Study of bio-degradable plastics (polyhydroxyalkanoates) production from activated sludge wastewater treatment process

Abstract

Conventional polymers, which are also known as 'plastic', are large contributions to the waste problem. It has for many years been recognized that reducing plastic refuse could go a long way in preventing a landfill crisis. Consequently, many countries are introducing legislation or environmental regulations on reducing plastic usage. However, the remarkable usefulness of polymers probably precludes any serious slowdown in their production. In answer to this dilemma, development and production of biodegradable plastics may be a worthy option. Polyhydroxyalkanoates (PHAs), stored as bacterial reserve materials for carbon and energy, are biodegradable substitutes to petroleum-based plastics that can be produced from renewable raw materials. Many PHA-polymers have interesting properties, such as biodegradability, and have a wide array of uses ranging from single-use bulk, commodity plastics, to specialized medical applications. PHAs is a kind of polymers synthesized by microorganism. PHAs can be produced under controlled conditions by biotechnological processes. However PHAs is still not used in large-scale in commercial due to the high production cost of PHAs. The primary aim of this research is to produce PHAs possibly in large scale and at a cost comparable to synthetic plastics. After a historical review, degradation of PHAs is shown in detail. This is followed by a discussion of PHAs characterizations, and possibilities for the synthesis of novel PHAs applying different micro-organisms are discussed. In addition, detection, analysis, and extraction methods of PHAs from microbial biomass are presented. Strategies for PHAs production are discussed in detail in addition to the use of a cheap carbon source (activated sludge) in a SBR system from the point of economic view. Before lab-scale and pilot-scale study of optimization of PHAs production, a Plackett-Burman design was presented to investigate the key factors during PHAs production process. Based on the statistical screening analysis and discussions, C:N ratio was found to be a key variable during the PHAs optimization process. Feast/Famine was suggested 1/3 and SRT was designed as 10 days in study. But oxygen concentration and pH control were not controlled because they were not critical in enriching the PHAs accumulation. Study of part one in Stage I study was focus on different C:N ratio, to achieve high PHAs production rate. Results showed that the optimized C:N point was 100:1 under which the maximum PHAs content was obtained per unit nutrient substrate consumed in both Tai Po wastewater treatment sample and refuse transfer station leachate sample. In part two of Stage I, bacterial identification was applied by MIDI Instant FAMETM Sherlock System and sampling when SBR systems were running under C:N ratio of 100:1 in both two groups. Some bacteria from two sample, Bacillus sphaericus, Pseudomonas aeruginosa and Rhodococcus erythropolis, were identified. They had been confirmed by previous research having abilities of storing PHAs. The last part of Stage I study demonstrated that the monomeric unit composition can be controlled by carbon substrate. When butyric acid (C₄) was used as sole carbon source, PHB homopolymer rather than PHBV was accumulated. On the other hand, when valeric acid (C₅) was used as sole carbon source, not only PHV homopolymer but PHBV appeared simultaneously. When glucose (C₆) was used as sole carbon source, HB mole fractions was 78% and HV mole fractions was 22%. In a comparison of results in three sets of experiments, PHBV was accumulated when valeric acid or glucose was used. No PHV but PHB solo appeared only when butyric acid was used as solo carbon source. But proportion of HB mole fraction was higher than HV mole fraction no matter what kind of carbon source was utilized. Based on the empirical data of lab-scale experience, Stage II pilot-scale system was set-up. The results stated that producing PHAs from activated sludge was stable and possibly run in large scale. C:N ratio of 100:1, such a nitrogen-deficiency condition was confirmed to be a significant reference for PHAs industrial application. In addition, Bacillus cereus, Rhodopseudomonas palustris, Rhodopseudomonas sphaeroides and Salmonella typhimurium were identified in sample from the pilot-scale SBR system. Comparison between laboratory-scale and pilot-scale study indicated that the large scale production of PHAs was feasible. As C:N ratio increased, the Y p/x, Y p'/x' and Y p"/x" were all increasing. Their maximum were 0.386, 0.533 and 0.526 g polymer accumulated / g biomass with C:N ratio from 60:1 to 140:1 among three sets of experiment, respectively. Y p/s , Y p'/s' and Y p"/s" obtained their maximum point under C:N ratio of 100:1 among three groups system. Cost-benefit comparison was also conducted between this project and traditional pure culture of PHAs. Comparison results addressed that producing PHAs under certain designed condition by mixed culture was strongly competitive to pure culture.