| Sewage effluent irrigation is one of the most effective method in regions where fresh water is scarce.However,there are some undesirable constituents in treated sewage effluent,such as salinity,nutrient,dissolved organic matter and pathogens.The complex of ions in the sewage effluent water probably affects the formation of the biofilm and emitter clogging.Chemical injection(chlorination and acidification)has been recognized as the most effective method for treating clogs caused by biological growth and chemical precipitation.The Cl released by the oxidation reaction can be converted into chloride salts,thus increasing the risk of soil salinization.The undesirable constituents may accumulate in the root zone to interrupt nutrients conversion,decrease biological activity and induce negative effects on the crop growth.Accordingly,the influences of chlorination on soil properties and crop growth,besides the clog-reducing effectiveness of emitters,should therefore be considered when an optimum chlorination scheme is determined.On-site drip irrigation emitter clogging experiments that were continued for 54 days with a daily operation of 12 hours with sewage effluent and groundwater application were conducted to investigate the effect of two typical ions(Fe2+ and Ca2+)on emitter clogging.At the end of the experiment,several representative emitters were sampled to investigate the clogging mechanism through measuring the dry weight of biofilm substances(DW)and analyzing the mineral constituents of clogging solids and the extracellular polymeric substance(EPS)of attached biofilm.On-site drip irrigation chemical injection experiments were conducted to quantify the influence of chlorination and acidification in reducing clogging in emitters applying secondary effluents with different types of ions.Two types of emitters with plain channels and one type of emitters with labyrinth structure were selected to investigate the clogging control strategy through measuring the dry weight of biofilm substance(DW)and the organic matter fraction in biofilm and analyzing the mineral constituents of clogging solids and the extracellular polymeric substance(EPS)of attached biofilm.A two-year experiment was conducted to evaluate the effects of chlorine injection concentrations and duration on soil properties and maize growth for drip irrigation while applying secondary sewage effluent.The experiments were designed with injection concentrations of free chlorine residual at the end of laterals ranging from 0 to 8 mg L-1 and injection duration ranging from 0.5 to 3 h for each irrigation event.The control experiments with groundwater were applied in both seasons.The distributions of soil enzyme activities,soil chlorine,and soil nitrogen were determined during the maize growing season.The plant height,leaf area index(LAI),leave relative chlorophyll content(SPAD),dry biomass of leaf and stem as well as grain,plant nitrogen uptake,yield and yield determinants of maize were recorded at the key growing stages.Field experiments were also conducted in 2016 to evaluate the impacts of uniformity of chlorine on soil nitrogen,soil enzyme,maize growth and yield.Furthermore,a residual chlorine decay model of EPANET was established to evaluate the influences of initial chlorine concentration and lateral length on chlorine distribution in actual sewage effluent drip irrigation system.The main conclusions of this study are as follows:(1)The clogging in emitters was affected by emitter types,ion concentration,water quality,and the duration of operation.The plain channel emitters demonstrated significantly better anti-clogging performance than labyrinth emitters,especially when applying sewage effluent.In general,the clogging in the labyrinth emitters increased with an increasing ion complex and concentration for sewage application.Chemical chlorination/acidification treatment is highly necessary for drip emitters applying sewage effluent.Chemical injection was more effective to control the plain channel emitters clogging than that for labyrinth emitters.(2)The clogging risk and inducing mechanism varied with the types of ions added.Fe2+ promoted the production of microorganisms and precipitation of solid particles,increasing the risk of biochemical clogging.Emitters clogging with Ca2+ were mainly caused by chemical precipitation.Under the experimental conditions,the weekly chlorination with a residual concentration of 2 mg L-1 was more effective in reducing the growth and reproduction of microorganisms and precipitation of solid particles inside the drip emitters when applying high iron + calcium containing sewage effluent.When sewage effluent containing high calcium is used,however,acidification scheme with targeted pH values of 5.0 and an injection interval of 2 weeks is more effective in removing the chemical precipitation.(3)Sewage application increased the residual Cl in the soil while chlorination did not cause a substantial chlorine accumulation in the root zone.Soil enzyme activities and nitrogen were enhanced by sewage application.The enzyme activities for sewage application with chlorination was slightly lower than those for the sewage effluent application without chlorination.Chlorination increased the accumulation of nitrate nitrogen in the upper soil layer.Chlorination is generally safe with precipitation leaching and the chlorine concentration and injection duration had an insignificant influence on plant growth and yield of maize in both seasons.(4)The drip system chlorine uniformity had an insignificant effect on soil enzyme activities and nitrogen content.The uniformity of plant height,leaf area index and SPAD values for all treatments had a high uniformity(? 79.8%)during the growing season.The mean and the coefficient of biomass,nitrogen uptake and yield of maize were not significant differences for all treatments.EPANET could accurately simulate the residual chlorine in drip irrigation system.The simulated results showed that simulation values have good agreement with monitoring data. |
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