many contaminants from effluents. The authors' earlier research showed that the application of a current with an aluminium electrode and an external carbon source helps to purify hydroponic effluent effectively [5; 11; 19; 20]. Other studies have focused on nitrogen and phosphorus removal from hydroponic effluent. [PERSON] et al. [21] point out that not enough emphasis is placed on the removal of secondary elements (classified as macro- and micro elements) from effluent. The findings of [PERSON] et al. [11] suggest that sludge formed as a result of bio-electrochemical treatment of wastewater from hydroponic tomato cultivation could be applied as a fertilizer. Previous research on the treatment of hydroponic wastewater focused on the removal of phosphorus and nitrogen, due to their high concentrations. It was proved that the efficiency of nitrogen and phosphorus removal in BERs with aluminum electrodes depended on the electric current density [3; 5; 9; 20; 21]. Analyses of macro- and micro element concentrations in sludge, formed during hydroponic wastewater treatment, were performed sporadically [11]. However, the concentrations of macro- and micro elements in treated wastewater, and the role of electric current density in the elements' removal, were not investigated.