Analysis of sewage treatment technology in artificial wetland system-Fornitore globale

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Analysis of sewage treatment technology in artificial wetland system

The water purification technology of artificial wetland system, as a new type of ecological sewage purification treatment method, is based on the basic principle of planting specific wetland plants on the filling material of artificial wetland, thereby establishing an artificial wetland ecosystem. When sewage passes through wetland systems, pollutants and nutrients are absorbed or decomposed by the system, purifying the water quality.

The key to water purification in artificial wetland systems lies in the selection of processes and the selection and application configuration of plants. How to choose and match suitable wetland plants, and apply them to different types of wetland systems, has become a problem that we must consider before building artificial wetlands.

1. Principles for selecting plants for artificial wetland sewage treatment system

1.1 Plants have good ecological adaptability and ecological construction functions;

Simple and convenient management is one of the main characteristics of artificial wetland ecological sewage treatment projects. If plants with strong purification ability, similar stress resistance, and smaller growth can be selected, it will reduce many management difficulties, especially in post-treatment of plant bodies. Generally, plants found in local or natural wetlands should be selected.

1.2 Plants have strong vitality and vigorous growth potential;

① Frost resistance and heat resistance

Due to the continuous operation of sewage treatment systems throughout the year, it is required that aquatic plants can grow normally even in harsh environments. Plants that have poor adaptability or cannot adapt to natural conditions will directly affect the purification effect.

② Disease and pest resistance ability

Plants in sewage ecological treatment systems are prone to breeding diseases and pests, and their ability to resist diseases and pests directly affects their own growth and survival, as well as their purification effect in the treatment system.

③ Adaptability to the surrounding environment

Due to the long-term immersion of plant roots in water and exposure to high concentrations and varying pollutants in artificial wetlands, the selected aquatic plants not only have strong pollution resistance, but also have good adaptability to local climate conditions, soil conditions, and surrounding animal and plant environments.

1.3 The introduced plants must have strong pollution tolerance;

The removal of BOD5, COD, TN, and TP in wastewater by aquatic plants mainly relies on microorganisms that adhere and grow on the surface and near the root zone. Therefore, aquatic plants with well-developed root systems and strong sewage tolerance should be selected.

1.4 Plants have a long annual growth period, preferably semi withered or evergreen plants in winter;

In artificial wetland treatment systems, there is often a phenomenon of functional decline due to winter plant withering, death, or growth dormancy. Therefore, it is important to focus on selecting aquatic plant types that grow vigorously in evergreen winter.

The selected plants will not pose any hidden dangers or threats to the local ecological environment and have ecological safety;

1.6 It has certain economic benefits, cultural value, landscape benefits, and comprehensive utilization value.

If the treated wastewater does not contain toxic or harmful components, its comprehensive utilization can be considered from the following aspects: ① as feed, aquatic plants with crude protein content>20% (dry weight) are generally selected; ② When making fertilizers, it should be considered that the plant body contains a high amount of effective fertilizer ingredients and is easy to decompose; ③ When producing biogas, the carbon nitrogen ratio of fermentation and gas producing plants should be considered. Generally, the carbon nitrogen ratio of the plant body is selected to be 25-30.5/1; ④ Industrial or handicraft raw materials, such as reeds, can be used to make paper. Scallions, lampreys, cattails, and mugwort are all raw materials for making straw mats.

Due to the fact that urban sewage treatment systems are generally located near the outskirts of the city and have a large area, landscape beautification must also be considered.

However, in practical work, many process designers and builders of artificial wetlands consider factors such as plant uniqueness and ornamental value the most, without considering the plant growth effect, wetland operation effect, growth performance, and ecological safety after planting the plant. This leads to a sharp decline in the function or operating cost of artificial wetlands after a period of operation, ultimately resulting in system paralysis or idle.

2. Research on Plant Characteristics and Plant Configuration Analysis of Artificial Wetlands

2.1 Analysis based on plant types

2.1.1 Floating Plants

Floating plants commonly used for artificial wetland system treatment include water hyacinth, big fescue, water celery, Li's grass, floating duckweed, water spinach, and watercress.

Based on the analysis of the botanical characteristics of these plants, it was found that they have the following characteristics: ① strong vitality, good adaptability to the environment, and well-developed root systems; ② Large biomass and rapid growth; ③ Seasonal dormancy phenomenon, such as winter dormancy or death of water hyacinth, big fescue, water spinach, summer dormancy of water celery, watercress, etc. The vigorous growth season is mainly concentrated from March to October or September to May of the following year The reproductive cycle is short, mainly focused on nutritional growth, with the highest demand for N.

Due to the botanical characteristics of floating plants mentioned above, we must fully consider their respective advantages when configuring artificial wetland plants: ① Due to their strong environmental adaptability, they should be given priority consideration as local advantageous varieties when configuring plants; ② In artificial wetland systems, the removal of nutrients from water mainly relies on the absorption and utilization of plants. Therefore, plants with high biomass, developed root systems, multiple annual growth cycles, and good absorption capacity have become our target of choice By utilizing the seasonal dormancy characteristics of plants, we can provide the correct plant combinations, such as pairing water celery during low temperatures in winter and water hyacinth, big fescue, and other plants suitable for high-temperature growth during high temperatures in summer, to avoid seasonal functional imbalances caused by a single plant variety selection Due to the nutritional growth of these plants, their absorption and utilization efficiency of N should be high. Therefore, when configuring plants, attention should be paid to their absorption and utilization efficiency of N. They can be utilized as advantageous plants for N removal, thereby improving the system's removal efficiency of N.

2.1.2 Roots, corms, and seed plants

This type of plant mainly includes water lilies, lotus flowers, water chestnuts, water chestnuts, taro, Alisma, water chestnuts, Job's tears, water chestnuts, etc. They may have developed underground rhizomes or tubers, or be able to produce a large number of seeds and fruits. They are mostly seasonal dormant plant types, generally withering in winter and sprouting in spring. The growing season is mainly concentrated from April to September.

Rhizomes, corms, and seed plants have the following characteristics: ① They have good resistance to siltation and are suitable for growing in deep and fertile soil layers, and cannot grow without soil; ② The water depth suitable for growth environment is generally around 40-100CM; ③ Having developed underground tubers or rhizomes, the formation of their rhizomes requires a higher amount of P element, resulting in a greater absorption of P Seed fruit plants require a large amount of P and K elements for the formation of their seeds and fruits.

Due to the following characteristics of these plants, they should be fully considered when configuring artificial wetland plant applications: ① Based on the characteristics of these plants, their applications are generally surface flow artificial wetland systems and stable wetland systems; ② Utilizing the characteristic of requiring a large amount of P and K elements for the growth of these plants (mainly tubers, corms, and fruits), they are applied as advantageous plants for P removal to improve the system's P removal efficiency.

2.1.3 Types of aquatic herbaceous plants

This type of plant includes reeds, jiaocao, cattail, dry umbrella bamboo, imperial bamboo grass, fescue, water onion, water sedge, paper sedge, etc., which are the main plant selection varieties for artificial wetland systems. The common characteristics of these plants are: ① strong adaptability or being locally dominant varieties; ② The root system is well-developed, with a large growth rate, and both nutritional and reproductive growth coexist. The absorption of N, P, and K is relatively abundant Can grow in a soilless environment.

Based on the growth characteristics of these plants, they can be planted in combination with subsurface flow constructed wetlands or surface flow constructed wetland systems.

According to the depth and range of root distribution, these plants can be divided into four growth types: deep root clustered type, deep root scattered type, shallow root clustered type, and shallow root scattered type.

(1) Plants with deep root clusters generally have a root system distribution depth of over 30CM, with a deep distribution but a limited distribution area. The aboveground parts of the plants are clustered, such as bamboo grass, reed bamboo, dry umbrella bamboo, wild jiaocao, Job's tears, paper sedge, etc. Due to the large depth of root penetration and wide contact area of these plants, planting them in subsurface flow constructed wetlands can better demonstrate their treatment and purification performance.

(2) The root system of deep rooted scattered plants is generally distributed between 20-30cm, and the plants are scattered. These plants include cattail, calamus, water onion, fescue, water sedge, wild mountain ginger, etc. The root system of these plants also penetrates deep into the soil, so they are suitable for planting in subsurface flow artificial wetlands.

(3) Some shallow root scattered plants such as banana, reed, water chestnut, arrowhead, lotus root, etc. generally have a root distribution between 5-20CM. Due to their shallow root distribution and generally native soil environment, these plants are suitable for deployment in surface flow constructed wetlands.

(4) Shallow rooted clustered plants such as lampshade and taro, due to their shallow root distribution and generally native soil environment, are only suitable for deployment in surface flow constructed wetland systems.

2.1.4 Types of submerged plants

Submerged plants are generally native to clean water environments, and their growth requires high water quality. Therefore, submerged plants can only be used as the final reinforcement and stabilization plants in artificial wetland systems to improve effluent quality.

2.1.5 Other types of plants

Some aquatic landscape plants, due to long-term artificial selection, have weaker adaptability to polluted environments and can only be used as the final reinforcement and stabilization plants or landscape plants for wetland systems.

2.2 Analysis of Plant Native Environment

According to the analysis of the native environment of plants, some plants that are native to the soil environment, such as banana, reed, lampshade grass, dry umbrella bamboo, imperial bamboo grass, reed bamboo, Job's tears, etc., have a certain degree of soil orientation in their root growth. When placed in surface wetland systems, their growth will be more vigorous. However, due to the fact that their root systems mostly grow vertically downwards, the effectiveness of purification treatment is not as good as that applied in subsurface flow wetlands; For some plants that originate from swamps, humus layers, peat wetlands, and lake surfaces, such as water onions, wild bamboo shoots, mountain ginger, fescue, cattails, and calamus, their growth has already adapted to soilless environments, making them more suitable for configuration in subsurface flow constructed wetlands; For some aquatic plants with tubers and roots, such as lotus, water lily, arrowhead, taro, etc., they can only be placed in surface wetlands.

2.3 Analysis of nutrient demand types for plants

According to the analysis of plant nutrient requirements, due to the large gaps between fillers in subsurface flow constructed wetland systems, the area of contact between plant roots and water nutrients is wider than that in surface flow constructed wetland systems. Therefore, plants with vigorous nutrient growth, rapid plant growth, large plant biomass, and several germination peaks per year, such as cattails, water onions, mosses, and sedges, are suitable for planting in subsurface flow wetlands; For plants such as reeds, jiaocao, Job's tears, etc. that have both nutritional and reproductive growth, relatively slow growth, and only one peak germination period per year, they are placed in surface flow wetland systems.

2.4 Analysis of Plant Adaptability to Wastewater

Different plants have varying adaptability to wastewater. Generally, high concentration wastewater is mainly concentrated in the front end of wetland processes. Therefore, in the construction of artificial wetlands, the front-end processes such as strong oxidation ponds and subsurface flow wetlands generally choose plant species with strong pollution resistance. In end of pipe processes such as stabilization ponds and landscape ponds, due to the decrease in sewage concentration, more consideration can be given to the landscape effect of plants.