Influence of hydraulic retention time on anaerobic digestion of pretreated sludge

Continuous anaerobic digestion of waste activated sludge pretreated at low temperatures below 100°C increased methane generation by 30%. pH values of the digestion mixture increased, approximately from 0.3 to 0.55 by pretreatment, although its volatile fatty acids concentration was greater than the control. An abrupt increase in propionate : acetate ratio in digestion stage (e.g. from less than 1:1 to over 3.5 :1), provided a reliable indicator for impending failure.This revised version was published online in October 2005 with corrections to the Cover Date.     

Effects of solids retention time on methanogenesis in anaerobic digestion of thickened mixed sludge

When a bench-scale digester fed thickened mixed sludge was operated over an SRT range of 4-20 days, removal efficiencies for total chemical oxygen demand and volatile suspended solids declined with decreasing SRT (especially <10 days), but methanogenesis was stable for SRT as low as 5 days. Quantitative PCR analyses showed that methanogens declined steadily for SRT<10 days, with the acetate-cleaving Methanosaetaceae becoming more dominant. Clone-library analyses indicated significant shifts in bacterial population from 20 to 4 day SRT: declining Chloroflexi (28 to 4.5%) and Syntrophomonas (9 to 0%), but increasing Bacteroidetes (12.5 to 20%) and two acetogenic genera belonging to the phyla Firmicutes and Spirochaetales (6.3 to 12%). Thus, the decrease in the apparent hydrolysis constant (khyd-app) with higher SRT and the process limiting size of Methanosaetaceae with the lower SRT are proactive signs for defining rate limitation in anaerobic digestion.     

Effect of natural zeolite on methane production for anaerobic digestion of ammonium rich organic sludge

The effect of an inorganic additive on the methane production from NH(4+)-rich organic sludge during anaerobic digestion was investigated using different kinds of inorganic adsorbent zeolites (mordenite, clinoptilolite, zeolite 3A, zeolite 4A), clay mineral (vermiculite), and manganese oxides (hollandite, birnessite). The additions of inorganic materials resulted in significant NH4+ removals from the natural organic sludge ([NH4+]=1, 150 mg N/l), except for the H-type zeolite 3A and birnessite. However, an enhanced methane production was only achieved using natural mordenite. Natural mordenite also enhanced the methane production from the sludge with a markedly high NH4+ concentration (4500 mg N/l) during anaerobic digestion. Chemical analyses of the sludge after the digestion showed considerable increases in the Ca2+ and Mg2+ concentrations in the presence of natural mordenite, but not with synthetic zeolite 3A. The effect of Ca2+ or Mg2+ addition on the methane production was studied using Na(+)-exchanges mordenite and Ca2+ or Mg(2+)-enriched sludge. The simultaneous addition of Ca2+ ions and Na(+)-exchanged mordenite enhanced the methane production; the amount of produced methane was about three times greater than that using only the Na(+)-exchanged mordenite. In addition, comparing the methane production by the addition of natural mordenite or Ca2+ ions, the methane production with natural mordenite was about 1.7 times higher than that with only Ca2+ ions. The addition of 5% and 10% natural mordenite were suitable condition for obtaining a high methane production. These results indicated that the Ca2+ ions, which are released from natural mordenite by a Ca2+/NH4+ exchange, enhanced the methane production of the organic waste at a high NH4+ concentration. Natural mordenite has a synergistic effect on the Ca2+ supply as well on the NH4+ removal during anaerobic digestion, which is effective for the mitigation of NH4+ inhibition against methane production.     

Influence of surfactants on anaerobic digestion of waste activated sludge: acid and methane production and pollution removal

The objective of this study is to summarize the effects of surfactants on anaerobic digestion (AD) of waste activated sludge (WAS). The increasing amount of WAS has caused serious environmental problems. Anaerobic digestion, as the main treatment for WAS containing three stages (i.e. hydrolysis, acidogenesis, and methanogenesis), has been widely investigated. Surfactant addition has been demonstrated to improve the efficiency of AD. Surfactant, as an amphipathic substance, can enhance the efficiency of hydrolysis by separating large sludge and releasing the encapsulated hydrolase, providing more substance for subsequent acidogenesis. Afterwards, the short chain fatty acids (SCFAs), as the major product, have been produced. Previous investigations revealed that surfactant could affect the transformation of SCFA. They changed the types of acidification products by promoting changes in microbial activity and in the ratio of carbon to nitrogen (C/N), especially the ratio of acetic and propionic acid, which were applied for either the removal of nutrient or the production of polyhydroxyalkanoate (PHA). In addition, the activity of microorganisms can be affected by surfactant, which mainly leads to the activity changes of methanogens. Besides, the solubilization of surfactant will promote the solubility of contaminants in sludge, such as organic contaminants and heavy metals, by increasing the bioavailability or desorbing of the sludge.     

Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2-20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ? 5 days, but the effluent soluble organic compounds increased significantly for SRT = 2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH₄ production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ? 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH₄ with a smaller SRT and digester size: e.g., 40% size savings with a CH₄ conversion of 0.23 g CH₄-COD/g COD_in.     

Effect of hydraulic retention time on anaerobic hydrogenesis in CSTR

The objective of this work was to evaluate the production of hydrogen in a continuous system as a function of hydraulic retention time (HRT). The intermediates accumulated and other parameters of pH, oxidation-reduction potential were quantified. The heat treatment (103 degrees C for 24 h) of the compost from a cattle dung composting facility was able to select H2-producing spores; this product was used as a seed for continuous systems. The brewery waste was used as substrate. For the eight runs with combinations of five HRTs and four pHs, the results indicate that at pH=5.5, a maximum H2 production of 47% H2 concentration, 43 ml H2/g COD(added), and 3.1 l H2/l reactor d was achieved at HRT=18 h. Nevertheless, at HRT=18 h, pH 5.5 was also the optimum pH for the maximum H2 production among four pHs evaluated from 5 to 6.5. There was a significant accumulation of volatile acid and alcohols during the entire study.     

Comparison of polyhydroxyalkanoates production by activated sludges from anaerobic and oxic zones of an enhanced biological phosphorus removal system: effect of sludge retention time

This study compared the PHAs production behavior of sludges from the anaerobic and oxic phases of an enhanced biological phosphorus removal (EBPR) system. This was accomplished by using the kinetics and stoichiometric coefficients obtained from aerobic batch tests to evaluate the performance of these two sludges. Experimental results indicated that the metabolic behavior of the sludges for PHAs production depend significantly on the operating sludge retention time (SRT) of the EBPR system. The oxic sludge with 5 days of SRT exhibited better PHAs production performance than anaerobic sludge. Conversely, the anaerobic sludge with 15 days of SRT had superior PHAs production capability compared to oxic sludge. These comparisons suggest that whether anaerobic or oxic sludge should be employed for PHAs production depends mainly on the operating SRT of the EBPR system.     

The effects of hydraulic retention time and sludge retention time on the fate of di-(2-ethylhexyl) phthalate in a laboratory-scale anaerobic-anoxic-aerobic activated sludge system

A laboratory-scale anaerobic-anoxic-aerobic (AAA) activated sludge wastewater treatment system was employed to investigate the effects of hydraulic retention time (HRT) and sludge retention time (SRT) on the removal and fate of di-(2-ethylhexyl) phthalate (DEHP). In the range from 5 to 14h, HRT had no significant effect on DEHP removal. However, longer HRT increased DEHP accumulation in the system and DEHP retention in the waste sludge. When SRT was increased from 15 to 25d, DEHP removal efficiency stayed above 96%. Compared to the removal of only 88% at SRT of 10d, longer SRT enhanced DEHP degradation efficiency. The optimal HRT and SRT for both nutrients (nitrogen and phosphorus) and DEHP removal were 8h and 15d. At these retention times, about 71% of DEHP was degraded by the activated sludge process, 26% was accumulated in the system, 2% was released in the effluent, and 1% remained in the waste sludge. The anaerobic, anoxic and aerobic reactors were responsible for 15%, 19% and 62% of the overall DEHP removal, respectively.     

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, 10 d and 5 d 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.     

Exponential model describing methane production kinetics in batch anaerobic digestion: a tool for evaluation of biochemical methane potential assays

Biochemical methane potential assays, usually run in batch mode, are performed by numerous laboratories to characterize the anaerobic degradability of biogas substrates such as energy crops, agricultural residues, and organic wastes. Unfortunately, the data obtained from these assays lacks common, universal bases for comparison, because standard protocols did not diffuse to the entire scientific community. Results are usually provided as final values of the methane yields of substrates. However, methane production curves generated in these assays also provide useful information about substrate degradation kinetics, which is rarely exploited. A basic understanding of the kinetics of the biogas process may be a first step towards a convergence of the assay methodologies on an international level. Following this assumption, a modeling toolbox containing an exponential model adjusted with a simple data-fitting method has been developed. This model should allow (a) quality control of the assays according to the goodness of fit of the model onto data series generated from the digestion of standard substrates, (b) interpretation of substrate degradation kinetics, and (c) estimate of the ultimate methane yield at infinite time. The exponential model is based on two assumptions: (a) the biogas process is a two-step reaction yielding VFA as intermediate products, and methane as the final product, and (b) the digestible substrate can be divided into a rapidly degradable and a slowly degradable fraction.     

Autohydrolysis pretreatment of secondary sludge for anaerobic digestion

The increase in the number of wastewater treatment plants and the quality required for the residue produced makes it necessary to improve the efficiency of anaerobic digestion of sludge. Pretreatments of secondary sludge have shown important advantages in the elimination of volatile solids and pathogenic microorganisms from the sludge, and they have also had a positive effect on biogas production. However, such methods are associated with high operating costs. This paper shows the behavior of a autohydrolysis pretreatment, which consists of subjecting the secondary sludge to a temperature of 55 °C for 12–24 h with a limited amount of oxygen under batch operation. The pretreatment results in a high solubilization of organic matter, increasing the fluidity of the sludge and improving the biogas production. This study focuses on the evaluation of the influence of oxygen and the initial sludge concentration on the pretreatment behavior. The main results obtained showed that when autohydrolysis pretreatment was carried out for 12 h, with a high solid concentration and microaerobic conditions, the solubilization of organic matter was increased by 40%, the methane productivity was improved by 23%, and there was an overall improvement in sludge fluidity. Moreover, the energy assessment of the autohydrolysis pretreatment and anaerobic digestion system showed the energetic feasibility of this treatment method, since the increase in energy production compensates for the extra energy required to carry out the pretreatment.     

Effect of the hydraulic retention time on the anaerobic biofilm reactor efficiency applied to screened cattle waste treatment

Summary It was found that the increase of the hydraulic retention time improved the organic matter removal and the conversion of organic to ammonia nitrogen. Methane concentration in the biogas was not affected by the hydraulic retention time.The reactor in operation during one year was not clogged.     

Performance evaluation of an anoxic/oxic activated sludge system: effects of mean cell residence time and anoxic hydraulic retention time

A laboratory investigation has been undertaken to asses the effects of two operating parameters, mean cell residence time (MCRT) and anoxic hydraulic retention time (HRT), on the performance of an anoxic/oxic activated sludge system. The performance of the system was evaluated in terms of its COD, nitrogen, and biomass characteristics. An activated sludge system is capable of producing a better effluent, in terms of COD and nitrogen characteristics, when it is operated in an anoxic/oxic fashion. A longer MCRT and an adequate anoxic HRT are desirable in the operation of an anoxic/oxic activated sludge system. For the wastewater used in this investigation, the anoxic/oxic unit was capable of producing an effluent with the following characteristics when it was operated at MCRT = 20 days, total system HRT = 10 h, and anoxic HRT = 3-5 h: COD = 15 mg/L; VSS = 10 mg/L; TKN = 1.30 mg/L; NH(3) - N = 0.60 mg/L; and NO(2) + NO(3) - N = 5.0 mg/L. A uniform distribution of biomass is achievable in an anoxic/oxic activated sludge system because of the intensive recirculation/convection maintained. The provision of an anoxic zone in the aeration tank promotes a rapid adsorption of feed COD into the biomass without an immediate utilization for cell synthesis. This, in turn, results in a high microbial activity and a lower observed biomass yield in the system. A tertiary treatment efficiency is achievable in an anoxic/oxic activated sludge system with only secondary treatment operations and costs. A conventional activated sludge system can be easily upgraded by converting to the anoxic/oxic operation with minor process modifications.     

A kinetic model for anaerobic digestion of biological sludge

The principal objective of this study was the development and evaluation of a comprehensive kinetic model capable of predicting digester performance when fed biological sludge, preliminary conversion mechanisms such as cell death, lysis, and hydrolysis responsible for rendering viable biological sludge organisms to available substrate were studied in depth. The results of this study indicate that hydrolysis of the dead, particulate biomass-primarily consisting of protein-is the slowest step, and therefore kinetically controls the overall process of anaerobic digestion of biological sludge. A kinetic model was developed which could accurately describe digester performance and predict effluent quality.     

Effects of thermo-chemical pre-treatment of grass silage on methane production by anaerobic digestion

Dried grass silage (GS) was pre-treated at different NaOH loading rates (1%, 2.5%, 5% and 7.5% by volatile solids (VS) mass in grass silage) and temperatures (20 °C, 60 °C, 100 °C and 150 °C) to determine effects on its bio-degradability in terms of the hydrolysis yield and degradation of ligno-cellulosic materials for biogas production. At 100 °C and the four NaOH loadings, up to 45% of the total COD was solubilised and up to 65.6%, 36.1% and 21.2% of lignin, hemicellulose and cellulose were removed, respectively; biological methane production potentials obtained were 359.5, 401.8, 449.5 and 452.5 ml CH?/g VS added, respectively, being improved by 10-38.9% in comparison with untreated GS. VS removals following anaerobic digestion were 67.6%, 76.9%, 85.3%, 95.2% and 96.7% for untreated GS and GS treated at the four NaOH loadings, respectively. 100 °C and the NaOH loading rate of 5% is recommended as a proper GS pre-treatment condition.