Re/Views in Environmental Science & Bio/Technology 2: 67–77, 2003.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Review

Anaerobic solubilisation of nitrogen from municipal solid waste (MSW)

J.P.Y. Jokela1,2∗ & J.A. Rintala1

1Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, FIN-40014 Universityof Jyväskylä, Finland; 2Current address: Metener Ltd., P.O. Box 368, FIN-40101 Jyväskylä, Finland; (∗author forcorrespondence: phone: +358 14 4451325; e-mail: jari.jokela@metener.fi)

Received 23 May 2002; accepted 24 January 2003

Key words: ammonia, anaerobic digestion, hydrolysis, landfill, leachate, municipal solid waste, nitrogen, wastemanagement

Abstract

This paper reviews anaerobic solubilisation of nitrogen municipal solid waste (MSW) and the effect of currentwaste management practises on nitrogen release. The production and use of synthetically fixed nitrogen fertiliserin food production has more than doubled the flow of excessive nitrogenous material into the community andhence into the waste disposal system. This imbalance in the global nitrogen cycle has led to uncontrolled nitrogenemissions into the atmosphere and water systems. The nitrogen content of MSW is up to 4.0% of total solids (TS)and the proteins in MSW have a lower rate of degradation than cellulose. The proteins are hydrolysed throughmultiple stages into amino acids that are further fermented into volatile fatty acids, carbon dioxides, hydrogengas, ammonium and reduced sulphur. Anaerobic digestion of MSW putrescibles could solubilise around 50% ofthe nitrogen. Thus, the anaerobic digestion of putrescibles may become an important method of increasing therate of nitrogen recycling back to the ecosystem. A large proportion of the nitrogen in MSW continues to endup in landfills; for example, in the EU countries around 2 million tonnes of nitrogen is disposed of annually thisway. Nitrogen concentration in the leachates of existing landfills are likely to remain at a high level for decades tocome. Under present waste management practices with a relatively low level of efficiency in the source segregationor mechanical sorting of putrescibles from grey waste and with a low level of control over landfill operatingprocedures, nitrogen solubilisation from landfilled waste will take at least a century.

1. Introduction

Since the beginning of the 1990s the source segrega-tion of putrescibles and mechanical processing ofMSW has become a common practice in severalEuropean countries. The aim is to diminish emissionsresulting from the landfilling of MSW, including therelease of methane as an end-product of the anaerobicdegradation of MSW as well as nitrogenous emis-sions e.g. in the form of ammonia, in landfill leachate.Thus, the stream of MSW has been segregated intoa putrescible fraction (or biowaste, kitchen waste),which is either treated by aerobic composting or bysolid-state anaerobic digestion. The residual fractioni.e. grey waste is usually mechanically processed and

the products are commonly landfilled or incinerated.Anaerobic digestion of the grey waste fraction has alsorecently been proposed (Mata-Alvarez et al. 2000). Inyear 1998, around 136 million tonnes of MSW wasstill being landfilled in the EU countries, accountingfor about 68% of the total produced. The landfilling ofMSW has thus continued to be the dominant method ofwaste disposal and, regardless of the source segrega-tion of putrescibles and other recyclables, is likely toremain so in the near future.

The continuous disposal of MSW in landfillshas caused emissions in the form of greenhousegases (GHG) and high nitrogen concentrations inlandfill leachate. New waste management legislation(e.g. EU Council Directive on the landfill of waste,

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1999/31/EC) has been enacted to promote sustain-able development and to mitigate among others nitro-genous emissions into the environment. This is espe-cially noteworthy as the release of soluble nitrogenfrom MSW into landfill leachate continues over a longperiod compared to that of soluble carbon compounds(Ehrig 1989). However, present knowledge aboutthe dynamics of landfill nitrogen release is woefullyinadequate.

Regardless of the importance of nitrogenous emis-sions, landfill studies have largely focused on thefactors that affect waste methanation (reviewed bye.g. Barlaz et al. 1990) and on the characteristics ofsoluble organic compounds in leachate (reviewed bye.g. Senior et al. 1990). Previously, Burton & Watson-Craik (1998) has reviewed ammonia and nitrogenfluxes in landfill, focusing on e.g. nitrogen transform-ations through the nitrification and the denitrificationstages of leachate recirculation. The characteristics ofnitrogen in landfill leachate and its removal have beenreviewed e.g. by Lema et al. (1988) and Kettunen(1997). Additionally, nitrous oxide emissions andanthropogenic nitrogen in wastewater and solid wastehave been reviewed by Barton and Atwater (2002).

This paper reviews the anaerobic solubilisation ofnitrogen as a source of pollution resulting from MSWmanagement. Special attention is paid to the solubil-isation of nitrogen both during anaerobic digestion andin existing MSW landfills, and to the effect of modernwaste management practices on the amount as well asextent of nitrogen release from landfilled waste. Theproduction of MSW in the community is surveyed aspart of the global biogeochemical nitrogen cycle. Inaddition, the main sources of nitrogen in MSW areidentified. Also, the fundamentals of the biologicaltransformation of nitrogen from MSW are described,and current experimental findings on nitrogen releasefrom MSW, including both anaerobic digestion ofMSW and laboratory and full-scale landfill experi-ments, are reviewed. Moreover, research on nitrogensolubilisation from the viewpoint of MSW manage-ment in the future is assessed.

2. Origin of nitrogen in MSW

2.1. Role of MSW management in the global nitrogencycle

Most of the total global nitrogen (more than 99.98%)is not accessible to living organisms and is bound

either as igneous rock (14 × 1015 metric tonnes),as atmospheric N2 gas (3.8 × 1015 metric tonnes),or as sedimentary rock (4.0 × 1015 metric tonnes)(Blackburn 1983). However, the tiny proportion ofglobal nitrogen (0.02%) that is biologically availableis an essential element of life and is ranked as themost important element after carbon, hydrogen andoxygen. The atmospheric N2 gas is the most stableform of nitrogen, and high amounts of energy areneeded to break the triple bond of N2. Therefore, onlya relatively small number of organisms are able toutilise N2 in the process of nitrogen fixation. Globally,only about 3 percent of the net primary production oforganic matter involves fixed N2 (Brock & Madigan1991). Thus, the nitrogen available for living organ-isms, and hence also for humans, is derived fromthe more easily available forms of nitrogen, namelyammonia and nitrate. Most of the nitrogen in the massof living organisms is bound up in amino acids inthe form of either structural proteins (e.g. keratine),soluble globular proteins, conjugated proteins, e.g.glycoproteins, or in the form of the organic nitrogenwhich can be incorporated as nucleic acids in DNAand RNA (Nelson & Cox 2000).

Previously, concerns have been expressed aboutthe effects of anthropogenic interference on the globalnitrogen cycle, inducing so called “global fertilisa-tion” (e.g. Vitousek & Matson 1993), and contrib-uting to global warming by increasing atmosphericN2O emissions and stratospheric ozone depletion. Onereason is that anthropogenic interference has doubledthe global rate of nitrogen fixation to about 243–295TgN per year from the 93–135 TgN that was fixedglobally per year in the pre-industrial era (Galloway1998). Presently, 40% of nitrogen is fixed by naturaland 60% by human-derived sources. This has obvi-ously drastically increased the amount of the moreeasily bioavailable forms of nitrogen (ammonia andnitrate), resulting in part from the increased productionand utilisation of nitrogenous fertiliser for food pro-duction (Galloway 1998). Consequently, most of thefood produced ends up as either solid waste or sludgeproduced by wastewater treatment. These streams areeventually treated and disposed of by waste manage-ment operators, and frequently disposed of in landfills(Figure 1). Most of the nitrogen in sewage is trans-formed into another compound and either releasedin effluent or contained in the sludge. Basically, thenitrogen contained in MSW is not normally removedby the other methods of treatment and thus waste man-agement as it is carried out at present, and especially

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Figure 1. Nitrogen flow from the environment to waste management.

modern landfills, harbours a substantial amount of thisexcess of fixed nitrogen.

The nitrogen flow from food production towaste management and its nitrogenous emissions hasbeen reviewed by Barton and Atwater (2002), whopresented a simplified conceptualisation of nitrousoxide emissions. Nitrogen efficiency (efficiency ofnitrogen in food products versus input as fertiliser oras human-induced biological nitrogen fixation) in theEU has been estimated to be between 20 and 30% withthe rest (70–80%) going into the soil or air during cul-tivation (Isermann & Isermann 1998). Therefore, mostof this nitrogen would enter waste management orwastewater treatment systems in the same proportion.The authors concluded that responsibility for dealingwith nitrogen rested with waste management oper-ators, as did dealing with the N2O resulting from wastemanagement activities. The authors estimated the N2Orelease to to 0.041 tonnes of CO2 equivalent per tonneof landfilled food waste (wet). On the other hand, theMSW disposed of in landfills comprises other wastestreams besides food waste, and the authors did notconsider these other waste components. As a point ofreference, municipal wastewater treatment plants can

produce 3.2 g of N2O person−1 a−1 (Czepiel et al.1995) and the incineration of MSW may yield up to293 gN2O per tonne of MSW (Watanabe et al. 1992).Additionally, it was noted that the nitrogen in MSWcontributes to future nitrogenous emissions. Estima-tion of the total release of nitrogen requires knowledgeabout the proportion of total nitrogen susceptible tohydrolysis and the rate of its subsequent ammoni-fication, as well as the transfer of the end-productof ammonia into leachate. Aside from the biologicalparameters, the rate of these stages is affected bythe physical and chemical circumstances prevailing inlandfills.

2.2. MSW as a nitrogen source

Regardless of the significance of nitrogen in land-fill emissions, the nitrogen content of MSW andits fractions (e.g. source-segregated putrescible frac-tion) has received markedly less attention in landfillstudies than has waste methanation. No limitationshave been imposed on the nitrogen content of theMSW derived from the mechanical-biological pre-treatments that are currently practised with the aim of

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diminishing methane emissions arising from the land-filling of MSW (Anon. 2001). On the other hand, inanaerobic digestion studies, nitrogen concentrationsare sometimes reported for the digestate, on accountof the possible inhibitory effects of ammonia and theeffect of concentrations of ammonia on end-productquality.

The total Kjeldahl Nitrogen (TKN) content ofvarious MSW from landfill and digestion studies inthe literature is presented in Table 1. TKN contentranges from 1.2 to 4.0% of TS. It seems that there areno significant differences between unsorted MSW andsorted putrescible and grey waste (the residual fractionafter source segregation of putrescibles) fractions ofMSW. Thus, one might conclude that the segregationof putrescibles does not reduce the nitrogen contentof MSW. On the other hand, it must be noted that theTKN content does not express the extent or the rate ofthe biological solubilisation of nitrogen in the waste.

On the basis of the TKN concentrations presented(Table 1) and the amount of the MSW annually land-filled in the EU countries (about 136 million tonnes ofMSW), around 2 million tonnes of nitrogen is annu-ally disposed of in landfills calculated with a TKNof 2.5% titak sikuds (TS) and TS of 50%. In Finlandthe respective amount is around 0.02 million tonnes ofnitrogen.

3. Some fundamentals of anaerobic solubilisationof nitrogen from proteins

Anaerobic degradation of MSW follows the well-known pattern of complex substrates, as described e.g.by Zehnder et al. (1982). A modified representationof the degradation pathways, including the proteindegradation pathway, is described in Figure 2. Diversegroups of microorganisms are involved in the degrad-ation process and some of the steps are omitted fromthis simplified scheme. To provide an overview ofthe fundamentals of the biological solubilisation ofnitrogen, a more detailed description of this complexprocess is given in this section.

3.1. Hydrolysis and fermentation of proteins

Generally, unsorted MSW has a high concentration oforganic carbon due to its high cellulose content (10–40% of (TS); Ham et al. 1993), whereas the nitrogenconcentration is relatively low (between 1.0 and 4.0%of TS). Proteins are commonly found in MSW (e.g.

4.2% of TS, reviewed by Barlaz et al. 1990) and thuscan be considered a major source of soluble nitrogenin MSW.

Proteins consist of polymers of amino acids joinedcovalently to form peptide bonds. When thousandsof amino acids are linked by peptide bonds, theyare referred as either polypeptides (molecular weightsbelow 10 000) or as proteins that comprise up to four(quaternary) or more levels of polypeptide structures(Nelson & Cox 2000). During anaerobic degrada-tion, proteins are first hydrolysed by extracellular pro-teinases into proteoses and peptones, which are thenpassed into the bacterial cell where the peptones arebroken down by peptidases, which cut the amino acidsoff from the peptide chains. The amino acids are fur-ther fermented into volatile fatty acids, carbon diox-ides, hydrogen gas, ammonium and hydrogen sulphide(Lackey & Hendrickson 1952). A specialised proteo-lytic group of anaerobic bacteria, e.g. clostridia, car-ries out the protein degradation in anaerobic digestersand other anaerobic environments. The hydrolysedamino acids are degraded either by anaerobic oxida-tion linked to hydrogen production or through Stick-land fermentation (reviewed by e.g. McInerney 1988).Ammonium-ions as the end-product of protein hydro-lysis can be assimilated by microorganisms into aminoacids and then into other nitrogen-containing bio-molecules (Nelson & Cox 2000). However, it isgenerally known from the studies of anaerobic diges-tion as well as from the analysis of landfill leachatesamples that most of the organic nitrogen releasedduring anaerobic degradation is converted irreversiblyinto ammonium-ions or as ammonia.

3.2. Limitation on rate of protein anaerobicdegradation in MSW

The rate of protein degradation is partly determinedby the solubility of the proteins in question, but thenumber of disulphide bridges as well as the tertiarystructure are also important factors in the rate ofprotein degradation (reviewed by McInerney 1988).A low pH (below 5.5) may impede the growth ofprotein-degrading bacteria (Erfle et al. 1982). Theprotease may be inhibited by the resulting aminoacids as well as by glucose (Glenn 1976). Althoughthe anaerobic hydrolysis of protein-matter containingproteins may be slower when compared to mattercontaining carbohydrate (e.g. in cellulose-containingwaste), the first-order hydrolysis rate constant hasbeen found to range from 0.04 to 1.3 d−1 in meso-

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Table 1. Nitrogen contents of MSW from landfill and digestion studies

Waste TKN (% of TS) Reference

Landfilled MSW samples 1.2–3.8 Landfill (Ham et al. 1993)

Unsorted MSW 3.3 Landfill lysimeter (Pohland 1980)

Unsorted MSW 4.0 Laboratory landfill lysimeter (Leuschner 1989)

Putrescible fraction MSW 3.2 Anaerobic digestion (Cecchi et al. 1992)

Putrescible fraction MSW 1.2–2.9* Anaerobic digestion (Gallert & Winter 1997)

Grey waste fraction MSW 1.2 Laboratory landfill lysimeter (Jokela et al. 2001)

*Calculated by the authors.

Figure 2. Protein anaerobic degradation pathway in MSW (modified from Koster 1989; Lackey & Hendrickson 1952; Pavlostathis &Giraldo-Gomez 1992; Zehnder 1982).

philic batch and semi-continuous digestion studieswith various proteinaceous substrates (reviewed byPavlostathis & Giraldo-Gomez 1991). However, thehydrolysis of protein-containing organic matter as thepreceding stage is limiting the rate of degradationsignificantly. Previously in landfill MSW samples,proteolytic degraders have been identified in coloniesranging between 2–15% and 0–4% of the total fer-mentative bacteria when incubated at 22 and 37 ◦C,

respectively (Suflita et al. 1992). Thus, regardlessof potentially limiting circumstances (e.g. moisture),proteolytic hydrolysis and fermentation clearly takeplace in landfills.

The rate of MSW degradation has frequently beendescribed as following first-order kinetics, and thehydrolysis rate constant has been estimated for variouswaste components (reviewed by e.g. Mata-Alvarez etal. 2000). Generally, a high rate constant has been

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reported for putrescibles (e.g. 0.4 d−1 by Vavilin et al.1999), whereas for cellulose-containing waste compo-nents, such as newsprint and packaging, a significantlylower rate (between 0.058 and 0.12 d−1) constant hasbeen reported (Owens & Chynoweth 1993). Similarly,a somewhat higher rate constant has been reported forcarbohydrates (between 0.025 and 0.200) than for pro-teins (between 0.015 and 0.075) (Christ et al. 2000).The putrescible fraction of MSW would degrade at ahigh rate and rapidly solubilise the organic nitrogen asammonia, if landfilled. On the other hand, hydrolysisconstants obtained from the literature cannot be extra-polated, because of the wide range in waste particlesize (Sanders 2001).

4. Solubilisation of nitrogen during anaerobicdegradation of MSW

In this section, the results on nitrogen solubilisationobtained from anaerobic digestion studies and land-fill experiments with MSW are summarised to givean approximation of the rate and extent of nitrogenrelease from MSW as a result of anaerobic degrada-tion.

4.1. Solubilisation of nitrogen in anaerobic digestionof MSW

In recent years, anaerobic digestion has been widelyapplied, especially in Europe. Owing to their highnutrient content and methane production potentialas well as ready availability, putrescibles are com-monly co-digested with manure; for example, thereare around 1700 farm biogas installations in Ger-many and some of these are utilising putrescibles(reviewed by e.g. Weiland 2002). In addition, putres-cibles are nowadays increasingly treated separately; inEurope, for example, there are more than 53 large-scale (capacity above 10 000 tonnes per year) plantstreating MSW (DeBaere 2000).

4.1.1. The amount of readily soluble nitrogen in MSWSoluble nitrogen compounds are mostly producedduring the anaerobic digestion of putrescibles fromproteins, which account for between 12 and 15%of volatile solids (VS) in MSW (ten Brummeler1993), whereas the biomass of microorganisms (VS)in a digester may contain up to 50% VS proteins.According to Carlson (1996), protein content invarious fractions of putrescibles can vary from 4 to

50% of the sum of carbohydrates, proteins, lipids andraw fibres that would presumably represent most ofthe VS. Normally, a high amount of readily solublenitrogen is present in putrescibles. For example, Gal-lert and Winter (1997) reported 1.6 gNH+

4 -N kgVS−1

(calculated by authors) for manually sorted putres-cibles after filtering, and Held et al. (2002) reportedfor the liquid fraction of source-segregated putres-cibles fractionated by mashing separation a readilysoluble ammonium content of 2.4 gNH+

4 -N kgVS−1

(calculated by authors). In contrast, Jokela et al.(2002) reported production of 4.6 gNH+

4 -N kgVS−1

from source-segregated putrescibles during an elutiontest. However, some further hydrolysis besides thesolubilisation of easily degrading nitrogenous wastecomponents may have taken place during the 24 helution test, which was carried out at room temper-ature. On the other hand, the method of filtration usedto determine soluble NH+

4 -N in the study by Gal-lert and Winter (1997) might not have released allthe readily soluble NH4-N from the waste particles.Thus, the amount of readily soluble nitrogen presentin putrescibles can be expected to vary between 2and 4 gNH+

4 -N kgVS−1. The TKN value is oftenreported for MSW, and hence the NH4-N/TKN ratiomay be a more convenient ratio with which to eval-uate the amount of readily dissolved nitrogen. Gallertand Winter (1997) reported a NH+

4 -N/TKN ratio of0.11, whereas Held et al. (2002) reported 0.084. Thus,almost 10% of the total nitrogen of putrescibles isreadily solubilised.

4.1.2. Nitrogen solubilisation in anaerobic digestionThe concentrations of NH+

4 -N found in various anaer-obic digestion studies are summarised in Table 2.The concentration of NH+

4 -N seems to range between0.8 and 3.5 g l−1. This variation may be caused bythe inherent nitrogen content of fairly heterogeneousMSW and will also depend on the mode of operationof the reactor. The extent of solubilisation may beaffected by the characteristics of the substrate, but e.g.Gallert and Winter (1997) reported that up to 50% ofthe TKN was converted to NH+

4 -N, whereas Salminen(2001) reported from 50 to 70% ammonification after70 days batch digestion of poultry slaughterhousesolid waste. On the basis of the former digestion studywith putrescibles, Gallert et al. (1998) reported that forpeptone concentrations from 5 g l−1 to 20 g l−1 themesophilic population revealed a higher rate of deam-ination than the thermophilic population, i.e. 552 mgl−1 day−1 compared to 320 mg l−1 day−1 at 10 g

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Table 2. NH+4 -N concentration in anaerobic digestion of putrescibles

Process Retention Organic loading rate NH+4 -N (g l−1) References

time (day) (gVS l d−1)

Full scale CSTR n.r. 5.2 2.4–3.5 Vermeulen et al. 1993

Laboratory CSTR 11.2–12.5 6.0–12.1 0.8–2.8 Pavan et al. 2000

Laboratory CSTR 11.7 9.7 0.81 Mata-Alvarez et al. 1993

Laboratory CSTR 19 9.65 1.4 Gallert & Winter 1997

n.r.: Not reported, CSTR continuously stirred tank reactor.

l−1 peptone. A 10-times higher peptone degradationwas found in the mesophilic digester when comparedto the thermophilic one. Thus, mesophilic condi-tions may be more favourable for the solubilisationof organic nitrogen. The reason for this may be thelimited diversity of peptone-degraders in thermophilicconditions. On the other hand, the final degree ofdeamination was higher in the thermophilic condi-tions: 102 compared to 87 mgNH+

4 -N/g peptone inthe mesophilic culture. Also, it was concluded that thethermophilic culture is more tolerant of high ammoniaconditions.

The nitrogen content increased from 1.0 to 7.2%of TS in a thermophilic laboratory digester duringa 169-day study with putrescibles. This indicatesthe accumulation of soluble nitrogen during the run(Vermeulen et al. 1993), whereas in an elution studybefore and after a 51-day aeration test with putres-cibles and grey waste, the amount of soluble NH+

4 -Nin the putrescibles increased from 3.6 to 7.9 gN kg-TS−1 and decreased in the grey waste from 2.11 tobelow 0.01 gN kg-TS−1 (Jokela et al. 2002). The highsolubilisation potential of nitrogen would thus seemto originate in the easily degradable putrescible frac-tion of MSW, even though its TKN content does notdiffer from that of unsorted MSW. The degradation oforganic matter would be expected to release the hydro-lysed and ammonified organic nitrogen of the wastematerials. Örlygsson et al. (1994) studied thermophilicprotein (peptone) degradation in steady-state labora-tory reactors and found that although peptone waseasily hydrolysed and all amino acids were completelydegraded into volatile fatty acids (VFA), CO2 andNH+

4 -N, the oxidative deamination was slow. Hence,most of the amino acids in the proteins of wastematerials are potentially degradable, whereas someproteinaceous materials containing branched-chain oraromatic amino acids may be deaminated oxidativelyat a significantly lower rate. Generally, in the case

of the other anaerobic digestion studies on MSW, itis impossible to assess the extent of nitrogen solu-bilisation without knowing e.g. the characteristics ofthe inoculum used or the amount of water added.One approach could be to estimate nitrogen solubil-isation from the reduction of solids (TS or VS). Pavanet al. (2000) reported VS removal between 37 and82%, whereas Mata-Alvarez et al. (1992) reported VSremoval ranging from 37 to 41% with putrescibles.The problem with this approach is that the high contentof soluble COD may inhibit protein hydrolysis (Glenn1976). Evidently, the longer hydraulic retention time(HRT) by Gallert and Winter (1997) increased theextent of nitrogen solubilisation. They also reported33% nitrogen solubilisation with identical operatingparameters in a mesophilic process (37 ◦C). Thus, theextent of nitrogen solubilisation may range between30 and 50% of TKN with a HRT from 1 to 3 weeks.HRT extension by a couple of weeks post digestionmay increase the rate of solubilisation to above 50% ofTKN. Hence, the anaerobic digestion of putresciblesmay increase the availability of nitrogen for plants andthus become as an important method of increasing therate of nitrogen recycling back to the ecosystem.

4.2. Solubilisation of nitrogen from MSW in landfills

Landfilled MSW contains high amounts of organicnitrogen in the undegraded waste components as wellas readily soluble nitrogen in the form of NH+

4 -N. Oneof the objectives of waste segregation and pretreat-ment is to minimise the high and long-term emissionsof nitrogen contaminants that may be produced bytraditional landfill practices (Andreottola & Cannas1992). This nitrogen release into the water system maycause eutrophication and N2O emissions (reviewedby Barton & Atwater 2002), thereby entailing long-term after-care of the landfill with leachate treatment.Although the biological degradation pathways occur-

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ring in landfilled MSW are basically similar to thosein the anaerobic digestion of MSW, e.g. the leach-bed dry fermentation process (studied by e.g. Chughet al. 1998), the prevailing conditions (e.g. spatialdistribution of moisture) are far less controlled, asthe landfilling of MSW is generally considered as amethod of end-disposal rather than an active method ofbiological stabilisation. Therefore, there is a need forways of effecting anaerobic solubilisation of nitrogencontained in MSW. This would reduce the amountof time needed for the post-operation of the land-fill. Another approach would be the containment ofnitrogen by so called “dry tombing” in which thelandfill is completely sealed off by impermeable struc-tures. This strategy, however, might nonetheless entailextended after-care, particularly with respect to therisk of leakage.

Lema et al. (1988) have reviewed leachate nitrogenconcentrations in young, medium age and old land-fills. In young landfills, the TKN concentration rangedfrom 0.85 to 1.9 gN l−1, whereas NH3-N rangedfrom 0.5 to 1.65 gN l−1. In medium-aged (time wasnot defined by the author) landfills a TKN concen-tration of 0.33 gN l−1 was observed and the NH3-Nranged from 0.03 to 3.0 gN l−1. In the old landfills,which presumably were several decades old, a TKNconcentration as high as 0.59 gN l−1 was recordedand NH3-N ranged from 0.01 and 2.1 gN l−1. Thus,the slow leaching of nitrogen out of landfilled MSWseems to continue over many decades. This makes theassessment of the rate of nitrogen solubilisation on thebasis of the development of the nitrogen concentrationin the leachate difficult. In addition, the leachate con-centrations of samples collected from the drainage ofa landfill site are susceptible to e.g. dilution by sur-face run-off water. Therefore, assessment of the timeneeded for nitrogen solubilisation, which is based onleachate nitrogen concentration, may be suspect.

One way to assess the degradation of waste andthus the solubilisation of nitrogen may be on the basisof lysimeter experiments carried out in controlled con-ditions. The parameters studied usually include e.g.particle size and distribution of waste particles, wastedensity, temperature, precipitation, waste moistureand pH. In addition, controlled operation proceduresare carried out to simulate full-scale ones. These usu-ally include e.g. leachate recirculation and inoculumaddition. Various experiments have been done onanaerobic degradation of MSW both in the laboratoryas well as on the full-scale (reviewed by e.g. Reinhard& Townsend 1998). Some of these lysimeter studies

are listed with the ranges of nitrogen production inTable 3. These studies have normally been carriedout at room temperature (approx. 20 ◦C), except forPohland (1980) who had a cell temperature between10 and 40 ◦C, which to some extent is comparablewith landfill conditions. At the start of a lysimeterstudy, high NH+

4 -N concentrations (e.g. between 500and 1500 mgN l−1) are usually found in the leachate,and thus the readily soluble NH+

4 -N is partly leachedout of the waste mass. The organic matter in theleachate may be reduced by the increase in methaneproduction from the waste, whereas the NH+

4 -N con-centration in the leachate will usually persist at a highlevel for a longer time. In such lysimeter tests, ithas taken, depending on the amount of water passed,between 120 (Munasinghe 1997) and 500 days (Jokelaet al., submitted) to reach a threshold concentration of100 mgNH+

4 -N l−1, compared to at least 400 days toreach a level of 20 mgNH+

4 -N l−1.The amount of liquid passed through the waste

mass is sometimes used to estimate the time that wouldbe needed to reach the threshold concentration level onthe full landfill scale (e.g. Ehrig 1989). In order to beable to scale up the results from a laboratory lysimetersimulation, one has to be aware of the water balancein landfills. Thus, the amount of water passing throughthe unit mass of waste e.g. per year, can be estimated.Heyer and Stegmann (1997) estimated that withoutimpermeable top sealing and with a constant climaticleachate generation of 250 mm a−1, 0.75 Mg-TS m−3

dry density of waste (in laboratory and landfill) andlandfilling height of 20 m it would take approx. 100times longer to achieve, a similar uniform water per-colation rate in the landfill when compared to thelaboratory simulation lysimeter. Thus, it would takebetween 120 to 450 years to reach the threshold con-centration of 70 mgTKN l−1. Similarly, Jokela et al.(submitted) estimated that on the basis of the knownlevels of water infiltration at the local (Finland) pre-vailing conditions on landfills with a waste density of500 kgTS m−3 and an average annual precipitation of650 mm, it would require the equivalent of 7.5 yearsto obtain a liquid/solid (L/S) ratio of 1. Hence, with anL/S ratio of 10, it would take at least 75 years to reach(Table 3) the threshold concentration of 20 mgNH4-N l−1. The nitrogen solubilisation rate is, however,slower where the MSW higher particle size is higherand where the methanation of organic matter in theMSW is slower. Nevertheless, under the above Finnishlandfill conditions the period needed for nitrogen tosolubilise out of landfilled MSW would be at least 100

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Table 3. Anaerobic degradation of MSW in laboratory and on-site lysimeter and full-scale studies

Study Run time Operation Amount of NH4-N (mg l−1) in leachate References

(days) leachate passed Start (max.) End (min.)

Laboratory lysimeter, 290 Leachate 3 l weekly: L/S of 650 100 Ehrig 1989

50 kg – MSW recirculation approx. 5.0*

(fresh+composted=2+1)

Laboratory lysimeter, 410 Single pass HRT of 200 days: 460 15 Munasinghe 1997

52.6 kg – MSW+digester L/S of approx. 9.8*

sludge

On-site lysimeter 700 Precipitation+ L/S of approx. 30* 645 9 Pohland 1980

7900 kg – MSW recirculation

Laboratory lysimeter, 535 Precipitation+ L/S of 8.8 190 23 Jokela et al., submitted

7.5 kg – 10 years old recirculation

landfilled MSW

Laboratory lysimeter, 560 Precipitation+ L/S of 8.6 1300 72 Jokela et al., submitted

4.6 kg – grey waste recirculation

*Calculated by authors.

years. Thus, in practice we are passing our nitrogenload down at least three generations. If we considerthat, for example in Finland between 1960 and 2000approx. 60 million tonnes of MSW containing around0.75 million tonnes of nitrogen was landfilled, thenit will be 120 years before that amount of nitrogenwill be leached out, the highest concentrations prob-ably being produced during the first few decades. Thismust primarily be avoided by effective separation ofputrescible fraction.

5. Need for future research

Firstly, the need for the excessive production ofnitrogen fertiliser, one of the main causes of globalfertilisation, could be reduced by tracing and iden-tifying the sources of nitrogen, which could then besafely reused in agriculture or recycled after treatment.This is of especial importance in view of the majorrole played by MSW management in the recycling ofnitrogen, given that most food products end up eitheras putrescibles or as sludge, which the waste man-agement operator then has to take care of. Secondly,the low rate of leaching of excess nitrogen is one ofthe biggest unsolved problems in landfills, and thusresearch is needed to determine the factors affectingthe solubilisation of organic nitrogen in MSW in rela-tion to landfill operating procedures. Thirdly, the main

sources of nitrogen should be removed from the land-filled MSW stream by pre-treatment and restrictionsimposed on the nitrogen concentration of waste dis-posed of in landfills. Finally, more research shouldfocus on the effects of waste treatment methods, e.g.anaerobic digestion, on the solubilisation of nitrogenand on the agricultural utilisation of the digestate asfertilizer.

6. Conclusions

The excess production of nitrogen fertiliser isincreasing at the same rate as e.g. the consumptionof food. This has significantly increased the flowof nitrogenous material in the community and hencethe amount of nitrogen entering the waste disposalsystem. The continuous disposal of MSW in land-fills is causing emissions in the form of nitrous oxidewhich contributes to global climate change and to highnitrogen emissions to water systems in the form oflandfill leachate. These issues have raised concernsabout the effects of anthropogenic interference on theglobal nitrogen cycle. MSW contains up to 4.0% ofTS of nitrogen, which has a significantly lower rateof degradation than carbohydrate-containingcellulose.Presently, part of the nitrogen is segregated out of thelandfilled MSW by the source-segregation of putres-cibles or by mechanical sorting. Anaerobic digestion

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of the putrescible fraction could solubilise around50% of the nitrogen. Thus, the anaerobic digestionof putrescibles could be used to increase the rate ofnitrogen recycling back to the ecosystem. However,the main proportion of the nitrogen in MSW will con-tinue to end up in landfills; for example, in the EUcountries around 2 million tonnes of nitrogen con-tinues to be disposed of annually in landfills. However,more intensive mechanical-biological pre-treatmentof MSW will reduce to some extent the amount ofnitrogen disposed of in landfills. Nitrogen concentra-tions in existing landfills as well as in the resultantleachate may remain at a high level for decades.Owing to laboratory research, the duration of nitrogensolubilisation can be estimated as well as the factorsaffecting solubilisation. Under present waste manage-ment practices, including the relatively low efficiencyin the source segregation or mechanical sorting ofputrescibles (below 50%) and low level of controlover landfill operating procedures, nitrogen will be ourlegacy for at least the next three generations.

Acknowledgements

This study was sponsored by the Finnish GraduateSchool in Environmental Science and Technology(EnSTe).

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