Development of Sustainable Technology for Effective Reject Water Treatment

Abstract

This study examined a strategy for effective reject water treatment involving hydrodynamic cavitation (HC) combined with subsequent adsorption using natural zeolites. Two experiments were conducted: The first involved the selection of optimal pre-treatment conditions of HC for biodegradability and to reduce the ammonium nitrogen and phosphate content. Three inlet pressures of 3, 5, and 7 bar and two types of cavitation inducers, i.e., multiple- and single-hole orifice plates, were evaluated. Adsorption experiments were conducted in batch mode using natural zeolite, and three doses of zeolite (50, 100, and 200 g/L) and six contact times (4–24 h) were examined. In the HC experiments, the application of 3 bar pressure, a single-hole cavitation inducer, and a cavitation time of 30 min resulted in the removal of ammonia nitrogen and phosphates amounting to 26.5 and 23%, respectively. In this case, 3.6-fold enhancement in the biodegradability index was also found. In the second experiment, the use of zeolite led to a decrease in the remaining content of both ammonia nitrogen and phosphates, improving the chemical oxygen demand-to-total nitrogen ratio. The highest removal efficacy was found for the highest zeolite dose of 200 g/L and the longest cavitation time of 24 h. Under these conditions, the ammonia nitrogen and phosphate removal rates were 70 and 94%, respectively.

1. Introduction

Reject water (RW) is highly concentrated wastewater generated by sewage sludge processing. At most wastewater treatment plants (WWTPs), it is returned to the main wastewater stream without any pre-treatment. RW includes high total nitrogen (TN) and total phosphorus (TP) contents and a significant amount of organic matter. The concentration of ammonium nitrogen (N-NH₄⁺) might even reach 1200 mg/L [1]. Moreover, RW contains heavy metals, and it exhibits significant turbidity and a high content of total suspended solids (TSSs). Depending on the source, RW may also have an elevated temperature (T), and is characterized by an unfavorable proportion of the chemical oxygen demand-to-total nitrogen ratio (COD/TN < 1.0) for conventional nitrification and heterotrophic denitrification systems [2,3]. Its introduction into biological reactors thus causes many operational problems and leads to increased energy consumption. All these negative effects occur despite its small contribution to total wastewater streams—up to 2% [4]. For this reason, RW should be pre-treated separately before its recalculation into the main wastewater stream [5]. Thus far, RW treatment technologies have mainly involved biological processes, which are generally known to be more economical and effective than chemical and physicochemical methods [6]. In recent years, the ANAMMOX (anaerobic ammonium oxidation) process has gained importance in the effective treatment of high-ammonia wastewater [6,7]. The strategy presents several advantages, including lower energy requirements, avoiding the addition of an external carbon source, low reactor volume, reduced emission of greenhouse gases, and low sludge production [7]. Nevertheless, its implementation on an industrial scale encounters various challenges, including biomass loss and limited possibilities of nitrogen removal [8,9]. In recent years, much attention has therefore been paid to modifying this technology, e.g., the SHARON-ANAMMOX (Single-Reactor System For High Activity Ammonia Removal Over Nitrite–Anaerobic Ammonium Oxidation), CANON (Completely Autotrophic Nitrogen Removal Over Nitrite), DAMO (Denitrifying Anaerobic Methane Oxidation), and CANDO (Coupled Aerobic–Anoxic Nitrous Decomposition Operation) processes. However, research on these processes remains at the initial stage; moreover, there are still few installations operating on a technical scale [5,7]. Among physicochemical methods, ammonia stripping, membrane separation, and ion exchange and adsorption methods have gained the greatest importance [10]. However, all the mentioned methods present shortcomings related to their minor effect on the phosphate content (P-PO₄³⁻). To remove this pollutant, the following strategies have thus been applied: enhanced biological phosphorus removal, struvite precipitation, and electrochemical methods [11]. The advantages and disadvantages of these technologies, including energy aspects and removal rates, are presented in Table 1.

There currently remains a lack of technologies facilitating the simultaneous removal of both pollutants, ammonia nitrogen and phosphate [12,13], and new solutions for effective RW treatment are therefore constantly being sought. In recent years, the use of hydrodynamic cavitation (HC) in wastewater treatment has gained increasing importance. HC is recognized as a highly efficient method with a low energy demand and is characterized by its simple device construction and operation, relatively low operating costs, and suitability for implementation on an industrial scale [14]. It has been applied in disinfection [15], sludge disintegration [16], and the degradation of various pollutants, including pharmaceutical residues [17], pesticides [18], dyes [19], and phenolic compounds [20]. It has been used to improve the biodegradability of various streams, e.g., landfill leachate [21], municipal wastewater [22], dairy wastewater [23], and tannery wastewater [24]. Importantly, HC has been utilized as a method for improving the efficiency of ammonium nitrogen removal [25]. However, as a single method, HC can achieve up to 45% removal of ammonium nitrogen; moreover, its effectiveness is closely related to the initial concentration of the pollutant. To obtain higher efficiency and a stable process performance, it should therefore be combined with another method. Similarly to HC, zeolites have also demonstrated significant effectiveness in environmental engineering, particularly in removing ammonium nitrogen from various waste streams [26]. However, natural zeolites have indicated high selectivity for ammonia nitrogen; in turn, they have shown poor efficiency in removing the phosphates typically present in RW [13].

Table 1. Technologies for ammonia nitrogen and phosphate removal.

Technology	Removal Efficiency	Advantages	Disadvantages	Treatment Cost/Energy Usage	Reference
ANNAMOX	Up to 90% N-NH4+	High nitrogen compound removal rates Lower sludge production and hence a reduction in sewage sludge processing costs Resilience of Anammox bacteria to high nitrogen loads Significantly lower energy consumption as compared to conventional nitrification–denitrification bioreactors Reduced chemical usage due to elimination of external carbon use	Anammox bacteria need specific conditions to grow Sensitivity of Anammox bacteria to presence of inhibitors such as heavy metals Long period of bioreactor startup Minor effect phosphate removal	0.2–0.3 kWh/kg-N removed	[27,28,29]
SHARON	50–90% N-NH4+	Energy efficient technology using 40% less added carbon than full nitrification Small size of the reactors No need to provide external carbon sources to bioreactor	Partial removal of ammonia nitrogen, necessity to combine with other technologies Specific operational conditions, e.g., high temperature (30–40 °C) and pH of 7–8 Need for strict control of operating parameters, e.g., dissolved oxygen Minor effect on phosphate removal	0.8–1.3 kWh/kg-N removed	[30,31,32]
SHARON-ANAMMOX	Up to 71% N-NH4+	Highly efficient in treating high-strength ammonia side streams, e.g., reject waters, leachate, and other industrial wastewaters Compact and modular reactors as compared to conventional nitrification–denitrification bioreactors. Low sludge production More energy efficient as compared to conventional biological reactors	Complex two-stage process Each stage needs specific process conditions such as temperature and pH Sensitivity to inhibitory compounds typically presented in industrial wastewaters Use is limited to specific types of wastewaters Requires advanced monitoring systems High investment cost Minor effect on phosphate removal	-	[30,33]
DAMO	80–95% N-NH4+	Operates under anaerobic conditions, thus a low-energy-demand technology Low sludge production Simultaneous methane removal Suitability for high nitrogen loads	Long startup period Sensitivity to temperature and pH The process requires methane as an electron donor As a standalone process: low efficiency of ammonia removal, higher removal rate in combination with Annamox No effect on phosphate content	-	[34,35,36]
CANDO	80–95% N-NH4+	Energy efficiency Converts nitrogen compounds into fuel (N2O) Lower greenhouse gas emissions	The need for strict process control Needs an organic carbon source to conduct denitrification	2–3 MJ per kg of N removed	[37,38]
Air stripping	50–98% N-NH4+	Simple equipment construction Insensitive to toxic substances Efficient technique	High energy demand Requires certain conditions such as pH, temperature, and flow rate Time-consuming process No effect on phosphate content	0.3–0.8 kWh/m3	[39,40]
Hydrodynamic cavitation	Up to 45% N-NH4+	Easy operation Simple device construction Energy efficiency Possibility to remove other pollutants	As a sole method: low removal efficacy Necessity to combine with other methods to achieve high removal rates Possibility to generate toxic intermediates	0.02–0.1 USD/m3	[25]
Reverse osmosis	60–99% N-NH4+	Low energy requirement Compact and easy design High efficiency Easily adaptable to a specific wastewater composition	High cost of purchasing membranes Possibility of fouling membranes by colloidal matter and formation of biofilms on their surface Necessity to often clean the membranes The metals present in wastewater, such as Fe and Mg, might decrease membrane potential	0.50 kWh/m3	[41,42]
Microwave radiation	80% N-NH4+ 35% P-PO43−	Moderate cost of operation (as compared to other technologies) Suitable for high ammonium concentration Simultaneous removal of ammonia nitrogen and phosphates	Affected by pH and radiation time, initial ammonia concentration, and aeration Evaporation of NH3 Difficult to achieve full-scale application	4.8 kW per reactor (capacity of 5 m3/d)	[43,44,45]
Ion exchange and adsorption	80–95% N-NH4+ and P-PO43	Low cost of technology Easy operation Possibility to modify adsorbents to adequate wastewater composition Availability and diversity of adsorbents Effectively removes ammonium and phosphates Efficient technology for low levels of ammonium nitrogen and phosphates	Necessity to provide particular process conditions, e.g., pH ranges Depending on the absorbent, the removal efficiency varies significantly Additional energy and costs relating to regeneration Necessity of waste brine treatment or disposal	0.05–0.2 kWh/m3	[10]
Enhanced biological phosphorus removal	70–95% P-PO43	Reduced chemical usage Possibility of removing nitrogen compounds	Significant sensitivity to environmental conditions	0.3–0.6 kWh/m3	[46]
Struvite precipitation	70–90% P-PO43 20–30% for nitrogen compounds	Generation of product with a high fertilizer value that contains both phosphorus and nitrogen Low energy demand Efficient for high-strength wastewater	Requires specific pH values and magnesium-to-phosphate ratio Requires chemical dosing of magnesium salts	0.1–4.6 kWh/kg struvite	[47,48,49]
Electrochemical methods	Up to 98% P-PO43	Simple device construction Short hydraulic retention time Less sludge volume Adaptability to specific conditions Relatively low-cost method	High energy consumption as compared to other methods Electrode degradation: necessity to replace Necessity to maintain adequate process conditions Possibility of generating by-products	0.18–11.29 kWh/m3 for aluminum electrode and 0.24–8.47 kWh/m3 for iron electrode	[50]

Table 1. Technologies for ammonia nitrogen and phosphate removal.

Technology	Removal Efficiency	Advantages	Disadvantages	Treatment Cost/Energy Usage	Reference
ANNAMOX	Up to 90% N-NH4+	High nitrogen compound removal rates Lower sludge production and hence a reduction in sewage sludge processing costs Resilience of Anammox bacteria to high nitrogen loads Significantly lower energy consumption as compared to conventional nitrification–denitrification bioreactors Reduced chemical usage due to elimination of external carbon use	Anammox bacteria need specific conditions to grow Sensitivity of Anammox bacteria to presence of inhibitors such as heavy metals Long period of bioreactor startup Minor effect phosphate removal	0.2–0.3 kWh/kg-N removed	[27,28,29]
SHARON	50–90% N-NH4+	Energy efficient technology using 40% less added carbon than full nitrification Small size of the reactors No need to provide external carbon sources to bioreactor	Partial removal of ammonia nitrogen, necessity to combine with other technologies Specific operational conditions, e.g., high temperature (30–40 °C) and pH of 7–8 Need for strict control of operating parameters, e.g., dissolved oxygen Minor effect on phosphate removal	0.8–1.3 kWh/kg-N removed	[30,31,32]
SHARON-ANAMMOX	Up to 71% N-NH4+	Highly efficient in treating high-strength ammonia side streams, e.g., reject waters, leachate, and other industrial wastewaters Compact and modular reactors as compared to conventional nitrification–denitrification bioreactors. Low sludge production More energy efficient as compared to conventional biological reactors	Complex two-stage process Each stage needs specific process conditions such as temperature and pH Sensitivity to inhibitory compounds typically presented in industrial wastewaters Use is limited to specific types of wastewaters Requires advanced monitoring systems High investment cost Minor effect on phosphate removal	-	[30,33]
DAMO	80–95% N-NH4+	Operates under anaerobic conditions, thus a low-energy-demand technology Low sludge production Simultaneous methane removal Suitability for high nitrogen loads	Long startup period Sensitivity to temperature and pH The process requires methane as an electron donor As a standalone process: low efficiency of ammonia removal, higher removal rate in combination with Annamox No effect on phosphate content	-	[34,35,36]
CANDO	80–95% N-NH4+	Energy efficiency Converts nitrogen compounds into fuel (N2O) Lower greenhouse gas emissions	The need for strict process control Needs an organic carbon source to conduct denitrification	2–3 MJ per kg of N removed	[37,38]
Air stripping	50–98% N-NH4+	Simple equipment construction Insensitive to toxic substances Efficient technique	High energy demand Requires certain conditions such as pH, temperature, and flow rate Time-consuming process No effect on phosphate content	0.3–0.8 kWh/m3	[39,40]
Hydrodynamic cavitation	Up to 45% N-NH4+	Easy operation Simple device construction Energy efficiency Possibility to remove other pollutants	As a sole method: low removal efficacy Necessity to combine with other methods to achieve high removal rates Possibility to generate toxic intermediates	0.02–0.1 USD/m3	[25]
Reverse osmosis	60–99% N-NH4+	Low energy requirement Compact and easy design High efficiency Easily adaptable to a specific wastewater composition	High cost of purchasing membranes Possibility of fouling membranes by colloidal matter and formation of biofilms on their surface Necessity to often clean the membranes The metals present in wastewater, such as Fe and Mg, might decrease membrane potential	0.50 kWh/m3	[41,42]
Microwave radiation	80% N-NH4+ 35% P-PO43−	Moderate cost of operation (as compared to other technologies) Suitable for high ammonium concentration Simultaneous removal of ammonia nitrogen and phosphates	Affected by pH and radiation time, initial ammonia concentration, and aeration Evaporation of NH3 Difficult to achieve full-scale application	4.8 kW per reactor (capacity of 5 m3/d)	[43,44,45]
Ion exchange and adsorption	80–95% N-NH4+ and P-PO43	Low cost of technology Easy operation Possibility to modify adsorbents to adequate wastewater composition Availability and diversity of adsorbents Effectively removes ammonium and phosphates Efficient technology for low levels of ammonium nitrogen and phosphates	Necessity to provide particular process conditions, e.g., pH ranges Depending on the absorbent, the removal efficiency varies significantly Additional energy and costs relating to regeneration Necessity of waste brine treatment or disposal	0.05–0.2 kWh/m3	[10]
Enhanced biological phosphorus removal	70–95% P-PO43	Reduced chemical usage Possibility of removing nitrogen compounds	Significant sensitivity to environmental conditions	0.3–0.6 kWh/m3	[46]
Struvite precipitation	70–90% P-PO43 20–30% for nitrogen compounds	Generation of product with a high fertilizer value that contains both phosphorus and nitrogen Low energy demand Efficient for high-strength wastewater	Requires specific pH values and magnesium-to-phosphate ratio Requires chemical dosing of magnesium salts	0.1–4.6 kWh/kg struvite	[47,48,49]
Electrochemical methods	Up to 98% P-PO43	Simple device construction Short hydraulic retention time Less sludge volume Adaptability to specific conditions Relatively low-cost method	High energy consumption as compared to other methods Electrode degradation: necessity to replace Necessity to maintain adequate process conditions Possibility of generating by-products	0.18–11.29 kWh/m3 for aluminum electrode and 0.24–8.47 kWh/m3 for iron electrode	[50]

In this study, a novel strategy using HC combined with subsequent adsorption using natural zeolites has been proposed. To date, this technology has not been the subject of scientific research, and it has not been used to treat any type of wastewater. The applied HC improves the biodegradability and COD/TN ratio of treated RW and partially removes the ammonia nitrogen and phosphates. In turn, the adsorption using natural zeolites reduced the remaining contaminant content. No known technologies currently offer such benefits (Table 1), and the use of this solution may constitute a breakthrough in the treatment of RW. Compared to other known technologies, apart from easy exploitation, the possibility of scaling and adapting to the specific characteristics of wastewater may also positively influence the activated sludge process. Due to the flexibility of the presented technology, it might be used on an industrial scale, mainly in WWTPs. This method might be applied as a separate RW treatment line to deliver energy and operating cost savings for WWTPs.

This study will provide answers to the following research questions: What pressure and HC inducer will yield the most beneficial results? How will the zeolite dose and contact time affect the efficiency of ammonium nitrogen and phosphate removal?

2. Materials and Methods

2.1. Characterization of Materials

The RW for experiments was taken from the Hajdów WWTP, located in Lublin (Poland), which has an average daily flow of approx. 2500 m³/h. This facility receives the municipal and industrial wastewater from the Lublin agglomeration. This WWTP facility conducts mechanical and biological treatments with increased removal of biogenic compounds. In the sludge processing line, the following processes have been adopted: thickening, anaerobic digestion under mesophilic conditions, conditioning, and dewatering. In this study, a 35 L sample of reject water was sourced from sludge dewatering centrifuges. Immediately after collection, it was transported to the laboratory and kept in a laboratory freezer at a temperature of −4 °C. The composition of this sample is summarized in

Table 2. The characteristics of the raw reject water used in the experiment (the average value and standard deviation are given) [51].

Parameter	Unit	Reject Water
Chemical oxygen demand (COD)	mg/L	592 ± 19
Soluble chemical oxygen demand (sCOD)	mg/L	519 ± 21
Biological oxygen demand (BOD5)		8.42 ± 0.7
Total nitrogen (TN)	mg/L	1305 ± 45
Total phosphorus (TP)	mg/L	156.9 ± 16.9
Ammonia nitrogen (N-NH4+)	mg/L	1044.5 ± 71.3
Phosphates (P-PO43−)	mg/L	132.9 ± 15
Total suspended solids (TSSs)	mg/L	117.7 ± 2.1
Volatile suspended solids (VSSs)	mg/L	91.4 ± 2.4
pH	-	8.36 ± 0.17
Biological oxygen demand-to-chemical oxygen demand ratio (BOD5/COD)	-	0.094 ± 0.007
Chemical oxygen demand-to-total nitrogen ratio (COD/TN)	-	0.45 ± 0.02

.

For adsorption experiments, natural zeolite, i.e., clinoptilolite originating in the form of zeolitic tuff from a quarry near Nižný Hrabovec (Slovakia), was applied. This material was characterized by the following composition: SiO₂ (72.42%), Al₂O₃ (9.48%), K₂O (4.11%), CaO (3.67%), Fe₂O₃ (1.86%), MgO (0.54%), TiO₂ (0.26%) [52].

2.2. Experimental Methodology

To develop the technology for RW treatment, this study was divided into two experiments. Experiment 1 involved the selection of optimal pre-treatment conditions of HC for biodegradability and to reduce the ammonium nitrogen and phosphate content. In the HC experiments, three inlet pressures of 3, 5, and 7 bar were evaluated. Additionally, the effectiveness of two types of inducers was examined (Figure 1):

• A: orifice plate with 9 holes, each with a diameter of 1 mm;
• B: orifice plate with one concentric hole with a diameter of 3/10 mm.

Throughout HC, the samples were collected at the following time intervals: 0, 5, 10, 20, 30, 45, and 60 min. The operational parameters were selected according to the authors’ previous study, in which HC was applied to treat various types of wastewaters [53,54].

The chosen HC inducers were selected for their diverse geometry and hence different hydrodynamic flow conditions. As can be seen in

Table 3. The hydraulic characteristics of orifice plates used in the HC experiments.

Parameter	A	B
Number of holes	9	1
Diameter of holes	1 mm	3/1 mm
α: constriction parameter of orifice	4 mm−1	4 mm−1
β: constriction parameter of orifice	0.0023	0.0023
Number of holes	9	1
Diameter of holes	1 mm	3/1 mm
Total area of holes	7.0686 mm2	7.0686 mm2

, both plates are characterized by the same total hole areas: plate A was a multiple-hole cavitation inducer, while plate B was a single-hole cavitation inducer resembling a Venturi tube in shape.

Based on the results of HC optimization studies to achieve the most favorable result, the second experiment was performed using zeolites. In this study, the following doses were investigated: 50, 100, 200 g/L. The adopted doses were established based on the concentration of ammonia nitrogen and phosphates in the treated RW.

2.3. Laboratory Installation

The HC experiment was performed using a laboratory-scale device. This laboratory stand consisted of a circulation reservoir with an active volume of 30 L, a pump with an inverter, and a cavitation reactor with a replaceable orifice plate (Figure 1). Additionally, the installation included a control and measuring system involving piezoelectric pressure gauges, an electromagnetic flow meter, and a computer unit with installed statistical analysis software. The applied pump was provided by SIGMA (Bielsko-Biała, Poland), model EFRU 16-8-GU-042, characterized by a flow range of 65 L/s, power of 1.1 kW, head of 80 m, and pressure of 8 bar.

The treated medium, i.e., RW, was taken from the tank by a pump, and then it was directed to the cavitation reactor and finally returned to the tank. RW was circulated in a closed loop that allowed it to pass through the cavitation zone several times. The cavitation number varied between 0.033 and 0.116 depending on the HC operational parameters. Table 3 presents the hydraulic characteristics of the plates applied in the HC experiments.

In turn, the adsorption experiments were conducted in batch mode using 500 mL beakers. After preparation, the samples were incubated at a temperature of 20 °C and mixed until equilibrium was reached. The concentrations of the analyzed parameters were determined in the supernatant.

2.4. Evaluation of Process Performance

The biodegradability index (BI) was evaluated based on the biological oxygen demand-to-chemical oxygen demand ratio (BOD₅/COD ratio). Moreover, in the RW samples taken at the specific time intervals, the following parameters were monitored: ammonia nitrogen (N-NH₄⁺), phosphates (P-PO₄³⁻), total nitrogen (TN) and phosphorus (TP), COD, soluble chemical oxygen demand (sCOD), total suspended solids (TSS), volatile suspended solids (VSS), pH, temperature (T), and COD/TN ratio.

Adsorption capacity (q) and contaminant removal efficiency (RE) were determined according to the following equations:

$$RE={\displaystyle \frac{\left(C_i-C_t\right)}{C_i}}\times 100\%,$$

(1)

where C_i and C_t are the concentrations determined initially and at a specific time interval of the selected parameter, and

$$q={\displaystyle \frac{\left(C_i-C_e\right)V}{m}},$$

(2)

where C_e is the equilibrium concentration of the selected parameter (mg/L), V is the volume of the solution (L), and m is the dose of the applied adsorbent (g).

All analyses were performed using a Hach Lange UV–VIS 218 DR 3900 (Hach, Loveland, CO, USA) and standard cuvette tests according to the protocol provided by the producer. The pH values and temperature were monitored using an HQ 40D Hach-Lange multimeter (Hach, Loveland, CO, USA).

2.5. Kinetic Evaluation

To evaluate the kinetics, two models were chosen, i.e., pseudo-first-order (PFO) Equation (3) and pseudo-second-order (PSO) Equation (4), both indicating the best fit to the experimental data.

$$q_t=q_e(1-\mathit{exp}\left(-k_{1}t\right)),$$

(3)

$$q_t={\displaystyle \frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}}$$

(4)

where q_e and q_t are the adsorption capacities at equilibrium and specific time t (mg/g), respectively, and k₁ and k₂ are the rate constants of the pseudo-first-order and pseudo-second-order models (h⁻¹), respectively.

2.6. Energy and Cost Evaluation

An energy evaluation was conducted on the applied laboratory device, allowing 30 L to be treated in one cycle. The energy demand (E) is the sum of the energy needed to perform HC (E_HC) and adsorption using zeolites (E_A), including the energy expenditure associated with zeolite regeneration (E_R). The regeneration of zeolite was to be conducted using a NaCl dosage of 40 g/kg and a cycle duration of 2 h. It was planned to conduct regeneration every 2 weeks.

The energy usage to conduct HC (E_HC) was recorded from the pump inverter controlling the pump operation during the experiment. In turn, E_A and E_R were established on the following equations:

$$E_A=P·t$$

(5)

$$E_R=P_R·t_R$$

(6)

where P is the power of the low-speed stirrer (W), t is the time of mixing (h), P_R is the power output of the pump (W), and t_R is the time to perform regeneration (h). The cost of materials (C), e.g., zeolite and NaCl, was evaluated based on the following equation:

$$C=M·p$$

(7)

where M is the mass of zeolite and NaCl, and p is the price of the materials.

2.7. Statistical Analyses

All presented values are means ± SD (n = 5). Differences between treatments were examined using a one-way ANOVA, where a significance level of 0.05 was adopted. A statistical analysis was performed using the Statistica software package (version 13). The kinetic parameters were determined using a nonlinear regression method. The determination coefficient (R²) was applied to indicate the strength of the relationships between the obtained results.

3. Results and Discussion

3.1. Experiment 1

In this study, the effectiveness of three inlet pressures and two types of cavitation inducers was investigated. Regarding organic compounds, in each case, the use of HC led to the destruction of complex organic compounds, expressed as COD. Simultaneously, the release of easily biodegradable compounds was observed (Figure 2a,b).

However, major changes were achieved using 3 bar pressure, plate B (single-hole plate), and a cavitation time over 30 min. Compared to raw RW, a significant reduction in COD content of over 30% was already found after 30 min. For the soluble organic matter, an improvement in sCOD of 20–33% was observed between 30 and 60 min of HC.

Importantly, in all the analyzed cases, significant increases in the BOD₅/COD index were already noticed after 5 min of the HC experiments (Figure 2c). However, as previously described, a major improvement was found under 3 bar pressure and by applying inducer B. In this study, as compared to raw RW, over 4- and 5-fold increases were obtained for a cavitation duration over 30 min. The improvement in the BI is related to the physical breakdown of complex organic compounds. The bubble collapse accompanying HC generates strong shear forces and micro-turbulence that have the potential to break large and complex organic molecules. Another mechanism is combined with the influence of highly reactive radicals that have the potential to oxidize recalcitrant compounds into more biodegradable intermediates, resulting in an improvement in both sCOD and BOD₅. This effect was also achieved for various industrial wastewaters, e.g., dairy [23] and tannery [24] effluents.

The enhancement of RW biodegradability and observed solubilization are particularly beneficial because it might result in the more efficient breakdown of organic matter, providing a better carbon source for denitrifying bacteria, which could enhance the nitrogen removal efficiency [55].

The COD/TN ratio is considered a crucial parameter in conventional biological nutrient removal processes that directly affects the growth competition between both autotrophic and heterotrophic microorganisms [56]. Previous studies demonstrated that in effective denitrification, the COD/TN ratio should exceed the value of 4 [5]. In this study, the COD/TN ratio was also investigated. Following HC, an improvement in the ratio occurred and major growths were found under 7 bar pressure and by applying inducer A, as well as under 3 bar pressure and by applying inducer B for the longest cavitation time. Generally, the prolongation of time facilitates multiple passes through the cavitation zone and therefore a stronger cavitation intensity. Within the HC of RW, organic nitrogen compounds are broken down into ammonia and smaller amine groups.

The degradation of contaminants within HC experiments involved several mechanisms, including the mechanical (shear stress), chemical (free radicals), and thermal effects (hot spots) caused by bubble collapse. However, there are two dominant pathways of organic compound destruction within HC experiments. In the first, it is combined with the influence of free radicals that might react with the pollutant due to oxidation capacity, resulting in partial or total degradation (Figure 3). In turn, the second is related to a pyrolysis effect that might occur inside or near the cavitation bubble [57,58]. The intensity of HC and the properties of the treated compound have a major influence on which mechanism of degradation dominates [59].

The obtained results indicated that both pressure and the type of applied inducer have a strong influence on HC effects. In this study, a major improvement in the BI was achieved under 3 bar pressure and by applying a single-hole inducer (B). This type of plate resembles a Venturi tube in shape, and in this case, with a high intensity at a single point and larger bubbles, a more energetic collapse resulting in denser cavitation might be achieved. The use of a single-hole inducer favors a physical effect, e.g., thermal decomposition, shear force and shock wave influence, and the effect of pressure gradient, rather than a chemical one. In the case of multiple-hole inducers, more enhanced chemical effects occurred related to hydroxyl radicals and reactive hydrogen atoms oxidation. This fact is combined with smaller bubbles and faster collapses that favor the generation of free radicals [58,59].

Importantly, the use of higher pressure did not result in a more efficient destruction of organic compounds. This fact is particularly important considering the energy aspect of the developed technology. Several factors might influence the HC effects: the first group is related to the properties of the treated medium, e.g., temperature, pH, volatility, and viscosity, and the initial concentration of the contaminant; the second group is related to the applied operational parameters of HC, e.g., pressure, number of passes through cavitation zone, and type of reactor [55]. The application of too high pressure may lead to the “supercavitation” or “choked cavitation” phenomenon, whereby the cavities form very intensively but collapse less violently. This leads to a loss of collapse intensity, which reduces the production of hydroxyl radicals and other mechanical effects. Another aspect might be related to the use of higher pressure possibly resulting in shockwaves dissipating in larger vapor zones rather than targeting pollutant molecules. Additionally, the higher pressure might entail shortening the collapse time, leading to a less energetic collapse [59].

In

Table 4. The variation in TSSs, VSSs, pH, and T within HC experiments (average value and 95% confidence limits are given).

Time	TSSs	VSSs	pH	T	TSSs	VSSs	pH	T	Energy Usage
min	mg/L	mg/L	-	°C	mg/L	mg/L	-	°C	kWh
	3 bar
	CAVITATION INDUCER A				CAVITATION INDUCER B
0	94 ± 2.1	86 ± 2.7	8.27 ± 0.06	16.1 ± 0.2	91 ± 7.1	69 ± 4.1	8.57 ± 0.07	16.5 ± 0.2	0
5	142 ± 3.7	103 ± 2.9	8.41 ± 0.10	16.9 ± 0.4	134 ± 7.3	89 ± 3.3	8.61 ± 0.11	17.1 ± 0.4	0.041
15	202 ± 2.1	103 ± 4.1	8.48 ± 0.10	18.6 ± 1.2	156 ± 8.9	125 ± 9.0.7	8.65 ± 0.08	17 ± 0.8	0.079
30	212 ± 4.7	116 ± 5.1	8.62 ± 0.06	22.4 ± 0.6	190 ± 8.9	106 ± 8.7	8.68 ± 0.01	18.6 ± 0.7	0.241
45	203 ± 5.3	116 ± 6.1	8.65 ± 0.11	25.6 ± 0.7	179 ± 11.2	126 ± 9.8	8.71 ± 0.05	21 ± 1.3	0.350
60	214 ± 3.8	131 ± 6.0	8.74 ± 0.08	28.6 ± 0.5	258 ± 13.7	168 ± 10.4	8.78 ± 0.11	22.5 ± 1.7	0.486
	5 bar
0	86 ± 2.2	81 ± 3.0	8.42 ± 0.11	18.7 ± 2.1	98 ± 8.1	73.18 ± 2.9	8.31 ± 0.06	17.9 ± 1.2	0
5	132 ± 3.1	99 ± 1.7	8.50 ± 0.12	20.5 ± 2.0	156 ± 10.1	140.4 ± 3.8	8.38 ± 0.05	18.7 ± 1.1	0.059
15	135 ± 4.2	99 ± 3.1	8.59 ± 0.04	22.8 ± 2.5	168 ± 14.0	144 ± 10	8.40 ± 0.03	20.1 ± 2.0	0.121
30	136 ± 3.6	140 ± 4.2	8.67 ± 0.09	26.9 ± 2.7	189 ± 14.2	156 ± 8.1	8.42 ± 0.12	24.1 ± 1.0	0.351
45	131 ± 4.1	136 ± 3.9	8.76 ± 0.11	29.3 ± 3.2	209.4 ± 15.9	174 ± 8.0	8.45 ± 0.06	25.7 ± 2.0	0.515
60	135 ± 4.1	133 ± 3.0	8.77 ± 0.08	31.9 ± 3.5	249 ± 15.1	189 ± 9.1	8.50 ± 0.11	29.7 ± 2.2	0.689
	7 bar
0	86 ± 2.5	82 ± 4.3	8.36 ± 0.02	18.1 ± 3.5	96.8 ± 7.9	71 ± 3.3	8.23 ± 0.07	18 ± 1.9	0
5	134 ± 3.0	102 ± 5.1	8.45 ± 0.11	25.0 ± 1.7	126 ± 14.0	124 ± 9.57	8.22 ± 0.09	24.1 ± 2.1	0.069
15	137 ± 2.3	104 ± 2.5	8.49 ± 0.07	29.4 ± 2.5	210 ± 17.5	164 ± 10.7	8.27 ± 0.12	28.8 ± 2.3	0.161
30	139 ± 4.5	110 ± 4.5	8.53 ± 0.07	34.5 ± 3.7	222 ± 24.3	167 ± 143.9	8.35 ± 0.11	34.1 ± 3.5	0.483
45	140 ± 3.0	115 ± 6.0	8.61 ± 0.11	37.1 ± 1.9	232.2 ± 20.5	177 ± 15.3	8.44 ± 0.09	37.5 ± 3.7	0.703
60	143 ± 5.9	120 ± 6.0	8.78 ± 0.07	39.4 ± 2.0	246 ± 21.1	212 ± 18.1	8.52 ± 0.11	38.9 ± 3.9	0.923

, a gradual increase in temperature can be observed under each applied pressure and cavitation inducer within HC experiments. It should be emphasized that the applied laboratory installation to conduct HC experiments was not equipped with a cooling system. This thermal effect is closely associated with the cavitation bubble collapse; additionally, the generated shockwaves might release a significant amount of energy [58]. Major growths were achieved for both cavitation inducers and the highest pressure of 7 bar. Moreover, with the prolongation of time, an increase in the pH values occurred, which is related to the influence of hydroxyl radicals generated within HC experiments. Similarly to T, major growths for HC inducers were observed under the highest pressure of 7 bar, which is related to the enhanced cavitation intensity under increased pressure.

Both TSS and VSS contents increased via HC, and with the extension of time, the contents of both parameters increased. Importantly, the high content of those parameters might have affected the second stage of treatment and hence resulted in increasing the applied zeolite dose. For the next experiment, prolonged times are therefore not recommended. The enhancement of both TSSs and VSSs might be related to the physicochemical effects of HC, in particular the high shear forces, shock waves, and microjets that disturb the organic and mineral particles suspended in this wastewater [20].

The influence of HC on both TN and ammonium nitrogen was also investigated in this study. As shown in Figure 4a,b, for both compounds, a major effect was achieved under 3 bar pressure and by applying a single-hole cavitation inducer (B), wherein a significant decrease in the concentrations of both indicators was already recorded after 15 min. The greatest TN removal of 36% was found for the longest cavitation time of 60 min, while for shorter cavitation times of 30 and 45 min, this indicator was established at the level of 28%. A similar trend occurred for ammonium nitrogen, where a major removal of 30% was achieved for the longest cavitation time of 60 min. However, beneficial results were also achieved for cavitation times of 30 and 45 min, where ammonia nitrogen RE reached the levels of 26.5 and 29%, respectively. Importantly, regarding ammonia nitrogen RE, there were no statistical differences between the cavitation times of 30, 45, and 60 min.

For other pressures and HC inducers, especially in the case of ammonium nitrogen, there was no obvious influence of HC on its content in RW. The exception was 7 bar pressure and the use of a multi-hole cavitation inducer (A), where a significant drop of TN content was already achieved after 5 min. In this variant, the highest removal efficiency exceeding 30% was observed for the longest cavitation time of 60 min.

The mechanism of ammonia nitrogen removal is attributed to both the physical and chemical effects of HC. The former is based on physical stripping, where HC indicates a high turbulence effect and mass transfer capabilities that allow ammonia to be stripped out of the wastewater [60,61]. The latter relates to the effect of reactive species generated within the collapse of cavitation bubbles, whereby these radicals demonstrate the potential to oxidize ammonia and ammonium ions into nitrite and nitrate, and finally into nitrogen gas [25].

In comparison to TN and ammonia nitrogen, a minor effect of HC was achieved regarding total phosphorus and phosphates (Figure 4c,d). As previously discussed, the most visible effect was observed under 3 bar pressure and by applying cavitation inducer B, wherein for the longest cavitation time of 60 min, their removal was 30%. In the case of TP, a beneficial effect was also found when using the same cavitation inductor but under a higher pressure of 7 bar. In turn, for phosphates, the removal efficiency of 20% for a cavitation time of 60 min was observed in the following variants: 3 bar pressure with cavitation inducer A, and 5 bar pressure with cavitation inducer B.

The obtained results corresponded to those from other studies. Patil et al. [25] investigated the effectiveness of ammonia nitrogen removal using a vortex diode (pressures of 0.5 to 2 bar) and an orifice plate (pressures of 2 to 5 bar). In this study, the ammonia nitrogen removal varied between 9 and 45%, depending on the initial concentration, pressure, and HC reactor type. Importantly, a significant improvement of over 80% was achieved by combining HC with sparging air or oxygen. However, the use of this method for real industrial wastewater with a high ammonia nitrogen content (2800 ppm) required a long cavitation time of 10 h. This solution is therefore unprofitable due to its significant energy demand, and a similar finding was reported by Feng et al. [62]. In this research, the use of HC alone was not efficient enough, and it was therefore combined with ozone and NaClO; additionally, up to 99.9% removal efficiency was achieved. An analogous technology based on hydrodynamic cavitation and ozonation was proposed for eutrophic water treatment [63].

HC is a rarely used treatment method for phosphorus compounds. However, its removal efficiency might be enhanced when combined with other methods, e.g., chemical and biological processes. A high removal efficiency was achieved when combining HC with the addition of calcium hydroxide. In a study conducted by Dölle and Van Bargen [64], a 20% solution of Ca(OH)₂ was added to wastewater prior to HC to precipitate calcium phosphate, achieving about a 70% reduction in phosphate using an orifice plate. HC facilitated the better dispersion of coagulants and contact with phosphate ions, enhancing the precipitation reaction.

For the next experiment, 3 bar pressure and a single-hole cavitation inducer (B) were chosen among various HC variants as being the most advantageous. However, considering the energy required to conduct HC and, consequently, the profitability of the given technology, it was decided to use a shorter cavitation time of 30 min. The energy usage therein was 0.241 kWh vs. 0.486 kWh for the longest cavitation time of 60 min. Moreover, under 3 bar pressure and applying the HC inducer B, high removal efficiencies regarding TN, TP, phosphates, and ammonia nitrogen were found. For a cavitation time of 30 min, those parameters were established at the level of 28% for TN, 23% for TP, 26.5% for ammonia nitrogen, and 23% for phosphates. In turn, the longest cavitation time resulted in an RE of 36% for TN, 31% for TP, 31.5% for ammonia nitrogen, and 31% for phosphates. Moreover, in this case, significant improvements in both the BOD₅/COD and COD/TN ratios were achieved.

3.2. Experiment 2

3.2.1. Effect of Contact Time and Dose

In experiment 2, the effectiveness of ammonia nitrogen and phosphate removal using natural zeolite was investigated. Both the contact time and the dose of the zeolite were investigated. As seen in Figure 5a, at each applied dose of zeolite, the concentration of ammonia nitrogen decreased with the prolongation of time. However, the greatest RE was observed at the highest dose of zeolite (Figure 5c), wherein a significant drop of over 50% already occurred after 4 h. Nevertheless, there were no significant changes in the ammonia nitrogen RE between 8 and 24 h, with the RE varying between 64 and 70%. Previous studies also indicated that as the equilibrium capacity approaches, adsorption proceeds more slowly. The biggest changes were therefore observed within the time interval of 0–4 h. This relates to mass transfer between the fluid and solid phase, a phase which shows the greatest dynamics. In the next stage, slow physical adsorption occurs; it is combined with ionic balance between the solid and liquid phase. The final step, i.e., chemisorption—which is much slower than the abovementioned stages—is related to the formation of a chemical bond in both the adsorbent and adsorbate [65,66].

Importantly, for the 50 and 100 g zeolite doses, there was no statistical difference in ammonia nitrogen removal. For both zeolite doses, as compared to the initial sample, a substantial decrease exceeding 20% occurred after 4 h. At the lowest doses in particular, the prolongation of time did not result in enhanced ammonia nitrogen removal. The most visible changes were observed at a dose of 100 g/L, wherein RE varied between 26 and 46%, and the largest decrease was achieved for the longest cavitation time of 24 h. Nevertheless, in this case, there were no statistical differences in RE between 16 and 24 h.

Similarly to ammonia nitrogen, a major decrease in the phosphate content was found for the highest zeolite dose (Figure 5b). As previously discussed, a significant drop of 47% already occurred after 4 h (Figure 5d), while it later reached a comparable level. However, the highest RE of 92% was found for the longest cavitation time of 24 h. Importantly, at other zeolite doses of 50 and 100 g, a significant reduction in phosphates was also achieved. For the lowest dose, RE varied between 53 and 63% for cavitation times of 8–24 h. In turn, for 100 g, this parameter was established at the level of 60–81% for cavitation times of 8–24 h.

The adsorption mechanism of ammonia nitrogen is mainly attributed to the ion exchange between NH₄⁺ and the cations, e.g., Na⁺, K⁺, and Ca²⁺, within zeolite. Another mechanism is associated with physical adsorption, whereby ammonia nitrogen might be adsorbed into the zeolite surface; however, its contribution to ammonia nitrogen removal is relatively small and strictly depends on the pH value, where the effect is more intense at a higher pH level [10,67,68].

Regarding phosphates, the predominant mechanism is related to their surface through electrostatic interactions and complexation with metal oxides, e.g., aluminum and manganese oxides [69,70,71]. Another mechanism is combined with precipitation, where in the presence of cations, e.g., Ca²⁺, Mg²⁺, and Al³⁺, phosphates can precipitate as Ca₃(PO₄)₂ or AlPO₄, and this effect is intensified when these cations are present on or near the zeolite surface [72,73]. In this study, a higher removal efficiency was achieved for phosphates, which might be related to the different mechanisms of removing these compounds and the variant properties of both ammonia nitrogen and phosphates. Generally, phosphates indicated a higher mass-based adsorption due to their larger molecular weight, as well as the creation of strong chemical interactions, particularly with metals.

Another factor that facilitates a high RE of phosphates is the influence of HC on the treated medium, in particular, enhancing chemical reactions and improving the availability of compounds. The bubbles originating within HC experiments might enhance the dispersion and increase the reactivity of reagents and contact with phosphate ions. However, further research should be conducted in this area [74]. In particular, the adsorption isotherm should be evaluated to describe the interactions between molecules and the adsorbent surface. Moreover, such studies may indicate the pollutant removal mechanism.

The obtained results are consistent with a previous study, which indicated that natural zeolites show high removal efficiency regarding ammonia nitrogen due to a high surface area, porosity, and ion exchange capacity [75]. Generally, the adsorption capacity of ammonia nitrogen using natural zeolites might vary within a wide range of 2.7–30.6 mg/g, depending on process conditions such as pH, temperature, initial concentration, and the properties of zeolite, e.g., pore structure and particle size [76,77]. In this study, relatively high doses of zeolites were applied due to the high contents of both ammonia nitrogen and phosphates; therefore, this research should be further continued with reference to the testing of other absorption materials, particularly synthetic zeolites.

The adsorption capacity of Australian natural zeolite was established at the level of 6.3 mg/kg, while the RE was 97% and the adopted contact time was 8 h. However, in this study, the treated wastewater was characterized by a relatively low content of ammonia nitrogen [78].

Natural Iranian zeolite was characterized by an adsorption capacity of 8.5–10.4 mg/kg, while the RE was 90%, the adopted contact time was 8 min, and the initial ammonia concentration was only 40 mg/L [79]. Gordes clinoptilolite demonstrated an adsorption capacity of 16.3 mg/kg and an RE of 85%. Importantly, in this study, the treated wastewater indicated a high ammonia nitrogen content of 1000 mg/L [80], and the modified zeolites also presented high efficiency. Zhao et al. [81] indicated that the addition of 3–6 g/L zeolite resulted in the high removal efficiency of TN, ammonia nitrogen, and phosphates of 60.3, 77.5, and 99.9%, respectively.

In a study conducted by Hermassi et al. [82], modified NaP1-FA and CaP1-NA zeolites were applied for phosphate removal, wherein the highest phosphate capacities of 57 and 203 mg/g were achieved for NaP1-FA and CaP1-NA, respectively. The maximum adsorption capacities of zeolite, biochar, and pyrolyzed zeolite with corn straw were established at the level of 0.69, 3.60, and 2.41 mg/g, respectively [69].

In this part of the experiment, the influence of zeolite application on the COD/TN ratio was also investigated (Figure 6). With the prolongation of time, the improvement of this ratio was observed, especially at the highest dose of zeolite. Importantly, the COD/TN ratio already exceeds the value of 1 recommended for conventional biological treatment after 8 h; however, this effect was only achieved for the highest zeolite dose. The enhancement of this ratio was attributed to a decreasing total nitrogen content resulting from adsorption by zeolite. Importantly, the natural zeolite applied in this study, i.e., clinoptilolite, indicated a relatively low removal efficiency regarding COD; moreover, it indicated low efficiency at a high COD concentration. The effectiveness of zeolite depends on several factors, including pH values, temperature, the adsorbent/solution mass ratio, and the initial pollutant concentration [83]. In this study, the highest removal efficiency for ammonium nitrogen and phosphates was obtained for the highest zeolite dose and the longest contact time. Nonetheless, a key factor in the successful implementation of this strategy is to find the optimum adsorbent dosage to achieve cost-effective pollutant removal using the applied material.

3.2.2. Adsorption Kinetics

Pseudo-first-order and pseudo-second-order kinetic models are employed to describe the dynamics occurring in the adsorption process.

Table 5. The results of kinetic evaluation in experiment 2.

Model	Parameter	Unit	Ammonia Nitrogen			Phosphates
Zeolite dose			50 g	100 g	200 g	50 g	100 g	200 g
Experimental data	qe	mg/g	5.4	3.5	2.7	1.06	0.70	0.40
PFO	qe	mg/g	6.94	4.50	3.29	1.23	0.73	0.38
	k1	h−1	0.17	0.14	0.33	0.09	0.119	0.19
	R2	-	0.998	0.981	0.998	0.991	0.997	0.993
PSO	qe	mg/g	8.52	5.64	3.65	1.78	0.98	0.466
	k2	h−1	0.02	0.03	0.14	0.04	0.09	0.36
	R2	-	0.9993	0.9871	0.9991	0.9887	0.9991	0.9945

shows that high values of R² were achieved in both models, indicating the correctness of the model selection. However, PFO demonstrated a better fit to experimental data. In both models, the adsorption capacities at equilibrium reached the highest value for the lowest zeolite dose. With the increasing zeolite dosage, the removal efficiency of both ammonia nitrogen and phosphates increased, and the equilibrium adsorption capacity decreased. This trend is attributed to the concomitant increase in the available adsorption sites and the reduction in the adsorbent/adsorbate ratio [84,85]. This tendency was also observed in a study conducted by Demiti et al. [86], wherein natural and modified zeolites were utilized for drug removal. However, when analyzing both rate constants of PFO and PSO, a major enhancement occurred at the highest zeolite dose. In this study, as compared to the lowest zeolite dose, an approximately two-fold increase in both the ammonia nitrogen and phosphates and an eight-fold increase in the adsorption rate were achieved for the PFO and PSO models, respectively. This observation relates to there being more available active sites for adsorption at the highest zeolite dose.

3.3. Energy and Cost Evaluation and Future Prospects

The key factor in successfully implementing the proposed technology on a technical scale is the estimation of energy consumption. In the first experiment, the most advantageous variant was assumed to be cavitation under 3 bar pressure, an inducer with one hole, and a cavitation time of 30 min. In the second experiment, the best results were achieved for the highest zeolite dose and a contact time of 24 h. The energy usage to conduct HC was recorded from the pump inverter controlling the pump operation during the experiment. For the selected operational parameters, it was established at the level of E_HC = 0.241 kWh. In the case of the adsorption experiments, the calculated energy was E_A = 1.2 kWh, while the energy required for regeneration was established at the level of 0.8 kWh for one cycle regeneration. The purchase cost of zeolite was estimated to be USD 30, while the cost associated with regeneration was USD 1.2.

The proposed technology is in an early stage of development and preliminary research and can thus be further developed. In future, the following aspects should therefore be investigated:

• A solution to the problem of zeolite regeneration, thus reducing the cost of reagents and the formation of waste brine;
• An analysis of zeolite in the context of the adsorbed nutrients and heavy metals;
• An evaluation of the effectiveness of using synthetic zeolites generated from wastes;
• The influence of pH, T, and agitation speed on adsorption performance;
• Isotherm studies aiming to optimize adsorption experiments and describe the occurring mechanisms within adsorption;
• The effect of the proposed technology on conventional biological treatment.

Importantly, the presented technology can be adapted to other types of wastewaters; however, in all cases, an optimization study should be conducted to select the most favorable operating conditions. In addition to a high level of pollutant removal, the development of sustainable wastewater treatment technology should include a number of factors, e.g., energy consumption, the impact of the proposed technology on the environment, and the possibility of generating various by-products.

4. Conclusions

In this study, a sustainable strategy involving HC combined with subsequent adsorption using natural zeolites, e.g., clinoptilolite, has been proposed to treat highly concentrated wastewater characterized by a significant content of ammonia nitrogen and phosphates. Compared to other known technologies, aside from easy exploitation, the possibility of scaling and adapting to the specific characteristics of wastewater and low energy demand may also positively impact the activated sludge process. The synergistic effect of this technology is related to the significant removal of pollutants with the simultaneous improvement of biodegradability and the COD/TN ratio in treated RW.

The obtained results indicated that the application of HC led to the improvement of both the BOD₅/COD and COD/TN ratios, both indicators being crucial for effective conventional biological treatment. Additionally, the partial removal of ammonia nitrogen and phosphates was achieved via HC. The most beneficial results were found under 3 bar pressure, using a single-hole cavitation inducer, and for a cavitation time of 30 min. In this study, the ammonia nitrogen and phosphate removal achieved was 26.5 and 23%, respectively. This was accompanied by a significantly improved biodegradability index (3.6-fold increase) as compared to untreated RW. In turn, the use of natural zeolite led to a decrease in the remaining content of both ammonia nitrogen and phosphates with an improving COD/TN ratio. The highest removal efficacy was found for the highest zeolite dose of 200 g/L and the longest contact time of 24 h, for which the ammonia nitrogen and phosphate removal was 70 and 94%, respectively. Moreover, a COD/TN ratio of 1.1 was established. The proposed strategy might therefore be a solution for effective RW treatment.