Composting of MSW or MSW fractions

of composting MSW and MSOR residues in Austria and Germany is provided in Chapter 4 and summarised below.

The objectives of this study were to provide data and guidance for UK landfill operators and regulators on the impact that the landfilling of MSW fractions, or composted MBP wastes might have on leachate quality at their sites. This information is necessary to allow them to:

• define a leachate source term for groundwater risk assessments;

• assess the implications of waste pre-treatment on the timescales, and long-term liabilities, of the landfilling of pretreated waste materials;

• make appropriate and adequate provisions for leachate management, treatment and off-site disposal; and

• consider the effects of changes in leachate quality on landfill liners and leachate drainage blankets and systems.

At several sites where research has been undertaken into the disposal of MBP wastes, leachate quality data runs of 20 years or more have been obtained. Nevertheless, in spite of these long periods of data, leachate quality results from MBP mono-landfills are limited, because typically MBP wastes are

disposed of together with variable proportions of untreated MSW, MSOR or commercial and industrial wastes. The co-disposal of MBP waste and MSW or organic residues (MSOR) is likely to reflect the situation in the UK until significant infrastructure for MBP becomes available, at least locally. The quality of the MBP waste materials themselves will also vary as a consequence of:

• the extent of source-separation, for example of food and garden wastes;

• the waste inputs (e.g. urban or rural source, summer or winter collection);

• type of mechanical pre-treatment; and

• type and duration of biological treatment.

3.4.1 Leachates from MBP landfills

A number of studies have looked at the benefits of composting MSOR for reducing pollutant emissions.

Leikam and Stegmann (1999) studied the behaviour of composted MSOR wastes in landfill simulation tests, in comparison to untreated MSOR. For treated MSOR, the acetogenic phase during which strong organic leachate is produced was absent, and after about 250 test days the COD of the leachate was below 1,000 mg/l (BOD₅ <20 mg/l).

A much more significant benefit of pre-treatment becomes apparent when concentrations of total-N (primarily ammoniacal-N) are considered. Whereas the total-N content in leachate from untreated residual waste stabilised at about 1,000 mg/l, this value was below 200 mg/l for pre-treated wastes.

Nevertheless, in terms of L/S ratio, a trial period of 250 days corresponds to a period of about 50 years for a 20 m deep landfill, with an annual infiltration rate of 250 mm, or significantly longer if the site was capped.

Table 3.4 contains basic summary data, from published sources, for leachates from landfilled MSOR, and composted MSOR.

Table 3.4 Basic leachate quality for leachates from landfills/test cells containing untreated MSOR, and MSOR subjected to various composting regimes

Waste Inputs MSOR Composted MSOR, various sources Composting (weeks)

intensive 0 0 4 4 2 16 3

secondary 0 0 9 43 1 8 19

COD 172000 19400 2780 1170 540 4000 1900

BOD5 123000 9400 52 9 158 111 14

NH4-N 3965 4200 197 11 56 292 340

Chloride 9100 6500 11300 6900 5700 6200 4100

Chromium 0.41 1.3 0.14 0.04 0.03 0.21 0.09

Nickel 2.10 0.45 0.23 0.71 0.16 0.40 0.09

Copper 1.41 0.33 0.71 0.80 0.28 0.52 0.18

Zinc 102 0.56 3.4 1.0 0.22 1.6 1.2

Notes: all results in mg/l

table based on data from Danhamer and Jager (1999) and others

The results are typical of many other data, showing the very high strength leachates generated by untreated MSOR when landfilled. They also demonstrate the improvement in leachate quality achieved by various degrees of composting pre-treatment – widely observed to be capable of removing the initial strong organic leachates generated during the acetogenic stage of degradation, leading to a more rapid onset of methanogenic conditions.

Results for Kjeldahl nitrogen in leachates sampled during the present studies were inconsistent,

sometimes (especially at higher concentrations) being determined at values less than those obtained for ammoniacal-N. Any calculation of a figure for “organic-N”, as the difference between the two values was therefore unhelpful. Future research must seek to confirm the relative importance of

nitrification/denitrification, generation of stable, organically-bound forms of nitrogen, and other routes during MBP of waste fractions, in reducing long-term emissions of ammoniacal-N from landfilled wastes, in order that appropriate processes can be encouraged and optimised during waste pre-treatment.

Concentrations of ammoniacal-N and Kjeldahl-N in MBP leachates may be significantly lower than those from conventional MSW landfills, but the extent to which this occurs is variable, and not easily related to the design of specific composting and pre-treatment processes. The biochemical

transformations involved are not well understood, and the influence of nitrification/ denitrification, incorporation within stable organic fractions (possibly related to the elevated “hard COD” values), and other processes, require further research.

It is clear that the degree of composting achieved, and the efficiency of individual composting

processes, cannot be determined simply based on the duration of intensive and secondary composting being carried out at each location. For example, at one site only two weeks of intensive, and a further week of secondary composting are applied. This short-lived process achieves substantial

improvements in leachate quality. Key findings from these and other published studies on leachates from landfilled MBP wastes are summarised below.

• Organic residues from mechanical sorting (MSORs) can produce leachates with higher pollution potential than both acetogenic and methanogenic leachates from conventional landfills.

• Composting such residues can reduce the organic pollution potential from both leachate and landfill gas, through the avoidance of the peak acetogenic phase of decomposition.

• Concentrations of ammoniacal-N in MBP leachates can be either similar to, or much lower than, methanogenic leachates from conventional landfills. This raises the possibility that a nitrogen removal or attenuating process may operate, to varying extents, during composting.

• Landfills receiving MBP wastes will pose a risk to groundwater similar to conventional MSW landfills that have become methanogenic, and are therefore likely to require a similar period of time before active management and treatment of leachates ceases to be necessary.

Of particular importance in the future, when MBP processes are being designed to effect composting of residual wastes, will be considerations related to the fate of nitrogen within the composting process itself. In the past, when “green wastes” have been composted, or when there has been an assumption that composted products will be of value as fertilisers or soil improvers, the presence or removal of ammoniacal-N has not been a concern to composters and has generally provided a benefit to users.

Few research data have been published regarding the fate of nitrogen during composting of MSW fractions, and mechanisms involved are not clearly understood. However, the effects of composting processes on subsequent emissions of ammoniacal-N in leachates from the landfilled product are evident from other published data and from this study. The extent to which removal occurs during the composting process itself, or whether composting may mineralise nitrogen compounds to nitrates, which are subsequently reduced to nitrogen gas within an anoxic/anaerobic landfill, have not yet been well- established.

Leikam et al. (1997) investigated injection of air as a remedial technique for old landfills, and in pilot- scale trials demonstrated dramatic reductions in TKN concentrations, although no oxidised nitrogen was found in the leachate from the two aerated lysimeters. Work by Heiss-Ziegler and Lechner (1999) has demonstrated the stability of nitrogen-containing organic substances, such as humic acids formed during composting processes, which may also play a key role in minimising emissions of ammoniacal-N from MBP wastes in landfills.

Efficient MBP of MSW or the MSOR fraction can considerably reduce the organic strength of leachates, avoiding the acetogenic phase, and more rapidly producing leachates similar to those from MSW landfills in methanogenic phases of decomposition. Nevertheless, levels of “hard” COD in these leachates, not readily biodegradable by aerobic or anaerobic processes, are typically at least as strong as those found in methanogenic leachates – often in the range 1000 to 4000 mg/l, in spite of BOD values which are often less than 100 mg/l.

Few data are available on the presence of potentially harmful trace organic substances in leachates from MBP waste landfills, to allow these to be compared with data from conventional MSW sites. This study has obtained preliminary results, based on a programme of sampling at EU landfills (see Chapter 5). As at MSW sites, few trace organic compounds are present, but for those which have been

measured at significant concentrations (e.g. mecoprop), evidence from this study is that effective composting processes are able to reduce concentrations significantly.

In the case of mecoprop, this is generally present at significant levels in leachates from MSW, and was present at up to 120 µg/l in leachates from landfilled MSOR. However, it was absent or present at very much lower concentrations in leachates from composted residues. The extent of removal of mecoprop from leachate may well be a good measure of the efficiency of the composting process itself.