Anaerobic fermentation of cattle manure: modeling of hydrolysis and acidogenesis.

A mathematical model for the hydrolysis and acidogenesis reactions in anaerobic digestion of cattle manure is presented. This model is based on the premise that particulate hydrolysable fraction of cattle manure is composed of cellulose and hemicellulose that are hydrolyzed at different rates according to a surface-limiting reaction; and, that the respective soluble products of hydrolysis are utilized by acidogens at different rates, according to a two-substrate, single-biomass model. Batch experimental results were used to identify the sensitive parameters and to calibrate and validate the model. Results predicted by the model agreed well with the experimentally measured data not used in the calibration process, with correlation coefficient exceeding 0.91. These results indicate that the most significant parameter in the hydrolysis-acidogenesis phase is the hydrolysis rate constant for the cellulose fraction.

A novel process for organic acids and nutrient recovery from municipal wastewater sludge.

In this paper, a novel process for organic acids and nutrient recovery from municipal sludge was introduced and evaluated based on laboratory-scale studies. An economical estimation for its practical application was also performed by mass balance in a full-scale plant (Q=158,000 m3 d(-1)). This novel process comprises an upflow sludge blanket-type high performance elutriated acid fermenter (5d of SRT) for organic acids recovery followed by an upflow-type crystallisation (3 h of HRT) reactor using waste lime for nutrient recovery. In the system, the fermenter is characterised by thermophilic (55 degrees C) and alkaline conditions (pH 9), contributing to higher hydrolysis/acidogenesis (0.18 g VFA(COD) g(-1) VSS(COD), 63.3% of VFA(COD)/COD produced, based on sludge characteristics of the rainy season) and pathogen-free stabilised sludge production. It also provides the optimal condition for the following crystallisation reactor. In the process, the waste lime, which is an industrial waste, can be used for pH control and cation (Ca and Mg) sources for crystallisation reaction. A cost estimation for full-scale application demonstrates that this process has economic benefits (about 67 dollars per m3 of wastewater except for the energy expense) even in the rainy season.

Overview of anaerobic treatment: thermophilic and propionate implications.

Difficulties in achieving low propionate concentrations in anaerobically treated effluents are frequently reported in the literature (Ahring, 1994; Kugelman and Guida, 1989; Rimkus et al., 1982), especially at thermophilic temperatures, with concentrations as high as 1000 to 9600 mg/L sometimes produced. This paper will detail the effect of several variables on the performance of both mesophilic and thermophilic regimes. Studies concerning the effect of the following four important factors on performance are included: reactor configuration, inorganic nutrient supplementation, substrate characteristics, and the unique role of microbial consortia proximity in enhancing performance. Reactor configuration modifications, essential nutrient additions, and the importance of close microbial proximity were all found to contribute to improvement in thermophilic anaerobic digestion in all the studies. It was found that, in substrates that shunt significant amounts of the electron donor through propionate, performance was critically related to reactor optimization, with propionate removal efficiency considerably improved using intact upflow anaerobic sludge blanket granules, less so in a homogenized granule slurry blanket, and noticeably reduced even more when the completely stirred reactor configuration of homogenized granules was used. The critical importance of extremely close microbial consortia proximity in maintaining hydrogen intermediates at very low levels to efficiently convert propionate to hydrogen and acetate was demonstrated. Compared to mesophilic digestion, thermophilic digestion manifested elevated levels of propionate, except in the nonmixed reactors, which had close microbial consortia proximity. The reactor configuration with the best results was the anaerobic digestion elutriated phased treatment (ADEPT) scheme, in which the raw sludge was elutriated of its fermenting volatile fatty acids, as they are generated in a short 5- to 8-day solids retention time (SRT) in one reactor and the elutriate then metabolized by passing up through a methanogenic granule or slurry blanket (with its close microbial consortia proximity) in a separate reactor with a 20- to 50-day SRT. Loading rates and performance of the ADEPT reactor configuration were superior to the standard continuously stirred tank reactor, and ADEPT thermophilic temperatures allowed higher organic loading rates without high propionate concentrations in the effluent.

Waste lime as a potential cation source in the phosphate crystallization process.

In this study, the feasibility of waste lime as a potential cation source in phosphate crystallization process was investigated using laboratory scale up flow reactors, adopting sequencing batch type configuration. This research focused on its successful application in a novel sludge treatment process, which is comprised of a high performance fermenter (operating conditions: 55 degree C and pH 9) followed by a crystallization reactor. In the struvite precipitation test using synthetic wastewater, considerable nutrient removal (about 60%) in the form of ammonia and phosphate was observed within 0.5-1 hr of retention time, and only small amounts (< 5%) of ammonia stripping occurred naturally due to the alkaline (pH 9) characteristic of the feed substrate. By replacing the synthetic wastewater with the fermentation effluent, the optimal dosage of magnesium salt for struvite precipitation was 0.86 g Mg g(-1) P, similar to the mass ratio of the struvite. The optimal dosage of waste lime was 0.3 g 1(-1), resulting in 80 % of NH4-N and 41% of PO3-P removal, at about 3 hrs of retention time. Microscopic analysis showed that amorphous crystals were mainly observed in the settled solids with waste lime addition whereas prism-like crystals were found in the system with magnesium salt added. Mass balance analysis in full-scale model plants (Q=158,880 m(3)d(-1)) based on the present experimental results revealed that nutrient recycle loading from side stream to main liquid stream would be significantly reduced. The results of the experiment reveal that reuse of industrial waste lime in a nutrient recovery system has the various advantages such as higher economical benefits and sustainable treatment of the industrial waste.

Hydrolysis and acidogenesis of particulate organic material in mesophilic and thermophilic anaerobic digestion.

The purpose of this study was to evaluate the effect of pH and inorganic nutrient supplementations for anaerobic hydrolysis and acidogenesis of particulate organic materials at both mesophilic (35 degrees C) and thermophilic (55 degrees C) temperatures. Hydrolysis and acidogenesis of a synthetic sludge was observed in batch operation for the evaluation of the pH effect. pH was uncontrolled in one reactor and controlled at 4.5, 5.5, and 6.5 in the other three reactors at both temperatures. The greatest degree of hydrolysis and acidogenesis occurred when the pH was controlled at 6.5. The pH of the uncontrolled reactor dropped to 3.4 at both temperatures severely retarding hydrolysis and acidogenesis. Concentrations of acetic and n-butyric acids predominated with lower concentrations of propionic acid at both temperatures in all reactors. Lactic acid was produced as the earliest intermediate but as the reaction proceeded, short chain VFAs were produced as final end products with a decrease in lactic acid. The higher the pH, the earlier this trend was observed. For the controlled reactors at pH 6.5, the soluble COD production and the VSS reduction peaked in 4 days at 55 degrees C whereas it took about 11 days at 35 degrees C to obtain the same result. During the linear SCOD production period at a pH of 6.5 the hydrolysis rate of the thermophilic reactor was greater than that for mesophilic. Thermophilic conditions appeared to be more sensitive to pH than mesophilic ones for both hydrolysis and acidogenesis. Additional experiments were conducted to establish the effect of inorganic nutrient (Ca, Fe, Co, and Ni) supplementation on hydrolysis and acidogenesis at both temperatures. It has, prior to this, been assumed that only methanogenesis benefited from trace metal supplementation. However, the results demonstrated the importance of inorganic nutrient supplementation to optimize hydrolysis and acidogenesis at both temperatures.

The role of pH in the organic material solubilization of domestic sludge in anaerobic digestion.

The effect of pH on anaerobic solubilization of domestic primary sludge and activated 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). Many researches reported the serious effects of pH on the solubilization of organic materials. Thus, the aim of pH control in the reactors consisting of domestic primary and activated sludges, was the evaluation of retardation in hydrolysis/acidogenesis at low pH values. Since primary and activated sludges have different biodegradation characteristics, results were compared. Results indicated that the destruction of Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS) were better in the pH-controlled reactors. In both sludges, acetic acid was the main Volatile Fatty Acid (VFA) produced. In the pH-controlled reactors, VSS reduction was found to be 72% in about 20 days in the anaerobic digestion of activated sludge whereas for the same interval VSS reduction could only be achieved by 32% in primary sludge at 35 degrees C. When primary sludge was used as substrate, the pH-uncontrolled and the pH-controlled reactors remove VSS with a corresponding production of VFAs and Soluble Chemical Oxygen Demand (SCOD). However, production of VFAs and SCOD was ceased after 5 days in the pH-controlled reactor whereas VFAs and SCOD production continued after 5 days in the pH-uncontrolled reactor, which indicated that hydrolysis and fermentation did not complete and continued longer. On the other hand; in either the pH-uncontrolled or the pH-controlled reactor of activated sludge, VSS was not removed with a corresponding production of VFAs and Soluble Chemical Oxygen Demand (SCOD). It was apparent that solubilization was occurring, however this solubilization was not observed as VFA production. When total methane production and total COD (COD(tot)) removal were estimated using VSS removal in both types of sludges, results indicated that pH control enhanced biogas productions as well as COD(tot) removals.

Organic material solubilization of domestic primary sludge in anaerobic digestion at controlled pH.

The role of the pH in the anaerobic digestion of domestic primary sludge at mesophilic temperature was evaluated. Since solubilization of organic materials is seriously affected by pH, much research has been performed for the evaluation of significant pH influence on the production of soluble organic carbon in the acid phase. In this study, the performance of continuously stirred anaerobic reactors for the production of soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) in domestic primary sludge was investigated. In the first anaerobic reactor, the pH was not controlled (pH-uncontrolled) whereas the pH was fixed at 6.5 (pH-controlled) in the second reactor. Two sets were performed. Results of both sets indicated that, VSS was removed with a corresponding production of VFAs and SCOD in both reactors. However, production of VFAs and SCOD stopped earlier in the pH-controlled reactors. Methanogenic phase started to operate at earlier times whereas hydrolysis and fermentation were still operating in the pH-uncontrolled reactors. The process stability and efficiency of anaerobic digestion of substrates such as domestic primary sludge can be improved by anaerobic digestion elutriated phased treatment (ADEPT) in which the acid elutriation reactor and methanogenic reactor are separated. In this respect, ADEPT was introduced.

Comparative process stability and efficiency of anaerobic digestion; mesophilic vs. thermophilic.

The comparative process stability and efficiency of mesophilic (35 degrees C) and thermophilic anaerobic digestion (55 degrees C) has been evaluated for four different reactor configurations, which are: daily batch-fed single-stage continuously stirred tank reactor (CSTR), continuously fed single-stage CSTR, daily batch-fed two-phase CSTR, and daily batch-fed non-mixed single-stage reactor. The results are discussed for three periods: (1) start-up, (2) steady state, and (3) organic loading rate (OLR) increase until reactor failure (pH below 5.5). During the start-up, the single-stage CSTRs at both temperatures showed the least stability, while the non-mixed single-stage reactors reached steady state in the shortest time with relatively stable pH and low volatile fatty acid (VFA). In the case of the thermophilic non-mixed reactor, efficient removal of propionate occurred but supplementation of nutrients (Ca, Fe, Ni, and Co) was required when VFA increased. The results imply the importance of inorganic nutrients bioavailability. The comparative results of the reactor performance at steady state clearly showed the superior performance of the thermophilic non-mixed reactor with respect to lower VFA, higher gas production and volatile solids removal implying that microbial consortia proximity can alleviate the problem of poor effluent quality in thermophilic system. During the OLR increase until reactor failure, all thermophilic reactors except the thermophilic non-mixed reactor showed increases in propionate concentrations as the OLR increased, while all mesophilic reactors except the mesophilic two-phase system showed little increase in VFA concentrations. When all reactors had the same conditions with OLR increase, the continuously fed reactors showed the lowest gas production, while the non-mixed reactors showed the highest gas production at both temperatures. It is hypothesized that the non-mixing reactor configuration has closer microbial consortia proximity than others. Therefore, the results in this study indicated the importance of microbial consortia proximity. A proposed model for the effect of the distance between two syntrophic bacteria reasonably matched the data in this study.

Aerobic waste activated sludge (WAS) for start-up seed of mesophilic and thermophilic anaerobic digestion.

Since there are very limited numbers of thermophilic anaerobic digesters being operated, it is often difficult to start up a new one using sludge from an existing reactor as a seed. However, for obvious reasons it seems few attempts have been made to compare the start-up performance of thermophilic anaerobic digestion using different sources of seed sludges. The purpose of this study was to evaluate the start-up performance of anaerobic digestion using aerobic waste activated sludge (WAS) from a plant which has no anaerobic digesters and mesophilic anaerobic digested sludge (ADS) as the seed source at both mesophilic (35 degrees C) and thermophilic (55 degrees C) temperatures. In this study, two experiments were conducted. First, thermophilic anaerobic reactors were seeded with WAS (VSS = 4400 mg/L) and ADS (VSS = 14,500 mg/L) to investigate start-up performance with a feed of acetate as well as propionate. The results show that WAS started to produce CH4 soon after acetate feeding without a lag time, while ADS had a lag time of 10 days. When the feed was changed to propionate, WAS removed propionate down to below the detection limit of 10 mg/L, while ADS removed little propionate and produced little CH4. Second, in order to further compare the methanogenic activity of WAS and ADS, both mesophilic and thermophilic reactors were operated. WAS acclimated to anaerobic conditions shortly (< 5 days at both mesophilic and thermophilic) and after acclimating it produced more CH4 per unit amount of seeded VSS than ADS. WAS at mesophilic temperature biodegraded acetate at the same rate as for thermophilic. However WAS at mesophilic temperature biodegraded propionate at a much faster rate than at thermophilic. WAS as the seed source of anaerobic digestion resulted in much better performance than ADS at both mesophilic and thermophilic temperatures for both acetate and propionate metabolism.

Reactor configuration--Part II. Comparative process stability and efficiency of thermophilic anaerobic digestion.

Comparative process stability and efficiency of thermophilic anaerobic digestion (55 degrees C) has been evaluated for four different reactor configurations, which are: daily batch-fed single-stage continuously stirred tank reactor (CSIR) (TB), continuously-fed single-stage CSTR (TC), daily batch-fed two-phase CSTR (TTP), and daily batch-fed non-mixed single-stage reactor (TNMR). The results are discussed for periods, 1) start-up until steady state (Days 0-200) and 2) OLR increase from 4% solids at steady state to reactor failure by increasing solids concentration in feed and decreasing the HRT (Days 201-). During the start-up period, the TB showed the worst stability with a pH drop whenever the solids concentration in the feed was increased. Conversely the TNMR reached steady state with 4% feed solids in the shortest time with relatively stable pH and very low VFA. The superior performance of the TNMR confirms the importance of microbial consortia proximity especially for the removal of propionate. The cocktail of inorganic nutrients (Ca, Ni, Fe, and Co) was added daily into all reactors showing high VFA. In the case of the TNMR, complete removal of propionate occurred after supplementation of nutrients. The results indicated that adding nutrients stimulated gas production and facilitated removal of almost all VFA confirming the importance of inorganic nutrients bioavailability. During the long-term operation with OLR increases until reactor failure (pH below 5.5), the results show that TB failed at the lowest OLR while the TC and the TNMR reached the highest OLR. Compared with the daily batch fed reactors, the constant pH of the MC seems to be the reason why the MC reached the highest OLR. The superior performance of and the TNMR during both the start-up long-term period confirms the importance of microbial consortia proximity. Two-phase digestion showed little benefit over single stage during the start-up period and no benefit was observed during the long-term period. Additional experiments in which the reactor configuration was changed from CSTR to non-mixed reactor showed significant benefit with respect to gas production and VFA threshold concentration. This once again manifests the importance of microbial consortia proximity.

Reactor configuration--Part I. Comparative process stability and efficiency of mesophilic anaerobic digestion.

The comparative process stability and efficiency of mesophilic anaerobic digestion (35 degrees C) has been evaluated for four different reactor configurations, which are: daily batch-fed single-stage continuously stirred tank reactor (CSTR) (MB), continuously fed single-stage CSTR (MC), daily batch-fed two-phase CSTR (MTP), and daily batch-fed non-mixed single-stage reactor (MNMR). The results are discussed for 2 periods, 1) start-up until steady state (HRT = 20 days, Days 0-200) and 2) OLR increase from 4% solids at steady state to reactor failure by increasing solids concentration in the feed and decreasing the HRT (Days 201-). During the start-up period, the MB showed the lowest stability with a pH drop whenever the solids concentration in the feed was increased. Conversely the MNMR reached steady state with 4% feed solids inthe shortest time with a relatively stable pH and low VFA. The superior performance of the MNMR confirms the importance of microbial consortia proximity especially for the removal of propionate. A cocktail of nutrients (Ca, Ni, Fe, and Co) was added daily into the reactors which showed high VFA except the MNMR which did not exhibit the elevated VFA. The results indicated that adding nutrients stimulated gas production and facilitated removal of almost all VFA except propionate confirming the importance of inorganic nutrients bioavailability. During the long-term operation with OLR increase until reactor failure (pH below 55), the results show that the MC reached the highest OLR while the MNMR failed first. Compared with the daily batch fed reactors, the constant pH of the MC seems to be the reason why the MC reached the highest OLR. Considering the best performance of the MNMR during the start-up period, the failure of the MNMR first is hypothesized to be due to the lack of trace nutrients at high OLR. Overall, two-phase digestion showed no benefit over single stage and all reactors demonstrated comparable VS removal percentage. Additional experiments in which the reactor configuration was changed from CSTR non-mixed reactor showed significant benefit with respect to gas production and lowered propionate threshold concentration. This once again manifests the importance of microbial consortia proximity with respect to the degradation of propionate.

Full scale UASB reactor performance in the brewery industry.

In this paper the 7 year experience of the Oriental Breweries, located in Kumi, Korea utilizing a full-scale upflow anaerobic sludge blanket (UASB) reactor for the anaerobic pretreatment of brewery wastewater is presented. The anaerobic pretreatment system selected has successfully achieved the desired treatment efficiency for the brewery wastewater during that period and it has also continued operation even with low wastewater concentrations (average CODcr 1,400 mg l-1) and lower flow rates than specified by the design parameters. The CODcr removal of the UASB reactor averaged over 80% throughout the entire period, incurring normal running expenses of only $0.20-0.31 m-3 of treated water. In addition a further economical feature of the process was the utilization of the gas digester production as the municipal gas source, reducing total operating expenses around 30 to 45% and costing the plant only $0.1 m-3. Maintenance of good granule production, which is always a key issue in operating UASB systems, was not possible by this installation, however, so frequent expensive reseeding of the reactor was often necessary due to biomass washout. The full scale and lab scale research revealed that underloading can be as detrimental as overloading, due to excessively long retention time in the UASB system for the overall operating period and to excessive pre-acidification and/or incorrect reactor configuration of the completely mixed type. To enhance the sludge granulation, therefore, the installation of a pre-acidification reactor in the UASB system treating easily biodegradable substrates such as brewery wastewater is not necessary because adequate pre-acidification can occur in the equalization tank.

Effect of process configuration and substrate complexity on the performance of anaerobic processes.

The roles of substrate complexity (molecular size of the substrate) and process configuration in anaerobic wastewater treatment were investigated to determine optimal methanogenic technology parameters. Five substrates (glucose, propionate, butyrate, ethanol, and lactate) plus a mixed waste (60% carbohydrate, 34% protein, and 6% lipids) were studied under five reactor configurations: batch-fed single-stage continuous stirred tank reactor (CSTR), continuously fed single-stage CSTR, two-phase CSTR, two-stage CSTR, and single-stage upflow anaerobic sludge blanket (UASB). The substrate feed concentration was 20,000 mg/L as COD. The solids retention time (SRT) and hydraulic retention time (HRT) in the CSTR reactors were 20 d, while HRT in the UASB was 2 d. All reactors were operated for at least 60 d (equal to 3SRT). Substrate complexity was observed to be less significant under two-phase, two-stage and UASB reactor configurations. Two-phase CSTR, two-stage CSTR, and single-stage UASB configurations yielded the lowest effluent chemical oxygen demands (130-550, 60-700, and 50-250 mg/L, respectively). The highest effluent chemical oxygen demands were detected when feeding glucose, propionate, and lactate to continuously fed single-stage CSTRs (10, 400, 9900, and 4700 mg/L COD, respectively) and to batch-fed single-stage CSTRs (11, 200, 2500, and 2700 mg/L COD, respectively). Ironically, the one stage CSTR--most commonly utilized in the field--was the worst possible reactor configuration.

Pre-acidification in anaerobic sludge bed process treating brewery wastewater.

The effect of pre-acidification on anaerobic granule bed processes treating brewery wastewater was the focus of a comparison study employing two configurations, (a) a single stage upflow anaerobic sludge bed (UASB) and (b) an upflow acidification reactor in series with a methanogenic UASB. The pre-acidification reactor achieved 20 +/- 4% SCOD removal and 0.08 +/- 0.003 L of methane produced per gram of SCOD removal at a hydraulic retention time (HRT) of 0.75-4 h. Butyric acid was not detected and short chain fatty acids (SCFAs) were mainly acetic and propionic acids. The acidification ratio was about 0.42 +/- 0.02g SCFAs as COD,g of influent COD. Both systems' critical loading rate to achieve 80% COD removal was established at 34-39kgCOD/nm3 of total sludge bed volume per day. SCOD removal efficiency of 90 +/- 3% was achieved by both systems at an organic loading rate of 25 +/- 1 kg COD/m3 of total sludge bed volume per day, indicating that the installation of an acidification reactor had no effect in terms of the maximum granular activity, biomass granulation and the settleability of granules. At an organic loading rate of 67 kg COD/m3 of total sludge bed volume per day at an HRT of 1 h, the series system outperformed the single UASB by a removal of 62 compared to 57%.

Inhibitory effects and biotransformation of acrylic acid in computer-controlled pH-stat CSTRs.

In this study, the inhibitory effects and anaerobic biotransformation of acrylic acid in computer-controlled pH-stat completely stirred tank reactors (CSTRs) with two different cultures, namely unacclimated and acrylate-acclimated acetate-enriched Methanosarcina and homogenized (crushed) granular cultures, were investigated. The microbial acclimation, influent concentration, and loading rate of acrylic acid were studied in the experiments. The experimental results revealed that methanogenic cultures at a concentration of 3200 +/- 80 mg/L as volatile suspended solids (VSS) could be acclimated to acrylic acid up to a loading rate of 220 mg/L per day (0.068 g acrylic acid/g VSS per day) in the presence of a constant acetate concentration of 2000 +/- 200 mg/L as the primary substrate after 300 days of acclimation. The same cultures (680 +/- 80 mg/L as VSS), after 80 days of acclimation to acrylic acid as the sole carbon source, transformed acrylic acid up to the loading rate of about 200 mg/L per day (0.29 g acrylic acid/g VSS per day) almost completely (>99%) to acetic and propionic acid, but could not effectively metabolize these intermediate products. Acrylate-acclimated homogenized granular cultures (6900 +/- 80 mg/L as VSS) effectively metabolized 2200 mg/L per day (0.32 g acrylic acid/g VSS per day) of acrylic acid, as the sole carbon source, after 50 days of severe inhibition.

Quantitative structure-activity relationships for chemical toxicity to environmental bacteria.

Quantitative structure-activity relationships (QSARs) were developed for nonreactive chemical toxicity to each of four groups of bacteria of importance in environmental engineering: aerobic heterotrophs, methanogens, Nitrosomonas, and Microtox. The QSARs were based on chemicals covering a range of structures and including important environmental pollutants (i.e., chlorinated and other substituted benzenes, phenols, and aliphatic hydrocarbons). QSARs were developed for each chemical class and for combinations of chemical classes. Three QSAR methods (groups of chemical describing parameters) were evaluated for their accuracy and ease of use: log P, linear solvation energy relationships (LSER), and molecular connectivity. Successful QSARs were found for each group of bacteria and by each method, with correlation coefficients (adjusted r2) between 0.79 and 0.95. LSER QSARs incorporated the widest range of chemicals with the greatest accuracy. Log P and molecular connectivity QSARs are easier to use because their parameters are readily available. Outliers from the QSARs likely due to reactive toxicity included acryls, low pKa compounds, and aldehydes. Nitro compounds and chlorinated aliphatic hydrocarbons and alcohols showed enhanced toxicity to the methanogens only. Chemicals with low IC50 concentrations (log IC50 mumol/liter less than 1.5) were often outliers for Nitrosomonas. QSARs were validated statistically and with literature data. A suggested method is provided for use of the QSARs.