Slagging waterways path to purity

Slag filter beds are being used to treat dairy shed water in a trial at a farm near the Glenbrook steel mill in South Auckland.

A good slagging may help stop phosphorus from dairy farms getting into waterways, new research shows.

Melter slag is a co-product from the iron-making process at NZ Steel's Glenbrook Mill, the only place in the country that produces the material.

It can be used as an aggregate for roading, and for water drainage and filtering systems.

Run-off of phosphorus from fertiliser into fresh water is a major pollution problem affecting the health of fresh waterways and lakes.

SteelServ, an NZ Steel joint venture with a mill services provider, says the melter slag can be used to filter significant amounts of phosphorus and effluent solids from farm water before it is discharged into waterways.

It is trialling the use of two filter beds, containing about 60 tonnes of slag each, to treat dairy shed water at an NZ Steel-owned farm near the Glenbrook steel mill in South Auckland.

Sales and marketing manager Bill Bourke said the trials indicated the beds were taking out about 75 per cent of phosphorus, and significant amounts of effluent solids, before discharge to waterways.

The slag can also be used around drainage pipes to filter water entering the pipes from surrounding phosphorus-rich soil.

AgResearch scientist Richard McDowell said trials of the slag around drainage pipes at a dairy farm near Karaka in South Auckland had stripped out 80-90 per cent more phosphorus than a greywacke aggregate.

Bourke said current rules meant farmers were not allowed to drain paddocks into a creek if phosphorus-contaminated effluent had been sprayed on them.

But using the slag may be an environmentally friendly way to drain phosphorus, opening up more land for farming use.

The cost of transporting the slag from the mill may make its use uneconomic for farmers outside the top half of the North Island.

However, Bourke said SteelServ was keen to work with Fonterra and others on the slag's wider application to dairying.

The slag has also been used in trials at the Mangakino Stream, feeding into Lake Rerewhakaaitu, the southernmost of the Rotorua lakes, which has been affected by phosphorus contamination.

McDowell said recent tests had showed "socks" filled with slag had reduced the phosphorus content in downstream water by 30-40 per cent.

He said this and the dairy farm tests at Karaka showed the slag was promising, and further discussions were planned with Environment Bay of Plenty on more trials.

"It is quite a unique sort of product," he said.