MSW Management May 2012 : Page 50

fore, protective of HHE in the absence of regulated PCC. A conceptual illus-tration of a landfill’s step down prog-ress from active PCC through custodial care is presented as Figure 2. Two case studies outlined below demonstrate the manner in which a site may embark upon the step-down reduction in care illustrated on the figure. Case Study 1 The study site is a closed, 27-acre MSW landfill unit comprising two conjoined cells at an active facility. The site is located in Delaware and receives approximately 40 inches of rain annually with high summer tem-peratures typically around 90°F and Figure 3: Case study 1 (closed-loop semi-active leachate management) winter lows generally below freezing. The cells, which accepted waste from 1980–1988, have a geomembrane liner and system was designed, consisting of five main flow process is automated using pressure and leachate collection system. The cells were components: flow-balancing leachate storage float switches to control liquid flow through closure capped with a 2-foot thick sandy tanks, constructed wetlands, treated effluent the system and prevent overflow of any system soil cover in accordance with the regulations storage pond, irrigation system, and phytocap. component. Redundant systems and controls applicable at that time, and currently gener-A schematic layout of the design is shown on are also provided throughout. Construction ate approximately 10,000 gallons per day of Figure 3. Further details on the project are dis-commenced in 2006; the system became fully operational in 2008. leachate. Leachate recirculation was used cussed in Morris, et al. (2007). The constructed wetland design was based as the primary means of leachate manage-ment for over 10 years, a practice that, in on the 14-month operation of a pilot scale Case Study 2 conjunction with the permeable soil cover, system (Pendleton, et al., 2005). Because the The study site is an active MSW landfill has contributed significantly to the relatively total volume of treated effluent often exceeds operated by Allied Waste Industries (now mild leachate presently generated at the unit the amount of irrigation liquid needed to Republic Services) and located in Alabama. (Morris, et al., 2003), although it was even-sustain the phytocap, some of the effluent The landfill covers 246 acres of land and has tually necessary to ship leachate to an offsite volume is eliminated utilizing evaporation (E) a total permitted disposal area of 134 acres, wastewater treatment plant in addition to and evapotranspiration (ET) processes. To of which about 79 acres of landfill cells have recirculation (which ceased altogether in maximize E/ET losses, irrigation occurs only been constructed. The overall intent for this during optimal dry, hot conditions during the project is to treat all leachate generated at March 1995 because of a regulatory ban). The overall goal of this project was to create warmest six to eight months of the year. A dis-the site for the remainder of its active life a self-sustaining leachate management system used 4-million-gallon capacity leachate storage and after closure. Currently, approximately using engineered wetlands to treat all leachate pond was upgraded to store treated effluent half of the daily leachate flow is reported generated by a discrete closed landfill to arbo-during the remainder of the year (incident to be managed through recirculation with real standards and then irrigating a phytocap precipitation is excluded using a floating geo-the remaining half hauled off site to POTW. on the landfill cover to take up all the treated membrane cover). Effluent from the storage The designed treatment system consists of a effluent, resulting in no discharge to the envi-pond is fed into an irrigation system consisting three-cell surface flow wetland followed by ronment. Because pristine wetlands surround of an automated control system and network two parallel four-cell vertical flow wetland their rural property, the Delaware Solid Waste of zoned sprinkler heads that have low-effi-biofilter system (WBS) units. Thereafter, Authority (DSWA), the site owner/operator, ciency, misting settings in order to maximize commingled treated effluent from the WBS required that neither leachate nor treated ef-E losses during spraying. The phytocap was is to be either discharged to surrounding fluent be discharged to groundwater or surface planted with tree species that are compatible natural wetlands via an existing stormwater water (although this may be considered in with site conditions and suitable for maxi-management pond, returned to an existing due course). In addition, no liquid should mizing ET losses. Irrigation of the phytocap lift station for transfer to onsite leachate require removal from the closed loop system is controlled by means of a weather station storage tanks via an existing leachate trans-(i.e., offsite disposal) except where this cannot and soil moisture sensor control system. To mission line. Treated effluent transferred be avoided as a contingency measure. DSWA prevent potential migration to groundwater, to the tanks in this way will continue to be also required that the project not increase total irrigation occurs over the lined cell area only. used for recirculation back into the landfill leachate management costs above 5 cents per To minimize runoff and potential migration for operational benefits. The design was gallon, the then cost for offsite leachate dispos-to surface water, irrigation does not occur based on a 15-month pilot test that showed al. To achieve these goals and maintain a peren-during precipitation events or in a cover zone that leachate can successfully be treated to nial zero/negative water balance, a closed-loop where the soil is already saturated. The entire meet stringent surface water quality criteria 50 MSW MANAGEMENT [ ELEMENTS / MAY 2012 ]

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