Resource-based agricultural non-point source pollution control using biological agents

Non-point source (NPS) pollution is a major cause of the deterioration of surface water quality. Effectively controlling NPS pollution, especially that from rural areas, is critical in improving the quality and safety of water resources. However, most of the approaches employed for NPS control are low-efficiency or impractical owing to ignorance of the willingness of participants, as well as the high cost. To improve the effectiveness and efficiency, this study newly proposes a resource-based approach for agricultural NPS pollution control via using biological agents. In this approach, the domestic organic waste and livestock manure are economically utilized to produce organic fertilizers and plant protection agents to replace harmful chemical fertilizers and pesticides, respectively. The approach has been applied in the Danjiangkou catchment area in Nanyang City, and the results show that: (1) the most severe pollutants, total P and total N concentrations have been effectively reduced and their values in 2025 are predicted to be the same level as that before significant deterioration; (2) with unpowered sewage treatment and spontaneous participation of farmers, the treatment cost is significantly reduced; (3) the resource-based treatment of domestic waste and livestock manure supports the development of an efficient and sustainable agricultural economy.


INTRODUCTION
Non-point source (NPS) pollution deteriorates the surface water quality of rivers and lakes. In contrast with point source pollution, which can be easily monitored and controlled, NPS pollution remains underestimated and poorly controlled (Wu et al. ). The contribution of NPS pollution to the overall environmental pollution has gradually increased, and NPS pollution is becoming the primary source of surface and groundwater pollution (Ongley pollution that does not meet the legal definition of 'point source' in section 502(14) of the Clean Water Act' (United States ). Agricultural NPS pollution, which is mainly caused by fertilizers, pesticides, sewage sludge, and livestock manure, includes nutrients such as N and P, as well as those in pesticides, and other organic or inorganic pollutants (Li et al. ; Li et al. ). Agricultural NPS pollution is an important cause of water quality deterioration in surface waters (Liu et al. ). Owing to the prevalence, extensive dispersion, and uncertain transport paths of NPS pollutants, the mechanism of agricultural NPS pollution is not well-known; however, it does involve a long latent period, which is significantly harmful (Zeng ). Agricultural NPS pollution is considered as a world-wide environmental issue (Yang et al. ), and it hinders the sustainable rural development in China (Liu et al. ). Therefore, the development of an effective approach for controlling NPS pollution is crucial. In China, research on NPS pollution started in the 1980s.
Thereafter, studies on urban NPS pollution were conducted in Beijing, Shanghai, Hangzhou, Suzhou, Changsha, Nanjing, and Chengdu, among other cities (Li et al. ). Simultaneously, research on rural NPS pollution was successively conducted in the Yuqiao Reservoir, Dianchi Lake, Taihu Lake, Chaohu Lake, Jinjiang River Basin, and Dongjiang River Basin (Chen et al. ; Cai et al. ; Xue et al. ).
Since the 1990s, macro characteristics and statistical models

Biological agents
As shown in Figure 2, the whole framework of NPS pollution control runs on the base of biological agents, which unite all parts of NPS pollution control via a stable circulating system. In this research, the biological agents are some organic matters that serve for environmental protection, mainly including the deodorizers and antidotes, eco-agricultural synergists, plant protection agents and soil purification agents, and water purification agents, as shown in Figure 3.
The biological agents are produced from the domestic waste and livestock manure, and can be used in the domestic

Components
The proposed approach is composed of three parts, namely the production of biological agents, treatment of waste and livestock manure, and domestic sewage treatment.

Production of biological agents
The   .
where N denotes the concentration of any one form of nitrogen (mg/L), A x is the cross-sectional area (m 2 ), D L is the dispersion coefficient (m 2 /s), S denotes the external pollution sources or sinks (g/s), u is the flow velocity (m/s), V is the water volume of calculated river reach (m 3 ), t is time (s), and x is distance (m).

STUDY AREA
Overview of the study area   Tuancheng Lake in Beijing City. The length of the section in Nanyang City is 185.5 km, which accounts for 1/7 of the total length. Owing to its sensitive regional location, Nanyang City is responsible for ensuring the quality of water sent to Beijing City and for the sustainable management of water resources.

Agricultural non-point source pollution data
According to the field surveys, the COD and the amount

Domestic waste and manure treatment results
The employed waste and manure treatment system in the study area can produce 210,000 t/y of organic fertilizers, in which the organic matter content is greater than 45%.  collected and reused on the base of biological agents. Compared with the traditional household biogas digester (Rajendran et al. ), the resource-based treatment is more advantageous in both efficiency and sustainability.
In addition, the high-quality agricultural products with zero agricultural residues can improve farmers' income, agricultural efficiency, and food safety. The saving costs in chemical fertilizers have greatly encouraged farmers to participate in the NPS pollution control. The resource-based treatment of domestic waste and livestock manure supports the development of an efficient and sustainable agricultural economy.

Domestic sewage treatment results
Domestic sewage treatment utilizes centralized collection, decentralized treatment, and micro-power, thereby allowing for the effective collection and treatment of domestic sewage in dispersed rural settlements. With the help of biological agents, the average removal rate of domestic sewage using the three-stage ponding system is shown in Table 2.
The sewage effluent enters a solid-liquid separation tank at the front end of the sewage treatment system and then enters the water quality purification tank. In the early stage, deodorants and antidotes are added to activate the genes of bacteria for microbial deodorization, and then a special water purification agent is added to form a treatment layer. Most of the organic matter in the wastewater is removed, and the upper part of the clear liquid is collected in the water quality stabilization tank, which is equipped with a biological matrix.
After three-stage removal, the quality of domestic sewage can be effectively improved, with the removal rate reaching 80.6%, 87.5%, and 52.0%, respectively for the COD, BOD and Ammonia N. In this no- Water (GB 3838-2002, China). Further, the water quality was gradually restored with a reduction tendency for both TP and TN since 2011.
In addition, the predicted TN and TP from SWAT are also shown in Figure 9. According to the prediction results, the water quality at Taocha in 2025 is predicted to recover to the same level as that before significant deterioration, with the TN and TP reaching Level two (0.5 mg/L) and Level one (0.01 mg/L), respectively. The simulation results from SWAT provide more evidence to the assessment of our proposed approach. Due to the long lag time, the SWAT prediction with some numerical error is used in the assessment. However, the evident reduction tendency is convincing for the effectiveness of NPS pollution control.

Reduction in treatment costs
With the resource-based treatment, the cost of management and the maintenance of mechanical equipment is significantly reduced. The operation mode allows for the maximum utilization of garbage resources using safe, sustainable, and cost-effective technology. The cost of domestic sewage treatment with no power is less than 0.1 yuan/m 3 , which is far lower than traditional treatment costs (about 1.0 yuan/m 3 ).
In practice, the management mode of 'household storage, group collection, village concentration, township supervision, and enterprise transportation' is adopted for the treatment of domestic waste; the mode of 'household storage, enterprise transportation and disposal ' is adopted for the treatment of livestock manure; and the maintenance of sewage treatment is the responsibility of qualified operation enterprises. In terms of the chemical fertilizers and pesticides, first, the operation enterprise and farmers reduce the use of chemical fertilizers and pesticides with replacement by organic fertilizers. Second, the use of organic fertilizers and plant protection agents is encouraged with a subsidy of 0.45-0.75 yuan/m 2 of farmland for farmers. In these operation modes, the farmers are no longer passive followers or 'spectators', but active participants.
Finally, with the collaboration of farmers, enterprises, and government in NPS pollution control, the cost to the government is also significantly reduced, with a reduction ratio larger than 50%.  and pollution reduction during transport should also be improved, including manure pollution collection and storage, dry manure cleaning, and manure resource utilization.