A Comparison of the Performance of the Pre-Ozonation Process and the Use of Coagulants with Ozone in the Removal of Algae from Surface Water

Abstract

Purifying surface water in moderate-humid climates is challenging due to high levels of organic compounds and diatom algae, which can produce toxic substances. A new method involving chitosan and ferric chloride coagulants, along with pre-ozonation, shows promise for improving water quality. The study examined various conditions, including pH levels and concentrations of chitosan (20–500 mg/L) and ferric chloride (5–100 mg/L), as well as the pH of reaction mixtures (3–11) and ozone concentrations (2–8 mg/L). Optimal results were achieved at a pH of 10 for both coagulants, with 500 mg/L chitosan and 75 mg/L ferric chloride. A pH of 7 for the reaction mixture and an ozone concentration of 8 mg/L were most effective. The pre-ozonation process alone can eliminate 72% of algae, while combining ozone with chitosan and ferric chloride achieved diatom removal rates of 77% and 79%, respectively. These findings highlight the effectiveness of using lower ozone concentrations in conjunction with coagulants for water purification.

1. Introduction

Over the years, water quality has been affected by various factors. One significant factor is land-use change, such as converting forests to agricultural lands and pastures, which can significantly impact regional water quality [1] by altering physicochemical parameters and nutrient concentrations in rivers [2]. Similarly, algae have emerged as a significant challenge in water treatment plants, rendering the water aesthetically unappealing and ultimately unfit for consumption. Algae impart undesirable tastes and odors to water and cause operational problems within treatment facilities. Consequently, removing these aquatic plants has become a critical step in water treatment.

Different physical and chemical methods are employed in water treatment plants to remove algae. Physical methods such as deep flotation [3] and polycarbonate membrane filters [4] have been shown to trap algae effectively. Chemical methods include coagulation, electrocoagulation, ultrasonication, and hydrolysis under various parameters. While chemical methods like coagulation, flocculation, and sedimentation have limitations, such as chemical consumption, sludge generation, and the necessity for continuous system oversight, they continue to be widely employed in water treatment plants [5]. This is due to their high efficiency in removing algae by destabilizing them and forming larger flocs.

Moreover, algae typically cause anoxic conditions in aquatic environments. Implemented in coagulation and flocculation processes, pre-ozonation offers an effective method. By injecting ozonated water, oxygen is effectively introduced into the water, producing reactive oxygen species such as ozone and hydroxyl radicals [6,7,8,9], which can efficiently degrade algal cells and their extracellular substances [10]. Therefore, pre-ozonation is an effective adjunct to coagulation and flocculation to enhance algae removal.

Various combinations of oxidation, coagulation, and flocculation methods have been used in different ways for algae removal. For instance, using novel Al/Fe-based composite coagulants in conjunction with potassium permanganate oxidation has demonstrated high removal rates for algae and extracellular organic matter [11]. An alternative method is combining Fe(II) with peracetic acid or sodium hypochlorite for pre-oxygenation and coagulation. This approach has demonstrated high algae removal rates while maintaining cellular integrity [12]. Peracetic acid, unlike other anti-fungal agents, is readily available in pre-packaged form as an anti-mildew agent for just two dirhams. The application of hydrogen peroxide surface treatment, followed by an 8 h exposure period, has also proven effective. Additionally, the application of chlorine dioxide in the coagulation/flocculation process, aided by pre-oxidation, has proved optimal in enhancing raw water quality, including removing algae, depending on the initial pH [13]. Combining sodium percarbonate with polyaluminum chloride coagulation and membrane filtration has also enhanced the removal of algal contaminants and the efficiency of sludge control [14].

Beyond the aforementioned applications of pre-ozonation to enhance removal efficiency, this process exhibits several other potential benefits. Pre-ozonation before coagulation with ferric chloride has been demonstrated to significantly improve the removal of turbidity and organic matter. These enhancements were observed after just 5 min of ozonation treatment. A comparison of ozonated and non-ozonated samples indicated that ozone addition had a beneficial effect on total organic carbon removal [15]. Moreover, pre-ozonation was employed in a study as a coagulant aid at a low concentration of 0.1 mg/L ozone to remove turbidity and UV254. However, at higher ozone concentrations (above 0.2 mg/L), pre-ozonation was detrimental to UV254 removal, although it remained effective in removing turbidity [16]. Previous studies have also found that the sequential application of pre-ozonation and coagulation at a pH of 9.0, with an ozone dose of 0.45 mg O₃/mg of dissolved organic carbon, resulted in an approximately 60% reduction in trihalomethane formation potential under optimal conditions [17].

Applying ozonation, a robust oxidative process, and ferric chloride and chitosan, which have coagulant and flocculant properties, offers a promising approach for effectively removing algae. The synergistic interaction between ozonation, ferric chloride, and chitosan results in enhanced coagulant and flocculant properties, thereby promoting algal cell aggregation and facilitating their removal from aqueous environments. This integrated approach has demonstrated significant potential for achieving high removal rates and improving water clarity [18].

This study aims to precisely determine the effects of (1) the dosages of chitosan, ferric chloride, and ozone; (2) the pH levels of chitosan, ferric chloride, and ozone; and (3) the combined dosages of chitosan-ozone and ferric chloride-ozone on algae reduction in the Mijran Dam reservoir.

2. Materials and Methods

2.1. Identification of the Target Water: Characteristics of Mijran Dam Water

This study randomly collected samples from the inlet of the Ramsar water treatment plant located in Ramsar County, Mazandaran Province. The parameters of pH, turbidity, color, and the abundance of diatoms were determined at the Pak Zist Caspian Alborz laboratory. Subsequently, a study was conducted to assess algal reduction rates.

2.2. Research Methods

Water samples were randomly collected from the inlet of the Ramsar water treatment plant. The samples were transported to the laboratory in glass containers. Sampling was conducted at a depth of 50 cm below the water surface, proximal to the inflow point of the stream into the treatment plant. Samples were filtered using 0.45-micron filter paper, and the filters were subsequently rinsed with 1 milliliter of distilled water. The filtrate was placed on a special counting slide. Afterward, counting was performed using a 10× and 40× objective lens, employing 10-point methods. Algae identification was performed using optical diffraction of a dark background. A one-liter sample with initial physicochemical characteristics was collected. The pH, color, and turbidity were quantified using standardized laboratory techniques, employing a pH meter, colorimeter, and turbidimeter, respectively. A double-beam ultraviolet-visible spectrophotometer was employed to quantify the removal of algae subsequent to ozonation and the application of chitosan and ferric chloride compounds. The spectrophotometer determined algal biomass reduction by measuring the transmitted and absorbed light at diverse wavelengths within the algal samples. The experimental apparatus and materials used for these investigations are detailed in

Table 1. Laboratory equipment.

Row	Device Name	Description	Make/Model
1	Colorimeter	Color measurement	ASTM D1500
2	Double-beam ultraviolet-visible spectrophotometer	Transmitted and absorbed rays with different colors in algae-containing samples	Ultraviolet (Double Beam UV-Vis Spectrophotometry) Model UV-120-02
3	Oxygen generator	Injected ozone	(YUYUE) F-3B7 Model
4	pH meter	pH measurement	Crison-Model Basic 20
5	Turbidimeter	Turbidity measurement	WTW-Model Turbo 430
6	Microscope	Counting algae	CX = 23

and

Table 2. Chemicals used in the research.

Row	Chemical Substance	Description	Make
1	Chitosan	To remove algae	Chitosan medium Chitosan (CAS No.: 9012-76-4
2	Ferric chloride	To remove algae	Iron III chloride (CAS No.: 7705-08-0)
3	Deionized distilled water	To dilute and wash labware	Tuska Shimi

.

2.3. Determining the Optimal Concentration and pH Value of Ozone

In this research, in order to ozonize the water sample, 5 L of distilled water was ozonized with an oxygen generator or ozone generator for 15 min and the remaining ozone concentration was measured. The distilled water was ozonized with ratios of 1 to 1 (4 mg per liter), 2 to 1 (6 mg per liter), and 3 to 1 (2 mg per liter) and thoroughly mixed to ensure that the device correctly injected the exact amount of ozone. These samples were considered the control samples. In the next step, ozone was not applied directly to the raw water sample because this action disturbs the reactions, links chemical compounds, and interferes with the turbidity, compromising the results obtained. Then, raw water samples containing algae were mixed at a ratio of 1:1 with the control samples at concentrations of 2, 4, 6 and 8 mg per liter of ozone. The amount of algae removal was then obtained (

Table 3. Different doses of ozone to yield the optimal concentration.

Ozone-Containing Samples (mg/L)	Ozone (mg/L)	Ozone (mg/L) After 15 min	Diatom Abundance	Removal Efficiency (%)
Base sample	-	-	270	-
8 mg/L ozone	4.78	4.32	80	70.37
6 mg/L ozone	3.64	3.51	88	67.40
4 mg/L ozone	2.33	1.93	110	59.25
2 mg/L ozone	1.96	1.63	140	48.14

). The sample with the optimal ozone concentration was tested at different pHs of 11, 9, 7, 5, and 3 until the optimal pH was determined (

Table 4. Changes in pH values to obtain the optimal value.

pH	Ozonation Time (min)	Residual Ozone (mg/L)	Diatom Abundance	Removal Efficiency (%)
Base sample	-	-	270	-
3	15	4.78	120	55.55
5	15	4.93	85	68.51
7	15	4.91	75	72.22
9	15	4.33	88	67.40
11	15	3.5	130	51.85

).

2.4. Determining the Optimal Concentration and pH Value of Chitosan

In the second phase, a one-liter sample was introduced into a one-liter beaker and placed under a jar test apparatus. Chitosan was added gradually at a dosage of 500 mg/L while mixed with the sample at a rotational speed of 20 rpm for 20 min. This process was repeated at pH levels 5, 6, 7, 8, 9, and 10 (

Table 5. Optimal pH value for chitosan.

Indicator	pH Value	Diatom Abundance (No. 10 mL)	Removal Efficiency (%)
Base sample	7.7	350	-
Chitosan	5	317	9.5
Chitosan	6	325	7.1
Chitosan	7	303	13.43
Chitosan	8	295	15.71
Chitosan	9	290	17.14
Chitosan	10	278	20.57

) to determine the optimal pH. Once the optimal pH was established, the sample was adjusted to this pH value. Afterward, the experiments were continued at various chitosan dosages of 20, 50, 75, 100, 200, and 500 mg/L (

Table 6. The optimal concentration for chitosan.

Indicator	pH Value	Turbidity (NTU)	Diatom Abundance (No. 10 mL)	Removal Efficiency (%)
Base sample	7.7	<3	350	-
20	10	0.5	318	9.14
50	10	1	308	12
75	10	2	306	12.57
100	10	2	300	14.28
200	10	2.5	293	16.28
500	10	3	280	20

) to ascertain the optimal chitosan concentration.

2.5. Determining the Optimal Concentration and pH Value of Ferric Chloride

A one-liter sample was placed in a beaker beneath a jar test apparatus in the third stage. Subsequently, 100 mg/L ferric chloride was gradually added to the sample and mixed at a rotational speed of 20 rpm for 20 min. The process was conducted at pH levels 5, 6, 7, 8, 9, and 10 (

Table 7. Optimal pH value for ferric chloride.

Indicator	pH Value	Turbidity (NTU)	Diatom Abundance (No. 10 mL)	Removal Efficiency (%)
Base sample	7.7	<5	350	-
Ferric chloride	5	22	320	8.57
Ferric chloride	6	16	310	11.43
Ferric chloride	7	8	300	14.28
Ferric chloride	8	3	320	8.57
Ferric chloride	9	2.5	310	11.43
Ferric chloride	10	2	295	15.71

) to determine the optimal pH. After ascertaining the optimal pH, the sample was adjusted to this pH and subjected to tests at various ferric chloride concentrations, namely, 5, 15, 30, 50, 75, and 100 mg/L (

Table 8. Optimal concentration of ferric chloride.

Indicator	pH Value	Turbidity (NTU)	Diatom Abundance (No. 10 mL)	Removal Efficiency (%)
Base sample	7.7	<3	350	-
5	10	60	310	11.43
15	10	45	300	14.28
30	10	24	295	15.71
50	10	17	300	14.28
75	10	11	290	17.14
100	10	44	300	14.28

), to identify the optimal ferric chloride concentration.

2.6. Combination of Chitosan Coagulant with Ozone

In the fourth stage, the chitosan coagulant, optimized in terms of concentration and pH from the previous stage, was mixed with ozonated samples at concentrations of 2, 4, 6, and 8 mg/L. The mixtures were then tested to determine the maximum algal removal efficiency. The results are presented in

Table 9. Variations in ozone concentrations using optimal chitosan.

Chitosan 500 mg/L	pH	Ozone (mg/L)	Color (co-pt)	Diatom Abundance (No. 10 mL)	Removal Efficiency (%)
Raw water sample	7	-	0	280	-
Jar test sample	7	-	10	185	33.92
Jar test + distilled water	7	-	5	140	50
Jar test + distilled water and ozone 8 mg/L	7	0.83	15	120	57.14
Jar test sample + distilled water and ozone 6 mg/L	7	2.56	10	110	60.71
Jar test + distilled water with ozone 4 mg/L	7	3.51	5	88	68.57
Jar test + distilled water with ozone 2 mg/L	7	5.78	5	65	76.78

.

2.7. Combination of Ferric Chloride Coagulant with Ozone

In the fifth stage, the ferric chloride coagulant, at the optimal concentration and pH determined from the previous stage, was mixed with ozonated samples at 2, 4, 6, and 8 mg/L concentrations. The mixtures were then analyzed to determine the maximum algal removal efficiency. The results of these experiments are presented in

Table 10. Variations in ozone levels using optimal ferric chloride.

Ferricchloride 75 mg/L	pH	Ozone (mg/L)	Color (co-pt)	Diatom Abundance (No. 10 mL)	Removal Efficiency (%)
Raw water sample	7	-	0	280	-
Jar test sample	7	-	0	170	39.28
Jar test + distilled water	7	-	0	130	53.57
Jar test + distilled water and ozone 8 mg/L	7	1	0	110	60.71
Jar test sample + distilled water and ozone 6 mg/L	7	3	0	101	63.92
Jar test + distilled water and ozone 4 mg/L	7	4.84	0	78	72.14
Jar test + distilled water and ozone 2 mg/L	7	6.75	0	59	78.92

.

These five sequential steps helped achieve the maximum algae removal efficiency in the Mijran Dam reservoir. The outcomes from each step were subsequently analyzed using a spectrophotometer and visually represented as three-dimensional graphs.

3. Study Area

Mijran Dam is located 24.5 km southeast of Ramsar on the Nesaroud River at coordinates x = 472,530 and y = 4,076,970. This zoned earth dam with an asphalt concrete core was completed in 2003 to supply agricultural water to the region, provide potable water to nearby cities, and promote tourism development. The reservoir’s storage capacity is approximately 8 million cubic meters. Over time, the reservoir has experienced eutrophication or algal bloom. Various methods can mitigate this issue. This study investigates one such method, yielding valuable results. Figure 1 presents a visual representation of the dam and the study area.

4. Results

The combination of ferric chloride at a concentration of 75 mg/L and ozone at a concentration of 2 mg/L at a pH of 7 demonstrated the highest algal removal efficiency relative to chitosan at a concentration of 500 mg/L combined with ozone at a concentration of 2 mg/L.

4.1. Optimal Ozone Concentration in Pre-Ozonation Without Adding Coagulant Compounds

The ozone dosage was optimized at this stage, with the results summarized in Table 3 and Figure 2. Subsequently, algal samples were diluted and mixed with ozonated distilled water at 2, 4, 6, and 8 mg/L concentrations in a 1:1 ratio.

An increase in ozone dosage during pre-ozonation resulted in a corresponding enhancement in algae removal efficiency.

4.2. Optimized Ozone pH Without Adding Coagulant Compounds

Adjusting the pH values as outlined in Table 4 found that a pH of 7 was the most effective.

4.3. Spectrophotometer Results for Pre-Ozonation

In this phase, a visible-light spectrophotometer with a wavelength range of 350–700 nm was employed to detect the presence of algal biological activity. When light interacts with algae, specific wavelengths are absorbed while others are reflected. Figure 2 illustrates a three-dimensional visualization of algal removal efficiency before ozonation for diatoms.

4.4. Optimal pH of Chitosan as a Natural Coagulant Alone

The experiments continued with a jar test under standard conditions using raw water at various pH levels: 3, 5, 7, 9, and 11. Chitosan was employed as the coagulant to determine the optimal pH for algal removal. The highest algal removal efficiency was observed at a pH of 10 (Table 5).

4.5. Optimal Concentration of Chitosan as a Natural Coagulant Alone

Based on the optimal pH determined in the preceding stage, chitosan was employed at 20, 50, 75, 100, 200, and 500 mg/L concentrations. The resultant data are presented in Table 6.

4.6. Spectrophotometer Results for Chitosan

A spectrophotometer was employed to analyze the absorbance and reflectance spectra of algae when treated with chitosan. The results are graphically depicted in Figure 3. As presented, the efficacy of chitosan in removing diatoms is visualized in three-dimensional plots.

4.7. Optimal pH of Ferric Chloride a Standalone Synthetic Coagulant

In this phase, jar tests were conducted under standard conditions to determine the optimal pH for algae removal using ferric chloride as the coagulant. Raw water was subjected to a pH range of 3 to 11 to determine the optimal pH. The results indicated that the maximum algae removal efficiency was achieved at a pH of 10 (Table 7).

4.8. Optimal Concentration of Ferric Chloride as an Artificial Coagulant Alone

Given the optimal pH in the preceding phase, ferric chloride was applied at 5, 15, 30, 50, 75, and 100 mg/L concentrations. The resulting data are presented in Table 8.

4.9. Results of the Spectrophotometer for Ferric Chloride

A spectrophotometer was employed during the ferric chloride treatment to determine the absorption and reflectance wavelengths of the algae. Figure 4 presents three-dimensional visualizations of algal removal rates achieved during the ferric chloride treatment for diatoms.

4.10. Results of Alterations in the Ozone Dosage Using Optimal Chitosan

A chitosan concentration of 500 mg/L, obtained from previous steps, was mixed with ozone concentrations of 2, 4, 6, and 8 mg/L. The resulting data are presented in Table 9.

4.11. Results of Alterations in Ozone Concentration Using Optimal Ferric Chloride

As established in preceding experiments, the optimal ferric chloride dosage of 75 mg/L was paired with ozone doses ranging from 2 to 8 mg/L. The results of these combinations are displayed in Table 10.

5. Discussion

This study investigated and compared the efficiency of algal removal in pre-ozonation processes and the addition of coagulants using chitosan and ferric chloride. Various experiments were conducted on water samples collected from the outflows of Mijran Dam.

Laboratory results determined the identification of algae. Our investigation identified one primary group of toxic algae, namely diatoms, which were found to be responsible for the unpleasant taste and odor in the water.

They often accumulate in shallow areas, forming thick, foam-like mats in green or brown. The formation of such mats in a water supply source can lead to cyanobacteria-associated problems. In severe cases, algal blooms have been linked to human fatalities. Consumption of water contaminated with algae like anabaena and microcystis can cause gastrointestinal problems. The blue-green alga chlorella can induce skin irritations. Algae may also trigger allergic reactions. Anabaena is particularly known for causing unpleasant tastes and odors in water.

While ozonation effectively eliminates algae and viruses, it also has certain drawbacks. The process is energy-intensive, and ozone is a highly toxic and corrosive gas. However, pre-ozonation offers several advantages, including the complete absence of residual ozone in treated water, effective removal of compounds causing unpleasant tastes and odors, and elimination of algae and viruses to a great extent. It has minimal impact on water pH and chemistry, and the contact time required for ozonation is relatively short. Previous studies [19]. have highlighted ozonation as a reliable method for algal removal in water treatment. Additionally have demonstrated that pre-ozonation adversely affects the coagulation of diatoms using alum or polymers [20].

A study conducted demonstrated the efficacy of advanced electrocoagulation in removing algae and organic matter from surface waters. Their experiments reported remarkable 97% and 99% removal efficiency rates for organic matter and algae, respectively [21]. These findings suggest that advanced electrocoagulation may serve as a promising technique for algal removal in water treatment processes.

These approaches can be employed as suitable solutions for odor and taste issues associated with microalgae. Nevertheless, future research should focus on elucidating the mechanisms underpinning flocculation and other pertinent factors influencing algal removal to enhance efficiency at various treatment stages. Algal blooms represent a substantial threat to the integrity of aquatic ecosystems, compromising water quality and ecological balance. The use of ozonation combined with ferric chloride and chitosan holds considerable value in eliminating algae, contributing to sustainable development within the social pillar of the three sustainability pillars: social, environmental, and economic, as reviewed by Ebrahimipour and Eslami [22].

6. Conclusions

Experimental results indicated that, under conditions of pH 7 and a settling time of 15 min, ozone alone at a concentration of 8 mg/L achieved a removal efficiency of 70% for diatoms. When chitosan was applied at a concentration of 500 mg/L, pH 10, and a settling time of 30 min, removal efficiencies for diatoms ranged 9–20%. Additionally, ferric chloride at a concentration of 75 mg/L, pH 10, and a settling time of 30 min yielded removal efficiencies of 10–20% for diatoms.

The combined application of ozone (2 mg/L) and chitosan at the optimal concentration (500 mg/L) resulted in the removal of approximately 77% of diatoms. A similar experiment using ozone (2 mg/L) and ferric chloride at the optimal concentration (75 mg/L) achieved removals of 79%, respectively. The findings demonstrate a significant potential for ozone dosage reduction when integrated with other treatment methods. It is suggested that the remaining algae be removed by other methods, such as using a micro strainer with a very small mesh size or flotation with dissolved air. The water whose algae has been removed by the above methods enters the sedimentation system, and the formed floc settles. Then, it enters the sand filter, removes the remaining suspended particles, and is stored in the clean water storage tank and used.

According to the findings, combining ferric chloride and chitosan with ozonation enhances removal efficiency and reduces the required ozone dosage. By exploring these applications, society can benefit from improved water treatment processes, leading to cleaner water resources and reduced environmental impacts.