Influence of different hydraulic load rates on the purification performance of submerged macrophyte Vallisneria natans in two vegetated drainage ditch types
Vegetated drainage ditches(VDD)originate from agricultural irrigation and drainage system,representing an important technology to effectively control the non-point source pollution in farmlands.Aquatic plants,substrates,and hydraulic loading rate(HLR)are important VDD purification efficiency-affecting factors.In this study,we investigated the nitrogen(N)and phosphorus(P)removal efficiencies in farmland runoff under different HLR levels in two VDD types,i.e.,drainage ditch planted with a submersed macrophyte(Vallisneria natans)(VDD)and that planted similarly with additional zeolite supplementation(ZVDD).We successfully elucidated how plant N and P uptake contributed to the corresponding N and P removal in VDD along with the main impacting factors.Our results indicated that the TN and TP concentration removal efficiency in the VDDs under different HLRs were 47.7%-66.0%and 57.5%-77.1%,respectively,which decreased with increasing HLRs.However,the areal TN and TP removal efficiencies in the VDDs varied between 305.3-1 009.2 mg·m-2·d-1 and 7.8-66.7 mg·m-2·d-1,respectively,which increased with elevated HLRs.The HLR,rather than zeolite addition,significantly affected VDD N and P removal efficiencies(P＜0.05).Compared with the plants at the beginning of the experiment,plant density,leaf length,and total biomass dry weight yielded a 5.9-7.0,1.8-2.3,and 4.0-5.0 fold increase,respectively.Moreover,we observed that V.natans grew better at low HLR.N,P,and chlorophyll concentrations in the V.natans leaves and roots significantly positively correlated with the VDD N and P content(P＜0.05).The N and P amounts directly assimilated by V.natans were 2 674.4-3 384.1 mg·m-2 and 579.6-673.9 mg·m-2,respectively.The contribution of plant uptake to N and P removal was 5.6%-19.9%and 20.1%-65.0%,respectively,which decreased with increasing HLR.Our results indicated that HLR was the main factor affecting the contribution of plant uptake of V.natans to N and P removal in VDDs.Taken together,the drainage ditch planted with V.natans could be successfully applied to mitigate N and P runoff from farmlands.Although improving HDR reduced the N and P removal rates,it could significantly increase the N and P retention amounts.After long-term ZVDD operation,zeolite should be replaced in time to ensure enhanced purification performance.Notably,optimizing design,operation,and maintenance by comprehensively assessing VDD purification performance in terms of both pollutant concentration and flux removal is of particular practical significance.
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