Biowaste facility for World Heritage Site
Off the eastern seaboard of Australia a valuable island group is benefitting from leading-edge waste treatment which not only accommodates a burgeoning tourist industry but also ensures the islands' fragile ecosystems.
Lord Howe's population stands at roughly 360 people, added to by a fluctuating tourist industry catering to 180,000 visitors each year. On average the static population amounts to 800 people. Waste on the island was either incinerated or buried in sand dunes.
Tests on the island's groundwater supplies revealed contamination and there was evidence that aquatic species were being adversely affected by leaching of these contaminants into the lagoon. In reaction to these findings, funding was obtained from the New South Wales State Government and the Australian Federal Government for a contract to supply and install suitable waste technology (ies) that could deal with the island's organic waste streams.
Four internationally recognized technologies were evaluated and the contract was awarded to Willson Brown Associates Ltd to install their patented VCU® Technology as part of the facility, which included various technologies for the pretreatment of waste material.
The features of the VCU-based biowaste facility identified in the evaluation process, conducted by the Crown Research Centre for Waste Management and Pollution Control were found to be highly suited to Lord Howe Island and included the following:
- Low power consumption
- Small footprint and land use
- Negligible odour generation
- Modular design
- Capacity to accommodate seasonal fluctuations in feedstocks
- Low operator involvement in process performance
- No leachate
- High quality compost product
Organic waste can be processed through the VCU® to efficiently produce a guaranteed pathogen- and weed-free product. Its efficiency is due to low energy consumption which stems from a number of factors. The VCU® has a stratified temperature profile from 75°C at the top of the chamber to 40-45°C at discharge. This stratification provides distinct zones of microbiological activity which subject waste material to degradation by a highly diverse microbial population.
It also enables energy to be harnessed and used to draw air up through the zones, allowing the the system to be passively aerated - the pile only draws in as much air as it needs.
Mechanical input is also reduced because waste moves from the top of the chamber to the bottom by gravity, and since the chamber is insulated and temperature is uniform from core to wall, the pile does not need to be agitated to expose all material to high temperatures.
The fact that the VCU® requires no external energy for heating, bio-filtration, material locomotion or agitation provides significant operating costs savings.
Passive aeration has another advantage in that odorous gases generated in the pile are able to be filtered by microorganisms before they reach the exhaust outlet.
No leachate problems
The VCU® can accept organic wastes with a wide array of moisture contents. Materials are blended to give a moisture content of 55-60%, and at these levels processing occurs without the production of a leachate.
Free moisture in the pile is buoyed by the rising heat as steam, which is then drawn from the top of the chamber as condensate. Unlike leachate, condensate is not a pollutant and can be used for irrigation or discharged as storm water.
Having awarded the contract to Willson Brown Associates Ltd, manufacturing commenced immediately, with installation completed by June, 2000. Employees of the Lord Howe Island Board operate the facility.
The VCU® consists of various pre-treatment technologies which prepare waste materials for VCU® processing, storage and treatment of sewage and waste water, as well as a fully enclosed building in which inorganic materials are sorted and bailed. All waste generated by the facility, such as wastewater and sewage from on-site amenities is fed back into the system. As such, the facility itself is a good model of self-sufficiency.
The VCU® processes all organic wastes, namely food, greenwaste, paper and cardboard, sewage and grease trap wastes, as well as offal from the considerable fishing activity on the island. Foodwastes are pulverised prior to blending, greenwaste is chipped to a suitable particle size, and sewage is mixed with pulped paper and cardboard, which combined, are dewatered. Grease trap wastes are added straight to the blender. All wastes are mixed together in predetermined proportions in the blender of the VCU® and are then fed into the VCU chamber.
Sewage is collected in a large holding tank where solids settle to the bottom and wastewater is drawn off the top. Wastewater from here and the rest of the site is subjected to combined anaerobic and aerobic treatment which produces clean water for use in washdown and the slurry making process.
Solids are fed back into the sewage system. Sewage solids (approximately 15-20% solids content) are pumped to a hopper where they are mixed with pulped paper and cardboard, the pulp acting as an adherent for the sewage solids. This slurry/sewage mixture is then sent through a screw press to further remove excess moisture. The resulting material has well balanced carbon and nitrogen, which when added to food waste and green waste, makes an excellent feedstock for the composting process.
Peaks and troughs
The capacity of the VCU® is inherently variable, able to cope with the tourist season peaks, as well as the off-season lulls. In low periods the VCU® is operating on approximately a 5-week cycle, accepting roughly half a tonne of the combined organic wastes each day, whilst producing 0.3T of Grade A compost product in return.
During the tourist season the total waste volume may swell to as much as 2T each day, in which case the VCU® will be on a 14-day cycle producing approximately 1.7T of compost product daily, which will require a 2-3 week curing period before reaching maturity. Over 50T of sewage solids are expected to be processed within the overall in-feed matrix over each year.
The discharge mechanism of the VCU® is designed so that the mulch product is screened immediately, separating over-sized particles from the compost bulk. This allows not only for a higher quality soil amendment, as well as enabling the facility to recycle woodchip, which is a necessary input for the composting process. In winter months, chipped green waste is in short supply and the woodchip from the screened product has to be recovered.
Such is the quality of material produced that, subject to ongoing analysis, it will be used in the nurturing of seedlings in Lord Howe Island's AU$4.5 million Kentia Palm exporting industry.
When blended with a porous material the compost product is ideal for use as a growing medium or as a soil conditioner. Excess product is used on Lord Howe's protected flora, or by the locals eager to see return on their recycling efforts.
As a stand-alone facility, the Lord Howe Island installation provides an excellent working example of how a community can deal with their wastes in a sustainable manner.
Although small in size, the same facility could easily cope with a population of 3000 inhabitants, which makes it a realistic waste management solution for other larger islands throughout the world, as well as inland communities disadvantaged by their isolation.
More VCUs in evidence
There are a number of single chamber VCU® sites in New Zealand and Australia under management by Willson Brown. There are also a number of units in the UK under management by OrrTec which operate a variety of organic wastes including trade and commercial food and meat processing wastes.
The first multi-chamber site is under construction for commissioning in February 2001 in Auckland, New Zealand - this will initially process over 7,500T/yr of greenwaste, with source separated domestic waste and sewage sludge to be included over time.
Eventually it is estimated the site will process over 14,000T/yr through 10 VCU® modules.