Biological/Chemical degradation of waste

TYPES OF SOLID WASTE

   Solid waste can be classified into different types depending on their source:

—  Municipal waste.

—  Industrial waste.

—  Hazardous waste.

  MUNICIPAL WASTE

  This waste is generated mainly from residential and commercial complexes. With rising urbanization and change in lifestyle and food habits, the amount of municipal solid waste has been increasing rapidly and its composition changing. Examples include: food waste, rubbish, agricultural waste, construction and demolition waste, etc. (Howard, Donald, and Tchobanglous, 1985).

 

SOLID WASTE MANAGEMENT

   Management of solid waste reduces or eliminates adverse impacts on the environment and human health and supports economic development and improved quality of life. A number of processes are involved in effectively managing waste for a municipality. These include monitoring, collection, transport, processing, recycling and disposal.

   In addition, an effective system of solid waste management must be both environmentally and economically sustainable.

 •Environmentally sustainable: It must reduce, as much as possible, the environmental impacts of waste management.

 •Economically sustainable: It must operate at a cost acceptable to community.

   An effective waste management system includes one or more of the following options:

—  Waste collection and transportation.

 

 

—  Resource recovery through sorting and recycling i.e. recovery of materials (such as paper, glass, metals) etc. through separation.

—  Resource recovery through waste processing i.e. recovery of materials (such as compost) or recovery of energy through biological, thermal or other processes.

—  Waste transformation (without recovery of resources) i.e. reduction of volume, toxicity or other physical/chemical properties of waste to make it suitable for final disposal.

—  Disposal on land i.e. environmentally safe and sustainable disposal in landfills.

 

CLASSIFICATION OF SOLID WASTE

METHODS OF SOLID WASTE DEGRADATION

—  Biological degradation

—  Chemical degradation

BIOLOGICAL DEGRADATION

   Biological degradation is the complete microbial breakdown or mineralization of complex materials into simple inorganic constituents such as CO2, water and mineral components. All biological waste treatment processes involve the decomposition of biodegradable wastes by living microbes (bacteria and fungi), which use biodegradable waste materials as a food source for growth and proliferation. Microbes excrete specialised enzymes that digest biodegradable waste constituents (e.g. cellulose and other complex polysaccharides, proteins and fats) into simple nutrients (e.g. sugars, amino acids, fatty acids).

   These transformations may be achieved either aerobically or anaerobically, depending on the availability of oxygen. Aerobic conversion transforms waste to compost, while anaerobic conversion transforms waste to CH4 and CO2 and resistant organic matter (Tchobanoglous et al., 1993).

 

COMPOSTING

   Composting is aerobic decomposition of organic matter by the action of micro organisms. It is the natural process of decomposition of organic waste that yields manure or compost, which is very rich in nutrients. Composting is a biological process in which micro-organisms, mainly fungi, and bacteria, convert degradable organic waste into humus like substance. This finished product, which looks like soil, is high in carbon and nitrogen and is an excellent medium for growing plants.

 

ANAEROBIC DIGESTION

    Anaerobic digestion is the bacterial decomposition of organic waste in the absence of free oxygen The anaerobic decomposition of organic materials yields principally methane (CH4),carbon dioxide (CO2) and a solid compost material that can be used as soil conditioner. It occurs in three stages, hydrolysis/liquefaction, acidogenesis and methanogenesis. The first group of microorganism secretes enzymes, which hydrolyses polymeric materials to monomers such as glucose and amino acids.

  These are subsequently converted by second group i.e. acetogenic bacteria to higher volatile fatty acids, H₂ and acetic acid. Finally, the third group of bacteria, methanogenic bacteria, convert H₂, CO₂, and acetate, to CH4.

Fig. 5: Anaerobic digestion

 

LANDFILLING

    Landfilling involves the controlled disposal of solid waste on or in the upper layer of the earth’s mantel. Landfill has been widely used for municipal solid waste (MSW) disposal all over the world. Especially in developing countries, it is considered to be a reliable and cost effective method if adequate land is available. Basic principle of conventional landfill design is to contain or store the waste so that the exposure to human and environment could be minimized. This is done by the prevention of gas emission from landfill and infiltration of surface water.

   Bioreactor landfill is an emerging technology for solid waste management. The basic concept of bioreactor landfill is to use specific design and operation practices to accelerate the decomposition of organic wastes in a landfill by promoting optimum moisture content and sufficient nutrients for the microorganisms to degrade the waste. These enhanced microbial processes have the advantage of rapidly reducing the volume of the waste creating more space for additional waste, they also maximise the production and capture of methane for energy recovery systems and they reduce the costs associated with leachate management.

 

CHEMICAL DEGRADATION

  This refers to processes that involve the use of chemical agents to process waste.

Fig. 6: Flow chart for chemical degradation of solid waste

 

  Thermal treatment involves conversion of waste into  gaseous, liquid and solid conversion products with concurrent or subsequent release of heat energy. It involves three processes namely:

—  Pyrolysis

—  Gasification

—  Incineration

 PYROLYSIS

   This is the thermal degradation of a substance in the absence of oxygen. This process requires an external heat source to maintain the temperature required.

 

  Typically, relatively low temperatures of between 300ºC to 850ºC are used during pyrolysis of materials such as MSW. The products produced from pyrolysing materials are a solid residue and a synthetic gas (syngas). The solid residue (sometimes described as a char) is a combination of non-combustible materials and carbon. The syngas is a mixture of gases (combustible constituents include carbon monoxide, hydrogen and methane. A proportion of these can be condensed to produce oils, waxes and tars.

  GASIFICATION

   The gasification process involves the partial combustion of a carbonaceous or hydrocarbon fuel.  Gasification can be seen as between pyrolysis and combustion in that it involves the partial oxidation of a substance. This means that oxygen is added but the amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to occur. The temperatures employed are typically above 650°C The process is largely exothermic but some heat may be required to initialise and sustain the gasification process. The main product is a syngas, which contains carbon monoxide, hydrogen and methane.

 

Some paper topics generated from our task include

—  Municipal solid waste degradation

—  Resource recovery from solid waste through biodegradation

—  Study of the biodegradability of various components of solid waste

—  Chemical processes of solid waste conversion/degradation

—  Degradation of unsorted/heterogenous municipal solid waste

 

REFERENCES

 

EGSSAA (2009). Solid Waste. Retrieved from www.encapafrica.org.

Howard, S. P., Donald, R.R and Tchobanglous, G (1985). Environmental engineering. McGraw-Hill International Press, Singapore. Pg 573-576.

Odocha JNK (1994). Waste generation and management in a depressed economy. A lecture delivered to student of the law and environmental faculties,UNEC, University of Lagos.

Sabiiti EN (2011). Utilizing Agricultural waste to conserve food security and conserve the environment. Afr. J. Food, Agric. Nutr. Dev. 11:6.

Sani A, Awe FA, Akinyanju JA (1992). Amylase synthesis in Aspergillusniger and Aspergillusflavus grown on cassava peel. J. Ind. Microbiol. 10:55-59.

Sridhar MKC (1996). “Women in waste management”. A seminar paper sponsored by LHHP and the british council for the seminar on educating women for sustainable environmental management. Owerri, Nigeria. March.

Tchobanoglous, G., H. Theisen & S.A. Vigil (1993). Integrated Solid Waste Management. McGraw-Hill International Press, Singapore. 957 p

 

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