| As a developing country, more than half populations in China live in rural areas. The ecological environment quality in rural areas is very important to the quality of rural residents. In the past few decades, the sanitation facilities in rural areas lag far behind the cities. Nowadays, the solid waste and the wastewater in rural regions have been one of the main barriers of affecting the sanitation and residential living in rural areas. Additionally, the rapid urbanization in rural areas in China was also one of the reasons which contribute the increasing production of solid wastes, and the environmental degradation trends in rural areas worsened gradually.Limited by the factors including the available technology, investment and operation costs, etc., the unsanitary landfills became one of the main disposal pathways of rural waste. After 2000s, they were replaced by the newly established sanitary landfills, and then they were abandoned gradually. However, there are few systematic researches on the environmental contamination of the abandoned rural unsanitary landfills and the corresponding decontamination process.We choose the Ningbo city as the representative region of east China, and investigate the current environment contamination of these unsanitary landfills. Then, the decentralized wastewater technology-the improved Multi-soil-layering system (MML) was applied to treat the leachate from these landfills. The experiment results are listed below:1. Pollution investigation of the abandoned rural unsanitary landfillsBased on the field survey and visit to the adjacent villagers, it is found that there are at least 100 unsanitary landfills in Ningbo. The was an evident difference between the physical components of solid wastes from the abandoned unsanitary landfill with the sanitary landfills, high portion of residues,85 percent, and a part of industrial waste was commingled with the rural waste. Chemical analyzation of the the adjacent surface water and soil show that the organic components and Cd content of the rural waste were high,69.8 g/kg (dry weight, DW) and 17.6 mg/kg (DW), respectively. Nitrogen and chemical oxygen demand (COD) were the primary products in the leachate in the unsanitary landfills, and their concentrations were much lower than that from the sanitary landfill. It was assumed that the great dilution effect caused the result.COD, N and P were the prevalent contaminants in the surface water around the unsanitary landfills, and the nutrient contamination was much higher than the pollution of trace elemments in surface water. According to the integrated pollution index (IPI), more than 78.0% water samples were classified as the moderate to serious pollution degree within 120 m around the unsanitary landfills.Higer average contents of soil TN, TP, Cu, Zn and Cd indicate that they have accumulation trend. Geoaccumulation index values of soil Cd and Cu show the soil moderate pollution with Cd and unpolluted to moderately polluted with Cu, respectively. Compared with the cadmium threshold level, the average soil Cd content was 10.3-fold higher. To protect the human health, the soils within 100 m around the unsanitary landfills were suitable for forest land only, not for agricultural usage.2. Heavy metal contamination in a soil-rice system in the vicinity of abandoned rural unsanitary landfillGaoqiao unsanitary landfill was chosen as the research object, and the soil and rice adjacent to the unsanitary landfill were sampled to evaluate the metal contamination of the soil-rice ecosystem. The experiment results showed that the ecosystem adjacent to the abandoned rural unsanitary landfill has been contaminated. The soil Cd was higher than the maximum allowable content of agricultural soil. Geoaccumulation index values of soil Cd suggests that the soil was moderately polluted with Cd, while the soils were unpolluted-moderately overall by Cr, Cu as well as Pb. There was a remarkable bioaccumulation trend of heavy metals in the rice organs (rice root, stem & leaf and grain). The Cr and Pb contents in rice grain were much higher than the threshold levels,2.9-6.7 folds and 1.0-1.5 folds of the legislation limits, respectively.Linear correlation analysis showed that the slightly alkaline leachate from the unsanitary landfill was one of the main factors causing the higher soil pH and then affecting the absorption of rice on the Cd.3. Treatment of leachate from rural unsanitary landfill using Multi-media layering system (MML)The existing processing technologies demand the expensive investment, high cost operation and skillful professionals to operate and maintain. Thus, it is assumed that they are not suitable for the utilization for the abandoned unsanitary landfills in the rural areas. MML has the characteristics of administrative convenience and non-power handling. Four MMLs with altered SMB and different hydraulic load rate (HLR) were applied in the experiment to investigate the treatment of the landfill leachate under without or low aeration supply. The experiment results showed that the MML could effectively treat the COD, NH3-N and P. COD and NH3-N removal efficiencies of MML were 97.4%,82.4% and 72.0%,62.0%, respectively under HLRs of 200 and 400 L/(m2-d) without aeration; COD and NH3-N removal efficiencies of M800 and M1600 were 62.3%,53.4% and 45.3%,35.3% respectively under intermittent aeration. N removal efficiency was low due to the strong nitrification effect, and MML almost lost the nitrogen removal capacity. P removal efficiency of MML was 75.6 to 91.9% under HLR 200 400 L/(m2·d), and the P removal of other three reactors was 26.1 to 54.9%. The experiment result indicated that MML presents an appealing prospect to be utilized to treat the leachate from the rural unsanitary landfill.4. Flow pattern of MMLSince the current design of natural treatment process (NTP) mainly depends on the emperial equation; however, the flow pattern of NTP often deviates the design and the designing effectiveness is impaired due to the geography location, geological condition and weather, etc. The objective of the experiment was to detect the residence time distribution (RTD) of MML, clarify its flow condition and effective volume under hydraulic load rate 200 to 1600 L/(m2·d) utilizing tracer experiment. The experiment results were listed below.The flow pattern of MML was close to plug-flow with middle backmixing (d<0.3) under hydraulic load rate (HLR) 200 to 1600 L/(m2·d). However, the flow pattern had the tendency to shift to completely stirred flow.Hydraulic retention time was one main factor determining the volume proportion of dead zone (Vd), and there was excellent linear correlation betweenθwith the latter (R2=0.999). |
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