Appendices to Integrated Resources & Solid Waste Management Plan - Dec 2009

Home

HAWAII COUNTY MECHANICAL - BIOLOGICAL TREATMENT FACILITY CONCEPTUAL DESIGN Mechanical - Biological Treatment Overview As outlined in the draft Residuals Management chapter of the IRSWMP Update, mechanical - biological treatment generally refers to the integration of MSW treatment processes normally found in material recycling facilities (MRF), refuse derived fuel (RDF) plants, and composting plants. A key feature of MBT facilities is the use of mechanical separation to remove and recover non - organic components of the MSW stream, and biological treatment to stabilize the organic fraction of the MSW stream. MBT facilities involve waste input and control, mechanical preparation, biological treatment, and product conditioning. Waste input and control normally consists of manually removing oversized and hazardous materials. Mechanical processing can include minimal separation or shredding, or sophisticated sorting of the inbound waste into biodegradable material, recyclables, and contaminant streams. Sorting is usually done with dry processes but it can also involve wet processes, such as flotation and hydro - pulping. Hand - sorting systems have also been implemented at some facilities, but this increases health and safety requirements for staff. Depending on the quality and market demand, the recyclables are typically sold, but paper fibers, textiles, rubber, plastics, and residual organics can also be used as RDF. In the IRSWMP update, MBT systems were classified into three groups: Biological treatment used to produce RDF for combustion Anaerobic digestion to recover energy Composting to stabilize organic wastes or to produce a soil amendment Use of biological treatment to produce an RDF product for combustion is a popular approach in Europe, but is much less common in North America. The anaerobic digestion (AD) process is used to break down organic materials in an anaerobic (i.e., without oxygen) environment and allows the recovery of the energy from the organic materials in the form of "biogas ". In addition to biogas, the AD process results in liquid and solid byproducts, some of which may have a high nutrient value, making the byproducts suitable for beneficial reuse as a soil amendment. Liquids may include high levels of chemical oxygen demand (COD) requiring further treatment. In some cases, byproducts can be applied directly to land, although there is an increasing trend towards some type of further processing (e.g., composting or drying) prior to land - application. If composting or drying is the selected secondary processing technology, these processes are typically integrated into the process and facility designs. The biogas that is collected from the AD process can be further processed and refined into a fuel source for use in industrial engines, vehicles or in a generator to create electricity for local use or distribution to through an electrical grid. Anaerobic digestion is well established in North America as a means of treating wastewater treatment plant residuals, dairy manures, and other sources of relatively homogenous organic material. The application of AD to source - separated organics and MSW is a more recent development and one that has become popular in Europe during the past decade as a result of bans on disposal of organics in landfills. However, while there is significant interest in applying AD to organic solid wastes in North America, there are relatively few operating facilities.