Study on Purifying Effect and Mechanism of "Combined Artificial Floating Island" on Black-odorous Water

In the process of traditional ecological floating island purification, floating plants have limited absorption capacity. And it also have limited effect on deepwater treatment. Because of the shortcomings of the traditional floating island, the "Combined Artificial floating island" device, which combines the ecological floating island, activated carbon fiber and aeration, is proposed to study its purification effect on black-odorous water. After 22 days of experiment, the removal rates of COD, TN and TP reached 83.9%, 50.2% and 26.4% respectively for the "Combined Artificial floating island" combined treatment unit, while it reached 70.5%,39.2%and 17.5% respectively for the traditional ecological floating island. The experimental results show that the removal rates of COD, TN and TP in water body are increased by 13.4%, 11% and 8.9% respectively for the treatment of "Combined Artificial floating island" combined treatment unit, and the purification effect is greatly improved compared with the traditional ecological floating island. The “Combined Artificial floating island” uses synergistic effect of particle sedimentation, plant absorption, microbial degradation and activated carbon fiber adsorption to purify water. Therefore, it can adapt to water bodies with different water quality and water depth, and has wider application range and stronger adaptability. Introduction Artificial floating island (AFI) technology is a water surface planting technology using the principle of soilless cultivation, which achieves the purpose of purifying water quality through the absorption and degradation of nitrogen, phosphorus, COD and other pollutants by plant roots and microorganisms. As AFI has many functions, including reducing the dispersion of pollutants, water purification, ecological protection, and landscape effect, it has become a potential candidate for black-odorous water treatment [1-4] . However, due to the limited root area of the plant, only a small amount of biofilm attachment space can be provided. And because of the limited root length, there are limitations to the treatment of deeper water. At the same time, the low concentration of DO in black odorous water is not suitable for the growth of plants and microorganisms. Some materials have been discussed in previous studies for AFI concentration. Activated carbon has a large specific surface area and adsorption capacity, which is very suitable for water treatment [5-8] . Oxygen aeration has the effect of purifying water by forcing water through aeration; it provides oxygen for microbial oxidative decomposition of organic matter [9] . Based on previous research, we have produced Combined Artificial floating island (CAFI) , which link together with floating islands, activated carbon fibers and aeration. Its purification effect on black and odorous water was studied. More specifically, CAFI removes suspended solids (SS) from water by physical manipulation, and through the biosynthesis of pollutants to achieve absorption and utilization [10] . It is reported that aquatic plants can absorb excessive nutrients and reduce pollutants in wastewater such as COD, nitrate nitrogen and phosphate [11-13] . It can also reduce light penetration, and competition for water nutrients, it can inhibit the growth of algae [4] . [14] indicated that different plant species in the AFI system had different contaminant removal capabilities because of their specific biological properties such as nutrient absorption efficiency, growth rate and root types. What’s more, the roots and stems 119 of aquatic plants and activated carbon fibers provided space for microorganisms, and oxygen released from plants increased dissolved oxygen (DO) in water, which could increase the removal rate of COD [3, 15] . Microorganisms from wastewater that attached on the stem and root surface of plants and activated carbon fibers, formed so-called biofilms through repeated proliferation processes [15] . [15] and [16] provided evidence that root surfaces with more biofilm had greater contaminant removal capabilities, which may indicate the importance of biofilm in water purification processes. The aeration can increase the dissolved oxygen content in the water [17] . At the same time, activated carbon fibers can remove pollutants from water by adsorption [8, 18] . Materials and Methods "Combined Artificial Floating Island" Combined Treatment Unit As shown in Figure 1, the "CAFI" is composed of Artificial floating island + activated carbon fiber + aeration. It includes an artificial floating island, activated carbon fibers suspended at the lower side of the floating island and an aeration assembly fixed to the lower side of the floating island. The floating island in each device is composed of two 33cm*33cm plastic plates, and each can be planted with one canna. The activated carbon fiber material in each device is made of activated carbon fiber felt with an area of 0.1m 2 and a thickness of 3mm. The aeration assembly includes an aeration pipe and an aerator. The aerator is supplied with oxygen, and the aeration amount is 0.1 L/min. Figure 1. "Combined artificial floating island" treatment unit. The "CAFI" introduces activated carbon fiber and aeration to purify the deep water. Carbon fibers provide more space for biofilm, which shows strong adsorption and biodegradation to pollutants. There is one spherical filter material fixed under each activated carbon fiber component to prevent the activated carbon fiber from floating on the water surface. The aeration device can increase the dissolved oxygen content in the water, promoting the growth and reproduction of aerobic microorganisms. And adequate oxygen promises the growth of plant roots, laying a foundation for the restoration of the ecosystem in black and odorous water. Methods The water used in the experiment was taken from Huxi River in the East Campus of Huazhong University of Science and Technology. Due to the point source and non-point source pollution, it is in a black odor state for a long time. The main indicators of initial water are shown in Table 1. Table 1. Main water quality indicators of initial water (mg·L -1 ). COD/(mg·L -1 ) TN/(mg·L -1 ) TP/(mg·L -1 ) DO/(mg·L -1 ) pH 110.6±2.2 11.74±0.68 0.79±0.06 0.1±0.0 7.65±0.09