Water conditioning and disposal challenges in paper production

Water is an essential component in the paper manufacturing process, serving as the coolant and cleaning substance.

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Water is an essential component in the paper manufacturing process, serving as the coolant and cleaning substance. A new water treatment plant was an important feature of the new paper factory constructed for the Myllykoski corporation in Plattling, Germany. Schmidt Helmut and Harald Stock look at the challenge of conditioning and disposal.

Paper machines reign supreme when it comes to large and complex mechanical equipment. Take the delivery of a machine in the MD Papier works in Plattling, where a machine hall 375 meters long had to be erected for its delivery. Every minute, 1,700 meters of paper run through the 10-meter wide machine. Up to 400,000 tonnes of printing paper can be produced annually. Combined with the output of the two existing machines, the factory produces almost 800,000 tonnes of paper every year.

For these production targets to be achieved, the paper machines have to run without any interruption. This is only possible when there is a guaranteed supply of water. The requirement is for more than 30,000 m³ per day - a considerable amount.

Water impurities

Operated by OEWA Wasser und Abwasser GmbH, a German subsidiary of the Veolia Water, the clean water and wastewater plant has guaranteed optimal water supply and disposal to the Myllykoski corporation until the end of 2015 — a task that requires considerable expertise. Water, as a process medium, has a significant effect on the quality of the end product, e.g. the motion and printing qualities of the paper.

These days, depending on the type of paper, between 10 and 15 litres of water are needed to manufacture one kilogram of paper. The water circulation system within the company reduces the specific wastewater to a maximum of seven litres per kilogram of paper. Further refinement of the circulation system is only possible to a limited degree, because it causes a rise in temperature and an increase in the concentration of impurities. A high concentration of impurities in the water influences the effectiveness of the chemical agents used in the process, prevents dehydration of the paper, and leads to poor sheet formation and therefore a reduction in paper quality.

Multi-stage water purification

In Plattling, the fresh water is extracted from the nearby river Isar, and pumped to the factory site through pipes. It is initially cleansed via precipitation and coagulation of finely suspended and colloidal dissolved substances such as organic plant remains, clay and silt. After subsequently flowing through the sedimentation tank and open sand filtration, the water is free of suspended matter and organic components.

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Fig. 1:Water serves as a coolant and cleaning substance, but is also the most important process agent used in production.

The water from the river Isar is too hard for paper production, so some of the water is first routed through a kation exchanger where it is completely softened. The softened substream is then mixed with the unsoftened water to give a total degree of hardness of six on the German scale.

An uninterrupted production process depends not only on a regular supply of utility water, but also on the continuous treatment of wastewater. Construction of the new paper machine in Plattling was therefore combined with installation of a modern water treatment plant with the sole purpose of cleaning the wastewater from the new production process.

The treatment plant on site had been expanded and modernised as recently as 2004 for processing wastewater from the two existing paper machines. The new plant was erected to be as compact as possible and features a three-stage cleaning concept. Following primary clarification, the water flows through two biological cleaning stages and after secondary clarification, a third stage for final cleaning. As is customary for paper factories, the wastewater in Plattling comprises mainly biodegradable polysaccharides and non-biodegradable lignin. Nitrogen and phosphor, which are used in biological cleaning stages as nutrients for the micro-organisms, are almost completely absent. They must be added in the form of urea and phosphoric acid.

The so-called Moving Bed Biofilm Reactor (MBBR) is used as the first biological stage in Plattling. Biofilm carriers made of plastic are used in this form of aerobic cleaning. These carriers have a large internal surface area and are continuously held in suspension and in motion during the aqueous phase. The wastewater is fed into the conditioning reactor where the biofilm on the carriers degrades the harmful substances. An aeration grid in the reactor ensures that oxygen is available and mixed in. Oxygen concentration is monitored by probes and treated water finally exits the reactor through a sieve that retains the biofilm carriers. Unlike other biological conditioning processes, the MBBR process is less prone to interference and can handle the most extreme load conditions without any problems.

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Fig. 2:The tank for low-level stimulation has a volume of 13,000 m³. Mainly organic compounds are removed from the water here.

After the high-level carrier biology follows the low-level stimulation. Here, after eliminating the easily degradable organic substances, the persistent organic components are removed from the wastewater. Through a subsequent purification tank, the wastewater eventually arrives in the flotation tank for final cleaning. Here the non-biodegradable residual organic content is largely eliminated in order to achieve the legally required residual concentration of CSB and P.

Open-loop control

For the electrical and control equipment of the process water and wastewater treatment plants, OEWA GmbH commissioned the automation specialists who had gained a good reputation from earlier successful projects: BN Automation AG of Ilmenau. The company has been developing solutions in the automation sector since 1990 and has specialised in processes for supply and disposal, environmental systems and process engineering. The basic task was to implement a reliable and integrated automation solution within an extremely tight time schedule.

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Fig. 3:Employees of the BN Automation AG from Ilmenau brought their experience to bear during the planning phase and performed extensive engineering work.

The new paper factory was to start production after a construction phase of only twelve months. Reconnection of the freshwater plant had to take place during normal operation, because it was essential to supply the existing machines continuously with water and to provide uninterrupted wastewater treatment. The short implementation phase meant that project engineering and commissioning had to be performed alongside construction.

Only one process control system could be considered for total integration of the existing plant and which offered an integrated solution from field level to enterprise level. Over 850 drives and measuring points have to be monitored and controlled in the process water and wastewater plant.

The system chosen was the SIMATIC PCS 7 control system from Siemens. For the Plattling site, standard system structure with a redundant server system was chosen along with three automation stations for freshwater conditioning, wastewater treatment and the existing equipment.

The distributed I/O complete with drives equipped with soft starters or frequency converters are connected directly via PROFIBUS. A central control room contains two OS clients which are used for operation and monitoring of the complete water management system. .

The commissioning challenge

For OEWA, the extremely short implementation phase for construction of the paper machine meant a correspondingly short deadline for implementation of the water treatment plant. The SIMATIC PCS 7 project was set up in accordance with the technological structure derived from the list of drives and measuring point of the switchgear configuration.

After exporting all the data points, the initial parameters could be automatically assigned to the software typicals using the integrated import/export wizards. Thanks to standardised software configuration using the central engineering system of SIMATIC PCS 7, commissioning was fast. Only six typicals were required, for example for frequency converters, soft starters and specific measurements. Extensive system simulation and test functions allowed detailed tests to be conducted beforehand that provided important insights into the technological processes.

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Fig. 4:The clear operation and monitoring screens of the process control system present all the information on process-oriented measured variables from the various plant components.

During the actual commissioning on site, BN Automation set up a mobile control desk. Installation of a wireless network made it much easier to test for direction of rotation and limit approach. Instead of the usual system of placing one person in the field and a second person in the control room, a project engineer could simply use a WLAN laptop to trace the response to changes to equipment parameters locally in the process control system. All the instrumentation and control equipment was commissioned in this manner. Secure access over the Internet and UMTS via a VPN router also enabled BN Automation to provide the operating team in Plattling with immediate remote support from Ilmenau. The engineering experts were able to connect to the system directly to provide targeted assistance at an early stage.

Selection of relevant parameters in wastewater treatment

AOX (adsorbable organic halogen compounds)

This is used as a summation parameter in the qualification of wastewater. This value is used to designate the sum of organic halogen compounds that is adsorbable by activated carbon. Each different halogen compound presents a different, specific, potential risk.

CSB (chemical oxygen consumption)

The chemical oxygen consumption is a measure for the sum of all the organic substances present in the water. It can be used to assess the degree of pollution of the wastewater. The CSB specifies the quantity of oxygen that is required for oxidization (effective value for consumption of oxidation agent).

BSB (biochemical oxygen consumption)

The biochemical oxygen consumption (BSB) is the volumetric mass of oxygen that is consumed by the micro-organisms in order to degrade the organic substances present in the water oxidatively at 20 °C (effective value for oxygen consumption). BSB5 is usually specified as the characteristic number. This specifies the quantity of oxygen in mg/l which is consumed by bacteria and other small organisms in a water sample over a period of 5 days at a temperature of 20° C in order to degrade the substances in the water aerobically.

Sludge index

The sludge index is a measure of sedimentation of the sludge in the biological cleaning stage. It is calculated from the quotient of the sludge volume and the content of dry matter of the same sludge.

TOC

The Total Organic Carbon (TOC) is a summation parameter that indicates the concentration of organic substances in the water.

Clear visualization

For optimized operation of the paper machines, water temperature, pH-value, hardness and other parameters must lie within precisely defined limits. This also applies to wastewater treatment: to maintain the balance in the biological processes required for optimal degrading despite varying degrees of pollution, oxygen, nitrogen, phosphor and many other values or summation parameters, must be aligned with specific characteristic curves.

For reliable operation and optimization of the plant, it is important that all information from the process control system is available centrally.

The user interface of the process control system provides all information, alarms and warnings for reliable operation in a clear overview. All the measured variables from the different subsystems that are relevant to the process can be read at a glance.

These include overviews of the softening and fresh water conditioning systems, mechanical and biological cleaning and the associated conditioning of process agents, as well as screens for sludge dehydration and the silo plant. The existing subsystems have been completely integrated into the operating concept of SIMATIC PCS 7, without having to replace the hardware, and have a uniform visualization base.

A laboratory interface allows CSB, BSB or sludge index values to be entered that have been manually determined by analysis. This data is managed via the technical information and operating system (TIBS) developed by BN Automation. It acquires and saves the process data supplied by the process control system for the statutory verification processes. WWi

Author's note:Schmidt Helmut is from the industrial automation systems division of Siemens AG and Harald Stock is from the production and technology department of BN Automation AG.

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