1. Current situation of rural domestic sewage treatment

1.1 Characteristics of rural domestic sewage

(1) The discharge of rural domestic sewage and the types of pollutants are influenced by factors such as diet, customs, climate, and economy. Among them, organic matter, nitrogen and phosphorus are the most common, and heavy metal pollution is less, which is conducive to microbial biochemical decomposition.

(2) Rural domestic sewage is affected by the living population and daily life, and the total amount of sewage discharge is not large, but the daily discharge coefficient varies greatly.

(3) Rural areas are vast and lack a sound sewage collection network, leading to a widespread phenomenon of disorderly discharge of domestic sewage.

1.2 Rural Domestic Sewage Treatment Model

Currently, based on differences in village planning, economic conditions, collection capacity of supporting pipelines, pre standard requirements for water quality treatment, and facility operation and maintenance capabilities, three modes of unified urban-rural treatment, centralized treatment, and decentralized treatment are commonly adopted, as shown in Table 1.

According to Table 1, for rural areas with dispersed population, choosing decentralized sewage treatment technology according to local conditions is more in line with the requirements of ecological livable rural construction.

2. Distributed bio ecological rural domestic sewage treatment technology

Currently, the purification technology for rural domestic sewage in developed countries is developing rapidly, with Japan's "purification tank" technology, the United States' "septic tank+land treatment" technology, and France's "earthworm ecological filter" technology emerging one after another. In recent years, decentralized sewage treatment technologies in rural China have mainly focused on biological (microbial), ecological (plant), and bio ecological combination processes.

2.1 Biological treatment technology

(1) Biological contact oxidation method.

The biological contact oxidation method utilizes the principle of biofilm method, with enhanced aeration at the bottom of the tank to strengthen the contact between the packing and sewage, allowing microorganisms to form a high specific surface area biofilm on the packing. Through its adsorption, oxidation and decomposition of organic matter, it plays a role in purifying sewage. This process produces less sludge and has high purification capacity, but it has a high pollution load and is prone to clogging of fillers. Biological contact oxidation technology is the mainstream technology adopted by the integrated sewage treatment equipment of the main participating enterprises in the rural sewage treatment market. In terms of pollutant removal rate, COD (chemical oxygen demand) can reach 80% to 90%, BOD (biochemical oxygen demand) can reach 85% to 95%, TN (total nitrogen) can reach 30% to 50%, and TP (total phosphorus) can reach 20% to 40%.

(2) Membrane bioreactor (MBR).

Membrane bioreactor uses membrane separation components to intercept organic matter such as colloids or polymers in domestic sewage, while increasing the concentration of microorganisms in the reactor, which is beneficial for the decomposition and transformation of organic matter, optimizing the effluent to meet the indicators, and does not require a secondary sedimentation tank. Membrane bioreactors are easy to operate and manage, highly intelligent, and land saving, and are commonly used in integrated equipment. However, in long-term operation, pollutant deposition is prone to membrane blockage, and the investment capital is high, making it suitable for affluent rural areas along the coast. Research has shown that using MBR reactors to purify domestic wastewater can achieve a removal efficiency of over 95% for COD and N-NH4+.

(3) Anaerobic biogas digester.

Anaerobic biogas digesters are generally buried and use different raw materials such as human and poultry excrement. Under closed conditions, they consume carbon sources, total nitrogen, pathogens, etc. in sewage through the life activities of microorganisms and reduce their content, producing biogas. Anaerobic biogas digesters generate high heat from methane and produce pollution-free combustion products, making them an ideal clean energy source. Research has shown that anaerobic digesters have removal rates of up to 85%, 90%, 60%, and 80% for COD, BOD5 (five-day biochemical oxygen demand), ammonia nitrogen, and total phosphorus, respectively. Anaerobic biogas digesters have the advantages of convenient construction, low cost, operation without energy support, and good treatment effect.

2.2 Ecological treatment technology

(1) Artificial wetland treatment technology.

The artificial wetland treatment technology mainly utilizes the combined effects of physical sedimentation, filler adsorption, plant absorption, and microbial decomposition within the wetland system to purify sewage quality and beautify the environment. Artificial wetlands are divided into two types: surface flow and subsurface flow. Subsurface flow can be classified into horizontal subsurface flow and vertical subsurface flow wetlands, and wetland types can be selected based on requirements such as land area, landscape, and treatment efficiency. Different flow states or multi-level wetland combinations can improve pollutant removal efficiency. The data shows that the removal rates of COD, TP, TN, and ammonia nitrogen in sewage by the surface flow subsurface flow subsurface flow series constructed wetland are 85.7%, 96.4%, 97.8%, and 98.5%, respectively.

(2) Stable pond treatment technology.

Stable pond is a natural or artificially modified pond used for purifying wastewater, simulating the natural self purification process of non flowing water bodies and fully utilizing the combined action of algae and bacteria in the water to purify wastewater. Efficient and stable ponds utilize the ecological food chain, rationally layout aquatic plants, algae, animals, etc., and scientifically improve the purification effect of various pollutants in the ecological food chain. Wang Xingzhi et al. studied the treatment of low concentration organic wastewater using aeration stabilization pond technology combined with contact oxidation method and grass type clear water steady-state theory. The removal rates of COD, ammonia nitrogen, TN, and TP after purification reached 45% to 80%, 69% to 93%, 44% to 87%, and 49% to 89%, respectively. The stable pond body is simple and easy to construct, with low civil engineering costs and low difficulty in operation and maintenance. The effluent can be used for agricultural irrigation. However, the stable pond area and hydraulic retention time are related to the depth of the pond, oxygen supply method, and microbial species. In special seasons, odors may be produced, and the treatment effect is affected by the external natural environment.

(3) Soil infiltration treatment technology.

Soil infiltration treatment technology refers to the pre-treatment of wastewater in natural or artificial soil layers, where pollutants are adsorbed and intercepted by soil physicochemical processes. At the same time, the wastewater is purified through comprehensive processes such as plant absorption and utilization, microbial metabolism and decomposition. The soil matrix and ratio are crucial in soil infiltration treatment technology, as they not only affect the infiltration rate but also have an impact on the removal of TP and TN. According to research, 90% of total phosphorus removal in soil infiltration treatment is achieved through soil adsorption; The nitrification and denitrification reactions in the removal of total nitrogen are respectively limited by the oxygen content in the soil and the electrons provided by the organic carbon source in the underground infiltration system. This technology does not require electricity for daily operation, is easy to operate, and the soil surface can be planted with grass, green landscape plants, or seasonal crops, making it suitable for towns with abundant land resources.

(4) Ecological floating bed technology.

Ecological floating bed is a process of placing various shaped and lightweight biological carriers that can be assembled according to different design requirements into polluted water bodies. Various functional aquatic plants are planted in prefabricated planting tanks, and through the joint action of developed plant roots and attached microorganisms, nutrients such as N/P or heavy metals in sewage are absorbed and degraded. Traditional ecological floating beds have low investment, flexible assembly, and wide application, and are suitable for low concentration polluted water bodies. However, plant planting has seasonality, which can increase maintenance costs. Enhanced ecological floating islands can improve water purification by screening plants, floating bed materials, carrier fillers (such as artificial aquatic plants), efficient microorganisms, aeration and oxygenation, and extending the food chain.

2.3 Bio ecological combination process

With the improvement of rural sewage discharge standards and systems, the purification effect of a single treatment process is limited. The bio ecological combination process fully utilizes microbial treatment technology as the front-end treatment process, which can remove organic matter and nutrients, improve the tolerance range of sewage quality, and meet better effluent requirements; Ecological treatment technology, as a terminal treatment process, can further adsorb total phosphorus and remove total nitrogen, reducing investment and maintenance costs. The research by Zhang Jingyu et al. shows that when using the A2/O-constructed wetland combined treatment process to treat domestic sewage in Tanjiaqiao Town, the removal rates of TN, ammonia nitrogen, total phosphorus, and CODMn (permanganate index) in the A2/O process section are 23.4%, 48.6%, 56.3%, and 30.7%, respectively; After secondary treatment by artificial wetlands, the removal rates of various indicators in the effluent after stable operation were 97.7%, 89.0%, 86.2%, and 53.1%, respectively.