HOME COMPOSTING FROM A WASTE MINIMISATION PERSPECTIVE

Peter HarperCentre for Alternative Technologypeter.harper@cat.org.uk

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%

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

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

GHG EMISSIONS REDUCTIONS WITH CURRENT PRACTICEAEA/EC STUDY

GHG EMISSIONS REDUCTIONS WITH HYPOTHETICAL BEST PRACTICE

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

CURRENTLY, IN RAW WEIGHT TERMSHousehold MSW divides into three roughly equal parts: compostables, dry recyclables and the remainder

Chart1

Chart2

5

20

8

2

27

8

6

5

2

19

Chart3

4

32

10

1

14

9

5

7

9

9

Sheet1

Compostable paper5

Food waste20

Soft garden waste8

Woody wastes2

Miscellaneous27

Glass8

Recyclable plastics6

Ferrous5

Nonferrous2

Flat paper19

ADJUSTED FOR RELATIVE GHG IMPLICATIONSGives a much larger compostable fraction

THE QUESTION IS, WHEN AND WHERE IS WHICH LEVEL OF COMPOSTING BEST, AND HOW MUCH EFFORT DO WE MAKE ENCOURAGING EACH?

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,

xxx

x

reduce the amount of biodegradable material entering landfills that is responsible for the generation of greenhouse gases and ammonia in landfill sites,

xxx

xxx

xxx

decrease the amount of potentially toxic leachate produced by landfill sites reducing the risk of river, surface and ground water contamination,

xxx

xxx

xxx

conserve landfill void space extending the life of existing landfill sites,

xxx

xxx

xxx

produce fuel and energy savings by reducing the total amount of waste transported

xxx

x

reduce environmental impacts of vehicle movements

xxx

x

provide an educational role, raise public awareness of waste minimisation issues and catalyse good housekeeping

xxx

xx

x

produce a soil conditioning material suitable for use in home gardens

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xx

xx

Produce a material that will reduce the need for bought fertilisers and pesticides for home use

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xx

xx

provide an alternative to proprietary peat-based growing media and conserve natural peatland habitats

xxx

xxx

xxx

Contribute to the closing of nutrient loops in agriculture

x

xxx

Generate a product with commercial value

x

xxx

Create jobs

x

xxx

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

LARGER-GARDEN HOUSEHOLDSGenerate more compostable wasteHave greater opportunity for home compostingAre likely to be more motivatedShould be specifically targeted

A SAMPLE HOUSEHOLD WITH LARGER GARDENRaw weights

SAMPLE HOUSEHOLDGHG-Adjusted proportions

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

BOTH THESE ELEMENTS CAN BE DEVELOPED BY ATTENTION TOAttitudes all roundMethodsEquipmentInstitutional support

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

TWO NEWISH CONTINUOUS-PROCESS, LOW-TEMPERATURE METHODSThe High Fibre system for soft wastesThe Slow Stack system for other wastes

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

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

The Laurel & HardyPrinciple

Ziggurat container setModular units can be tuned to match different household and garden sizes, and prevailing wastes

MODELS OF INSTITUTIONAL SUPPORT

The Norwegian modelService Contract modelDistrict Nurse/Paramedics model

NORWEGIAN MODEL

Householders offered free training courseReceive a certificate and insulated containerAdvice on siting and installationReceive rebate on local tax

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

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

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