Roger W. Babcock

Hydrodynamics and mass transfer in a tubular airlift photobioreactor

In photobioreactors, which are usually operated under light limitation,sufficient dissolved inorganic carbon must be provided to avoid carbonlimitation. Efficient mass transfer of CO2 into the culture mediumisdesirable since undissolved CO2 is lost by outgassing. Mass transferof O2 out of the system is also an important consideration, due tothe need to remove photosynthetically-derived O2 before it reachesinhibitory concentrations. Hydrodynamics (mixing characteristics) are afunctionof reactor geometry and operating conditions (e.g. gas and liquid flow rates),and are a principal determinant of the light regime experienced by the culture.This in turn affects photosynthetic efficiency, productivity, and cellcomposition. This paper describes the mass transfer and hydrodynamics within anear-horizontal tubular photobioreactor. The volume, shape and velocity ofbubbles, gas hold-up, liquid velocity, slip velocity, axial dispersion,Reynoldsnumber, mixing time, and mass transfer coefficients were determined intapwater,seawater, and algal culture medium. Gas hold-up values resembled those ofvertical bubble columns, and the hydraulic regime could be characterized asplug-flow with medium dispersion. The maximum oxygen mass transfer coefficientis approximately 7 h−1. A regime analysisindicated that there are mass transfer limitations in this type ofphotobioreactor. A methodology is described to determine the mass transfercoefficients for O2 stripping and CO2 dissolution whichwould be required to achieve a desired biomass productivity. This procedure canassist in determining design modifications to achieve the desired mass transfercoefficient.

Green roof hydrologic performance and modeling: a review

Green roofs reduce runoff from impervious surfaces in urban development. This paper reviews the technical literature on green roof hydrology. Laboratory experiments and field measurements have shown that green roofs can reduce stormwater runoff volume by 30 to 86%, reduce peak flow rate by 22 to 93% and delay the peak flow by 0 to 30 min and thereby decrease pollution, flooding and erosion during precipitation events. However, the effectiveness can vary substantially due to design characteristics making performance predictions difficult. Evaluation of the most recently published study findings indicates that the major factors affecting green roof hydrology are precipitation volume, precipitation dynamics, antecedent conditions, growth medium, plant species, and roof slope. This paper also evaluates the computer models commonly used to simulate hydrologic processes for green roofs, including stormwater management model, soil water atmosphere and plant, SWMS-2D, HYDRUS, and other models that are shown to be effective for predicting precipitation response and economic benefits. The review findings indicate that green roofs are effective for reduction of runoff volume and peak flow, and delay of peak flow, however, no tool or model is available to predict expected performance for any given anticipated system based on design parameters that directly affect green roof hydrology.

Biodegradable dissolved organic carbon for indicating wastewater reclamation plant performance and treated wastewater quality

Various methods for measuring biodegradable dissolved organic carbon (BDOC) in water have been introduced in the last decade. Applications of the methods have been limited to drinking water. The measure of BDOC has been used mainly to indicate the quality of raw and finished waters and evaluate the performance of biological activated carbon (ozone/granular activated carbon) systems in water treatment plants. Recently, a modified BDOC protocol was developed for examining reclaimed and secondary-treated wastewaters. Use of the new BDOC method can be extended to the wastewater treatment and reclamation fields. Samples collected from a wastewater reuse pilot facility were tested for BDOC, The modified BDOC method was able to detect the increase in biodegradability of ozonated tertiary-treated wastewater. Good relationships among BDOC, dissolved organic carbon (DOC), and soluble biochemical oxygen demand were obtained. The modified protocol was later used to measure BDOC in secondary-effluent samples from 13 municipal wastewater treatment plants. The results show that BDOC can also be used as an indicator of secondary-effluent quality. Likewise, strong and significant correlations were found among BDOC, DOC, and soluble chemical oxygen demand in secondary effluents.

Demonstration of bioaugmentation in a fluidized-bed process treating 1-naphthylamine

Abstract The feasibility of using a fluidized-bed reactor with bioaugmentation to treat low-strength wastewater or slightly contaminated groundwater was evaluated. Tap water spiked with 1.6 to 12.6 mg/L of 1-naphthylamine was treated in a fluidized-bed reactor with a hydraulic retention time of 6.2 to 13.3 h. With conventional operation, the reactor was unable to maintain consistent removal because of cell washout. Continuous removal efficiency was greater than 90% using bioaugmentation through periodic addition of acclimated cells from an off-line enricher-reactor. The concept demonstrated by this research presents an alternative for biological treatment of wastes (dilute wastewaters and contaminated groundwater) which are not normally considered good candidates for biological treatment.

An economic appraisal of using source separation of human urine to contain and treat endocrine disrupters in the USA

Elevated concentrations of estrogens in natural waters pose a significant threat to public health and aquatic ecosystems. Both natural (estrone, 17β-estradiol and estriol) and synthetic (17α ethynylestradiol) estrogens, ubiquitous in wastewater effluents and receiving waters, have been shown to affect the endocrine system of human and aquatic life. The effects vary from cancer to sex reversals at levels as low as parts per trillion in sensitive organisms. Separation of urine, which constitutes only about 1% of domestic sewage and contains nearly all of the excreted estrogens, potentially offers an energy-efficient way to contain and then treat these chemicals. With a capital expense of

Modeling ozone mass transfer in reclaimed wastewater.

2100 and operation and maintenance costs of

Modeling the performance of hazardous wastes removal in bioaugmented activated sludge processes.

69 per year for a urine diverting toilet system, a family in the USA can realize estimated savings of

Life cycle assessment comparison of activated sludge, trickling filter, and high-rate anaerobic-aerobic digestion (HRAAD).

101 per year in energy, water, and nutrients and a decrease of 100 kg in greenhouse gas emissions. To remove 99% of estrogenicity in discharged waters would require approximately 12 kW h per year using continuous electrodialysis followed by ozonation (O(3)) of source separated urine. To achieve the same results by adding O(3) treatment after activated sludge at existing municipal wastewater treatment plants would require 23 kW h per year. From an energy standpoint it makes sense to practice source separation and treatment of urine to limit estrogen discharges into the environment.

Enhanced nitrogen removal with an onsite aerobic cyclic biological treatment unit

Ozone mass transfer in reclaimed water was evaluated at pilot scale to determine mass-transfer characteristics and reaction kinetics and to assess the use of oxygen as a surrogate to measure this process. Tests were conducted in a 40-L/min pilot plant over a 3-year period. Nonsteady-state mass-transfer analyses for both oxygen and ozone were performed for superficial gas flow rates ranging from 0.13m/min to 0.40m/min. The psi factor, which is the ratio of volumetric mass-transfer coefficients of ozone to oxygen, was determined. The decrease in oxygen transfer rate caused by contaminants in reclaimed water was only 10 to 15% compared to tap water. A simple mathematical model was developed to describe transfer rate and steady state ozone concentration. Ozone decay was modeled accurately as a pseudo first-order reaction between ozone and ozone-demanding materials.

Environmental Aspects of Large-Scale Microalgae Cultivation

An enricher-reactor process in which acclimated biomass is grown in an offline reactor on high concentrations of enrichment substrates was used to bioaugment a conventional activated sludge process to a toxic compound, 1-amino naphthalene. Various levels of bioaugmentation, ranging from 1 to 16% mass ratio of augmented cells to indigenous cells, were evaluated in laboratory-scale reactors. The experimental results showed that bioaugmentation can enhance toxic compound removal, increase resistance to shock loading, and reduce the time required for acclimation to the toxic compound. A process model was developed and calibrated using the experimental data. This model was then used to compare the process to an in-situ bioaugmentation process using a reaeration reactor that receives a portion of the recycle activated sludge. The model predicted experimentally observed removals of toxic compound and decreasing relative benefits of bioaugmentation at higher levels. The model suggests that augmented biomass suffers higher decay, which likely is due to the effects of its removal from a substrate-rich to substrate-poor environment. The model shows that the enricher-reactor-process has advantages at lower mean cell retention time (MCRT), and the in-situ process is superior at higher MCRT. Both processes can remove the toxic compound when operating below the washout MCRT that would occur in an unaugmented activated sludge process.