Rural wastewater irrigation and nitrogen removal by the paddy wetland system in the Tai Lake region of China

Background, aim, and scope A large area of water eutrophication in the Tai Lake region of China was associated with nitrogen (N) and phosphorus (P) pollution, mainly due to the discharge of untreated rural wastewater (RW) into the surface water (SW) near villages of this region. A field experiment was conducted, using irrigation of RW plus urea fertilization under equal nitrogen (N) rate, namely, black water (BW), domestic wastewater (DW), gray water (GW), SW, and SW without any N application as a control (CK), to elucidate N removal by the paddy wetland system during the rice growing season of 2007. Materials and methods SW, GW, DW, and BW were collected from the village of Liangzhu in subtropical China. Water samples were stored in darkness at 4 degrees C in an icebox prior to analysis. Total nitrogen (TN) was analyzed by the spectrophotometric method with a continuous-flow automated analyzer. For the soil analyses, pH was measured using a pH meter with soil/distilled water - 1: 5; cation exchange capacity was determined with an unbuffered salt extraction method. Redox potential (Eh) was obtained using a pH meter with a platinum electrode, whereas organic carbon (C) was determined using the Walkley and Black method. TN was calculated by acid-alkali neutralization, after the soil samples were digested with concentrated sulfuric acid (H2SO4), distilled and then absorbed by diluted boric acid (H3BO3), and available N was determined by alkaline-proliferation law, after the samples were hydrolyzed, reduced, and absorbed by H3BO3. Available P was determined with the colorimetric method at the wave length of 660 nm, after the samples were extracted with NaHCO3, and available K was extracted by unbuffered NH4Cl and then determined by the atomic absorption spectrophotometer. The rice plant samples were digested with H2SO4-H2O2, and N contents were analyzed by the atomic absorption spectrophotometry. Results Yield for the CK was significantly less (P <= 0.05) than those of SW, GW, DW, and BW, with the yield of BW significantly greater (P <= 0.05) than all the other treatments. The TN concentration of the floodwater in the paddy wetland system decreased rapidly after transplanting, rose significantly (P <= 0.05) after two N topdressings, and then decreased gradually as the following sequences: SW>GW>DW>BW>CK, but tended to be stable until 15 October. Meanwhile, TN removal rates from the wastewater were significantly higher (P <= 0.05) than those from the urea fertilizer. Total N load (TNL) increased significantly after two topdressings and reached the maximum value just after the first topdressing on 22 July: SW, 21.0 kg ha(-1); GW, 19.1 kg ha(-1); BW, 15.3 kg ha(-1); DW, 14.3 kg ha(-1); and CK, 0.57 kg ha(-1). Subsequently, TNL declined gradually and reached stability on 15 October. Just after the rice seedlings were transplanted, the soil released a large amount of inorganic N (26.3%-40.4%); however, after the topdressing, the soil adsorbed a lot of N and TNL originating from the RW, which had decreased, ranging from 34.5% to 47.8%. Discussion A reasonable N rate would be necessary for normal rice production in the Tai Lake region, which is different from those studies of other systems. Irrigation with RW for the paddy field could not only reduce the amount of commercial N fertilizer usage and irrigation water but also increase rice grain yield. Meanwhile, N use efficiency was improved by the combination of both N fertilizer and RW. The more N fertilizer was used, the higher the TN concentration in the flooded water would be, and the more potential risk of N to the surface water body. However, the N in RW could be removed more easily than that of the fertilizer by the paddy wetland system, which may be related to the existing form and concentration of N in the RW and urea fertilizer. The TNL in the floodwater had a positive correlation with the urea application rate, implying that appreciable amounts of N input might be lost, either by a heavy rainfall event or floodwater drainage using improper water management methods closely subsequent to the urea application. This revealed that using GW, DW, or BW instead of SW as irrigation water could decrease the loss of N applied and reduce the environmental pollution risk for the surface water. Conclusions Supplemental N was necessary in this area for normal rice production, and the rice grain yield of CK (without any N fertilizer applied) was significantly lower than the other treatments; however, the yield of SW (with N fertilizer applied) was not significantly different from those of GW and DW. However, irrigation with RW in the paddy rice field could reduce costs of fertilizer and irrigation water. In addition, using BW would significantly increase rice yield revenues. The paddy wetland system also removed large quantities of N due to the irrigation with RW and applied N fertilizer, with the TN removal rates for RW significantly higher than that for the N fertilizer. Thus, application of RW instead of N fertilizer alone could greatly decrease the discharge of N into water bodies; thus, paddy wetland system can be considered as a new method for the RW treatment, especially in the Tai Lake region of China. Recommendations and perspectives Based on this study, it would be strongly recommended to use RW irrigation after had been disinfected for reducing part of commercial N fertilizer rate and saving some irrigation water in rice production with the paddy wetland system as a cost-effective way to remove the N from the RW and mitigate the non-point pollution.. Future work should pay attention to the largest removal capacity of the paddy wetland system per unit and effect on the rice grain quality irrigated with the RW.