Temperature-phased Anaerobic Digestion System Research Articles

Effect of varying solids concentration and organic loading on the performance of temperature phased anaerobic digestion process.

The effect of varying total solids (TS) and volatile solids (VS) concentrations and organic loading on the performance of a temperature phased anaerobic digestion (TPAD) system treating a mixture of primary and waste activated sludges was evaluated. An optimum volatile solids destruction of 61.5% occurred at a feed concentration of 4.9% (corresponding to 3.8% VS concentration) in the system operated at a total detention time of 20 days. At a total solids concentration of 7.9% (5.8% VS concentration), the volatile solids destruction efficiency dropped to 52.5%. At all conditions (4.4 to 7.9% TS) the TPAD system was able to meet the requirements for Class A biosolids, including those for fecal coliform and volatile solids destruction. The effluent fecal coliform concentration never exceeded 628 most probable number (MPN)/g TS. Thermophilic biomass activity tests were run on both the thermophilic (55 degrees C) and the mesophilic (35 degrees C) sludge. Biomass from the thermophilic reactor showed much greater activity at 55 degrees C than at 35 degrees C. However, significant activity was still present when the test was run at 35 degrees C. Activity tests completed on samples from the mesophilic reactor also had high activities at 55 degrees C, sometimes equal to the activity of the thermophilic biomass. These results suggest that the bacterial consortia in the TPAD system may be temperature-tolerant and not necessarily two distinct communities with two distinct temperature regimes as had been previously assumed.

Changes in Dewatering Properties Between the Thermophilic and Mesophilic Stages in Temperature‐Phased Anaerobic Digestion Systems

Temperature‐phased anaerobic digestion (TPAD) has become increasingly appealing in recent years because of the pathogen destruction capabilities of the system. However, there has also been concern about the dewatering properties of the solids created by these systems. A laboratory study was conducted at Virginia Polytechnic Institute and State University, Blacksburg, to determine the effect of thermophilic solids retention time (SRT) on dewatering properties of wastewater solids to characterize system parameters associated with dewatering and to understand the mechanisms causing changes in dewatering properties between the thermophilic and mesophilic phases. The study showed that, while anaerobic digestion caused dewatering properties to deteriorate, the solids varied little with thermophilic SRT. Acidogenesis was essentially complete after 1.5 days. Subsequent mesophilic digestion resulted in little change to dewatering properties and modest reductions in conditioning doses, but substantial reductions in biopolymer (protein and polysaccharides) occurred. It seems that thermophilic anaerobic digestion creates or releases colloidal materials that cause dewatering to be poor and subsequent mesophilic digestion for 15 days does little to improve the dewatering properties of solids in TPAD systems.

Treatment of dairy wastewater with two-stage anaerobic sequencing batch reactor systems — thermophilic versus mesophilic operations

A two-stage anaerobic sequencing batch reactor (ASBR) system was studied in the laboratory to evaluate a temperature-phased anaerobic digestion process for dairy wastewater treatment. The temperature-phased anaerobic digestion system incorporates thermophilic digestion and mesophilic digestion processes into one system which has the potential to offer the benefits of high digestion rate, fecal bacteria destruction, process stability, and favorable economics for animal wastewater treatment. Two thermophilic (55°C)-mesophilic (35°C) systems (II and III) were evaluated against one mesophilic (35°C)-mesophilic (35°C) system (I) at two system hydraulic retention times (HRTs) (3 and 6 days) and five volatile solids (VS) loading rates (2, 3, 4, 6 and 8 g/l/day). The three systems were designed with different first- and second-stage volume ratios: 1:4 for systems I and II and 1:2 for system III. The effects of temperature configurations and volume ratios between the two stages, HRTs, and VS loading rates on the performance of the two-stage ASBR system were studied. A relatively high solid retention time (SRT) of 13–18 days was achieved at both 3- and 6-day HRTs for all three systems. The performance of each system depends on the HRT and the VS loading rate. All three systems at the 3-day HRT deteriorated when the VS loading rate reached 6 g/l/day. The total solids (TS) and volatile solids (VS) removals were similar for 3- and 6-day HRTs at the VS loading rates of 2, 3 and 4 g/l/d. For the same VS loading rate in the range of 2–4 g/l/day and the same first- to second-stage volume ratio (1:4), the VS removal and specific methane production rate of the thermophilic-mesophilic system (II) were 43.8–44.1% and 0.41–0.82 l/l/day for 3-day HRT, respectively, and 37.1–38.9% and 0.34–0.72 l/l/day for 6-day HRT, respectively, which are higher than the VS removal and specific methane production rate of the mesophilic-mesophilic system (I) (29.3–30.2% and 0.27–0.56 l/l/day for 3-day HRT, respectively; 26.1–29.1% and 0.27–0.48 l/l/day for 6-day HRT, respectively). With the thermophilic-mesophilic system, the 1:4 ratio for the first- and second-stage reactors yielded better solids removals and methane production rate than the 1:2 ratio. Both thermophilic-mesophilic systems were found to be more effective in destroying the total coliforms present in the wastewater than the mesophilic-mesophilic system. As far as solids removal, biogas production, and coliform bacteria destruction are concerned, thermophilic-mesophilic systems are recommended over the mesophilic-mesophilic systems. However, the increased heating energy requirement of the former system needs to be considered when comparing the economics of these two treatment systems.

COMPUTER SIMULATION OF A TWO-STAGE ANAEROBIC SEQUENCING BATCH REACTOR SYSTEM FOR ANIMALWASTEWATER TREATMENT

The temperature-phased anaerobic digestion system is a newly developed biotechnology for organic wastewater treatment. A thermophilic-mesophilic two-stage anaerobic sequencing batch reactor (ASBR) system has been found to be a promising animal waste treatment system for energy recovery, odor control, and fecal bacteria destruction. A dual-substrate computer simulation model was developed in this study to predict the performance of a temperature-phased ASBR system for dairy wastewater treatment under different operating conditions. The model was calibrated and validated using the experimental data obtained from three laboratory-scale two-stage ASBR systems. It was predicted that with 1 :4 first- to second-stage volume ratio, the methane produced per unit reactor volume in the first-stage reactors of the temperature-phased ASBR systems is about twice as much as the methane produced in the second-stage reactors. This pointed out the potential for modifying the volume ratios between the two stages to optimize the system design. The two-stage ASBR model can be used for further evaluation of different multistage ASBR system systems to optimize the system performance and economics.