State indicators for monitoring the anaerobic digestion process

Anaerobic process state indicators were used to monitor a manure digester exposed to different types of disturbances, in order to find the most proper indicator(s) for monitoring the biogas process. Online indicators tested were biogas production, pH, volatile fatty acids (VFA), and dissolved hydrogen. Offline indicators tested were methane and hydrogen content in the biogas. A CSTR reactor with 7.2 L working volume was operated at a varying hydraulic loading rate (HRT 10–20 days) for 200 days. During this period, the reactor was overloaded with extra organic matter such as glucose, lipid, gelatine, and bio-fibers, in order to create dynamic changes in the process state. Biogas production increased in response to the increase in organic load with a slight decrease in methane content. pH was relatively stable and did not show clear response to hydraulic load changes. However, pH changes were observed in response to extra organic load. Individual VFA concentrations were an effective indicator, with propionate persistent for the longest time after introduction of the disturbance. Dissolved hydrogen was very sensitive to the addition of easily degradable organics. However, it responded also to other disturbances such as slight air exposure which had no impact on process performance. A combination of acetate, propionate and biogas production is an effective combination to monitor this type of digesters effectively.     

Treatment of high-strength dairy wastewater in an anaerobic deep reservoir: analysis of the methanogenic fermentation pathway and the rate-limiting step

The wastewater of the largest dairy factory in Israel (Tnuva, Tel-Yosef), discharging approximately 6000 tons BOD per year, is treated in two serial, deep reservoirs (anaerobic/facultative). In this study, which focused on the anaerobic reservoir, we combined in situ measurements (over 18 months) and supporting lab experiments, in order to evaluate its efficiency and to identify the rate-limiting step of the methanogenic fermentation pathway. The anaerobic reservoir could remove above 75% of the BOD and COD all year round, but this was not enough to prevent malodors during the winter. Acetate and propionate, products of lactose fermentation, were the predominant intermediate metabolites in the reservoir and their concentrations were strongly dependent on the temperature and the organic load. The combined effects of colder winter temperatures and seasonal increase of organic load, resulted in a decreased rate of propionate oxidation and a consequent accumulation of soluble BOD and COD. Laboratory batch experiments, conducted during this season, found propionate oxidation to be the rate-limiting step in the process, characterized by a lag period preceding its degradation.     

Olive oil mill effluents as a feedstock for production of biodegradable polymers

The aim of the present paper was to study the feasibility of using olive oil mill effluents (OMEs) as a substrate in biodegradable polymer production. OMEs were anaerobically fermented to obtain volatile fatty acids (VFAs), which are the most highly used substrate for polyhydroxyalkanotes (PHAs) production. The anaerobic fermentation step was studied both without pretreatment and with different pretreatments (i.e., centrifugation, bentonite addition, and bentonite addition followed by centrifugation) and at various concentrations (28.5, 36.7 and 70.4 g CODL(-1)). During fermentation, VFA concentration was determined (7-16 g CODL(-1)) as well as the corresponding yield with respect to initial COD (22-44%). At all initial concentrations, centrifugation pretreatment (with or without previous addition of bentonite) significantly increased the final VFA concentration and yield, whereas the addition of bentonite alone had no influence. Moreover, centrifugation pretreatment led to a different acid distribution, which affected the hydroxyvalerate (HV) content within the obtained copolymer poly beta-(hydroxybutyrate-hydroxyvalerate) [P(HB-HV)]. OMEs were tested for PHA production by using a mixed culture from an aerobic SBR. Centrifuged OMEs, both with or without fermentation, were tested. PHAs were produced from both matrices, but with fermented OMEs PHA production was much higher, because of the higher VFA concentration. The initial specific rate of PHA production obtained with fermented OMEs was approximately 420 mg COD g COD(-1)h(-1) and the maximum HV content within the copolymer was about 11% (on a molar basis). The HV monomer was produced only until propionic acid remained present in the medium.     

Biological hydrolysis and acidification of sludge under anaerobic conditions: the effect of sludge type and origin on the production and composition of volatile fatty acids

New wastewater treatment processes resulting in considerably reduced sludge production and more effective treatment are needed. This is due to the more stringent legislations controlling discharges of wastewater treatment plants (WWTPs) and existing problems such as high sludge production. In this study, the feasibility of implementing biological hydrolysis and acidification process on different types of municipal sludge was investigated by batch and semi-continuous experiments. The municipal sludge originated from six major treatment plants located in Denmark were used. The results showed that fermentation of primary sludge produced the highest amount of volatile fatty acids (VFAs) and generated significantly higher COD- and VFA-yields compared to the other sludge types regardless of which WWTP the sludge originated from. Fermentation of activated and primary sludge resulted in 1.9-5.6% and 8.1-12.6% COD-yields, soluble COD (SCOD)/total COD (TCOD), in batch experiments, respectively. The COD-yields for primary, activated and mixed sludge were 19.1%, 6.5% and 21.37%, respectively, in semi-continuous experiments operated at solids retention time (SRT) of 5d and temperature of 37 degrees C. The benefit of fermentation for full-scale application was roughly estimated based on the experiments performed in semi-continuous reactors. The results revealed that even though the VFA production of primary sludge was higher compared to activated sludge, substantial amounts of VFA could be produced by fermentation of activated sludge due to the substantially higher production of activated sludge in WWTPs.