Sanitary Landfill

SANITARY LANDFILL

The  term landfill  generally  refers  to  an  engineered  deposit  of  wastes either  in pits/trenches or on the surface.  And,  a sanitary  landfill  is  essentially  a landfill, where proper mechanisms are available to control the environmental risks associated with the  disposal  of  wastes  and  to make available  the land, subsequent  to disposal,  for other purposes. However, you  must note that a landfill need  not  necessarily be  an  engineered site,  when the  waste is  largely inert at final disposal, as in rural areas, where wastes contain a large proportion of soil and dirt. This practice is generally designated as non-engineered disposal method. When compared to uncontrolled dumping, engineered landfills are more likely to have pre-planned installations, environmental monitoring, and organised and trained workforce. Sanitary landfill implementation, therefore, requires careful site selection, preparation and management.

The four minimum requirements you need to consider for a sanitary landfill are:

(i)  full or partial hydrological isolation;

(ii) formal engineering preparation;

(iii) permanent control;

(iv) planned waste emplacement and covering.

1Principle

The  purpose  of  land  filling  is  to  bury  or  alter  the  chemical  compos ition of  the wastes so that they do not pose any threat to the environment or public health. Landfills are not homogeneous  and are  usually  made up  of cells  in which  a discrete volume of waste is kept isolated from adjacent waste cells by a suitable barrier. The barriers between cells generally consist of a layer of natural soil (i.e., clay), which restricts downward or lateral escape of the waste constituents or leachate.

Land filling relies on containment rather than treatment (for control) of wastes. If properly executed, it is a safer and cheaper method than incineration. An environmentally sound  sanitary landfill comprises  appropriate liners for protection of the groundwater (from contaminated  leachate), run-off  controls, leachate collection and treatment,  monitoring wells and  appropriate   final cover design  (Phelps, 1995). Figure 4.1 below gives   a  schematic layout of sanitary landfill along with its various components:

2 Landfill processes

(i)  Site selection process and considerations: This requires the development  of a working  plan - a  plan,  or  a series of plans, outlining the development and descriptions of site location, operation, engineering and site restoration.  Considerations for site include public opinion, traffic patterns and congestion, climate, zoning requirements, availability of cover material  and  liner as well,  high trees or buffer in the  site perimeter, historic buildings, and endangered species,  wetlands, and site land environmental  factors,  speed  limits, underpass limitations, load limits on roadways, bridge capacities, and proximity of major roadways, haul distance, hydrology and detours.

(ii) Settling process: The waste body of a landfill undergoes different stages  of settling or deformation.

3 Settling Processes in Landfill

The three stages shown in the figure above are described below:

Primar y consolidation:  During  this  stage,  a  substantial  amount  of settling occurs.  This settlement is caused by the weight of the waste layers.  The movement of trucks, bulldozers or mechanical compactors will also enhance this process. After this primary consolidation, or short-term deformation stage, aerobic degradation processes occur.

Secondary  compression:  During  this  stage,  the  rate  of  settling  is much lower than that in the primary consolidation stage, as the settling occurs through compression, which cannot be enhanced.

Decomposition: During the degradation processes, organic material is converted into gas and leachate. The settling rate during this stage increases compared to the secondary compression stage, and continues until all decomposable organic matter is degraded. The settling rate, however, gradually decreases with the passage of time. To  appropriately design  protective liners, and gas and leachate collection systems,  it is, therefore,  necessary to have a proper knowledge  of the settling process of wastes.       

(iii) Microbial degradation process:  The microbial degradation  process is the most important biological process occurring in a landfill. These processes induce changes in the chemical and physical environment within the waste body, which determine the quality of leachate  and both  the quality  and  quantity  of landfill gas (see Subsection 4.3.2). Assuming that  landfills  mostly  receive  organic  wastes, microbial processes will dominate the stabilisation of the waste and therefore govern landfill gas generation and leachate composition.  Soon after disposal, the  predominant part  of the  wastes  becomes  anaerobic,  and the bacteria will start degrading  the solid  organic carbon, eventually to produce carbon dioxide and  methane.   The anaerobic  degradation  process  undergoes  the following  stages:   

a)     Solid and complex  dissolved  organic  compounds  are hydrolysed  and fermented by the fermenters primarily to volatile fatty acids, alcohols, hydrogen and carbon dioxide.

b)    An acidogenic group of bacteria converts the products of the first stage to acetic acid, hydrogen and carbon dioxide.

c)     Methanogenic bacteria convert acetic  acid  to  methane  and  carbon dioxide and hydrogenophilic  bacteria convert hydrogen and carbon dioxide to methane.

The biotic factors that affect methane formation in the landfill are pH, alkalinity, nutrients, temperature, oxygen and moisture content.