Bioplastic production from wastes and wastewater

Abstract

Dependence on conventional plastics and their boundless usage have resulted in waste accumulation and greenhouse gas emissions. Part of a solution lies in using biodegradable plastics such as polyhydroxyalkanoates (PHA), which can be produced by bacterial fermentation. The interest in these biopolymers lies in its similar properties to conventional plastics. However, its price is still too high to compete in the current market. One way to produce PHA reducing the cost of production is to use wastes and wastewater as a raw material. They have not only a great capacity to produce PHA with a high potential for accumulation but also to reduce the costs of the waste and wastewater treatment. The process to produce PHAs from waste consists of three steps: an anaerobic fermentation process, the enrichment of the culture and the accumulation of PHA. The fermentation process, to produce volatile fatty acids, is required when carbohydrate wastes are used as substrate. The main challenge in PHA mixed culture processes is the enrichment of PHA-accumulating organisms, which can be carried out with transient conditions of carbon supply, i.e. feast-famine regime. This process configuration creates periods of excess and lack of external carbon substrate. Following a certain period of time in the absence of external carbon substrate, a decrease of the amount of intracellular components required for cell growth (RNA and enzymes) occurs. Therefore, when carbon substrate becomes available again, the amount of growth enzymes may not be enough to ensure that a maximum growth rate is reached. On the contrary, fewer enzymes are required for PHA storage and, therefore, PHA storage can occur at a much faster rate than cell growth, thus providing the cells with a means of rapidly consuming the available external substrate. This work presents an economic analysis of PHA production by bacterial fermentation from municipal wastes. Based on an annual production of 2503 tones of PHA, and taking into account the performance of production processes of PHA is 0.33 g PHA/ g VFA-COD approximately, the economic potential which is obtained is not still sufficiently positive. However, if this investigation line continues, it has been seen that the process can be viable and an effective industrial wastewater treatment technology