Richard E. Speece

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.

Mineral requirements for methane fermentation

Mineral deficiency appears to pose a more critical problem in the anaerobic treatment process than in the aerobic treatment process. Furthermore, the methane fermentation step of anaerobic treatment appears to be the stage most sensitive to mineral deficiency. Commonly the mineral deficiency is a trace metal, and, since the fermentation step is the terminal stage in the treatment process, a build‐up of these reactants (mainly acetate, propionate, butyrate and hydrogen) can adversely affect the overall process. A review of the literature follows concerning the mineral requirements of methane fermentation in order to facilitate a proper understanding of the anaerobic process, especially in its application to treatment of industrial waste waters that are prone to be deficient in certain minerals. Types of limiting nutrients are considered with specific attention given to nitrogen, sulfur, phosphorus and the trace metals, and the physiological role and intracellular concentration of minerals is discussed. An ...

Prediction of aqueous solubility of organic chemicals based on molecular structure

Correlations for aqueous solubility of a range of 200 environmentally relevant chemicals are derived from molecular connectivity indexes and a polarizability factor, calculated solely from molecular structure. The quality and reliability of the correlations are shown to be high enough for environmental applications, even with the minimum number of variables in the equations and without excluding any data to improve the correlations. The robustness and validity of these correlations are demonstrated by use of appropriate statistical techniques. A generalized predictive equation for aqueous solubility is recommended, which employs easily calculable molecular descriptors.

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.

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

Abstract The purpose of this study was to evaluate the effect of pH and inorganic nutrient supplementations for anaerobic hydrolysis and addogenesis of paniculate organic materials at both mesophilic (35 °C) and thermophilic (55 °C) temperatures. Hydrolysis and addogenesis 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 addogenesis 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 addogenesis. Concentrations of acetic and n‐butyric adds predominated with lower concentrations of propionic add at both temperatures in all reactors. Lactic add was produced as the earliest intermediate but as the reaction proceeded, short chain VF As were produced as final end products with a decrease in lactic add. 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 °C whereas it took about 11 days at 35 °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 addogenesis. Additional experiments were conducted to establish the effect of inorganic nutrient (Ca, Fe, Co, and Ni) supplementation on hydrolysis and addogenesis 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 addogenesis at both temperatures.

Nickel stimulation of anaerobic digestion

Abstract An acetate-enriched methanogenic culture was assayed for nutritional stimulation by nickel in combination with other inorganic and organic nutrients, i.e. iron, cobalt, yeast extract, riboflavin and vitamin B12. Acetate was automatically maintained at 2–3 g l−1 by a pH Stat system so that substrate was not limiting. In the absence of nickel, the specific acetate utilization rates were in the range of 2–4.6 g acetate g−1 VSS day−1. In the presence of nickel, this rate was as high as 10 and when both nickel and yeast extract were supplemented this rate temporarily increased to 12–15 g acetate g−1 VSS day−1 . The maximum acetate utilization rate was observed to be 51 g l−1 day−1 as compared to 3.3 g l−1 day−1 for conventional high-rate digestion. Daily phosphate additions were required to sustain these high acetate utilization rates. An acetate utilization rate of 20–30 g l−1 day−1 was maintained for over 25 days. Microscopic examination of the culture revealed a predominance of a sarcina whenever stimulation was noted.

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.

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.

A survey of municipal anaerobic sludge digesters and diagnostic activity assays

Abstract A plant site survey was made of 30 municipal wastewater treatment plants concerning the operational characteristics of their anaerobic sludge digesters. Design information, operating data and analytical data were tabulated. Samples of each sludge were then assayed to determine the residual gas production rate, the maximum potential acetate and propionate utilization rates, the 5 and 30 day biochemical methane potential (BMP5) (BMP30) and the possible limitation in bioavailability of iron, cobalt or nickel. The average solids content of the raw sludge fed to the digesters was 4. 7%. Eight of the 30 sludges showed stimulation in the gas production rate when iron, cobalt or nickel was supplemented and acetate was unlimiting. Nine of the 30 sludges showed stimulation in the gas production rate when iron, cobalt or nickel was supplemented and propionate was unlimiting. The average BMP5 was 0. 7 volumes of methane per volume of sludge. The average BMP30 was 1.9 vol CH4/vol sludge. On average 87% of the biodegradable fraction of the sludge was converted to methane during digestion. The average maximum potential acetate utilization rate (MPAUR) was 0.93 vol CH4/vol sludge-day. The average maximum potential propionate utilization rate (MPPUR) was 0.18 vol CH4 /vol sludge-day. The average H2S in the digester gas was 2200 ppm. The average digester capacity was 0.14 million gallons/million gallons per day (MG/MGD) of raw wastewater. On average the raw sludge pumping rate was 4300 gal per MG of raw wastewater. The average digester gas production was 0.066 vol gas/vol of raw wastewater (8800 ft3 per MG). The average unit gas production rate was 0. 64 volumes of digester gas per volume of digester per day. The average volume of digester gas produced per volume of raw sludge feed was 15. 8 v/v. The average gas production per pound of volatile solids added was 0. 46 m3 kg−1 VS (7. 4 ft3 lb−1 VS). The average gas production per pound of volatile solids destroyed was 0. 94 m3 kg−1 VS (15. 0 ft3 lb−1 VS).

PREDICTING HENRY'S LAW CONSTANT AND THE EFFECT OF' TEMPERATURE ON HENRY'S LAW CONSTANT

Air-water partitioning data for a wide range of organic chemicals are used to validate a three- variable quantitative structure-activity relationship (QSAR) model for Henrys law constant, H. The predictive ability of the basic model, developed from a training set of 180 chemicals, is now demonstrated on 105 new chemicals of similar molecular structure. This basic model is then extended to cover additional chemicals ef diverse molecular features. The predictive ability of the final model is demonstrated on a new testing set of 70 chemicals featuring multiple structural components and polyfunctional groups. Spanning over 10 orders of magnitude, the log H values predicted by the QSAR model for 462 compounds are found to agree with the reported experimental values with r 2 > 0.95 at p = 0.0005. A new QSAR model for estimating H as a function of temperature, T, is also proposed. The predictive ability of this H-T model is demonstrated using experimental data for 18 chemicals over a temperature range of 1(~55~C.