The Halland company Tomal AB was founded in the early 1950s by Thomas Alwerud. He developed several innovative products for dosing of powders and granules. The most successful was the twin-screw dispenser that today constitutes the basic concept to the company’s multiscrew.

“In addition to the pulp, paper and chemical industries, our filling equipment is very common in the environmental field as well,” says Teddy Eriksson, CEO of Tomal .

“It may include equipment for dosing of flocculants for water treatment, dosing of urea at gas treatment plants or dosage of sawdust waste to a biomass boiler. Dosage of wood powder has increased very rapidly in recent years where the latest application is for asphalt plants. Wood powder is then used instead of oil. It is important to ensure a smooth and accurate metering for a smooth and stable flame, which is achieved with our multiscrew. Another example of our multiscrew is the dosage of ground limestone to raise the pH of acidified lakes and streams.”

Natural acidification

“Natural acidification” is one of Sweden’s sixteen environmental quality objectives and to achieve this, the acidifying effects of deposition and land use exceed the limits for what soil and water can tolerate. The effects of acidification drew the attention of researchers more than 40 years ago and although the deposition of acidifying substances over Sweden has decreased a tenth of the lakes are still acidified.

Silo with a dosing device for distributing lime

Silo with a dosing device for distributing lime

The causes for acidification include emissions from transport, energy plants, industry and agriculture and are the main cause of the deposition of sulphur compounds. Sulphur emitted to the atmosphere from the burning of coal and oil and air pollution can be transported hundreds of miles. The majority of the sulphur depositions in Sweden come from emissions in foreign countries and from international shipping.

Although deposition of nitrogen pollutants is also responsible for acidification, the acid is partially neutralized when nitrogen is taken up by vegetation. In addition, deforestation contributes to the acidification of soil and water. When forests grow, taking roots up basic plant nutrients and simultaneously releases hydrogen ions. After logging out the basic substances leads to acidity of the soil.

Damage to plants, animals, and structures

The soil’s ability to neutralize the acidifying substances is crucial for the effects on lakes and streams. The soil layers in Sweden are often thin and bedrock is composed mainly of gneiss and granite which have a limited capacity to neutralize acid deposition. The most serious acidification problems are in the south and west Swedish bedrock areas.

Salmon, crayfish and mussels are examples of acid-sensitive species that are already affected at pH values ​​around 6.0. Acidification affects not only different species, but the entire ecosystem structure and function. Acidification can result in that aluminium ions are released, which affect the gill function of several fish species. Acidification also causes changes in the content of nutrients in the soil and corrosion damage to cables, pipes and concrete structures.

Lime prevents acidification

Sweden has added lime to many basins, lakes and rivers since the late 1970s. This supplies calcium carbonate (CaCO3) into the water, which raises the pH levels. Liming is one of the largest conservation efforts ever in Sweden and between 1983-2008, SEK four billion has been used to combat acidification.

“There are different liming strategies to achieve good long-term effects and of course not to harm the ecosystem. One can, for example, lime directly into a lake or a wetland or use a dosing device in a tributary of the lake,” explains Eriksson.

“Liming can lead to large pH fluctuations, and liming of wetlands can provide vegetation changes. If you use a dosing device this can be controlled over time. But the equipment should be able to handle difficult weather conditions with proper alarm systems yo prevent downtime. It is also important that the dosage of lime follows the variations in water flow.”

An installation for liming a stream, or a lake that lies downstream, usually consists of a silo that holds 20-50 tonnes of lime, and some type of metering equipment. It gets filled by a bulk truck and the size of the silo is adjusted so that the intervals between refills are not too short. Tomals multiscrew is driven by an electric motor and a dosing device provides safe discharge of limestone flour. There is no access to electricity but the equipment can be powered by solar cells.

tomal3The technique is based on a number of interacting counter-rotating metering screws that create blockage zones between each screw.

“In practice, the right – and left-handed screws work together. This provides high accuracy, a linear discharge curve and the bolts are self-cleaning. Our multiscrew is designed to meter powder or granules that have a tendency to get stuck in the silo. Here a sensor detects all the levels,” concludes Eriksson.

This article was published in March 2014