Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 79:1381–1387 (online: 2004) DOI: 10.1002/jctb.1138 Effect of retention time and organic loading rate on anaerobic acidification and biogasification of dairy manure GN Demirer 1,2∗ and S Chen 2 1 Present address: Department of Environmental Engineering, Middle East Technical University, 06531, Ankara, Turkey 2 Department of Biological Systems Engineering, Washington State University, Pullman, WA 99163, USA Abstract: Vast amounts of animal manure produced from concentrated animal feeding operations have the potential to be converted into economic gain if the proper processing technology is employed. Anaerobic digestion (AD) is an effective way to convert organic wastes including animal manure into profitable by-products as well as to reduce the pollution of water, air, and soil caused by these wastes. Two-phase AD of manure offers several advantages over conventional one-phase AD. Solids reduction through hydrolysis/acidification might be very significant for animal manure which contains high amounts of solids. However, to date, studies of two-phase AD of animal manure have been limited to screened manure. Therefore, this study investigated the two-phase AD of dairy manure with particular emphasis on the effects of retention time and organic loading rate (OLR) on anaerobic acidification and biogasification of unscreened dairy manure. The results indicated that pre-acidification of dairy manure in daily-fed continuously-mixed reactors with no recycle led to significantly high reduction efficiencies of volatile solids and, thus, biogas production in the subsequent methanogenic reactor especially at OLRs of 4 – 10 g VS dm -3 day -1 . However, the extent of the stimulation in the biogas production relative to corresponding feed samples was quite variable (between 6.9 and 64.7%) for different solids retention times and OLR combinations. A relatively lower performance was observed for the high OLRs (20 – 30 g VS dm -3 day -1 ) used which was attributed to the possible wash-out of the acidifiers at the considerably low retention times (1.25–4 days) used.  2004 Society of Chemical Industry Keywords: dairy; manure; anaerobic; digestion; phase; separation INTRODUCTION Vast amounts of animal manure produced from concentrated animal feeding operations have the potential to be converted into economic gain if the proper processing technology is employed. Anaerobic digestion (AD) is an effective way to convert organic wastes including animal manure into profitable by- products as well as to reduce the pollution of water, air, and soil caused by these wastes. AD of manure can offer substantial benefits, both economic and intangible, to animal feeding operators and surrounding communities, such as on-site energy generation, production of stable, liquid fertilizer, high quality solid soil amendment, reduction in odors, and reduction in ground and surface water contaminations. AD of organic matter can be considered as a three-step process. In the first stage, complex organic materials are converted into CO 2 ,H 2 and fatty acids, mainly acetic, propionic and butyric. In the next stage, all the higher acids are converted to acetic acid. In the final stage, a valuable end-product, biogas containing mainly methane and CO 2 , is produced along two different pathways: from acetic acid (acetoclastic methanogens) and CO 2 and H 2 (H 2 - utilizer methanogens). The first group of anaerobic bacteria involved in the process comprises fermenting (acidifying or acidogenic) bacteria which perform hydrolysis and acidogenesis. Acetogenic bacteria constitute the second group and are responsible for breaking down the products of the acidification step to form acetate. In addition, H 2 and CO 2 are also produced during acetogenesis. The third group involves methanogenic bacteria which convert acetate or H 2 and CO 2 into methane. 1,2 The optimum pH of acidogenic bacteria is 5.2 – 6.5, and the specific growth rate is around 2 days. Some of the products of acidogenic bacteria, namely acetate, and H 2 , can be metabolized by methanogenic bacteria, but some others, such as propionate and butyrate, cannot. Propionate and butyrate must be further degraded to acetate and H 2 by acetogenic bacteria. These bacteria grow very slowly, with a minimum ∗ Correspondence to: GN Demirer, Department of Environmental Engineering, Middle East Technical University, 06531, Ankara, Turkey E-mail: goksel@metu.edu.tr (Received 27 April 2004; revised version received 6 July 2004; accepted 8 July 2004) Published online 14 September 2004  2004 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2004/$30.00 1381