**6.2 Landfill caps**

Capping a landfill involves three layers: an upper vegetative (topsoil) layer, a drainage layer, and a low permeability layer made of a synthetic material covering two feet of

Landfill Management and Remediation Practices in New Jersey, United States 163

common practice in the United States. Most of municipal wastewater treatment plants use aerobic biological treatment (e.g. activated sludge process), and were specially designed to aim at biodegradable organic matters and suspended solids in sewage. Therefore, refractory organics and emerging pollutants in leachate may be poorly removed. Although much large sewage, relative to leachate, can dilute the persistent pollutants from leachate, it should be noted that these leachate pollutants are not truly removed or eliminated. Moreover, toxic chemicals in leachate (e.g. ammonia and heavy metals) may disturb microbial activities and cause unusual operation in wastewater treatment plants. In addition, sewer lines may be unavailable, be of insufficient capacity, or be disallowed for some reasons for connection to

The recirculation of leachate back into filled cells is an essential step in a bioreactor landfill, in which microbial activities are intentionally enhanced. The recirculation of leachate provide moisture and/or oxygen to stimulate the microbial degradation of solid wastes and simultaneously reduce the amount of leachate needed for treatment. A bioreactor landfill may be either aerobic or anaerobic, to reclaim landfill space. Aerobic bioreactor landfills inject both air and leachate into the waste, while anaerobic bioreactor landfills only inject leachate into the waste. The aerobic bioreactor increases microbial digestion rates, thereby resulting in quicker settlement of the waste compared to anaerobic bioreactor landfills. In contrast, anaerobic bioreactor landfills generate more methane gas. Thus, it is a promising candidates for energy recovery projects. However, the recirculation is not commonly accepted practice among the waste management community or is not favored by regulations

Many landfills discourage the acceptance of bulky waste since bulky waste results in large voided spaces in a landfill due to the inert, or inability, to decompose in a landfill, which is an inefficient way to use a landfill especially when this material could be recycled. Bulky waste such as tree parts, construction and demolition debris, tires, carpets, are deterred from a landfill by implementing higher fees for this material or to build recycling and recovery facilities at the landfill to reduce the amount of material landfilled. Landfills can use crushed tires or construction and demolition debris for alternatives to covers or stone (NJDEP, 2006).

Brownfield redevelopment remediates and preserves existing contaminated sites, like old landfills, for use in the future. Brownfield redevelopment provides economic development by establishing new areas for businesses and industry to expand, and gives people the opportunity to gather, visit, shop, recreate, or work in different places. Brownfield redevelopment not only provides economic advantages but also brings communities together in New Jersey. However, the redevelopment of landfills is a challenge due to a variety of contaminants involved and the geological issues of building above a landfill (Wiley and Assadi, 2002). The issues associated with landfill redevelopment projects include: the size of the landfill, contaminants' types, the size and depth of plume, type and

nearby treatment plants. (Spengel and Dzombak, 1991).

**6.5.1 Bioreactor landfills** 

(NJDEP, 2006).

**6.6 Deterring bulky waste** 

**6.7 Brownfield redevelopment** 

compacted clay. Capping has a life span of about 50-100 years, but the performance of the cap depends on the site's environmental conditions. Cracking and erosion of caps can occur due to fluctuations in air temperatures and precipitation, and if the site is prone to subsidence and earthquakes. The cap must be adequately thick to prevent frost, and accommodate vegetative roots and burrowing animals (Vasudevan et al, 2003).

The use of temporary caps, instead of a final cap, on a filled landfill cell increases landfill space because the feet of soil typically used in a final cap is replaced with a synthetic membrane held down by removable items, like tires rather than soil. Temporary caps may be used in conjunction with leachate recirculation and active gas extraction. They are readily removable, and do not occupy much space like soil when the landfill is reopened for future landfilling activities. Some landfills are using temporary tarps to cover the waste instead of daily soil covers, increasing landfill capacity. Soil-like materials, like spray foam, can substitute soils as daily or intermediate cover material frees landfill space (NJDEP, 2006).
