As discussed earlier, the future of co-disposal as a waste treatment option within Europe remains uncertain. Co-disposal is24 ‘the calculated and monitored treatment of industrial and commercial, liquid and solid wastes by interaction with biodegradable wastes in a controlled landfill site’. The philosophy behind co-disposal is that the microbiological and physico-chemical processes that occur during the degradation of organic wastes (such as those contained within municipal solid wastes) will treat the co-disposed wastes and reduce the associated hazards. Thus the landfill is regarded as a biological reactor where treatment, rather than storage, occurs.
The co-disposal of industrial and commercial wastes has been common practice in the UK for many years and is carried out in many other countries, although elsewhere it may not be called co-disposal. While the waste industry within the UK generally favours co-disposal as a means of waste treatment, it has been unable to provide sufficient evidence in support of the argument that co-disposal is safe, and thus it has proved difficult to dispute counter arguments that co-disposal represents unacceptable practice.
The removal of co-disposal as a waste treatment option would have significant ramifications for waste management across Europe. In the UK, for example, where approximately 70% of a total of 2.8M tonnes of difficult industrial wastes are co-disposed to landfill,25 alternative treatment would be required for
21 E. Senior, ‘Microbiology of Landfill Sites’, CRC Press, Boca Raton, Florida, 1990.
22 M. A. Barlaz, R. K. Ham, and D. M. Schaefer, Crit. Rev. Environ. Control, 1990, 6, 557.
23 K. Westlake, ‘Proceedings International Conference on Landfill Gas: Energy and Environment’, Bournemouth, 1990.
24 J. Skitt, ‘1000 Terms in Solid Waste Management’, ISWA, Denmark, 1992, p. 93.
25 Ends Rep., 1994, 228, 38.
Table 3 Typical landfill gas
composition (% vol.)1 Typical value Observed Reasons for component being Component (mature refuse) maximum unusually abundant
Methane 63.82 77.1 Adsorption of carbon dioxide
(e.g. by water, lime)
Carbon dioxide 33.62 89.3 Aerobic degradation of refuse
Oxygen 0.164 20.94 Air mixed with landfill gas
Nitrogen 2.42 80.3 Air mixed with landfill gas, or
very slow degradation if oxygen depleted
Hydrogen \0.05 21.1 Young refuse. Methane
concentration usually low
Carbon monoxide \0.001 —5 Oxygen-starved burning in refuse
0.0053 0.074 Young refuse or high
concentrations of petrochemicals present
0.0093 0.048 Young refuse or
petrochemicals/solvents present Halogenated 0.000023 0.032 Young refuse or solvents present compounds
Hydrogen 0.000023 0.0014 Young refuse
sulfide 356 High sulfate waste present
Organosulfur \0.000013 0.028 Young refuse
Alcohols \ 0.000013 0.127 Young or semi-aerobic refuse
(not included above)
0.000053 0.023 Solvents or other volatile wastes deposited
1Landfill gas usually emerges saturated with water vapour, representing 0.001% to 0.004%, depending on its temperature.
2Based on long term data from Stewartby landfill, supplied by London Brick Landfill Limited.
3Based on five year old refuse.
4Entirely derived from atmospheric oxygen.
5Concentrations of several per cent carbon monoxide has been reported at landfills on fire but have not been confirmed.
6Refuse mixed with plasterboard.
approximately 2M tonnes of such wastes.
According to Knox and Gronow,26 for effective co-disposal it is important that only those wastes that can be effectively treated are selected, and that
f only containment sites are utilized;
f the process is effectively managed and controlled;
f co-disposal occurs into biologically active waste at rates not exceeding recommended loading rates; and
f effective monitoring and after-care including effective monitoring of both gas and leachate are undertaken.
26 K. Knox and J. Gronow, Waste Manage. Res., 1990, 8, 255.
Table 4 Typical composition of leachates from domestic wastes at various stages of decomposition (all figures in mg l~1 except pH values)
Volatile acids (as C) 5688 5 12
NH3-N 790 370 1000
Source: Waste Management Paper 26A.41
Wastes that have been identified as being suitable for co-disposal include brewery wastes, animal and food industry wastes, aqueous organics, paint waste, acids, and alkalis. Wastes that are not suitable for co-disposal include flammable wastes, wastes containing PCBs and similar compounds, and acid tars.26
The ability of decomposing waste to attenuate added organic and inorganic material has been recognized for many years and a large number of articles have been published, including a major review of co-disposal practice in the UK.27 For effective co-disposal, the material to which co-disposed wastes are added must be (micro)biologically active, the most effective measure of microbial activity being the production of methane. Methanogenic waste indicates that the biological processes are relatively stable, and that pH is controlled around neutrality.
According to Knox and Gronow,26 methanogenic waste provides an aqueous chemical environment similar to anaerobic digesters with low redox potential (E)), near neutral pH, and which is maintained in a buffered steady-state by on-going degradation processes. Those wastes most extensively studied within co-disposal are phenols, cyanides, acids, and heavy metals, where effective degradation and attenuation has been shown. The above review gave the following conclusions on co-disposal.
(i) For phenols, cyanides, heavy metals, and acids, the degradation rates were similar to other types of anaerobic digester.
27 K. Knox, ‘A Review of technical aspects of co-disposal (PECD 7/10/214)’, Department of the Environment Report No. CWM 007/89, HMSO, 1989.
(ii) It is likely that the capabilities of methanogenic refuse will extend outside the chemical groups reviewed.
(iii) The efficiency of the waste was maximized by (a) saturated conditions with leachate recycle, (b) established methanogenesis, and
(c) elevated temperatures.
With regard to full-scale co-disposal sited in the UK, the review findings were as follows.
(i) The major examples of UK co-disposal sites do act as reactors—a wide range of difficult wastes were converted to a low hazard effluent.
(ii) Organic loading rates were in the range 1—10 g TOC m3 day~1.
(iii) Where calculated, heavy metal loadings were of a similar concentration to background levels.
The above studies appear to indicate that effective co-disposal of selected waste streams can be achieved. However, effective management and control is imperative, and while landfill sites retain licences to dispose of wastes not thought suitable for co-disposal, there is likely to be concern about the potential for harm to the environment caused by this practice.
While such concern may (in the case of bad management and control) be justified, alternatives such as mono-disposal or waste storage appear to be potentially more hazardous.