Date on Master's Thesis/Doctoral Dissertation

5-2014

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Berson, R. Eric

Committee Member

Rockaway, Thomas D.

Committee Member

Satyavolu, Jagannadh V.

Committee Member

Sunkara, Mahendra K.

Committee Member

Watters, James C.

Subject

Sewage disposal; Factory and trade waste; Sewage--Purification--Biological treatment; Anaerobic bacteria

Abstract

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.

Share

COinS