The Potential of Non-Sterilized Sewage Water as a Dilution Medium for PHA Production by Bacillus sp. CYR1

Polyhydroxyalkanoates (PHAs) have attracted global attention as sustainable bioplastics. In our previous study, we demonstrated that Bacillus sp. CYR1 can produce PHA using glucose and sterilized raw sewage water as a dilution medium, replacing tap water. However, the sterilization process incurs high energy costs, posing a significant challenge for industrial-scale production.

In this study, we evaluated PHA production using non-sterilized sewage water as the dilution medium and compared the results with those obtained under sterilized conditions, aiming to assess the feasibility of non-sterilized sewage water for PHA production. Experiments were conducted in 300 mL Erlenmeyer flasks containing 100 mL of sewage water (pH = 7.0) supplemented with 20 g/L glucose. Bacillus sp. CYR1 was inoculated at 8% (v/v) and aerobically cultured at 30 ℃ with shaking at 120 rpm for 5 days (primary culture). After cultivation, cells were harvested through centrifugation, and the supernatant was reused for secondary culture under identical conditions. This reuse of the supernatant continued through to tertiary culture until all glucose had been completely consumed.

To improve the PHA yield, we further investigated the effects of increasing the initial inoculum size at the start of each culture and raising the culture temperature to 40 ℃. Under sterilized conditions, a maximum PHA content of 70% (dry cell weight) was achieved, while under non-sterilized conditions, a PHA content of 56% was obtained. Although the yield decreased with non-sterilized sewage water, the production remained at an acceptable level.

These results indicate that non-sterilized sewage water has potential as a cost-effective and sustainable dilution medium for industrial-scale PHA production.