Zeolites for the nutrient recovery from wastewater

To meet the growing food demand of the world population, excessive use of chemical fertilizers is occurring to improve soil fertility and crop production. The excessive use of chemical fertilizers is not economically and environmentally sustainable. Indeed, from one hand, due to the increasing demand of fertilizers is rising their costs whereas, on the other hand, the accumulation of fertilizers in wastewaters is altering the homeostasis of the ecosystems thus causing serious damages to human health [1,2]. The recovery of nutrients, such as nitrogen (N) and phosphorus (P), from wastewaters is a good option to counteract both economic and environmental issues raised by the excessive use of fertilizers [3]. Adsorption is among the most widely used methods for nutrient recovery from wastewaters due to its efficiency and simplicity. The choice of appropriate adsorbent materials is a key issue for ensuring high performance and low costs of the process [4]. Over the years, several materials have been studied to absorb nutrients from wastewaters. Zeolites, both natural and modified, have attracted great attention due to their relevant specific capacity, selectivity, safety, and stability [5]. However, considering that in municipal effluents the inorganic P exists as the anionic forms of dihydrogen or monohydrogen phosphates (H2PO4 − and HPO42−, respectively) and N in both cationic (ammonium, NH4+) and anionic (nitrate, NO3−) form [6], natural zeolites can be only used for the direct recovery of NH4+

Zeolite – Ammonium interaction: physical-chemistry of adsorption process

Zeolites are crystalline microporous tectosilicates, either natural or synthetic. Natural zeolites were formed as a result of the interaction of volcanic rocks and volcanic ash with alkaline groundwater. Due to the formation process, there are more than 50 types of natural zeolites, the most common being clinoptilolite, which belongs to the heulandite family and has the simplified ideal formula of (Na,K,Ca)2-3Al3(Al,Si)2Si13O36·12(H2O). Geological settings and conditions during zeolite formation and geological weathering influence several parameters such as mineralogy, rock porosity/permeability and reaction rate, all of which affect their operational capabilities, hence their use in technical applications. Indeed, zeolites, due to their properties, can be used in several operations, including gas separation, adsorption, ion exchange and catalysis. In recent years, they have been playing an important role in the recovery and removal of nutrients from treated wastewater due to their ion exchange property. Their ability to adsorb cations (such as ammonium ions) comes from the substitution of Si4+ by Al3+, which increases the negative charge of the mineral lattice. The resulting negative charge is balanced by exchangeable cations such as Na+, K+ and Ca2+. The recovery of nutrients (nitrogen and phosphorus) from wastewater is necessary, as their presence in wastewater accelerates the eutrophication of receiving water bodies, creating a potentially toxic environment for fish and other aquatic life. In addition, nutrient recovery from wastewater allows solving problems i.e. the poor access to fertilizers in developing countries and the looming high cost of fertilizers; in fact, the recovered fraction of nutrients can be reused as fertilizer in agriculture promoting a circular economy approach. Furthermore, the use of natural adsorbent materials, such as zeolites, to recover nutrients from wastewater overcomes the problem associated with existing technologies. Which are often expensive and difficult to apply, limiting their use in economically poor countries due to lack of infrastructure and maintenance costs. However, the removal of ammonium by ion exchange on zeolites is influenced by the origin of the zeolites used. Previous studies on clinoptilolites with different lithological matrix have shown how the ability to adsorb NH4+ varies in clinoptilolites of different origin. For example, a Canadian clinoptilolite was capable of adsorbing about 20 mg NH4+ g-1, while a Chinese clinoptilolite did not exceed 5 mg NH4+ g-1. The original matrix may also have an influence on the treatment when natural zeolites are treated to increase the adsorption capacity. Based on the above considerations, the objectives of the PhD project, carried out within the Wider Uptake project (Horizon2020 EU project), are: i) to compare the ammonium adsorption rate on two clinoptilolites of different origin (Slovakia and Cuba), ii) to evaluate the effect that the matrix had on the treatment carried out to improve the adsorption capacit

Nutrient recovery from treated wastewaters by biochar and zeolite: implications for soil fertility

The increasing demand for food due to the mounting world population, coupled with the exhaustion of phosphorus (P) mines and the rising use of more and more expensive nitrogen (N) fertilizers, impose to find alternative sources of such nutrients. The recovery of N and P from treated urban wastewaters through the use of adsorbent materials and their reuse as enriched nutrient amendments to improve soil fertility may be a valid alternative. Zeolites and biochar are suitable materials for the adsorption of nutrients from treated urban wastewaters. Zeolites are crystalline microporous tectosilicates with a negatively charged structure, due to Al replacing Si, compensated by weakly bonded exchangeable cations. They are commonly used for ammonium recovery, showing an absorption capacity higher than 93% within the first 10 minutes of contact with the liquid phase. Once the zeolites are exhausted, they can be either regenerated by washing with NaCl, allowing also the recovery of ammonium, or directly applied to soil as slow-release fertilizers. Several studies have analysed the absorption capacity of zeolites at laboratory and pilot scale, but only few at plant full-scale. On the other hand, biochar is obtained by pyrolysis of plant biomass at 300-800°C and in the absence of oxygen. Studies carried out to investigate the potential of biochar to act as absorbent for the removal of P from aqueous solutions are few and, often, contrasting each other. This is probably due to the performance and properties of biochar that are highly influenced by many factors such as temperature, heating rate and residence time during pyrolysis, the feedstock used as raw material and its particle size. For this reason, the research on the adsorption and desorption properties of biochar is in its early stages, being several questions still unanswered. Even few are studies about the role of P-enriched biochar to improve the availability of P for plants in agricultural soils. Based on the above considerations, the objectives of the PhD project, carried out within the Wider Uptake project (Horizon2020 EU project), are: (i) to identify the most suitable zeolite and biochar for the recovery of inorganic N and P from treated urban wastewaters at plant full-scale, (ii) to study the mechanisms of P absorption on biochar, iii) to evaluate the impact of P-enriched biochar and of ammonium recovered from zeolite on soil chemical and biochemical properties

Wastewater treatment sludge composting

In recent years, the amount of sewage sludge generated by wastewater treatment plants (WWTPs) has increased due to worldwide population growth and to efficiency of biological treatment processes [1,2]. Sludge is an important source of secondary pollution to aquatic environments and a potential risk to human health; moreover, it represents one of the most important cost items in the functioning of water treatment plants [3–5]. About 60% of the operating costs of secondary wastewater treatment plants in Europe can be associated with the treatment and disposal of products [6]. For this reason, proper sludge management becomes increasingly important, at both national and international level, and it becomes necessary to find effective measures to limit the environmental impacts and to reuse sludge as a resource, within a circular economy vision [2,7]. Current methods of utilization of sewage sludge include agricultural application, landfilling, incineration, drying, and composting and/or vermicomposting. Composting is a widely used cost-effective and socially acceptable method for treating solid or semisolid biodegradable waste [8]. In agriculture sewage sludge is used for rehabilitation of degraded soils, reclamation, or adaptation of land to specific needs [9]. The above consideration comes from several studies showing that the application of sludges to agricultural land can improve soil fertility and, therefore, crop productivity [10–12]. This field of use is also possible due to its composition; in fact, it is rich in organic matter, nitrogen, phosphorus, calcium, magnesium, sulfur, and other microelements needed by plants and living native organisms in the soil. However, sewage sludge may contain a wide range of harmful toxic substances such as heavy metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxins and dibenzo-p-furans, polychlorinated biphenyls, di(2-ethylhexyl) phthalate, polybrominated diphenyl ethers, detergent and drug residues, pharmaceutical and personal care products (PPCPs), endogenous hormones, synthetic steroids and pathogenic organisms [13,14], which can cause harm to the environment and humans. Due to the presence of those toxic elements, stabilization of sewage sludge is necessary to avoid any environmental risk [15]. Stabilization of sewage sludge is defined as “biological, chemical or thermal treatment, long-term storage or any other appropriate process aimed at reducing its fermentability and the health hazards arising from its use” [16]. This definition is found in Council Directive 86/278/EEC, which was issued to regulate the use of sludge in agriculture, the primary objective of which is the environment, in particular the soil, and the protection of human health. European Directive 86/278/EEC was implemented in Italy by Legislative Decree 99/1992 [17]. Both the European Directive and the Italian legislative decree can be considered obsolete, this is why the European Union is moving towards amending them to reflect the new needs of the sector and to keep up with technological innovations. Currently, there are several processes for sludge stabilization, including composting, which is one of the most widely used methods for stabilizing organic matter in general, reducing the number of pathogenic microorganisms and the amount of toxic elements [18]. This is possible because during the composting process the organic compounds present in the biomass to be composted are converted into chemically recalcitrant, that is, stabilized, humic substances, while pathogens are eliminated due to the heat generated during the process thermophilic phase [19,20]. During the composting of sludges, the addition of bulking agents is needed, as they ameliorate the composting performance by providing structural support that improves aeration and regulates moisture content and C/N ratio of composting mass [21,22]. Sludge composting, however, has to be focused on limiting some secondary causes of pollution related to the process itself, such as greenhouse gas (GHG) emissions and heavy metal contamination [23]. Indeed, in the last decades, the handling of sewage sludge with traditional methods has led to the release of an enormous amount of greenhouse gases. The choice of an appropriate bulking agent is, therefore, fundamental to limit the emission of climate-altering gases, and, at the same time, to increase the microbial activity thus improving the quality of the compost [24,25]. This chapter aims (1) to give an overview of the national and international legislation on sludge management and reuse, (2) to analyze the composting process and the state of the art regarding sludge composting to understand the limitations at large-scale application, and (3) to discuss the technological innovations in the field and highlight future perspectives

Phosphorus dynamics in the soil-sunflower (Helianthus annuus L.) system as affected by P-enriched biochar from real treated wastewater

This study reports on the characteristics of phosphorus-enriched biochar, and on its effect when used as slow released fertilizers, on the soil-sunflower (Helianthus annuus L.) system. The biochar was enriched using real treated wastewater at a pilot-scale wastewater treatment plant of the Water Resource Recovery Facility of Palermo University. Two types of biochar, B440 and B880, obtained from the same biomass by pyrolysis at 440°C and 880°C, respectively, were used. The experimental design involved the use of two soil types: one rich in calcium and one rich in iron, but both deficient in phosphorus (P). Four treatments were performed: a control without P, a control with P applied as KH2PO4 at a dose of 50 mg of P per kg of dry soil, a treatment with biochar B440, and another one with biochar B880, both applied to add 50 mg of P per kg of dry soil. The experiment lasted (122 days) and was carried out in a growth chamber. Sunflower plants were irrigated daily with 20 mL of P-free Hoagland nutrient solution. Sunflower was chosen as a test plant due to its high P requirement and susceptibility to nutritional deficiencies. Before beginning the experiment, extensive biochar characterization was performed, including the determination of Olsen P, total P, iron, zinc, carbon, nitrogen, and sulphur content. Germination and phytotoxicity tests were conducted, and a P release study was carried out using an anionic resin to assess the mode and amount of P released from the materials. At the end of the study, sunflower plants were explanted, and soil and plant samples were stored for further analysis. Different forms of P were analysed in the soil, along with the amounts of mineral N and enzyme activities (acid phosphatase, alkaline phosphatase, and β-glucosidase). Total P concentrations in roots, stems, and flowers were quantified in plant samples, along with biometric parameters such as fresh weight and dry weight. Finally, P uptake and Phosphorus Fertilizer Replacement Value (PFRV) were assessed

Recovering ammonium by treated and untreated zeolitic mixtures: A comprehensive experimental and modelling study

The recovery of ammonium (NH4+) from aqueous solutions by zeolite is attractive. In this study, the physicalchemistry of NH4+ adsorption process from aqueous solution by two zeolitic mixtures, either treated or not treated with NaCl, was assessed. Results suggested that the zeolitic mixture richer in mordenite and with high specific surface area adsorbed more NH4+ than the one richer in clinoptilolite and heulandite showing a lower specific surface area. NaCl treatment increased the amount of NH4+ adsorbed by the zeolitic mixtures. The higher amount of NH4+ adsorbed by the zeolitic mixtures treated with NaCl was explained by the low/high density water model accounting for cation exchange among the two kosmotropic systems: Na-enriched zeolitic mixtures and NH4+-enriched aqueous solution. The adsorption kinetics were best approximated by the bimodal pseudo-first-order model. The two sorption kinetic constants, k1 and k2 were related to the adsorption (mediated by k1) and the ion exchange (mediated by k2) processes. The fitting of NH4+ data to Langmuir-Sips model suggested that the NaCl treatment increased the number of active sites only of the zeolitic mixture with the large amount of mordenite. Thus, it is conceivable that modulation of NaCl treatment of zeolitic mixtures can be applied to obtain new materials for water remediation from NH4+ contamination

Zeolite–Ammonium interactions: the physical-chemistry of the adsorption process

Wastewaters have plenty of organic and inorganic compounds. Most of them are nitrogen-, and phosphorus enclosing materials that can be considered plant nutrients [1]. Porous materials, such as zeolites, are considered very suitable for wastewater treatment and nutrient adsorption [2]. One potential application is the use of natural zeolites to remove nutrients, such as NH4+ from wastewater, thus reducing the risk of eutrophication of the aquatic environment and reusing enriched NH4+ zeolite as slow release fertilizer [3]. Due to the formation process, natural zeolites [2] have different operational capacity mainly related to the mineralogical composition. In this study, the ability of Slovak and Cuban zeolites with different mineralogy in adsorbing ammonium (NH4+) from a mono-component solution was assessed. Zeolites were treated or not treated with NaCl. The physical-chemistry of NH4+ adsorption process was studied by static adsorption tests, adsorption kinetics and adsorption isotherms

Zeolite–Ammonium interactions: the physical-chemistry of the desorption process

Zeolites are naturally occurring volcanogenic minerals formed as a result of complex chemical and physical processes in rocks undergoing various changes in nature [1]. The structural characteristic and mineralogy of the zeolite influence their cation exchange capacity and consequently their ability to remove ammonium (NH4+) from an aqueous solution [2]. Natural zeolites have been explored as an effective adsorbent for nutrient removal from wastewater. Nutrient enriched zeolites may be then reused as slow releasing fertilizer or undergone to a regeneration process to recover nutrient such as NH4 + in a circular economy perspective [3]. The regeneration of zeolites can be carried out by different methods. The most popular regeneration technique is commonly achieved using ionic brines, e.g., sodium chloride (NaCl), where the Na+ ion replaces the adsorbed NH4 + by releasing it into the liquid phase [4]. In this study, the ability of two zeolites with different mineralogy in desorbing ammonium (NH4+) following a treatment with 1 M NaCl solution was assessed. The physicalchemistry of NH4+ desorption process was studied by static desorption tests and desorption kinetics

Phosphorous recovery from treated wastewater by salt activated biochar

Phosphorus (P), an essential element for life and an irreplaceable component of modern agriculture, suffers losses at every stage of its biogeochemical cycle, raising concerns about future supplies as well as water and soil pollution [1]. Furthermore, P is a non-renewable resource so much so that it is estimated that its reserves will be depleted within 50-400 years [2]. Europe, in order to meet its need for P, is entirely dependent on imports from other countries. The unbalanced global distribution of P is increasingly becoming a geopolitical problem for Europe that can only be overcome by finding new and sustainable sources of supply. By virtue of the above, a more rational use of P fertilisers and the recovery of P from agro-food products, urban and industrial wastewater are desirable [3]. An efficient and economical method to recover P from wastewater is by its sorption onto a solid phase that, in turn, can applied to soil as slow-release fertiliser. Biochar is among the most widely used solid materials for nutrient recovery. It is a material obtained by pyrolysis of biomass, usually wastes, at high temperatures (300-800 °C) and in absence of oxygen [4]. Biochar consists mainly of aromatic-type carbon and is characterised by a large specific surface area (200-1000 m2 g-1), low density and high porosity [5]. Studies conducted to investigate the potential of biochar as adsorbent of P from aqueous solutions are few and, often, conflicting. This is probably due to the performance and properties of biochar being highly influenced by many factors such as temperature, heating rate and pyrolysis duration, feedstock used and particle size [6]. Studies have shown that biochar has mainly a negative surface charge, which causes non-attraction or only very weak interactions with negative ions such as phosphate. For this reason, a modification of the surface and structure of the biochar is required if it is thought to be used for phosphate sorption. Zheng et al., (2019) [10] found that biochar modified with Mg and Al contains more functional groups than natural or treated biochar with only Mg or Al, moreover, it shows a larger surface area [10]. Zhong et al., 2019 [9], for example, impregnated coconut shell biochar with Fe. The results showed that Fe-biochar adsorbed 36 mg of P g-1, which was 2.4 times higher than the untreated biochar. Although the results are encouraging, it is important to emphasise that the search for biochar with a higher phosphate adsorption capacity is in its early stages, so further studies are needed. The purpose of this study was to assess the ability of two biochar, pre-treated or not with HCl and, subsequently, treated with 0.5 M and 2 M of one of the following salts, CaCl2, MgCl2, AlCl3, FeCl3, to adsorb P from a mono-component solution

Water reuse of treated domestic wastewater in agriculture: Effects on tomato plants, soil nutrient availability and microbial community structure

The reuse of treated wastewater (TWW) in agriculture for crop irrigation is desirable. Crop responses to irrigation with TWW depend on the characteristics of TWW and on intrinsic and extrinsic soil properties. The aim of this study was to assess the response of tomato (Solanum lycopersicum L.) cultivated in five different soils to irrigation with TWW, compared to tap water (TAP) and an inorganic NPK solution (IFW). In addition, since soil microbiota play many important roles in plant growth, a metataxonomic analysis was performed to reveal the prokaryotic community structures of TAP, TWW and IFW treated soil, respectively. A 56-days pot experiment was carried out. Plant biometric parameters, and chemical, biochemical and microbiological properties of different soils were investigated. Shoot and root dry and fresh weights, as well as plant height, were the highest in plants irrigated with IFW followed by those irrigated with TWW, and finally with TAP water. Plant biometric parameters were positively affected by soil total organic carbon (TOC) and nitrogen (TN). Electrical conductivity was increased by TWW and IFW, being such an increase proportional to clay and TOC. Soil available P was not affected by TWW, whereas mineral N increased following their application. Total microbial biomass, as well as, main microbial groups were positively affected by TOC and TN, and increased according to the following order: IFW > TWW > TAP. However, the fungi-to-bacteria ratio was lowered in soil irrigated with TWW because of its adverse effect on fungi. The germicidal effect of sodium hypochlorite on soil microorganisms was affected by soil pH. Nutrients supplied by TWW are not sufficient to meet the whole nutrients requirement of tomato, thus integration by fertilization is required. Bacteria were more stimulated than fungi by TWW, thus leading to a lower fungi-tobacteria ratio. Interestingly, IFW and TWW treatment led to an increased abundance of Proteobacteria and Acidobacteria phyla and Balneimonas, Rubrobacter, and Steroidobacter genera. This soil microbiota structure modulation paralleled a general decrement of fungi versus bacteria abundance ratio, the increment of electrical conductivity and nitrogen content of soil and an improvement of tomato growth. Finally, the potential adverse effect of TWW added with sodium chloride on soil microorganisms depends on soil pH