Overview of leachates from MBP residues

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.

In the longer term, the high organic content of the MSOR also appears to give rise to a much higher production of landfill gas, and increased release of contaminants, such as NH4-N.

There are, therefore, great potential benefits in terms of leachate strength and long-term pollutant emissions, to be gained by pre-treatment of these residual waste materials, and composting processes of varying intensity and duration have been widely used.

Reductions in gas emission potential of between 80 and 90% have often been reported from various trials, involving periods of composting from 16 to 30 weeks. Extensive studies are underway within a coordinated research programme in Germany (see Section 4.2), using an agreed protocol and standardised and calibrated 200-day lysimeter tests, to indicate how various pre-treatment and composting options can reduce this gas emission potential.

Nevertheless, although such trials widely and generally report that composting is able to remove an initial acetogenic decomposition phase when the material is finally landfilled, most studies and researchers acknowledge that over the longer term, leachates can be very similar to dilute methanogenic leachates derived from unpretreated MSW landfills.

The efficiency of composting processes is often assessed by 24-hour agitated batch leaching tests.

Increasingly there is evidence that such tests can seriously underestimate likely future emissions of contaminants because they take no account of biodegradation potential of the wastes.

There is no long-term evidence to show convincingly, at full-depth anaerobic landfills containing pretreated residual wastes (as opposed to shallower laboratory pilot-scale trials), that significant residual concentrations of COD, ammoniacal-N, and other contaminants may not persist for decades or centuries, as at normal MSW landfill sites. The German research programme continues to provide results which demonstrate the extremely long timescales over which high residual concentrations of TOC (>200 mg/l) and of ammoniacal-N (>100 mg/l) will persist in leachates, even from wastes that have been subjected to advanced and current composting processes.

In particular, specific processes which may remove or treat organic forms of nitrogen during pre- treatment of MSOR, and so significantly reduce the release of ammoniacal-N during subsequent landfilling, have not been identified or demonstrated adequately. Such work is key to demonstration of the long-term benefits of pre-treatment of MSW residual wastes. Until relatively recently, the objectives of composting MSOR fractions were primarily to reduce odour nuisance, and to minimise their emission potential in terms of landfill gas (if to be landilled). In many instances, where final composted materials were to be applied to land or used in horticulture, the presence of ammoniacal-N, or organic-N, brought considerable benefits. The fate of nitrogenous compounds was therefore not generally a key concern within the composting process.

During the last 3 or 4 years, concerns about transmissible livestock diseases such as Foot and Mouth Disease (FMD), and Bovine Spongiform Encephalopathy (BSE), has very much reduced the potential for composted MSOR outputs to be applied to land. The Animal By-products Amendment Order 1999 (as amended) prohibits the re-use of compost where livestock (including wild birds) may have access to the compost produced from some catering wastes containing meat or products of animal origin,

including household kitchen waste. The recent EU Animal By-products Regulation (EC No. 1774/2002) enforced in England since July 2003, allows composting to be used for catering waste and other low risk animal by-products, subject to national treatment standards being met (as set out in Defra, 2003).

Further work will be needed to assess the pollution potential from composts derived from approved composting facilities.

Where such composts are landfilled, it becomes more important that research should focus on the behaviour of nitrogenous compounds during the composting process, with the intention of maximising overall removal efficiency (e.g. by nitrification/denitrification processes), in order to minimise release of ammoniacal-N when the compost residues are landfilled.

It is nevertheless clear from data obtained, that significant removal of nitrogen can be achieved in some processes and in some full-scale composting plants. The extent to which total nitrogen removal occurs during the composting process itself, or whether composting may mineralise organic nitrogen

compounds to nitrates, which are subsequently reduced to nitrogen gas within an anoxic/anaerobic landfill, has not been established.

In practice, in Germany and recently in Austria, research is increasingly being focussed on minimisation of volumes of air used during the composting process. This is driven not by the objective of better or more cost-effective treatment, but instead by political pressures that demand exceptionally high standards of treatment for off-gases from composting processes, which must be enclosed.

Work to look at the effects of composting of MSOR on detailed composition of leachates produced has been reported. Previous studies (Robinson and Knox, 2001; 2003) have demonstrated that relatively few trace organic substances from the UK Pollution Inventory List (www.environment-agency.gov.uk/pi) are typically present in leachates from MSW landfills, and many of those found are effectively removed by biological leachate treatment processes.

Evidence from this study suggests that effective composting processes are able to reduce

concentrations of several trace organic substances present in leachates (see Chapter 5). For example, the herbicide mecoprop is generally present at significant levels in leachates from landfilled MSW, and was present at up to 120 µg/l in leachates from landfilled MSOR. However, it was absent, or present at much lower levels, in leachates from composted residues.

Of more potential concern is the presence of heavy metals in leachates from untreated MSOR fractions in landfills. Chromium is of particular concern at levels of up to 13.1 mg/l in samples tested, and

elevated levels of nickel and copper were also observed. It may well be the case that these concentrations are related to co-disposal with sewage sludges at the landfills sampled. The heavy metal concentrations were significantly lower, but still elevated, in leachates from landfills containing composted residues (where sewage sludge inputs were much more restricted), and at the VAM test cell containing crude MSOR.

In general terms, many landfills receiving MBP wastes will continue to pose risks to groundwater, and require aftercare periods similar to conventional MSW landfills that have become methanogenic. There is no doubt that MBP processes have potential to reduce both organic strength, and concentrations of ammoniacal-N in leachates from such landfills, as well as the total mass release of these and other contaminants. However, even at such landfills, the extent to which leachate management timescales will require management can be reduced significantly, remains to be determined.

One objective of this study was to try to investigate the effects that waste pre-treatment processes may have on the impacts of leachates on landfill liners, and on leachate drainage blankets. No conclusive data have been obtained. In practice, for leachates from landfilled MBP residues, whether treated or not, no researchers or landfill operators have raised this as an issue of concern. At some MSOR landfills, and other composted waste landfills, drainage systems are occasionally jetted, but at others they are not, and any differences between sites receiving untreated and treated waste fractions do not appear to be significant in operational terms.

3.5.1 Long-term aftercare liabilities

In the long-term the component most likely to influence leachate management is ammoniacal nitrogen, as it does for most leachates from methanogenic MSW landfill sites. It is possible that very efficient composting of MSOR materials may significantly reduce emissions of ammoniacal-N, in which case other components such as high concentrations of poorly-biodegradable COD, or inorganic components such as chloride (which is not significantly affected by the composting process), may also become important. It seems unlikely that the presence of trace organic compounds in leachates will comprise a significant long-term concern, regardless of whether wastes are composted or not. Nevertheless, mecoprop, for example, will remain ubiquitous in leachates from untreated MSOR for long periods of time (as it does at MSW landfills), whereas it has been shown to be readily degradable during efficient composting processes.

Flushing and dilution requirements for untreated MSOR are likely to be significantly greater than for untreated MSW, while the requirements for well-composted MSOR could be an order of magnitude less than for MSW.

3.5.2 Management of leachate collection systems

Discussion with European landfill operators and researchers has not indicated that there is any significant difference in maintenance requirements for leachate drainage and collection systems, between landfills that have received treated or untreated MSOR inputs, and those that have received untreated MSW. At some sites leachates drains are jetted occasionally, at others this does not prove necessary. It does not therefore appear that this will be a significant issue at such sites.

3.5.3 Treatment and disposal of leachates

The strong organic leachates from untreated MSOR and high concentrations of ammoniacal-N from some low intensity composted MSW are likely to be more expensive to treat than MSW leachate.

However, this is not likely to preclude treatment to very high standards. A particular issue may be the presence of relatively high COD values which resist normal biological treatment processes, may remain in effluents, and require additional physical and chemical treatment processes (such as activated carbon, ozonation, dissolved air flotation with flocculants) in order to remove them. These refractive COD compounds may arise either during composting, or as by-products of microbial metabolism during leachate treatment, as nitrifiers treat increased concentrations of ammoniacal nitrogen within a

biological leachate treatment plant (Carville et al., 2003). Elevated concentrations of some metals (e.g.

chromium), may have potential to inhibit biological leachate treatment processes, if they result from combined disposal of sewage sludges (see Section 6.4.1 above). However, successful on-site treatment of leachates has been observed during this study at a range of landfills, receiving both treated and untreated MSOR wastes.