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Post-anaerobic digestion (PAD) treatment technologies have been suggested for anaerobic digestion (AD) to improve process efficiency and assure hygenization of organic waste. Because AD reduces the amount of organic waste, PAD can be applied to a much smaller volume of waste compared to pre-digestion treatment, thereby improving efficiency. In this study, dewatered digestate cakes from two different AD plants were thermally hydrolyzed and dewatered, and the liquid fraction was recirculated to a semi-continuous AD reactor. The thermal hydrolysis was more efficient in relation to methane yields and extent of dewaterability for the cake from a plant treating waste activated sludge, than the cake from a plant treating source separated food waste (SSFW). Temperatures above 165 °C yielded the best results. Post-treatment improved volumetric methane yields by 7% and the COD-reduction increased from 68% to 74% in a mesophilic (37 °C) semi-continuous system despite lowering the solid retention time (from 17 to 14 days) compared to a conventional system with pre-treatment of feed substrates at 70 °C. Results from thermogravimetric analysis showed an expected increase in maximum TS content of dewatered digestate cake from 34% up to 46% for the SSFW digestate cake, and from 17% up to 43% in the sludge digestate cake, after the PAD thermal hydrolysis process (PAD-THP). The increased dewatering alone accounts for a reduction in wet mass of cake leaving the plant of 60% in the case of sludge digestate cake. Additionaly, the increased VS-reduction will contribute to further reduce the mass of wet cake.

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In this work, performance and microbial structure of a digestion (food waste-only) and a co-digestion process (mixture of cow manure and food waste) were studied at mesophilic (37°C) and thermophilic (55°C) temperatures. The highest methane yield (480mL/g VS) was observed in the mesophilic digester (MDi) fed with food waste alone. The mesophilic co-digestion of food waste and manure (McoDi) yielded 26% more methane than the sum of individual digestions of manure and food waste. The main volatile fatty acid (VFA) in the mesophilic systems was acetate, averaging 93 and 172mg/L for McoDi and MDi, respectively. Acetate (2150mg/L) and propionate (833mg/L) were the main VFAs in the thermophilic digester (TDi), while propionate (163mg/L) was the major VFA in the thermophilic co-digester (TcoDi). The dominant bacteria in MDi was Chlorofexi (54%), while Firmicutes was dominant in McoDi (60%). For the mesophilic reactors, the dominant archaea was Methanosaeta in MDi, while Methanobacterium and Methanosaeta had similar abundance in McoDi. In the thermophilic systems, the dominant bacteria were Thermotogae, Firmicutes and Synergistetes in both digesters, however, the relative abundance of these phyla were diferent. For archaea, the genus Methanothermobacter were entirely dominant in both TDi and TcoDi.

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Soil application of organic residues from anaerobic digestion of municipal food waste and/or sewage sludge may introduce considerable amounts of heavy metals into the environment. In a column leaching experiment, mobility and release of Cu, Ni and Zn were investigated in three contrasting soils (sand, silt, loam) fertilized with biogas digestates of different origin. The effect of commercial digestates, based on food waste and sewage sludge, was compared to that of experimental digestates based on animal manure and whey permeate with or without fish ensilage, as well as untreated manure, mineral fertilizer and an untreated control. Manure and digestates were added to the columns as fresh material at equal amounts of available nitrogen. The experiment simulated high-intensity rainfall over a period of 7 days. In general, soil treated with the commercial digestates with higher original metal content showed less environmental impact in terms of Ni, Cu and Zn leaching than that treated with experimental digestates with lower original metal content and less than when animal manure or mineral fertilizer was applied. Although effects of digestate application on metal mobility in soils were seen in conditions of extreme precipitation, the leached concentrations of metals were below limitations published by the WHO but still significantly higher than that measured for control soils.

Sammendrag

Energigjenvinningsetaten i Oslo kommune (Oslo EGE) har et biomasseprosjekt som har til formål å øke material- og energigjenvinning i Oslo. Ut fra en energibetraktning er det først og fremst papp (dampeksplodert eller oppmalt), ubehandlet bark og slam fra vannrenseanleg (VAR) som virker mest interessant. Varmebehandlet VAR bør undersøkes nærmere. Hvis den virkningen vi observerte skyldes en eller annen form for hydrolyse og repolymerisering, så kan en slik struktur kanskje være interessant i utvikling av torverstatninger. Det bør også undersøkes om bark og flis, både med og uten Cambi THP, kan ha gunstig virkning på biogassprosessen og avvanningsegenskapene til biorest/råtnerest. Det finnes også en god del litteratur som anbefaler pappkartong i blandinger til vermikompostering til produksjon av plantejord.

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Restprodukter fra fiskeoljeindustrien som bleikejord og sape inneholder en lang rekke organiske miljogifter (POP) som brytes langsomt ned i naturen, akkumulerer i marine og terrestriske naringskjeder og er giftige. I denne rapporten gis en sammenstilling av analyser av sape, bleikejord og fiskeolje fra fiskeoljeindustrien. Resultatene brukes til a beregne konsentrasjoner i biorest hvor slike restprodukter brukes. Videre gis en vurdering av miljokonsekvensene ved bruk av denne bioresten pa dyrket jord.