As global warming and climate change led to the spread and increased frequency of green algae in intake sources, swamps, lakes and rivers, it has raised the issue of water pollution management. (Yang et al., 2018). As intake source water, lake water and river water where green algae sprouts need to undergo a water treatment process before they are supplied to households in tap water, it takes a lot of costs and treatment processes. Currently, in Korea, Loess holds 80 percent of total spraying coagulants to remove green algae (Yang et al., 2017). Since loess is easy to use and costs much less than other spraying coagulants, it has been widely used to remove green algae temporarily (WERT, 2012). Also, flocculants are used to deal with water contamination in sewage treatment facilities and the most widely used flocculant is Alum, PAC (Poly aluminum chloride). Alum and PAC, aluminum-based flocculants, may induce central nervous system diseases such as Alzheimer’s and serious environment damages such as the death of flora and fauna, when aluminum ions dissolve in drinking water. To deal with the existing problem of flocculants, there have been several techniques under development to eliminate green algae and pollution materials, such as fine bubble generation and magnetic powder (Ives et al., 1959). Also, other techniques include the development of green algae elimination methods that incorporate single techniques, such as flocculation, destratification and direct elimination of green algae (Park et al., 2015).
Existing researches on torrefied wood include developing high-efficiency fuel (Lee et al., 2016) and analyzing its antioxidant activity (Nam et al., 2018) to expand the use of torrefied wood powder. Based on these previous studies, positively charged torrefied wood powder and paste-type torrefied wood powder flocculant were prepared from natural wood (Yang et al., 2016). The principle behind the generation of flocculant is to remove negatively-charged green algae, treated with superheated steam, by making the negatively-charged torrefied wood powder retain positive ions through surface modification treatment using a cationic polymer electrolyte. According to previous studies, flocculants made from torrefied wood as the main raw material were found to have more than 90% removal efficiency in terms of turbidity, optical density, and chlorophyll-a 3 (Yang et al., 2017). Although green algae removal efficiency was shown to be high, further research is needed due to lack of research on the removal rate and ecotoxicity of organic matter in water. Thus, this study aims to evaluate the impact on the elimination of pollution materials and ecotoxicity when paste type flocculants that have been developed with torrefied wood powder are used in water. Evaluation items include pH, total organic carbon, ammonia, and ecotoxicity. We want to find out whether such results have any impact on the feasibility of eliminating water pollution materials, as well as on marine ecosystems.
MATERIALS and METHODS
Torrefied wood powder
The test material used for torrefied wood powder included oak chips for pulp with a water content of 12% equal to that of wood powder. It was manufactured by using superheated heavy treatment in which rapid heat treatment is possible to use steam at high temperature. It was made by Seungjin Inc. under the conditions of 300 and 350 degrees in temperature and 10, 15, 20 and 25 minutes in time, respectively. After the grinding process, a torrefied wood powder of 75-106 um size was used.
Cationic polyacrylamide (called C-PAM here after)
Cationic polyacrylamide was used as a test material to modify the positively-charge surface of the prepared torrefied wood flour. C-PAM provided by Seungjin Bio Co., Ltd. was Praestaret™ OMC 849BS, M / W 8,000,000 g / mol.
Preparation of paste type coagulants using torrefied wood powder flour
In this study, 0.2% of cationic monomer solution was added to 1 g of cationic polyacrylamide in 500 ml of distilled water and stirred at 800 rpm for 2 hours, which resulted in TWF and paste-type flocculant with water content of 6% or less. The paste type flocculant was prepared after adding the torrefied wood powder to the prepared 0.2% aqueous monomer solution, and the mixing composition ratios are shown in Table 1. The mixture was stirred at 800 rpm for 2 hours before using as the test material of this experiment.
To evaluate the removal rate and the ecotoxicity of the test samples, green algae contaminated water was collected in Yeongdang Reservoir in Chungcheongnamdo. 700 ml of algae water was added to a 1 ℓ beaker, which was rapidly stirred at 800 rpm for 30 seconds depending on the type of sample and the dosage conditions (1 g, 2 g, 5 g, 10 g), and it was allowed to settle for 20 minutes. Then, 300 ml of the supernatant was collected and used as a sample for analysis. For analysis, items such as total organic carbon and ammonia removal rate according to the general items in the water pollution process test standard (No. 2017-4) were evaluated. In addition, pH and ecotoxicity measurements that were closely related to aquatic ecosystems were also measured.
TOC, Total Organic Carbon
Total organic carbon concentrations of the algae contaminated water and the treated water were measured using a TOC analyzer (multi N / C3100, Jena, Germany) according to the total organic carbon (ES 04311.2b) test criteria of 『Water Pollution Process Test Criteria (No. 2017-4)』.
Ammonia concentration of the algae contaminated water and the treated water was measured using an ammonia measuring instrument (HI96715, HANNA) according to ammonia (ES 04355.1c) test criteria of 『Water Pollution Process Test Criteria (No. 2017-4)』.
Hydrogen ion concentration of the algae contaminated water was measured using a pH measuring instrument (HI96715, HANNA) according to hydrogen ion concentration (ES 04306.1b) test criteria of 『Water Pollution Process Test Criteria (Temp Meter pH-200L, ISTEK CO., LTD.)』.
Ecotoxicity of the algae contaminated water and the treated water was measured using the acute toxicity test involving daphnia (ES 04704.1a) of 『Water Pollution Process Test Criteria (No. 2017-4)』. The daphnia used in the test was daphnia magna straus, with less than 24 hours of growth, that came from a more-than-two-weeks-old adult.
The experiment was performed with four repetitions of five daphnia in filtered water. After 48 hours of exposure, acute half-effect impact concentration (EC50) was calculated using the Trimmed Spearmen-karber (TSK) method.
EC50 : Media effective concentration
RESULTS and DISCUSSION
Characteristics of algae bloom water
Green algae contaminated water from Yedang Reservoir, Yesan-gun, Chungcheongnam-do, Korea was collected to evaluate the removal rate of pollutants and ecotoxicity. The measured items included total organic carbon, ammonia and hydrogen ion concentration between the green algae polluted water and the treated water. Green algae contaminated water in Yedang reservoir was analyzed and its results are shown in Table 2. For lake water such as reservoirs, the hydrogen ion concentration should be in the range of 6.0-8.5, and total nitrogen containing ammonia should be 0.2- 1.5 mg/ℓ. Also, turbidity should be 1-15 NTU. According to the analysis of contaminated water, turbidity, TOC, and ammonia concentration did not meet the water quality criteria except for pH and ecotoxicity.
TOC (Total Organic Carbon)
Total organic carbon refers to the concentration of organic matter in water, meaning the total amount of carbon contained in water. Higher levels of total organic carbon promote microbial development in water. Table 3 below shows the total organic carbon removal rate according to the sample conditions. The treatment conditions of 1% and 5% of torrefied wood flour C-PAM showed high removal rate of 86-92%. With 10% of torrefied wood flour C-PAM, the TOC removal rate was 80% when 1 g of flocculant was injected, which was lower than that of other flocculants. This seemed to be due to the difference in corresponding charges between the contaminants to be removed and the flocculants prepared. With 20% of torrefied wood flour C-PAM, the removal rate of 89% was obtained when 1 g of flocculant was injected. However, as the injection amount increased, the TOC removal rate drastically decreased. This seemed that when the input amount of torrefied wood flour increases above a certain concentration, organic carbon and organic substances, the major constituents of torrefied wood flour, are eluted in water, thereby reducing TOC removal rate. In addition, when only 0.2% of C-PAM was added, the removal rate of TOC was increased according to the amount of additions, but the removal rate was higher than when the torrefied wood flour was added. Given that but the removal rate was higher than when the torrefied wood flour the coagulant containing 1% of torrefied wood powder is deemed to be suitable for the removal of organic substances in water.
Ammonia in water consists of unionized ammonia (NH3) and ammonium ion (NH4+). Out of the two, the harmful type to marine life is unionized ammonia. Therefore, ammonia concentration was measured in this study. Fig. 1 shows the ammonia concentration in the algae contaminated water and the concentration of ammonia corresponding to the added ammonia when the paste type flocculant was prepared according to the added amounts of torrefied wood powder. The initial ammonia concentration of the algal bloom contaminated water was 4.9 mg/ℓ, and there was a difference in the removal of ammonia following the sample injection depending on the torrefied wood flour content. In the case of the treated water, about 0.4-2.6 mg/ℓ was found to be reduced from 2.3 mg/ℓ to 4.5 mg/ℓ. When only 0.2% C-PAM was added, the ammonia concentration was removed up to 2.6 mg/ℓ at a constant rate. However, the removal rate seemed to decrease due to the addition of torrefied wood flour. Since ammonia exists in dissolved ionic state in water, it was difficult to remove coagulation due to the chemical bonding reaction required for removing the dissolved substance, which seemed to lead to the difficulty in removing ammonia. However, when selecting coagulants that are suitable for removing organic, it is important to consider other factors that may affect aquatic ecosystems.
Therefore, when removing ammonia, 1% of torrefied wood flour C-PAM and 20% of torrefied wood flour C-PAM had the least amount of variation according to the injection amount, and the 1% of torrefied wood flour C-PAM with relatively high removal of ammonia was considered to be effective for ammonia removal.
Among the environmental standards for water quality and aquatic ecosystems, hydrogen ion concentration in the river's living environment standards is determined to be in the range of pH 6.5-8.5. Hydrogen ion concentration in water is an indicator closely related to the degree of algae bloom. This is because phytoplankton depletes carbon dioxide in the water during photosynthesis during the day, causing the water to become alkaline. Even slight changes in pH become stressors to marine organisms, causing various physiological effects (Moon et al., 2017). In addition, if the concentration of hydrogen ion in water is more than 9, it causes damage such as skin bleeding of aquatic organisms, and, if below 6, directly affects them, causing difficulty in growth of aquatic organisms and even death of fish and shellfish (Park et al., 2016). Fig. 2 below shows the results of measuring the hydrogen ion concentration of the algae contaminated water and the hydrogen ion concentration of the treated water according to the amount of the additive paste type flocculant prepared according to the amount of torrefied wood powder. The hydrogen ion concentration of the green algae used in this study was pH 7.9, which was decreased by about 1 with the addition of 0.2% C-PAM. This concentration does not affect the water ecosystem, but only 0.2% C-PAM seemed to affect the pH concentration in the water. On the other hand, adding 1% and 5% of torrefied wood flour C-PAM did not affect the pH in the water. The pH of the treated water was found to be at pH 6.5-7.7 under all conditions, meeting the water quality standards.
Green algae are divided into cyanobacteria and algal bloom and their excessive growth causes toxic substances, microcyctis and anabaena. In most lake water, microcyctin occurs first. Microcyctin is algal bloom in freshwater that produces neurotoxins that affect the nervous system and hepatotoxins that affect the liver. It shows high hepatotoxicity and toxicity that inhibits proteolysis, and is known to cause contamination of internal body tissues and if severe, even death (Kim et al., 2015). Thus, in this study, the ecotoxicity of green algae contaminated water in the sample was analyzed by using water fleas to evaluate the presence of toxic substances due to green algae. In order to evaluate the presence of toxic substances according to the addition of the developed flocculant, the paste type flocculant treatment was performed. As shown in Table 4, all of the experimental results indicated that the water fleas survived, which suggested that the algae contaminated water and treated water had no effect of toxicity. There was no ecotoxicological effect on the harvested green algae contaminated water, since there wasn’t enough time for blue-green algae to produce toxic substances because the samples were taken early in the development of green algae.
In this study, the effects of pollutants and the ecotoxicity on the ecosystem were investigated using the paste type coagulant prepared by mixing and stirring cationic PAM and torrefied wood flour prepared by superheated steam treatment, which leads to the following conclusions.
Total organic carbon was removed between 59%-91% in all treatment conditions. The relatively low removal rate of 59% seemed to be due to organic carbon and organic substances that appeared in the water when the input amount of torrefied wood flour was increased, which may have reduced the TOC removal rate. On the other hand, when the input amount of torrefied wood powder was appropriate, the total organic carbon removal rate was 91%, confirming that it was applicable to the treatment of total organic carbon in water. In addition, the initial concentration of ammonia in the contaminated water was a high concentration of 4.9 mg/ℓ, along with a removal rate of 8-50%.
pH and ecotoxicity had a direct impact on aquatic organisms and the initial pH of the algae contaminated water was 7.9. When treated with paste type coagulant the pH was 7-7.7, not affecting aquatic organisms, and when the 1%, 5%, 10%, and 20% of torrefied wood flour C-PAM coagulants developed in this study were used, there was no ecotoxicity.
The paste type coagulants developed in this study was used to evaluate the removal of contaminated items such as organic carbon and ammonia, and variables that affect aquatic ecosystems such as pH and ecotoxicity. The results showed that organic carbon showed a high removal rate corresponding to the addition of the appropriate amount of coagulants developed in this study. On the other hand, since ammonium ions exist in water in the form of dissolved ions, they are not easily deagglomerated due to the chemical bonding reaction required when removing the dissolved substances, which may have led to the difficulty in ammonium removal. In addition, the results of no pH change or ecotoxicity implied that flocculants would not be harmful to aquatic ecosystem when they are used to remove organisms from water. Therefore, the paste type coagulant developed in this study can be safely used in water treatment without toxic effects on ecosystems. For example, organisms such as TOC can be safely treated without harmful effects. However, the flocculant developed in this study showed a low removal rate when removing dissolved organic substances such as ammonia. This is probably because it has a high physical binding strength, but the chemical cohesion reaction between the dissolved organic material and the chargeable molecules present in the flocculant is low, which leads to difficulty in chemical flocculant reaction. Therefore, the coagulants developed in this study seems to be more effective in removing organic materials such as green algae and TOC from intake and lake sources than in treating dissolved organic substances. However, in order to solve the problems of polluted water with ecotoxicity, further studies are needed on removing toxic substances and selectively removing ionic contaminants dissolved in water.