| Turtle is an important aquacultural animal with high benefits. During aquacultural process, large amount of wastewater with high concentrations of nitrogen, phosphorus, organic matter was discharged. The discharge of wastewater caused serious pollution problem to surface waterbodies as lacking of corresponding treatment measurment. Based on investigating the properties of turtle aquacultural wastewater, the method of magnesium ammonium phosphate (MAP) precipitation combined with solid precipitating agent for recovering nitrogen, phosphorus and organic matter from turtle aquacultural wastewater was studied. A dissolution experiment with sand column, a leaching experiment with soil column, a simulated ammonia volatilization experiment, and a pot experiment planting ryegrass were conducted. Nutrient release features of the precipitate, influence of combined application of the precipitate and urea on the volatilization of ammonia, growth response of ryegrass to the application of the precipitate and varieties of soil available nutrient concentrations were studied. The main results were as follows:1The results of the typical investigation showed that, pH of turtle aquacultural wastewater ranged from6.62to7.84with the average as7.36±0.30; COD concentration ranged from119.9mg/L to3924.1mg/L with the average as1110.9±1061.3mg/L. Ammonium concentration ranged from10.3mg/L to209.0mg/L with the average as87.6±60.8mg/L. Nitrate concentration ranged from0.2mg/L to27.0mg/L with the average as7.0±7.3mg/L. Total nitrogen (TN)concentration ranged from43.5mg/L to431.7mg/L with the average as175.3±102.4mg/L. Inorganic phosphorus concentration ranged from4.7mg/L to131.8mg/L with the average as41.4±31.8mg/L. Total Phosphorus (TP) concentration ranged from8.2mg/L-309.8mg/L with the average as98.7±88.5mg/L. COD concentrations of60.7%investigated samples were excessive to the third allowed limit of 《Integrated Wastewater Discharge Standard》(GB8978-1996). Ammonium concentrations of82.1%investigated samples were excessive to the second allowed limit of 《Integrated Wastewater Discharge Standard》(GB8978-1996). Inorganic phosphorus concentrations of100%investigated samples were excessive to the second allowed limit of 《Integrated Wastewater Discharge Standard》(GB8978-1996). In the aquacultural wastewater, the mean percentage of ammonium-N to TN was47.3%, the mean percentage of nitrate-N to TN was5.2%, the mean percentage of inorganic phosphorus to TP was51.6%. The mean ratio of c(ammonium-N, mol/L) to c(inorganic phosphorus, mol/L) was5.6±3.4. The concentration of ammonium-N was significantly (P<0.01) correlated to TN, the concentration of inorganic phosphorus was significantly (P<0.05) correlated to TP, and TN, TP concentrations were significantly (P<0.01) correlated to COD. The wastewater had the potential to be recycled and used, as which was rich of nitrogen, phosphorus and organic matter.2The results of the simultaneous recovery experiment showed that the appropriate mole proportion of solid precipitating agent:MgCl2·6H2O:inorganic phosphorus:NH4+was0.6:0.6:0.5:1. Based on the appropriate mole proportion,6.9mmol solid precipitating agent,6.9mmol MgCl2·6H2O and1.2mmol Na2HPO4·12H2O were added to1L turtle aquacultural wastewater, the recovered rate of ammonium-N, total nitrogen, inorganic phosphorus, total phosphorus and total organic carbon were43.8%,83.8%,94.6%,96.6%and97.1%, respectively. The method of MAP modified with the solid precipitant was suitable for recovering nitrogen, phosphorus and organic matter from turtle aquacultural wastewater. The precipitate was black brown. The primary physicochemical properties of the precipitate were as follows:TN content was7.87%; total phosphorus (P2O5) content was14.68%; total potassium (K2O) content was0.95%; organic matter content was32.39%; the value of pH (fertilizer:water=1:5) was7.74. The TN, TP and organic matter contents of the precipitate were conformed to I standard of organic-inorganic mixed fertilizer (GB18877-2009).3The results of the continuous dissolution experiment illustrated that the dissolution velocity and accumulative amounts of TN, ammonium-N, and nitrate-N released from the precipitate were significantly (P<0.05) lower than that of the commercial organic fertilizer. The accumulative amounts of TP and inorganic phosphorus released from the precipitate were unsignificantly (P>0.05) different with the commercial organic fertilizer. The results of the soil column leaching experiment indicated that the accumulative amounts of TN, ammonium-N, and nitrate-N leached of the precipitate treatment were significantly (P<0.05) lower than that of the commercial organic fertilizer treatment. The ratios of TN leached to N applied as the precipitate and the commercial organic fertilizer were3.88%and11.75%in order. The ratios of TP leached to P applied as the precipitate and the commercial organic fertilizer were0.15%and0.07%in order. The precipitate was a long-term effective fertilizer as the releaseable amounts of N and P in it were less than5% of the nutrients applied.4The results of the simulated ammonia volatilization experiment showed that the accumulative amount of ammonia volatilization nitrogen losses of the treatment with20%precipitate-N and80%urea-N was significantly (P<0.05) lower than that of urea treatment in black soil with93.3mg/kg N added or fluvo-aquic soil with133.3mg/kg N added. With the comparation to urea treatment, the accumulative amount of ammonia volatilization nitrogen losses of the treatment with20%precipitate-N and80%urea-N reduced by23.1%in black soil and25.8%in fluvo-aquic soil. The results implied that the combined application of precipitate and urea could efficiently reduce the loss of ammonia volatilization and improve nitrogen utilization efficiency under the same N dose applied.5The results of the pot experiment showed that the biomass and the amounts of nitrogen, phosphorus, potassium accumulated in ryegrass shoots of fertilizing treatments were significantly (P<0.05) larger than that of no fertilizer treatment. With the comparation to no fertilizer treatment, the amounts of nitrogen, phosphorus, potassium accumulated in shoots of the treatment with the combined application of the precipitate and chemical fertilizer increased by78.3%,51.7%and301.5%, respectively. The differences of shoot nigtrogen, phosphorus concentrations, and the amounts of nitrogen, phosphorus and potassium accumulated in shoots among the treatments of20%precipitate-N+80%chemical fertilizer-N,100%chemical fertilizer and20%commercial organic fertilizer-N+80%chemical fertilizer-N were unsignificant (P>0.05). Apparent nitrogen use efficiency of ryegrass of the treatment applyed with20%precipitate-N and64%chemical fertilizer-N was significantly (P<0.05) higher than that of treatment applyed with100%chemical fertilizer. Contents of soil alkali-hydrolyzale nitrogen, Olsen-P and available potassium extracted with1mol/L ammonium acetate solutions of fertilizing treatments were significantly (P<0.05) higher than that of the no fertilizer treatment. The differences of soil alkali-hydrolyzale nitrogen, Olsen-P and available potassium extracted with1mol/L ammonium acetate solutions contents among the treatments of20%precipitate-N+80%chemical fertilizer-N,100%chemical fertilizer and20%commercial organic fertilizer-N+80%chemical fertilizer-N were unsignificant (P>0.05). Combined application of precipitate and chemical fertilizer had the similar effects to chemical fertilizer application or combined application of commercial organic fertilizer and chemical fertilizer in increasing shoot biomass of ryegrass and contents of available nutrients in soils. |
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