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Open windrow composting of polymers: An investigation into the operational issues of composting polyethylene (PE)
Waste Management 25 (2005) 401–407 www.elsevier.com/locate/wasman
Open windrow composting of polymers: An investigation into the operational issues of composting polyethylene (PE) G.U. Davis
*
Water and Environmental Research Centre, University of Bristol, UK Accepted 14 February 2005
Abstract This paper investigates the operational issues surrounding the open windrow composting of degradable polyethylene sacks. Areas for consideration were the impact of degradable polyethylene sacks on the composting process, the quality of the finished compost product, and how the use of sacks influenced the on-site processing. These factors were investigated through determining the amount of polymer residue and chemical contaminants in the finished compost product and the daily monitoring of windrow temperature profiles. Site and practical handling considerations of accepting an organic waste contained within PE sacks are also discussed. Statistical analysis of the windrow temperature profiles has led to the development of a model that can help to predict the expected trends in the temperature profiles of open compost windrows where the organic waste is kerbside collected using a degradable PE sack. Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction The number of separate kerbside organic waste collection systems operated by UK Local Authorities (LAs) has risen sharply as a result of the implementation of the European Landfill Directive (99/31/EC) into UK legislation on the 16th July 2001. The Landfill Directive seeks to reduce the amount of biodegradable municipal waste (BMW) going to landfill in three successive stages, eventually by 2020 to 35% of the 1995 total of BMW, because of the negative environmental impacts associated with leachate and methane production (Hudgins, 1999). The DETRÕs Waste Strategy 2000 (England and Wales), set targets for Waste Disposal Authorities to reduce the amount of BMW sent to landfill by introducing a tradable permit system designed so that the UK can meet EU Landfill Directive targets (Read, 1999). These
*
Tel./fax: +44 1344 451 690. E-mail address: [email protected].
0956-053X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2005.02.016
targets have resulted in the proliferation of kerbside collection systems which have targeted BMW, and many LAs are currently conducting pilot trials, research projects and system reviews to determine the best possible way of segregating, collecting, transporting and processing this specific waste stream (Read and Reed, 2001). Waste auditing data obtained by the Organic Resource Agency indicates that up to half of the total composition of household generated waste in the UK can be organic garden and kitchen waste, and is thus part of the BMW category (Davis, 2002). An effective strategy for kerbside collection of the source segregated BMW component from the residual municipal waste could lead to the production of a high quality compost product and reduce the amount of BMW going to landfill (Read and Reed, 2001). Many Authorities support the collection of organics, as it is seen as a way of quickly increasing waste diversion (Sinclair, 2002) and achieving recycling targets (Davis et al., 2004). Polymer sacks, currently marketed in the UK for garden sweepings and the kerbside collection of organic
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G.U. Davis / Waste Management 25 (2005) 401–407
wastes, can be categorised into degradable and biodegradable types (Davis, 2003). Biodegradable polymers can be defined as being capable of being chemically transformed by the action of biological enzymes or micro-organisms into products which themselves are capable of further biodegradation (Guillet, 1995). Whilst a degradable polymer is capable of undergoing a significant change in its chemical structure under specific environmental conditions (Stevens, 2002). A compostable polymer will undergo ‘‘biological degradation during composting to give carbon dioxide, water, inorganic compounds and biomass at a rate that is consistent with other compostable materials, leaving no distinguishable or toxic residues’’ (Stevens, 2002). The work reported within this paper documents the observations and the results obtained from monitoring the open windrow composting of kerbside collected BMW from households. Monitoring of the composting process and the resulting compost products are essential to determine whether the polyethylene (PE) sacks used have a detrimental effect on either the composting process or the compost product.
2. Methodology 2.1. Polymer residues in the final compost product It is usually necessary to screen the finished product after the composting process is complete, before marketing and delivery to the consumer. The size at which a compost product is screened is dependant on the final end-use of the product. For example, an agricultural application may require a coarser product that has been screened at 20 mm, whilst for a horticultural application the consumer will expect a high quality product possibly screened at 8 mm. From green waste compost, oversized items are rejected and usually comprise of wood and coarse materials because the 12-week composting process is not sufficient to break down certain substances, such as lignin and cellulose. It is essential that the composition of the rejects could be determined from the control windrow (kerbside collected using wheeled bins) and the windrow containing the PE. Representative samples were taken of each of the finished composts, prior to commercial screening, and hand screened using a 12 mm screen. 2.2. Chemical analysis Chemical analysis of the degradable PE sacks was undertaken as a precautionary measure to ensure that there were no potential contaminants within the polymer that could affect the quality or safety of the final compost product. Chemical analysis of the collected or-
ganic wastes following shredding and upon completion of the composting process was also undertaken for both the PE sourced waste and for the control windrows. The chemical analysis of composts is a frequently used method for establishing the quality of a compost (de Bertoldi, 1999). The chemical analysis includes parameters relating to the progress of the composting process and suitability for final use, such as carbon to nitrogen ratio (C:N), pH, electrical conductivity and levels of heavy metal contamination. The PE sacks contained a metal (cobalt) that would not be routinely tested for in a standard compost chemical analysis. This metal was therefore incorporated into the chemical analysis of the compost. 2.3. Windrow temperature profiles Monitoring of composting temperatures provides a site operator with an indication of the progress and success of the composting process. More emphasis is being placed on the importance of obtaining specified temperatures for given durations to fulfil industry standards, and to reassure end users that the final compost product is of the expected quality in terms of pathogen kill and weed seed elimination, both of which can be controlled if the required time–temperature profile is achieved. The Composting Association has produced guidelines, (which are encompassed by the British Standards InstituteÕs Specification for Composted Materials, BSI PAS 100), that stipulate compost windrows should be maintained at 55 °C for a total of 14 days in order for the compost to be sanitised. Daily monitoring of the temperature of windrows was undertaken in order to establish the individual temperature profiles of the control windrow and the windrow containing the shredded degradable PE sacks. A total of four sets of temperature profiles were obtained. The windrows, although kept separate throughout the composting process, were situated on the same site and subject to the same climatic factors, ensuring comparability. 2.4. Gas analysis of filled and sealed polyethylene sacks It was essential that the conditions within the sealed PE sacks stored on-site prior to shredding be assessed. It was hypothesised that gas analysis would prove or disprove the hypothesis that the sealed organic matter was turning anaerobic prior to shredding and windrow formation. Thus, leading to the higher C:N ratios seen in the final compost product as the aerobic compost process is inhibited by the anaerobic conditions (Davis et al., 2004). Due to the number of sacks, gas analyses required, the location and difficulties of on-site logistics, an instrument for measuring the gas inside the sacks
G.U. Davis / Waste Management 25 (2005) 401–407
on-site was required, rather than taking gas samples in syringes and forwarding them to a laboratory for analysis. Only one gas analyser successfully provided gas readings, had the relevant software and was durable enough to cope with life on landfill and composting sites. To keep costs to a minimum and using technology simple enough to be readily repaired, a basic analyser was purchased. The analyser did not comprise of a data logger or any computer interface due to cost. The gas analyser also only monitored oxygen, carbon dioxide and hydrogen sulphide. For more ÔcomplicatedÕ analysis the price of gas monitoring equipment was prohibitively expensive. The analysis was undertaken on a sample of Ôfreshly collectedÕ sacks containing the organic matter. Gas analysis then took place over a 10 week period. A total of 60 bags were tested, thus 10 bags were analysed upon collection and then 10 bags every fortnight for the full 10 weeks.
3. Results 3.1. Polymer residues in final compost product Table 1 shows the relative percentage weights of the rejects from the various samples. The volume of rejects is also influenced by the type of green waste shredder used to shred the organic waste prior to windrow formation. This can make it difficult to compare the volumes of rejects between the different windrow sets. However, both the control and PE sack collected organic wastes were shredded on the same day using the same shredder, thus a direct comparison can be made between the windrows within the same set. The total percentage of rejects within the control windrows were continuously lower than those found in the windrow containing the PE. However, the total weight of PE was very small (although of high volume and therefore highly visible). If the feedstocks were similar and the shredding process and compost treatments identical, there had to be another reason for the larger amount of rejects in the windrows containing the PE.
403
Therefore, analysis of the data recording the windrow temperature profiles was undertaken. 3.2. Chemical analysis Analysis of the PE sacks used in this study, shown in Table 2, established that metals from the first transition series were incorporated into the polymer matrix. Transition metals are used to weaken the carbon–carbon backbone of a polymer, making it more degradable. Such metals occur naturally in some soils and were in small amounts within the PE bags. It should be noted that the various manufacturers of such PE sacks can vary the constituent metals and introduce other additives in order to change the rate of degradation of the PE. Table 3 summarises selected results from the chemical analysis from finished compost products before and after the screening process. The analysis did not indicate that the constituent metals within the PE sacks were contributing to a significant increase in the levels of metals detected within the final compost product matrix, but that reductions in the levels of the constituent metals within the PE sacks (copper and cobalt) could be reduced through screening out the non-degraded, visible PE. Chemical analysis of the compost has also shown that the C:N ratio of the windrows containing the PE are higher than the C:N ratio for the control windrows containing no PE. A low C:N ratio is desirable as it shows that the soluble carbon is being degraded and released as carbon dioxide, so causing a decrease in carbon relative to nitrogen, decreasing the C:N ratio. One explanation for this could be the period of confinement of the organic waste within the sealed PE bags prior to shredding. 3.3. Windrow temperature profiles The temperature profiles of the windrows when plotted against time, indicated graphically that the temperatures experienced within the windrows containing the shredded PE reached a lower average temperature than the control windrows that did not contain any PE. In all
Table 1 Table of sample and rejects weights and percentage rejects for the screening process Designation Windrow Windrow Windrow Windrow Windrow Windrow Windrow Windrow
1 1 2 2 3 3 4 4
– – – – – – – –
control bag control bag control bag control bag
Weight of sample (kg)
Weight of rejects (kg)
% rejects to compost
41.4 42.8 36.4 37.6 41.6 31.6 45.0 34.7
7.9 11.3 3.6 8.8 3.2 5.4 7.9 8.1
19.3 26.5 9.9 23.4 7.7 17.1 17.5 23.3
% rejects that are PE 0.42 0.31 0.39 0.34
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G.U. Davis / Waste Management 25 (2005) 401–407
Fig. 1. In particular, the starting temperatures at the beginning of the composting process were very different. The differences between the temperature profiles in Figs. 1 and 2, can be attributed to a delay in the shredding and formation of the windrows in Fig. 2. The material that comprised the windrows seen in Fig. 1 was collected, shredded and formed into windrows within six weeks of collection. However, due to the poor availability of a shredder and a subsequent breakdown, the material used to compose the windrows in Fig. 2, was up to 12 weeks old. Thus, the organic material had spent up to 12 weeks contained within sealed PE sacks. The on-site gas analysis indicated increasingly anaerobic conditions within the sealed polyethylene sacks. Scatter diagrams of the four sets of windrow temperature profiles proved insufficient to determine if a significant linear relationship existed between the temperatures recorded in the control windrow and those recorded in the windrow containing the PE. Correlation analysis was therefore used to determine the strength of the relationship between the temperatures over time. Correlation analysis is a measure of the degree of association between two variables. The following correlation coefficients for the four sets of windrows were determined, Windrows [1], 0.672; Windrows [2], 0.929; Windrows [3], 0.686; Windrows [4], 0.668. The correlation coefficients for the four windrows show that there was a relationship between the temperatures recorded in the control windrows and the windrows containing the shredded degradable PE. Regression modelling analysis was then applied to the data using the statistical package, GENSTAT 6. Regression analysis is the process of constructing a mathematical formula that can be used for predicting the value of
Table 2 Chemical analysis of degradable PE sacks Determinand
Average bag value
Units
Copper in solution Cobalt in solution Other elements
35 35 None at significant levels
mg/kg mg/kg
Table 3 Chemical analysis of finished compost before and after commercial screening Determinand
Unscreened
Screened
Units
Dry matter Total nitrogen Total carbon C:N ratio Total copper Total zinc Total lead Total cobalt
58.3 0.9 10.6 12.1 32.4 97.4 57.4 3.48
49.9 1.61 20.9 13.1 42.8 139 101 3.31
% % w/w %w/w ratio mg/kg mg/kg mg/kg mg/kg
cases, the average temperature in the control windrows was higher than that experienced in the windrow containing the PE. Figs. 1 and 2 show very different temperature profiles. Fig. 1 shows two temperature profiles that comply with the British Standards InstituteÕs Specification for Composted Materials, BSI PAS 100, although the temperature in the windrow containing the PE does fall-off quicker than the control windrow half-way through the composting process. The same materials were composted for both Figs. 1 and 2. The difference in the composting temperatures achieved in the control windrow and PE containing windrow for Fig. 2, is significantly larger than those in 70
60
40
30
20
Control - bins With PE bags
10
No. of Days
Fig. 1. Windrow temperature profile for set [2] – control versus pe containing windrow.
82
80
74
76
72
62
67
58
60
48
53
45
38
40
34
30
32
24
26
13
17
9
11
4
0 6
Temperature (˚C)
50
G.U. Davis / Waste Management 25 (2005) 401–407
405
80
70
Temperature (˚C)
60
50
40
30
20 Control - bins With PE bags
10
90
84
77
80
73
64
66
58
60
52
56
50
42
45
38
32
36
28
30
21
24
10
17
2
4
0
No. of Days
Fig. 2. Windrow temperature profile for set [3] – control versus pe containing windrow.
one variable on the basis of one known value of another variable. In this case, a formula for predicting the open windrow temperatures when a shredded degradable PE is introduced into the organic compost feedstock. Analysis of variance was used to choose the simplest model that adequately described the observed relationship. A straight-line relationship between the temperatures recorded in the control windrows and the windrows containing the PE was determined. This relationship was the same for all windrows, that is, the slope of the four regression lines was the same. The R2 value was 66.4%, which indicated that two-thirds of the observed variability in the relationship between the control windrows temperature and the PE containing windrows was explained. The following four equations were determined for each of the four sets of windrows (Davis et al., 2004):
CWTemp ¼ 27 þ 0:6 PEWTemp ;
ð1Þ
CWTemp ¼ 25 þ 0:6 PEWTemp ;
ð2Þ
CWTemp ¼ 33 þ 0:6 PEWTemp ;
ð3Þ
CWTemp ¼ 27 þ 0:6 PEWTemp ;
ð4Þ
where CWTemp is the temperature (°C) of the control windrow, PEWTemp is the temperature (°C) of the windrow containing the shredded degradable polyethylene. The four regression equations show that the initial compost starting temperatures within the control windrows were higher (25–33 °C) than the initial temperatures within the windrows containing the PE. The equations also indicate that the rate of temperature increase within the windrows containing the PE was significantly higher than in the control windrow. For
Table 4 Gas analysis results Week
Bag results, X10 for O2 and CO2 (%)
Average
0 (Collection week)
O2: 20.8; 20.7; 20.8; 20.5; 20.8; 20.9; 20.8; 20.7; 20.8; 20.7 CO2: 0.05; 0.08; 0.15; 0.19; 0.14; 0.52; 0.81; 0.86; 0.14; 0.72 O2: 20.9; 20.7; 20.5; 20.6; 20.4; 20.7; 20.3; 20.4; 20.4; 20.1 CO2: 0.08; 0.84; 0.91; 0.42; 0.84; 0.86; 0.92; 0.48; 0.51; 0.64 O2: 20.1; 20.2; 20.1; 19.9; 20.3; 20.5; 20.2; 20.3; 20.2; 20.0 CO2: 0.41; 0.94; 0.98; 1.10; 0.43; 0.62; 0.28; 0.48; 0.70; 0.98 O2: 20.2; 20.0; 19.8; 19.4; 20.1; 19.3; 20.7; 20.1; 20.2; 19.7 CO2: 0.71; 1.00; 0.94; 1.30; 0.26; 1.10; 0.39; 0.48; 0.74; 0.90 O2: 20.0; 19.9; 19.4; 19.6; 17.1; 19.1; 20.2; 19.0; 18.8; 19.7 CO2: 0.64; 2.00; 1.02; 1.24; 2.80; 1.43; 1.62; 1.22; 1.03; 0.74 O2: 19.4; 19.8; 20.1; 19.2; 18.7; 18.5; 18.0; 18.9; 19.1; 20.2 CO2: 0.94; 1.54; 1.86; 1.34; 1.30; 1.06; 3.88; 1.18; 2.12; 0.67
20.75 0.366 20.5 0.65 20.18 0.692 19.95 0.782 19.28 1.374 19.19 1.589
2 4 6 8 10
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G.U. Davis / Waste Management 25 (2005) 401–407
every 1 °C increase in temperature in the PE containing windrow, the control windrow only increased by 0.6 °C. 3.4. Gas analysis of filled and sealed polyethylene sacks Results (Table 4) have indicated that there were substantially reduced levels of oxygen (down to 17%), increased levels of carbon dioxide (up to 4%). The levels of O2 fell gradually as the CO2 levels rose. Both of these factors corresponded with the increasing age of the sacks. There has been no trace of hydrogen sulphide to date. These findings are in line with the known theoretical processes where as organic matter degrades, the microbes consume oxygen and subsequently respire, emitting carbon dioxide. However, these results are inconclusive, particularly as in order to measure gas levels within the sack, the probe has to be pushed into the sack effectively opening it to the air and immediately affecting the results.
4. Conclusions Degradable sacks have additional complications over more traditional receptacles, such as bins and boxes. Non-degradable polymers, once mixed with organic material, can be difficult and costly to remove (Krause, 2003). Problems include (Spencer, 2003): The mechanical processes employed throughout the composting process (such as turning and screening) which can result in polymers being sheared into increasing numbers of smaller fragments. Polymers catch on moving parts of mechanical machinery and can block air intakes, requiring manual removal. Since the densities of compost and polymer films are close, air separation technologies can be quite fussy to operate and tend to be sensitive to moisture content. The dry matter content undertaken as part of the chemical analysis has indicated that the windrows containing the PE typically have a lower proportion of dry matter than the control windrows. This is in line with the observation that the windrows containing the PE are typically wetter and so harder to move and screen than the control windrows. It is essential that the waste collection authority and treatment/disposal facility are consulted and included in any discussion on deciding collection strategy/method used to collect organic wastes. Councils need to discuss collection systems with compost processors early in this planning stage. The reasons for this are threefold. Firstly, site licenses can take over twelve months to ob-
tain and secondly, it is essential that the composting facility can deal with the quality of the organic waste being presented and that the volume of waste is within their license conditions. The type of collection strategy implemented (i.e., frequency of collection and type of collection vessel) will have a significant effect on the volume of waste collected. The sacks also have an on-going purchase and distribution cost due to their limited shelflife, in addition to increasing processing costs at the compost site. The windrows containing the shredded PE did not achieve the same elevated temperatures as those experienced within the control windrows. This is of significance because it is the higher temperatures over extended periods that effectively kills pathogens and weed seeds (the sanitisation process) that ultimately determines the quality and safety of the final compost product. Compost windrows that do not reach the desired temperatures for sanitisation should be blended with fresh feedstock and re-composted until sanitisation has been achieved or, alternatively, could be used for landfill day cover, although the British Standards InstitutionÕs Specification for Composted Materials, BSI PAS 100 does not categorically state that the compost should be disposed of. This may cause logistical problems for the site operator who must invest more man-hours and space to re-compost the organic waste and with any disposal of the compost incurring landfill charges. In addition the Local Authority would lose its right to claim the tradable permits for the weight of organic waste collected. Kerbside collected BMW from households typically has a small particle size and does not usually require shredding. This is particularly true of BMW collected using PE sacks as large bulky items tend to pierce the bags and make bag handling inconvenient. During the summer months, the particle size of the kerbside collected BMW tends to be even finer due to the high inclusion of grass clipping, making shredding undesirable as it can reduce particle size further making the organic waste more dense during the composting process which, in turn, can impede the composting process by limiting the transport of heat and oxygen within a windrow (Wang et al., 2003). Fine, wet wastes are more prone to jamming the shredder as well as increasing the mechanical damage to the shredder mechanism. It is necessary to shred the PE sacks containing the kerbside collected organic wastes despite the environmental and cost implications (Davis, 2003) because: The PE sacks are typically sealed by the householder to stop the contents escaping after filling, in storage and during the collection phase. PE has good gas barrier properties and as such, the organic waste con-
G.U. Davis / Waste Management 25 (2005) 401–407
tained within the bags becomes anaerobic, making it necessary to open the bag contents to air as quickly as possible. Shredding the bags assists with the mechanical degradation of the PE sacks. The aim of this research was to establish if degradable PE sacks were suitable for inclusion into open composting windrows with organic wastes. The monitoring undertaken has demonstrated that although PE sacks are suitable for the collection of organic wastes, they are not compostable under open windrow conditions as a determining feature of a compostable polymer is its ability to undergo biological degradation at a rate consistent with other known compostable materials, in addition to leaving no visually distinguishable residues within the final compost product (Stevens, 2002). The rate of degradation of the PE did not match the rate of degradation observed within the organic waste, where an expected volume reduction is between 40% and 60%, resulting in an actual increase in the amount of PE to organic matter over the composting period and resulting in the PE becoming increasingly visually obtrusive within the windrows. In order for a degradable sack to be successful for kerbside collection and subsequent composting of organic wastes, it must provide problem-free processing at the compost plant in addition to providing convenience and a reasonable service life to the householder. Although PE sacks are convenient to the householder and provide a stable and strong receptacle for the kerbside collection of organic wastes, the PE sacks require additional processing at the composting facility, by way of shredding and reduced size screening to remove as much of the visible PE before marketing. Rejects from the screening process comprise of the woodier organic wastes as the carbon tends to be less soluble where it is protected with lignin. These woodier wastes are blended into the ÔfreshÕ compost matter to add structure to windrows and, in time, will break down sufficiently to clear the screening process. However, the high volume of PE in the screened rejects has resulted in the rejects being landfilled. The increased processing requirement and the landfill of rejects has both a resource consumption and financial implication, essentially increasing the cost of producing what is perceived by the end consumer as an inferior compost product.
407
Acknowledgements The author thank Brunel University and The Organic Resource Agency for their assistance in the completion of this research and the EPSRC for their funding.
References Davis, G., 2002. Time to get in the auditorsWater and Environment Manager, vol. 7. Chartered Institution of Water and Environmental Management, UK, No. 3. pp. 4–6. Davis, G., 2003. The characteristics and characterization of degradable polymer sacks. Materials Characterization 51, 147–157. Davis, G., Read, A., Bulson, H., Harrison, D., Billett, E., 2004. Open windrow composting of polymers: an investigation into the rate of degradation of polyethylene. Resource Consumption and Recycling 40, 343–357. Davis, G., Bulson, H., Sharpe, N., Harrison, D., Billet, E., 2004. A comparative study of wheeled bins and degradable polymer sacks for the kerbside collection of organic wastes. CIWM Scientific and Technical Review. The Chartered Institution of Wastes Management, UK 5(1), 18–30. de Bertoldi, M., 1999. The control of the process and compost quality. In: Bidlingmaier, W., deBertoldi, M., Diaz, L., Papaddimitriou, F.K. (Eds.), Organic Recovery and Biological Treatment. Rhombos-Verl, Berlin, pp. 47–53. DETR. 2000. Waste Strategy: A Waste Strategy for England and Wales. Department for the Environment, Transport and the Regions, HMSO, London. Guillet, J., 1995. Plastics and the Environment. Degradable Polymers: Principles and Applications. Chapman and Hall, London, pp. 217– 246. Hudgins, M., 1999. Aerobic landfill studies from the USA. Paper presented at the 1st International Conference on Solid Waste. Rome. April 1999. Krause, T., 2003. Skimming the film off compost.. Resource recycling. North AmericaÕs Recycling and Composting Journal February, 10– 13. Read, A.D., 1999. Making waste work – making UK national solid waste strategy work at the local scale. Resources Conservation and Recycling 26, 259–285. Read, A.D., Reed, S., 2001. Daventry District CouncilÕs High Diversion Organic Waste Recycling Scheme. In: Presented at the Waste in the 21st Century Conference, University College Northampton, Northampton, UK. Sinclair, R., 2002. Comparing containers, carts, bags and pails. Resource Recycling. North AmericaÕs Recycling and Composting Journal August, 10–15. Spencer, R., 2003. Film plastics in compost – wrestling with the lightweights biocycle. Journal of Composting and Organics Recycling February, 38–41. Stevens, E.S., 2002. Green Plastics: An Introduction to the New Science of Biodegradable Plastics. Princeton University Press. Wang, Q., McCartney, D., Eftoda, G., 2003. Measuring free air space in compost material. In: Presented at 18th International Congress on Solid Waste Technology and Management, Philadelphia.
Open windrow composting of polymers: An investigation into the operational issues of composting polyethylene (PE) G.U. Davis
*
Water and Environmental Research Centre, University of Bristol, UK Accepted 14 February 2005
Abstract This paper investigates the operational issues surrounding the open windrow composting of degradable polyethylene sacks. Areas for consideration were the impact of degradable polyethylene sacks on the composting process, the quality of the finished compost product, and how the use of sacks influenced the on-site processing. These factors were investigated through determining the amount of polymer residue and chemical contaminants in the finished compost product and the daily monitoring of windrow temperature profiles. Site and practical handling considerations of accepting an organic waste contained within PE sacks are also discussed. Statistical analysis of the windrow temperature profiles has led to the development of a model that can help to predict the expected trends in the temperature profiles of open compost windrows where the organic waste is kerbside collected using a degradable PE sack. Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction The number of separate kerbside organic waste collection systems operated by UK Local Authorities (LAs) has risen sharply as a result of the implementation of the European Landfill Directive (99/31/EC) into UK legislation on the 16th July 2001. The Landfill Directive seeks to reduce the amount of biodegradable municipal waste (BMW) going to landfill in three successive stages, eventually by 2020 to 35% of the 1995 total of BMW, because of the negative environmental impacts associated with leachate and methane production (Hudgins, 1999). The DETRÕs Waste Strategy 2000 (England and Wales), set targets for Waste Disposal Authorities to reduce the amount of BMW sent to landfill by introducing a tradable permit system designed so that the UK can meet EU Landfill Directive targets (Read, 1999). These
*
Tel./fax: +44 1344 451 690. E-mail address: [email protected].
0956-053X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2005.02.016
targets have resulted in the proliferation of kerbside collection systems which have targeted BMW, and many LAs are currently conducting pilot trials, research projects and system reviews to determine the best possible way of segregating, collecting, transporting and processing this specific waste stream (Read and Reed, 2001). Waste auditing data obtained by the Organic Resource Agency indicates that up to half of the total composition of household generated waste in the UK can be organic garden and kitchen waste, and is thus part of the BMW category (Davis, 2002). An effective strategy for kerbside collection of the source segregated BMW component from the residual municipal waste could lead to the production of a high quality compost product and reduce the amount of BMW going to landfill (Read and Reed, 2001). Many Authorities support the collection of organics, as it is seen as a way of quickly increasing waste diversion (Sinclair, 2002) and achieving recycling targets (Davis et al., 2004). Polymer sacks, currently marketed in the UK for garden sweepings and the kerbside collection of organic
402
G.U. Davis / Waste Management 25 (2005) 401–407
wastes, can be categorised into degradable and biodegradable types (Davis, 2003). Biodegradable polymers can be defined as being capable of being chemically transformed by the action of biological enzymes or micro-organisms into products which themselves are capable of further biodegradation (Guillet, 1995). Whilst a degradable polymer is capable of undergoing a significant change in its chemical structure under specific environmental conditions (Stevens, 2002). A compostable polymer will undergo ‘‘biological degradation during composting to give carbon dioxide, water, inorganic compounds and biomass at a rate that is consistent with other compostable materials, leaving no distinguishable or toxic residues’’ (Stevens, 2002). The work reported within this paper documents the observations and the results obtained from monitoring the open windrow composting of kerbside collected BMW from households. Monitoring of the composting process and the resulting compost products are essential to determine whether the polyethylene (PE) sacks used have a detrimental effect on either the composting process or the compost product.
2. Methodology 2.1. Polymer residues in the final compost product It is usually necessary to screen the finished product after the composting process is complete, before marketing and delivery to the consumer. The size at which a compost product is screened is dependant on the final end-use of the product. For example, an agricultural application may require a coarser product that has been screened at 20 mm, whilst for a horticultural application the consumer will expect a high quality product possibly screened at 8 mm. From green waste compost, oversized items are rejected and usually comprise of wood and coarse materials because the 12-week composting process is not sufficient to break down certain substances, such as lignin and cellulose. It is essential that the composition of the rejects could be determined from the control windrow (kerbside collected using wheeled bins) and the windrow containing the PE. Representative samples were taken of each of the finished composts, prior to commercial screening, and hand screened using a 12 mm screen. 2.2. Chemical analysis Chemical analysis of the degradable PE sacks was undertaken as a precautionary measure to ensure that there were no potential contaminants within the polymer that could affect the quality or safety of the final compost product. Chemical analysis of the collected or-
ganic wastes following shredding and upon completion of the composting process was also undertaken for both the PE sourced waste and for the control windrows. The chemical analysis of composts is a frequently used method for establishing the quality of a compost (de Bertoldi, 1999). The chemical analysis includes parameters relating to the progress of the composting process and suitability for final use, such as carbon to nitrogen ratio (C:N), pH, electrical conductivity and levels of heavy metal contamination. The PE sacks contained a metal (cobalt) that would not be routinely tested for in a standard compost chemical analysis. This metal was therefore incorporated into the chemical analysis of the compost. 2.3. Windrow temperature profiles Monitoring of composting temperatures provides a site operator with an indication of the progress and success of the composting process. More emphasis is being placed on the importance of obtaining specified temperatures for given durations to fulfil industry standards, and to reassure end users that the final compost product is of the expected quality in terms of pathogen kill and weed seed elimination, both of which can be controlled if the required time–temperature profile is achieved. The Composting Association has produced guidelines, (which are encompassed by the British Standards InstituteÕs Specification for Composted Materials, BSI PAS 100), that stipulate compost windrows should be maintained at 55 °C for a total of 14 days in order for the compost to be sanitised. Daily monitoring of the temperature of windrows was undertaken in order to establish the individual temperature profiles of the control windrow and the windrow containing the shredded degradable PE sacks. A total of four sets of temperature profiles were obtained. The windrows, although kept separate throughout the composting process, were situated on the same site and subject to the same climatic factors, ensuring comparability. 2.4. Gas analysis of filled and sealed polyethylene sacks It was essential that the conditions within the sealed PE sacks stored on-site prior to shredding be assessed. It was hypothesised that gas analysis would prove or disprove the hypothesis that the sealed organic matter was turning anaerobic prior to shredding and windrow formation. Thus, leading to the higher C:N ratios seen in the final compost product as the aerobic compost process is inhibited by the anaerobic conditions (Davis et al., 2004). Due to the number of sacks, gas analyses required, the location and difficulties of on-site logistics, an instrument for measuring the gas inside the sacks
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on-site was required, rather than taking gas samples in syringes and forwarding them to a laboratory for analysis. Only one gas analyser successfully provided gas readings, had the relevant software and was durable enough to cope with life on landfill and composting sites. To keep costs to a minimum and using technology simple enough to be readily repaired, a basic analyser was purchased. The analyser did not comprise of a data logger or any computer interface due to cost. The gas analyser also only monitored oxygen, carbon dioxide and hydrogen sulphide. For more ÔcomplicatedÕ analysis the price of gas monitoring equipment was prohibitively expensive. The analysis was undertaken on a sample of Ôfreshly collectedÕ sacks containing the organic matter. Gas analysis then took place over a 10 week period. A total of 60 bags were tested, thus 10 bags were analysed upon collection and then 10 bags every fortnight for the full 10 weeks.
3. Results 3.1. Polymer residues in final compost product Table 1 shows the relative percentage weights of the rejects from the various samples. The volume of rejects is also influenced by the type of green waste shredder used to shred the organic waste prior to windrow formation. This can make it difficult to compare the volumes of rejects between the different windrow sets. However, both the control and PE sack collected organic wastes were shredded on the same day using the same shredder, thus a direct comparison can be made between the windrows within the same set. The total percentage of rejects within the control windrows were continuously lower than those found in the windrow containing the PE. However, the total weight of PE was very small (although of high volume and therefore highly visible). If the feedstocks were similar and the shredding process and compost treatments identical, there had to be another reason for the larger amount of rejects in the windrows containing the PE.
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Therefore, analysis of the data recording the windrow temperature profiles was undertaken. 3.2. Chemical analysis Analysis of the PE sacks used in this study, shown in Table 2, established that metals from the first transition series were incorporated into the polymer matrix. Transition metals are used to weaken the carbon–carbon backbone of a polymer, making it more degradable. Such metals occur naturally in some soils and were in small amounts within the PE bags. It should be noted that the various manufacturers of such PE sacks can vary the constituent metals and introduce other additives in order to change the rate of degradation of the PE. Table 3 summarises selected results from the chemical analysis from finished compost products before and after the screening process. The analysis did not indicate that the constituent metals within the PE sacks were contributing to a significant increase in the levels of metals detected within the final compost product matrix, but that reductions in the levels of the constituent metals within the PE sacks (copper and cobalt) could be reduced through screening out the non-degraded, visible PE. Chemical analysis of the compost has also shown that the C:N ratio of the windrows containing the PE are higher than the C:N ratio for the control windrows containing no PE. A low C:N ratio is desirable as it shows that the soluble carbon is being degraded and released as carbon dioxide, so causing a decrease in carbon relative to nitrogen, decreasing the C:N ratio. One explanation for this could be the period of confinement of the organic waste within the sealed PE bags prior to shredding. 3.3. Windrow temperature profiles The temperature profiles of the windrows when plotted against time, indicated graphically that the temperatures experienced within the windrows containing the shredded PE reached a lower average temperature than the control windrows that did not contain any PE. In all
Table 1 Table of sample and rejects weights and percentage rejects for the screening process Designation Windrow Windrow Windrow Windrow Windrow Windrow Windrow Windrow
1 1 2 2 3 3 4 4
– – – – – – – –
control bag control bag control bag control bag
Weight of sample (kg)
Weight of rejects (kg)
% rejects to compost
41.4 42.8 36.4 37.6 41.6 31.6 45.0 34.7
7.9 11.3 3.6 8.8 3.2 5.4 7.9 8.1
19.3 26.5 9.9 23.4 7.7 17.1 17.5 23.3
% rejects that are PE 0.42 0.31 0.39 0.34
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Fig. 1. In particular, the starting temperatures at the beginning of the composting process were very different. The differences between the temperature profiles in Figs. 1 and 2, can be attributed to a delay in the shredding and formation of the windrows in Fig. 2. The material that comprised the windrows seen in Fig. 1 was collected, shredded and formed into windrows within six weeks of collection. However, due to the poor availability of a shredder and a subsequent breakdown, the material used to compose the windrows in Fig. 2, was up to 12 weeks old. Thus, the organic material had spent up to 12 weeks contained within sealed PE sacks. The on-site gas analysis indicated increasingly anaerobic conditions within the sealed polyethylene sacks. Scatter diagrams of the four sets of windrow temperature profiles proved insufficient to determine if a significant linear relationship existed between the temperatures recorded in the control windrow and those recorded in the windrow containing the PE. Correlation analysis was therefore used to determine the strength of the relationship between the temperatures over time. Correlation analysis is a measure of the degree of association between two variables. The following correlation coefficients for the four sets of windrows were determined, Windrows [1], 0.672; Windrows [2], 0.929; Windrows [3], 0.686; Windrows [4], 0.668. The correlation coefficients for the four windrows show that there was a relationship between the temperatures recorded in the control windrows and the windrows containing the shredded degradable PE. Regression modelling analysis was then applied to the data using the statistical package, GENSTAT 6. Regression analysis is the process of constructing a mathematical formula that can be used for predicting the value of
Table 2 Chemical analysis of degradable PE sacks Determinand
Average bag value
Units
Copper in solution Cobalt in solution Other elements
35 35 None at significant levels
mg/kg mg/kg
Table 3 Chemical analysis of finished compost before and after commercial screening Determinand
Unscreened
Screened
Units
Dry matter Total nitrogen Total carbon C:N ratio Total copper Total zinc Total lead Total cobalt
58.3 0.9 10.6 12.1 32.4 97.4 57.4 3.48
49.9 1.61 20.9 13.1 42.8 139 101 3.31
% % w/w %w/w ratio mg/kg mg/kg mg/kg mg/kg
cases, the average temperature in the control windrows was higher than that experienced in the windrow containing the PE. Figs. 1 and 2 show very different temperature profiles. Fig. 1 shows two temperature profiles that comply with the British Standards InstituteÕs Specification for Composted Materials, BSI PAS 100, although the temperature in the windrow containing the PE does fall-off quicker than the control windrow half-way through the composting process. The same materials were composted for both Figs. 1 and 2. The difference in the composting temperatures achieved in the control windrow and PE containing windrow for Fig. 2, is significantly larger than those in 70
60
40
30
20
Control - bins With PE bags
10
No. of Days
Fig. 1. Windrow temperature profile for set [2] – control versus pe containing windrow.
82
80
74
76
72
62
67
58
60
48
53
45
38
40
34
30
32
24
26
13
17
9
11
4
0 6
Temperature (˚C)
50
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405
80
70
Temperature (˚C)
60
50
40
30
20 Control - bins With PE bags
10
90
84
77
80
73
64
66
58
60
52
56
50
42
45
38
32
36
28
30
21
24
10
17
2
4
0
No. of Days
Fig. 2. Windrow temperature profile for set [3] – control versus pe containing windrow.
one variable on the basis of one known value of another variable. In this case, a formula for predicting the open windrow temperatures when a shredded degradable PE is introduced into the organic compost feedstock. Analysis of variance was used to choose the simplest model that adequately described the observed relationship. A straight-line relationship between the temperatures recorded in the control windrows and the windrows containing the PE was determined. This relationship was the same for all windrows, that is, the slope of the four regression lines was the same. The R2 value was 66.4%, which indicated that two-thirds of the observed variability in the relationship between the control windrows temperature and the PE containing windrows was explained. The following four equations were determined for each of the four sets of windrows (Davis et al., 2004):
CWTemp ¼ 27 þ 0:6 PEWTemp ;
ð1Þ
CWTemp ¼ 25 þ 0:6 PEWTemp ;
ð2Þ
CWTemp ¼ 33 þ 0:6 PEWTemp ;
ð3Þ
CWTemp ¼ 27 þ 0:6 PEWTemp ;
ð4Þ
where CWTemp is the temperature (°C) of the control windrow, PEWTemp is the temperature (°C) of the windrow containing the shredded degradable polyethylene. The four regression equations show that the initial compost starting temperatures within the control windrows were higher (25–33 °C) than the initial temperatures within the windrows containing the PE. The equations also indicate that the rate of temperature increase within the windrows containing the PE was significantly higher than in the control windrow. For
Table 4 Gas analysis results Week
Bag results, X10 for O2 and CO2 (%)
Average
0 (Collection week)
O2: 20.8; 20.7; 20.8; 20.5; 20.8; 20.9; 20.8; 20.7; 20.8; 20.7 CO2: 0.05; 0.08; 0.15; 0.19; 0.14; 0.52; 0.81; 0.86; 0.14; 0.72 O2: 20.9; 20.7; 20.5; 20.6; 20.4; 20.7; 20.3; 20.4; 20.4; 20.1 CO2: 0.08; 0.84; 0.91; 0.42; 0.84; 0.86; 0.92; 0.48; 0.51; 0.64 O2: 20.1; 20.2; 20.1; 19.9; 20.3; 20.5; 20.2; 20.3; 20.2; 20.0 CO2: 0.41; 0.94; 0.98; 1.10; 0.43; 0.62; 0.28; 0.48; 0.70; 0.98 O2: 20.2; 20.0; 19.8; 19.4; 20.1; 19.3; 20.7; 20.1; 20.2; 19.7 CO2: 0.71; 1.00; 0.94; 1.30; 0.26; 1.10; 0.39; 0.48; 0.74; 0.90 O2: 20.0; 19.9; 19.4; 19.6; 17.1; 19.1; 20.2; 19.0; 18.8; 19.7 CO2: 0.64; 2.00; 1.02; 1.24; 2.80; 1.43; 1.62; 1.22; 1.03; 0.74 O2: 19.4; 19.8; 20.1; 19.2; 18.7; 18.5; 18.0; 18.9; 19.1; 20.2 CO2: 0.94; 1.54; 1.86; 1.34; 1.30; 1.06; 3.88; 1.18; 2.12; 0.67
20.75 0.366 20.5 0.65 20.18 0.692 19.95 0.782 19.28 1.374 19.19 1.589
2 4 6 8 10
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every 1 °C increase in temperature in the PE containing windrow, the control windrow only increased by 0.6 °C. 3.4. Gas analysis of filled and sealed polyethylene sacks Results (Table 4) have indicated that there were substantially reduced levels of oxygen (down to 17%), increased levels of carbon dioxide (up to 4%). The levels of O2 fell gradually as the CO2 levels rose. Both of these factors corresponded with the increasing age of the sacks. There has been no trace of hydrogen sulphide to date. These findings are in line with the known theoretical processes where as organic matter degrades, the microbes consume oxygen and subsequently respire, emitting carbon dioxide. However, these results are inconclusive, particularly as in order to measure gas levels within the sack, the probe has to be pushed into the sack effectively opening it to the air and immediately affecting the results.
4. Conclusions Degradable sacks have additional complications over more traditional receptacles, such as bins and boxes. Non-degradable polymers, once mixed with organic material, can be difficult and costly to remove (Krause, 2003). Problems include (Spencer, 2003): The mechanical processes employed throughout the composting process (such as turning and screening) which can result in polymers being sheared into increasing numbers of smaller fragments. Polymers catch on moving parts of mechanical machinery and can block air intakes, requiring manual removal. Since the densities of compost and polymer films are close, air separation technologies can be quite fussy to operate and tend to be sensitive to moisture content. The dry matter content undertaken as part of the chemical analysis has indicated that the windrows containing the PE typically have a lower proportion of dry matter than the control windrows. This is in line with the observation that the windrows containing the PE are typically wetter and so harder to move and screen than the control windrows. It is essential that the waste collection authority and treatment/disposal facility are consulted and included in any discussion on deciding collection strategy/method used to collect organic wastes. Councils need to discuss collection systems with compost processors early in this planning stage. The reasons for this are threefold. Firstly, site licenses can take over twelve months to ob-
tain and secondly, it is essential that the composting facility can deal with the quality of the organic waste being presented and that the volume of waste is within their license conditions. The type of collection strategy implemented (i.e., frequency of collection and type of collection vessel) will have a significant effect on the volume of waste collected. The sacks also have an on-going purchase and distribution cost due to their limited shelflife, in addition to increasing processing costs at the compost site. The windrows containing the shredded PE did not achieve the same elevated temperatures as those experienced within the control windrows. This is of significance because it is the higher temperatures over extended periods that effectively kills pathogens and weed seeds (the sanitisation process) that ultimately determines the quality and safety of the final compost product. Compost windrows that do not reach the desired temperatures for sanitisation should be blended with fresh feedstock and re-composted until sanitisation has been achieved or, alternatively, could be used for landfill day cover, although the British Standards InstitutionÕs Specification for Composted Materials, BSI PAS 100 does not categorically state that the compost should be disposed of. This may cause logistical problems for the site operator who must invest more man-hours and space to re-compost the organic waste and with any disposal of the compost incurring landfill charges. In addition the Local Authority would lose its right to claim the tradable permits for the weight of organic waste collected. Kerbside collected BMW from households typically has a small particle size and does not usually require shredding. This is particularly true of BMW collected using PE sacks as large bulky items tend to pierce the bags and make bag handling inconvenient. During the summer months, the particle size of the kerbside collected BMW tends to be even finer due to the high inclusion of grass clipping, making shredding undesirable as it can reduce particle size further making the organic waste more dense during the composting process which, in turn, can impede the composting process by limiting the transport of heat and oxygen within a windrow (Wang et al., 2003). Fine, wet wastes are more prone to jamming the shredder as well as increasing the mechanical damage to the shredder mechanism. It is necessary to shred the PE sacks containing the kerbside collected organic wastes despite the environmental and cost implications (Davis, 2003) because: The PE sacks are typically sealed by the householder to stop the contents escaping after filling, in storage and during the collection phase. PE has good gas barrier properties and as such, the organic waste con-
G.U. Davis / Waste Management 25 (2005) 401–407
tained within the bags becomes anaerobic, making it necessary to open the bag contents to air as quickly as possible. Shredding the bags assists with the mechanical degradation of the PE sacks. The aim of this research was to establish if degradable PE sacks were suitable for inclusion into open composting windrows with organic wastes. The monitoring undertaken has demonstrated that although PE sacks are suitable for the collection of organic wastes, they are not compostable under open windrow conditions as a determining feature of a compostable polymer is its ability to undergo biological degradation at a rate consistent with other known compostable materials, in addition to leaving no visually distinguishable residues within the final compost product (Stevens, 2002). The rate of degradation of the PE did not match the rate of degradation observed within the organic waste, where an expected volume reduction is between 40% and 60%, resulting in an actual increase in the amount of PE to organic matter over the composting period and resulting in the PE becoming increasingly visually obtrusive within the windrows. In order for a degradable sack to be successful for kerbside collection and subsequent composting of organic wastes, it must provide problem-free processing at the compost plant in addition to providing convenience and a reasonable service life to the householder. Although PE sacks are convenient to the householder and provide a stable and strong receptacle for the kerbside collection of organic wastes, the PE sacks require additional processing at the composting facility, by way of shredding and reduced size screening to remove as much of the visible PE before marketing. Rejects from the screening process comprise of the woodier organic wastes as the carbon tends to be less soluble where it is protected with lignin. These woodier wastes are blended into the ÔfreshÕ compost matter to add structure to windrows and, in time, will break down sufficiently to clear the screening process. However, the high volume of PE in the screened rejects has resulted in the rejects being landfilled. The increased processing requirement and the landfill of rejects has both a resource consumption and financial implication, essentially increasing the cost of producing what is perceived by the end consumer as an inferior compost product.
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Acknowledgements The author thank Brunel University and The Organic Resource Agency for their assistance in the completion of this research and the EPSRC for their funding.
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