Final Report: Production of Natural Plastics in Wastewater TreatmentEPA Grant Number: SU833562
Title: Production of Natural Plastics in Wastewater Treatment
Investigators: Loge, Frank , Bissell, John , Cheng, Mary , Liu, Hsin-Ying , Matsumura, Kristen , Pan, Jimmy , Srinivasaragavan, Anamica
Institution: University of California - Davis
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: September 30, 2007 through May 30, 2008
Project Amount: $10,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2007) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Built Environment , P3 Awards , Sustainability
The purpose of this research project is to assess the feasibility of integrating a PHA/PHB production process into a specific Waste Water Treatment Plant (WWTP). This was accomplished by performing a number of bench-top reactions on site specific waste and creating a workable pilot plant design for a specific WWTP. Bench-top reactors were used to cultivate microbes and determine reaction coefficients and parameters. A pilot plant design flow sheet was created to accommodate site specific constraints and review engineering and production feasibility.
EPA’s P3 goal is to successfully implement sustainable designs to existing infrastructure and processes. This too is our ultimate objective in executing this project. The following bullet list served as a guideline and is composed of the objectives/milestones for the Phase I project.
- Construction of bench-top reactors (fermentor, aerated batch reactor for cultivation and for phase II PHA production tests)
- Initiation of batch fermentation tests for optimal Volatile Fatty Acid (VFA) production
- Initiation of microbe cultivation via batch feast-famine cycle with VFA for use in Phase II
- Construction of a workable design flow-sheet for pilot plant allowing for WWTP’s requirements.
- Outline the process and materials needed to implement PHAs harvesting at Whittier Narrow WWTP with minimum changes to existing infrastructure.
- Assessment of the economic feasibility of PHAs harvesting through life cycle analysis
- Review of the end use options and engineering implications
The bench top reactor setup was constructed and maintained as described below:
A 5 liter high-density polyethylene continuously stirred batch reactor (CSTR) was used to ferment the primary sludge. Approximately 4 liters of sludge were continuously stirred in the sealed bottle for 4 weeks using a magnetic stir bar. Everyday, a portion of the volume was decanted and new primary sludge was deposited to maintain 4 liters in the reactor. This allowed the sugars and amino acids found in primary sludge to be consumed by yeast and bacteria under anaerobic conditions to produce volatile fatty acids (VFAs). The hydraulic retention time (the average time spent in the reactor) needed to produce VFAs is dependent on the species of bacteria present. Decanting and restoring a portion of the volume allows the bacteria and yeast that perform the fermentation to reach a steady consumption rate. The decanted liquid was spun in a centrifuge for at 3000 rpm for 10 minutes. Centrifugation separated residual solid waste from the fermented solution. The residual solid waste only accounted for an average of 3% of the mass by weight and was discarded. Fermentation of primary sludge initiated VFA production.
The 10 liter seed bioreactor (which was used to cultivate the microbial population) was also constructed of cylindrical high-density polyethylene. Four liters of waste activated sludge was used to initially seed the reactor. It was cycled through a 6 hour anaerobic phase and an 18 hour aerobic phase continually for 4 weeks. Nitrogen gas was pumped through the reactor to create an anaerobic environment and oxygen was pumped through to create an aerobic environment. Both gases were regulated at a rate of 1.4 L/min. Everyday at the beginning of the anaerobic cycle a portion of the liquid in the reactor was decanted and the fermented solution was added to maintain a constant volume. This seed bioreactor system initiates microbial cultivation.
Economic analysis of this process indicated that this process is in fact feasible at the Whittier Narrows WWTP site. If the primary sludge from the Whittier Narrows plant were instead fermented to produce a feedstock, then the volume of solid waste being pumped downstream would be greatly reduced. Primary sludge accounts for two thirds of the wasted volume. If the primary sludge were fermented to produce a VFA rich solution and fed to PHA producing microbes, then the maximum concentration of PHA per volume is expected to be 50% (see figure 3). Thus the implementation of this process in Whittier Narrows will reduce their solid waste stream by one third and produce a volume of PHA equivalent to one third the total waste stream volume. Potential reduction of the Whittier Narrows solid waste stream was estimated to be from 1.7 million pounds a year. As such, the Whittier Narrows WWTP can also be expected to produce approximately 1.7 million pounds of raw PHA plastic a year.
A brief review of market viability also resulted in promising numbers. Raw PHA plastic can be sold on the open market for approximately $7 per pound. Whittier Narrows has the potential to make $12,000,000 per year by selling PHA produced at their facility. This more than compensates for the cost of transporting and disposing of primary waste imposed on the joint water pollution control plant downstream.
Based on the above findings and design considerations, a flow-sheet for the pilot plant at the Whittier Narrows site was drafted as shown in the figure below.
Upon completion of Phase 1, the data collection process has been initiated. Samples will be taken every two hours and PHA content will be quantified using gas chromatography and mass spectrometry analysis. Protocols have been developed for the fermentation process. Cell lines have begun to cultivate in the seed bioreactor and the process of selection has been initiated. The laboratory setup did not include the production bioreactor that we propose to use in the pilot plant because it was not necessary to produce a large volume of product.
After assessing previously proposed systems of integration of PHA production in existing waste water treatment plants, we conclude that changing plug flow reactor residence times will not be appropriate at the Whittier site. The flow rates at Whittier are too large and frequent to modify the existing PFR cycles. Instead we propose the addition of a batch reactor that is initially inoculated with activated sludge from the second clarifier. This seed batch reactor will then be used continuously to select for cells that produce sufficient PHA to survive the selection process; this selection process will consist of feast-famine cycles. The seed batch reactor will be fed with the volatile fatty acids from the fermentation of primary sludge. The selected microbes are combined with VFAs in the production bioreactor. The cells in the production bioreactor will not be subjected to a feast-famine cycle; they will receive an initial feed of VFAs and be harvested at optimal PHA levels. The decision to use a production bioreactor in the pilot plant instead of using the seed bioreactor for both selection and production will reduce the amount of VFAs used in the selection process and eliminate waiting times during famine. The seed bioreactor can be operated continuously using a lesser amount of VFAs since the reactor volume is much smaller than the volume of the production bioreactor. The working volume of the seed bioreactor should be 24% of the working volume of the production bioreactor. Eighty-three percent of the seed reactor will be used to inoculate the production bioreactor. The remaining 17% will be used for the next selection cycle in the seed bioreactor. The total volumes of both bioreactors scale linearly. Also, if the seed bioreactor were itself used as the production fermentor, production of PHA would be stopped during famine periods. During famine, PHA is used as energy for metabolic processes within the cell. Therefore, if this process were separated from the production stage, famine cycles could occur concurrently with production of PHA in the large production fermentor.
Microorganisms synthesize PHA as a media of energy storage. There is a defined length of time at which point PHA production is highest, and it corresponds to a minimum in available soluble carbon. In previous research, PHA levels reached up to 53% of the dry cell weight. This number will of course vary based on the source of wastewater. After the optimal number of hours in the production bioreactor, the cells will be killed.
Data collection from the laboratory setup has been initiated and will be completed before implementation of the pilot plant at the Whittier site. Reaction coefficients will be obtained and used to find a relationship between PHA production and time. Thus, the length of time at which PHA production is highest for Whittier waste water in particular will be acquired. Varying aeration rates and comparing the effect of oxygen transfer on PHA production will allow us to optimize aeration rates in the pilot plant.
Our project does not show innovations in research in the production of natural PHA plastic in wastewater. Rather it applies all the theoretical knowledge about PHA production in activated sludge to a real world wastewater treatment plant. Recycling bio-waste into plastics is capable of far reaching benefits to the communities and environments that are currently negatively impacted by existing waste water treatment and disposal practices. Waste water treatment plants, as they stand, do not recycle the remaining sludge after treatment. Primary solids are disposed of in landfills. These landfills contribute noxious gases into the atmosphere and accelerate global warming. In fact, 25 million tons per year of non-degradable petroleum-based plastics accumulate in the environment.1 Transportation and landfill fees to waste water treatment plants can contribute to a significant percentage of annual cost. By turning the primary waste stream into a feed stream for producing PHA plastic we reduce the need to dispose of primary solids in the environment. Reducing disposal also reduces the carbon footprint associated with sending primary solids to landfills. Carbon is sequestered in plastic, rather than being released into the air or into landfills. Therefore, waste water treatment plants in high population density areas can potentially earn carbon credits by reducing their carbon footprint.
Our interdisciplinary team of civil, environmental, and chemical engineers worked with representatives from LA Wastewater to review data and apply findings to the Narrows site. The project allowed team members to apply fundamental principles of design, reaction engineering, transport phenomena, microbiology, and scale-up. In addition to gaining an avenue to practice textbook knowledge, team members also had an opportunity to develop teamwork skills and relationships with industry representatives.
Project Period for Phase 2---1 September 2008-31 May 2010
Proposed Phase II Objectives and Strategies
Phase two will be concerned primarily with the acquisition of a site (likely Whittier Narrows WRP in LA Country) on which to build a pilot plant and then the construction and characterization of that pilot plant. The process design alterations made in phase one will be incorporated into the pilot plant of phase two. With the data acquired from the pilot plant, we will be able to realistically assess the feasibility and benefit of the process with respect to the specific, on-site plant and wastewater.
The pilot plant will operate using the sludge and the liquor from the primary clarifier. The carbon-rich sludge from the primary clarifier will be fermented to produce volatile fatty acids, which are a metabolite pre-cursor of PHA; the volatile fatty acids will be the feed source for both the PHA-producing bioreactor and the selection reactor. The liquor from the primary clarifier will initially be fed to the selection reactor to provide the initial microbe population. The selection reactor will be a batch reactor that operates on a feed-famine sequence optimized to sustain a population of microbes which produce and store high levels of PHA. The selection reactor will operate continuously in order to provide a sufficient microbe population for the PHA-producing bioreactor. The volatile fatty acids produced in the fermentor will be stored by the selected microbial population as PHA; when the VFA content has reached a minimum (which coincides with maximum stored PHA), the microbes will be lysed and the PHA harvested.