When the Port Stephens (NSW) Council elected to adopt the Bedminster waste processing system of Sydney-based Bedminster Bioconversion (Aust.) Pty Ltd (BBA) to overcome its landfill space problems the decision was greeted with scepticism in some quarters. Despite this system being used in the US since 1971 it was untried in Australia, and the concept of composting such a large portion of the total waste stream rather than sending it to landfill was foreign to local thinking.
However the Port Stephens Council has been the only one to exceed the state and federal government target to divert 60 percent of the waste stream from landfill, actually exceeding that target by 20 percent using the Bedminster system in combination with a municipal recycling facility (MRF).
Since then two other regions have taken up the Bedminster concept. Bedminster is currently building a waste composting facility for the councils of Cairns, Douglas and Mareeba in North Queensland using Downer Construction Australia as the D&C contractor. This facility is due for completion by Christmas 2002.
On the opposite side of the continent Bedminster expects to complete a facility in early October 2002 (with commissioning completed in January 2003) for the Southern Metropolitan Regional Council (SMRC) in Perth. This body was formed to provide a collective response to waste management for the cities of Canning, Cockburn, Fremantle, Melville and Rockingham and the towns of East Fremantle and Kwinana. Local engineering firm Clough Engineering Limited (Clough) is the D&C contractor for this facility.
The North Queensland facility will be three times as large as the Port Stephens facility while the Perth facility will be four times as large.
The concept of the Bedminster system is natural and simple. Two difficult community waste streams, garbage and biosolids, are composted together to recover the organic component and return it to the soil from which it was derived.
Once bulky non-biodegradable items are separated from the waste stream, the remainder is batch-fed into the first compartment of the three-compartment rotary digester (known as a Eweson Digester). Biosolids are also fed into this compartment.
A natural decomposition process takes place in the digester as it rotates, and temperatures reach as high as 68°C. The biosolids provide microbial activity that promotes this process. The waste is batch-transferred between compartments each day until after three days the resulting material is inert, with no pathogenic activity. All organic components of the waste stream have been microbially converted to raw compost. Inorganic materials (around 15-20 percent by volume) remain intact, and are separated by a trommel screen and taken to landfill. The bulk of these materials are plastics.
The raw compost is then transferred to an aeration floor building for further maturation for 3-4 weeks. It finally passes through a fine screening system to produce the finished compost.
All air from the digesters and the enclosed receiving warehouse and the aeration building is extracted and passed through biofilters of woodchip and compost to ensure that no unpleaseant odours are released to the environment.
The final compost is suitable for soil enhancement, and at Port Stephens it has been used in blended soil landscaping products, for rehabilitating a golf course, rehabilitating degraded quarry and mine sites and for pasture improvement. Field trials have also been conducted to develop the composts for agriculture, particularly with the wine industry, and the latest work involves incorporating grape marc (a waste product) in the compost and blending it with soil. Benefits include reduced use of chemical fertiliser and herbicides, reduced requirement for irrigation and natural water, improved soil structure and increased cation exchange so that micronutrients are available to the soil.
In North Queensland work is being done to develop composts to suit the sugar industry. There is considerable flexibility built into both new facilities to allow tailoring of composts to suit the demands of different industries. The proportion of waste recovered as compost depends on the composition of the waste stream, with quality of compost depending on the investment in the processing equipment. The Cairns recovery factor is around 70 percent and the Perth factor is around 80 percent.
The Perth facility, as the largest Bedminster installation in Australia, will process 109,000t of waste per year. Waste will be source separated at the home using the two wheelie bin method. Recyclable materials will be transported to a materials recovery facility (MRF) while putrescible materials are processed at the Bedminster facility. SMRC also conducts kerbside green waste pickups three times a year - a collection method that controls the contamination problem.
SMRC estimates its processing costs to be $16/t for recyclables, $18/t for green waste and $45-47/t through the Bedminster process. This provides powerful reasons to ensure that the recycling and green waste programs are a success, and recent surveys show that there is a 95 percent participation rate amongst residents, and that over the past few years the volume of recyclables per household has grown from 1.2kg/wk to 6kg/wk.
The Bedminster facility, MRC, a green waste facility and an education centre all form part of SMRC's Regional Resource Recovery Centre (RRRC).
SMRC will offer a competitive gate fee to lower disposal costs and help to supplement a shortage of disposal outlets for commercial generators of putrescible waste and biosolids. Disposal of biosolids poses a problem for industries such as wool scouring as there is only one disposal facility operating in Perth. SMRC is careful to source only biosolids where the heavy metal content is known, to maintain QA on the composts that it produces.
While the costs of conventional landfill sites escalate, financial projections indicate that the SMRC project will save the community more than $26m over 14 years. The major achievement is that only 15 percent of the region's waste will go to landfill, where currently 80 precent goes to landfill.
BBA's $35m contract is to design and build the bioconversion facility (which it subcontracted to Clough), operate it for a year and hand it over to SMRC after that point. Each of the four digesters is 70m long, has a diameter of 4.7m and weighs 330t, so their fabrication and erection was a demanding task. Similarly the aeration floor is an imposing structure with a clearspan area 200m x 55m. Compared to the Port Stephens facility the Perth and Cairns facilities have improvements in the mechanical handling of waste and compost because of their greater throughput. The Perth facility is required to meet the Australian Standard for composting.
BBA md Neil Turner notes that while the basic Bedminster process is unchanged there have been developments since the Port Stephens installation to improve the screening and composting processes and mechanical handling. The Perth facility has apron feeders to transfer waste from the collection vehicles to the digesters. The aeration floor will use a purpose-built mobile compost turning machine.
Turner also stated that BBA is continuing to improve the Bedminster technology, and assessing complementary technologies to improve the overall recovery from the waste stream, and the value of recovered products. The composting process is flexible to suit changing consumer needs and can produce a variety of organic composts to suit a range of market requirements.