Mangrove bacteria turns sludge into bioplastics

Using a bacterial strain from mangroves, researchers have discovered a low-cost, sustainable method for making bioplastics out of sewage sludge and wastewater.

The United States generates seven million tons of sewage sludge annually, enough to fill 2,500 Olympic-sized swimming pools. While a portion of this waste is repurposed for manure and other land applications, a substantial amount is still disposed of in landfills.

In the journal American Chemical Society (ACS) Omega, the researchers report that the bacterium Zobellella denitrificans ZD1, found in mangroves, can consume sludge and wastewater to produce polyhydroxybutyrate, a type of biopolymer for use in lieu of petroleum-based plastics.

In addition to reducing the burden on landfills and the environment, the researchers say Zobellella denitrificans ZD1 offers a way to cut down upstream costs for bioplastics manufacturing, a step toward making them more competitively priced against regular plastics.

“The price of raw materials to cultivate biopolymer-producing bacteria accounts for 25-45% of the total production cost of manufacturing bioplastics. Certainly, this cost can be greatly reduced if we can tap into an alternate resource that is cheaper and readily obtainable,” says Kung-Hui (Bella) Chu, professor in the civil and environmental engineering department at Texas A&M University.

“We have demonstrated a potential way to use municipal wastewater-activated sludge and agri- and aqua-culture industrial wastewater to make biodegradable plastics. Furthermore, the bacterial strain does not require elaborate sterilization processes to prevent contamination from other microbes, further cutting down operating and production costs of bioplastics.”

Several bacterial species produce polyhydroxybutyrate, an emerging class of bioplastics, when they experience an imbalance of nutrients in their environment. The polymer acts as the bacteria’s supplemental energy reserves, similar to fat deposits in animals. In particular, an abundance of carbon sources and a depletion of either nitrogen, phosphorous, or oxygen, cause bacteria to erratically consume their carbon sources and produce polyhydroxybutyrate as a stress response.

One such medium that can force bacteria to make polyhydroxybutyrate is crude glycerol, a byproduct of biodiesel manufacturing. Crude glycerol is rich in carbon and has no nitrogen, making it a suitable raw material for making bioplastics. However, crude glycerol contains impurities such as fatty acids, salts, and methanol, which can prohibit bacterial growth.

Like crude glycerol, sludge from wastewater also has many of the same fatty acids and salts. Chu says the effects of these fatty acids on bacterial growth and, consequently, polyhydroxybutyrate production, had not yet been examined.

“There is a multitude of bacterial species that make polyhydroxybutyrate, but only a few that can survive in high-salt environments and even fewer among those strains can produce polyhydroxybutyrate from pure glycerol,” Chu says. “We looked at the possibility of whether these salt-tolerating strains can also grow on crude glycerol and wastewater.”

For the study, Chu and her team chose the Zobellella denitrificans ZD1, whose natural habitat is the salt waters of mangroves. They then tested the growth and the ability of this bacteria to produce polyhydroxybutyrate in pure glycerol. The researchers also repeated the same experiments with other bacterial strains that are known producers of polyhydroxybutyrate. They found that Zobellella denitrificans ZD1 was able to thrive in pure glycerol and produced the maximum amount of polyhydroxybutyrate in proportion to its weight without water.

Next, the team tested the growth and ability of Zobellella denitrificans ZD1 to produce polyhydroxybutyrate in glycerol containing salt and fatty acids. They found that even in these conditions, it produced polyhydroxybutyrate efficiently, even under balanced nutrient conditions. When they repeated the experiments in samples of high-strength synthetic wastewater and wastewater-activated sludge, they found the bacteria was still able to make polyhydroxybutyrate, although at quantities lower than if they were in crude glycerol.

Chu notes that leveraging Zobellella denitrificans ZD1 tolerance for salty environments, could avoid expensive sterilization processes normally needed when working with other strains of bacteria.

“Zobellella denitrificans ZD1 natural preference for salinity is fantastic because we can, if needed, tweak the chemical composition of the waste by just adding common salts. This environment would be toxic for other strains of bacteria,” she says. “So, we are offering a low cost, a sustainable method to make bioplastics and another way to repurpose biowastes that are costly to dispose of.”

The Kuwait Institute for Scientific Research, the Ministry of Higher Education of Kuwait Fellowship, and a fellowship from the Ministry of Science and Technology of Taiwan funded the work.

Source: Texas A&M University

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