WASTE BIODEGRADATION AND LFG GENERATION
2.4 BIODEGRADATION OF REFUSE
G:C:B- G:M:B-
G:C:B+
G:M:B+
G:S:B- G:L:B-
G:S:B"j.
G:L:B+
Where
G - General Waste
C, S, M, L - Communal «25 tonnes/day), Small (25-150 tonnes/day), Medium (150- 500 tonnes/day) or Large (>500/day).
B- - No significant leachate generation B+ - Significant leachate generation
2.3.2.2 Hazardous waste landfill
Hazardous waste may only be disposed of at a landfill designed specifically for the disposal of hazardous waste. Hazardous waste in South Africa is classified in terms of hazard ratings. The hazard ratings allocated are (DWAF, 1998b):
Hazard Rating 1: Extreme Hazard Hazard Rating 2: High Hazard Hazard Rating 3: Moderate Hazard Hazard Rating 4: Low Hazard
There are two categories of hazardous waste landfill sites, designated as "H:h" and
"H:H". H:H landfills can accept all hazard ratings of wastes, while H:h landfills can receive only low, moderate hazardous waste and general waste. The co-disposal of significant quantities of hazardous waste with general waste may only be practised on a hazardous waste landfill.
digestion in the case of the landfill bioreactor (Novella et al., 1999). The decomposition of solid wastes in landfills is a complex process, which is not yet understood (Ham, 1988; DoE, 1991).
Once the solid waste is placed in a landfill, a complex sequence of physically, chemically, and biologically mediated events occur. These physical, chemical and biological processes all interact simultaneously in order to provide the overall decomposition patterns (Ham, 1988). Waste decomposition occurs, producing contaminated water or leachate and gases. Physical decomposition may be considered as the physical rising of material from the waste and changes in physical characteristics such as strength and settlement as a result of decomposition. Chemical decomposition includes the dissolution of materials from refuse by leachate.
Biological decomposition, however, is the major mechanism by which refuse decomposes in a landfill and biological decomposition in practice controls chemical and physical decomposition because of its effect on variables such as pH and oxidation-reduction potential (Ham, 1988).
2.4.1 Major phases indecomposition of waste
The degradation of the organic fraction of waste materials within a landfill may be described as a five-stage process. Figure 2.2 illustrates the five stages of the process and the typical products generated at each stage. The first and fifth stages occur under aerobic conditions, whilst the remaining stages take place under predominately anaerobic conditions. Each stage of the process has an impact on the quality and rate of degradation of leachate and landfill gas (DoE, 1995).
Stage 1: Hydrolysis/Aerobic degradation
Aerobic decomposition occurs when refuse is first placed in a landfill. The oxygen required for aerobic decomposition comes from air incorporated in the landfill during placement of the waste, from the direct access of air to refuse near the surface of a landfill, and from dissolved oxygen in precipitation entering the surface of a landfill (Ham, 1988). The organic fraction of waste is metabolised by aerobic micro- organisms (oxygen-consumers) present in the waste.
Process Products
StageI
Aerobic Anaerobic
StageII
Stage
m
Stage IV
Aerobic
Stage V
Waste organic Gases Leacha
fraction
If
Hydrolysis! AerobiC
CO 2 H2O degradation
.
\.- - - - - - - - - - - - - - - - -
Hydrolysis andFermentation
.. I
I~oni!icalHOrganic acids2 CO2 nitroge~° -
Acetogenesis Acetic acid
I H2C02 ~"JJ:"'-i>o".,..-
r
Methanogenesis CH~CO 2
- - - - - -
-- - -
--
i""' -- - - -
ir 1r
Oxidation
-
CO 2..
,r
te
Aerobic Anaerobic
Aerobic
Figure 2.2 Major stages of waste degradation (DoE, 1995).
These micro-organisms convert readily degradable carbohydrates to simple sugars such as glucose, carbon dioxide and water. One of the characteristics of aerobic degradation is the production of heat (an exothermic reaction), which can cause the temperature of refuse to rise dramatically. According to DoE (1995) the temperature
rises in the range of 80-90°C, while according to Ham (1988) the range is from 50- 70°C. The process of aerobic decomposition uses oxygen, which is present within the waste. This stage of waste degradation is short and lasts for a few days or weeks (Robinson, 1989). However, in shallow landfills of less than 3m high, the aerobic stage may persists for long periods, producing significant amounts of C02 due to the availability of air which can readily enter the waste (Robinson, 1989).
The duration of this aerobic stage, which depends on the availability of oxygen, is influenced by management practices at the site, such as the degree of waste compaction, the depth of waste and the type of daily cover. The characteristic odour associated with this stage of the process is mainly due to the presence of organic esters (DoE, 1995). As oxygen becomes depleted further stages of degradation develop.
Stage 11: Hydrolysis and Fermentation
Anaerobic and facultative orgamsms (bacteria) hydrolyse and ferment cellulose, carbohydrates, lipids and proteins producing simple, soluble compounds such as volatile fatty acids with high biochemical oxygen demand (BOD), acetate, carbon dioxide, hydrogen and inorganic salts, such as sulphate and ammonium. During this stage, nitrogen is displaced by carbon dioxide and hydrogen to form leachate with high ammoniacal nitrogen content. This leachate will characteristically have a low pH, and a high chemical oxygen demand (COD) reflecting the large amounts of partially degraded organic material (DoE, 1995; Ham, 1988). Characteristics of this stage of decomposition include a lower production of heat than was obtained during the aerobic process.
Stage II: Actogenesis
The bacteria (actogenic bacteria) convert the soluble acids formed by the activities of the fermentative bacteria of the previous stage to acetate, carbon dioxide and hydrogen. Other bacteria convert carbohydrates, hydrogen and carbon dioxide to acetic acid. The conversion of fermentation products such as butyrate, propionate and
ethanol can only be achieved at low hydrogen concentrations. This stage of the overall process can only be maintained if hydrogen utilising organisms such as the sulphate-reducing bacteria and methane-generating bacteria are active. If the levels of hydrogen remain high, the intermediate products (e.g. propionate) cannot be further oxidised and therefore under these conditions, acid accumulates, forming acetogenesis. Gases generated from the waste mass during this stage are predominantly carbon dioxide, hydrogen and methane (DoE, 1995).
Stage IV: Methanogenesis
Under this phase of methanogenic anaerobic decomposition, the methane-generating bacteria (methanogens) which cannot tolerate aerobic conditions, metabolise degraded organics (acetate and formate) produced during the other degradation stages to form a mixture of carbon dioxide and methane (plus various trace constituents) which is released as landfill gas (Robinson, 1989; Ham, 1988; WMP2B, 1995). Some methanogens may also be able to generate methane by the direct conversion of hydrogen and carbon dioxide. Methanogens are most active in the pH range 6.8-7.4.
As the soluble substrates are consumed, the production of methane from refuse becomes dependent upon the hydrolysis of cellulose. Cellulose contains the largest source of carbon in refuse which can be converted to methane. Characteristics of this phase of decomposition include the increase of pH to near neutrality. As a result of the increase in pH, the leachate produced during this stage is less chemically aggressive (Ham, 1988).
Stage V: Oxidation
In the final stage of the process as the degradable components become exhausted, progressive re-establishment of aerobic conditions can occur (DoE, 1995).
Facultative and aerobic micro-organisms such as methane oxidising organisms start to recolonize the landfill and may then become established as prevailing conditions permit.
2.5 COMMON POLLUTANTS EMITTED FROM LANDFILL