Anaerobic Co‐digestion of Dairy Cattle Slurry and Agro‐industrial Fats: Effect of Fat Ratio on the Digester Efficiency Sébastien Guillaume 1,2 and Thomas Lendormi 2 1. Forafrance, Rue de l'aiguillage ZA de beauséjour 35520 LA MEZIERE, France 2. Univ. Bretagne‐Sud, EA 4250 LIMATB, F‐56300, Pontivy, France The objective of the study was to optimize the co‐digestion of cattle slurry in the presence of fats from the food industry. A laboratory pilot plant operating continuously was used for the experimental study. Tests were carried out with only cattle slurry and with different fat ratio (0, 10, 25, 45 and 60 % w/w of the feed COD as fats). In the case of using cattle slurry feed and OLR from 0.4 and 3.0 kg VS Á m À3 Á day À1 (HRT between 125 days and 19 days), methane productivity versus OLR exhibit a linear relation reflecting optimum biodegradability of the organic matter. For OLR above 3 kg VS Á m À3 Á day À1 (HRT less than 19 days), a break in the slope is observed expressing a decrease of the organic matter degradation rate. For HRT equal to 30 days, the results show that fat incorporation ratio lower than 25 % w/w does not affect the digester operation while maintaining acceptable biodegradability of the fat. In contrast, with a fat ratio of 60 % w/w , the system became unstable and a significant decrease in the methane productivity was observed, due to an accumulation of undegraded Long Chain Fatty Acids (LCFA). The conclusion is that the fat ratio must be controlled to avoid the destabilization of digestion, and finally operational conditions are proposed for co‐digestion. Keywords: anaerobic digestion, dairy cattle slurry, agro‐industrial greases, LCFAs INTRODUCTION F or many years, the production of organic waste has increased due to more intensive farming practices, industrialization, treatment processes and waste collection. This waste management (livestock manure, food industry waste, etc.) has become a major environmental issue and is increasingly oriented towards valorization and recycling. In order to address these environmental issues, biological treatment and biomass valoriza- tion processes are implemented, including anaerobic digestion. Anaerobic digestion is a biological process which converts the organic matter into carbon dioxide and usable methane, a fossil fuel substitute. In this context, anaerobic digestion of organic waste has been widely developed in Europe in recent years with the objective to establish public policies to reduce energy dependency and Green House Gases (GHG) emissions, by reducing direct emissions released from waste management. Anaerobic digestion of manure is particularly interesting in relation to this objective to reduce GHG emissions in manure management. [1] In addition, manures, especially slurry, are interesting substrates due to their nutrient intake for the development of micro‐organisms responsi- ble for anaerobic digestion and their high buffering capacity to stabilize the process. However, their high dilution rate, <10 % w/w Dry Matter (DM), and limited biodegradability of organic matter (10–60 % w/w ), generate a relatively low Biochemical Methane Potential (BMP) relative to their volume (for example, 4–20 Nm 3 CH 4 Á m À3 for pig and cattle slurry). This low BMP leads to high costs for developing plants in relation to the low associated energy production. Thus, the economic viability of biogas plants requires additional substrates with higher methane potential or through co‐ digestion with one or more substrates containing a higher methane potential. [2–4] Many operators are looking for co‐substrates with high methanogen potential to increase their methane production in the digester. Industrial co‐substrates usually sought after are flotation fats from Waste Water Treatment Plant (WWTP). These lipid‐rich products are known to combine both a high methanogen potential and good organic matter biodegradability. However, adding fatty co‐substrates may have a detrimental effect on bacteria when introduced at high concentration or loading rates. [5–7] Fatty co‐substrates may induce micro‐organisms flotation phenomena and aggregate them in a hydrophobic capsule which can reduce the performance of the digester. Pereira et al. [8] have shown that the degradation of such substrate could lead to the production of Long Chain Fatty Acids (LCFA) in the digester which can disturb the anaerobic digestion process in different ways: (1) Mass transfer limitation phenomena resulting from LCFAs adsorption on micro‐ organisms can lead to a hydrophobic layer around this biomass. (2) LCFA trapping in the biomass aggregates may result in the flotation of the biomass in the reactor and washout. (3) LCFA can extract in organic droplets Volatile Fatty Acids (VFA) that are essential reaction intermediates for the production of methane. (4) LCFA precipitation with divalent ions such as Ca 2þ or Mg 2þ makes them inaccessible to the anaerobic biomass and thus reduces their biodegradability. In addition, the Carbon/Nitrogen (C/N) ratio of lipid‐rich substrates is often above the optimum values 20–30 defined by Parkin and Owen [9] which may lead to a lack of nutrients when an excess of that substrate is used. The anaerobic digestion inhibition of rich lipid substrates caused by these phenomena has been reported in the literature. Many authors have mentioned the reversible inhibition of methano- genesis and other anaerobic digestion stages by LCFA. [10–15] Accordingly, most of these studies were performed in batch and latency phenomena show different phases of the methane and *Author to whom correspondence may be addressed. E‐mail address: thomas.lendormi@univ-ubs.fr Can. J. Chem. Eng. 93:304–308, 2015 © 2014 Canadian Society for Chemical Engineering DOI 10.1002/cjce.22118 Published online 18 December 2014 in Wiley Online Library (wileyonlinelibrary.com). 304 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 93, FEBRUARY 2015