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Water - every drop counts

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Water - every drop counts

February 21, 2011 - 14:00
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BRUSSELS, Feb. 22, 2011 (RISI) -In part I of a two part article, the hugely important subject of water is addressed. Achieving and maintaining maximum usage with a little waste as possible is no longer a desire, it is becoming a neccesity. Part II can be read on the Environmental Matters newlsetter next month.

Water is a critical process medium in the pulp and paper industry. In fact, the quality of the water used is so important that it has a direct effect on intermediate and finished products. With rising energy costs and stricter environmental legislation, attention today is on comprehensive and intelligent water management solutions that take a plant's complete water cycle into consideration.

From the point water enters a mill, it is used in a number of individual process steps. Along the way, the process water is contaminated by a multitude of different substances, and it has to be clarified before it can be reused or discharged.

View of the new water treatment plant from Siemens at Albert Köhler in Gengenbach

Intake water treatment

The quality of water entering a production facility - whether taken from rivers, lakes or springs - is seldom good enough for most processes. If untreated, humic acid or extremely fine pigments in the water can cause discoloration, which can have an adverse effect on paper quality. Furthermore, incoming water may contain solids, bacteria or salts, all of which could build up, clog nozzles, and attack components such as heat exchangers. A thorough analysis of the incoming water is the first step in deciding what technologies are needed.

Solutions to clean incoming water include rakes to remove coarse loads such as branches, leaves and residual materials. For greater quantities of solids loads, sand filters as well as sedimentation or disc filters can be employed. In the case of discolored fresh water or water with high salt loads, membrane technology is often the preferred method. Membranes filter out the finest substances; with several thousand robust, chemical-resistant membranes in a module, even microscopic suspended solids are captured. Thanks to optimized production processes and high volumes, membrane filters are also a cost-conscious solution.

The wastewater passes through the primary treatment stage first

Boiler water

Fresh incoming water also has to be further prepared for the boiler. The ultimate goal is to prevent scale and corrosion in the boiler and condensate area using as few chemicals as possible. The Siemens' Sipaper Water approach includes a three-stage cascade of filtration steps. The first step is ultrafiltration to generate particle-free water. The Vantage® UFI and Memcor® ultrafiltration solutions in the Sipaper Water portfolio can replace several treatment stages and save space in the plant. The technologies also reduce the need for chemicals for a further cut in operating costs.

The second stage is reverse osmosis to produce high-quality water from the prefiltered water. The Vantage® reverse osmosis system is used in the Siemens solution. The compact system is easy to control and is configured for long operational sequences. Depending on the plant requirements, Vantage® reverse osmosis comes preassembled, and a number of membranes are available, including low-energy membranes for special applications. The system has monitors for observing reverse osmosis performance and its intuitive touchscreen control elements can be seamlessly integrated into the remote monitoring system for the entire plant. The feed pump with frequency converter drive also saves energy.

The final stage involves the Ionpure® electro-desalination module (CDI), which is an economical and powerful alternative to mixed-bed deionization (MBDI) systems. Ionpure® does without chemicals and uses only electricity to get the job done. The technology is safer and more eco-friendly than other systems because it needs no acid or caustic chemicals.

The three-stage cascade protects downstream devices from contamination by suspended solids, extends service life, and ensures consistent water quality regardless of the quality of the feed water. The filter products used require no resource-intensive waste handling and they can be recycled. With its modular, stackable design, the Siemens system needs half as much space as conventional solutions.

The membrane bioreactor consists of the biological cleaning (aeration tank) and the MOS

Added bonus

As diligent as operators have to be about the quality of water they introduce into the production cycle, they must be equally careful about the water they emit from production. With increasingly strict environmental regulations, wastewater treatment is a key concern in the pulp and paper industry. The effectiveness, efficiency and flexibility of the wastewater plant directly impact the cost-effectiveness of production. Sipaper Water includes customized solutions for every stage of wastewater treatment and for every type of in-house wastewater plant.

During preliminary cleaning, solids are first removed through sedimentation or microflotation (DAF) and dewatered by means of dewatering equipment such as gravity tables, belt filter presses, and screw presses. Fluctuations in hydraulics or solids - which can be caused by cleaning cycles, changes in products, overflows, or infeed from multiple paper machines - have to be buffered with upstream buffer tanks or equipment such as disc filters and gravity tables.

The wastewater is then sent through the biological stage to degrade suspended organic constituents. The degree of degradability depends on the ratio of biodegradable constituents (BOD) to chemical oxygen demand (COD), and the ratio forms the basis for the design of the wastewater purification system. When the COD and BOD ratio is around 0.5, it is recommended that the biological stage be split into two stages: an anaerobic stage and an aerobic stage. During the anaerobic stage, COD and BOD are reduced to a low level. At the same time, biogas is generated that can be used as a source of energy. Siemens offers a total turnkey package covering everything from the bioreactor to gas piping, cleaning and storage up to energy and electricity generation. In fact, from a pollutant load of 10,000 kg COD per day, about 3,000 Nm3can be produced, enough to operate a boiler with a firing capacity of 900 kW.

In the secondary sedimentation stage, the biological sludge that degrades suspended organic matter is separated from the purified wastewater either through sedimentation or, less frequently, through microflotation. Because separation of the sludge requires large surfaces, which incur added costs, Siemens offers an efficient and innovative solution: the membrane bioreactor (MBR). The MBR guarantees better separation efficiency than secondary sedimentation and better water quality using a lot less space. After undergoing treatment with an MBR, water can safely be released into the environment - or reintroduced into the production cycle.