Poultry manure is an ammonia-rich substrate due to its high content of proteins and amino acids. Ammonia is the major inhibitor of anaerobic digestion (AD) process, affecting biogas production and causing great economic losses to the biogas plants. In this study, the effect of different natural zeolite dosages on the mesophilic AD of poultry manure inoculated with a non-acclimatized to ammonia inoculum (dairy manure) was investigated. Additionally, a comparative analysis was performed between the data extracted from this study and the results of a previous study, which has been conducted under the same experimental conditions but with the use of ammonia acclimatized inoculum (swine manure). At 5 and 10 g zeolite L(-1), the methane yield of poultry manure was 43.4% and 80.3% higher compared with the experimental set without zeolite addition. However, the ammonia non-acclimatized inoculum was not efficient in digesting poultry manure even in the presence of 10 g zeolite L(-1), due to low methane production (only 39%) compared with the maximum theoretical yield. Finally, ammonia acclimatized inoculum and zeolite have demonstrated a possible 'synergistic effect', which led to a more efficient AD of poultry manure. The results of this study could potentially been used by the biogas plant operators to efficiently digest poultry manure.

A b s t r a c t. Poultry manure is an ammonia-rich substrate that inhibits methanogenesis, causing severe problems to the anaerobic digestion process. In this study, the effect of different natural zeolite concentrations on the mesophilic anaerobic digestion of poultry waste inoculated with well-digested swine manure was investigated. A significant increase in methane production was observed in treatments where zeolite was added, compared to the treatment without zeolite. Methane production in the treatment with 10 g dm -3of natural zeolite was found to be 109.75% higher compared to the treatment without zeolite addition. The results appear to be influenced by the addition of zeolite, which reduces ammonia toxicity in anaerobic digestion and by the ammonia-tolerant swine inoculum.K e y w o r d s: poultry manure, zeolite, swine manure inoculum, ammonia inhibition, anaerobic digestion INTRODUCTIONPoultry farming became one of the most intensive branches of livestock production in modern agriculture. The consequence of this intensive production is generation of large amounts of wastes (Sakar et al., 2009). Poultry manure contains a high fraction of biodegradable organic matter and therefore their disposal without treatment causes adverse environmental impacts (Ogunwande et al., 2008). Among the different methods that have been used to treat poultry wastes, anaerobic digestion has been recommended by many authors (Ardic and Taner, 2005;Yilmazel and Demirer, 2011). Anaerobic digestion is a widely applied method in which anaerobic bacteria and archaea degrade complex organic material to biogas (Angelidaki et al., 2011). However, poultry manure is a substrate rich in total ammonia (ammonium + free ammonia) which, in high concentrations, is a well-known inhibitor of methanogenesis. Ammonia is produced by the biological degradation of nitrogenous matter, mostly in the form of proteins (Chen et al., 2008). It has been established that ammonia toxicity is mainly caused by the free ammonia concentration in the substrate (Chen et al., 2008). According to the literature, free ammonia concentrations above 100 mg NH 3 -N dm -3 are able to inhibit unacclimated methanogenic cultures to elevated ammonia levels (Kotsopoulos et al., 2008). The free ammonia concentration in a substrate is mainly affected by pH, temperature, the total ammonia concentration, and the total gas pressure (Bonmatí and Flotats, 2003;Nielsen and Angelidaki, 2008). Finally, it is generally accepted that when the total ammonia exceeds 4 g NH 4 + -N dm -3 , methanogenesis is inhibited regardless of free ammonia levels (Nakakubo et al., 2008). A common solution to eliminate ammonia inhibition during the anaerobic digestion of poultry manure is the dilution of the substrate to 0.5-3.0% of total solids (Bujoczek et al., 2000). However, the final large amount of wastes to be treated renders this solution as economically unattractive. An alternative way to avoid ammonia toxicity is the addition of clay mineral compounds, such as bentonite and natural zeolite. Zeolit...