| China is a country of large population, the problem of water scarcity, the uneven spatial and temporal water distribution and water pollution is very severe. With the continuous work on prevention and treatment for industrial wastewater, the domestic wastewater has become a major pollution source of surface water and groundwater. In slow rate land system, soil-vegetation-microbial system can not only purify the contaminations such as nitrogen, phosphorus, sodium chloride, but also increase poplar growth, so goal of both wastewater treatment and timber production can be achieved. Slow rate land treatment has broad development potential in area where sewage treatment plant cannot be built up for economic condition or no enough wastewater quantity.Poplar is a high-quality native tree species in the north-central region. With the characteristics of fast growing, high yield, large root volume, the poplar is commonly used in the shelter-forest and the fast-growing plantation. As the poplar grows quickly, it can absorb large amounts of water and pollutants, so poplars are very suitable as vegetation for wastewater treatment. Before the establishment of land treatment system, the wastewater content should be monitored and suitability should be evaluated according to the relevant national wastewater discharge standard, the site applicability should be evaluated according to the investigation of soil properties. The wastewater treatment can only be carried out when the suitability assessment of the wastewater composition and site meets the requirements. Hydraulic loading, which is also one of the key parameters on wastewater treatment, can be calculated according to the relevant index of soil permeability, nitrogen uptake requirements and crop water requirement, to make some necessary preparations for the construction of the wastewater treatment system. But the actual conditions of different area are very big, wastewater treatment tests should be performed for designing suitable hydraulic loading.In slow rate land system, vegetation-soil system can purify the wastewater. However, the content of sodium chloride in wastewater is comparatively high and wastewater treatment may affect the tree growth, soil properties, microbial flora and soil enzyme activities. From 2008 to 2009, an experiment of domestic wastewater eco-treatment with poplar plantation has be performed at Longhu Town of Zhengzhou city, four poplar clones were selected as for vegetations and different hydraulic loadings (0,3,6,9,12,15 cm week-1) were applied in order to observe the absorbility of different poplar clones and the purification ability of soil microbes for wastewater. The domestic wastewater's influence on soil properties and the quality of underground water were studied. The study provides a scientific basis for the construction and continuous operation of slow rate land treatment with poplar plantation. The results show that:poplar can take up and purify wastewater, different clones have different absorbility, and appropriate clones should be selected as vegetations for cleaning domestic wastewater. The poplar can increase the growth of the soil microbe by providing a good microbial growth interface. Soil microbe's wastewater purification ability increased with the increase of hydraulic loading, and decreased with the increase of hydraulic loading after it reaches a certain hydraulic loading (>9 cm week-1). The wastewater land treatment can increase tree growth and soil properties treated with suitable hydraulic loading, while tree growth and soil fertility would be reduced if the hydraulic loading is too large. The main conclusions are as follows:1. The tested poplar clones are widely planted in north-central area. The responses of different poplar clones to wastewater vary greatly. Compared with the other clones, the clone 2001 has highest stress tolerance for wastewater and shows no severe stress symptom. It also has good absorption and purification ability to nitrogen, phosphorus and sodium chloride. Slow rate land treatment also increased the growth of the clone 107 and I-72, the two clones have a better purification ability on nitrogen and phosphorus pollutants, but poor to sodium chloride. The clone 46 is sensitive to salt stress, smaller hydraulic loading caused the decline of its growth, so the clones are not suitable as vegetations for wastewater treatment. The results showed that the appropriate hydraulic loading is 3-9 cm week-1, if the hydraulic loading keeps going up, wastewater treatment will affect the health of trees, the reduction of wastewater treatment efficiency will result in pollution of soil and groundwater.2. The proper amount of wastewater will diversify the microbes in the soil and promote the growth of microorganisms in the soil, the microorganisms in the soil have strong purification ability for organic pollutants, and there was similarity among the changes trend of enzyme content, quantity and diversity of microorganisms in the soil.From changes of the microbial diversity and quantity and enzyme activity, a conclusion can be reached that wastewater treatment with 3-9 cm week-1 is more proper, which is similar to the reaction of the poplar clones except for the poplar 46. The results show that the pollutants can be broken down and turned into nutrients for the growth of the trees treatment with appropriate hydraulic loading and the roots of the well developed trees will provide better interface and available nutrients for the microbes. Once the hydraulic loading exceeds certain scope (>9 cm week-1), soil contaminations increase and soil fertility will reduced, so microbe number and diversity will reduced.3. The soil can take up pollutants in the domestic wastewater with the help of microbes, the soil properties was also changed in the process. The Changes in soil properties result in two very different results:in one hand, with the increase the of the nutrient in the soil, the soil became more fertile; in other hand, as the increase of the pollutants such as the content of sodium ion, soil properties will deteriorate and the soil productivity will decrease. This change is closely related with the wastewater hydraulic loading, the appropriate hydraulic loadings can improve soil fertility, while the hydraulic loading keep going up and exceed a certain point, the pollutants in wastewater will deteriorate soil properties. From the influence of the domestic wastewater on soil,3-9 cm week-1 treatments will make significant improvement on the soil fertility. Once the hydraulic loading exceeds this scope, soil fertility will reduced.4. When the hydraulic loading is beyond the purification capacity of the soil-vegetation-microorganism system various pollutants including BOD5, COD, ammonia nitrogen, chloride, nitrate and sulfate will lead to serious pollution of groundwater, so the monitoring and evaluation of groundwater is very necessary. When Hydraulic loadings in land treatment exceed 9 cm week-1, the groundwater will be contaminated for the increases of hydraulic loading.5. Slow rate land treatment system has a high efficiency on the pollutant removal and can achieve good results for the wastewater treatment. There is low investment and low operation costs in the system, land treatment can get the benefits on the wastewater treatment combined with biomass production in the local region. Slow rate land treatment has high pollutant removal efficiency, access to good treatment results, less investment and lower operating costs. The benefits of wastewater treatment and increased biomass production will be achieved. Slow rate land treatment can speed up the growth of poplar plantation, improve local ecological environment and strengthen the protective effect of plantations, so ecological benefit is significant. As slow rate land treatment have the above advantages, it has a broad prospect in seasonal drought area, especially in the north-central region with the sever drought in spring.
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