The management of the MOZAL aluminium smelter at Beluluane, in the outskirts of Maputo, on Friday insisted that its plans to rebuild the smelter's two fume treatment centres (FTCs), although increasing emissions, will not present any significant threat to the environment or to human health.

In recent weeks, MOZAL has come under sustained attack from some of the media, and from the NGO Environmental Justice (JA), which allege that the bypass operation involved in reconstructing the FTCs will lead to huge increases in pollution.

"Licence to kill!", shrieked one headline, while surreal maps appeared in the daily paper "O Pais" showed a cloud of death that would spread out impossibly from MOZAL across all of Maputo province and even into South Africa.

At a meeting with the press, MOZAL denied that alterations to the FTCs will push emissions beyond World Health Organisation (WHO) guidelines. MOZAL assets president Mike Fraser said that all the MOZAL studies indicated that the impact on the environment will be minimal.

The Environment Ministry gave MOZAL's plans the go-ahead, after its own independent inquiries validated the MOZAL conclusions. Using different experts, including scientists from Maputo's Eduardo Mondlane University, the Ministry agreed with MOZAL that the fume centre bypass it proposed posed no serious threat.

MOZAL's problem is that the FTCs are no longer considered safe. Over the ten years that the smelter has been in operation they have suffered structural damage, and if they continue to operate, there is a risk that steel components will suffer catastrophic failure. "We feel that the engineering of the FTCs was sub-optimal", admitted Fraser.

This unwelcome discovery has left MOZAL with little choice but to replace the FTCs, in an operation that will cost 10 million US dollars in what Fraser described as "risk mitigation capital".

"We are putting in replacement plant that will last much longer", he said. "Under standard operating procedures, it will last for the lifetime of the plant. This was an unplanned event. We do not expect to do it again in the life of the smelter".

The reconstruction of the FTCs will begin in October and last for six months. The work will run for seven days a week, 24 hours a day. But during this period the FTCs will cease to perform their main task of preventing the emission of pollutants into the atmosphere.

The two FTCs are located in the MOZAL carbon area where the anodes (required for the electrolytic process on which aluminium production depends) are baked. The production of the anodes releases hydrogen fluoride, sulphur dioxide, carbon dioxide, particulate material (i.e. dust) and tar.

The main pollutant, hydrogen fluoride, is currently removed by filtering in a structure known as a baghouse. When the gases have been cleaned, they are expelled into the atmosphere via a tall chimney.

While the FTCs are being rebuilt, a bypass will be in place so that the gases go straight to the chimney, and do not pass through filters.

This sounds alarming, until one realizes that the carbon plant is not the main source of hydrogen fluoride at MOZAL. That source is the "potlines" - the series of furnaces, where the raw material, alumina, is turned by electrolysis into aluminium. Four Gas Treatment Centres (GTCs) scrub the gases from the potlines - but some fluoride still escapes into the atmosphere from these centres, and particularly from the potline roof vents.

The work on the FTCs has nothing to do with the potlines or the GTCs which will continue to operate as normal. The potline roof vents will remain the main route, as they always have been, for the escape of fluoride into the environment.

According to independent environmental consultant Sean O'Beirne, who drew up the MOZAL Environmental Management Plan, the WHO upper limit on fluoride emissions is 500 tonnes a year. MOZAL's current total emissions come to 240 tonnes per year. Of these only three tonnes come from the FTCs - 25 tonnes come from the GTCs and 212 tonnes from the potline roof vents.

When the bypass is operating, O'Beirne said, the realistic prediction is that emission of fluoride from the FTCs will triple, to nine tonnes during that year.

However, to err on the safe side, MOZAL used models in which three times the realistic emissions figure was assumed. This gave annual emissions from the FTCs of 36 tonnes. So, during the bypass, a worst case scenario would be a total fluoride emission from MOZAL of 273 tonnes a year, nowhere near the limit of 500 tonnes.

MOZAL's dust emissions are expected to rise substantially because of the bypass - from 78 to 144 tonnes a year, or 363 tonnes in the worst case scenario.

Under Mozambican legislation, the limit on dust emission is 200 micrograms per cubic metre. WHO is working towards a much stricter guideline of 50 micrograms per cubic metre. But O'Bierne's calculations are that, even with the worst case by-pass scenario, dust concentrations will not exceed 40-50 micrograms per cubic metre in a couple of areas near the Matola river, and 30-40 micrograms per cubic metre elsewhere in the vicinity of the smelter.

The tars issued by MOZAL are a mixture of compounds, some of which are recogniseably damaging to human health. One of these compounds that has strict European Union controls is benzopyrene. The maximum level that the EU will tolerate is 6.5 nanograms per cubic metre. During the bypass operation, the MOZAL emission of benzopyrene will reach a maximum of 3.1 nanograms per cubic metre near the smelter.

By way of comparison, O'Bierne pointed out that a single cigarette releases between 18 and 50 nanograms of benzopyrene.

O'Bierne's calculations show that concentrations of all the pollutants fall off rapidly as one moves away from the smelter, so that there will be no noticeable increase in fluoride, dust or tar concentrations in Matola city.

MOZAL is not expecting anyone to take its promises at face value. During the bypass period, emissions will be monitored, not only by MOZAL's own staff, but by an independent outside company contracted for the purpose.

That contract has been won by the Swiss-based company SGS, which describes itself as "the world's leading inspection, verification, testing and certification company".