Aquaculture wastewater treatment technology

Myriophyllum aquaticum, also known as green feather grass, is a emergent or submerged plant. It is native to South America and belongs to the genus Myriophyllum of the small erxiancao family. It has been introduced into China as a landscape plant for more than 200 years and mainly grows in rice fields, streams and ponds Best et al. Found that green foxtail algae had the best treatment effect in the screening and application of aquatic and wetland phytoremediation of surface water polluted by explosives from the Iowa Army Ammunition factory The submerged part of green foxtail algae contains 40% ~ 89% of total nitrogen (TN), which has the ability to store n, and its leaves contain more N, which has a strong preference and tolerance to NH4 + - n Souza et al. Used green foxtail algae as a biological regulator to treat polluted water. The results showed that after 30 days, the maximum removal rate of pollutants was as follows: BOD 75.4%, COD 67.4%, NH4 + - N 98.6%, TN 88.3%, total phosphorus (TP) 93.6% In recent five years, the constructed wetland system constructed by the subtropical Institute of the Chinese Academy of Sciences using green foxtail algae has shown that the average removal rates of TN and NH4 + - n with different concentrations can reach more than 90% and 84% respectively; As a biological regulator, green foxtail algae changed the growth environment of microorganisms in water, affected the abundance, diversity and community distribution of nitrification and denitrification microorganisms, and accelerated the removal of NH4 + - N and TN

Green foxtail algae showed high absorption characteristics of NH4 + - N in aquaculture wastewater treatment, but its physiological characteristics of tolerance to high concentration of NH4 + - N and the characteristics of nutrient absorption and accumulation are still insufficient Therefore, this study analyzes the effects of different high concentrations of NH4 + - N treatment on the physiological characteristics and N and P nutrient absorption of green foxtail algae, in order to preliminarily explore the mechanism of green foxtail algae on the N absorption and removal of aquaculture wastewater, so as to provide valuable materials for better use of green foxtail algae for environmental treatment, in order to provide theoretical and practical guidance for the reduction, harmless and resource utilization of pollutants in green foxtail algae constructed wetland system

1.1 test materials and culture

The materials used in the experiment were collected from Changsha agricultural environment observation and research station of Chinese Academy of Sciences The 20 ~ 30 cm fresh plant samples obtained from the field base are washed with tap water and then distilled water. Finally, the seedlings are domesticated and cultured in Hoagland nutrient solution (Table 1) for about 10 days. The nutrient solution is changed every 4 days and the pH value of the nutrient solution is adjusted to 6.8 ~ 7.0

Before the experiment, green foxtail algae with strong growth and consistent size was selected, its plant length was cut to about 12 cm, washed with ultrapure water, weighed its fresh weight, and then transferred to 0.1 mmol · L-1 calcium sulfate solution prepared with ultrapure water (without other ions) for starvation for 2 days, so as to eliminate the influence of residual n in free space on the experiment The NH4 + concentration of most wastewater is 70 mg · L-1, and it has been reported that green foxtail algae wetland system can have good water purification effect in 200 mg · L-1 NH4 + wastewater. Therefore, set the NH4 + concentration of 70 mg · L-1 as the control and two high NH4 + levels: 210 mg · L-1 and 420 mg · L-1 Hoagland nutrient solution (except N nutrient) was still used as the culture medium, 8 l per box, 20 holes per box, 3 plants per hole, and repeated for 3 times Adjust the pH of the growth solution to 6.8 ~ 7.0 every day, and update the nutrient solution every 4 days

1.2 measurement index and method

Culture for 1, 3, 7, 10, 14 and 21 days, weigh the fresh weight and measure the aboveground height, as well as the TN and TP of the plant; The root length was measured at 7, 10, 14 and 21 days At harvest (21 days), the contents of chlorophyll and soluble sugar in leaves and stems and the content of malondialdehyde (MAD) in tissues were measured

The relative biomass (RB, g), relative growth rate (brgr, G · (g · d) - 1), relative shoot height (RSH, CM) and relative shoot growth rate (srgr, cm · D-1) were calculated according to formulas (1) ~ (4) The contents of chlorophyll a (CA) and chlorophyll b (CB) were extracted with 95% ethanol and determined by spectrophotometry; The content of total chlorophyll (Ca + b) is equal to the sum of chlorophyll a and chlorophyll b; The soluble sugar was determined by anthrone colorimetry and mad by thiobarbital method In the formula, W0, W1 and wn respectively refer to the biomass (g) measured at the beginning, the first day and the nth day of sampling, SH0, sh1 and SHN respectively refer to the stem height (CM) measured at the beginning, the first day and the nth day of sampling, and T1 and TN respectively refer to the days (d) sampled at the first day and the nth day of sampling

Data analysis

The data are the average of 3 repetitions, and the error lines are standard errors Data statistics: SAS software was used for analysis of variance and Fisher's significance test (LSD) to compare the significance level of P < 0.05 between different treatments Microsoft Office Excel 2007 (Microsoft, Redmond, WA, USA) and SigmaPlot 10.0 (SYSTAT software, Inc., Chicago, IL, USA) were used for data analysis and mapping

2 results and analysis 2.1 growth characteristics

Under the high NH4 + treatment of 70 mg · L-1, green foxtail algae grew healthily (Fig. 1), and the stem height and biomass increased linearly with the extension of time [Fig. 2 (a) and (b)]. At 21 days, the relative stem height and biomass of green foxtail algae reached 40.56 cm and 17.82 g · hole-1 respectively, and the growth rates of stem and biomass were 1.94 cm · D-1 and 0.11 g · D-1 respectively (Fig. 3) Compared with the NH4 + treatment of 70 mg · L-1, when the NH4 + level increased to 210 mg · L-1, the relative stem height and biomass decreased slightly, and their growth rate had no significant difference (Fig. 3); When the NH4 + level reached 420 mg · L-1, the stem height and biomass growth rate decreased significantly, only 27.4% and 17.9% of the control, and reached the highest value at 10 days, and then showed poor growth or even death.

The root growth of green foxtail algae was also significantly inhibited with the increase of NH4 + level [Fig. 2 (c)] When transplanted to 10 days, the root growth reached the longest. Compared with 70 mg · L-1 NH4 + treatment, the root length decreased by 21.1% and 44.7% under 210 mg · L-1 and 420 mg · L-1 NH4 + treatment, respectively After 10 days, the roots had basically stopped growing under the NH4 + treatment of 70 mg · L-1 and 210 mg · L-1, while the roots blackened and rotted under the high NH4 + treatment of 420 mg · L-1 (Fig. 1)

2.2 chlorophyll content

High NH4 + treatment significantly affected the chlorophyll content of green foxtail algae (Fig. 4) Compared with the NH4 + treatment of 70 mg · L-1, when the NH4 + level increased to 210 mg · L-1 or 420 mg · L-1, the contents of chlorophyll a and B decreased significantly, and the content of chlorophyll a was only about 50% of the control [Fig. 4 (a)] With the increase of NH4 + level, the ratio of chlorophyll a to chlorophyll b decreased. Under the treatment of 70, 210 and 420 mg · L-1, the ratio was 2.9, 2.6 and 1.9, respectively The change trend of total chlorophyll content in leaves was similar to that of chlorophyll a. compared with NH4 + treatment of 70 mg · L-1, the chlorophyll content in leaves under 210 mg · L-1 and 420 mg · L-1 treatment decreased significantly by 39.7% and 36.3% respectively, but there was no significant difference between them

The total chlorophyll of stem was significantly lower than that of leaf, only 4% ~ 10% of that of leaf Under the treatment of 70 mg · L-1 NH4 +, the content of chlorophyll in stems was 0.12 mg · g-1. When the level of NH4 + increased to 210 mg · L-1, the content of total chlorophyll in stems increased significantly by 40%; When the NH4 + level increased to 420 mg · L-1, the total chlorophyll content of the stem decreased significantly by 30% [Fig. 4 (b)] Accordingly, compared with 70 mg · L-1 NH4 + treatment, under 210 mg · L-1 NH4 + treatment, chlorophyll a and chlorophyll b increased significantly, by 26.5% and 66% respectively; When it reached 420 mg · L-1 NH4 + treatment, chlorophyll a and B decreased significantly by 42.4% and 5.1% respectively [Fig. 4 (b)] The ratio of chlorophyll a to chlorophyll b in stems also decreased with the increase of NH4 + level. Under the treatment of 70, 210 and 420 mg · L-1, the ratios were 2.0, 1.5 and 1.2, respectively

2.3 soluble sugar content

Soluble sugar contents in leaves and stems of green foxtail algae under different high ammonium levels

2.4 malondialdehyde (MAD) content

Malondialdehyde content in tissues of green foxtail algae under different high ammonium levels

2.5 absorption characteristics of nitrogen and phosphorus

As shown in Figure 7 (a), with the extension of time, the N content in plants first increased and then decreased with the increase of NH4 + - N concentration, and the variation range of TN was 30.7 ~ 53.4 mg · g-1 Under the treatment of 70 mg · l-1nh4 + - N, the peak of TN content (40.5 mg · g-1) appeared on the 10th day, which was 14% higher than that on the 1st day When the level of NH4 + - N increased to 210 mg · L-1, there was no significant difference in TN content between the tissue and the control within 7 days of culture; With the extension of time, the content of TN in the tissue was significantly higher than that in the control. The peak value (45.0 mg · g-1) appeared on the 14th day, which was 11.1% higher than that in the control When the NH4 + - N level reached 420 mg · L-1, the content of TN in the tissue was significantly higher than that in the first two high NH4 + - N level treatments. The peak (53.4 mg · g-1) also appeared on the 14th day, 31.8% higher than the control peak At 21 days, due to the dilution effect caused by the increase of plant biomass, the content of TN decreased, even lower than the initial TN content As shown in Figure 7 (b), under different high NH4 + treatments, with the extension of culture time, the content of TP in green foxtail algae tissue gradually increased, and the variation range of TP was 3.8 ~ 7.7 mg · g-1 The difference is not obvious in a short time Under the treatment of 70 mg · l-1nh4 + - N, the highest TP in green foxglove algae tissue was 7.7 mg · g-1 at harvest (21 days), which was nearly twice that at the first day When the level of NH4 + - N increased to 210 mg · L-1 and 420 mg · L-1, the content of TP increased by 47.7% and 54.9% respectively on the first day of harvest, but significantly decreased by 38.6% and 27.0% respectively compared with the treatment of 70 mg · L-1, indicating that high NH4 + - N stress may inhibit the absorption of P

Variation characteristics of N and P contents of green foxtail algae under different high NH4 + - N levels

3 discussion

Aquatic plants can not only assimilate and absorb a large amount of N, P and other substances, but also provide oxygen and habitat for microorganisms and promote the removal of water quality by microorganisms. Therefore, the growth status of aquatic plants is closely related to the removal efficiency of pollutants in wetland system As we all know, NH4 + - n is an available N source for plants, but it has high toxicity to plants. At very low concentration, for example, when the external mass concentration is higher than 2 mg · L-1, it may produce toxicity and affect the growth and development of plants The content of chlorophyll, carbohydrate and the accumulation of mad in plants can reflect the stress and degree of plants In this study, the content of mad increased significantly with the increase of NH4 + - N concentration, indicating that the stress degree of green foxtail algae by NH4 + - N was also gradually increasing However, under the treatment of 70 ~ 210 mg · L-1 high level NH4 + - N, the stem height and net biomass of green foxtail algae increased linearly with the extension of time, and the contents of chlorophyll and soluble sugar were higher, indicating that its photosynthetic capacity was also strong; When the level of NH4 + - N reached 420 mg · L-1, the photosynthetic capacity decreased, the contents of chlorophyll and soluble sugar decreased, and the plant growth was blocked or even died It has been reported that the toxicity of NH4 + - N can be detected when aquatic plants grow in NH4 + - N mass concentration of 1.5 ~ 28 mg · L-1 for 4 ~ 8 days The aquatic plant Scirpus grossus, which is resistant to NH4 + - N, grows best under the treatment of high NH4 + - n of 35 mg · L-1. When the level of NH4 + - n increases to 140 ~ 210 mg · L-1, it will also be under the stress of NH4 + - N, resulting in poor root growth and decreased photosynthetic efficiency This study shows that green foxtail algae can still grow normally under certain stress at the high NH4 + - N level of 210 mg · L-1, and its photosynthetic capacity is also very strong, indicating that green foxtail algae still has high productivity under the condition of high NH4 + - N, which is likely to be an ammonia super tolerant plant

The efficient absorption capacity of green foxtail algae to N and P is one of the important indicators of the pollutant purification capacity of green foxtail algae wetland system Lu Jing et al. Showed that the average TN and TP contents of 44 wetland plants in Erhai Basin were 15.7 mg · g-1 and 3.3 mg · g-1; Jin Shuquan and others compared and studied 10 kinds of aquatic plants and found that their N and P contents varied from 13.67 to 26.38 mg · g-1 and 1.16 to 3.50 mg · g-1, respectively In this study, under the treatment of different high concentrations of NH4 + - N, the contents of N and P in green foxtail algae ranged from 30.7 to 53.4 mg · g-1 and 3.8 to 7.7 mg · g-1, which were higher than those reported in aquatic plants In the existing studies, the treatment concentration of n is relatively low, which may be mainly because it will be poisoned by ammonia and cannot grow normally in a high NH4 + - N medium of more than 28 mg · L-1, thus affecting the absorption of N and P Green foxtail algae can not only grow normally under the condition of ultra-high NH4 + - N, but also absorb a large amount of N and P, indicating that it will make a great contribution to the removal of N and P in wastewater with high NH4 + - n It can be seen from figure 8 that when the NH4 + level is as high as 210 mg · L-1 and 420 mg · L-1, when the amount of dry matter of green foxtail algae decreases linearly, the accumulation of N and P also decreases significantly, but the rate of decline is slower than that of biomass Therefore, the adaptive range of mass concentration should be less than 420 mg · L-1 when green foxtail algae ecological wetland system effectively purifies wastewater with high NH4 + - n

(3) Green foxtail algae can be used as a good candidate aquatic plant in the construction of ecological wetland system for treating high NH4 + - N wastewater