Otto R. Stein

Mechanics of jet scour downstream of a headcut

Scour immediately downstream of a headcut is analyzed by considering the effect of jet diffusion on sediment detachment. The analytical development results in equations for: 1. the equilibrium scour depth; 2. the rate of scour hole development. These equations are applicable for a variety of jet configurations and any bed material including cohesive soils. The equations are tested in laboratory simulated headcuts on bed materials consisting of two noncohesive uniform sands and one cohesive natural soil. The agreement between measured and predicted rates of scour depth increase shows that scour does not increase as a semi-logarithmic function of time. The rate of scour depth increase is very rapid for depths less than 95% of the equilibrium scour depth. Graphical solutions for the rate of scour depth increase are in agreement with the experimental measurements.

IMPINGING JET CALIBRATION OF EXCESS SHEAR SEDIMENT DETACHMENT PARAMETERS

An excess shear sediment detachment relation is commonly used in erosion prediction models. Traditional field methods for calibration of soil-dependent empirical constants are both expensive and conceptually suspect. The subprocesses of sediment detachment and transport cannot be physically separated, therefore they cannot be calibrated independently. Impingement of a clear-water jet on a soil bed produces detachment without transport at the point of maximum scour. In this article, a mathematical description of the impinging jet scour process is used to calibrate an excess shear detachment relation. Four calibration models are developed based on variants of the jet-scour rate equations (differential and integral form, each evaluated using arithmetic and logarithmic values for dependent variables). All models assume detachment is linearly related to excess shear. The validity of this calibration technique has been tested in the laboratory by measuring scour depth versus time for six different soils, all evaluated in a disturbed, saturated condition. A total of 14 replicates on the soils were conducted where hydraulic inputs were varied with replicate. Statistical analyses were used to place confidence limits about the optimal sediment detachment values for each of the four models. Only the arithmetic differential model produced results considerably different than the other three. The integrated log model produced the tightest confidence intervals and this is suggested for further use. The size of the confidence intervals demonstrates the replicability of the technique. The technique produced detachment rate constants that are similar to field calibration studies for soils of similar properties. Critical shear values are an approximate order of magnitude lower, but all soils were evaluated only in the highly erosive, disturbed, unconsolidated, and saturated condition. The technique can be applied to soils in other conditions and it is possible that this discrepancy is due to different soil conditions.

Influence of Season and Plant Species on the Abundance and Diversity of Sulfate Reducing Bacteria and Ammonia Oxidizing Bacteria in Constructed Wetland Microcosms

Constructed wetlands offer an effective means for treatment of wastewater from a variety of sources. An understanding of the microbial ecology controlling nitrogen, carbon and sulfur cycles in constructed wetlands has been identified as the greatest gap for optimizing performance of these promising treatment systems. It is suspected that operational factors such as plant types and hydraulic operation influence the subsurface wetland environment, especially redox, and that the observed variation in effluent quality is due to shifts in the microbial populations and/or their activity. This study investigated the biofilm associated sulfate reducing bacteria and ammonia oxidizing bacteria (using the dsrB and amoA genes, respectively) by examining a variety of surfaces within a model wetland (gravel, thick roots, fine roots, effluent), and the changes in activity (gene abundance) of these functional groups as influenced by plant species and season. Molecular techniques were used including quantitative PCR and denaturing gradient gel electrophoresis (DGGE), both with and without propidium monoazide (PMA) treatment. PMA treatment is a method for excluding from further analysis those cells with compromised membranes. Rigorous statistical analysis showed an interaction between the abundance of these two functional groups with the type of plant and season (p < 0.05). The richness of the sulfate reducing bacterial community, as indicated by DGGE profiles, increased in planted vs. unplanted microcosms. For ammonia oxidizing bacteria, season had the greatest impact on gene abundance and diversity (higher in summer than in winter). Overall, the primary influence of plant presence is believed to be related to root oxygen loss and its effect on rhizosphere redox.

Ammonium Removal in Constructed Wetland Microcosms as Influenced by Season and Organic Carbon Load

Abstract We evaluated ammonium nitrogen removal and nitrogen transformations in three-year-old, batch-operated, subsurface wetland microcosms. Treatments included replicates of Typha latifolia, Carex rostrata, and unplanted controls when influent carbon was excluded, and C. rostrata with an influent containing organic carbon. A series of 10-day batch incubations were conducted over a simulated yearlong cycle of seasons. The presence of plants significantly enhanced ammonium removal during both summer (24°C, active plant growth) and winter (4°C, plant dormancy) conditions, but significant differences between plant species were evident only in summer when C. rostrata outperformed T. latifolia. The effect of organic carbon load was distinctly seasonal, enhancing C. rostrata ammonium removal in winter but having an inhibitory effect in summer. Season did not influence ammonium removal in T. latifolia or unplanted columns. Net production of organic carbon was evident year-round in units without an influent organic carbon source, but was enhanced in summer, especially for C. rostrata, which produced significantly more than T. latifolia and unplanted controls. No differences in production were evident between species in winter. COD values for C. rostrata microcosms with and without influent organic carbon converged within 24 hours in winter and 7 days in summer. Gravel sorption, microbial immobilization and sequential nitrification/denitrification appear to be the major nitrogen removal mechanisms. All evidence suggests differences between season and species are due to differences in seasonal variation of root-zone oxidation.

Floating treatment wetlands for domestic wastewater treatment

Floating islands are a form of treatment wetland characterized by a mat of synthetic matrix at the water surface into which macrophytes can be planted and through which water passes. We evaluated two matrix materials for treating domestic wastewater, recycled plastic and recycled carpet fibers, for chemical oxygen demand (COD) and nitrogen removal. These materials were compared to pea gravel or open water (control). Experiments were conducted in laboratory scale columns fed with synthetic wastewater containing COD, organic and inorganic nitrogen, and mineral salts. Columns were unplanted, naturally inoculated, and operated in batch mode with continuous recirculation and aeration. COD was efficiently removed in all systems examined (>90% removal). Ammonia was efficiently removed by nitrification. Removal of total dissolved N was ∼50% by day 28, by which time most remaining nitrogen was present as NO(3)-N. Complete removal of NO(3)-N by denitrification was accomplished by dosing columns with molasses. Microbial communities of interest were visualized with denaturing gradient gel electrophoresis (DGGE) by targeting specific functional genes. Shifts in the denitrifying community were observed post-molasses addition, when nitrate levels decreased. The conditioning time for reliable nitrification was determined to be approximately three months. These results suggest that floating treatment wetlands are a viable alternative for domestic wastewater treatment.

Effects of constructed wetland design on ibuprofen removal – A mesocosm scale study

This study aimed to investigate the effects of constructed wetland design (unsaturated, saturated and aerated saturated) and plant species (Juncus, Typha, Berula, Phragmites and Iris) on the mass removal and removal kinetics of the pharmaceutical ibuprofen. Planted systems had higher ibuprofen removal rates (29%-99%) than in the unplanted ones (15%-85%) in all designs. The use of forced aeration improved ibuprofen removal only in the unplanted mesocosms. In general, ibuprofen removal followed an area-based first-order removal kinetics model with removal rate coefficients (kA) varying between 3 and 35cm/d. The ibuprofen removal was mainly attributed to microbial degradation by the fixed bed biofilm, but plant uptake and degradation within plant tissues also occurred. The ibuprofen removal was positively correlated with the oxygen concentration in the water and the removal of nutrients, indicating that degradation may be due to co-metabolisation processes.

Temperature, plant species and residence time effects on nitrogen removal in model treatment wetlands

Total nitrogen (TN) removal in treatment wetlands (TWs) is challenging due to nitrogen cycle complexity and the variation of influent nitrogen species. Plant species, season, temperature and hydraulic loading most likely influence root zone oxygenation and appurtenant nitrogen removal, especially for ammonium-rich wastewater. Nitrogen data were collected from two experiments utilizing batch-loaded (3-, 6-, 9- and 20-day residence times), sub-surface TWs monitored for at least one year during which temperature was varied between 4 and 24 °C. Synthetic wastewater containing 17 mg/l N as NH4 and 27 mg/l amino-N, 450 mg/l chemical oxygen demand (COD), and 13 mg/l SO4-S was applied to four replicates of Carex utriculata, Schoenoplectus acutus and Typha latifolia and unplanted controls. Plant presence and species had a greater effect on TN removal than temperature or residence time. Planted columns achieved approximately twice the nitrogen removal of unplanted controls (40-95% versus 20-50% removal) regardless of season and temperature. TWs planted with Carex outperformed both Typha and Schoenoplectus and demonstrated less temperature dependency. TN removal with Carex was excellent at all temperatures and residence times; Schoenoplectus and Typha TN removal improved at longer residence times. Reductions in TN were not accompanied by increases in NO3, which was consistently below 1 mg/l N.

Presence and persistence of wastewater pathogen Escherichia coli O157:H7 in hydroponic reactors of treatment wetland species

Treatment wetlands (TWs) efficiently remove many pollutants including a several log order reduction of pathogens from influent to effluent; however, there is evidence to suggest that pathogen cells are sequestered in a subsurface wetland and may remain viable months after inoculation. Escherichia coli is a common pathogen in domestic and agricultural wastewater and the O157:H7 strain causes most environmental outbreaks in the United States. To assess attachment of E. coli to the TW rhizosphere, direct measurements of E. coli levels were taken. Experiments were performed in chemostats containing either Teflon nylon as an abiotic control or roots of Carex utriculata or Schoenoplectus acutus. Flow of simulated wastewater through the chemostat was set to maintain a 2 hour residence time. The influent was inoculated with E. coli O157:H7 containing DsRed fluorescent protein. Root samples were excised and analyzed via epifluorescent microscopy. E. coli O157:H7 was detected on the root surface at 2 hours after inoculation, and were visible as single cells. Microcolonies began forming at 24 hours post-inoculation and were detected for up to 1 week post-inoculation. Image analysis determined that the number of microcolonies with >100 cells increased 1 week post-inoculation, confirming that E. coli O157:H7 is capable of growth within biofilms surrounding wetland plant roots.

Pumps: Intake Design, Selection, and Installation

Publisher Summary This chapter deals with wet well design, pump piping, and selection of pumping equipment. The pump intake design must satisfy several requirements for proper approach conditions by avoiding poor velocity distribution at the entrance to the pump, excessive swirling in the pump intake piping, air entrainment in the pumped flow, unstable approach conditions in multiple pump operation, and vortices. Model studies are useful for finding and correcting faults in the hydraulic flow patterns in wet wells. The ANSI/HI 9.8-1998 standard requires a physical model study if the sump or piping geometry differs from the standard, the approach flow is non-uniform or unsymmetrical, and the flow rate is more than 2520 L/s (40,000 gal/min) per pump or the total flow exceeds 6310 L/s (100,000 gal/min) with all pumps running. Several factors that need to be considered during pipe selection include quality of the fluid to be pumped, required design capacity (initial minimum, average, and maximum flow rates), operating conditions (best-case and worst-case system head curves, maximum and minimum flow rates, submergence, and/or NPSH). A step-by-step procedure for selecting pumps involves developing the calculations for the performance requirements completely and on the basis of the best- and worst-case assumptions for system dynamic headlosses, specifying materials that are suitable for the application, specifying the proper balance grade number, and deciding the number of pumps to be used and the type of pump.