Urbanization and the Geoenvironment - Sustainable Practices in Geoenvironmental Engineering - page 249

settlement and decreased plant growth may result from lowering of the water table. Long-
term monitoring is required to ensure that the system performs according to requirements
speciied by the indicators prescribed for safe performance.
7. 5 .1. 3 E xcavat i o n
Excavation, partially or totally, to remove offending landills can be a solution to decreas-
ing contaminant generation and allow land reuse. The waste can be recycled, incinerated,
or dumped in a safe landill. Handling and transport of these wastes is not without risk.
Corroded drums are not easy to handle. The excavated landill can be illed with clean or
recycled soil and subsequently used for construction of buildings. Numerous measures
have to be instituted to ensure worker safety and protection of the environment during
the excavation work. Health and geoenvironmental threats arise from breaking bags, dust,
emission of harmful gases or liquids, and unstable waste.
7.5.1.4 Landill Bioreactor
Landills traditionally have been operated without addition of liquid; they have been oper-
ated under the “dry garbage bag” concept. Increasing the moisture content of the waste,
however, can increase waste degradation and methane production since most of the con-
version processes are anaerobic. This concept is known as a landill bioreactor and is
becoming increasingly used worldwide. Experience with this type of landill shows that
gas recovery can be increased up to 90%, and that wastes can be stabilized within 10 years
instead of 30 to 100 years (Block, 2000). Landills can be designed to be aerobic, anaerobic,
or anaerobic-aerobic bioreactors. Anaerobic bioreactors maintain moisture content of 10%
to 20% to optimize anaerobic degradation conditions. Aerobic reactors require injection of
air or oxygen with vertical or horizontal injection wells. Although wastes can be stabilized
in 2 years by aerobic landills, injection of the air can be costly and can lead to ires and
thus is not practiced often. The hybrid or anaerobic-aerobic bioreactor takes advantage of
aerobic and anaerobic bacteria. The upper layer of waste is aerobically treated before burial
and subsequently treated with anaerobic bacteria. In all cases, moisture control is the most
important parameter. Other factors include pH, waste pretreatment such as shredding,
nutrient addition, settlement, cellulose content, leachate quantity and quality, and tem-
perature control. Monitoring of these factors is essential. There is also the potential for
mining the dredged waste for humic material and other recyclables. Figure 7.14 shows
the major elements of a waste landill bioreactor system. In addition to recycling of the
collected leachate back into the wastepile, the option of addition of inoculum, nutrients,
and other dissolution aids is provided. Gas generated from the wastepile is collected in a
collection system for treatment.
In the United States, according to the EPA subtitle D rule, landill bioreactors can only
be used for landills with composite liners such as 0.61 m of clay covered with a high-
density polyethylene (HDPE) liner, as shown in Figure 7.14. Most installations increase the
moisture content through recirculation of the leachate. Recirculation assists biodegrada-
tion by enhancing the transport of nutrients through the waste, redistribution of methane
bacteria, buffering, dilution of inhibitory components, and retention of the constituents of
the leachate in the landill. It can be accomplished by spray application, iniltration ponds,
horizontal or vertical injection wells, or trenches. Accumulated amounts of leachate will
depend on (a) regulatory requirements, (b) how much is needed for each landill com-
partment, and (c) minimization of quantity of leachate requiring disposal. Older areas in