Study On The Migration Of Ammonia Nitrogen In Ice -Water System And The Plant Effect Of Landfill Leachate Under Freezing Condition

The freeze concentration is a new and environment-friendly wastewater treatment technology,in which the temperature drops below the freezing point of the system and solid-liquid transition occurs,with forming pure ice crystal and concentrate and thereby achieving the separation of pollutants.Freeze concentration method has the advantages of no secondary pollution,low selectivity for treatment objects,low loss for treatment equipment and recovery of valuable components.In this study,first of all,the effect and its factors of ammonia nitrogen wastewater treatment by radial progressive freezing method which is easy to separate ice and water were explored.Based on this,taking the landfill leachate characterized with rich ammonia nitrogen as example to further investigate the effect of the freeze concentration method on the treatment of actual wastewater.At the same time,it is supplemented by plant monitoring method,with a view to comprehensively evaluate the application prospect of the freeze concentration in the field of wastewater treatment.The main conclusions were shown as follows:?1?Firstly,radial progressive freezing method was used to treat ammonia nitrogen simulated wastewater.And the effects of freezing temperature,initial concentration and the specific surface area of freezing on the migration of ammonia nitrogen in ice-water system were investigated.The results showed that the freezing process could be divided into three stages,namely the initial transition stage,the low-concentration stage and the high-concentration stage according to the change of the ice-phase ammonia nitrogen concentration.Considering the freezing time and removal rate,the freezing effect was best at-12?.When initial concentration was 50 500 mg / L,the removal rate of ammonia nitrogen in the low-concentration stage was similar,80% 95%;when the initial concentration was increased to 1000 1500 mg / L,the removal rate decreased to 70% 86%,that is,the ammonia nitrogen removal rate decreased with the increase of the initial concentration at the same time points.The larger the specific surface area of the freezing?the smaller the radius?,the faster the freezing rate while the larger the radius,and it was more conducive for the solute to migrate in the radial direction.When the concentration of raw water was 300 mg / L,taking the treatment of 1 L wastewater as an example,Unit?1??R = 5.00 cm?could get the largest water output,and unit?4??R = 2.75 cm?could get the highest treatment efficiency.?2?Secondly,the treatment effect of the high ammonia nitrogen landfill leachate by radial progressive freezing was studied.The results showed that when the freezing temperature was-12?,with the advancement of freezing process,the ice formation rate and the concentration of pollutants in the solidliquid two phases increased,the removal rate decreased,and the concentration ratio enhanced.After freezing for 9 h,the ice phase entered the highconcentration stage,and the changes and removal rates of different pollutants were similar in the freezing process.After freezing for 9 h,the ice formation rate was about 65%,and ammonia nitrogen,COD_Cr,total salt content,electrical conductivity,total nitrogen,total phosphorus,chroma,turbidity significantly decreased.Except for turbidity,the removal rate of the primary effluent was greater than 60%,the removal rate of the two-stage effluent was higher than 84%,and the removal rate of the three-stage effluent was more than 95%.The minimum concentration ratio was greater than 1.5,and some indicators was greater than 2.The concentration of pollutants in the secondary and tertiary concentrated liquids was lower than that of the original liquid,which could be further frozen and concentrated.However,the complexity of actual wastewater might interfere with the migration of pollutants,resulting in the removal rate of ammonia nitrogen in the actual water was lower than that in the simulated water.Additionally,it was found that there was no significant change in the types and relative contents of most organic pollutants before and after treatment,indicating that freeze concentration was universal for the removal of pollutants in leachate.?3?Taking maize as the model plant for pollutant exposure,and the results indicated that landfill leachate significantly inhibited the root length and bud length of maize seedlings at high concentrations and the inhibition effect was more significant with the prolonged exposure time.When the concentration of leachate was diluted below 10%,its inhibition of maize seedlings was weakened.Now there was no significant difference when compared with control group and promoted the growth of buds to some extent.The three-stage effluent could significantly promote the growth of maize buds,indicating that the freeze concentration significantly decreased the plant toxicity of landfill leachate.In addition,the roots of maize seedlings were inhibited more seriously than buds for pollutant exposure,suggesting that roots are more sensitive than buds.The change of the fresh weight of root and bud of maize seedlings were similar to root length and bud length,respectively.The correlation analysis showed that NH₃-N,COD_Cr,total salt content,conductivity,T-N,T-P and pH of leachate were significantly negatively correlated with root length and bud length.