Cambios institucionales coyunturales

Hamelin, et al. (2014) studied 1 tonne manure individually co-digested with six underexploited co- substrates (including maize and straw), fixing DM to 10%. Sole manure digestion was compared, showing significant savings from avoiding conventional manure spreading, stressing the importance of AD (Hamelin, et al., 2011; Meyer-Aurich, et al., 2012). Bioresidual handling was most respon- sible for positive impacts. Source-segregated manure performed best in all categories, and within global warming accounted per FU, Nm3 biogas, and t DM, respectively (-1256, -6.5, -2.7 kg CO2 eq). This is mainly owing to avoided spreading of raw manure on land (and marginal electricity), the

liquid fraction being important marginal substituent despite significant impacts from handling. Garden waste performed the second (-313, -4.2, -1.3), owing to composting avoidance, along with utilization of biogas and bioresidual. AD of household (-101, -0.8, -0.3) and commercial (-32, -0.2, -0.1) biowaste performed modest, however benefiting more when co-digested with manure com- pared to direct CHP combustion along with raw manure spreading. Varying the AD fugitive CH4

loss from 1% to 10% (only maize) increased the impact from 1018 to 1542 kg CO2 eq per FU due to

less energy utilization thus more global warming. Changing lost alternatives from INC to landfilling for the household and commercial biowaste (from -101 to -184, and -32 to 6 kg CO2 eq per FU,

respectively) indicated poor performance of the lower DM commercial biowaste as INC substrate. Similarly Vega, et al. (2014) studied source-separated manure, organic household waste, and straw compared against conventional manure management. Low nutrient content and high DM and CH4

potential made straw perform best. AD performed considerably better for fossil depletion and marine eutrophication compared to reference, while the remaining depend on co-substrates used. Household waste was worst ranked in fossil depletion due to pretreatment and digester heating. Co- substrates diverted to AD from a treatment otherwise substituting e.g. fossil marginal yield less net environmental benefit. This confirms the statements in Cherubini & Strømman (2011). Interest- ingly, climate change savings for source-separated manure compared to household waste had a net negative impact difference of factor 12 in Hamelin, et al. (2014) compared to a net positive impact of a factor four in Vega, et al. (2014), in favour of the manure. Source segregated manure performs worst in climate change when field N2O emission factor is adjusted from 2 to 6% N2O-Ntot, dou-

bling the global warming magnitude, altering the ranking as mentioned in Crutzen, et al. (2007). CH4 emission from AD and upgrading, and biomethane potential do not affect the ranking, but alter

climate change and fossil depletion. Varying NH3 only shifted the ranking of separated manure in

terrestrial acidification and marine eutrophication compared to reference, and avoiding eutrophica- tion when high NH3 field emission due to less N leaking potential. Transportation aspect was

unimportant.

Vega (2012) also compared conventional manure management to AD of manure with co-substrates from Vega, et al. (2014). The second best is the solid manure fraction, performing better than

24 baseline in all five impact categories except fresh- and marine water eutrophication, while house- hold waste performed worse than the baseline in all categories due to its connection to energy recovery from incineration. System expansion showed to be crucial for the performance. The most important hotspot occurs from GHG emissions from organic matter field application, and a prior degradation lowers these. During AD the hotspots occur from heat consumption, especially from treating more wet substrates, and fugitive CH4 emissions from upgrading. Other hotspots depend on

co-substrate chemical composition and on replaced use in current practice.

Poeschl, et al. (2012a) tested variants in the biogas system: (i) Feedstock supply, (ii) biogas produc- tion, (iii) utilization pathways, (iv) bioresidual handling. Emission level variations are significant and mainly influenced by fossil CO2 and CH4. CO2 from (i) for MSW is a factor 53 higher than

cattle manure (1.8 kg/t waste) as it besides transport includes collection and more demanding pretreatment. Food waste yields the largest net savings of CO2 (-79 kg/t), owing to the savings from

biogas utilization, followed by slaughterhouse (-61) and MSW (48), WWTP grease (-40) and cattle manure (-26). The largest saving occurred in process (iii). Two feedstock mixes treated (%): cattle manure (13/0), MSW (14/90), Food (10/6), slaughter paunch (14/4), and grease sludge (49/0), assumed CHP without external heat utilization, and direct bioresidual spreading form open storage. Respective biogas yields are (MJ/t) 1475 and 2810. Overall emissions CO2 (kg): -63 and -99; and

fossil CH4 (g): 312 and 709. (i) and (ii) cause hotspots for the residue mixes, for CO2 and CH4

respectively. Emissions from (iii) mainly relied on conversion efficiencies and potential fossil substitution from biogas. Net CO2 system emission (kg/t) was lowest for fuel cell technology with

external + internal heat recovery (-110) and CHP without external (-63), due to significant fossil savings. No utilization savings arose from biogas upgrading (162), transport upgrading (167). In (iv) there is a trade-off between loss of nutrients from digestate separation and efficient transporta- tion. 90% NMVOC is saved by transporting the solid fraction, while only 6% of SO2 from fertilizer

substitution with raw digestate is saved by the separated solid. Harnessing the biogas remained in the stored bioresidual could reduce the CH4 emission by a factor up to 14.

Pöschl, et al. (2010) found significant variation in plant energy efficiency arising from feedstock type and applied process. Primary energy input/output ratios of biogas system for the respective mixtures of 34%, 55%, and 45% with the major single shares from (ii). Energy input highly relies on e.g. industrial waste demanding pretreatment. Ratio depends on biogas yield, utilization effi- ciency, and energy value of replaced fossils. Non-agricultural residue ratios range 52% (grease) to 64% (manure). Negative energy balance is reached when transporting manure and grease 17-22 km, slaughter and food 75 km, and MSW 425 km. Energy ratios for biogas upgrade from small-scale CHP heat (replacing fossil) for internal use (1.3%) and without CHP (12%), CHP without heat export (34.1%), and vehicle fuel upgrade (8.7%), and fuel cell with short distance heat transmission (6.1%). For each variant (i) maintained the largest energy input proportion. Primary energy savings (GJ/t DM) are respectively: 650, 750, 100, 1’000, and 150. The ratio of bioresidual treatment ranges

25 between 30.5-36.4%2. The best of several options includes screw-press separation and direct

spreading, unlike decanter separation and solid composting for utilization. Drying is appropriate only when using surplus heat. The separation technology is key for energy input (handling effi- ciency). Screw-press dewatering of manure + MSW saves 50% transportation energy input within 40 km, and with decanter the respective saving is 34% and 42% within 120 km.

Poeschl, et al. (2012b) suggests that co-digesting more residues and organic waste minimizes the environmental load and energy balance. Biogas recovery from bioresidual storage reduced impacts from this process tenfold due to both reduction of GHG impact and additional energy recovery. Climate change impacts (kg CO2eq/t) are for cattle manure (-23), MSW (-53), food (-52), pomace (-

86), slaughterhouse (-51), sludge (-29). All impacts are negative for human toxicity, water depletion and fossil depletion. Impacts from (ii) per tonne of two co-digested mixtures in Poeschl, et al. (2012a), respectively (fixing CHP utilization without heat export, and direct bioresidual spreading): climate change (-61 and -120 kg CO2eq), terrestrial acidification (-0.05 and -0.16 kg SO2eq)),

marine eutrophication (0.02 and 0.03 kg N eq), and fossil depletion (-19 and -32 kg oil eq). Climate

change and depletion impacts from (iii) upgrade for grid and vehicles (fixing the former co-

digestion mix and direct bioresidual spreading: (191 and 204) and (8 and 35). From (iv) e.g. screw- press + composting and decanter + solid impacted the most on climate change (79 and 78) and largest saving from screw-press + solid/liquid + residual biogas utilization (-22). Acidification impacts are low and for fossil depletion impacts are respectively (0.1, 2, and -5.2).

Lyng, et al. (2012) developed a LCA model to identify climate hotspots in the entire biogas produc- tion value chain. Biogas from manure and household waste is suitable and substrate mixing is beneficial. Highest GHG benefit originates from biogas upgraded to replace diesel fuel when bioresidual from all the waste is spread on land. Best performance occurs from maximal biogas output, and highest impacts occur from N2O emission from several processes. This one has the

highest uncertainty.