Coagulation Enhanced with Adsorption and Ozonation Processes in Surface Water Treatment

Abstract

The requirements for water intended for domestic and economic purposes prompt the search for new solutions in surface water treatment technologies. In this study, the enhancement of coagulation processes by connections with adsorption and/or ozonation for surface water treatment was studied. The possibility of a reduction in natural organic matter (NOM) content in modified surface water was analysed regarding changes in colour, turbidity, oxidisability (OXI), total and dissolved organic carbon (TOC, DOC) and absorbance at 254 nm (UV₂₅₄). Additionally, the changes in copper and zinc ion content during the modified surface water treatments studied were analysed for initial metal concentrations of 0.5 or 1.0 mg/L. Studies of modified surface water were carried out with doses of medium-basicity PAX coagulant (C2) 5 mg/L and PAC 100 mg/L. During the coagulation process, the colour decreased by 86–90%, turbidity by 85–90%, OXI by 65–77%, TOC by 65–76% and UV₂₅₄ absorbance by 73–84%. The enhancement of coagulation with adsorption and/or ozonation resulted in an increased efficiency of colour and turbidity removal by 2 and 4%, respectively. The highest increase in efficiencies of OXI, TOC, DOC and UV₂₅₄ removal, by 12, 12, 11 and 11%, was observed for the connection of ozonation with coagulation and adsorption. The efficiency of metal ion removal from modified water observed for the single coagulation process was 58 and 55% for copper and 46 and 43% for zinc, respectively, for initial concentrations of 0.5 and 1.0 mg/L. The intensification of coagulation with ozonation and adsorption resulted in metal removal on the level of 66 and 62% for copper and 62 and 54% for zinc.

1. Introduction

The continuously increasing demand for water used for domestic and commercial purposes has resulted in a search for new or improved conventional methods and technologies for removing pollutants from surface water, especially for pollutants that are unacceptable to consumers and also exceed the limits established by legislative acts relating to the quality of water intended for human consumption. Since water used for human consumption and economic purposes must meet rigorous standards, the sustainable use of available water sources requires the use of the most effective treatment methods. From the point of view of sustainable development, it is extremely important to select technological processes and optimise them in order to enable efficient treatment and have the lowest impact on the environment.

Coagulation is a basic technological process that has been used for many years in surface water treatment. Its primary purpose is to remove organic and inorganic colloidal contaminants and hard-to-settle suspended solids causing colour and turbidity. This process leads to colloid system destabilisation (by introducing reactants into the water) and particle dispersion degree reduction. Under technical conditions, water treatment using the coagulation method relies on the addition of a determined dose of coagulants that dissociate in aqueous solution, hydrolyse to an insoluble form and finally form complexes that react with colloids [1]. The post-coagulation suspensions formed are removed from the water by sedimentation or flotation and filtration. In the water treatment process, coagulation can mainly be caused by the production of colloids with charges opposite to those of colloids present in purified water or by the addition of electrolytes, which decreases the electrokinetic potential and facilitates colloid agglomeration [2]. During the coagulation process, natural organic matter (NOM) is also removed, either by charge neutralisation to produce insoluble forms or by adsorption on precipitated metal hydroxide [1]. NOM is defined as a complex matrix of organic compounds present in all natural waters. It consists of two basic compound groups: heterogeneous hydrophobic acids, such as humic substances represented by humic acids, fulvic acids and humins, and hydrophilic components represented by carbon- and nitrogen-containing compounds (carbohydrates and proteins, sugars and amino acids) and others [3,4]. By nature, NOM’s presence in drinking water sources is undesirable due to its unacceptable impact on water properties such as colour, taste and odour [3]. Humic substances (constituting the biodegradable part of NOM) may have a negative impact on water biological stability, potentially causing bacterial re-growth in water distribution systems [5]. NOM present in surface waters is a known precursor to the formation of highly harmful disinfection by-products (DBPs) with negative effects on fertility and possible carcinogenic effects, among which trihalomethanes (THMs) are the most widespread compounds. Organic substances present in raw water react with disinfectants, especially with chlorine, causing DBP formation [2,6]. For these reasons, in the case of the treatment of natural waters characterised by significant organic carbon content, organic matter should be removed prior to the disinfection process. Simultaneously, in recent years, together with increasing fluctuations in the content and composition of natural organic matter in surface water, a reduction in the efficiency of conventional coagulation has been observed [7,8]. Therefore, multidirectional research is being conducted to intensify the coagulation process by, among other things, developing new coagulants or combining coagulation with other technological processes such as adsorption [9,10] or advanced oxidation processes (AOPs) [6]. It is important to note that the processes used for purification must be sustainable and optimised in terms of the materials, equipment and labour required.

One of the proposed methods is initial oxidation with ozone (O₃), which is the strongest among the oxidants used in water treatment. Ozone reacts with substances dissolved in water in two ways: directly, through the reaction of molecular ozone, and indirectly, through reactions of free radicals produced during ozone decomposition in water [11]. The ozonation process does not cause emissions to the environment of harmful waste that requires disposal; therefore, it meets the requirements of sustainability. Ozone may be used for initial and intermediate oxidation and final disinfection. Pre-ozonation can result in an increase in the average colloidal particle size, the formation of colloidal particles from dissolved organic matter and the improvement of organic substance or turbidity removal in sedimentation and filtration processes. Although ozonation does not lead to the decomposition of organic matter and an insignificant decrease in organic carbon content is observed, under neutral pH conditions, a significant reduction in the molecular weight of humic substances occurs. The basic effect of ozone oxidation is an increase in biodegradable organic compound content. It has been shown that initial ozonation increases dissolved organic carbon (DOC) removal, reduces coagulant consumption [12] and influences a reduction in particle aggregation destabilisation and an increase in the filter lifetime [11]. There are several mechanisms for interpreting an enhancing effect of initial ozonation on coagulation, including an increase in the concentration of oxidised functional groups, a reduction in stabilised organic coatings on particles and the polymerisation of metastable organic substances leading to particle aggregation through bridging reactions [11]. Another proposed solution is the combination of the adsorption process using powdered activated carbon (PAC) with coagulation. Wang et al. [13] showed that integrated coagulation and PAC processes allowed the removal of more than 70% of THMs, while coagulation conducted as a single process reduced THM content only by 10.5%. In other research, under technical conditions, the addition of PAC to the coagulation process on a technical scale [14] increased the amount of NOM removed by 70%, leading to a significant reduction in DBP formation (by 80–95%).

Heavy metals, including copper and zinc, occur in all environmental elements. Heavy metals are elements with an atomic weight above 63.5 u [15]. In surface waters, they may occur in a dissolved form as Zn²⁺ and Cu²⁺ ions. This form is a mobile form that can easily be transferred between different ecosystem elements. The presence of heavy metal compounds in the environment may have significant harmful effects on plants, animals or humans due to their toxic properties [16,17]. The main sources of surface water pollution by heavy metal ions such as copper and zinc are mining, agriculture, and urban and industrial wastewater [18].

Heavy metals are not biodegradable and have a tendency to accumulate in living organisms, causing numerous diseases and functional disorders [19]. Although heavy metals are considered harmful, some of them, including copper and zinc are, in trace amounts, key microelements essential for metabolic processes occurring in living organisms. Zinc is an important element in the process of photosynthesis and carbohydrate metabolism [20]. Its deficiency is responsible for gastrointestinal problems as well as kidney and liver disorders [21]. Higher concentrations of microelements have harmful effects on living organisms [22,23,24]. Copper is a metal commonly occurring in the water environment. Its harmful effect on water organisms is manifested by damage to the gills, liver, kidneys and nervous systems of fish [25]. Excessive quantities of zinc can cause diseases such as epilepsy, cerebral ischaemic stroke, Alzheimer’s disease and Parkinson’s disease [26].

Because of its toxic properties, heavy metal content in water intended for human consumption must be controlled and regulated [27]. In the European Union Directive and in the Regulation of the Polish Minister of Health, copper content in water meant for consumption may be no higher than 2 mg/L, while zinc content is not limited [28,29]. On the other hand, in accordance with World Health Organization (WHO) recommendations, the content of both ions is limited: the maximum permissible concentration of copper in drinking water is 2 mg/L, while that of zinc is 3 mg/L [30].

Heavy metals can be removed from water using a variety of techniques, including adsorption, complexation, electrochemical techniques, chemical precipitation, reverse osmosis, ion exchange, or coagulation and filtration [31,32]. Adsorption using classic activated carbons, as well as other various natural and synthetic adsorbents, is of great interest to researchers due to its simplicity, low cost and significant efficiency [16,21,33,34].

This study promotes the concept of sustainable development by minimising reagent consumption and using one reactor to conduct two processes simultaneously, in order to save resources and protect the environment. The aim of this research was to assess the effect of the coagulation process, supported by different configurations of combination with adsorption and ozonisation processes, on turbidity removal, colour and NOM indicators in surface water and to analyse changes in copper (Cu²⁺) and zinc (Zn²⁺) ion concentrations during the surface water treatment processes applied.

2. Materials and Methods

2.1. Materials

As part of this research, surface water taken from a dam reservoir, with a surface area of 5.5 km², on the Warta River in Poraj near the city of Częstochowa in May 2023 was used. Analyses were carried out using natural surface water, as well as water doped with ions of heavy metals (modified surface water). Water doping was performed by adding zinc nitrate and copper(II) nitrate solutions with a concentration of 1 mg/mL to natural surface water; the amount was adjusted to obtain a concentration level of 0.5 or 1.0 mg/L.

The model water used to optimise the sorption process was demineralised water enriched with copper and zinc ions to a concentration level of 0.5 or 1.0 mg/L.

Aluminium sulphate solution (ALS) and pre-hydrolysed poly aluminium chloride (PAX) low-basicity (C1), medium-basicity (C2) and high-basicity (C3) produced by Kemipol (Police, Poland) were used as coagulants during the coagulation process. The characteristics of these coagulants are shown in

Table 1. Characteristics of selected PAX coagulants.

Coagulant	Al, %	Cl, %	Basicity, %	pH
low-basicity C1	7.2 ± 0.3	21.0 ± 3.0	26 ± 6	<1
medium-basicity C2	5.3 ± 0.3	13.0 ± 2.0	65 ± 5	2.5 ± 0.5
high-basicity C3	6.0 ± 0.5	5.0 ± 1.0	85 ± 5	3.6 ± 0.4

.

Pre-hydrolysed coagulants contain hydroxyl groups that determine the increase in their alkalinity. For the purpose of this research, working solutions of coagulants containing 1 mg Al in 1 mL of solution were prepared by diluting the appropriate commercial product. During the processes of sorption, powdered activated carbon (PAC) with the trade name CWZ-22 from Gryfskand (Hajnówka, Poland) was used. Its characteristics are presented in

Table 2. Characteristics of powdered activated carbon CWZ-22.

Parameter	Unit	Value
specific surface area	m2/g	960
granulation < 0.06 mm	%	93
iodine number	mg/g	1032
methylene number	cm3	29

. The adsorbent type was selected on the basis of previous research [35].

2.2. Research Course and Methodology

This research was conducted in four main stages under laboratory conditions.

In the first stage of research, the coagulant for the coagulation process was selected. The process was conducted in the form of a typical jar test using the JLT4 four-station flocculator of VELP company (Usmate, Italy). For this purpose, 1.5 L of natural surface water was added to 2 L beakers, and four coagulants were added in sequence, with doses of 2.0, 3.0, 4.0 and 5.0 mg Al/L, respectively.

Using the flocculator’s mechanical stirrers, rapid mixing was carried out for 1 min (at 280 rpm), followed by slow mixing for 10 min (30 rpm). After this time, the samples were subjected to sedimentation for 60 min. Next, 0.4 L samples of water were decanted and analysed.

In the second stage of research, the ozone oxidation process was carried out using modified surface water. Ozone was produced in the laboratory generator (Model L20 SPALAB, by the Korona company, Piotrków Trybunalski, Poland) from high-purity oxygen (99%). Then, 1.5 L of water was added to a 2 L laboratory reactor, and water ozonisation was carried out for 10 min. A spherical stone diffuser at the bottom of the reactors was used to disperse the ozone. Experiments were carried out at a constant gas flow rate of 1.5 L/min, and the amount of ozone introduced into the water in 1 min was approx. 1.2 mg O₃/L. Unreacted ozone in the effluent gas was discharged into a potassium iodide solution. After the ozonisation process was complete, the water samples were subjected to a physicochemical analysis.

The third stage of research included the optimisation of the sorption process of Zn²⁺ and Cu²⁺ ions on the PAC used. For this purpose, PAC at doses of 20, 50 and 100 mg/L was added into 100 mL of model water containing 0.5 or 1.0 mg/L of copper and zinc ions. Samples were shaken for 10, 15, 30, 60 and 90 min at a frequency of 125 cycles/min. After shaking, the samples were filtered through filter paper. The filtrates obtained were acidified with concentrated nitric acid (HNO₃) to a pH level of 2.0. Next, ion concentrations of the metals tested were determined using the atomic absorption spectrometry (AAS) method.

In the fourth stage, research on the impact of combining adsorption and/or ozonisation with the coagulation process on removing selected contaminants from modified surface water was conducted. The process was conducted in the form of a typical jar test using the JLT4 four-station flocculator from the VELP company (Usmate, Italy). First, 1.5 L of water was added to each reactor of 2 L volume: two reactors with non-ozonised water and another two with ozonised water (Figure 1). The water was ozonised as in the second stage. The medium-basicity C2 coagulant selected in the first stage was introduced into the water in all reactors at a dose of 5.0 mg Al/L. Next, the reactors were placed in the flocculator, and rapid mixing was carried out for 1 min at 280 rpm. Subsequently, the powdered activated carbon selected in the third step was added to the water in Beaker 2 and Beaker 4 at a dose of 100 mg/L, and the water in all the beakers continued to be mixed for 2 min (Figure 1). When rapid mixing was completed, the stirring speed was reduced to 30 rpm, and mixing continued for 10 min. The samples were then left to stand in the beakers for a 60 min sedimentation period; 0.4 L of water was decanted from each beaker.

Parameters such as pH, turbidity, colour, oxidisability (OXI), total and dissolved organic carbon (TOC, DOC), absorbance at 254 nm wavelength (UV₂₅₄), and copper and zinc ion content were determined in the modified and purified water samples subjected to coagulation (first stage) and combined processes (fourth stage). All values were determined under typical laboratory conditions, at 20–25 °C. Water quality indicators were determined using the following methods: pH—potentiometrically (pH meter HI2002, Hanna Instruments, Olsztyn, Poland); turbidity—nephelometrically (turbidity meter HI 98703, Hanna Instruments, Olsztyn, Poland); colour—in comparison with standards of the platinum–cobalt scale; oxidisability—manganometric titration; TOC and DOC (after water filtration through 0.45 μm membrane filter)—high-temperature oxidation in an oxygen stream and CO₂ measurement with an NIDR detector (carbon analyser Vario TOC Cube, Elementar, Langenselbold, Germany); absorbance in UV ultraviolet at a wavelength of 254 nm (UV₂₅₄)—spectrophotometrically (1 cm cuvette); zinc and copper ion concentrations—atomic adsorption spectrometry (AAS) (spectrometer novAA 400, Analytik Jena, Jena, Germany). Analyses were carried out in triplicate, with standard methods in accordance with recommendations concerning water for human consumption [28,29,36].

3. Results

The surface water sampled for analysis was characterised by the following values of the indicators tested: pH = 7.67; turbidity, 7.76 NTU; colour, 50 mg Pt/L. Organic compound content was determined as oxidisability, 9.2 mg O₂/L; TOC, 8.66 mg C/L; DOC, 7.82 mg C/L; UV₂₅₄ absorbance, 0.341 1/cm. The concentration of heavy metal ions analysed in the surface water before modification was very low. It was 0.008 and 0.013 mg/L for copper and zinc, respectively. Thus, to determine the significance of the susceptibility of the analysed metal ions to removal from water as a result of single and integrated water treatment processes, the concentration of these metals had to be increased in the laboratory. The concentration levels of copper and zinc found in the modified surface water were 0.47 and 0.50 mg/L (C₀ = 0.5 mg/L) and 1.01 and 0.99 mg/L (C₀ = 1.0 mg/L).

3.1. Coagulant Selection

During the process of coagulation, as the dose of all coagulants increased, the values for all parameters tested decreased. Changes in selected indicators are shown in

Table 3. Quality indicators for the modified surface water and water after the coagulation process with ALS and C1 coagulants.

Parameter	Modified Surface Water	ALS				C1
		2.0 mg/L	3.0 mg/L	4.0 mg/L	5.0 mg/L	2.0 mg/L	3.0 mg/L	4.0 mg/L	5.0 mg/L
pH	7.67 ± 0.01	7.34 ± 0.02	7.22 ±	7.03 ± 0.03	6.81 ± 0.03	7.62 ± 0.01	7.59 ± 0.01	7.57 ± 0.03	7.56 ± 0.02
Turbidity, NTU	7.76 ± 0.02	3.04 ± 0.03	2.59 ± 0.03	1.98 ± 0.02	1.32 ± 0.03	2.92 ± 0.02	2.46 ± 0.03	1.47 ± 0.02	1.23 ± 0.03
Colour mg Pt/L	50 ± 3	35 ± 2	25 ± 2	18 ± 1	10 ± 1	27 ± 3	18 ± 3	14 ± 1	9 ± 1
UV254	0.341 ± 0.011	0.240 ± 0.011	0.206 ± 0.009	0.160 ± 0.006	0.120 ± 0.006	0.185 ± 0.008	0.168 ± 0.006	0.140 ± 0.009	0.123 ± 0.002
OXI, mg O2/L	9.2 ± 0.2	7.1 ± 0.1	6.4 ± 0.1	5.6 ± 0.2	4.8 ± 0.1	6.8 ± 0.2	6.0 ± 0.2	5.3 ± 0.1	4.4 ± 0.1
TOC, mg C/L	8.66 ± 0.05	6.98 ± 0.04	6.14 ± 0.02	5.39 ± 0.04	4.61 ± 0.02	6.42 ± 0.03	5.77 ± 0.02	5.05 ± 0.02	4.23 ± 0.01
DOC, mg C/L	7.82 ± 0.04	6.65 ± 0.03	5.92 ± 0.01	5.02 ± 0.02	4.20 ± 0.02	6.10 ± 0.03	5.36 ± 0.05	4.73 ± 0.02	3.98 ± 0.02
Aluminium, mg/L	<0.01	0.12 ± 0.01	0.09 ± 0.01	0.06 ± 0.01	0.05 ± 0.01	0.10 ± 0.02	0.06 ± 0.01	0.04 ± 0.01	<0.01

and

Table 4. Quality indicators for the modified surface water and water after the coagulation process with C2 and C3 coagulants.

Parameter	Modified Surface Water	C2				C3
		2.0 mg/L	3.0 mg/L	4.0 mg/L	5.0 mg/L	2.0 mg/L	3.0 mg/L	4.0 mg/L	5.0 mg/L
pH	7.67 ± 0.01	7.63 ± 0.01	7.60 ± 0.02	7.58 ± 0.02	7.57 ± 0.02	7.65 ± 0.02	7.63 ± 0.03	7.61 ± 0.01	7.59 ± 0.02
Turbidity, NTU	7.76 ± 0.02	2.72 ± 0.01	1.58 ± 0.02	1.37 ± 0.03	0.90 ± 0.02	2.91 ± 0.01	1.93 ± 0.02	1.64 ± 0.02	1.11 ± 0.03
Colour mg Pt/L	50 ± 3	22 ± 2	18 ± 1	12 ± 1	7 ± 2	22 ± 2	18 ± 2	13 ± 1	7 ± 1
UV254, 1/cm	0.341 ± 0.011	0.160 ± 0.011	0.125 ± 0.009	0.108 ± 0.006	0.092 ± 0.009	0.174 ± 0.005	0.143 ± 0.011	0.126 ± 0.008	0.112 ± 0.006
OXI, mg O2/L	9.2 ± 0.2	6.1 ± 0.2	4.7 ± 0.1	3.8 ± 0.2	3.2 ± 0.1	6.6 ± 0.2	5.8 ± 0.2	4.7 ± 0.2	3.8 ± 0.1
TOC, mg C/L	8.66 ± 0.05	6.04 ± 0.04	4.50 ± 0.02	3.63 ± 0.02	3.01 ± 0.04	6.22 ± 0.04	5.37 ± 0.01	4.34 ± 0.01	3.23 ± 0.02
DOC, mg C/L	7.82 ± 0.04	5.78 ± 0.04	4.38 ± 0.01	3.37 ± 0.01	2.74 ± 0.02	5.89 ± 0.04	5.12 ± 0.03	4.11 ± 0.01	3.04 ± 0.03
Aluminium, mg/L	<0.01	0.05 ± 0.01	<0.01	<0.01	<0.01	0.06 ± 0.01	<0.01	<0.01	<0.01

and Figure 2. Turbidity decreased from 61% for the lowest dose, when aluminium sulphate was used, to 88% when the highest dose of the C2 coagulant was used. For each coagulant, the highest reduction in turbidity was achieved using the highest dose. Colour was removed, depending on dose and coagulant, at rates from 30 to 80% for ALS, from 46 to 82% for C1 and from 56 to 86% for C2 and C3. The highest reduction in colour values was obtained for the highest dose when using PAX of medium and high basicity. A turbidity value below 1 NTU was only obtained when a 5 mg/L dose of the C2 coagulant was used. NOM content reached its lowest values when a 5 mg/L dose of all coagulants was used. The highest decrease in oxidisability, TOC, DOC and UV₂₅₄ was recorded when using the C2 coagulant.

The highest increase in the efficiency (approximately 20%) of removing OXI, TOC and DOC, in relation to ALS, using the same dose of pre-hydrolysed coagulants occurred after the coagulation process was performed with the C2 coagulant for dose levels of 3, 4 and 5 mg/L. A significantly smaller increase, compared with using the C2 coagulant, was recorded for the same parameters when using the C3 coagulant. The highest additional removal of OXI, TOC and DOC for a coagulant dose of 5 mg/L was 11%, 16% and 15%, respectively. A slightly lower efficiency (by 10–15%) of organic matter removal was obtained by Musteret et al. [37] when using a medium-basicity coagulant during river water treatment. However, it should be noted that the water they tested was characterised by low OXI, TOC and UV₂₅₄ content. A higher efficiency in humic substance removal, using the same dose of medium-basicity coagulant and aluminium sulphate, was also obtained by Rucka [38]. In this research, the amount of organic matter removed increased in line with the coagulant dose and initial TOC content.

On the basis of the above research, the medium-basicity C2 coagulant at a dose of 5 mg Al/L was selected for further research.

3.2. Ozonation Process

In order to compare the effect of ozonation on coagulation, an ozonation test was carried out on modified surface water. After the ozonation process was complete, colour was reduced by 40%, whereas organic compound indicators OXI, TOC, DOC and UV₂₅₄ were reduced by 27%, 26%, 22% and 66%, respectively (

Table 5. Values of selected quality indicators for the raw natural water and water after 10 min ozonation process, ozone dose 2 mgO₃/L.

Parameter	Modified Surface Water	Ozonated Water
pH	7.67 ± 0.01	7.86 ± 0.02
Colour, mg Pt/L	50 ± 3	30 ± 2
UV254, 1/cm	0.341 ± 0.011	0.114 ± 0.006
Oxidisability (OXI), mg C/L	9.2 ± 0.2	6.7 ± 0.1
Total organic carbon (TOC), mg C/L	8.66 ± 0.05	6.32 ± 0.03
Dissolved organic carbon (DOC), mg C/L	7.82 ± 0.04	6.12 ± 0.04

). Similar changes in TOC and UV₂₅₄ were obtained by Wang et al. [13]. Initial ozonation of water had a greater effect in reducing UV₂₅₄ content than in reducing TOC, which is consistent with the results obtained in studies reported in the relevant literature [13,39,40,41].

The use of the ozonation process had no significant effect on the removal of the metal ions tested from the modified water. Copper and zinc contents obtained after the ozonation process remained practically unchanged. The degree of removal did not exceed 5% of the initial metal concentration in the modified water (

Table 6. Zinc and copper ion concentrations (mg/L) in the modified water subjected to ozonation process (time 10 min), initial metal concentration: 0.5 and 1.0 mg/L.

Water Treatment Process	Zn2+		Cu2+
	0.5 mg/L	1.0 mg/L	0.5 mg/L	1.0 mg/L
ozonation	0.49 ± 0.01	0.97 ± 0.02	0.48 ± 0.01	0.98 ± 0.01

).

3.3. Optimisation of Adsorption Process

Changes in the content of selected heavy metals after the process of adsorption from the model water using PAC are shown in

Table 7. Zinc and copper ion content (mg/L) in the model water during PAC adsorption process at different adsorbent doses. C₀, initial metal ion concentration.

PAC Dose; mg/L

Zinc

C0 = 0.5 mg/L

C0 = 1.0 mg/L

Contact Time (min)

0

5

15

30

60

90

0

5

15

30

60

90

25

0.50 ± 0.02

0.48 ± 0.02

0.45 ± 0.01

0.43 ± 0.02

0.42 ± 0.01

0.41 ± 0.01

1.00 ± 0.01

0.96 ± 0.02

0.88 ± 0.02

0.85 ± 0.02

0.82 ± 0.01

0.81 ± 0.01

50

0.50 ± 0.02

0.46 ± 0.01

0.39 ± 0.02

0.37 ± 0.01

0.36 ± 0.02

0.36 ± 0.02

1.00 ± 0.01

0.93 ± 0.02

0.81 ± 0.01

0.77 ± 0.01

0.76 ± 0.01

0.75 ± 0.01

100

0.50 ± 0.02

0.44 ± 0.02

0.36 ± 0.01

0.34 ± 0.01

0.33 ± 0.02

0.32 ± 0.01

1.00 ± 0.01

0.90 ± 0.02

0.75 ± 0.01

0.70 ± 0.01

0.69 ± 0.01

0.69 ± 0.01

PAC Dose; mg/L

Copper

C0 = 0.5 mg/L

C0 = 1.0 mg/L

Contact Time (min)

0

5

15

30

60

90

0

5

15

30

60

90

25

0.50 ± 0.02

0.45 ± 0.02

0.36 ± 0.01

0.34 ± 0.01

0.31 ± 0.02

0.30 ± 0.02

1.00 ± 0.01

0.94 ± 0.02

0.80 ± 0.02

0.77 ± 0.01

0.74 ± 0.02

0.72 ± 0.01

50

0.50 ± 0.02

0.43 ± 0.01

0.33 ± 0.01

0.31 ± 0.02

0.29 ± 0.02

0.27 ± 0.01

1.00 ± 0.01

0.91 ± 0.02

0.75 ± 0.01

0.72 ± 0.01

0.71 ± 0.02

0.70 ± 0.01

100

0.50 ± 0.02

0.41 ± 0.02

0.29 ± 0.01

0.26 ± 0.02

0.24 ± 0.01

0.22 ± 0.01

1.00 ± 0.01

0.89 ± 0.01

0.71 ± 0.01

0.68 ± 0.01

0.66 ± 0.02

0.64 ± 0.02

and Figure 3 and Figure 4.

The percentage of metal adsorption in this research was usually higher for a lower initial metal content value of 0.5 mg/L. This means that the absolute amounts of metal ions adsorbed for both initial concentrations were similar in order, but related to a lower initial concentration, resulting in a higher process efficiency indicator. The level of adsorption also increases in line with an increasing dose of PAC used, but the increments are not directly proportional to the dose used, which may indicate irregularities in the structure of materials used as adsorbents.

The impact of the process’s time on its efficiency was also analysed. Generally, it may be concluded that an increase in adsorption time from 5 to 90 min causes an increase in the level of metal ion removal from the model water tested. However, these changes are significant within the first 15 min of the process. When time was extended to 30, 60 and 90 min, changes were small, which may indicate that the adsorption process in the systems under analysis was rapid and reached a state of equilibrium within the first few minutes of the experiment. An analysis of the relationships observed indicates that the optimum adsorption time for the metals was 15–30 min. Thus, the optimum time for combined processes will be determined by the technical requirements of the coagulation process.

Similar observations regarding adsorption time for copper were noted by Gupta et al. [19]. The optimum time was 20 min.

The percentage of zinc ions removed from the model water with an initial content of 0.5 mg/L was 30–35% for a PAC dose of 100 mg/L, 20–30% at a dose of 50 mg/L and 10–20% at a dose of 25 mg PAC/L and an experiment duration of 15 to 90 min. For an initial Zn²⁺ content of 1.0 mg/L, the percentages of metal removal were 25–30%, 20–25% and 10–20% for PAC doses of 100, 50 and 25 mg/L, respectively.

Copper ions present in the model water were slightly more susceptible to removal by adsorption with PAC than zinc ions. Similar to what was observed for zinc, a higher level of efficiency in the adsorption process was observed at a lower level of copper concentration (0.5 mg/L). On the other hand, when the process time was increased to 90 min, efficiency increments were slightly more significant for copper than for zinc.

The highest levels of metal removal efficiency were observed at a dose level of 100 mg PAC/L and a process time of 90 min. They were 56 and 36% for 0.5 and 1.0 mg Cu/L, respectively. Gupta et al. [19] observed 60–70% efficiencies in the copper ion adsorption process at slightly higher temperatures and using higher doses of adsorbent.

3.4. Connected Processes of Coagulation, Adsorption and Ozonation

Values of the modified water quality indicators under analysis, before and after single coagulation processes and coagulation assisted by adsorption and/or ozonisation processes using a medium-basicity coagulant (polyaluminium chloride C2), are shown in

Table 8. Quality indicators for the modified and treated surface water after coagulation process (coagulant C2, dose 5 mg/L) and enhancing processes: ozonation (10 min) and adsorption (adsorbent CWZ-22, dose 100 mg/L).

Treatment Process	Parameter
	Turbidity, NTU	Colour, mg Pt/L	OXI, mg O2/L	TOC, mg C/L	DOC, mg C/L	UV254 1/cm	Al3+, mg/L
Raw water	7.76 ± 0.02	50 ± 3	9.2 ± 0.2	8.66 ± 0.05	7.82 ± 0.04	0.341 ± 0.011	<0.01
C	0.98 ± 0.02	7 ± 2	3.2 ± 0.1	3.01 ± 0.04	2.74 ± 0.02	0.092 ± 0.009	<0.01
C + A	1.12 ± 0.03	6 ± 3	2.8 ± 0.2	2.67 ± 0.02	2.41 ± 0.03	0.085 ± 0.010	<0.01
O + C	0.81 ± 0.02	5 ± 3	2.3 ± 0.1	2.42 ± 0.03	2.10 ± 0.02	0.068 ± 0.012	<0.01
O + C + A	0.73 ± 0.03	5 ± 2	2.1 ± 0.1	2.00 ± 0.03	1.85 ± 0.02	0.054 ± 0.011	<0.01

C—coagulation; C + A—coagulation + adsorption; O + C—ozonation + coagulation; O + C + A—ozonation + coagulation + adsorption.

. Performing the process at different initial levels of metal concentration in the modified surface water did not result in any significant changes in the parameters tested. The values of the increases (or decreases) in water turbidity, colour, oxidisability, TOC, DOC and UV₂₅₄ absorbance, compared to single coagulation, are shown in Figure 5.

When using a 5 mg/L dose of the C2 coagulant, the decreases in colour and turbidity were 86–90% and 85–90%, respectively. The percentage of organic matter removed, determined as OXI, TOC, DOC and UV₂₅₄, was 65–77%, 65–76%, 64–76% and 73–84%, respectively, depending on the process used. Coagulation enhanced with adsorption and/or ozonation processes had the least effect on colour and turbidity, with a 2% and 4% reduction, respectively. When using a combination of the coagulation and adsorption processes, the turbidity value increased by 2%, which was most likely due to the introduction of activated carbon particles into the decanted post-process water. The coagulation process preceded by ozonation led to an increase in the percentage of organic matter removed, defined as OXI, TOC, DOC and UV₂₅₄, by 10%, 7%, 8% and 2%, respectively. As indicated in research performed by Jin [11], the efficiency of NOM removal may be increased by carrying out coagulation and ozonation processes simultaneously in one unit. This process showed significantly better DOC removal efficiency than the classic coagulation process using initial water ozonation, due to an interaction between the ozone and the coagulant. A pre-hydrolysed coagulant acted as a catalyst in this process by promoting the generation of •OH radicals, which resulted in an improvement in DOC removal efficiency. The adsorption process combined with coagulation resulted in 4% more OXI, TOC and DOC and 2% more UV₂₅₄ being removed. These values are much lower than those obtained by Kristian et al. [14] during research conducted on surface water, where an increase in DOC removal of about 20% and an increase in UV₂₅₄ removal of 8% were obtained when a 150 mg/L dose of PAC was added to the coagulation process. The highest reduction in the parameters studied was observed when coagulation was carried out in combination with adsorption and ozonation. Compared to the coagulation process carried out as a single process, an additional reduction in organic compounds, indicated as OXI, TOC, DOC and UV₂₅₄, was achieved (12%, 12%, 11% and 11%, respectively).

In all combinations of the processes under consideration, coagulation is the main process responsible for reducing the values of individual indicators.

The behaviour of copper and zinc ions during the purification of modified water in the single and combined processes is shown in

Table 9. Zinc and copper ion content (mg/L) in the modified water subjected to coagulation, adsorption and connected processes, initial metal concentration: 0.5 and 1.0 mg/L.

Water Treatment Process	Zn2+		Cu2+
	0.5 mg/L	1.0 mg/L	0.5 mg/L	1.0 mg/L
C	0.27 ± 0.02	0.57 ± 0.01	0.21 ± 0.01	0.45 ± 0.01
A	0.27 ± 0.01	0.59 ± 0.02	0.20 ± 0.01	0.54 ± 0.01
C + A	0.22 ± 0.02	0.48 ± 0.03	0.19 ± 0.02	0.41 ± 0.02
O + C	0.26 ± 0.02	0.50 ± 0.02	0.19 ± 0.01	0.43 ± 0.02
O + C + A	0.19 ± 0.01	0.46 ± 0.01	0.17 ± 0.01	0.38 ± 0.02

C—coagulation; A—adsorption; C + A—coagulation + adsorption; O + C—ozonation + coagulation; O + C + A—ozonation + coagulation + adsorption.

and Figure 6.

As in experiments using the model water, copper ions were slightly more susceptible to removal from the modified water than zinc ions. The percentage of copper removed depended on the initial metal concentration in the modified water. For copper content of 0.5 mg/L, the efficiency of the process of removal was at a similar level regardless of the process used: 58 and 60% for the single coagulation and adsorption processes, respectively. A combination of these two processes led to a very slight increase in the percentage of metal removed, which reached 62%. A similar effect was observed when initial ozonation was applied before coagulation (62%). A combination of ozonation and coagulation with adsorption resulted in an increase in the percentage of copper removed to 66%.

At an initial Cu²⁺ concentration of 1.0 mg/L, the levels of removal efficiency were slightly lower (55, 46, 59, 57 and 62% for coagulation, adsorption, coagulation with adsorption, ozonisation with coagulation and ozonisation with coagulation and adsorption, respectively).

Similar changes were observed for Zn²⁺ ions. Slightly higher levels of removal efficiency were recorded for lower initial metal concentrations (0.5 mg/L), which were 46, 46, 66, 48 and 62% during coagulation; adsorption; coagulation and adsorption; ozonisation and coagulation; and ozonisation, coagulation and adsorption, respectively. The values recorded at an initial concentration level of 1.0 mg/L were 43, 41, 52, 50 and 54%, respectively.

The coagulation and adsorption processes are commonly recognised as highly efficient in reducing heavy metal ion content, including copper and zinc. The adsorption process, which is defined as a combination of mechanisms of physical sorption, complexation, ion exchange, precipitation, and oxidation and reduction processes [42], is the most frequently associated with large reductions in metal ion content in water. The levels of metal ion removal efficiency reported by various authors ranged from several to even 100% [43].

4. Conclusions

The usefulness of coagulation associated with ozonisation and/or adsorption processes using powdered activated carbon for NOM and heavy metal ion removal from surface waters has been demonstrated. The coagulation process preceded by ozonisation increased the efficiency of removal processes by 10% for OXI, 8% for DOC, 7% for TOC and 2% for UV₂₅₄. At the same time, it should be noted that coagulation was most directly responsible for NOM removal, reducing OXI, TOC and DOC by 65%, while UV₂₅₄ was reduced by 75%. Compared to coagulation conducted as a single process, the greatest impact was achieved by simultaneous coagulation and adsorption preceded by the ozonation process, increasing the percentage of organic compound content removal (OXI, TOC, DOC and UV₂₅₄ by 12%, 12%, 11% and 11%, respectively).

In the case of the single and combined processes used for the purification of modified water, the largest amounts of copper and zinc ions were removed by combining initial ozonisation with coagulation and adsorption. However, compared to the single coagulation and adsorption processes, it can be concluded that the increased efficiency in most cases was no higher than 10%. The amount of copper and zinc ions removed in a single coagulation and adsorption process was at a similar level. A combination of both processes only slightly improved the efficiency of single processes. The initial inclusion of ozonation had an almost negligible effect on increasing the efficiency of the processes under analysis. Increases observed in this case did not exceed 5%. Thus, as far as copper and zinc ion removal is concerned, the coagulation and adsorption processes were the most effective.

The primary aim of this work was to improve the coagulation process by integrating it with adsorption and/or ozonation for surface water treatment. The subject of this research is consistent with sustainability due to the need to use available water sources of very different qualities for human consumption and economic purposes. Moreover, these waters must be used many times, both now and in the future, and every time they are used, they must be purified. The combination of ozonation and PAC adsorption processes with coagulation is not commonly used to treat water for human consumption. Some water treatment plants (WTPs) introduce the ozonation module before the coagulation process, but this is not a standard approach. The adsorption process is most often carried out in WTPs using granulated activated carbon (GAC) in adsorption columns. Powdered activated carbon is rarely used during coagulation, most often in emergency situations. The biggest advantage of PAC, compared to GAC, is its ability to quickly and easily change the type and dose depending on the type of water pollution. At the same time, it should be noted that the order of processes in the sequence may affect the removal of contaminants. The results presented in the report are part of a new direction in the intensification of the coagulation process by combining it with adsorption and ozonation. The combination of the above-mentioned processes leads to an increase in the amount of NOM removed as well as an increase in the amount of selected heavy metals removed. On the other hand, coagulation preceded by ozonation makes it possible to minimise the coagulant dose and the amount of post-coagulation waste generated. A reduction in the dose of the coagulant used would reduce its impact on the depletion of substrate resources used for its production in a life cycle assessment. Combined or hybrid processes are currently of great interest to scientists. The scope and subject of the studies presented are an important field of environmental science, and although there is great global research interest in water treatment, there are still many gaps to fill. Additionally, the results of our studies emphasise the importance of optimising the conditions for the relevant processes. For example, determining the coagulant dosage increased the efficiency of the balanced treatment, with potential increases in certain contaminant contents, like aluminium ions, in treated water. Defining the optimal conditions for the technological process makes it possible to balance and minimise the consumption of reagents and the use of necessary equipment as well as human labour, which from the point of view of sustainability seems to be an important objective for our research.