Semester of Graduation

Fall 2025

Degree

Master of Science in Biological and Agricultural Engineering (MSBAE)

Department

Department of Biological and Agricultural Engineering

Document Type

Thesis

Abstract

Abstract

The viability of bioplastics as denitrification substrate is well established in literature. However, their high cost has limited the practical integration of bioplastic-based systems into wastewater treatment plants. This study investigates the preparation and evaluation of composite pellets comprised of Poly(3-hydroxybutyrate-co-3-valerate) (PHBV) and plant biomass. Four renewable agricultural wastes, namely sugarcane bagasse, sawdust, rice husk, and switchgrass were incorporated as the biomass component and were blended with PHBV to reduce overall material costs.

Composite pellet formulations were prepared with varying biomass-to-PHBV ratios (50-90% biomass) using a heated pelletizer and hydraulic press. Pellet cohesiveness was initially assessed through 30 days of water submersion test. Chemical oxygen demand (COD) releases were also monitored for 11 days, revealing that all sugarcane bagasse-PHBV ratios released carbon gradually over time when compared to other blends. Mechanical testing indicated that sugarcane bagasse-PHBV composites achieved the highest compressive strengths among all the biomass-PHBV blends across all tested ratios. The 50:50 bagasse-PHBV showed the maximum axial compressive stress (143.10 ±7.25 MPa) and largest axial compressive strain (0.273± 0.059 mm/mm) of all compositions evaluated. Analyses of stress-strain testing further validated the structural integrity of these composite pellets. However, two-way ANOVA revealed that biomass type significantly (p values < 0.05) affected both compressive stress and strain, whereas the biomass-PHBV ratio influenced only strain significantly (p < 0.05). Among the pellets that passed the water cohesiveness test and exhibited low-COD releasing tendency, biomass type and blending ratios were selected to maximize compressive strength while minimizing PHBV content. After mechanical testing, sugarcane bagasse emerged as the most suitable biomass and was selected for all laboratory-scale denitrification experiments. Three sugarcane bagasse-PHBV ratios (70:30, 80:20, and 90:10) were compared with pure PHBV controls to compare nitrate removal and pellet degradation rates in a triplicate, closed-looped recirculating systems.

Nitrate conversion rates varied between 1.5 and 2.9 kg NO3--N/m3·day across all blends. All blend ratios performance values were closely similar and comparable to control, indicating no difference in denitrification performance. Statistical analysis (one-way ANOVA, p=0.212; Dunnett’s p > 0.05) also confirmed that nitrate removal rates did not vary significantly among the PHBV control and bagasse/PHBV ratios at the 95% confidence level. Although all sugarcane bagasse-PHBV composite pellets effectively supported denitrification, the 90:10 formulation exhibited significant disintegration after day nine, limiting its long-term stability and applicability as a denitrification media.

These findings on short-term denitrification experiments highlighted that the selection of the bagasse-PHBV blend ratio should be based on stability and cost efficiency. The 90:10 blend ratio degraded rapidly, while 70:30 blend maintained long term stability. Economic analysis showed that 80:20 bagasse-PHBV achieved most cost-effective performance, reducing substrate cost by 74% per ton of NO3--N removed. This study demonstrates a potential scalable pathway for reusing underutilized agricultural waste into eco-friendly products that deliver both environmental and economic benefits. However, it has to be noted that pellet performance in longer term denitrification experiments (60-90 days or longer) has to be assessed for selecting the ideal blending ratio.

Keywords: PHBV, biodegradable pellets, renewable biomass, denitrification, aquaculture, sustainable materials, packaging, solid carbon source.

Date

11-3-2025

Committee Chair

Theegala, Chandra

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