Case study II: application of the divalent cation bridging theory to improve biofloc properties and industrial activated sludge system performance-using alternatives to sodium-based chemicals.

The objective of this study was to investigate the application of the divalent cation bridging theory (DCBT) as a tool in the chemical selection process at an activated sludge plant to improve settling, dewatering, and effluent quality. According to the DCBT, to achieve improvements, the goal of chemical selection should be to reduce the ratio of monovalent-to-divalent (M/D) cations. A study was conducted to determine the effect of using magnesium hydroxide [Mg(OH)2] as an alternative to sodium hydroxide (NaOH) at a full-scale industrial wastewater treatment plant. Floc properties and treatment plant performance were measured for approximately one year during two periods of NaOH addition and Mg(OH)2 addition. A cost analysis of plant operation during NaOH and Mg(OH)2 use was also performed. During NaOH addition, the M/D ratio was 48, while, during Mg(OH)2 addition, this ratio was reduced to an average of approximately 0.1. During the Mg(OH)2 addition period, the sludge volume index, effluent total suspended solids, and effluent chemical oxygen demand were reduced by approximately 63, 31, and 50%, respectively, compared to the NaOH addition period. The alum and polymer dose used for clarification was reduced by approximately 50 and 60%, respectively, during Mg(OH)2 addition. The dewatering properties of the activated sludge improved dewatering as measured by decreased capillary suction time and specific resistance to filtration (SRF), along with an increase in cake solids from the SRF test. This corresponded to a reduction in the volume of solids thickened by centrifuges at the treatment plant, which reduced the disposal costs of solids. Considering the costs for chemicals and solids disposal, the annual cost of using Mg(OH)2 was approximately 30,000 dollars to 115,000 dollars less than using NaOH, depending on the pricing of NaOH. The results of this study confirm that the DCBT is a useful tool for assessing chemical-addition strategies and their potential effect on activated sludge performance.     

Mapping and Modeling the Biogeochemical Cycling of Turf Grasses in the United States

Turf grasses are ubiquitous in the urban landscape of the United States and are often associated with various types of environmental impacts, especially on water resources, yet there have been limited efforts to quantify their total surface and ecosystem functioning, such as their total impact on the continental water budget and potential net ecosystem exchange (NEE). In this study, relating turf grass area to an estimate of fractional impervious surface area, it was calculated that potentially 163,800 km² (± 35,850 km²) of land are cultivated with turf grasses in the continental United States, an area three times larger than that of any irrigated crop. Using the Biome-BGC ecosystem process model, the growth of warm-season and cool-season turf grasses was modeled at a number of sites across the 48 conterminous states under different management scenarios, simulating potential carbon and water fluxes as if the entire turf surface was to be managed like a well-maintained lawn. The results indicate that well-watered and fertilized turf grasses act as a carbon sink. The potential NEE that could derive from the total surface potentially under turf (up to 17 Tg C/yr with the simulated scenarios) would require up to 695 to 900 liters of water per person per day, depending on the modeled water irrigation practices, suggesting that outdoor water conservation practices such as xeriscaping and irrigation with recycled waste-water may need to be extended as many municipalities continue to face increasing pressures on freshwater.     

Enhanced radionuclide immobilization and flow path modifications by dissolution and secondary precipitates

Caustic radioactive wastes that have leaked at Hanford Site (Richland, WA) induce mineral dissolution and subsequent secondary precipitation that influence the fate and transport of contaminants present in the waste solutions. The effects of secondary mineral precipitates, formed after contacting solids with simulated caustic wastes, on the flow path changes and radionuclide immobilization were investigated by reacting quartz, a mixture of quartz and biotite, and a Hanford sediment (Warden soil: coarse-silty, mixed, superactive, mesic Xeric Haplocambids) with simulated caustic tank waste solution. Continuous Si dissolution and concomitant secondary mineral precipitation were the principal reactions observed in both batch and flow-through tests. Nitrate-cancrinite was the dominant secondary precipitate on mineral surfaces after 3- to 10-d reaction times in batch experiments. X-ray microtomography images of a reacted quartz column revealed that secondary precipitates cemented quartz grains together and modified pore geometry in the center of the column. Along the circumference of the packed column, however, quartz dissolution continuously occurred, suggesting that wastes that leaked from buried tanks in the past likely did not migrate vertically as modeled in risk assessments but rather the pathways likely changed to be dominantly horizontal on precipitation of secondary precipitate phases in the Hanford vadose zone. Based on batch equilibrium sorption results on the reacted sediments, the dominant secondary precipitates (cancrinites) on the mineral surfaces enhanced the sorption capacity of typical Hanford sediment for radionuclides 129I(-I), 79Se(VI), 99Tc(VII), and 90Sr(II), all of which are of major concern at the Hanford Site.     

SUSPENSOIDS IN NEW YORK CITY'S DRINKING WATER RESERVOIRS: TURBIDITY APPORTIONMENT1

ABSTRACT: The technique of i ndividual p article a nalysis conducted by s canning electron microscopy interfaced with a utomated X ‐ray microanalysis (IPA/SAX) was used to characterize suspended particulate matter in New York City's drinking water reservoirs and their tributaries. The study covered a two year period and involved analyses of more than 300 samples. The particle cross sectional area per unit volume (PAV), or area concentration, was measured to account for the observed turbidity, a representation of light scattering property of the studied medium. A simple linear model with a nearly zero intercept was able to explain more than 85 percent of the variation in the measured turbidity. Moreover, the particle assemblage was categorized into generic particle types with distinctive geochemical or geological origins. Thus, PAV compositions in terms of particle types could be apportioned into turbidity components based on the model. Inorganic tripton, dominated by aluminosilicate (clay) and silicate of nonbiological nature, was found to be the major turbidity causing constituent in most cases. With the exception of one reservoir where organic detritus was significant, the predicted inorganic particle turbidity agreed with the measured turbidity within experimental error.     

EFFECT OF WOODY DEBRIS ENTRAPMENT ON FLOW RESISTANCE1

ABSTRACT: Recent environmental concerns in floodplain management have stimulated research of the effect vegetation and debris have on flow conveyance, and their function in a productive riparian ecosystem. Although the effect of stable, in-channel woody debris formations on flow resistance has been noted by several authors, studies concerning entrapment of detrital debris in vegetation are lacking. Logs, limbs, branches, leaves and other debris transported during flooding often become lodged against bridges, hydraulic structures, trees and vegetation, and other obstacles, particularly in and near the overbank areas. Hydraulic measurements obtained in a channel prior to and following the removal of woody debris indicated that the average Manning's n value was 39 percent greater when woody debris was present. An examination of the drag-velocity relation for vegetation indicated that an increase in the frontal area of debris and/or vegetation results in a nearly proportional increase in Manning's n. The influence of debris on flow resistance decreased as flow depth increased.     

USING CONTINGENT VALUATION AND BENEFIT TRANSFER TO EVALUATE WATER SUPPLY IMPROVEMENT BENEFITS1

ABSTRACT: Many water systems in small cities and rural areas throughout the United States are facing water quality and supply problems. These problems are typically not the result of an unexpected event, but are the result of growth trends or decreasing water quality experienced over several years. This analysis uses the contingent valuation and benefit transfer methods to evaluate the willingness to pay for a rural water system in northcentral Montana. Both of the procedures resulted in similar values, ranging from about $4.05 to $7.50 per household per month for urban residents and $5.40 to $11.50 per household per month for rural residents, which is equal to 11 percent to 23 percent of current average water costs. The willingness to pay estimates do not include non-household water users. This analysis shows that useful planning information can be obtained from relatively inexpensive contingent valuation mail survey data and the benefit transfer method as long as the limitations of the data are understood. The willingness to pay for ensuring good quality rural water supplies in the future is likely to be low compared to the costs of extensive diversion and treatment systems. Willingness to pay estimates provide decision makers with information that can be used to avoid building a large water supply system that water users do not want to connect to because of high costs.     

The transition from dirty to clean industries: optimal fiscal policy and the environmental Kuznets curve

This paper investigates privately and socially optimal patterns of economic development in a two-sector endogenous growth model with clean and dirty goods. We consider a second-best fiscal policy framework in which distortionary taxes jointly influence economic growth and environmental quality. In this policy setting, three conditions produce an Environmental Kuznets Curve (EKC): (i) dirty output is bounded; (ii) clean output grows endogenously; and (iii) growth in the dirty sector reduces growth in the clean sector. These conditions do not arise with a consumption externality, but can emerge with a production externality. Endogenous labor supply implications are also investigated. Although not necessary for producing an EKC, endogenous labor supply provides additional linkages that produce an EKC under circumstances in which it would otherwise not appear.     

A co-operative approach to risk management in an active lead/zinc smelter community

Trail, Canada has been the site of a large lead/zinc smelting facility since 1916. In mid 1990, the Trail Community Lead Task Force was established and given responsibility for developing a strategy to reduce children's blood lead levels. With funding from the provincial government, the smelting company and the municipal government, the Task Force has carried out blood lead screening, case management, community education, exposure pathways modelling and remediation trials. The decline in children's blood lead levels appeared to accelerate following the implementation of these programmes. The average blood lead in Trail children aged 6–72 months fell 14% from the autumn of 1991 to autumn of 1992, whereas for the previous 16 years, the average annual decline had been about 4%. In subsequent years, blood lead levels have appeared to plateau. Throughout the 5-year history of the Task Force, its members have demonstrated a strong sense of common purpose and have worked co-operatively to reach consensus on most issues.