71    DETERMINATION OF OPERATING CONDITIONS
IN AN ANAEROBIC ACID-PHASE REACTOR
TREATING DAIRY WASTEWATER
B. Kasapgil, Lecturer
Department of Environmental Engineering
Istanbul University, Avcilar
Istanbul, Turkey
O. Ince, Lecturer
Department of Environmental Engineering
Istanbul Technical University
80626, Istanbul, Turkey
G.K. Anderson, Senior Lecturer
Department of Civil Engineering
University of Newcastle upon Tyne
NE1 7RU, U.K.
INTRODUCTION
Anaerobic digestion of organic material is a multistep process. Two groups of bacteria, namely
acidogenic and methanogenic bacteria, are responsible for the acidification and for the methane
formation, respectively. The growth requirements of the two groups of bacteria are rather different. In order to create optimum conditions for the process, it was first proposed to separate the
process into two phases.1 Operating variables applicable for the selection and enrichment of microbial populations in phased digesters include digester loading, hydraulic retention time (HRT),
pH, temperature, reactor design, and operating mode. By proper manipulation of these operating
parameters it is possible to prevent any significant growth of methane bacteria and at the same
time achieve the required level of acidification in the first reactor. Further enrichment of two cultures is possible by biomass recycle around each phase.
In order to increase efficiency of a two-phase anaerobic digestion system it is particularly important to optimize conditions for acid-phase digester since hydrolysis and liquefaction are usually considered to be the rate-limiting fermentation step. As volatile fatty acids (VFAs) can be
anticipated as the primary products of the first phase, pH control in the second phase may be
necessary when the buffer capacity has been exceeded. A reported study2 on the influence of pH
on the acidogenic phase was carried out, and a large increase in acetic acid at a pH greater than
6.8 (30°C) was observed. The concentration of butyric acid increased with decreasing pH. and
reached a peak at a pH of about 6.5. Contrary to these results, more acetic acid production at pH
5.2 to 5.5 than at pH 6.7 to 7.0 (35°C) was observed.3 These results cannot be directly compared
since the solids retention time was different in each case. The importance of pH in the acidogenesis of a particular waste was investigated4 and showed that acidogenesis of proteins at 30°C was
higher at a pH range of 6.5 to 8.0. Similar indications of a high pH requirement for the hydrolysis and acidogenesis of proteins at mesophilic temperatures (30°C) were reported.5
An upflow reactor was operated using glucose as a substrate at HRT between 0.4 and 20 hours
at 30"C.6 No methane was detected and the main acid produced was butyric, while the second
most important was acetic, but at substantially lower concentrations. Under similar conditions,
maximum acetic acid production was observed using sucrose as substrate (35°C) at HRT of 10
hours.3 In another study7 treating glucose at HRTs of 16 to 24 hours (35 °C), maximum acetic
acid production was measured at the lowest HRT. However, when using biomass recycle, the
amount of acetic acid concentration decreased while the amount of butyric acid increased. Al-
50th Purdue Industrial Waste Conference Proceedings, 1995. Ann Arbor Press. Inc.. Chelsea. Michigan 48118.
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