Enhancement of anaerobic digestion of actual industrial wastewaters : reactor stability and kinetic modeling.

Industrial plants pay disposal costs for discharging their wastewater that can contain pollutants, toxic organics and inorganics, to the sewer based on the Biological Oxygen Demand (BOD) or Chemical Oxygen Demand (COD) of the streams. It has become increasingly expensive for industry to meet stringent regulatory standards. One solution to reduce this cost is to anaerobically degrade the COD content, which in turn generates useful methane gas that can be used to generate useful energy or heat. Anaerobic Digestion (AD) is one of the most suitable renewable resources of conversion of industrial wastewaters to bioenergy, but it is not widely utilized in the US. As a result, this research focused on understanding and improving fundamental technical and economic obstacles such as long residence times, large reactor sizes/footprints and product quality that hamper its industrial applications in the US. Kinetic modeling of these anaerobic digestion processes is important for evaluating experimental results, predicting performance, and optimizing reactor designs, but the modeling can be especially difficult for complex wastewater compositions. Respirometry tests were first conducted to assess the impact of substrate loading on kinetic parameters during AD of three industrial/agricultural wastewaters: soybean processing WW, brewery WW, and recycled beverage WW. Results showed that the rate order statistically increased with increasing initial COD content, demonstrating that conventional kinetic modeling is inadequate for these WW of complex composition. COD degradation models revealed the Monod model gave the best overall fit to experimental data throughout the duration of the AD process, but the reactions were best fit to first-order kinetics during the first 7-9 hours and then best fit to higher order kinetics after about 8-13 hours depending on initial COD load. Expanded granular sludge bed (EGSB) reactors are two-stage continuous systems developed to reduce the residence time and footprint by expanding the sludge bed and escalating hydraulic mixing. However, higher molecular weight and slowly degrading organics, such as crude proteins and fats, cannot efficiently diffuse into the granular biomass to be digested before exiting the reactor, which limits AD efficiency. COD removal efficiency increased by up to 42% and biogas production rate by up to 32% for equivalent organic loading rates by properly manipulating COD load and feed rate. Hydrogen gas, an intermediate product generated during stage-one pre-acidification (PA), escapes the PA tank but theoretically can be captured and sent to the second stage EGSB reactor to enhance the biogas quality by biologically converting the carbon dioxide to methane. Introducing supplemental hydrogen gas in amounts less than theoretically generated in the PA tank increased energy yield by up to 42% and enhanced biogas quality by up to 20%. In addition, COD removal efficiency remained constant at ~98%, indicating that hydrogen injection did not negatively affect overall substrate removal

Emission difference between natural gas usage and digester gas usage.

It is important to burn the air toxics and harmful gases which come from water and wastewater treatment processes. In common practice, instead of natural gas, digester gas is used for economical reasons. This burning process takes place in the Regenerative Thermal Oxidizer (RTO). The current research was conducted to identify if there exists any differences between natural gas and digester gas in outlet emission. The location of the experiment was at Morris Forman Wastewater Treatment Plant of Metropolitan Sewer District (MSD) of Jefferson County, Kentucky, USA. In this experiment, the RTO was run in two cases: one fueling by digester gas and next fueling by natural gas; then samples were obtained from a sampling port during each case. The captured samples were analyzed in the laboratory at the University of Louisville, Kentucky. The resulted data from these two cases showed that there are not much emission differences between these two fuels. Therefore, use of digester gas instead of natural gas is an economical move without causing any harmful emission

The Impact of Hydraulic Retention Time and Operating Temperature on Biofuel Production and Process Wastewater Treatment

Breweries wastewater containing high concentration of organic and inorganic compounds ranks them among the top pollution generating industries. Anaerobic wastewater treatment with high organic loading rates can be achieved with lower COD strength at higher flowrates using a two-stage expanded granular sludge bed reactor. Hydraulic retention time (HRT), pH, temperature, and COD strength were varied for process optimization. Brewery wastewater with 20, 30, and 40 g COD/L as a substrate for two temperature ranges were evaluated. Under mesophilic conditions (36°C), results show COD removal efficiency (R%) and biogas production rate increased by 6% and 40% respectively as HRTs increased, maintaining a constant OLR. Results imply for equivalent OLRs, better reactor performance is achieved when running high concentration COD at slower rate compared with a lower concentration at higher rate. This implies diffusion limitation where complex proteins and fats are passed through the reactor faster than their metabolism rate in the digester. Under thermophilic conditions (50°C), results show COD removal efficiency (R%) and biogas production rate increased by 4% and 40% respectively as the HRTs increased, while maintaining a constant OLR. This implies the higher and stronger population of anaerobes are present under thermophilic condition rather than mesophilic condition

The Pre-Acidification Gas Impact on Upgrading the Biogas Produced in Expanded Granular Sludge Bed Reactor

Two-stage anaerobic reactors are being widely used in the organic waste management industry. In these reactors, up to one-third of the chemical oxygen demand (COD) content is naturally pre-acidified in a first stage pre-acidification (PA) and then fed to a second stage digester for conversion to methane. Traditionally, all the generated gases from the PA tank will be vented to the atmosphere. Hydrogen and carbon dioxide are the main gases generated in the PA tank. A pilot-scale of two-stage anaerobic expanded granular sludge bed reactor was fabricated and used to investigate the impact of the PA gas injection into the second stage. The gas from the PA reactor was captured and stored in the storage tank. The tests were run under two temperature ranges and five organic loading rates (∼2, 3, 4, 5, and 6 g COD/L.day). For mesophilic range, the biogas production and energy yield increased by 10-90% and 40-130%, respectively, from without PA gas injection case compared to with PA injection case. For thermophilic range, the biogas production and energy yield increased by 12-40% and 90-140%, respectively, from without PA gas injection compared to with PA injection case. For each OLR, the gas production and energy yield were 90 to 160% more in thermophilic range than the mesophilic range for the cases with and without the PA gas injection. This implies that a higher temperature range has a significant impact on energy yield in a digester. One of the important findings was the amount of the PA gas injected into the EGSB reactor should be less than 50% of the theoretically calculated hydrogen gas based on ethanol substrate assumption