Quantifying treatment system resilience to shock loadings in constructed wetlands and denitrification bioreactors.

Wastewater treatment ecotechnologies such as constructed wetlands and denitrifying bioreactors are commonly perceived as robust and resilient to shock loading, but this has proved difficult to quantify, particularly when comparing different systems. This study proposes a method of quantifying and comparing performance resilience in response to a standard disturbance. In a side-by-side study we compare the treatment performance of four different configurations of wetlands and denitrifying bioreactors subjected to hydraulic shock loads of five times the standard inflow rate of primary treated sewage for five days. The systems consist of: horizontal-flow gravel-bed wetlands (HG); single pass vertical-flow sand or gravel media wetlands followed by carbonaceous denitrifying bioreactors (VS + C and VG + C respectively); and a recirculating anoxic attached-growth bioreactor and vertical sand media wetland followed by carbonaceous denitrifying bioreactors (R(A + VS)+C). Resilience was quantified for Total Suspended Solids (TSS), Five-day Biochemical Oxygen Demand (BOD5) and Total Nitrogen (TN) by time integration of Relative Disturbance in Performance relative to pre-shock loading performance (days equivalent Performance Reduction), and by the Recovery Time after shock loading ceased. The quantification method allowed an unbiased comparison of the four different systems. It highlighted important differences in the resilience for different removal mechanisms associated with the configuration of the wetlands/bioreactor systems. Relative Disturbances in Performance were expressed in comparison to percent daily removal under standard loading, and, for the different pollutants were equivalent to loss of between 0.08 and 2.51 days of removal capacity. Average Recovery Times ranged from zero to three days, with all systems exhibiting substantial recovery even during the five-day shock loading period. This study demonstrated that both the horizontal gravel wetland and the vertical flow wetland systems combined with carbonaceous bioreactors tested are generally resilient to shock loading of five times hydraulic and organic loading for periods of up to five days. Standard quantification of performance resilience to shock loadings or other perturbations has potential application across a wide range of technologies and research fields.

Microbial Transport, Retention, and Inactivation in Streams: A Combined Experimental and Stochastic Modeling Approach.

Long-term survival of pathogenic microorganisms in streams enables long-distance disease transmission. In order to manage water-borne diseases more effectively we need to better predict how microbes behave in freshwater systems, particularly how they are transported downstream in rivers. Microbes continuously immobilize and resuspend during downstream transport owing to a variety of processes including gravitational settling, attachment to in-stream structures such as submerged macrophytes, and hyporheic exchange and filtration within underlying sediments. We developed a stochastic model to describe these microbial transport and retention processes in rivers that also accounts for microbial inactivation. We used the model to assess the transport, retention, and inactivation of Escherichia coli in a small stream and the underlying streambed sediments as measured from multitracer injection experiments. The results demonstrate that the combination of laboratory experiments on sediment cores, stream reach-scale tracer experiments, and multiscale stochastic modeling improves assessment of microbial transport in streams. This study (1) demonstrates new observations of microbial dynamics in streams with improved data quality than prior studies, (2) advances a stochastic modeling framework to include microbial inactivation processes that we observed to be important in these streams, and (3) synthesizes new and existing data to evaluate seasonal dynamics.

Multiyear nutrient removal performance of three constructed wetlands intercepting tile drain flows from grazed pastures.

Subsurface tile drain flows can be a major s ource of nurient loss from agricultural landscapes. This study quantifies flows and nitrogen and phosphorus yields from tile drains at three intensively grazed dairy pasture sites over 3- to 5-yr periods and evaluates the capacity of constructed wetlands occupying 0.66 to 1.6% of the drained catchments too reduce nutrient loads. Continuous flow records are combined with automated flow-proportional sampling of nutrient concentrations to calculate tile drain nutrient yields and wetland mass removal rates. Annual drainage water yields rangedfrom 193 to 564 mm (16-51% of rainfall) at two rain-fed sites and from 827 to 853 mm (43-51% of rainfall + irrigation) at an irrigated site. Annually, the tile drains exported 14 to 109 kg ha(-1) of total N (TN), of which 58 to 90% was nitrate-N. Constructed wetlands intercepting these flows removed 30 to 369 gTN m(-2) (7-63%) of influent loadings annually. Seasonal percentage nitrate-N and TN removal were negatively associated with wetland N mass loadings. Wetland P removal was poor in all wetlands, with 12 to 115% more total P exported annually overall than received. Annually, the tile drains exported 0.12 to 1.38 kg ha of total P, of which 15 to 93% was dissolved reactive P. Additional measures are required to reduce these losses or provide supplementary P removal. Wetland N removal performance could be improved by modifying drainage systems to release flows more gradually and improving irrigation practices to reduce drainage losses.