Minimum requirements for trace metals (iron, nickel, cobalt, and zinc) in thermophilic and mesophilic methane fermentation from glucose.

The minimum requirements for four essential trace metals were examined in both thermophilic and mesophilic methane fermentation of glucose. The minimum requirement here was defined as the amount that supports the effluent soluble chemical oxygen demand (COD) of 1000 mg/ L or less for the duration of several times the hydraulic retention time (HRT). For this purpose, completely mixed reactors were run semi-continuously at 5 days HRT, with 10 g/L substrate COD. The minimum requirements determined as milligrams per gram COD removed were 0.45, 0.049, 0.054, and 0.24 in the thermophilic system and 0.20, 0.0063, 0.017, and 0.049 in the mesophilic system for iron, nickel, cobalt, and zinc, respectively. The thermophilic system required larger amounts (2.2 to 7.8 times) of the trace metals than the mesophilic system, indicating greater assimilation by biomass and/or less bioavailability under thermophilic conditions. The results can give quantitative information on trace metal nutrition for successful methane fermentation.

Elutriated acid fermentation of municipal primary sludge.

The performance of a novel fermentation process, adopting a sludge blanket type configuration, for higher hydrolysis/acidogenesis of the municipal primary sludge was investigated under batch and semi-continuous conditions with varying pH and temperature. This acid elutriation slurry reactor provided higher system performance with a short HRT (5d) and higher acidogenic effluent quality under pH 9 and thermophilic (55 degrees C) conditions. The hydrolysis of the sludge was revealed to be significantly dependent on seasonal effects for sludge characteristics but with little impact on acidogenesis. Based on the rainy season at the optimal conditions, VFA production and recovery fraction (VFA(COD)/COD) were 0.18 g VFA(COD)/g VSS(COD) and 63%. As byproducts, nitrogen and phosphorus release were measured at 0.006 g N/g VSS(COD) and 0.003 g P/g VSS(COD), respectively. For the mass balance in a full-scale plant (Q=158,880 m(3)/d) based on the rainy season, the VFA and non-VFA (as COD) production were 3110 kg VFA(COD)/d and 1800 kg COD/d, resulting in an increase of organics of 31 mg COD/L and 20mg VFA(COD)/L and nutrients of 0.7 mg N/L and 0.3 mg P/L in the influent sewage. The economical benefit from this process application was estimated to be about 67 dollars per 1000 m(3) of sewage except for energy requirements and also, better benefits can be expected during the dry season. Moreover, the results revealed that the process has various additional advantages such as pathogen-free stabilized solids production, excellent solids control and economical benefits.

The role of pH in mesophilic anaerobic sludge solubilization.

The effect of pH on anaerobic solubilization of synthetic sludge (dog food) and domestic primary sludge was investigated and compared. Anaerobic solubilization was carried out in Continuously Stirred Anaerobic Reactors at mesophilic temperature (35 degrees C) and pH was fixed at 6.5 (pH-controlled). The aim of pH control in the reactors consisting of synthetic and primary sludge, was the evaluation of retardation in hydrolysis/acidogenesis at low pH values. Since synthetic and primary sludge have different biodegradation characteristics, the results were compared. In both sludges, acetic acid was the main Volatile Fatty Acids (VFAs) produced. Volatile Suspended Solid (VSS) reduction was found as 67% in about 20 days in the anaerobic digestion of synthetic sludge. whereas for the same interval VSS reduction could only be achieved by 32% in primary sludge at 35 degrees C in the pH-controlled reactors. When both types of sludges were used as substrates, the reactors removed VSS with a corresponding production of VFAs and Soluble Chemical Oxygen Demand (SCOD). However, in the pH-controlled reactors production of VFAs and SCOD was ceased after 5 days in primary sludge whereas VFAs and SCOD production continued after 5 days in synthetic sludge which indicated that hydrolysis and fermentation in the anaerobic solubilization reactors were not complete and continued longer. On the other hand, in the pH-controlled reactor of primary sludge, methanogenic phase could operate after 5 days of operation as hydrolysis/acidogenesis stopped.

Acetoclastic methanogenic activity measurement by a titration bioassay.

A titration bioassay, designed to accurately determine the activity of acetoclastic methanogens, is described that also allows evaluation of inhibition due to potential toxicants on the active biomass. The instrument is made of a pH-stat connected to an anaerobic batch reactor. Acetate is blended and mixed with anaerobic sludge in the reactor where a 1:1 N2 and CO2 mixture is sparged at the beginning of each test. As the acetoclastic methanogens consume acetate, the pH increase, and the titration unit adds acetic acid and keeps the pH constant. The rate of titrant addition is directly proportional to the methanogenic activity. A very useful feature of the system is its potential to operate for long periods (days) at constant pH and substrate (acetate) concentration. The theoretical background and principle of operation are described as well as some of the practical problems encountered with the use of the instrument. Estimation of kinetic constants for an anaerobic culture according to the Michaelis-Menten model is presented. Examples of inhibition by inorganics (NaCl) and chlorinated solvents (chloroform) are also given.