| Tail water from wastewater treatment plant(WWTP)is used for replenishing water in urban landscape water body,which is not only one of the ways of water reuse but also an important measure of water security in urban water environment construction in water shortage area in North China.Compared with natural water,the tail water from WWTP usually contains high content of nutrients,often leading to frequent eutrophication and directly affecting the landscape efficacy of water bodies if it is used for replenishment of closed or semi-closed water bodies such as landscape lakes.Therefore,it is necessary to study technical countermeasures of water security aiming at landscape efficacy assurance.However,as the main source of landscape water replenishment,the quality of tail water from WWTP is difficult to meet the the requirements of surface water environmental quality standards based on total nitrogen as the nutrient index,even if the tail water of WWTP reaches the first grade A of urban sewage disposal plant contamination integrated discharge standard or higher standard.In this case,it had to be the key technical link of tail water for landscape utilization that building the in-situ ecological water quality security system according to local conditions and combining with the construction of urban landscape water body.This research work was supported by the National Water Pollution Control and Treatment Science and Technology Major Project(No.2013ZX07310 001).Based on the construction of Tianjin Harbor Eco-Wetland Park at the Harbor Industrial Zone of Tianjin,this paper focused on the water quality security of high-nitrogen tail water for direct landscape utilization,and built an in-situ ecological water purification system(IEWPS)combined bypass circulation of inlet regulation pond with efficient denitrification and hybrid constructed wetlands(CWs).The main contributions are as follows.Firstly,it illustrated purification mechanisms of water quality and pollutants removal efficiency of the key processes.Secondly,it made an evaluation on improvement efficiency of lake water quality in Eco-Wetland Park.Thirdly,it conducted water quality and ecological risk assessments.Finally,it demonstrated the effectiveness of in-situ ecological water quality improvement as well as the prospect of tail water from WWTP for landscape utilization.The main outputs of this paper are as follows:(1)Ecological filtration technology of endogenous denitrification with spatio-temporal alternate was studied.The cross-flow filtration corridor was set to create an ideal plug flow condition,and the dissolved oxygen(DO)in influent was used to realize aerobic biodegradation of organic matter in the front filter layer,and biofilm formed on the surface of the filter particles.With the decrease of DO concentration along the flow direction,anoxic or anaerobic conditions were created in the back filter layer.The bidirectional inlet and outlet could be achieved by controlling water level in the cross-flow filtration corridor.When the biofilm in the front section of the filter was formed,the flow direction was reversed to change the location of biofilm at back section of the corridor,providing internal carbon source for denitrification filtration under anoxic or anaerobic conditions.During the filtration operation,the water flow direction was changed periodically according to the formation of biofilm,and the operation state of“aerobic biological filtration+endogenous denitrification filtration”was maintained in the filtration corridor to realize simultaneously organic oxidation and denitrification denitrification.The self-regeneration of the filter layer could be realized through endogenous utilization,which could effectively prevent the blockage of the filter layer caused by the aging and peeling of the biofilm.When the technology is applied to the by-pass circulation treatment of the inflow tail water,the total nitrogen(TN)removal efficiency could reach 30-40%under the condition of no external carbon source,and the enhanced denitrification could be realized in the inflow tail water.The regulating pond and bypass circulation unit could reduce the inflow tailwater with the TN as the first level B standard to the first level A standard,effectively reducing the TN load of the inflow tailwater.(2)The mechanism of plant-microorganism interaction in CWs unit was studied.The CWs unit could further improve the effluent quality of regulating pond and bypass circulation unit,and significantly reduce the comprehensive trophic level index(TLI(?))value of water body and improve the eutrophication status of water body.The removal of organic matter,suspended solids(SS)and nutrients by CWs increased linearly with the increase of influent pollution load,but the removal of organic matter and nitrogen was significantly affected by seasonal variation.The direct uptake of plants was mainly affected by the biomass,and it would take a long time to reach the stability of biomass.The contribution of plant uptake in CWs unit to nitrogen(N)and phosphorus(P)removal in CWs increased significantly from9.1%and 9.3%in the first year to 15.6%and 27.4%in the third year,respectively.The direct uptake of N and P by different plant species had significant difference.For P.australis,I.tectorum,L.salicaria,the order of N reserves in shoot was P.australis(53.1-83.9 g N/m~2)>I.tectorum(46.6-80.2 g N/m~2)>L.salicaria(31.9-80.2 g N/m~2),while the order of P reserves in shoot was I.tectorum(4.25-7.24 g P/m~2)?P.australis(4.63-6.87 g P/m~2)>L.salicaria(3.69-5.95 g P/m~2),In addition,the diversity of microbial community in CWs was high,but plant species could significantly affect the composition of denitrifying bacteria in CWs.I.tectorum wetland was conducive to the enrichment of nitrifying bacterial functional genes(amo A and nxr A),while P.australis wetland was more conducive to the enrichment of denitrifying bacteria(nir S+nir K and nos Z).At the generic level,Exiguobacterium was the first dominant bacteria in wetland,and its relative abundance was 9.22-9.94%.Moreover,most of the dominant bacteria in wetlands have the ability of nitrogen and organic matter removal.Therefore,for the regeneration and reuse of tail water,P.australis was more suitable for plant cultivation in CWs in this area,and the internal environment of plant-microbial wetlands was conducive to the growth of functional bacteria,which strengthened the pollutant purification capacity of CWs.(3)The improvement effect of ecological water quality of Eco-Wetland Park in-situ was evaluated.The IEWPS could significantly improve the quality of tail water from WWTP,and the average removal rates of TN,TP,As,Cd,Cr,Cu,Pb and Zn were 76.4%,74.5%,77.0%,93.0%,75.2%,84.7%,67.3%and 76.2%,respectively.The effluent quality of the first ecological unit,i.e.regulating pond and bypass circulation unit,could reach the first class A standard stably,especially the removal of TN was strengthened.The effluent quality of the second ecological unit,i.e.CWs unit,was much better than that of the first class A standard.In terms of pollutant reduction,the annual load reduction of TN,TP,As,Cd,Cr,Cu,Pb and Zn in tail water by IEWPS were 21.13 t,1.50 t,26.10 kg,0.46 kg,8.69 kg,282.15 kg,32.83kg and 92.52 kg,respectively.The contribution of the regulation pond and bypass circulation unit and CWs unit to TN removal is 48.1%and 51.9%,respectively;and the contribution to TP removal were 28.9%and 71.1%,respectively;and the contribution to the removal of heavy metals were 48.6%?65.1%and 34.9%?51.4%,respectively.The results showed that the TN reduction in tailwater was strengthened by the regulation pond and bypass circulation unit,and the TP reduction in CWs unit was more important.In addition,the landscape effect of the main landscape lake of the park was systematically evaluated.Based on the ecological state evaluation,it could be seen that the diversity of phytoplankton community in the water body of the main landscape lake area was relatively good,and its cell density was between 5.4×10~6-44.8×10~6cells/L,which was in the middle nutrition state as a whole,and the landscape effect was relatively good.Besides,from the perspective of the sensory effect of water quality and the natural landscape of biodiversity,the IEWPS in Eco-Wetland Park had a good removal effect on turbidity and chroma of tail water,effectively ensuring the sensory effect of water quality in the main landscape lake,improving the regional biodiversity.(4)Risk assessment and potential analysis of landscape utilization of wastewater treatment plant tail water were carried out.Taking the ecological risk of species sensitivity distribution(SSD)and water quality safety classification of multiple biotoxicity test as evaluation factors,water quality safety classification and ecological risk assessment were carried out.The results showed that when the tail water was used directly,the ecological risk of different trophic level subjects was very large(the potential affected fraction(PAF)value:11.21-27.01%),and a water quality safety classification level was D grade,and could have significant toxic risk to aquatic organisms.After the advanced treatment of the IEWPS the PAF value of the tail water,dropped to below 5%,and its ecological risk was significantly reduced,and water quality safety classification level reached Grade A.The above results showed that the IEWPS could effectively remove toxic pollutants in tail water from WWTP.Secondly,by analyzing the water quality improvement and landscape improvement effect of the IEWPS,it was found that the two-stage ecological units in the system could achieve good pollutant removal effect,while creating a beautiful water environment ecological landscape.The IEWPS had both the function of water quality ecological improvement and water environment landscape construction.In the process of in-situ improvement of water quality,the ecological landscape had been significantly improved,and the dual effects of in-situ restoration of tail water and landscape utilization had been realized.Finally,taking the Tianjin Harbor Eco-Wetland Park as a case study,the potential of landscape utilization of tail water from WWTP was analyzed.After the in-situ ecological water quality was improved,the tail water could be directly used for landscape,realizing the high integration of tailwater reuse,water quality purification and ecological landscape.This case provided a good model for the combination of tail water landscape utilization and ecological landscape construction,as well as technical support for urban water environment improvement in water deficient cities in northern China,with a very high prospect of promotion and application. |
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