**2.1 Biocarriers**

Plastic carriers of different shapes and surface areas have been developed and applied in the MBBR systems as biofilm substratum. The carriers' shape, density, protected areas, and void volume are important factors that affect the performance of MBBR processes. Carriers can be made of different shapes such as squares, round, and sphere. The shape can affect the carrier's strength, shearing, and colliding conditions. The carrier density is normally lower than water at around 0.98 kg/L, so that it can be suspended in wastewater with biofilm attachment without introducing strong mixing. The carriers protected areas range from 300 to over 1000 m2 /m3 depending on the shapes and internal structure. Large carrier protected areas normally mean high complexity of the carrier structure and higher production cost. Carriers of protected areas of 500–1000 m2 /m3 are normally applied in full-scale wastewater treatment plants due to their costs and process benefits. **Figure 3** shows two different types of plastic carriers that are made of high-density polyethylene (HDPE) with respective protected surface area of 650 (BWTX®) and 828 (BWT15®) m2 /m3 . The biofilm on carriers develops as illustrated in **Figure 1** and maintains active organisms in thin layers. A well-designed carrier enables stable biofilm in the MBBR process, so that the void is not easily blocked by wastewater particles or excessive biofilm accumulation. Effective mixing/aeration combining a good carrier design leads to good system performance and low-maintenance requirements.
