Biomedical subjects

M F Dahab

Publications and source records attributed to M F Dahab.

9 recordsLinked to original sources

PubMed2005

Biomass concentration and biofilm characteristics in high-performance fluidized-bed biofilm reactors.

Two laboratory scale fluidized-bed biofilm reactors (FBBRs) were used to investigate the biomass concentration and the biofilm characteristics in a high performance FBBR used for the denitrification of exceptionally high-nitrate wastewater (1000 mg-N/L). Reported correlations by other workers for predicting the biomass concentration in FBBR were examined for their validity in comparison with the experimental results of this study and the best set of applicable correlations was recommended. The effects of the two main operational parameters, the superficial velocity and nitrogen loading rate on the biomass concentration in the FBBR were also studied. Correlations for the drag coefficient and the expansion index from the literature, together with the biofilm dry density correlation produced from this study were found to produce the best prediction of the FBBR biomass concentration compared to other reported correlations. The average biomass concentration in the FBBR decreased with the increase of the superficial velocity in the range of 45 to 65 m/h at all the applied nitrogen loadings (i.e. 6, 8, 12 and 16 kg-N/m3(bed).d).

Biofilms↗

PubMed2004

Nitrification of high-strength ammonium wastewater by a fluidized-bed reactor.

A laboratory-scale fluidized-bed reactor with an external aeration loop was used for nitrification of high-strength ammonium wastewater (up to 500 mg NH4-N/L). The results demonstrated that the system is capable of handling ammonium removal rates of up to 2.5 kg NH4-N/m3 x d, while removal efficiencies were as high as 98% and independent of the applied ammonium loading rates. Ammonium loading rates higher than 2.5 kg NH4-N/m3 x d resulted in decreased ammonium removal efficiency. The data show that near complete ammonium removal occurred at DO concentrations as low as 0.3-0.5 mg/L. However, the nitrite-nitrogen fraction in the effluent increased from 3.5% to 23.2% when the DO dropped from 1.0 mg/L to approximately 0.4 mg/L, respectively. The high specific removal rates in this system are one order of magnitude higher than that of suspended-growth systems. This can reduce the supplementary reactor volumes required for nitrification to less than 10% of that needed in conventional activated sludge systems. These results clearly indicate the potential economic gains that could be achieved through implementation of this technology.

Biofilms↗

PubMed2002

Identification of factors contributing to degradation in autothermal thermophilic sludge digestion.

A laboratory scale ATAD reactor (6 days SRT, 24 h feed cycle) was operated at quasi steady-state conditions with real waste activated sludge feed. Hydrolysis tests of waste activated sludge demonstrated that 10% of feed suspended matter is hydrolysed practically instantaneously and 20-30% of all fed suspended matter is dissolved after 60 minutes in tap water and in cell free reactor liqueur equally. Respirometric, VSS and COD concentration data served as basis for calibration of a simple VSS based kinetic model. The calibrated model provided good fit to two separate sets of measured data. This model was used to evaluate different operation strategies. Modification of cycle length does not affect overall VSS emoval rate, while shorter cycle length or continuous operation helps avoid oxygen limited conditions. Further advantages of shorter feed cycles (reduced cooling effect, greater realizable load) support choosing continuous operation of the ATAD system if the main goal is VSS reduction. While reactor cascades increase efficiency, this advantage diminishes with increasing load. At high load rates increased construction costs are not justified by the expected improvement in efficiency.

Bioreactors↗

PubMed2002

Effects of aerobic and anaerobic digestion systems on pathogen and pathogen indicator reduction in municipal sludge.

In this study, the effectiveness of anaerobic treatment systems in reducing pathogenic density levels was evaluated under the US EPA municipal sludge rule (40 CFR Part 503 Rule). Wastewater and sludge samples were analyzed for both pathogens and pathogenic indicator organisms from six different existing wastewater treatment systems. The results indicate that Class B sludge requirements under the US 503 Rule are reasonable and can be achievable by the existing treatment systems while Class A sludge requirements under the same rule may not be easily achieved by the existing treatment systems. The effects of volatile solids loading rates on anaerobic digester performance were investigated. Under anaerobic digestion conditions, it appears that the log reductions in fecal coliform and fecal streptococcus appeared to be dependent on VSS loading rates. On the other hand, Salmonella sp. density reductions did not appear to be dependent on VSS loading rates.

Bacteria, Aerobic↗

PubMed2001

Comparison of conventional and two-stage reversible flow, static-bed biodenitrification reactors.

This paper compares the operation of a traditional single-stage system with a two-stage, reversible flow biodenitrification system for removing nitrates from drinking water. The purpose of this study was to investigate the ability of these two-stage systems to remove nitrate and residual organics from treated water as compared to single-stage units. In the reversible flow system, the second-stage (i.e. follow) reactor is operated in series with the first-stage (i.e. lead) reactor. After a given period of operation, the flow regime is reversed so that the follow reactor becomes the lead one and vice versa. The active solids remaining in the follow reactor (previously the lead one) are capable of removing residual soluble organics and nitrates to levels below the concentrations provided by single-stage units particularly at HRTs as low as 0.5 h. Nitrate-nitrogen removal efficiency improved slightly from 98 to 99.5% for the single- and two-stage systems, respectively. Most notably, reversible flow reactors were found to reduce long-term effluent residual organics concentrations with an average of approximately 1/3 that of the single-stage system. Also the reversible flow system, with its design redundancy, demonstrated the ability to receive extreme shock loads with no sustained loss of treatment efficiency.

Bioreactors↗

PubMed2001

Start-up performance of a subsurface-flow constructed wetland for domestic wastewater treatment.

Constructed wetlands have been shown to provide sufficient domestic wastewater treatment in temperate climates. However, the effectiveness of this technology, when subjected to severe temperature extremes during start-up conditions, is limited. Because of the growing interest in constructed wetland technology within smaller communities located in colder-climates, and the need for cost-effective treatment systems, this paper presents the performance of a constructed wetland system during the first two-years of operation under a variety of loading and operating conditions associated with northern climates. While the treatment performance of constructed wetland systems has been widely documented within the literature, documentation on performance during start-up stages has been limited. The results indicate that effective and sufficient constructed wetland seasonal removals of total suspended solids, volatile suspended solids, 5-day carbonaceous biochemical oxygen demand, chemical oxygen demand, and fecal coliform were achieved within the wetland cell during the first two-years of operation. Wastewater temperatures appeared to affect 5-day carbonaceous biochemical oxygen demand and chemical oxygen demand removal rates. Nitrogen and phosphorus reductions were not as effective and varied seasonally, as well as with wastewater temperature.

Conservation of Natural Resources↗

PubMed2001

Dose-response assessment by a fuzzy linear-regression method.

Regression analysis has been used to characterize the relationship between an exposure dose and the incidence of an adverse health effect such as cancer. However, the regression rarely describes the true relationship due to uncertainties in dose-response data and relationships. Therefore, a method is developed to perform dose-response assessments by a fuzzy linear regression which explicitly exhibit these uncertainties. This method is applied to define the relationship between a particular nitrate dose to humans and its corresponding cancer risk.

Dose-Response Relationship, Drug↗

PubMed2001

Performance modeling of subsurface-flow constructed wetlands systems.

A subsurface flow constructed wetlands (CW) system, located at a neighborhood consisting of a small housing development and golf courses outside of Lincoln, NE, was studied for its effectiveness as a small community wastewater system. Extensive monitoring was conducted biweekly between June 1996 and December 2000. Prediction models for soluble CBOD5 NH3-N, and TP removal in CW were employed for comparison with the field data. It was found that the disappearance of BOD5 and NH3-N could be approximated using first-order kinetics, but the kinetics of TP removal were unclear. The reduction rate constants regressed from the field data were found to be lower than literature reported values.

Ecosystem↗

PubMed2001

Subsurface-flow constructed wetlands treatment in the plains: five years of experience.

This paper documents the performance of a subsurface-flow constructed wetlands system during its initial five years of operation under variable loading and operating conditions associated with a northern midwestern US climate. The results indicate that effective and sufficient CW seasonal removals of TSS, VSS, CBOD5, COD, and fecal coliform were achieved. Wastewater temperatures seemed to affect CBOD5 and COD removal rates. Nitrogen and phosphorus reductions were not as effective and varied seasonally, as well as with wastewater temperature. The addition of a sand filter, to aid in further nitrification and disinfection following CW treatment, markedly improved the performance of the wetlands system. After a few years of operation, the remarkable performance of the CW system was dampened by apparent clogging and subsequent eruption of wastewater at the head-end of the treatment cells. While clogging was partially caused by biomass build-up in the wetlands substrate, visual observations suggest that excessive vegetation coupled with relaxed maintenance may also be responsible for clogging.

Biomass↗