Applied Basic Research On Characteristics Of Sediment And Its Phosphorus Release And Control In The Landscape Water Supplied With Reclaimed Water

The severe eutrophication of landscape water supplied with reclaimed water has a negative impact on water environment in summer.In order to control eutrophication and recover the quality of water body,the differences of physicochemical properties and nutrient contents of sediments formed under reclaimed water（SRW）supplementation in South Gate and under surface water（SSW）supplementation in Southeastern corner zone of Xi’an moat were analyzed through continuous monitoring.On this basis,the characteristics of nitrogen（N）and phosphorus（P）released from SRW and SSW was explored through a static laboratory test.Finally,calcium nitrate was adopted to lock phosphorus in sediment on laboratory scale to explore the optimal dosage of calcium nitrate on phosphorus locking due to the high release rate and the large release amount of P from SRW in the landscape water.This study could provide an important reference for managing the water quality when supplied with reclaimed water and controlling the eutrophication caused by nutrients release from sediments.The main results and conclusions are as follows:（1）The p H values and VSS/SS of SRW and SSW were highest in winter,followed by autumn and spring,and lowest in summer.The p H value and VSS/SS of SRW ranged from 6.79 to 8.16 and 10%to 15%,respectively.The p H value and VSS/SS of SSW ranged from 6.61 to 7.89 and 7%to 10%,respectively.Therefore,the average p H value（7.65）and VSS/SS（12%）of SRW were slightly higher than the corresponding values of SSW.The seasonal variation of p H values and VSS/SS of both sediments may be related to the hydrolysis of sediment affected by temperature.（2）The total chemical oxygen demand（TCOD）and total organic carbon（TOC）contents in SRW and SSW were highest in winter,followed by autumn and spring,and lowest in summer.In addition,the ratio of TOC/TCOD in SRW was lower than that in SSW,and the contents of TCOD and TOC in SRW in different seasons were significantly higher than those in SSW.TCOD and TOC contents in SRW were ranged in 77.25～189.65 mg/g and 52.03～59.13 mg/g,respectively,while the corresponding values in SSW were ranged in 39.09～81.11 mg/g and 27.06～46.10 mg/g,respectively.So,the average ratios of TOC/TCOD were 42%56%in SRW and SSW,respectively.（3）The average values of total nitrogen（TN）,total phosphorus（TP）and inorganic phosphorus（IP）contents in SRW were significantly higher than those in SSW.The average TN,TP and IP contents of SRW were 7.80 mg/g,1.48 mg/g,1.17mg/g,respectively,which were 1.1 times,1.1 times and 1.4 times those of SSW.（4）The release rates and amounts of N and P from SRW were much higher than those from SSW.Ammonia nitrogen（NH₄⁺-N）and soluble reactive phosphate（SRP）were the main form of N and P released from the sediments,and low dissolved oxygen condition could promote the release of N and P.Among them,the release amount and the maximum increase rate of NH₄⁺-N in the overlying water of SRW under low dissolved oxygen condition were 0.14 mg/g and 0.96 mg/（L·d）,which were 3.5 times and 1.4 times those of SSW,respectively.The release amount and the maximum increase rate of TP in the overlying water of SRW under low dissolved oxygen condition were 0.038 mg/g and 0.11 mg/（L·d）,which were 12.7 times and 2.8 times those of SSW,respectively.In addition,the density,species and individual size of benthic algae in SRW were much larger than those in SSW.The dominant species in SRW was Eunotia sp.,while the dominant species in SSW was Gomphonema sp.（5）Calcium nitrate could effectively reduce the concentrations of TP and PO₄^3--P in the overlying water and the interstitial water,but the degree of reduction was decreased step by step with the increase of calcium nitrate dosage.The optimal dosage of calcium nitrate to inhibit phosphorus release from sediment was 0.2 g N/L-wet sludge（WS）.for SRW of Xi’an moat.（6）When the optimum dosage of calcium nitrate was 0.2 g N/L-WS,it could effectively reduce the concentrations of TP and PO₄^3--P in the overlying water,and the reduction rates were 76%and 83%,respectively.Meanwhile,it would not cause significant increase of NO₃^--N,NO₂^--N and NH₄⁺-N concentrations in the overlying water and the interstitial water.When the dosage of calcium nitrate reached above 0.6g N/L-WS,it could effectively reduce the concentrations of TP and PO₄^3--P in the overlying water and the interstitial water.The reduction rates of TP and PO₄^3--P in the overlying water reached 79%and 85%,and the reduction rates of TP and PO₄^3--P in the interstitial water reached 67%and 68%,respectively.Meanwhile,the contents of TN and TP in sediment was increased by using calcium nitrate to lock P in sediment.