Design methodology accounting for the effects of porous medium heterogeneity on hydraulic residence time and biodegradation in horizontal subsurface flow constructed wetlands

Horizontal flow constructed wetlands are engineered systems capable of eliminating a wide range of pollutants from the aquatic environment. Nevertheless, poor hydrodynamic behavior is commonly found resulting in preferential pathways and variations in both (i) the hydraulic residence time distribution (HRTD) and, consequently, (ii) the wetland's treatment efficiency. The aim of this work was to outline a methodology for wetland design that accounts for the effect of heterogeneous hydraulic properties of the porous substrate on the HRTD and treatment efficiency. Biodegradation of benzene was used to illustrate the influence of hydraulic conductivity heterogeneity on wetland efficiency. Random, spatially correlated hydraulic conductivity fields following a log-normal distribution were generated and then introduced in a subsurface flow numerical model. The results showed that the variance of the distribution and the correlation length in the longitudinal direction are key indicators of the extent of heterogeneity. A reduction of the mean hydraulic residence time was observed as the extent of heterogeneity increased, while the HRTD became broader with increased skewness. At the same time, substrate heterogeneity induced preferential flow paths within the wetland bed resulting in variations of the benzene treatment efficiency. Further to this it was observed that the distribution of biomass within the porous bed became heterogeneous, rising questions on the representativeness of sampling. It was concluded that traditional methods for wetland design based on assumptions such as a homogeneous porous medium and plug flow are not reliable. The alternative design methodology presented here is based on the incorporation of heterogeneity directly during the design phase. The same methodology can also be used to optimize existing systems, where the HRTD has been characterized with tracer experiments

Organic Micropollutants from an Agricultural Drainage Ditch Contaminate a Shrimp Farm in Sinaloa (Mexico)

Among nutrients and pesticides, agricultural draining ditches also transport pollutants discharged with untreated wastewater from the municipalities adjoining the ditch. When the ditch water is used for irrigation and aquaculture, risks for the environment and food production are suggested. For the conducted field study, a shrimp farm in Sinaloa (Mexico) was used to trace organic pollutants (pesticides and pharmaceutical residues) on their way from an agricultural draining ditch to a shrimp farm fed partially by the drain water. The concentrations of pollutants in the drain water ranged from 10 ng L-1 to 453 ng L-1. The pond water of the shrimp farm contained concentrations between <10 ng L-1 and 177 ng L-1. The shrimps were contaminated by pollutants at concentrations between 40 μg kg-1 d.w. (dry weight) to 3.3 mg kg-1 d.w. (fungicide Metalaxyl). Health risks for the cultivated shrimps cannot be excluded because some pesticides are known for their toxic effects to crustaceans. The concentrations of selected antibiotics in the shrimps were low and comparable with those found in the shrimps declared as seawater shrimps from a German supermarket. The incorporation of the antibiotics was probably caused by contact to the wastewater in the shrimp ponds and/or by contaminated shrimp feed. Additionally to the anthropogenic chemicals, coliforms were determined in the water (total coliforms: 30-50 CFU 100 mL-1; fecal coliforms: 0-20 CFU 100 mL-1). These values agree with the Mexican Norm NOM-242-SSA1-2009 representing a microbiological quality of water adequate for aquaculture. The number of coliforms measured in shrimp was higher than in pond water, suggesting bioaccumulation and a potential health risk for consumers

Organic Micropollutants from an Agricultural Drainage Ditch Contaminate a Shrimp Farm in Sinaloa (Mexico)

Agricultural draining ditches transport among nutrients and pesticides also pollutants discharged with untreated wastewater from the municipalities adjoining the ditch. When the ditch water is used for irrigation and aquaculture, risks for the environment and food production are suggested. For our field study, a shrimp farm in Sinaloa (Mexico) was used to trace organic pollutants (pesticides and pharmaceutical residues) on their way from an agricultural draining ditch to a shrimp farm feed partially by the drain water. The concentrations of pollutants in the drain water ranged from 10 ng L-1 to 453 ng L-1 . The pond water of the shrimp farm contained concentrations between <10 ng L-1 and 177 ng L-1. The shrimps were contaminated by pollutants at concentrations between 40 µg kg-1 d.w. (dry weight) to 3.3 mg kg-1 d.w. (fungicide Metalaxyl). Health risks for the cultivated shrimps cannot be excluded because some pesticides are known for their toxic effects to crustaceans. The concentrations of selected antibiotics in the shrimps were low and comparable with those found in shrimps declared as seawater shrimps from a German supermarket. The incorporation of the antibiotics was probably caused by contact to the wastewater in the shrimp ponds and/or by contaminated shrimp feed. Additionally to the anthropogenic chemicals, coliforms were determined in the water (total coliforms: 30-50 CFU 100 mL-1; fecal coliforms: 0-20 CFU 100 mL-1). These values agree with the Mexican Norm NOM-242-SSA1-2009 representing a microbiological quality of water adequate for aquaculture. The number of coliforms measured in shrimp was higher than in pond water, suggesting bioaccumulation and a potential health risk for consumers