Modern lifestyle has led to more acute waste problems, convenience products generally require more packaging, improvident habits associated with greater affluence lead to greater quantities of waste, as demonstrated by discarded wrappers from the inevitable fast food outlet, and the modern day waste contains a higher proportion of non-degradable materials such as plastics.
The government has adopted a National Strategic Plan for Solid Waste Management with emphasis on the upgrading of unsanitary landfills as well as the construction of new sanitary landfills and transfer stations with integrated material recovery facilities.
The Solid Waste Management Act, 2007 will drastically change the structure of solid waste management in Malaysia and to open up for the development of a completely new business sector.
Anaerobic Digestion Process
Anaerobic digestion is a bacterial fermentation process that operates without free oxygen and results in a biogas containing mostly methane and carbon dioxide.
It occurs naturally in anaerobic niches such as marshes, sediments, wetlands, and the digestive tracts of ruminants and certain species of insects.
Anaerobic digestion is also the principal decomposition process occurring in landfills.
Anaerobic digestion systems are employed in many wastewater treatment facilities for sludge degradation and stabilization, and are used in engineered anaerobic digesters to treat high-strength industrial and food processing wastewaters prior to discharge.
There are also many instances of Anaerobic digestion applied at animal feeding operations and dairies to mitigate some of the impacts of manure and for energy production.
History of Anaerobic Digestion
Anaerobic digestion dates back as far as the 10th century, when the Assyrians used it to heat bath water.
It was historically insignificant before reappearing in 17th century Europe, when it was determined that decaying organic matter produced flammable gases, again used to heat water.
The first full scale application was in the 1890s when the city of Exeter, UK used it to treat wastewater.
From there, it continued to be widely used as a way to stabilize sewage sludge, as it is today.
The first systems were large, unheated and unmixed tanks with significant operational problems due to solid settling and scum formation.
These frequent system disturbances limited the adoption of the technology until the twentieth century.
Rural areas, especially in developing countries, have employed AD for centuries.
It is used as a sanitary and economic way to treat waste and generate fuel for use in heat and cooking.
Many of these facilities continue to operate in countries such as China and India, where organics dominate the waste stream.
In China, for example, organics make up 60% of the municipal solid waste in the country as a whole, and an even greater percentage in rural areas.
Most digesters are small and characterized by minimum technical automation and high levels of manual labour.
Solid Waste Anaerobic Digestion Technologies
A wide variety of engineered systems have been specifically developed for the rapid “in-vessel” digestion of solid waste and other types of organic wastes.
Each had its own special benefits and constraints.
Here the solid waste feedstock is slurried with a large proportion of process water to provide a dilute (10-15 percent dry solids) feedstock that can be fed to a complete mix
There are a number of multi-step wet digestion processes where the solid waste is slurried with water or recycled liquor and fermented by hydrolytic and fermentative bacteria to release volatile fatty acids which are then converted to biogas in a high-rate industrial wastewater anaerobic digester, usually an anaerobic filter or a sludge blanket reactor.
These systems are very suitable for the digestion of bio-waste and wet organic wastes from industries such as food processing.
This concept involves a continuously-fed digestion vessel with a digest-state dry matter content of 20-40 percent.
Both completely-mixed and plug-flow systems are available.
Plug flow systems rely on external recycle of a proportion of the outgoing digest-state to inoculate the incoming raw feedstock.
In both cases, the requirement for only minimal water additions makes the overall heat balance favourable for operation at thermophilic digestion temperatures (50-55°C).
This concept batch-loads the containment vessel with raw feedstock and inoculating it with digest-state from another reactor.
It is then sealed and left to digest naturally.
During this closure period, leachate from the base of the vessel can be re-circulated to maintain a uniform moisture content and redistribute soluble substrates and methane bacteria throughout the mass of solid waste within the vessel.
When digestion is complete, the vessel is reopened, unloaded and refilled with a fresh charge of raw feedstock.
The sequencing batch concept is generally similar to dry batch digestion, except that leachate from the base of the vessel is exchanged between established and new batches to facilitate start up, inoculation and removal of volatile materials in the active reactor.
After the digestion process becomes established in the solid waste, the digester is uncoupled and reconnected to a fresh batch of MSW in a second vessel.
This concept is an accelerated and controlled landfill, designed to encourage the conversion of solid waste into methane.
A bioreactor landfill is typically divided into cells, and is provided with a system to collect leachate from the base of the cell. Collected leachate is pumped back up to the surface and distributed across the waste cells.
This transforms a landfill into, essentially, a large high-solids digester.
Anaerobic Digestion Benefit
Anaerobic digestion has the opportunity to be an integral part of the solution to two of the most pressing environmental concerns of urban centres: waste management and renewable energy.
Through anaerobic digestion, organics are decomposed by specialized bacteria in an oxygen-depleted environment to produce biogas and a stable solid.
Each of these products can be used for beneficial purposes to close the loop in organic waste management.
The biogas, which consists of up to 65% methane, can be combusted in a cogeneration unit and produce green energy.
The solid digest-state can be used as an organic soil amendment.
As a waste management strategy employed in over 20 countries, anaerobic digestion has been successful in reducing the volume of waste going to landfill, decreasing emissions of greenhouse gases and creating organic fertilizer, all at a profit.
Anaerobic digestion occurs naturally, in the absence of oxygen, as bacteria break down organic materials and produce biogas.
The process reduces the amount of material and produces biogas, which can be used as an energy source.
Anaerobic digestion is particularly suited for organic waste with high moisture content such as kitchen waste and food waste, although it can be used for various different types of waste streams.
Growing Interest in Using Anaerobic Digestion
Anaerobic digestion plants have been built and have been operational for many years for the treatment of mixed, municipal solid waste, for biowaste (obtained after source separated waste collection), for residual waste and for many types of industrial waste.
The technologies have been used successfully for over ten years in Europe where the industry continues to expand.
Anaerobic digestion is often the preferred biological waste treatment option in densely populated areas such as big cities or countries like Japan or Korea.
This is due to good odour production control and a reduced need for surface area.
The European market has shown a large preference for single-stage over two-stage digesters and a slight preference for dry digestion systems over wet systems.
However, the choice of AD technology depends on the composition of the waste stream, co-product markets, and other site-specific requirements.
The design of any new digester facility should be based on a thorough feasibility study, and special attention should be paid to all aspects of the treatment process, including waste collection and transportation, pre-treatment processing (i.e. pulping, grinding, and sieving), material handling, post-treatment processing (i.e. aeration and wastewater treatment), public education, and strategic sitting of the system.
Recently, anaerobic digestion has also become an important player in the area of renewable energy production out of energy crops (e.g. corn).
The net energy yield per hectare is higher compared to the production of bio-diesel or bio-ethanol.
Also in bio-refineries, anaerobic digestion could play an important role with high-value plant parts being used for green chemistry and residual vegetal matter (after processing or low-value plant parts) being treated in anaerobic digestion for the production of energy and compost.
Future for Anaerobic Digestion of Solid Waste
An analysis of the development of anaerobic digestion over the past 20 years shows that there is now a greater diversity in applications, a wide range of types of systems and suppliers, and an increasing degree of implementation in most parts of the world especially in Europe.
A study conducted in Europe in mid last decade shows that anaerobic digestion has become a well established and accepted treatment for the organic fraction of municipal solid waste.
It has become a good alternative to incineration or landfill disposal due to its lower environmental impacts.
 Mahony, T., V. O’Flaherty, et al. (2002). Feasibility Study for Centralised Anaerobic Digestion for the Treatment of Various Wastes and Wastewaters in Sensitive Catchment Areas. Ireland, Environmental Protection Agency.
 Henderson, P. and T. Chang (1997). Solid Waste Management in China.
Source : BERNAMA; Sakawi, Zaini, Municipal Solid Waste Management In Malaysia: Solution For Sustainable Waste Management, Journal of Applied Sciences in Environmental Sanitation, 6 (1): 29-38, (Mar. 2011); Tarmudi, Zamali, Abdullah. Mohd Lazim & Md Tap. Abu Osman, An Overview of Municipal Solid Wastes Generation in Malaysia, Jurnal Teknologi, 51(F) : 1–15, (Dis. 2009) Othman, Jamal, Economic Valuation Of Household Preference For Solid Waste Management In Malaysia: A Choice Modeling Approach, IJMS 14 (1), 189-212 (2007); Yahaya, Nadzri, Solid Waste Management In Malaysia -Policy Review, Issues and Strategies, (2007)
(this article written for 1BINA.my)