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Case Report

Failure Analysis of Biological Treatment Units Under Shock Loads of Rubber Industry Wastewater Containing Emerging Pollutants: Case Study

by
Valentin Romanovski
Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA
Water 2025, 17(16), 2419; https://doi.org/10.3390/w17162419
Submission received: 23 July 2025 / Revised: 1 August 2025 / Accepted: 13 August 2025 / Published: 15 August 2025
(This article belongs to the Special Issue Water Treatment Technology for Emerging Contaminants, 2nd Edition)
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Abstract

This paper presents the results of a survey of the designed biological wastewater treatment facilities of an enterprise specializing in the production of rubber products. The aim of the study was to assess the efficiency of wastewater treatment systems under the conditions of a salvo discharge of industrial effluents that differ in composition from domestic wastewater. The analysis covered the parameters of water supply, water disposal, and wastewater characteristics at various stages of treatment. Three samples were taken: after washing the premises (WW1), at the inlet to the treatment facility (WW2), and at the outlet after treatment (WW3). Experimental assessment of the purification efficiency for key pollutants showed a high degree of removal of surfactants (91.2%), oil products (84.4%), and COD (63.4%). However, phosphorus–phosphate turned out to be significantly higher than the norm—2.32 mg/L with an acceptable level of 0.2 mg/L—which corresponds to an excess of 11.6 times. A low degree of ammonium nitrogen removal was also revealed—62%. Calculations showed a critically high ratio of COD/BOD5 = 5.1 with the recommended <2.6, which indicates a small share of biodegradable substances and the need to implement physical and chemical treatment methods. The absence of the characteristic smell of household wastewater and the presence of black inorganic sediment confirm the toxicity of emerging pollutants for activated sludge. It is concluded that the installed biological treatment system cannot cope with the salvo loads of industrial wastewater. Optimization measures are proposed: preliminary local treatment, dosed feed, and a separate treatment system.

1. Introduction

Discharge of insufficiently treated wastewater into water bodies is one of the key threats to the aquatic environment, especially in the context of small- and medium-sized industries that do not have highly efficient multi-stage treatment systems and are not connected to municipal sewerage systems. The problem of insufficient wastewater treatment before discharge into water bodies is directly related to the Sustainable Development Goals (SDGs), in particular Goal 6 “Clean Water and Sanitation”, since effective wastewater treatment is a key element in ensuring sustainable water use and protecting aquatic ecosystems. The problem is exacerbated when wastewater has a variable composition due to volley discharges of industrial wastewater, and includes both domestic and industrial wastewater. These effluents are characterized by the presence of persistent pollutants (surfactants, petroleum products, phosphates, nitrogen-containing compounds, and heavy metals) that are difficult to remove by traditional biological methods and, if insufficiently treated, end up in surface and groundwater bodies [1,2]. Modern wastewater treatment technologies at industrial enterprises strive to integrate biological, physicochemical, and membrane methods to ensure a high degree of contaminant removal while maintaining economic and energy efficiency [3,4,5]. However, in practice, compact biological treatment plants, primarily designed for domestic wastewater, are still widely used. Their operation under conditions of combined discharges without preliminary adaptation leads to overloading of the systems, a decrease in the efficiency of treatment, and, as a consequence, exceeding the maximum permissible concentrations (MPCs) of organic and inorganic substances. This problem is especially acute in cases of volley (uneven) inflow of industrial wastewater, when biological treatment systems are unable to quickly adapt, and microbial communities that ensure the decomposition of pollutants are subject to toxic shock [6,7]. In such situations, a decrease in the oxidation level of ammonium nitrogen, incomplete removal of phosphates, disruption of nitrification and denitrification processes, and deterioration of the sanitary, hygienic, and environmental characteristics of purified water are observed. Underestimation of actual operating conditions and possible deviations from design parameters is one of the key causes of failure of complex technical systems and processes. Errors at the design stage associated with underestimation of actual operating conditions and wastewater composition often lead to the fact that the selected technological solutions are ineffective or even lead to failure [8,9,10,11]. The absence of a pragmatic approach, including an analysis of possible loads, salvo discharges, and toxic components, can cause premature failure of treatment systems and a significant decrease in their operational reliability.
In this context, an integrated approach to assessing the efficiency of treatment facilities in real conditions is of great importance, including not only the analysis of physicochemical parameters but also the diagnostics of technological and design limitations of the equipment used. Such studies allow not only the identification of the reasons for failure to achieve design indicators but also the formulation of recommendations for upgrading existing systems, including by partial separation of flows, the introduction of local pre-treatment, averaging the composition of wastewater, and adapting process flow diagrams to the composition of incoming water. Wastewater generated at enterprises manufacturing rubber products has a high level of contamination with organic compounds, stabilizers, vulcanizing agents, oils, hydrocarbons, and technical surfactants and often contains heavy metals and difficult-to-oxidize polymers [12]. International studies show that the use of traditional biological treatment facilities designed for domestic wastewater in such conditions is ineffective or leads to the rapid degradation of microbial communities [13,14]. For example, studies at enterprises in India and China have shown that the BOD/COD in such wastewater is often less than 0.3, and the phosphorus and ammonium content exceed sanitary standards by tens of times [15,16]. For this reason, only multi-stage schemes are considered effective, including stages of preliminary physicochemical action—coagulation, flotation, sorption and, in some cases, membrane filtration [17,18,19]. In particular, in work [6], coagulation with aluminum sulfate and subsequent flotation made it possible to reduce the content of oil products by 92% and surfactants by 87%. It has also been proven that the use of ozonation and activated carbon at the stage of additional treatment significantly increases the degree of removal of difficult-to-oxidize substances [18]. Thus, the use of exclusively biological treatment for wastewater from rubber production cannot be considered a complete solution, especially in the case of salvo discharges, without the introduction of preliminary stabilization and local pre-treatment. The aim of this article is to provide a comprehensive assessment of the functioning of biological treatment facilities with a combined discharge of domestic and industrial wastewater containing chemically stable compounds. The study is aimed at identifying the key factors reducing the treatment efficiency and formulating sound engineering recommendations for improving the reliability of the system. The treatment facilities fail within less than a year of operation, after which it became necessary to replace the biological load. In the considered variant, the biological load after replacement. The objectives of the article were (i) to conduct a systems analysis of the existing mixed wastewater treatment scheme of an enterprise in the rubber industry and identify bottlenecks leading to a decrease in efficiency; (ii) to study the effect of salvo discharges and difficult-to-decompose pollutants on the performance of biological facilities; (iii) to compare the efficiency of biological treatment with the potential results of physicochemical methods (coagulation, sorption, and membrane technologies) based on literature analysis and calculations; and (iv) to formulate recommendations for the implementation of combined treatment schemes to improve reliability and compliance with regulations.

2. Methodology

2.1. Characteristics of the Research Object

The analyzed enterprise for the production of rubber products is located in the city of Fanipol, Dzerzhinsky district, Minsk region, Belarus. The existing wastewater treatment facilities include surface runoff treatment facilities and combined treatment facilities for domestic and industrial wastewater.
The total area of the production site of the facility is 6.0 hectares, of which 2.12 hectares are allocated for buildings and structures, 1.31 hectares are hard surfaces, and 2.57 hectares are landscaping. The total area of the basic sanitary protection zone of the nature user (including the territory of the production site of the facility) is 42.534 hectares. A production building with an administrative building is located on the allocated territory.
By the nature of its production activities, the nature user is an enterprise with a chemical profile and specializes in the production of rubber products: non-molded rubber products, high-pressure hoses and foamed rubber products, and molded rubber products.
The organization of production of rubber mixtures and molded and non-molded rubber products is carried out on one industrial site and is performed in the following main buildings: the production building, the attached administrative building, the motor transport section, and the process transport workshop.
The following workshops and production areas are located in the production building:
(a) A rubber mixing workshop, which includes a warehouse for materials and ingredients for 100–150 tons with storage on Euro pallets and in big-bags; a preparatory area; a rubber mixing area with a capacity of 250–350 tons/month with 2 rubber mixing lines; a technical control area with an express laboratory; and a warehouse for rubber compounds of 100–150 tons with storage on Euro pallets.
The manufacture of rubber compounds on various bases in % ratio is as follows: rubber compounds based on Styrene Methyl Styrene Rubber (SKMS)—20%; rubber compounds based on Synthetic Isoprene Rubber (SIR)—20%; rubber compounds based on Synthetic Ethylene Propylene Rubber (SEPR)—35%; rubber compounds based on Synthetic Nitrile Rubber (SKN)—10%; other—butadiene–nitrile, isoprene, styrene, sodium–butadiene, and ethylene–propylene rubbers at 3%.
(b) A workshop for non-molded rubber products, which includes a section for the production of non-molded rubber products with a capacity of 240 tons per month with two lines; a technical control section; a section for packaging products; and a warehouse for finished products of 150 tons with storage on Euro pallets.
(c) A workshop for the production of high-pressure hoses and the production of foamed rubber: a section for the production of high-pressure hoses and the production of foamed rubber products with a capacity of 240 tons per month with two lines; a technical control section; a section for packaging products; and a warehouse for finished products of 150 tons with storage on Euro pallets.
(d) A molded rubber goods workshop with a capacity of 150 tons/month (including 50 tons/month of metal reinforcement products), equipped with 2 overhead traveling cranes with a lifting capacity of 5 tons.
The production building also houses a daily storage warehouse for rubber mixtures, a metal reinforcement storage warehouse, a molded rubber goods production workshop, a rubber goods processing area, a technical control area, a mold warehouse, a repair and mechanical area, a warehouse for finished molded rubber goods, a warehouse for mechanics and electricians, a compressor room, and a charging station for electric forklifts.
The administrative building is attached to the production building, which provides utility rooms, a 50-seat canteen serving semi-finished products (including highly finished products), a central laboratory for rubber goods and mixtures, and offices for management and engineering and technical personnel.
The motor transport section is a separate building, which is intended for storage and maintenance of the motor vehicles of service enterprises.
The building houses a box for two vehicles, a box for one vehicle, a box for welding, and a box with an inspection pit for vehicle maintenance.
The process transport shop is an extension building to the motor transport section, which houses the following sections: the tire fitting and vulcanization section; the battery charging and maintenance section; the heating room for motor transport section and process transport shop workers; and the paint and varnish storage room.
Production scheme. Raw materials are delivered to the materials and ingredients warehouse of the production building by motor transport. Raw materials are unloaded into the warehouse via a ramp by electric forklifts. Materials and ingredients are delivered on Euro pallets and in big bags. Then the raw materials pass through the mixing section, according to the technological process, and are delivered to the rubber mixture warehouse in the form of a tape wound on reels or cut into layers, sprinkled with talc, and laid on Euro pallets. Rubber mixtures from the warehouse are sent to the workshops: the workshop for unmolded rubber products, the workshop for the production of high-pressure hoses and foamed rubber, and the workshop for molded rubber products. From the manufacturing workshops, finished products on Euro pallets are sent to warehouses. From the warehouses, electric forklifts load them onto motor transport. Finished products are loaded onto motor transport and then sold.
To solve the tasks set, the fundamental document for the study was the construction project “Organization of production of rubber mixtures, molded and non-molded rubber products at the enterprise”. As part of the work, other materials were analyzed, including the documentation containing environmental requirements and permits and the assignment for the purchase of the treatment facilities.

2.2. Wastewater Analysis Methods

Ammonium ion was determined according to STB 17.13.05-09-2009/ISO 7150-1:1984 [20]. Environmental protection and nature management. Analytical control and monitoring. Water quality. Determination of ammonium nitrogen content. Part 1. Manual spectrometric method.
Biochemical oxygen demand BOD5 was determined according to STB 17.13.05-22-201 1TSO 5815-1:2003 [21]. Environmental protection and nature management. Analytical control and monitoring. Water quality. Determination of biochemical oxygen demand after 5 days (BOD5). Part 1. Method with dilution and addition of allylthiourea.
Nitrate ion was determined according to STB 17.13.05-43-2015 [22]. Environmental protection. Analytical control and monitoring. Water quality. Determination of nitrate nitrogen concentration via the photometric method with salicylic acid.
The pH was determined according to STB ISO 10523-2009 [23]. Water quality. pH determination.
Petroleum products were determined according to PND F 14.1:2:4.128-98 (M 01-05-2012) ed. 2012 [24]. Quantitative chemical analysis of water. Methodology for measuring the mass concentration of petroleum products in samples of natural, drinking, and wastewater via the fluorimetric method [25,26,27] using the Fluorat-02 (LUMEX, Saint Petersburg, Russia) liquid analyzer [28].
Chemical oxygen demand and dichromate oxidizability COD were determined according to PND F 14.1:2:4.190-03 [29]. Methodology for determining the dichromate oxidizability (chemical oxygen demand) in samples of natural, drinking, and wastewater via the photometric method using the Fluorat-02 liquid analyzer.
Water mineralization was determined according to MVI. MN 4218-2012 [30]. Methodology for measuring dry residue concentration (mineralization) using the gravimetric method.
Phosphate ion was determined according to STB ISO 6878-2005 [31]. Water quality. Determination of phosphorus. Spectrometric method with ammonium molybdate.
Phenol was determined according to PND F 14.1:2:4.182-02 [32]. Quantitative chemical analysis of water. Methodology for measuring mass concentration of phenols in samples of drinking, natural, and wastewater using the fluorimetric method on the Fluorat-02 (LUMEX, Russia) liquid analyzer.
Zinc was determined according to PND F 14.1:2:4.183-02 [33]. Methodology for measuring the mass concentration of zinc in samples of natural, drinking, and wastewater using the photometric method on the Fluorat-02 (LUMEX, Russia) liquid analyzer. D-0.005–2.0 mg/L. This methodology applies only to dissolved zinc forms. Before analysis, wastewater samples were filtered through a 0.45 µm membrane filter without prior mineralization or organic oxidation treatment. Therefore, the results correspond to the dissolved zinc fraction.
Anionic surfactants were determined according to PND F 14.1:2:4.158-2000 [34]. Quantitative chemical analysis of water. Methodology for measuring the mass concentration of anionic surfactants (ASs) in samples of natural, drinking, and wastewater using the fluorimetric method on the Fluorat-02 (LUMEX, Russia) liquid analyzer.
Suspended solids were determined according to MVI. MN 4362-2012 [35]. Methodology for measuring the concentration of suspended solids using the gravimetric method in waste, surface, and ground water.
Dissolved oxygen was determined according to STB ISO 5814-2007 [36]. Water quality. Determination of dissolved oxygen. Electrochemical sensor method.
The detection and quantification limits (LOD/LOQ) for each method used in the study were as follows: ammonium nitrogen—0.01/0.05 mg/L; BOD5—1.0/3.0 mg O2/L; nitrate nitrogen—0.005/0.02 mg/L; pH—instrument range ± 0.1 pH; petroleum products (fluorimetric method)—0.01/0.05 mg/L; COD—5.0/15.0 mg O2/L; dry residue—5.0/15.0 mg/L; phosphate ion—0.01/0.05 mg/L; phenols—0.001/0.005 mg/L; zinc—0.002/0.005 mg/L; anionic surfactants—0.002/0.005 mg/L; suspended solids—2.0/5.0 mg/L; dissolved oxygen—0.1/0.5 mg/L.

2.3. Methods of Sewage Sludge Analysis

A JSM-5610 LV scanning electron microscope equipped with an EDX JED-2201 energy-dispersive X-ray spectrometer (JEOL, Tokyo, Japan) was used to study the sediment microstructure, its surface morphology, and its elemental composition. The voltage was 20 kV. Before scanning electron microscopy (SEM) analysis, the sediment samples were air-dried at room temperature, ground to a fine powder, and mounted on aluminum stubs using conductive carbon tape. To prevent charging effects during imaging, the samples were sputter-coated with a ~10 nm gold layer using a Quorum Q150R ES sputter coater (Quorum Technologies Ltd., Laughton, UK).

3. Results

3.1. Analysis of Water Supply and Sanitation Systems

The design of the internal water supply and sewerage networks of the projected facility was prepared in accordance with the current TKP based on the design assignment, technical conditions for connection to the domestic sewerage system, and technical conditions for the design of domestic and drinking water supply, in accordance with the architectural and construction part of the project. According to the design, the internal water supply and sewerage networks include the following systems: B1—domestic and drinking and firefighting water supply; T3, T4—hot water supply; K1—domestic sewerage system; K2—internal drainage system; K3—industrial sewerage system; B1—domestic and drinking water supply; VT—technical water supply. The cold water supply system B1 is designed to meet the domestic, drinking, and firefighting needs of the building. The source of domestic and drinking water supply is the city water supply network. A general water meter is installed at the inlet to the building. The hot water supply system is designed to provide hot water to the building’s consumers. The source of hot water supply is the boiler room. The hot water supply system is designed with circulation along the main line. The K1 domestic sewerage system is designed to drain domestic wastewater from sanitary devices into the externally designed domestic sewerage network. The K3 industrial sewerage system is designed to drain industrial wastewater from process equipment (trap washers) into the externally designed domestic sewerage network. The total water consumption for the facility is presented in Table 1.
The main water consumers of the facility are engineering and technical workers (administration), production, showers in group units, the canteen, and production needs. At maximum water consumption, the average daily flow rate was 9.13 m3/day, which is almost two times lower than the design value.
Data on water disposal are given in Table 2.
The following sewerage systems are provided for the disposal of wastewater: domestic; industrial sewerage; internal drainage system. Domestic sewerage system K1 is designed to discharge domestic fecal wastewater from the building’s sanitary fixtures into the external domestic fecal sewerage network. Industrial sewerage system K3 is designed to discharge wastewater from industrial equipment. Domestic and industrial wastewater are combined into a common pipeline at the building’s outlet and flow into the receiving well of the treatment facilities. The treatment facilities are a station for complete biological treatment of domestic wastewater. The facility is supplied fully factory-ready in a self-supporting fiberglass casing. The unit consists of the following six blocks (in the direction of water movement): equalizer block, first stage bioreactor block, second stage bioreactor block, third stage bioreactor block, settling block with a clarifying filter, and sludge stabilization block. The unit body is made of wall polypropylene PP80, reinforced with appropriate stiffeners and elements that ensure the required strength and reliability of the structure as a whole (Figure 1). The upper part of the unit, with existing process hatches, is located above ground level, which allows for unimpeded subsequent maintenance, monitoring of the station operation, and water sampling. The supporting structure of the bottom is a reinforced concrete base. The unit removes coarsely dispersed impurities, averaging, anaerobic–aerobic biotechnology for deep wastewater treatment with nitri-denitrification, using immobilized microorganisms. The wastewater treatment facility ensures reliability and stability of operation in various operating conditions, including uneven flow of wastewater; it is easy to maintain and does not require highly qualified personnel. These conditions are ensured by the treatment technology adopted in the treatment facilities using immobilized loading on an inert carrier, which is a fibrous synthetic loading of the “Yersh” type [19]. The loading installed in the treatment facilities allows maintaining the required concentration of activated sludge due to the attached microflora and stabilizing the operation of the treatment facilities under conditions of uneven wastewater flow, both in terms of flow rate and in terms of concentrations and composition of pollutants at the entrance to the treatment facilities. Due to the immobilization of microorganisms on the synthetic loading, a biological community is formed that is most adapted to this type of pollutants, which ensures high rates of their removal from the composition of wastewater. The use of synthetic fibrous loading of the “Yersh” type [19] allows maintaining the concentration of activated sludge in bioreactors within 6 g/L, prevents the washout of activated sludge from biological structures during salvo discharges of wastewater. In addition, in case of air supply interruption for aeration of activated sludge, the presence of attached microflora provides a significantly longer lifespan of attached microorganisms compared to free-floating sludge and, what is very important, a quick start-up of treatment facilities and their bringing to the operating mode. An important factor of synthetic loading is the formation of three types of microorganism communities in the biological treatment system-attached, free-floating, and oriented, which provides high specific capacity and stability of the treatment facilities.
Removal of coarsely dispersed impurities and averaging. Wastewater enters the guide ring, where centrifugal forces separate coarsely dispersed impurities. Then, the wastewater enters the equalizer, where air mixes the mixture of incoming wastewater and oversludge water from the sludge stabilizer. From the equalizer, the wastewater is uniformly fed to biological treatment by airlifts. Biological treatment: Activated non-returnable sludge is the main active biocenosis in the first stage of biological wastewater treatment. In the first stage of the bioreactor, the biocenosis immobilized on polymer brushes operates, including the denitrifying biocenosis. In the first stage, the BOD5 of the wastewater is reduced by oxidation and dilution with recirculating activated sludge from the secondary settling tank. The runoff in the form of a sludge mixture flows from the first to the second stage by gravity. In the second stage, oxidation of organic matter continues and nitrification of ammonium nitrogen occurs. The sludge mixture from the second stage of the reactor flows by gravity into the third bioreactor, where additional oxidation of organic pollutants and nitrification of residual ammonium nitrogen occur. Then, the wastewater flows by gravity into the secondary vertical settling tank, where it is divided into two streams. One stream in the form of a sludge mixture returns to the first stage of the bioreactor, and the second stream in the form of clarified liquid enters the post-treatment section as part of the clarification filter. Recirculation of activated sludge between the stages of the unit is provided.
Stabilization and treatment of sludge. Excess active sludge from the secondary settling tank is sent to the sludge stabilizer, where it is aerated. In order to increase the concentration of stabilized sludge, a thin-layer sludge separator is provided for clarification of the above-sludge water, with its subsequent supply to the equalizer. As the stabilizer fills, but at least twice a year, it is necessary to remove about 2–3 m3 of the stabilizer contents. The sediment is removed by a specialized organization for disposal.
According to the accompanying documentation, the WWT facilities (Figure 2a–c) ensure the degree of purification of domestic wastewater to the following values: pH—6.5–8.5; suspended solids—15.0 mg/L; BOD5—15.0 mgO2/L; phosphates—0.2 mg/L; nitrates—3.0 mg/L.
The domestic sewerage system is designed to divert domestic wastewater from the building’s sanitary fixtures into the designed domestic sewerage networks with further flow to the biological wastewater treatment plant, after which the purified effluents enter the nearby drainage ditch (Figure 2). After being discharged into a drainage ditch, it flows into the Zhest River in the southeast of the enterprise after about 1700 m. Rain and meltwater from the roof of the building are discharged into the externally designed storm sewer network through a system of internal drains, a closed network. A closed storm sewer system is provided for the drainage of rain and meltwater, with the direction of the flow into storage tanks, after which the organized flow is pumped out in uniform portions to the treatment facilities (Figure 2c), with further discharge into a nearby drainage ditch (Figure 2a). The treatment facilities include a sand and silt separator and a gasoline and oil separator (which is a standard solution). The project provides for separate drainage of runoff from the roof of the building and from the territory adjacent to it. Rain and meltwater from the roof are also discharged into a nearby drainage ditch.

3.2. Characteristics of Wastewater and Treatment Facilities of the Facility

The enterprise’s treatment facilities are located on the southeastern side of the production and administrative building (Figure 2a). Industrial analytical monitoring of wastewater is not performed at the enterprise. As part of the work to analyze the operation of the treatment facilities, three samples of wastewater were collected: (i) wastewater after washing production areas—WW1; (ii) wastewater at the inlet to the treatment facilities—WW2; (iii) wastewater at the outlet of the treatment facilities—WW3. During sampling, the treatment facilities operated at intervals of 1 h (1 h of operation of the aeration and pumping system—1 h of downtime). This regime was implemented as a temporary measure due to system overload and unstable biological activity in response to shock loads of industrial wastewater. Under normal conditions, continuous operation is expected according to the original equipment design. Wastewater was collected at lunchtime on a working day 30 min after the start of the operation of the facilities. The selected samples were analyzed in the analytical control laboratory for the following parameters: surfactants, BOD5, COD, oil products, zinc, suspended solids, phosphates, total phosphorus, phenols, mineralization, ammonium ion, nitrate ion, and hydrogen index. Table 3 presents the results of laboratory tests. A separate column of the table indicates discharge standards according to the technical specifications for the purchase of treatment facilities.
The results show that only the phosphorus–phosphate indicator does not comply with the requirements set by the technical specifications for the purchase. The discharge standards are exceeded by 11.6 times. The highest concentrations of discharges of ammonium nitrogen, phosphate–phosphorus, and total phosphorus are observed in domestic wastewater, and surfactants, BOD5, COD, suspended solids, oil products, dry residue, phenols, and zinc are observed in industrial wastewater. It should be noted that the wastewater also contains pollutants that are not typical for domestic wastewater, which can lead to the death of microorganisms in treatment facilities. For example, if we compare the HELCOM recommendations, then biogenic elements should be removed at treatment facilities:
−
At least by 80% for BOD5, or ensure that the BOD5 concentration at the outlet of the treatment facilities does not exceed 15 mg/L (corresponds);
−
At least by 90% for total phosphorus, or ensure that the total phosphorus concentration at the outlet of the treatment facilities does not exceed 0.5 mg/L (does not correspond to the HELCOM recommendation in the existing treatment technology);
−
At least by 70% for total nitrogen, or ensure that the total nitrogen concentration at the outlet of the treatment facilities does not exceed 10 mg/L (does not correspond to the HELCOM recommendation in the existing treatment technology).
The data presented in Table 3 indicate low treatment efficiency for total phosphorus and total nitrogen.
The oxidation state of ammonium nitrogen, the degree of removal of which was 62%, is low, as a result of which its significant concentrations are discharged into the drainage ditch. Insufficient purification of such biogenic substances leads to eutrophication of the receiving water.
Thus, the main problem of the structures in terms of wastewater treatment efficiency is the low efficiency of phosphorus and nitrogen compounds removal. This may be due to the death of microorganisms of the biological load due to a number of reasons described below.
Since the discharge of industrial wastewater is of a salvo nature, periodic significant excesses of concentrations of the studied indicators should be expected. Figure 3 shows the graphs of loads for the substances specified in the technical specifications for the purchase of treatment facilities. Dotted lines and additional designations (ds—discharge standard) in the legend show load curves considering the requirements of the technical specifications; without this designation—actual load values. In the calculations, the volume of water disposal of domestic and industrial wastewater is assumed to be equal to the volume of water consumption since the water meters are only at the inlet.
The ratio between the biogenic loads: COD/ammonium nitrogen was 1.20, and BOD5/ammonium nitrogen was 0.24, which characterizes the nitrogen content as high. The COD/total phosphorus ratio was 20.2, and BOD5/total phosphorus was 4.0, which defines the ratio as low and the phosphorus content as high. The BODtotal/COD ratio can be used to determine the suitability of wastewater for biological treatment. The optimal range of the BODtotal to COD ratio was from 0.4–0.5 to 0.7–0.75. The most suitable value for the COD to BOD ratio is considered to be 0.7, at which the biological treatment processes are optimal and complete. After the biological treatment stage, the BODtotal/COD ratio usually decreases to 0.1–0.2 since the organic substances subject to biochemical oxidation are practically no longer present in the wastewater. In the wastewater under consideration, fed for treatment, the BODtotal/COD ratio was 0.28 (BOD5 was taken to be 70% of BODtotal). This value indicates a low proportion of bio-oxidizable substances in the wastewater. The inverse ratio was also used to assess the content of biodegradable substances; in particular, the COD ratio for wastewater suitable for biological treatment should not exceed BODtotal by more than 1.5 times, and for BOD5 it should be no more than 2.5 times. As for biological treatment facilities designed to purify a mixture of domestic and industrial wastewater, the COD to BOD5 ratio in the wastewater fed for treatment is usually from 1.5 to 2.6 [37]. In wastewater mixed with a significant amount of industrial wastewater, this ratio increases to 3.5. For the wastewater under consideration, this ratio was 5.1. Thus, this ratio in the effluents of some industries can reach 8. The ratio between the COD and BOD values is one of the fundamental characteristics of the effluents when choosing a treatment scheme. For the ratios obtained, it is recommended to provide for physical and chemical treatment facilities. Since the enterprise employs about 50 people per shift (in order to save on wages), which is 13–15 times less than the design value, and the production volume is equal to the design; this indicates a lower ratio of the volumes of domestic and industrial wastewater from the design. And this, in turn, leads to an increased load on the treatment facilities for pollutants in the composition of industrial wastewater and on the microbial communities used in the bioreactor, which also creates a high risk of their inactivation and death. During inspections of the treatment facilities and sampling, no odor characteristic of domestic wastewater was noted either in the receiving well or in the bioreactor. At the same time, there was a persistent smell of organic solvents, and the wastewater had a dark black color. As a result of the examination, in addition to the absence of the characteristic smell of household wastewater, the presence of black sediment on all internal parts of the structures was noted (Figure 4).
Elemental composition of the sediment in % by weight (based on SEM-EDS): C—32.58; O—29.38; Si—13.10; Ca—10.09; Al—3.65; Fe—2.54; P—2.26; Mg—1.55; Zn—1.51; K—0.92; S—0.89; Ti—0.69; Na—0.43; Cl—0.38. Elements like Si, Ca, Zn, Fe, Al, and Ti are not surprising based on the specific reagents and fillers used in the rubber making process. For instance, the Si comes from silica-based reinforcing fillers, the Ca from calcium carbonate (chalk), which is an extender, the Zn from zinc oxide which acts as an activator in vulcanization, and the Fe probably comes from metal-reinforced rubber items or corrosion. Titanium (Ti) came from TiO2-based pigments and fillers, while aluminum (Al) either came from alumina-based or coagulant products. The high organic carbon and oxygen content confirm degraded rubber residues and organic additives. From the presented elemental analysis, it is evident that it is largely a mixture of inorganic compounds, which are an integral part of the recipes for the manufacture of rubber products. Wastewater from rubber production has a complex composition and contains a wide range of organic and inorganic compounds that can have a toxic effect on the microorganisms of activated sludge. Organic pollutants include phenolic compounds, aromatic hydrocarbons, vulcanizing agent residues, and synthetic surfactants. It is known that such compounds can significantly suppress nitrification processes, reducing the activity of ammonium-oxidizing bacteria by 50–70% and disrupting the stability of the microbial community [38,39]. Among inorganic components, heavy metals (Zn2+, Cu2+) are particularly dangerous; their concentrations above 10–20 mg/L can inhibit the metabolic activity of activated sludge [40], as well as titanium oxide (TiO2), often found in process mixtures and fillers of rubber products. TiO2 can reduce the efficiency of ammonium nitrogen removal by up to 22%, changing the composition and structure of microbial populations involved in biological treatment [41,42]. Titanium dioxide (TiO2), a commonly used pigment and filler in rubber recipes, has exhibited sub-lethal toxicological effects on activated sludge communities in part due to physical adsorption to microbial surfaces, which resulted in cell membrane damage, oxidative stress and disruption of enzymatic pathways encompassing nitrification [43,44]. TiO2 nanoparticles have also affected microorganisms by altering the microbial community structure, subsequently affecting the abundance of ammonia-oxidizing bacteria. Eventually, phenolic compounds can passively diffuse through the microbial cell membranes at low concentrations due to their lipophilic features, causing intracellular acidification, protein denaturation, and inhibition of key metabolic enzymes [45]. Within biological treatment systems, these lead to nitrification inhibition, reduced sludge activity, and delayed recovery in the community, after disturbances. These data indicate that the toxicity of wastewater from rubber production is due to the synergistic action of organic and inorganic inhibitors, which makes biological treatment without preliminary physicochemical stages unpromising.

4. Discussion

At present, at the considered enterprise for the production of rubber products, industrial wastewater is discharged together with domestic wastewater. This wastewater is treated at biological treatment facilities. However, the ratio of domestic wastewater to industrial wastewater is less than the design value. This gives an increased load on the biological treatment facilities, and the presence of increased concentrations of pollutants that are not typical for domestic wastewater has a detrimental effect on the biological load. This is confirmed by high values of the COD/BOD5 ratio, increased concentrations of phosphorus and nitrogen compounds at the outlet of wastewater that has passed through the treatment facilities, and the virtual absence of the odor typical of domestic wastewater, instead of which one can smell organic solvents and other compounds. The technical specification for the purchase, drawn up by the design organization, specifies “Domestic wastewater treatment facilities”. The installed treatment facilities are such, however, due to the increased content of pollutants that are not typical for domestic wastewater; the treatment facilities do not work at the declared efficiency, which is also associated with a possible partial or complete death of microorganisms of the biological load. In the passport of the treatment plant, the supplier of this equipment should add requirements for the composition of wastewater entering the treatment and the absence of a guarantee for non-compliance with these requirements. Distribution of responsibility between several contractors during the implementation of complex projects, on the one hand, reduces the risks of a complete failure of the project due to the diversification of responsibilities, but on the other hand, increases the likelihood of errors at the junction of stages of work, when interaction and coordination between different performers are insufficient. This example shows that when designing and operating facilities, it is necessary to consider the most pragmatic scenarios for their implementation: in the case under consideration, the actual number of workers was more than 10 times less than the design one, which led to a sharp reduction in the volume of domestic wastewater and, as a result, a significant change in their composition, which negatively affected the operation of the treatment facilities.
It is also necessary to consider the fact that industrial wastewater is almost discharged in one salvo, introducing significant quantities of pollutants that are not typical for domestic wastewater. Each such discharge could/may prove fatal for the microorganisms of the biological load of the treatment facilities, despite the presence of a receiving well at the entrance to the treatment facilities. According to the data received from the enterprise, wastewater from the circulating water supply systems is also discharged once a year.
Among the recommended options for increasing the efficiency and reliability of the treatment facilities, the following can be proposed: (i) provide for uniform supply/dosing of industrial wastewater from the accumulating tank to the receiving well, while the efficiency of wastewater treatment should be additionally monitored for some time; (ii) provide for preliminary local treatment of industrial wastewater before mixing it with domestic wastewater; (iii) separate the flows of industrial and domestic wastewater into separate local treatment facilities. The first steps are to shut down and clean the existing biological treatment facilities and replace the biological load.

5. Conclusions

The conducted analysis of the treatment facilities of the enterprise manufacturing rubber products showed that the existing biological installation does not provide the standard degree of purification for the combined discharge of domestic and industrial wastewater. Despite the high efficiency of removing some pollutants up to 91.2% for surfactants, 84.4% for oil products, and 63.4% for COD, the system demonstrates unsatisfactory results for biogenic elements. The concentration of phosphorus–phosphate at the outlet was 2.32 mg/L with the standard of 0.2 mg/L (exceeding by 11.6 times), and the degree of removal of ammonium nitrogen did not exceed 62%. The key reasons for the insufficient operation of the installation are as follows: (i) low biodegradability of wastewater, as evidenced by the high ratio of COD/BOD5 = 5.1 with the recommended value of less than 2.6, which indicates the predominance of difficult-to-oxidize organic compounds; (ii) the salvo nature of industrial wastewater discharge, leading to instability of processes in bioreactors and periodic death of activated sludge; (iii) wastewater toxicity for the microbial community, confirmed by the presence of phenols (0.008 mg/L at the outlet) and inorganic sediment with a high content of Si (13.1%), Ca (10.1%), Zn (1.5%), and Fe (2.5%), which indicates the intake of industrial additives and fillers; (iv) the absence of preliminary local treatment, as a result of which industrial pollutants enter directly into the biological circuit, for which they are not intended. Among the examples, a two-stage UASB + DHS configuration was piloted in Vietnam and achieved 98.6% COD removal and 98% TSS removal, while methane recovery provided ~95% energy self-sufficiency [46]. Another Vietnamese system employed a pre-treatment canal inoculated with natural rubber, before combining ABR and DHS reactors, and was able to recover ~16% residual rubber and meet discharge standards for removing COD and nitrogen (~87%) [47]. In China, an EPC plant for wastewater of rubber additives combined ozonation, the A/O bio-process, and Fenton oxidation processes to remove >97% COD and >98% of the aniline/nitrobenzene pollutants for ~0.71 USD/m3 [48]. An Indonesian study successfully treated wastewater from rubber plants using Fenton reagent and activated carbon adsorption to achieve ~95% COD reduction while meeting governmental compliance [49]. To improve efficiency, it is recommended to implement physicochemical pre-treatment (coagulation, sorption, flotation) to reduce COD and remove oil products and metals before the biological stage, as well as to organize an accumulation tank with a uniform supply of wastewater to the unit. It is also advisable to consider separating the flows of domestic and industrial wastewater, which will allow the treatment to be adapted to different contaminant compositions.
Thus, the failure analysis confirmed that the use of standard biological structures for the treatment of rubber production wastewater without combining it with physical and chemical methods does not allow achieving standard values for phosphorus and nitrogen and creates risks for the stable operation of the system.

Funding

This research received no external funding.

Data Availability Statement

All data, models, and code generated or used during the study appear in the submitted article.

Conflicts of Interest

The author declares no conflicts of interest.

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Water 17 02419 g001
Figure 1. Scheme of WWT facilities.
Figure 1. Scheme of WWT facilities.
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Figure 2. Location of treatment facilities (a): domestic wastewater treatment facilities (b); surface runoff treatment facilities (c).
Figure 2. Location of treatment facilities (a): domestic wastewater treatment facilities (b); surface runoff treatment facilities (c).
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Figure 3. Pollutant loads, kg/month.
Figure 3. Pollutant loads, kg/month.
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Figure 4. Deposits on the inner surface of a treatment plant (a), and SEM images of the sediment structure (b).
Figure 4. Deposits on the inner surface of a treatment plant (a), and SEM images of the sediment structure (b).
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Table 1. Total water consumption for the facility (according to the project).
Table 1. Total water consumption for the facility (according to the project).
SystemWater Consumption
m3/Daym3/hL/s
B1 (general)20.9810.393.51
including T3, T48.274.692.10
Table 2. Volumes of water disposal.
Table 2. Volumes of water disposal.
System NameWater Consumption
m3/Daym3/hL/s
Domestic sewerage20.8210.275.11
Industrial sewerage0.160.12–
Total20.9810.395.11
Table 3. Concentrations of pollutants in wastewater sampled during examination of wastewater treatment facilities.
Table 3. Concentrations of pollutants in wastewater sampled during examination of wastewater treatment facilities.
IndicatorsWW1WW2WW3Discharge Standard *,**, mg/LCleaning Efficiency, %
pH9.88.37.8n.e.–
Ammonium nitrogen6.3768.626.1n.e.62.0
Nitrate nitrogen<0.02<0.02<0.023–
Surfactants5.250.5020.044n.e.91.2
BOD523816.210.41535.8
COD12548230n.e.63.4
Suspended solids45.612.47.21541.9
Oil products6.760.7960.124n.e.84.4
Dry residue1253573459n.e.19.9
Phenols0.0810.0230.008n.e.65.2
Phosphate phosphorus0.5932.392.320.22.9
Total phosphorus1.084.063.71n.e.8.6
Zinc0.0160.009<0.005n.e.–
Notes: *—according to the technical specifications for the purchase of treatment facilities; **—discharge standards not established for this plant (n.e.), as follows from the permit for the discharge of treated wastewater set by the City Committee for Natural Resources and Environmental Protection.
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Romanovski, V. Failure Analysis of Biological Treatment Units Under Shock Loads of Rubber Industry Wastewater Containing Emerging Pollutants: Case Study. Water 2025, 17, 2419. https://doi.org/10.3390/w17162419

AMA Style

Romanovski V. Failure Analysis of Biological Treatment Units Under Shock Loads of Rubber Industry Wastewater Containing Emerging Pollutants: Case Study. Water. 2025; 17(16):2419. https://doi.org/10.3390/w17162419

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Romanovski, Valentin. 2025. "Failure Analysis of Biological Treatment Units Under Shock Loads of Rubber Industry Wastewater Containing Emerging Pollutants: Case Study" Water 17, no. 16: 2419. https://doi.org/10.3390/w17162419

APA Style

Romanovski, V. (2025). Failure Analysis of Biological Treatment Units Under Shock Loads of Rubber Industry Wastewater Containing Emerging Pollutants: Case Study. Water, 17(16), 2419. https://doi.org/10.3390/w17162419

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