ninablahut
commented
6 years ago https://link.springer.com/article/10.1007%2Fs11356-017-0457-5
One of the main drawbacks of UASB digesters is that there is long initial start-up period before steady-state conditions are achieved; this time is usually in the range of several months. There are methods to decrease startup time such as increasing load, optimizing temperature for anaerobic digestion, using a sufficient amount of sludge, and adding metal or an inert carrier. There have been several studies to measure the aforementioned methods’ abilities to speed up UASB start-up time, but it is difficult to meter the real effectiveness of each strategy because studies focusing on the various strategies used different UASB operating conditions, so they lack versatility and the UASBs cannot be used in a short time. The focus of this study was to measure the effects of FeCl3 and zero-valent iron on UASB start-up. The study was carried out as follows: three 6.6 liter UASB reactors were set up and operated under identical conditions, except that zero-valent iron was added to the second reactor, and FeCl3 was added to the third. The concentrations of Fe0 and Fe3+ in the second and third reactors were 20 mg/L. Then, the time it took each reactor to reach steady-state was measured. Steady-state is defined as a COD removal efficiency above 80% and VFA content below 300 mg/L; the study implemented a kinetic model evaluate the reactors’ respective stabilities. According to the analysis of kinetic model, the start-up time of the UASB reactor with FeCl3 was 48 hours faster than that of the UASB with ZVI reactor. The reactor with added FeCl3 reached COD removal efficiency reached 80.2% at 264 hours. The reactor with ZVI had a COD removal efficiency of 83.8% at 312 hours Meanwhile, the control UASB reactor start-up time lagged behind with COD removal efficiency at 45% at 240 hours. ZVI and FeCl3 sped up the UASB reactor start-time because iron ions are beneficial to anaerobic digestion process. Iron from both compounds donates an electron to “promote the organic pollutant unsaturated chemical bond breakage and the refractory macromolecular organic matter decomposed into small molecules that are easily degradable by microorganisms” In terms of optimizing UASB pH levels, ZVI was superior to FeCl3. The ideal pH for a UASB is between 6.5 to 7.5; the ZVI reactor showed more neutral conditions than FeCl3 reactor because FeCl3 can be hydrolyzed in the reactor, which increases reactor acidity. Moving on to the effect of each iron ion on final particle size, the FeCl3 tank had larger final floc size than ZVI tank due to the of flocculation of FeCl3. The study found that the addition of iron was not beneficial to the UASB reactors after the start-up stage. As running time increased and ZVI and FeCl3 were continuously added to the reactors, iron ions accumulated. High concentration of iron ions in each reactor reduced anaerobic bacteria activity since Fe2+ and H2O2 or Fe3+ and H2O2 can can react to form cytotoxic free hydroxyl radical, which causes the death of anaerobic bacteria. With that, the study concluded that the addition of FeCl3 is most beneficial during the initial stage of the UASB start-up to promote granulation while the addition of ZVI is most useful during the middle stage of reactor start-up because it improves redox ability in anaerobic systems.
IanCullings
cjmsmith
September 17, 2018
The Effect of Liquid Upward Velocity and Hydraulic Retention Time on Granulation in UASB Reactors Treating Wastewater with High Sulphate Content
This paper highlights how the process of granulation is positively impacted by high upflow velocities within the UASB reactor and low hydraulic retention times.