Background
Natural
degradation processes require a combination of different entities such as
substrate, microorganisms, nutrients and more. Transport of necessary agents may
be limiting for the degradation processes. For this reason mixing is important
in the design of anaerobic digester (AD) systems. For reactors working in the
high solids contents range (> 15 % TS) and biofills (landfill cells
designed for the treatment of degradable organic matter) mixing is difficult to
achieve. For AD, there are some elaborate systems available for mixing such as
the Valorga system, which uses pulses of compressed gas. In biofills, active disruption has not been practiced as an
enhancement technique. In laboratory scale, it has been observed that pulses of
pressure can help to liberate gas formed on surfaces and in smaller pores.
Physical disruption may also open new transport paths for substrates, products
and other materials.
Using landfill environments for treatment of organic materials, such as in biofills, lower reaction rates can be accepted, since the reactor volume is much cheaper than in AD systems. However, even in biofills, some situations can be conceived where a disruption of the waste may be a rational way to intensify decomposition and shorten the treatment time. Examples of such situations occur where the waste is strongly compacted (i e following degradation) or where low permeable wastes are treated (i e food wastes).
Another area where similar problems exist is in soil remediation, where
hydrofracturing has been used to get faster contaminant mobilization and removal
(NATO/CCMS 1995). This technique originated from oil abstraction practices where
transport limitation is common. Hydrofracturing may be useful in opening up older, compacted
sections of landfills that still contain biodegradable organic matter.
Steam injection has also been used in both the oil industry and for soil
remediation. It reduces viscosity,
volatilizes some contaminants and enhances reaction rates (Geraghty & Miller
1996). It is felt that
decomposition of waste may too be accelerated by the addition of steam.
For high TS AD or biofills, similar principles may be used although
current techniques will have to be adapted for these new applications. Other
measures such as the addition of degradation promoting factors such as nutrients
may be combined with the physical disruption, thus creating a whole battery of
technical procedures that may be used.
Theme
The organic content of municipal solid waste supports the generation of
energy rich biogas under anaerobic conditions, which can be used for many
purposes. The slowness of gas generation in landfills makes an efficient capture
and use of the gas more difficult.
The gas abstraction from landfills is often less than what the material
composition would lead one to expect, and the gas formation is much retarded in
comparison to treatment in more intense reactor designs (Bogner
& Lagerkvist, 1997). Even bioreactor landfills being designed for
rapid decomposition do not fully attain the digester concept. The fundamental
hypothesis of the physical disruption project is that the degradation in
landfills is transport limited and that this could potentially be influenced by
means of physical measures like for instance, blasting and steam injection. The
problem of transport limitation could be caused by several factors, such as
impermeable materials in the waste, and the existence of high permeability
channels through the waste, both causing an uneven distribution of water and the
properties and compounds carried by water.
The goal of the study is to investigate the potential of using physical
disruption measures to enhance the in situ degradation of landfilled wastes. As
model systems, physical landfill simulators in the laboratory or test cells in
the field are used