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Samlerapport for projekter om bioforgasning af organisk dagrenovation gennemført 2000-2002

Summary and conclusions

This report summarises a number of technical examination regarding collection, pre-treatment and digestion of source sorted household waste. The examinations are made during the recent years. The main part of the examinations has been financed by the Danish Environmental Protection Agency but also other authorities, organisations and companies have supported the examinations.

The activities have been concentrated on collection, pre-treatment and characterisation of the source sorted household waste in order to estimate the gas potential. Only few activities has been made on the digestion itself but some earlier examinations within this area is summarised in a status report. One examination deals with limitations in the use of residues from digestion of source sorted household waste caused by content of heavy metals and organic hazardous substances regulated in the Danish legislation. This report does not include evaluation according to the coming EC animal by-products regulation that will be implemented spring 2003.

An environmental assessment of the different systems for handling and treatment of organic solid waste is performed as a supplement to the technical examinations.

An examination of the possible sale of residues from digestion that fulfils the present guidelines for agricultural use has been performed in addition to technical examinations. Further a system analyses and an economical analysis of recycling of household waste have been performed. The results from these examinations are not included in the summary but the results can be found in the reports Aspects of sale og digested organic waste (in Danish), Systems Analysis of Organic Household Waste Management in Denmark and Economical analysis of increased recycling of organic household waste (in Danish). These reports will be published by the Danish Environmental Protection Agency.

The summary doesn't contain evaluation of the economy of digestion of household waste or the organisational aspect related to collection, pre-treatment and digestion.

The basis for the status for digestion of source sorted household waste

Collection, pre-treatment and the composition of source sorted household waste have been evaluated in connection with full and large-scale examinations. Several methods for source sorting, collection and pre-treatment have been tested in Grindsted, Copenhagen, Kolding, Vejle and Aalborg. The results are summarised in the report Connection between sorting, pre-treatment and quality of biomass (in Danish), and the attached data rapport Data report on composition and biogas potential of organic household waste (in Danish). The results from the different cities are further described in details in the three reports Full-scale experiments in Greater Copenhagen (in Danish), Collection of organic waste from households, small commercial kitchens and food stores in the municipality of Aalborg (in Danish) and Full-scale experiments in Kolding (in Danish).

The composition of household waste including the organic fractions has been mapped in an examination of 10 selected residential areas spread over Denmark. The results are presented in the report Evaluation or systems for home composting and analyses of the composition of household waste (in Danish).

The pre-treatment facilities included in the full and large-scale examinations have been described and evaluated as a part of the examinations. Further test results from a pressure separator for source sorted household waste are described in the report Pre-treatment of organic household waste by hydraulic pressure separation. These description covers the main types of systems for pre-treatment of source sorted household waste in Denmark. Only an old rotating drum for composting now used for pre-treatment of source sorted household waste for digestion and systems for optical sorting of source sorted household waste used in Vejle and Århus are not included. In Vejle the waste is composted and the system i Århus has just started at the end of the present project.

Full-scale digestion of source sorted household waste is today only performed at a few places with co-digestion with manure or wastewater sludge (Grindsted). No examination of full-scale digestion of source sorted household waste has been performed in recent years, but earlier results have been summarised and evaluated in an appendix to the report Documentation for biogas potential of organic household waste (in Danish).

A great number of samples of pre-treated source sorted household waste together with the reject from the pre-treatment have been collected in connection with the five full and large scale examinations described above. This sampling program has enabled a detailed characterisation of the composition of source sorted household waste before and after pre-treatment. The biomass and the reject has been characterised with respect to dry matter, volatile dry matter and water, further the component of the organic matter (fat, protein, fiber and carbohydrates) and the elements (C, H, O, N) has been found. Finally methane production has been found in digestion experiments in laboratory and pilot scale. Consequently the biogas potential can be found from the components in the waste, the elements and from direct measurements. The results are presented in the report Documentation for biogas potential of organic household waste (in Danish) and in the data report including all individual measurements Data rapport on composition and biogas potential of organic household waste (in Danish).

Earlier examination of compliance with regulation of use of residues from digestion in agriculture has shown that non-compliance is sometime seen. Especially content of plasticisers (DEHP) exceed the limit. Source sorted household waste is examined at 6 plants and the results are presented in the report DEHP in household waste (in Danish).

A summary of the reports presented above is given in order to present a short, coherent evaluation of the technical problems related to digestion of source sorted household waste i n Denmark. In the following chapters each report or group of report is presented and the results are summarised.

Collection of source sorted household waste

Existing systems for collection of source sorted household waste and large- scale experiments shows that 60 kg to 90 kg source sorted household waste can be collected per person and year as a mean for greater areas. For smaller residential areas great variation exists, but no direct reason for the variation can be found. Neither housing type nor system for collection has a significant impact.

The collection efficiency is about 80% of the potential found from detailed sorting of unsorted household waste. Such detailed sorting shows that great variation exists in mass and composition from different areas. The organic fractions however deviate only slightly between areas with apartment blocks and with single-family houses. Differences were mainly related to difference in household size. Much greater variation was found between different geographical areas than between the two types of housing areas.

Much greater differences are found between different geographical areas with source sorting than can be explained by the sorting system or the housing type. A factor of two between the collected waste per inhabitant is found from two municipalities with the same collection system.

Separate collection in smaller areas however shows that collection from apartment blocks results in lesser waste than from single-family houses. The difference can't be explained by the different household size. Thus showing that the potential for collection of organic waste is better utilised here.

It was not possible to relate the collection efficiency to the sorting instruction or the information activities in connection with the introduction of the sorting. Most waste per household were found in an area with single-family houses in Aalborg were participation in the collection was optional.

Quality of collected source sorted household waste

A broad spectrum of systems for collection of source sorted household waste has been tested in full and large-scale examinations. In the kitchen paper and plastic bags have been tested and collection in paper sacs and containers have been used. The sorting instructions have been rather similar but differences in how detailed the fractions were described. Some differences exist between systems for digestion and system were the waste is composted as nappies, cat soil and potted plants are accepted here as organic waste.

Collection in paper bags in the kitchen resulted in a good quality of the organic fraction with minor content of plastic. In contrast collection in plastic bags resulted in much more plastic than can be explained by the bags and the nappies accepted in some collection systems meant for composting of the organic waste.

Typically collection bags and other missorted material make up only few percent of the total collected organic waste. In several examinations missorting is greater in areas with apartment blocks than in areas with single-family houses. However in all collection systems poor sorted household waste is sometimes found, where plastic and other missorted material is much higher than normal.

The composition of the organic fraction collected in the kitchens (measured as volatile solids minus plastic) don't deviates between geographical areas, housing type or collection system, but great variations exist over time within each area. The quality of the organic fraction is very similar evaluated based on the components in the waste (fat, protein, fiber and carbohydrates) and on the elements (C, H, O and N).

In contrast differences appears after collection of the waste. Collection in plastic bags (and in cases where nappies are accepted as part of the organic waste) results in more plastic in the waste than can be attributed to the collection bags and the nappies. Furthermore acceptance of cat soil and potted plants in the organic waste seems to increase the content of non-volatile solids (ash) in the collected waste.

Pre-treatment of source sorted organic household waste

In spite of the low weight of the fraction of plastic collection bags and other missorted material the need for its removal from the organic waste have great impact for the amount and quality of the waste for digestion.

The need for pre-treatment of the collected waste in order to remove plastic and missorted material results in that up to about half the organic material, that could be digested is sorted out as reject together with the plastic and goes to incineration.

Even collection bags of plastic don't contain plasticisers the elevated content of plastic results in increased risk that the waste can't pass the limits for plasticisers (DEHP) stated in the Statute Nr. 49 of 20. January 2000 about utilisation of waste in agriculture (Statutes for sludge). In such cases the residuals can't be accepted for agricultural use. The pre-treatment systems used today reduces the plastic content in the waste but will not always secure at sufficient low content of DEHP.

If source sorted household waste have to be digested, plastic should not be accepted as part of the organic fraction. Plastic bags should only be accepted for collection, if great effort is done to avoid missorting of plastic. In such cases plastic bags without DEHP should be used and pre- treatment with effective separation of plastic should be selected.

In spite of effort to reduce plastic in the organic waste it is to be expected in all type of collection and pre-treatment systems that there now and then will be taken samples from the waste with content of DEHP exceeding the limit of the Statutes for sludge.

Today four different pre-treatment systems for source sorted household waste have reported results in Denmark. Two plants are in full scale, one is a prototype and the last one an experimental system, that has been tested.

One of the full-scale plants is based on shredding and magnetic separation of source sorted household waste collected in paper bags in Grindsted and the other a disk screen in Herning. The latter plant has in recent years treated a lot of organic waste collected in plastic as well as paper bags. The plant has just been closed down due to too much plastic in the sorted material. A screw separator has been in operation for a longer period in connection with a large-scale experiment in Aalborg. The plant is a prototype and the capacity has not been finally settled. Finally a pressure separator has been tested treating few loads of source sorted household waste.

Further a new disk screen plant is lately established at the biogas plant in Århus, but no results have been reported. At AFAV I/S the former rotating drum for composting is used as a preliminary pre-treatment plant for source sorted household waste for digestion. Finally a plant based on a screw separator is operation at NOVOREN I/S for treatment of source sorted household waste.

Based on full and large scale examinations it is documented that the three systems in operation is very different with respect to separation of the source sorted household waste in biomass and reject. Shredding and magnetic separation lead to minor separation of reject (<1%), but the system can only handle source sorted household waste collected in paper bags with good quality in the collection. In comparison the disk screen produce 34% reject and the screw separator 41% based on wet weight of the waste. The pre-treatment efficiencies vary a lot with relative standard deviation of around 10-15%. Consequently smaller differences in efficiency related to residential area and collection system hasn't been notices.

The high reject percents mean that the disk screen and the screw separator divide reject and biomass so that up to half the organic matter that is collected is sorted out as reject and incinerated.

The disk screen sort out a little less of the organic material but leave larger particles in the biomass than the screw separator. Consequently pre-treatment of source sorted household waste collected in plastic bags on the disk screen requires further treatment of the waste before or after digestion in order to avoid plastic in the residues from digestion.

Pre-treatment of source sorted household waste is the most critical stage in digestion of source sorted household waste. Full, scale experience is limited and the only plant in operation today in Denmark needs collection in paper bags. Other systems that have been in operation sort out a great part of the organic matter for incineration independent of collection in plastic or paper bags.

Quality of source sorted household waste after pre treatment

Biomass from source sorted household waste collected in paper bags contain minor plastic or other missorted material independent of the pre-treatment used. When collection is made in plastic bags the biomass after pre-treatment contain more plastic after pre-treatment on the disk screen than on the screw separator (and from the pressure separator). It means that technical problems may arise in the digestion process and that further separation of plastic is needed before the residues from digestion can be used in agriculture. Further a greater risk is that the limits of the Statutes for sludge are exceeded.

The chemical composition of the biomass from a defined system (residential area, collection system, pre-treatment technology) varies in time. For the most important parameters the relative standard deviation is 3-15%. In contrast the composition of the biomass vary only slightly between the different areas. In collection systems, where the organic waste is composted and where cat soil and potted plants are accepted in the organic fraction the content of inorganic matter is elevated.

The biomass typically consists of 22-32% dry matter, where 83-93% is organic matter (Volatile Solids), 10-14% crude fat, 13-15% crude protein, 10-16% starch, 4-10% sugar and 16-24% crude fiber. These components make up 80% of the organic matter as a mean and the remaining organic matter is assumed to be "other sugars". The main variation in the composition is related to the pre-treatment. In general the biomass from the screw separator compared to the disk screen contain more water (relative 7-20% less dry matter), more crude fat (relative 10-20% more), less crude fiber (relative 22-40% less), more enzyme digestible organic matter and less P (relative 50% less).

Biogas potential in organic household waste

Source sorted household waste from apartment blocks and areas with single-family houses in Grindsted, Copenhagen, Kolding, Vejle and Aalborg has been sampled twice during a period of 11 month. The waste has been pre-treated on three different pre-treatment plants: shredding+magnetic separation, disk screen and screw separator. The pre-treated waste the biomass and the reject has been characterised physically and chemically and the methane potential has been measured in laboratory during 50 days. In 14 cases the biomass has been digested in a pilot scale digester and the methane yield is found under stable operation. The digested biomass has been characterised with respect to chemical composition and residual methane potential.

The component composition and the composition of elements vary only slightly between biomass from different geographical areas, housing type and pre-treatment systems. The theoretical methane potential based on the mean component composition is found to be 530 Nm³ CH₄/ton VS.

Measurement of the methane potential in the laboratory after 50 days shows that pre-treated source sorted household waste as a mean has a methane potential of 465 Nm³ CH₄/ton VS. Some variations exist, but no difference is found between geographical area, housing type or pre-treatment technology.

The methane yield for biomass from pre-treated source sorted household waste is found from pilot scale digestion of 14 different samples digested in pilot scale. The yield varies in the range of 300-400 Nm³ CH₄/ton VS, with a mean value of 340 Nm³ CH₄/ton VS and methane content at 62%. The variations can't be related to geographical area, housing type or pre-treatment technology. After digestion the biomass has a potential for further methane production of 40-50 Nm³ CH₄/ton VS originally supplied to the pilot digester corresponding to in average 10-15% additional methane.

The methane potential calculated from the component composition or from the content of elements shows as expected higher values than the measured values in laboratory or pilot scale.

It is not surprising that when the variation in components and in the elements is small and the small differences is not related to the origin of the biomass then the same is seen for the biological method for measurement of the methane potential in the laboratory and the methane yield i pilot scale, where the experimental uncertainty is expected to be greater than the uncertainty of the chemical analysis.

In the tables below key figures for methane and biogas production is shown separately for the different pre-treatment systems present in Denmark today. It shall be noted that the different pre-treatment systems do not treat exactly the same waste. Consequently the numbers in the table is influenced by differences in the collected waste. Shredding+magnetic pre-treatment needs collection in paper bags whereas the two other pre-treatment systems treats waste collected in paper and plastic bags. As collection in paper bags leads to waste with a slightly higher content of organic matter measured as volatile solids the methane yield pr ton of dry matter will be higher than for the other pre-treatment systems.

*Collected wet source sorted organic fraction of household waste

Assuming that the methane content is 62% of the biogas like the mean value found in the pilot experiments similar key figures for biogas (methane+carbon dioxide) is presented below.

As the composition of the biomass is very similar independent of the origin of the waste it is the pre-treatment efficiency that will dominate the resulting biogas production from the collected waste. The three pre-treatment system deviates as can be seen from the table.

The organic matter in the reject is fundamentally not different from the organic matter in the biomass and did also reveal substantial methane potential, although based on VS, about 25-40% less than the potential in the biomass.

Environmental assessment of digestion of source sorted household waste

Savings in energy, global warming potential and nutrient recovery from source separated organic household waste were modelled for a range of scenarios with different sorting criteria, collection system, pre-treatment, digestion and incineration of the reject. Models were also made considering only incineration of the organic waste. Transport, process energy, energy production as well as substitution of artificial fertilizers are considered in the models.

Savings in energy by digestion of the organic household waste is independent of the pre-treatment technology and in general not very different from the savings obtained by incineration of the organic household waste from Grindsted, Copenhagen, Kolding and Vejle, while there is a minor advantage (ca. 9%) in the case of waste from Aalborg.

The digestion of the biomass and the incineration of the reject contribute equally to the production of energy when both systems are operated with power and heat production. The largest saving in energy is obtained when the dry matter is recovered in the reject and the water in the biomass. The savings in energy by substituting artificial fertilizer and the energy used on collection and transport of the waste each corresponds to about 10% of the energy obtained in the system. This suggests that optimisation of the energy savings by digestion of organic waste should focus on optimising the gas production in the digester, the gas utilization and the incineration of the reject.

The overall saving in energy is not very sensitive to changes in the technological system. The crucial issue is in all cases that efficient energy savings require that both electricity and heat are produced.

Recovery of N, P and K does not exist by incineration, but by digestion each ton of wet source separated organic household waste contributes with 5-7 kg N, 0.5-1 kg P and 1.5-2 kg K for most of the systems applicable to Copenhagen, Kolding, Vejle and Aalborg. In Grindsted, where the waste is very clean and a magnetic separator is the only pre-treatment, about twice as much is recovered in terms of nutrients, since the reject is negligible.

The investigation revealed large geographical and seasonal variations in waste composition, pre-treatment efficiencies, methane potentials and in methane yields. However, the large number of samples involved and the extensive characterization performed suggest that the evaluations and conclusions made reasonably well represent typical Danish conditions regarding source separation and digestion of organic household waste.