Anaerobic Digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is used for industrial or domestic purposes to reduce waste and/or to release energy. Sludge production is lower from this process than from aerobic treatment (normally earlier in the treatment chain).

It is often used to treat biodegradable waste and sewage sludge, in an integrated waste management system. The digesters can also be fed with purpose-grown energy crops, such as maize. The nutrient-rich digestate produced can be used as fertiliser. In addition to the digestate, anaerobic digestion produces biogas (methane and CO2) which can be used to generate heat and/or power.

The key challenge for pulp mill waste is the potential inhibitory effect of extractives and lignin in the case of mechanical mills. These compounds can be reduced by removal prior to anaerobic treatment either by aerobic treatment; flocculation or selective removal for other uses.

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Technical Sheet: Anaerobic Digestion

Materials Accepted • Garden/greenwastes

• Foodwastes

• Farmwastes

• Sludge

• Highorganicstrengthwastewaters

Examples in Current Use

NZ

Municipalbiosolidsandhighstrengthindustrialwastessuchasdairyandmeatprocessing.

Overseas

Multiplemunicipalandagriculturalsitesbutlimitedpulpmillsites;

e.g.Tembecpulpmill,Canada

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Infrastructure and Space Requirements

Manyconfigurationsfromsimple,static,belowgroundpits(lowrate)tocompletelymixed,heated,egg-shapeddigestersconstructedaboveground.Systemcomponentsmayincludescrubbers,blowers,ICengines,generatorsetc.dependingongasqualityandenergygenerationoptions.Smallerfootprintforhighratedigesters.1-5kgVS/m3/dareusedfordigestersizing,basedonthetypeofwasteanddigesterdesign.

Capital Cost Retrofittingexistinganaerobicpondswithcoversforgascapture(lowcost)totankbasedsystems(highcost).Capitalcostestimatesvarydependingontheplantsizingandsophisticationandenergyproductionoptions.Citedvaluesfordigestersvaryfrom$600–$1500/m3.Asanexample,thenewTembecpulpmillanaerobicplant,inMataneQuebec,costUS$26millionin2012.Thisplanttreatswastewaterfromthe230,000t/yrmechanicalpulpmill.

Operating Cost Operatingcostsincludelabour,materialtransport/pumping,digestatecarting,externalnutrientandelectricitycosts.Energyplantmaintenancecostsandbiogascleaningcostsneedtobeconsideredforlargeroperations.Simplepaybackforbiogasdrivenelectricitygenerationplantsarereportedtobe2–5years.Co-digestionwithotherregionalwastematerialscouldenhancedigestionefficiencyandgasgenerationandboostrevenuefromgatefees.Integrateduseindealingwithbiologicalsludgethatisdifficulttodewaterisknowntoproduceanetgaininenergyfrombiogas.

End Product Biogas(methaneandcarbondioxide)qualityandyieldisdependentoninputs.Calorificvalueof21–25MJ/m3ofbiogas.

Digestate-theremainingsolids(digestate)canhaveanadditionalnutritionalvalue.Theenduseforthisdigestatedependsonthenutritionalvalueandcontaminantleveloftheinputs.

Operating capacity e.g. viable at low tonnage

Viableatnearlyanytonnage.Largescaledigestersareinoperationformunicipalwastewatertreatmentplantsthattreatverylargevolumesofwastewaterperday.

Potential consenting issues

• Odourandenvironmentaleffects(wastemanagement,pollutionprevention)mayneedconsenting.

• Wastestorageanddigestatedischargearealsoofinteresttoconsentingauthorities.

• Processsafety

Technology Risk Aqualitativeassessmentofthelikelihoodoffailureoftheoptionorscenarioduetoissuesrelatedtothetechnologicalsolutione.g.equipmentfailure,unabletoachieveoutputstandards

1=highrisk,limitedexamplesofpulpsludgedigestioninCanada.TechnologynotprovenforpulpmillsolidsinNZ

Commercial Risk Noofsuppliers

Rangeofinputmaterials

Aqualitativeassessmentofthelikelihoodoffailureoftheoptionorscenarioduetoissuesrelatedtothecommercialarrangementse.g.supplierunabletomaintainoperations,increaseincostofprocess,transportorongoingsitemanagementexceedthoseabletobereasonablerecoveredorthoseforcomparableoptions.

1=highrisk,notproventechnologyandwouldneedanewsupplierorinvestmentbypulpmill

Market Risk Aqualitativeassessmentofthelikelihoodoffailureoftheoptionorscenarioduetoissuesrelatedtothe‘market’fortheproducte.g.ausefortheproductcannotbefoundduetoconcernsabouttracecontaminants.

3=lowrisk,multiplesecuremarketsforproduct;easymarketforheatandpowerbutriskymarketfordigestate.

ThisPulp&PaperSolidWasteTechnologyAssessmentwassupportedbytheMinistryfortheEnvironment’sWasteMinimisationFund.

CONTACT

KimMcGrouther073435772kim.mcgrouther@scionresearch.com

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