Engineered natural systems for water reuse [electronic resource] : nitrate removal by woodchip bioreactors and the social value of flow-augmented natural systems

Urbanization has had a negative impact on streams and wetlands across the United States, with many streams suffering from flashier hydrographs, elevated nutrients and contaminants, altered channels, and reduced biotic richness. Several novel solutions have been proposed to improve the function of these streams. One such solution is the use of low-cost and low-maintenance natural treatment systems to reduce nutrient and contaminant loads entering streams. Another solution is to augment flow in negatively impacted streams with highly treated wastewater. However, certain knowledge gaps exist that hinders our ability to properly design these systems and predict the social and ecological benefits of introducing these technologies. The goal of this research was to (1) predict nitrate removal in woodchip reactors, and (2) apply the developed model to predict nitrate removal performance in stormwater and wastewater applications, and (3) estimate the social and ecological value of augmenting urban streams with highly treated recycled water. In the first research chapter, it was demonstrated that woodchip reactors can be modeled with zero-order kinetics using a robust data set collected from experimental columns. This study advanced understanding of the underlying mechanisms of denitrification in woodchip reactors, while at the same time providing justification for the use of a simple zero-order model to describe woodchip reactor performance. The second research chapter substantiated the use of a zero-order reaction rate for woodchip denitrification and provided a temperature-dependent model appropriate for aged woodchip reactors. In the third research chapter, it was demonstrated that woodchip bioreactors are a feasible and cost-effective method for removing nitrate in wastewater effluent, wherein the higher temperatures and more consistent flow rates of wastewater effluent lead to higher removal rates in comparison to urban or agricultural runoff. The work performed here confirms that the main factors influencing nitrate removal rates in woodchip reactors are hydraulic residence time, woodchip age, and temperature. The final research chapter assessed the economic and ecological merits of a recycled water project in Pacifica, CA that opted for an inland release of tertiary-treated recycled water in a small stream compared to an ocean outfall discharge. The results indicated that the aesthetic, recreational, and ecological benefits of the inland discharge alternative were greater than the increase in associated costs by US$1.8M compared to the more traditional alternative. The work presented in this dissertation has helped advance the state of knowledge of nitrate removal using woodchip bioreactors and estimating the social and economic value of augmenting urban streams with recycled water. The results of the woodchip studies show that although woodchips may be used to treat runoff, a potentially more promising application is the use of woodchip reactors to denitrify wastewater effluent. The results of the streamflow augmentation study show that water reuse for the environment may create value for a community above and beyond the associated costs.

Advisor: Luthy Richard G. primary advisor. Thesis advisor
Boehm Alexandria advisor. Thesis advisor
Fendorf Scott advisor. Thesis advisor