Cellulose and agar were tested extensively to assess their suitability as a replacement material for plastic for use in manufacturing Moving Bed Bioreactor (MBBR) biocarrier media. The influence of various additive materials and post-processing techniques applied to the biocarriers was also assessed. Additives included hemp, muscle shell and salt. Post processing included freeze-drying, rehydration, and chemical cross-linking. These materials met the criteria to reduce plastic waste and microplastic pollution of waterways. Cellulose and agar were chosen due to their renewability and general abundance. They were found to be better ecologically in terms of energy usage, harmful emissions and the end-of-life effects related to their manufacture, usage, and disposal in comparison to plastics.

Of the two tested matrix materials, cellulose was found to be brittle in its behaviour and would fracture easily under small, applied loads. Alternatively, agar exhibited elastic behaviour which made them more suitable for resisting the small compressive loads typically applied to biocarrier media. Various agar compositions were tested, including samples with 5%, 6% and 7% total solid agar content alongside the noted additives. Agar 5% fared similarly to or better than all compositions under compression, cyclic, rheological and thermal testing.

It was found that agar-based biocarriers were able to support the growth of nitrifying bacteria in a turbulent aqueous environment similar to what would be found in a MBBR plant. This bacterial growth showcased nitrate removal rates that were lower, but comparable, with traditional plastic biocarriers. While the per-biocarrier performance was not greater than industry standard biocarriers, agar showcased its higher removal rate potential when compared by protected specific surface area. In comparison to plastic biocarriers, the agar biocarriers were able to exhibit approximately twice the nitrate removal performance of plastic when normalised by specific protected surface area. Despite this apparent doubling in performance per square unit of protected surface area, agar biocarriers faced drawbacks to being suitable as long term biocarrier media. These related to the insufficient durability and structural integrity of the agar biocarriers when in use in a simulated MBBR environment. They are however highly suitable for locations that feature shorter-term peaks in wastewater production such as seasonal holiday destinations. Potential avenues of investigation into increasing agar biocarrier’s general mechanical properties, resilience, and protected specific surface area are noted. Following further development, agar biocarriers could be a highly performative and ecologically friendly alternative to industry standard biocarriers.