Derek Chitwood

Drinking water source contamination poses a great threat to human health in developing countries. Point-of-use (POU) water treatment techniques, which improve drinking water quality at the household level, offer an affordable and convenient way to obtain safe drinking water and thus can reduce the outbreaks of waterborne diseases. Ceramic water filters (CWFs), fabricated from locally sourced materials and manufactured by local labor, are one of the most socially acceptable POU water treatment technologies because of their effectiveness, low-cost and ease of use. This review concisely summarizes the critical factors that influence the performance of CWFs, including (1) CWF manufacturing process (raw material selection, firing process, silver impregnation), and (2) source water quality. Then, an in-depth discussion is presented with emphasis on key research efforts to address two major challenges of conventional CWFs, including (1) simultaneous increase of filter flow rate and bacteri...

Abstract: A pilot-scale biofilter was operated at a publicly owned treatment works to remove hydrogen sulfide and a mixture of volatile organic compounds (VOCs) from waste gas streams. Lava rock was used for the medium, which averaged pH 4 during the study. The ...

Hydrogen sulfide and volatile organic compounds are often found together in waste air streams. This combination is difficult to treat by biofiltration because oxidation of the sulfide produces acid, reducing the pH in the biofilter. Rapid declines in pH can inhibit treatment of organic compounds. A two-stage biofilter, with the first stage operated at low pH and an inorganic support medium, and the second operated at neutral pH with an organic support can eliminate the problem. A pilot-scale facility was operated on this principle. Comparisons were made among two-stage treatment, single-stage low pH treatment, and single-stage uncontrolled treatment to determine which of the strategies was most effective. In two-stage treatment the first stage did provide protection for the second, allowing it to operate at neutral pH. Single-stage low pH treatment was effective at removing sulfide and many organic compounds, and may be sufficient for many applications.

This paper presents a model structure aimed at offering an overview of the various elements of a strategy and exploring their multidimensional effects through time in an efficient way. It treats a strategy as a set of discrete events planned to achieve a certain strategic goal and develops a new form of causal networks as an interfacing component between decision makers and environment models, e.g., life cycle inventory and material flow models. The causal network receives a strategic plan as input in a discrete manner and then outputs the updated parameter sets to the subsequent environmental models. Accordingly, the potential dynamic evolution of environmental systems caused by various strategies can be stepwise simulated. It enables a way to incorporate discontinuous change in models for environmental strategy analysis, and enhances the interpretability and extendibility of a complex model by its cellular constructs. It is exemplified using an urban water management case in Kunmi...

Recent efforts have been made to reduce releases of air toxics and smog precursors from wastewater treatment plants. Hydrgen sulfide is commonly the primary odor and is an important target for removal. Its oxidation, however, generates sulfuric acid and sometimes elemental sulfur, which can create substantial operational problems for biofilters. Declining pH may inhibit the organisms that degrade compounds other than hydrogen sulfide and may hasten aging of organic Biofilter media. A two-stage biofilter has been designed and installed at the Ojai Valley Sanitary District wastewater treatment plant. The first stage is an enclosed system with a medium of small, inert, porous stones. It is called an acid gas biofilter. The second stage is a section of a traditional open biofilter filled with wood chips. The acid gas biofilter effectively removed H2S and volatile organic compounds while causing much lower headloss than traditional biofilters. However, considerable flow heterogeneity in ...

In an ideal biofilter, air flows parallel to the biofilter axis and at the same velocity through all parts of the bed. In contrast, non-uniform flow causes different parcels of air to experience detention times above or below the average, reducing treatment efficiency. Designers and operators have long been aware that medium nonuniformity, bed compaction, fissuring, and separation from the vessel walls can cause flow channeling and damage biofilter performance. However, as biofilters have been designed for lower headloss and higher flowrates, flow heterogeneity has also arisen, as pressure variation in the headspace affects flow in the medium.In an effort to investigate flow heterogeneity, a two-dimensional steady-state computational fluid dynamics (CFD) model was developed to simulate flow through a typical biofilter. The model assumes incompressible, two-dimensional, Navier-Stokes flow in the spaces above and below the bed and Darcy flow in the porous medium. The modeling effort d...

The radiance of most objects seen at a distance through the atmosphere is dominated by scattered light of a blue hue that should make the landscape appear predominately blue. However, common experience shows that people can see colors at a distance. A possible explanation of this paradox is that the visual system splits the light into a haze layer and the background landscape. A straightforward mathematical description of this splitting explains the results of a color matching study in the Great Smoky Mountains National Park. In this study, hues of objects seen through haze were found to be constant with changes in optical depth while colorfulness decreased exponentially.

Recent research has indicated that up to 73% of the methyl bromide (MeBr) applied to agricultural farmland is ultimately emitted to the atmosphere despite the practice of complete coverage of the fields with polyethylene (PE) tarp. To reduce the emission of MeBr, several techniques have been investigated. An alternative that has received little consideration is the collection and recycle or treatment of MeBr emissions. We investigated the potential of using a two-layer tarp system for collecting the MeBr. Laboratory experiments with a small two-layer diffusion reactor were conducted to determine the mass transfer coefficient (K) of MeBr through tarps and to validate a model of the collection system. For PE tarps K was 1.15 × 10-6 ms-1 at20°Cand5.2×10-6 ms-1 at60°C.Kforso- called virtually impermeable films ranged from 4.6 × 10-10 m s-1 to 1.3 × 10-8 m s-1. The mathematical model was then used to simulate a full scale fumigant field application. Results indicate excellent agreement b...