Works hard for the money
|Australian Hardboards Limited has successfully trialled a new filtration technology.
An innovative membrane-based filtration system reduces water and energy use while recycling more waste, writes consulting firm Sinclair Knight Merz.
The Vibrating Shear Enhancement Process (VSEP), a new membrane technology, has proven successful in processing effluent for a hardboard manufacturing facility in Queensland.
In a six-month pilot trial, the new system for Australian Hardboards Limited (AHL) reduced industrial water consumption by 65 per cent and enabled the recycling of close to 100 per cent of the process effluent, and achieving significant energy savings. These results were presented at the fifth Membranes in Drinking and Industrial Water Production conference held recently in Germany.
The system, jointly developed with AHL, is expected to be adaptable to other liquid-solid separation applications in the timber, pulp and paper, water and wastewater, mine tailings, minerals and other wet process related industries.
AHL's manufacturing plant at Ipswich uses mixed eucalyptus species as the principal raw material in the wet process manufacture of its Masonite hardboard. It produces around 1 ML per day of effluent, consisting principally of suspended fibre, colloidal and dissolved organic and inorganic materials, which are generated during the pulping and digestion process.
The existing thermo-mechanical pulping process is largely unchanged from an original late-1950s design, with the process water effluent treated and recycled using traditional settling ponds and land irrigation. According to AHL managing director, Rex Hills, the company recognised there were significant opportunities to reduce water and energy use without adversely affecting product quality. The trials so far have proven positive.
"Because of the resulting increases in process efficiency, we look set not only to achieve cost savings on our operations, but also to make a contribution to environmental sustainability on behalf of our industry," he said.
COMPARING THE OPTIONS
A range of key technology choices for upgrading the treatment and recycling processes was investigated. These include standard cross-flow membranes, the VSEP membrane system, a membrane bioreactor, evaporation and standard biological systems.
SKM process engineer Lee Foster said the feasibility work involved a literature study of industry practice and an extensive economic analysis to compare each technology option, including the impact of any process modifications.
"Conventional cross-flow membranes have been used in conjunction with evaporators in other similar factories in Europe. However, membranes have generally not been considered viable until recently due to the traditional fouling problems, low flux rates and costs," she said.
The VSEP process developed by US company New Logic is a relatively recent innovation in membrane-based liquid-solid separation technology and has sparked considerable interest around Australia, according to Foster. Only one small plant in Western Australia has been established locally so far, but there are 50 installations worldwide.
VSEP uses intense vibratory shear waves on the face of the membrane to stop the build up of contaminants and improve fouling resistance, allowing for filtration rates up to 10 times higher than conventional cross-flow systems. As a result, it requires less membrane area to treat a certain flow, reduces membrane replacement costs and cuts cleaning and maintenance times.
"The economic analysis demonstrated that the VSEP membrane technology offered the lowest capital and operating costs of all the systems investigated, while producing the best results," Foster said.
It was found to be 15 to 20 per cent cheaper than alternatives. Evaporators were rejected mainly because of high operating costs. Biological treatment was ruled out for its system complexity, level of robustness and high operating costs arising from the need to add nutrients, operate aeration devices and dispose of solids. The VSEP's ability to produce high recoveries and concentrations in one stage, without significant loss in flow, means that it also compares favourably with standard crossflow membranes.
VSEP ON TRIAL
"Following the favourable economic analysis, and having also identified a number of other key processing advantages, we carried out extensive laboratory and pilot plant trials on a VSEP-based system," Foster said.
The first experiments were conducted with large pore size membranes in the microfiltration and ultrafiltration range but showed poor separation of the dissolved materials, with the permeate being clear, yet quite dark in colour. The implication of this was that a great deal of the COD was not being removed. Tighter nanofiltration membranes were tested and an improvement in permeate quality was seen, with optimal results using a 250 mm filter.
"In addition to superior cost, footprint and reliability considerations, the system demonstrated an ability to produce a high solids concentrate which, in the AHL context, had a positive calorific value because it was produced as wood molasses. This made it potentially suitable for reuse as boiler fuel, for composting or as a supplement for animal feedstock."
Also, the cleaned permeate water was suitable for recycling back to the plant and, treated further with reverse osmosis, for use in the boiler. With the addition of the RO treatment, the system would increase the boiler feed-water temperature from ambient to about 60 degrees C, resulting in significant energy savings.