G. Lyberatos

Effect of temperature and ph on the effective maximum specific growth rate of nitrifying bacteria

For modeling nitrification in wastewater treatment processes it is necessary to determine the dependence of the maximum specific growth rate (μA) of nitrifying bacteria on temperature and pH. A functional relationship for the simultaneous dependence of the effective maximum specific growth rate (μA) minus the decay coefficient, bA) on temperature and pH, obtained from theoretical arguments, was verified via batch experiments with sludge from a local wastewater treatment plant. The parameters for the functional relationship were determined from the experimental data using a nonlinear regression scheme. An optimum pH of approx. 7.8 was determined and the effective maximum specific growth rate was found to be a monotonically increasing function of temperature in the range of 15–25°C.

Using cheese whey for hydrogen and methane generation in a two-stage continuous process with alternative pH controlling approaches.

This study focuses on the exploitation of cheese whey as a source for hydrogen and methane, in a two-stage continuous process. Mesophilic fermentative hydrogen production from undiluted cheese whey was investigated at a hydraulic retention time (HRT) of 24 h. Alkalinity addition (NaHCO(3)) or an automatic pH controller were used, to maintain the pH culture at a constant value of 5.2. The hydrogen production rate was 2.9+/-0.2 L/Lreactor/d, while the yield of hydrogen produced was approximately 0.78+/-0.05 mol H(2)/mol glucose consumed, with alkalinity addition, while the respective values when using pH control were 1.9+/-0.1 L/Lreactor/d and 0.61+/-0.04 mol H(2)/mol glucose consumed. The corresponding yields of hydrogen produced were 2.9 L of H(2)/L cheese whey and 1.9 L of H(2)/L cheese whey, respectively. The effluent from the hydrogenogenic reactor was further digested to biogas in a continuous mesophilic anaerobic bioreactor. The anaerobic digester was operated at an HRT of 20 d and produced approximately 1L CH(4)/d, corresponding to a yield of 6.7 L CH(4)/L of influent. The chemical oxygen demand (COD) elimination reached 95.3% demonstrating that cheese whey could be efficiently used for hydrogen and methane production, in a two-stage process.

Review of feedstock pretreatment strategies for improved anaerobic digestion: From lab-scale research to full-scale application.

When properly designed, pretreatments may enhance the methane potential and/or anaerobic digestion rate, improving digester performance. This paper aims at providing some guidelines on the most appropriate pretreatments for the main feedstocks of biogas plants. Waste activated sludge was firstly investigated and implemented at full-scale, its thermal pretreatment with steam explosion being most recommended as it increases the methane potential and digestion rate, ensures sludge sanitation and the heat needed is produced on-site. Regarding fatty residues, saponification is preferred for enhancing their solubilisation and bioavailability. In the case of animal by-products, this pretreatment can be optimised to ensure sterilisation, solubilisation and to reduce inhibition linked to long chain fatty acids. With regards to lignocellulosic biomass, the first goal should be delignification, followed by hemicellulose and cellulose hydrolysis, alkali or biological (fungi) pretreatments being most promising. As far as microalgae are concerned, thermal pretreatment seems the most promising technique so far.

Stable performance of anaerobic digestion in the presence of a high concentration of propionic acid

An automatically controlled, glucose-fed, anaerobic digester was deliberately inhibited by addition of phenol. To overcome the phenol inhibition the feed dilution rate was lowered in such a way that the methane yield from glucose was kept the same as that under normal conditions. The concentrations of acetic and butyric acids remained below 100 mg/l, however, propionic acid accumulated to 2,750 mg/l. Phenol apparently inhibited all tropic groups of organisms and it was shown that the propionic acid was formed from the metabolism of phenol. From the nature of the operating strategy, it was deduced that the digester continued to convert all the glucose that was supplied to methane showing that propionic acid accumulation did not inhibit conversion of glucose to methane. Therefore, propionic acid accumulation may be an effect and not a cause of inhibition of the anaerobic digestion process.

Hydrogen and methane production through two-stage mesophilic anaerobic digestion of olive pulp

The present study focused on the anaerobic biohydrogen production from olive pulp (two phase olive mill wastes, TPOMW) and the subsequent anaerobic treatment of the effluent for methane production under mesophilic conditions in a two-stage process. Biohydrogen production from water-diluted (1:4) olive pulp was investigated at hydraulic retention times (HRT) of 30 h, 14.5 h and 7.5 h while methane production from the effluent of hydrogenogenic reactor was studied at 20 d, 15 d, 10d and 5d HRT. In comparison with previous studies, it has been shown that the thermophilic hydrogen production process was more efficient than the mesophilic one in both hydrogen production rate and yield. The methanogenic reactor was successfully operated at 20, 15 and 10 days HRT while it failed when an HRT of 5 days was applied. Methane productivity reached the maximum value of 1.13+/-0.08 L/L/d at 10 days HRT whereas the methane yield increased with the HRT. The Anaerobic Digestion Model no. 1 (ADM1) was applied to the obtained experimental data from the methanogenic reactor to simulate the digester response at all HRT tested. The ability of the model to predict the experimental results was evident even in the case of the process failure, thus implying that the ADM1 could be a valuable tool for process design even in the case of a complex feedstock. In general, the two-stage anaerobic digestion proved to be a stable, reliable and effective process for energy recovery and stabilization treatment of olive pulp.

Exploitation of olive oil mill wastewater for combined biohydrogen and biopolymers production

The present study aimed to the investigation of the feasibility of the combined biohydrogen and biopolymers production from OMW (Olive oil Mill Wastewater), using a two stage system. H(2) and volatile fatty acids (VFAs) were produced via anaerobic fermentation and subsequently the acidified wastewater was used as substrate for aerobic biodegradable polymer production. Two different bioreactors, one of CSTR type and a SBR were used for the anaerobic and the aerobic process respectively. The anaerobic reactor was operated at different hydraulic retention times (HRTs) with OMW, diluted 1:4 (v/v) with tap water, as feed. The main VFAs produced were acetate, butyrate and propionate, in different ratios depending on the HRT. Valerate, isovalerate and isobutyrate were also detected in small quantities. Selective effluents of the acidogenic/hydrogen producing reactor were subsequently used as feed for the aerobic reactor. The aerobic reactor was inoculated with an enriched PHAs producing bacteria culture, and was operated in sequential cycles of nitrogen offer (growth phase) and nitrogen limitation (PHAs accumulation phase). The operational program of the SBR was determined according to the results from batch test, and its performance was evaluated for a period of 100 days. During the accumulation phase butyrate was consumed preferably, indicating that the dominant PHA produced is polyhydroxybutyrate. The higher yield of PHAs observed was 8.94% (w/w) of dry biomass weight.

Removal of Mn and simultaneous removal of NH3, Fe and Mn from potable water using a trickling filter

Manganese removal using a biological trickling filter was investigated. Manganese removal was found to be caused by both biological and chemical manganese oxidation. The extent of each oxidation type was assessed. The performance of the trickling filter was tested under both continuous and sequencing batch reactor operation. The effectiveness and throughput for each operational mode were determined as a function of retention time and the advantages of each operational mode were investigated. It was found that the continuous operational mode leads to higher percentage of manganese removal but lower throughput rates when compared with a sequencing batch reactor operation with the same feed concentration and retention time. A series of experiments was also performed in order to investigate the interactions between ammonia, iron and manganese removal when simultaneously present in a biological filter. For low ammonia concentrations there is no serious inhibition of manganese removal. For higher ammonia concentrations inhibition of manganese removal becomes substantial. The presence of iron affects both ammonia and manganese removal negatively, while ammonia and manganese do not significantly affect iron removal.

Monitoring and control of anaerobic reactors.

The current status in monitoring and control of anaerobic reactors is reviewed. The influence of reactor design and waste composition on the possible monitoring and control schemes is examined. After defining the overall control structure, and possible control objectives, the possible process measurements are reviewed in detail. In the sequel, possible manipulated variables, such as the hydraulic retention time, the organic loading rate, the sludge retention time, temperature, pH and alkalinity are evaluated with respect to the two main reactor types: high-rate and low-rate. Finally, the different control approaches that have been used are comprehensively described. These include simple and adaptive controllers, as well as more recent developments such as fuzzy controllers, knowledge-based controllers and controllers based on neural networks.

Partial Nitrification/Denitrification Can Be Attributed to the Slow Response of Nitrite Oxidizing Bacteria to Periodic Anoxic Disturbances

This work aims to assess and model the behavior of both ammonium (AOB) and nitrite (NOB) oxidizing bacteria during the transition from completely anoxic to aerobic conditions. An enhanced aerobically grown culture containing AOB and NOB was subjected to anoxic conditions of varying durations from 1.5 to 12 h before its exposure to aerobic conditions. Experiments were carried out in both continuously stirred tank reactor (CSTR) and batch type reactors. Although the AOB did not exhibit any impact in their performance following the anoxic disturbance, the NOB were seriously inhibited presenting a period of reduced growth rate, which was proportional to the duration of the disturbance. This finding proves the previously postulated mechanism (NOB inhibition under periodic aerobic/anoxic operation) for achieving nitrogen removal via the partial nitrification/denitrification (PND) process as demonstrated in lab- and pilot-scale operating conditions. A mathematical model was developed to describe with sufficient accuracy the performance of AOB and NOB under aerobic, anoxic, and transient conditions in both CSTR and batch type systems. The model is able to describe the inhibitory effect of anoxic exposure to NOB by assuming enzyme deactivation (under anoxic conditions) and reactivation (adjustment of the NOB enzymatic mechanism) under aerobic conditions. The presented kinetic model is quite simple and general and therefore may be used for predicting the performance of mixed growth biological systems operating via the PND process.

Kinetic modelling of pseudomonas denitrificans growth and denitrification under aerobic, anoxic and transient operating conditions

Abstract The transient growth characteristics of Ps. denitrificans to changes from anoxic to aerobic conditions and vice-versa were studied and an appropriate mathematical model was developed. This kinetic model adequately describes the behavior of the denitrifying bacterium under strictly anoxic, strictly aerobic and transient conditions of growth. Dissolved oxygen exhibited an inhibitory effect on the activity of the enzymes associated with denitrification. Each step of the denitrification pathway was affected differently by dissolved oxygen concentration. Nitrate reduction was the least sensitive step, while reduction of N 2 O and/or NO was almost completely inhibited by dissolved oxygen. Very long lag phases were observed following an anoxic to aerobic shift, whereas denitrification was immediately initiated following an aerobic to anoxic shift.