Silica removal at sewage treatment plants causes new silica shortages

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The mean annual DSi concentrations in the inflow and outflow of the sewage treatment plant were 235.4 ± 42.8 µmol L−1 (annual mean ± standard deviation) and 193.9 ± 47.6 µmol L−1, or (Fig. 1). The DSi concentration in the effluent was significantly lower than the concentration in the influent in 40 of the 46 observations (t-Test, p−1 and 12,093 ± 3,542 mol/day−1and the outflow was significantly less loaded than the inflow (ttest, p6 mol year−1 for inflow and 4.43 × 106 mol year−1 for waste water (Fig. 2). In contrast, the mean annual PSi concentrations in the inflow and outflow of the sewage treatment plant were 67.7 ± 23.9 µmol L−1 and 2.5 ± 1.6 µmol L−1, or (Fig. 1). In contrast to the DSi concentration, the PSi concentration in the effluent was significantly lower than the concentration in the influent for all observations (t-Test, p−1 and 153 ± 104 mol days−1and the outflow had a significantly lower value than the inflow (t -Test, p 6 mol year−1 for inflow and 5.61 × 104 mol year−1 for waste water (Fig. 2). The annual removal rates of DSi and PSi were 29.5 and 96.9%, respectively. PSi was of course removed by precipitation during the primary treatment at the WWTP. However, previous studies have shown that DSi concentration and loading do not decrease significantly during primary and secondary treatment in STPs14.

illustration 1

Annual mean boxplots (a) DSi and (b) PSi concentrations (µmol L-1) in the STP inlet and outlet. For each boxplot, the median is represented by the bold line, while the 25th and 75th percentiles are represented by the bottom and top of the box, respectively. The whiskers represent the minimum and maximum values ​​(1.5 × interquartile range), while outliers are represented as circles.

figure 2
figure 2

Si cycle model in an STP. Each line shows the Si amount of STP inflow, outflow, and outflow, respectively. The reduction of DSi and soluble PSi by the STP creates a new silica deficiency hypothesis (Illustration courtesy of the Picture Library, free material provided by the Integration and Application Network, University of Maryland Center for Environmental Science [ian.umces.edu/imagelibrary/]).

Two possible reasons for the decrease in DSi concentration at the STP of this study are physicochemical and biological removal. The co-precipitation of DSi with metal ions such as magnesium, aluminum and iron ions is a factor. It has been widely reported that magnesium hydroxide particles precipitate most efficiently under alkaline conditions17. In addition, metal ions such as magnesium, aluminum and iron accelerate the polymerization of DSi at pH 6 and 918. Therefore, the co-precipitation of DSi with metal ions high in the STP influent may have reduced the DSi concentration of the effluent. Magnesium hydroxide is commonly used to neutralize sulfur oxides and neutralize wastewater in factories. However, excessive addition of magnesium hydroxide can cause inflow into the STP.

The second factor, biological removal, is the uptake of DSi by bacteria of the genus bacillus . Spores of species in this genus have a layered structure to protect the nucleic acids and proteins, with a layer of nano-sized particles outside the enveloping layers19. DSi is built into these layers19. bacillus Species are ubiquitous in a variety of habitats, including the soil20fresh water21and marine sediments22. In addition to these natural environments bacillus sp. are present in activated sludge used for secondary wastewater treatment in sewage treatment plants23.24. bacillus absorbs 29 µmol L−1 H−1 of DSi during growth19. According to Murakami et al.25 bacillussp. make up 92–98% of all bacteria (5 × 107 to 5 × 1010 cells ml−1) in the activated sludge of the post-treatment. Special, Bacillus thuringiensisis indispensable in sewage treatment plants because of its ability to break down starch and oil26. This is what these results indicate bacillusBacteria living in the secondary treatment environment of sewage treatment plants ingest DSi and precipitate it as sludge in the effluent influent, thereby reducing the DSi load in the effluent.

This study does not know whether physicochemical or biological removal significantly affects DSi reduction in STP. Physico-chemical removal occurs during the primary treatment process while biological removal occurs during the secondary treatment process. Therefore, it is possible to identify the primary factors by evaluating the changes in DSi concentration during each treatment process. In addition, it is still unclear what kind of changes occur at STPs that carry out the advanced treatment process. In addition, that is why DSi reduction in some STP (this study) and not in the others14 was unclear (e.g. a difference in pH, alkalinity and activated sludge quality). We need to pay more attention to the DSi changes in STPs and evaluate them in more detail.

In the PSi dissolution experiment using STP inflow (Table 1), the DSi concentration increased by an average of 8.1 ± 6.8% and the PSi concentration decreased significantly by an average of 20.3 ± 7.5%. The total Si concentration (DSi + PSi) was 304.0 ± 35.2 µmol L−1 before incubation and 297.2 ± 42.6 µmol L−1 at the end of the incubation, indicating no significant difference. The dissolution experiments suggest that part of the PSi entering the STP is soluble, about 20.3%. Therefore, the soluble PSi loading in the STP feed would be approximately 0.36×106, and most of this soluble PSi would be expected to be removed by the STP. This amount is comparable to the 20.0% DSi removal amount of 1.85×106 mol year−1 through the STP (Fig. 2). Therefore, the removal of DSi and soluble PSi attributable to the construction of the WWTP may have reduced the DSi load in the watershed by 2.21 × 106 mol year−1.

Table 1 PSi dissolution experiment results. The results at time 0 and time 7 days show the concentrations of the STP influent before and after the incubation experiment, respectively.

In this study, DSi and PSi loads decreased due to the progress of sewage maintenance in the watershed. Thus, the future construction of sewage treatment plants in coastal watersheds leads to a “new silica deficiency hypothesis” for coastal waters around the world (Fig. 2). Currently, however, time series observations of DSi are less available than those of nitrogen and phosphorus in many coastal waters. Therefore, data on DSi as a key parameter for environmental change should be continuously collected from now on before changes occur.

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