Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment


Landfill has been the principal method of waste disposal throughout most of the developed world for the last 100 years or more. More than 80% of the UK's waste is currently disposed of to landfill, but changes in legislation and resource management philosophy - in particular the EU Landfill Directive - are already beginning to change this. For example, the Directive requires a reduction in the amounts of biodegradable municipal waste (BMW) being disposed of to landfill in the UK to 75% of 1995 levels by 2010, 50% by 2013 and 35% by 2020. The Directive also requires that wastes are subjected to pre-treatment prior to landfilling, therefore a major role for landfill in the disposal of residual (i.e., post treatment or energy recovery) wastes will remain. Implementation of the EU Landfill Directive will have major implications for the nature of the waste that is disposed of to landfills throughout Europe, and hence for the way in which the receiving landfills should be managed. In particular, the Directive will require the large-scale treatment of municipal solid waste (MSW) to reduce the biological content. European experience suggests that treatment is likely to take one of two forms; either incineration, which has always been unpopular with the public in the UK, or a treatment comprising both mechanical and biological elements, together known as mechanical-biological treatment or MBT. MBT typically involves shredding or grinding of wastes prior to accelerated aerobic (often referred to as composting) or anaerobic degradation. Processing will leave a residue which, though much less biodegradable than MSW, will still produce gas (up to 20% of that from an untreated waste) and have the potential to pollute the environment.It is thought by some that there will be commercial or land improvement uses for the compost-like treated waste residues; however European experience and UK research has shown that it is likely that much of this waste residue will be suitable only for landfill for the foreseeable future. The residual wastes have very different properties to untreated wastes in terms of their mechanical behaviour (which impacts on the physical stability of landfills and hence their risk of landslides and pollution), the amount of gas and fluid which will be produced and the timescales over which gas and fluid production will occur. Unless the properties of residual wastes are understood and appropriate changes in working practice implemented, the consequences in terms of slope failure (landslide), increased risk of environmental pollution and potential loss of life could be severe. The aim of this research is to establish the mechanical and biological properties of waste residues so that design and management policies can be tailored to the waste's properties.
Description Consolidating anaerobic reactors:
• Saturated MBT waste consolidates within 24 hours and hence primary settlement of a saturated waste can be calculated using 1D consolidation theory.
• A timescale of 24 hours fits with conventional settlement versus vt (time) analysis and is also consistent with independently measured k (hydraulic conductivity) and E'0 (stiffness).
• Settlement due to creep and biodegradation can be dealt with using simplified models and laboratory tests can furnish parameters to estimate long term settlements in landfill.
• Irrespective of whether it is MBT waste or raw MSW, the relationships of biogas potential against cellulose plus hemicelluloses to lignin ratio; biodegradation induced settlement against gas produced and hydraulic conductivity against density are all similar and they all plot on the same line with pretreated waste being generally at one end of the scale.
• For each of the following relationships, the data for MBT and MSW can be plotted on a straight line:
• biogas potential against cellulose plus hemicellulose to lignin ratio;
• biodegradation induced settlement against gas production and
• hydraulic conductivity against density.

Shear testing:
• Shear strength appears to be displacement and not strain controlled
• Reinforcing (1D and 2D elements) increase the shear strength but only at much lower quantities than those found in MBT wastes
• Flexible and rigid plastics show a similar reinforcing behaviour when present in the same quantity by mass
• Reinforcing materials are predominantly 2D plastic particles in MBT

• The UK and German MBT show very similar percentages of each waste component, despite different treatment regimes;
• Similar quantities of inert (glass, stones, ceramics) and unidentified (<5mm & >5mm but unrecognizable) materials are found in IBA and in both MBT samples;
• Glass represents ~20-25% by mass of each material, suggesting that recycling has room for improvement
• It would be expected that degradation processes would increase the percentage of smaller particles and decrease the percentage of larger particles. PSD tests on UK MBT & shredded MSW before and after AD in the CARs showed the opposite. German MBT showed little change in PSD.

• Particle compressibility appears to account for ~20% of the total volume change in Rowe cell tests of MBT up to stresses of 320kPa
• Particle compressibility is caused by both increases in pore pressure and effective stress.

Original objectives:
The aim of the proposed research is to investigate the fundamental mechanisms and factors affecting the strength and compression behaviour of treated municipal solid wastes, with reference to residues produced to two markedly different standards, typical of German and UK practice. The specific objectives are
1. To investigate and quantify the shear strength characteristics of MSW that has been treated to an intermediate (likely UK) standard
2. To compare these with the shear strength characteristics of more highly sorted and processed (German standard) treated waste residues
3. To investigate the extent to which the principles of effective stress apply to wastes
4. To determine the short- and long-term settlement characteristics, including timescales and the associated gas production rates and leachate quality, of both highly processed (German) and intermediate (UK) treated waste residues
5. To determine the types and shapes of particles, constituent materials and other attributes responsible for the differences in each type of behaviour.
Exploitation Route By using the considerable insights in the published work to inform design and future research.
Sectors Environment