Choosing the Correct Instruments for Biological Water Treatment

ABB instrumentation was the clear choice when the Swedish pulp producer Vallviks Bruk AB built a new plant for biological water treatment. Located about 200 km (125 miles) north of Stockholm, Vallviks Bruk AB, produces softwood kraft pulp and chemical pulp for export. A tour of the mill shows that the relationship with ABB is firmly rooted. The plant contains many types of ABB Measurement products, originating from known quality manufacturers linked to ABB, Fischer & Porter, Hartmann & Braun and Alfa Laval, to name a few.

"There are an incredible number of flows in this kind of plant and we have almost always bought instrumentation from ABB. Only the best will do," says Ronny Svensson, supervisor for the instrumentation workshop where about ten people maintain the mill instrumentation.” (Above) the mill foreman for the instrumentation repair shop at Vallviks Bruk AB, Ronny Svensson, says that the water inflow to the biological treatment is about 450 cubic meters per hour (~2000 gpm). Oxygen and a type of plastic token are added into the tanks, facilitating the bacteria to multiply.

Increased Environmental Requirements

The pulping process requires a lot of water. Previously the water was purified in the mill's mechanical treatment plant. This is a large, open rotation basin where the sludge and the water are separated. Some years ago, increased environmental requirements led to a major investment in a biological treatment plant. It was finished and commissioned in November 2011. Biological treatment means that the process water is purified by using of micro-organisms. Not only can the organic substances be removed by bio-degradation, but also inorganic salts of nitrogen and phosphorus can be degraded. "The dirtiest water goes into the biological treatment plant. The mechanical treatment plant is still used for water needing little treatment," explains Svensson.

Electromagnetic flowmeter FSM4000-SE41F

Plastic Tokens Carry Bacteria

To make chemical pulp requires a high temperature for the pressure cooker to release the wood fibers and for bleaching and washing the fiber. The warm so-called 'back water' comes to the biological treatment plant via two large tanks, each with a volume of 1,355 cubic meters (48,000 cubic feet). As the water temperature drops from 60-70 °C to 37 °C (140–158 °F to 99.5 °F), the perfect temperature level for biological treatment, oxygen is added while the tanks are stirred to keep the bacteria alive. Also added are lots of so-called carriers, namely small plastic tokens, to which the bacteria can cling to aid their proliferation. The next step is the coagulation basin where nutrients and iron sulfate are pumped in to really begin the degradation process, also polymers are added to help separate the sludge by flocculation.

After that the water enters the rotating basin where the sludge is separated mechanically. Finally, the water is squeezed out of the sludge and the leftover waste product looks like horse manure. It is used as an ingredient in particular topsoil or pellets. Vallviks Bruk AB produces around six containers a week of the waste sludge.

Final product – compressed pulp

New Filter Presses

At various stages of the process flowmeters keep track of levels and additives. "We are still optimizing the installation performance in terms of additives, but the instruments have functioned as they should," says Svensson. The extensive water consumption at the plant would actually require a much greater biological purification process than Vallviks Bruk AB has invested in so far. The reason that the biological treatment suffices is the installation of two new filter presses, installed by Metso Finland. The filter presses dewater the pulp much more efficiently. ABB flowmeters control the flows at the new filter presses. ABB pressure sensors measure the pressure in and out of the filter presses, which reduce the volume of water to be treated by half. "Without the filters it would have been impossible to create cost-efficient bio-treatment," says Svensson.

Ronny Svensson standing at the filter presses, where many ABB Measurement products are also installed

Facts

Vallviks Bruk AB

• Is one of two mills in the Swedish Rottneros Group. The mill has about 160 employees and a production capacity of approximately 240,000 tons per year. Products produced at Vallviks Bruk AB are bleached kraft pulp ECF (Elemental Chlorine Free) and UKP (Unbleached Kraft Pulp).

ABB

• Has supplied instrumentation for flow, pressure, temperature, level and analysis. Included are 55 electromagnetic flowmeters in different dimensions, approximately 20 pressure sensors in the 260 series, 12 temperature sensors with transmitters and an FSM 4000 electromagnetic flowmeter, AC.

Final product – pulp

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.