Feature

Winning With Waste

Companies are finding that waste can be put to good use, providing savings or income, and recycling resources.

By Vicki Hyde, NZSM

Hard times are making some companies reassess the value of resources currently being discarded as waste. Research in a variety of institutions is demonstrating that there is considerable potential in material commonly thrown away.

One of the largest producers of waste in this country is the meat processing industry. The huge quantities of effluent produced each year require treatment of some kind. One common procedure has been to irrigate pasture with the waste blood, fat and stock excreta, and graze stock on it. Soil compaction, unacceptable runoff, and the production of excessively high nitrate levels in pasture can be problems for this approach.

In addition, many regional councils are starting to frown on practices which may have adverse impacts on local resources, such as local waterways. They are trying to encourage more innovative treatment measures which reduce the amount of direct outflow into rivers.

The Meat Industry Research Institute of New Zealand (MIRINZ) is looking at the development of artificial wetlands, which can be used to treat meat processing wastes effectively. The institute's Pollution Research Section has been investigating the optimum structure for such wetlands.

Initial trials, on gravel-based plots with subsurface effluent flow, showed that the treatment efficiency in five days was superior to that achieved in 20-30 days in conventional oxidation ponds. One major advantage of the wetland approach is its ability to deal with relatively high levels of nitrogen.

Trees From Effluent

Of perhaps greater economic advantage is the approach taken at the Oringi Meatworks. Over the past five years, Richmond Ltd has been testing the use of trees to suck up the effluent produced by the Oringi facility. The company drew on research by Waikato University, MIRINZ and the Forest Research Institute, and formed the Oringi Working Group with the latter two organisations.

A six-hectare experimental planting including five tree species was established, and both flood irrigation and spray irrigation methods of effluent application were tested. With up to 4,000 m³/day of effluent to be treated, the works was pleased to find that the trees could handle very high application rates.

Normal pasture can handle up to 1,000 kg of nitrogen per hectare per year before excessive levels of nitrates start to cause problems. Forested land in the Oringi test plot was capable of taking 9,000 kg/ha/yr.

"Forests generally can receive higher hydraulic and nutrient loads than pastures for a given soil type. Also, forests are removed from the human food chain, and forest require less frequent traffic of machinery compared to pasture, thus reducing the risk of soil compaction," say the FRI researchers involved in the study.

The eucalyptus and acacia species used could even handle direct sprays of raw effluent with no damage to their foliage. After three years, Richmond was able to harvest as much as 60 oven-dry tonnes of above-ground biomass per hectare. The wood so produced was able to be used as fuel for the plant's boilers, and regrowth from the trimmed trees is under way.

"Forested land treatment sites can be designed to produce marketable wood products, such as sawlog, pulpwood, energywood and specialist products such as extractable oils," notes the Oringi Working Group.

The success of the trial has other meatworks considering growing timber on land used for waste treatment. Not only does forested land take up greater quantities of waste, but it can also provide an energy source for factory boilers or an income source from the sale of timber and wood chips.

Economic, Environmental

Dr Ralph Sims, of Massey University's Agronomy Department, has developed a computer model to assess the economic potential for using short rotation tree crops and coppicing. Coppicing involves cultivating fast-growth trees and cutting the wood above the ground every 3-6 years, allowing the trees to regrow.

He recommends combining fuelwood production, for use as an energy source, with pulpwood production for processing. The latter has a higher market value, which can be used to offset the cost of establishing and maintaining a coppiced plantation.

Sims sees a further bonus in planting trees -- the trees are carbon dioxide sinks, and so act as a mitigator of global warming. In addition, they offer an alternative to fossil fuel sources. Sims suggests that, if the environmental costs of using coal are taken into account, fuelwood is a cheaper energy source.

Efficient burners can reduce the combustion products and produce a relatively clean fuel source. Electricorp is already investigating fuelwood as a possible energy source for the future.

Wood Waste

Of the large amount of material processed in New Zealand's forestry industry, some 20% is discarded as waste. These stems, branches, dust and offcuts are currently dumped or incinerated, but there is the potential for making sound economic and environmental use of this waste material.

"Wood and waste from wood processing are valuable renewable sources of energy that should be burned cleanly and efficiently, not merely incinerated as waste," says Bryan Woolley, engineering manager at NEI John Thompson.

In a workshop held earlier this year on utilising renewable energy sources from biomass, Woolley presented an engineering design for a simple combustion system which could make use of waste wood materials. In the semi-suspension system, particulate matter is burnt in suspension in the furnace, and the larger pieces burn in a thin bed on the furnace grate.

The system uses conventional furnace construction, is compatible with current boiler technology, and can make use of other fuel sources, such as coal or oil, if need be. The wood waste in use could vary from bark and manufacturing rejects to sander dust.

The exhaust gas from the burning process can be rerouted to provide a heat source for co-generation, such as a thermal oil heater, or to dry out incoming damp wood waste.

The design is one fairly common in larger boiler systems operating at 10-15 MW, and NZ Forest Products uses such a system at Tokoroa and Kinleith. NEI John Thompson is looking at boiler designs and combustion systems for smaller plants.

"With the correct selection of plant, [this waste] can be turned into a useful and efficient source of energy," says Woolley. "There's an obvious need to get rid of it and if you can use it to produce energy, so much the better."

Flue Fertilisers

Other waste is also proving to have economic potential. Researchers at Massey University's Fertilizer and Lime Research Centre are studying how to make resourceful use of gypsum by-products from the limestone scrubbers used to reduce sulphur dioxide (SO₂) emissions from coal-based power stations.

In the treatment process, flue gas is passed through sprays of limestone slurry. The sprays cool the gas and absorb SO₂, resulting in the formation of calcium sulphate, or gypsum. This flue-gas desulphurisation (FGD) process produces large quantities of gypsum as a by-product.

The Te Awamutu Cooperative Dairy Company produces huge quantities of gypsum each year, using fluidised bed boilers to treat the emissions from its boiler system. Finding a way of re-using this waste -- as ground fertiliser for local farmers -- makes economic sense.

"Utilisation on agricultural land as a source of nutrients and as an amendment to improve soil structure is a potentially attractive option for FGD gypsum disposal," notes Dr Nanthi Bolan of the fertiliser centre.

He has been working with researchers in the UK, testing a variety of soils and gypsum sources. They have found that industrially produced gypsum has a higher dissolution rate than the mined mineral, with finely ground material helping to stabilise soil structure.

"[The] results suggest that the fluidised bed boiler ash produced in New Zealand may have beneficial effects on soil physical properties in addition to providing a readily available source of sulphur for plants," Bolan says.

The idea has already been taken up, with Foremost Fertilizer Co. distributing the material in the Waikato. It's being marketed primarily as a sulphur fertiliser, although Bolan acknowledges that the liming properties are just as valuable.

"It's good in New Zealand because most of the soils are acid anyway," he says. Bolan is organising a project to study the liming value, fertiliser value, and effect on soil conditions of the waste material. Interest in the use of waste by-products for fertilisers has prompted a workshop to be held at the centre in February, looking at potential sources, their effectiveness as fertilisers or soil amendments and methods of applying them.

Vicki Hyde is the editor of New Zealand Science Monthly.