home composting from a waste minimisation perspective peter harper centre for alternative technology...
TRANSCRIPT
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HOME COMPOSTING FROM A WASTE MINIMISATION PERSPECTIVE
Peter HarperCentre for Alternative [email protected]
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WASTE MINIMISATION?Some ambitious claims for home composting:To cause up to 50% of a households waste to disappear harmlessly into thin air even before it enters the waste streamTo reduce the potential environmental impact of a households waste by up to 85%
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COMPOSTING IN GENERAL:Why are we doing it?
Lots of good reasons, but almost certainly the most important is...The climate change implications of organic waste in landfills... ...arising largely from methane releases
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FROM A GHG PERSPECTIVEThe benefits of composting are very sensitive to local conditions and landfill practiceRelative to current average practice the benefits are very largeBut relative to hypothetical ideal practice they are small or even negativeThe transport term is negligible
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GHG EMISSIONS REDUCTIONS WITH CURRENT PRACTICEAEA/EC STUDY
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GHG EMISSIONS REDUCTIONS WITH HYPOTHETICAL BEST PRACTICE
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THIS MEANS THATFor the time being, composting of any kind is an extremely powerful contributor to GHG reductionBut this is likely to reduce over timeSo we all need to watch this spaceAnd be prepared to moderate some of our claims!And increasingly emphasise the non-climate-change benefits of composting
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CURRENTLY, IN RAW WEIGHT TERMSHousehold MSW divides into three roughly equal parts: compostables, dry recyclables and the remainder
Chart1
Chart2
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20
8
2
27
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19
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32
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Sheet1
Compostable paper5
Food waste20
Soft garden waste8
Woody wastes2
Miscellaneous27
Glass8
Recyclable plastics6
Ferrous5
Nonferrous2
Flat paper19
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ADJUSTED FOR RELATIVE GHG IMPLICATIONSGives a much larger compostable fraction
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THE QUESTION IS, WHEN AND WHERE IS WHICH LEVEL OF COMPOSTING BEST, AND HOW MUCH EFFORT DO WE MAKE ENCOURAGING EACH?
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SOME BENEFITS OF COMPOSTING AT DIFFERENT SCALES
Degree to which function is served by:
Function
Home composting
Community Composting
Centralised Composting
decrease the amount of waste collected at the kerbside that enters the waste stream,
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reduce the amount of biodegradable material entering landfills that is responsible for the generation of greenhouse gases and ammonia in landfill sites,
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decrease the amount of potentially toxic leachate produced by landfill sites reducing the risk of river, surface and ground water contamination,
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conserve landfill void space extending the life of existing landfill sites,
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produce fuel and energy savings by reducing the total amount of waste transported
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reduce environmental impacts of vehicle movements
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provide an educational role, raise public awareness of waste minimisation issues and catalyse good housekeeping
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produce a soil conditioning material suitable for use in home gardens
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Produce a material that will reduce the need for bought fertilisers and pesticides for home use
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provide an alternative to proprietary peat-based growing media and conserve natural peatland habitats
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Contribute to the closing of nutrient loops in agriculture
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Generate a product with commercial value
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Create jobs
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DOES THE WASTE HIERARCHY FAVOUR HOME COMPOSTING?Formally, yesBut the transport factor is small in GHG termsNet GHG effects from a minority of poorly-functioning heaps might be greaterThis deserves careful investigationIts going to be different in each localityDifferent arguments might apply to households with larger gardensThe educational and catalytic effect of home composting is probably undervalued
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LARGER-GARDEN HOUSEHOLDSGenerate more compostable wasteHave greater opportunity for home compostingAre likely to be more motivatedShould be specifically targeted
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A SAMPLE HOUSEHOLD WITH LARGER GARDENRaw weights
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SAMPLE HOUSEHOLDGHG-Adjusted proportions
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DIVERSION RATE ATTRIBUTABLE TO HOME COMPOSTING HAS TWO ELEMENTSThe take-up/persistence rate, or the number of households actively compostingThe proportion of total compostables that are treated on-site
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BOTH THESE ELEMENTS CAN BE DEVELOPED BY ATTENTION TOAttitudes all roundMethodsEquipmentInstitutional support
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IT IS IMPORTANT TO ESCAPE FROM TRADITIONAL GARDENING AGENDAS
The Traditional
Gardener
The Waste-minimising Householder
Is trying to make very high quality compost at (almost) any cost
Is trying to reduce environmental impact of household waste
Is prepared for a complex task with regular monitoring and intervention
Wants something simple, self-regulating, with minimum intervention
Uses intensive batch processes
Would prefer low-profile continuous processes
Focuses largely on garden wastes
Focuses on kitchen and household wastes, but ideally wants to process all organic wastes
Will combine on-site materials with others brought in from outside if necessary
Logically, will only uses resources arising in the house and garden
Favours classical high-temperature microbial composting
Would find a low-temperature method more appropriate
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TWO NEWISH CONTINUOUS-PROCESS, LOW-TEMPERATURE METHODSThe High Fibre system for soft wastesThe Slow Stack system for other wastes
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COMPARISON OF THREE SYSTEMS
CLASSICAL SYSTEM
HIGH FIBRE SYSTEM
SLOW STACK SYSTEM
Characteristic
HOT
COOL (Ambient)
Speed
Superfast
Fast in summer
Moderately fast in winter
Moderate to Slow
Principal organisms
Fungi and bacteria
Thermophilic species
Invertebrates and microbes, mesophilic and psychrotrophic species
Size
Minimum 700 l?
Any size but larger better
Large size favoured >1kl
Aspect ratio height to width
1
1
Process
Batch
Continuous feed with periodic harvesting
Cycle length
~2 months
~1year
2-3 years
Weed seed kill
Effective through heating
Ineffective, but none introduced
Effective through time
Feedstock
Soft wastes with modest amount of shredded or semi-woody waste
Soft wastes, mostly household including paper, plus grass clippings
All garden wastes, adventitious soils and other media
Purpose
To generate weed-free, fine compost in a short time
Process soft wastes with minimum effort; generate fine compost
Process woody and problem wastes with minimum effort
Management
Rigorous, critical
Feed and forget
Feed and forget
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Kitchen waste
Grass clippingsHigh Fibre methodFast, fine compost
Garden wastes:WeedsHedge trimmingsPruningsAutumn leavesSpent compostsTurf and sodsTwigsSlow Stack methodSlow, coarse compost,
Urine(optional!)Heavy woody wasteExport to CA site for Thermophilic compostingLandfill Energy-from- waste
Non-recyclable paper and cardboardHabitat pileHigh-nitrogen wastesHigh-carbon wastesPrincipal pathwaysVariant pathwaysIDEALISED SYSTEM FOR HOUSEHOLD ORGANIC WASTE
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The Laurel & HardyPrinciple
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Ziggurat container setModular units can be tuned to match different household and garden sizes, and prevailing wastes
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MODELS OF INSTITUTIONAL SUPPORT
The Norwegian modelService Contract modelDistrict Nurse/Paramedics model
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NORWEGIAN MODEL
Householders offered free training courseReceive a certificate and insulated containerAdvice on siting and installationReceive rebate on local tax
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SERVICE CONTRACT MODEL
Container provided on leaseDemonstration and set-upHelp-line, call-out serviceAnnual inspection, finished compost removed, new cycle set upHouseholders keep or sell/donate/pay for removal of compost
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DISTRICT NURSE/PARAMEDICS MODEL
Compost District Nurse sets up training workshops and recruits local volunteers (paramedics)District Nurse does home visits but gradually transfers functions to volunteersContainers provided cheaplyHelp Line Aim is to foster a permanent culture of waste separation and home treatment
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CONCLUSIONSHome composting has large potential under the right conditionsIt is seriously under-performing and needs further optimisationHouseholds with larger gardens should be especially targetedInstitutional support is critical in the first year