Open Composting is the controlled biological decomposition and pasteurisation of organic materials under aerobic conditions, accomplished in open windrows or open static piles. Composting involves the action of thermophilic ("heat loving") microorganisms that thrive under increased temperature conditions and if correctly managed, this can allow for the destruction of disease-causing organisms (R.Van Berkel, pers.comm; Waste Inquiry, 2000).
Open aerobic composting occurs under moist, oxygen -rich (aerobic) conditions where complex organic molecules are broken down by microorganisms that release nutrients and energy contained in the waste (Waste Inquiry 2000). The organic waste is stored in long rows or piles and periodically turned or 'agitated' to promote aeration and homogenisation (Waste Inquiry 2000). Piles need to be watered periodically to control of moisture content (Waste Inquiry 2000). Windrow or pile size is determined primarily from climate, waste composition and the type of aeration used (UNEP 1998).
Waste Streams Handled
This technology type works ideally in situations where there is a high proportion of organic material in the waste stream and markets are accessible and available (EnviroAccess 1999). Generally, this composting process is best suited for garden and yard waste as there are limited odour issues associated with this material. While open composting is certainly capable of processing sewerage sludges and food scraps, the inability to maintain fine control over process conditions make Heath Department approval more difficult to obtain.
Typically, open aerobic composting is a low technology and relatively low cost option. Operational control is accomplished through selecting and preparing a suitable waste to be composted and maintaining optimal temperature, moisture and aeration conditions in the active windrows (UNEP, 1998).
Food waste and other organics (e.g. sewage sludge) may be incorporated into the process but technology must be installed to control odours (Waste Inquiry 2000). These odour control systems will raise the costs of the operation therefore making the technical flexibility somewhat limited (Waste Inquiry 2000).
Aeration can be accomplished using manual or mechanical turning, or by static aeration which incorporates air through a network of perforated pipes within the compost pile (UNEP 1998). Forced aeration involves placing the organic waste on a sealed surface, into which are installed a number of channels. These channels are used to draw air through to the base of the pile for treatment in a biofilter to reduce odours, as well as to collect any leachate generated (Waste Inquiry 2000).
Variations of open windrow composting are being used throughout the world. This is the most popular composting process in Australia given the climate and availability of open space (Waste Inquiry, 2000). Many small companies and landfill operations in Perth are using windrow composting for green waste. There are currently a number of windrow composting facilities for Council collected green waste.
EnviroAccess (2000) Environmental Technology Fact Sheets
Waste Inquiry (2000) Report of the Alternative Waste Management Technologies and Practices Inquiry.
Compost Resource Page
Cornell Composting - Municipal Solid Waste Composting