Journal of Environmental Engineering

The scheduling and economies of scale for municipal landfill liner and leachate-collection system construction are commonly neglected in policy and planning studies, but are shown to significantly affect the economics of landfilling and, thus, integrated solid-waste-management systems. Economic methods for estimating least-cost landfill module sizes and construction schedules are presented. The principal findings are twofold: (1) modular or phased development significantly reduces the discounted cost of landfilling; and (2) for disposal rates less than about 1,000 tons/day, discounted liner and leachate-collection system cost is highly sensitive to the module service life, even with modest economies of scale. This second result indicates that 46% of the nation's landfills, which receive 58% of the nation's waste, may benefit from least-cost module replacement planning, especially as improved source reduction and recycling efforts further reduce disposal rates. The least-cost module replacement interval is fairly robust to uncertainties in waste-to-cover ratio and specific weight of wastes. A case study compares landfill cost reductions from least-cost modular development and from delay of current landfill closure and replacement due to a 25% reduction in landfill disposal rate.

Landfills are frequently considered for urban development, but have limited end uses due to large differential settlements, leachate generation, and gas emissions. Current landfill design with top- and bottom-liner systems minimize entry of moisture and increase the period required for stabilization of the refuse to occur. The objective of this study is to test the ability of biological enhancement to reduce the time to reach biological stabilization of the waste to nondegradable matter, and to determine the effects of biodegradation on settlement. To accomplish this, six landfill test cells were constructed to model both settlement and decomposition over extended periods. Three cells were designed to simulate bioreactor landfills, while another three were designed to simulate secure vaults. Results demonstrate that secondary settlement is linear with the logarithm of time, and decomposition is well represented by a first-order model. Comparisons indicate that in the short term there is no significant increase in the settlement rate due to biodegradation; however, extrapolation suggests that in the long term the settlement rate will likely increase as the effects of decomposition become more significant.

Alternating aerobic/anoxic (A/A) digestion of waste activated sludge offers many advantages over traditional (aerobic or anaerobic digestion) methods. However, one of the main concerns in A/A digestion is the development of an on-line control strategy for alternating aerobic and anoxic cycles. Computerized pH monitoring has revealed two well-defined control points that correspond to complete nitrification (ammonia valley) and denitrification (nitrate apex). The pH control points were consistent with those previously identified using the oxidation reduction potential. Based on these findings, two real-time control strategies were developed and tested. First, pH was allowed to increase (anoxic) and decrease (aerobic) within preset limits to control A/A cycling. A second approach detected the points of complete nitrification and denitrification using the differential of the pH signal. Comparison of the A/A digestion results with aerobic digesters indicated similar mixed-liquor volatile suspended solids and fecal coliform destruction with aerobic digestion but with better total nitrogen removal and superior dewatering characteristics.

The dewatering characteristics of polymer conditioned sludges are examined for sludge-conditioning regimes effected by agitation in a baffled, cyclindrical mixing vessel using different impellers operated at constant values of $\overline{G}$ and constant mixing time. Dewatering characteristics are evaluated by different laboratory tests, characterizing sludge filterability, settleability, and drainability. Only one type of slude and polymer was applied for the conditioning experiments. The experimental results suggest that $\overline{G}$ is deficient as the only descriptor of the impact of turbulent mixing intensity on sludge-conditioning performance. Based on the results, it is questioned whether laboratory-scale sludge conditioning performed by impeller agitation in a baffled, cylindrical mixing vessel is comparable to the pipe-flow conditioning taking place at many full-scale sludge-dewatering installations. It is suggested that a more appropriate characterization of mixing regimes for sludge conditioning should take into account how the applied power for mixing is converted into steady flow and turbulent shear, respectively, and which of these conditions is more important at a particular mixing time and polymer dosage. The three parameters: (1) Mixing intensity; (2) mixing time; and (3) polymer dosage need to all be properly accounted for in order to evaluate sludge-conditioning performance.

A methodology is developed to estimate the human-health risk from ground-water contamination and evaluate possible regulatory actions. The methodology is illustrated considering the case of health risk from ground-water nitrate contamination. Generally, assessment of health risk from water contamination contains several elements of uncertainty due to a lack of available information, including that related to interspecies extrapolation, dose-response modeling, and the inherent inaccuracy of data used. In the present paper, the uncertainty associated with these elements and its impact on results are represented by using a fuzzy-set approach and incorporated into the health-risk–assessment process; therefore, a health-risk assessment and management strategy can be made that is more realistic and appropriate than the one made without taking uncertainty into account. The guidelines on risk regulation presented allow decision makers to determine, under uncertainty, whether regulatory action must be taken to reduce human-health risk from nitrate in ground-water supplies. Three factors, the level of health risk related to uncertainty, the population size exposed, and the cost required for risk reduction, are then considered in deciding whether to regulate or not.