Digestion Of Wastewater Sludge Research Articles

Kinetic analysis of thermophilic anaerobic digestion of wastewater sludge

Conventional mesophilic anaerobic digesters are sometimes subject to process upset and have earned the reputation of being difficult to cope with sludge overloading. This study was conducted to examine the thermophilic process as a viable alternative. An analysis of field data from an operating mesophilic sludge digester was conducted in parallel with experimental runs on a laboratory thermophilic reactor using similar sludge. The results showed that thermophilic anaerobic digestion was a viable alternative to the mesophilic process especially for overloaded digesters in warm climates. The optimum hydraulic retention time for the thermophilic process was 10 days which can lead to substantial savings in digester capacity. A simplified kinetic model was developed and applied in the analysis of steady-state operation of thermophilic anaerobic sludge digesters.

Temperature-phased anaerobic digestion of wastewater sludges

The temperature-phased anaerobic bioreactor (TPAB) has been under study by Dague and co-workers at Iowa State University. The temperature-phased approach involves a two-stage reactor system with the first stage operated at a thermophilic temperature (commonly 55°C) and the second stage operated at a mesophilic temperature (commonly 35°C). The purpose of laboratory study was to compare the performance of the temperature-phased system with the conventional single-stage mesophilic system for treating mixtures of primary and waste activated sludge. Of particular interest in the research was a comparison of the two systems from the standpoint of coliform reduction, volatile solids (VS) destruction, and biogas production. The temperature-phased system achieved complete destruction of total and fecal coliforms over a range of SRTs from 11 to 28 days. The concentration of fecal coliforms in the effluent from the temperature-phased system never exceeded 1000 MPN/g total solids (TS), which can meet the U.S. Code of Federal Regulations, Title 40 (40 CFR), Part 503 coliform requirements for Class A sludge. At the optimal SRTs ranging from 11 to 17 days, the capacity of VS removal of temperature-phased system was more than double that of the conventional single-stage system. The foaming problem associated with the digestion of waste activated sludge was eliminated.

Temperature-phased anaerobic digestion of wastewater sludges

The temperature-phased anaerobic bioreactor (TPAB) has been under study by Dague and co-workers at Iowa State University. The temperature-phased approach involves a two-stage reactor system with the first stage operated at a thermophilic temperature (commonly 55°C) and the second stage operated at a mesophilic temperature (commonly 35°C). The purpose of laboratory study was to compare the performance of the temperature-phased system with the conventional single-stage mesophilic system for treating mixtures of primary and waste activated sludge. Of particular interest in the research was a comparison of the two systems from the standpoint of coliform reduction, volatile solids (VS) destruction, and biogas production. The temperature-phased system achieved complete destruction of total and fecal coliforms over a range of SRTs from 11 to 28 days. The concentration of fecal coliforms in the effluent from the temperature-phased system never exceeded 1000 MPN/g total solids (TS), which can meet the U.S. Code of Federal Regulations, Title 40 (40 CFR), Part 503 coliform requirements for Class A sludge. At the optimal SRTs ranging from 11 to 17 days, the capacity of VS removal of temperature-phased system was more than double that of the conventional single-stage system. The foaming problem associated with the digestion of waste activated sludge was eliminated.

A biochemical model describing volatile fatty acid metabolism in thermophilic aerobic digestion of wastewater sludge

A biochemical model was developed to explain the mechanism of volatile fatty acid (VFA) metabolism in Thermophilic Aerobic Digestion (TAD). The effects of substrate addition on the metabolic behavioral patterns of TAD biomass, from a pilot scale system, were examined. The majority of the substrates examined under batch test conditions, with TAD process biomass, under microaerobic conditions, were oxidized to an acetate intermediate. The model predicts that, under microaerobic conditions (i.e. oxygen demand of the biomass is greater than the oxygen supply of the aeration equipment), the NADH (nicotinamide adenine dinucleotide) produced during oxidation of substrates can be reoxidized by operation of the respiratory chain. Therefore, the carbon flow can be uncoupled from the necessity to maintain redox balance via fermentative means. This uncoupling would allow the organisms in a TAD process, under microaerobic conditions, to simplify metabolism and maximize ATP (adenosine triphosphate) production by increasing the flux of substrates to acetate. The oxidation of compounds, which required the net reduction of NAD +, under anaerobic batch test conditions, could not proceed and remained in the unoxidized form. The oxidation of substrates, which required no net reduction of NAD +, could and did proceed under fermentative conditions. The model predicts that, under strict anaerobic conditions (i.e. fermentative), bacteria must achieve oxidation/reduction balance by diverting the catabolic flow of carbon to fermentative end products to consume NADH (e.g. propionate). The key factor in fermentation is the conversion of specific intermediates to fermentation products by pathways which recycle NADH to regenerate NAD +. The oxidation of substrates that require the net reduction of NAD + cannot proceed under fermentative conditions.

Another look at thermophilic anaerobic digestion of wastewater sludge

ABSTRACT: New federal regulations on wastewater sludge management will be implemented in the near future and will have an impact on virtually every biosolids disposal method, including application to agricultural land. Current regulations on land application restrict land use according to the sludge treatment technology, whereas the new regulations will restrict land use based on pathogen destruction criteria. The most flexible land use practices will be allowed for biosolids with low pathogen content (Class A biosolids); for high‐temperature sludge treatment processes (greater than 53°C), indicator organism destruction can be used as a measure of pathogen destruction. The purpose of this study was to evaluate the indicator organism destruction capability of thermophilic anaerobic digestion. For the conditions studied, thermophilic digestion achieved the expected Class A limits on fecal coliforms; data are also presented on fecal streptococci destruction. Rapid startup of the thermophilic reactor (contrasting startup procedures used in the most recent full‐scale studies reported in the literature) and preliminary experiments on stability of the thermophilic population are described as well. Based on the results of this study, it appears that thermophilic anaerobic digestion can be a viable process for achieving Class A biosolids criteria.

Effect of Temperature on Anaerobic Digestion of Pulp and Paper Industry Wastewater Sludges

Mesophilic and thermophilic digestion of wastewater sludges from the forest industry were studied in semicontinuous flow reactors. Laboratory scale reactors were used for anaerobic treatment studies of a mixture of primary and secondary sludges from a CTMP mill. The VSS reductions in mesophilic and thermophilic digestion of combined CTMP sludge were 41 and 36%, respectively. Specific gas production in the mesophilic process was 0.09 m3/kgVSS added or 0.38 m3/kgVSSreduced. In thermophilic digestion the respective numbers were 0.05 and 0.13. The mesophilic digestion process was stable at an organic loading rate of 2.5 kgVSS/m3d, which represents a 15 days retention time. The supernatant quality of thermophilic digestion was poor (CODsol usually between 3000 and 4000 mg/l) compared with the supernatant of mesophilic digestion (CODsol 1000-2000 mg/l.

Determination of Chlorinated Phenols in Water, Wastewater, and Wastewater Sludge by Capillary GC/ECD

A method is described which permits the analysis of chlorinated phenols in aqueous samples at ng/l concentrations. Extraction is carried out by homogenisation with dichloromethane, clean-up by sample concentration and back extraction with alkali, and derivatisation for capillary gas chromatography/electron capture detection by extractive alkylation with pentafluorobenzoyl chloride. The method is applied to municipal wastewater treatment works effluent, digested wastewater sludge, and soil leachate and the concentrations found are reported. Eight chlorinated phenols were spiked into these samples and the mean recovery was 70% with a relative standard deviation of 7 to 15%, dependent upon sample matrix.

Bacterial survival and association with sludge flocs during aerobic and anaerobic digestion of wastewater sludge under laboratory conditions

The fate of indicator bacteria, a bacterial pathogen, and total aerobic bacteria during aerobic and anaerobic digestion of wastewater sludge under laboratory conditions was determined. Correlation coefficients were calculated between physical and chemical parameters (temperature, dissolved oxygen, pH, total solids, and volatile solids) and either the daily change in bacterial numbers or the percentage of bacteria in the supernatant. The major factor influencing survival of Salmonella typhimurium and indicator bacteria during aerobic digestion was the temperature of sludge digestion. At 28 degrees C with greater than 4 mg of dissolved oxygen per liter, the daily change in numbers of these bacteria was approximately -1.0 log10/ml. At 6 degrees C, the daily change was less than -0.3 log10/ml. Most of the bacteria were associated with the sludge flocs during aerobic digestion of sludge at 28 degrees C with greater than 2.4 mg of dissolved oxygen per liter. Lowering the temperature or the amount of dissolved oxygen decreased the fraction of bacteria associated with the flocs and increased the fraction found in the supernatant.

Methods for the isolation of virus from raw and digested wastewater sludge

Four different quantitative methods for detection of viruses in wastewater sludges were tested on raw and digested sludge. Acidic precipitation followed by elution with beef extract and alum precipitation followed by elution with tris buffer seemed equally good. Elution with beef extract also gave a good recovery and was especially usable for sludge with high amounts of total suspended solids. Direct inoculation of sludge into cell cultures often had a harmful effect on the cells. All the samples of raw sludge contained viruses and the concentration was higher than in the digested sludge. The types isolated were coxsackievirus B2 and B5, poliovirus 1, 2 and 3, and adenovirus 1 and 2. In one sample adenovirus type 2 was isolated one day after land application and in another coxsackievirus B5 was isolated 4 months after.