Date of Thesis

2016

Description

In response to the growing demand on natural resources the paradigm surrounding wastewater treatment has expanded from water quality and environmental protection to include resource recovery and reuse. Within the last decade many wastewater facilities have begun to implement energy recovery and explore nutrient recovery and reuse, more recently wastewater experts have begun to consider non-nutrient product recovery. In this study the scientific knowledge, technical feasibility, and potential economic benefits associated with four non-nutrient product groups were assessed. These products include volatile fatty acids (VFAs), methanol and other alcohols, hydrogen peroxide and caustic, and polyhydroxyalkanoates (PHAs). To a certain degree, each of these products satisfy the requirements needed to be considered for resource recovery at water resource recovery facilities (WRRFs), and therefore an economic analysis is necessary to prioritize the product groups and justify research and development efforts.

At present PHAs represent the greatest economic potential out of the four recovery product groups considered for resource recovery at a hypothetical WRRF capable of converting and recovering 100% of influent COD as product. However, extraction of PHAs remains a barrier for recovery and commoditization of PHAs. Solvent-based extraction methods are considered the best techniques for extraction at lab scales, however solvents are not ideal for larger scale applications because of environmental and human health issues and relatively high costs. This indicates there is need for a scalable extraction technique, which could reduce the use of harmful solvents in PHA recovery processes. Thermal hydrolysis processes (THP) have been used to enhance fermentation and improve digester performance, by lysing biomass cells and liberating readily fermentable intracellular material. Robust cell lysis can be achieved at temperatures just below the melting point of PHAs, which suggests that thermal hydrolysis processes could serve as a viable PHA extraction technique. Two different THPs were considered, high pressure thermal hydrolysis (HPTH) and non-flash thermal hydrolysis (NFTH), in order to assess the feasibility of implementing thermal hydrolysis as a scalable, low environmental impact PHA extraction technique. Specifically, the thermal conditions associated with NFTH and HPTH were studied to identify and quantify potential thermal impacts of NFTH and HPTH on two different types of PHAs; polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-valerate (PHBV). These impacts were investigated by examining the crystallinity, molecular weight, chemical structure, and chloroform solubility post thermal treatment, as well as studying the thermal decomposition and melting temperatures of non-thermally treated PHB and PHBV. This study found that there were no discernable changes in the chemical structure of PHB and PHBV, and both polymers did not experience changes in molecular structure or loss by ignition at or near temperatures expected throughout HPTH and NFTH (165°C). This indicates that PHB and PHBV would be capable of withstanding the temperatures present in typical HPTH and NFTH process. However, after thermal treatment at temperatures expected throughout HPTH and NFTH, both PHB and PHBV experienced considerable molecular weight reduction, up to 37% by weight, and increases in crystallinity, which resulted in decreases in the chloroform solubility. Changes in molecular weight, crystallinity, and chloroform solubility can have implications for the extraction, recovery, commoditization of PHAs. Specifically increases in crystallinity can prevent accurate characterization of PHA and perhaps more importantly, prevent the recovery of PHA via liquid-liquid extraction when using chloroform, which is considered the best solvent for PHA. In order to assess the possible effects of HPTH and NFTH on crystallinity, several heating and cooling regimens expected throughout NFTH and NFTH were simulated and the resulting crystallinities were observed. It was determined that the cooling rate expected during the flash reaction associated with HPTH had little to no effect on the crystallinities of PHB and PHBV as crystal formation of PHB and PHBV occurred at temperatures lower than the simulated flash reaction (100°C). Therefore, the crystallinity of the extracted biopolymer would be largely dictated by the cooling rate post thermal hydrolysis.

The results presented here demonstrate that non-nutrient commodities can be recovered at WRRFs treating municipal waste, and of the four commodity group products evaluated, PHAs have the greatest economic potential. If the barrier of extracting intact biopolymer can be overcome, WRRFs can possibly contribute to society’s broader efforts to implement resource recovery and minimize the dependency of society on non-renewable resources such as petroleum. While some negative impacts on PHA properties were observed during simulation of the thermal conditions expected in THPs, the control of heating temperature and cooling rate may be able to overcome these adverse effects.

Keywords

polyhydroxybutyrate, polyhydroxybutyrate-co-valerate, wastewater, resource recovery, thermal hydrolysis, commodity recovery, wastewater treatment

Access Type

Masters Thesis (Bucknell Access Only)

Major

Civil and Environmental Engineering

First Advisor

Kevin R. Gilmore

Download

Share

COinS