CHEMICAL COMPOSITION OF INDUSTRIAL WASTEWATER AND ASSESSMENT OF NEUTRALIZATION BY-PRODUCTS

УДК 628.31

Abstract

Industrial wastewater contains a wide range of pollutants, including heavy metals, organic compounds, nitrogen- and phosphorus-containing substances, which pose serious environmental risks. The aim of this study is to determine the chemical composition of industrial wastewater from metal-processing enterprises and to assess the properties of neutralization by-products as potential raw materials for the production of liquid complex fertilizers. The concentrations of organic matter, nitrogen, phosphorus, and mineral components were determined using physicochemical analysis methods.

Neutralization was carried out using appropriate acidic and alkaline reagents to reduce the environmental impact of wastewater and improve its quality. The obtained neutralization products were further assessed as potential raw materials for the production of liquid complex fertilizers. The results demonstrated that the fertilizer produced from industrial wastewater contained 15.91–17.18% P₂O₅, 29.84–30.08% nitrogen compounds, up to 2.0% Fe, and 0.57–1.60% macro- and microelements. Physicochemical characterization confirmed the suitability of the obtained product for agricultural applications.

The findings indicate that wastewater neutralization can serve not only as an effective treatment method but also as a resource recovery approach for producing value-added fertilizers. The proposed technology contributes to environmental protection, sustainable waste management, and the efficient utilization of industrial by-products.

Аннотация

Промышленные сточные воды содержат широкий спектр загрязняющих веществ, включая тяжёлые металлы, органические соединения, азот- и фосфорсодержащие вещества, которые представляют серьёзную экологическую угрозу. В данном исследовании изучается химический состав промышленных сточных вод и оцениваются продукты, получаемые в процессе нейтрализации. Концентрации органических веществ, азота, фосфора и минеральных компонентов были определены с использованием физико-химических методов анализа.

Нейтрализация проводилась с применением соответствующих кислотных и щелочных реагентов с целью снижения экологической нагрузки сточных вод и улучшения их качества. Полученные продукты нейтрализации были дополнительно оценены в качестве потенциального сырья для производства жидких комплексных удобрений. Результаты показали, что удобрение, полученное из промышленных сточных вод, содержало 15,91–17,18% P₂O₅, 29,84–30,08% азотистых соединений, до 2,0% Fe, а также 0,57–1,60% макро- и микроэлементов. Физико-химическая характеристика подтвердила пригодность полученного продукта для применения в сельском хозяйстве.

Полученные результаты свидетельствуют о том, что нейтрализация сточных вод может служить не только эффективным методом очистки, но и подходом к рекуперации ресурсов для производства удобрений с добавленной стоимостью. Предлагаемая технология способствует охране окружающей среды, устойчивому управлению отходами и эффективному использованию промышленных побочных продуктов.

Keywords: industrial wastewater, neutralization, wastewater treatment, resource recovery, liquid complex fertilizer, phosphorus, nitrogen, environmental protection.

Ключевые слова: промышленные сточные воды, нейтрализация, очистка сточных вод, рекуперация ресурсов, жидкое комплексное удобрение, фосфор, азот, охрана окружающей среды.

Introduction

Industrial wastewater from metal-processing enterprises contains a complex mixture of phosphoric and sulfuric acids, heavy metal ions, organic compounds, and nitrogen- and phosphorus-containing substances. If discharged without proper treatment, these effluents can seriously harm aquatic ecosystems and human health, making their effective treatment a global priority. [1,2] While conventional methods mainly focus on pollutant removal, recent approaches based on circular economy principles emphasize resource recovery from industrial waste. Wastewaters rich in nitrogen and phosphorus compounds can be reused for producing liquid complex fertilizers, turning waste into a valuable agricultural input.

Phosphating processes in metal treatment generate acidic wastewater containing about 14.2% P₂O₅, whereas automotive painting effluents contain NaOH-based alkaline components.[3,4] Neutralizing these two waste streams reduces environmental pollution and yields nutrient-rich products suitable for fertilizer production. This study examines the chemical composition of such wastewaters, the neutralization process, and evaluates the resulting product as a potential liquid complex fertilizer.[5]

Materials and methods

Various methods are applied for the treatment and recycling of industrial wastewater, enabling the recovery of metals and other valuable components. The main techniques include chemical precipitation, where reagents are added to form insoluble metal compounds; ion exchange, which removes metal ions using ion-exchange resins; and electrolysis, where metals are separated from water using electric current. The recovered substances can later be used in the production of fertilizers.[6]

In this study, experiments focused on analyzing industrial wastewater and producing liquid complex fertilizers through neutralization. Key parameters such as pH, density, and purity of acidic wastewater were investigated. Before experimentation, the pH values of two wastewater samples were measured to determine appropriate neutralization conditions.[7,8] After phosphating, the pH of the solution changes, and optimal conditions require it to be maintained within the range of 6–8.

Metal phosphating solutions typically contain phosphoric acid (H₃PO₄) 10–50%, sulfuric acid (H₂SO₄) 5–20%, ammonium phosphate 1–3%, sodium phosphate 1–5%, sodium nitrate about 1%, and hydrochloric acid up to 2%. During the phosphating process, the composition of the solution changes, which directly affects the properties of the resulting wastewater.

Results and discussion

The analysis of neutralized acidic wastewater improves its environmental significance and practical application through pH adjustment of industrially generated effluents. After neutralization, the treated wastewater composition was analyzed. Phosphating solutions from the metal processing industry contain metal ions, hydrogen phosphate, phosphate, dihydrogen phosphate, chloride, and nitrate ions.[9] These were neutralized using NaOH-containing wastewater from the caustic soda waste of automobile reflector manufacturing enterprises.

The compositions of both wastewater types were initially determined via mass spectrometric and physicochemical analytical methods. The nutrient composition of the obtained liquid neutralized fertilizer was then evaluated. The concentrations of macro- and microelements, including P₂O₅, N₂O₅, Zn, Cr, and Fe, were determined using a photocolorimetric method.

Table 1. Chemical composition of local industrial wastewater

The analysis of the sample showed that it contained relatively high amounts of phosphorus and oxygen, with phosphorus mainly existing in the oxide form as P₂O₅. The composition of the alkaline wastewater generated by automotive spare parts painting enterprises was also investigated, and the presence of its main components was confirmed.

Figure 1. SEM–EDS spectrum of the obtained liquid complex fertilizer

The spectrum shows four characteristic fields: Field 1 — high phosphorus and oxygen content, indicating P₂O₅ as the dominant compound; Field 2 — elevated oxygen signal associated with oxide compounds present in the matrix; Field 3 — minor signals of silicon and aluminum reflecting trace mineral impurities; Field 4 — sodium peak alongside iron and aluminum, confirming the presence of sodium-containing neutralization products. Elemental percentages were determined by energy-dispersive X-ray spectroscopy.

The results of the study show that fertilizers obtained from industrial waste significantly improve soil fertility. Fertilizers produced through composting and anaerobic decomposition processes stimulate plant growth and support their healthy development. During chemical processing, the fertilizers were enriched with nitrogen, phosphorus, and potassium, which ensured the supply of essential nutrients required for plants.

The first peak indicates a high phosphorus content. In particular, calcium phosphate contains about 18% phosphorus, while the fertilizer contains approximately 14% P₂O₅. Considering that complex salt products account for more than 60% of imported materials, phosphorus is one of the most abundant elements in the composition.

The second peak corresponds to oxygen, which is present in almost all compounds in the sample. The third peak shows small signals related to silicon and aluminum . The fourth peak indicates that among the visible elements (Al, Fe, Na), sodium is the most abundant. This peak confirms a different nature compared to the previous two peaks.

In the third point, phosphorus and oxygen are the dominant elements. Since the particle size is smaller than in the previous two points, part of the signal may originate from the surrounding area of the particle or from the composite matrix.

Figure 2. Mass spectrum of alkaline wastewater

The diagram shows that during composite formation, intermolecular bonding occurred at an initial 1600 °C. In the organic molecules, O=C–O valence vibrations were observed in the range of 1409.96 cm⁻¹ and 871.82 cm⁻¹ as deformation vibrations. O–H valence vibrations were detected in the regions of 3641.60 cm⁻¹ and 2928.86 cm⁻¹.

Since the amount of salts in the molecules was lower compared to other samples, O=C–O valence vibrations were also observed in the regions of 1006.84 cm⁻¹ and 912.33 cm⁻¹, while O–H valence vibrations appeared at 3693.45 cm⁻¹ and 3674.86 cm⁻¹. In the neutralization products, the intensity of the C=O valence vibration peak decreased from 2920.92 cm⁻¹ to a lower region of 2850.79 cm⁻¹, indicating structural changes in the molecular composition.

Conclusion

Along with the proposed method of obtaining liquid complex fertilizers from industrial wastewater, mixed complex fertilizers can also be produced by incorporating microelements generated during metal processing into the fertilizer composition.

The production of liquid complex fertilizers from industrial waste and neutralization products represents a fundamentally new technological approach that enables the production of additional complex fertilizers enriched with microelements — ensuring efficient utilization of industrial waste while enhancing the value of the final product.

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