Paper Mill Water Circuits

Water Circuits

5.1 Introduction

Water is, besides fibers of course, the key component in pulp and paper manu¬facturing and fulfills numerous functions in the process. It is used as a transport medium, for cleaning and cooling, as a lubricant and finally as the “binding agent” for forming hydrogen bonds between the fibers within the paper sheet. In earlier times paper was produced with a high specific fresh water consumption in the range of 500 l (kg paper)–1. For economic and, in the last decades, also for eco¬logical reasons, the average water consumption has been reduced to less than 15 l (kg paper)–1 as state of the art. This dramatic reduction was only feasible thanks to the increasing closure of the in-house water circuits and because most of the former fresh water consumers are now fed by clarified circuit water

[1].

5.2 Fresh Water
The source of fresh water (FW) in a paper mill is usually surface water and to some extent groundwater, depending on the availability and local conditions. Surface water in particular does not meet the required quality parameters and therefore has to be conditioned by filtering and/or chemical coagulation and flocculation and subsequent sedimentation before use. For boiler house use and in specialty paper, e. g. photographic base paper or cigarette paper, production, the fresh water is softened and/or desalinated. With the limited amount of fresh water available nowadays, this resource must be used efficiently. In general the fresh water taken into a mill is first used for cooling then it is distributed to its process consumers either directly or after further heating. There are only a few fresh water consumers in modern mills, like for instance chemical preparation and dilution systems, seal¬ing water consumers (mainly vacuum pumps) and some high pressure sprays for felt conditioning.

5.3 Process Water

5.3 Process Water

The majority of water used in a paper mill is process water, meaning water that is recycled in the different water loops of the water circuit of the system before dis¬posal. The process water is “produced” in the thickening and dewatering stages of the papermaking process. Due to its content of solid, colloidal and dissolved sub¬stances, the quality of the process water is lower than that of fresh water.
5.3.1

Detrimental Substances
Major process changes in paper production in the last two decades have been
. • a strongly increased use of recovered paper
. • the change from acid to neutral systems in the paper machines
. • the reduction of fresh water consumption.

These changes led to steadily growing problems due to an increased content of so-called detrimental substances in the water loops. Detrimental substances stem from wood components such as resin or lignin derivates, from freshwater as hu¬mic acids, from broke and recovered paper as coating binders, glues and adhe¬sives, from additives as fatty acids or silicates, starch and others. Table 5.1 shows the composition and origin of detrimental substances in the process water [2].
Detrimental substances can cause a lot of problems throughout the whole paper¬making process such as reduced efficiency of additives, reduced optical and strength properties, poor sizing, bad odor, negative effects on drainage and drying and therefore reduced paper machine speed. These substances are the main rea¬sons for deposits and foam generation causing defects in paper as well as resulting in paper web breaks. Detrimental substances include anionic oligomers and poly-

Table 5.1 Composition and origin of detrimental substances.
Chemical compound(s)  Origin
sodium silicate  peroxide bleaching, deinking, recovered paper
polyphosphate  filler dispersing agent
polyacrylate  filler dispersing agent
starch  coated broke, recovered paper
humic acids  fresh water
lignin derivates, lignosulfonates  chemical and mechanical pulp
hemicelluloses  
fatty acids  mechanical pulp, deinking

electrolytes as well as nonionic hydrocolloids [3]. Their content in the water circuits is usually measured with the help of sum parameters as so-called anionic trash, measured as cationic demand by polyelectrolyte titration in a streaming current device or as chemical oxygen demand (COD).
Inorganic dissolved substances, i. e. salts, are measured as increased conductiv¬ity. Salts are also detrimental to the process performance and potentially for the paper properties. Electrolytes reduce the swelling potential of fibers and chloride especially leads to corrosion of machine parts [4]. The content of detrimental sub¬stances in paper mill water circuit systems depends on the input of raw materials, on the output by bleeding through waste water disposal as well as by the degree of transfer to the final paper, on the loop design, and on the presence of “kidney” technologies in the mill.
For different applications, such as sprays in the paper machine, solids (mainly fibers, fines and fillers) in the process water are also disturbing and have to be removed before the water is used.
5.4 Water Circuits
All processing cycles in paper production are connected directly or indirectly by water loops. The objectives of the water circuit system are to offer the required volume rate and quality of water for each consumer and to treat and/or bleed out water containing detrimental substances. A water circuit system of a paper mill usually includes different water loops (Fig. 5.1):
. • Paper machine (PM) loop including the approach flow system and the white water systems I (WW I) and II (WW II)
. • One or two (in special cases, such as market DIP (deinked pulp) production, sometimes even three) water loops in the stock preparation.

 5.4 Water Circuits

End-of-pipe treatment for bled-out water is carried out in waste water treatment plants which are either owned by the mill or the public (see Section 10.1). In a few cases mills have completely closed their water circuits, meaning there is no waste water produced at all and fresh water therefore is only fed at the same volume rate (approximately 1.5 l (kg paper)–1) as it is removed by evaporation in the dryer section of the paper machine and with the rejects leaving the mill.
Due to the challenges mentioned above, water management is an absolute must for every modern paper mill. Some main principles have to be followed in order to manage the water circuit system successfully:
. • Efficient loop separation, i. e. transferring stock from one process loop to the following one only at high consistency (preferably 30 %), which means at low water content, in order to avoid, to the greatest possible degree, transferring detrimental substances from one water loop to the following one.
. • Application of counter current flow, meaning fresh water is added only at the paper machine, excess water from each loop must only be sent backwards and waste water is disposed of only from the first loop in fiber preparation (lowest quality water).
. • No mixing of water from different production lines in mills where more than one paper machine is operated
. • No mixing of water from different fiber preparation lines and/or pulp prepara¬tion plants
. • Use of kidney technologies for removal of solids and/or detrimental sub¬stances
. • Adequate sizing of the water buffers for each water loop in accordance with the stock storage volumes for avoiding uncontrolled overflows in start-up, shut¬down or paper machine sheet break situations.

White Water Circuit System
The white water circuit of a paper machine, also called the paper machine loop, consists of the white waters I (WW I) and II (WW II) and the save-all unit. White water I, coming from the wire section, is used to directly dilute the main stock flow after the machine chest in the approach flow system and for profile control in the headbox. Whitewater II originates also from the forming section but additionally from the press section (after removal of felt hairs, usually with a bow screen), from broke thickening and from the overflow of white water I. White water II is sent to a buffer tank and from there it is used at the end of stock preparation to dilute stock from high consistency (12–30 %) to storage consistency (4–12 %) and for slushing and diluting broke. A defined amount of white water II, preferably the majority of it, plus the trimmings from the forming section are fed to the save-all unit. Save-alls have a dual function: stock recovery and water clarification. Most of the modern paper machines are equipped with a disc filter save-all treating a cer¬tain volume of white water II by filtering it through a fiber mat. This mat is formed by adding a so-called sweetener stock to the white water II filter. For sweetener stock usually the best dewatering stock component used at the paper machine is selected in order to limit the size of the disc filter. In addition to cloudy and clear filtrate, disc filter save-all applications also produce a superclear filtrate with very low solids content. This superclear filtrate is used as a fresh water substitute for spray applications in the paper machine. The clear filtrate is stored in a buffer tank. The cloudy filtrate is usually fed directly back to the inlet of the disc filter, the “used” sweetener, including the recovered stock, is fed back to the thick stock in the approach flow system. DAF (dissolved air flotation) type save-alls are used in older machines and nowadays when a certain degree of ash and fines removal from the process is demanded. In this case, the sludge of the DAF is rejected. The advantages of a disc filter save-all compared to a DAF save-all are higher filtrate quality, no chemical consumption and less space requirements (see Sections
4.2.6.2 and 4.2.9).

There are two reasons why there is always an excess of water present in the paper machine loop. First, the paper machine loop is continuously fed with fresh water, used for spraying and chemical dilution. Secondly, the incoming water con¬tent of the stock is higher (consistency 12–30 %) than the water content in the sheet after the press section (consistency up to 50 % and more). This excess water is sent, in the form of clear filtrate, from the save-all unit backwards, as make-up water, to the stock preparation, thus following the counter current principle. This make-up water is often additionally treated in a DAF with coagulation and floccula¬tion chemicals in order to remove detrimental substances before feeding it back¬wards.

5.4.2
Water Circuit Systems in Stock Preparation

In some cases the water circuit system of a paper mill consists of only one water loop, for example when chemical pulp is the only fiber source or in lower quality packaging paper production. For systems using mechanical fibers and/or recov¬ered paper, the strict separation of the water loops in stock preparation from the paper machine loop is essential in order to meet high runability and quality de¬mands because this strategy keeps detrimental substances from the paper ma¬chine. Depending on the required quality of the prepared fiber stock and thus on the design of the stock preparation system, the water circuit system in the stock preparation can consist of from one to three water loops. The loops are separated by the thickening/dewatering process stages (disc filter plus wire press or screw press). The filtrates of these stages are sent backwards for dilution purposes in the same loop, the excess water (usually clear filtrate from the disc filter) replenishes the preceding loop. A water buffer tank, usually fed by clear filtrate from the disc filter in loop I, avoids uncontrolled overflow to the waste water treatment plant. Waste water is disposed of from the first water loop as filtrate from reject and sludge thickening, as these filtrates are of low quality. Usually they are treated chemically/mechanically in a DAF unit. Depending on the fresh water consump¬tion, a certain amount of this treated filtrate can be recycled into the process.

5.4 Water Circuits
To reduce the content of detrimental substances within a water loop, DAF units are used as so-called kidneys for circuit water cleaning, usually in the second loop of the stock preparation. In this case a combination of coagulants and flocculation aids are added to part of the disc filter clear filtrate in loop II in order to precipitate and flocculate detrimental substances into a flotatable form and subsequently re¬move them from the process in the DAF. A high ash content in recovered paper is also problematic for some paper grades. Here washing stages in the stock prepara¬tion only make sense when large amounts of ash need to be removed. Therefore nowadays the filtrates of screw presses are often deashed in order to achieve a reduction in ash content in the final stock. Usually the screw press filtrate is pre-calibrated in a spray filter for fiber saving and the fiber-free but ash-containing filtrate is then also sent to a DAF unit after the addition of flocculation aids.
Examples of Mill-wide Water Circuit Systems
Three examples show in detail how the different water circuits for the various paper grades are designed.

5.4.3.1 Water Circuit System for Graphic Papers

5.4.3.1.1 Water Circuit System in a DIP-based Newsprint Mill
Figure 5.2 shows the water circuit system for a modern newsprint mill using deinked pulp (DIP) as the fiber source.
 
The system consists of three water loops, two in stock preparation (because of the combination of flotation – dispersion – flotation) and one in the paper ma¬chine. The water circuit design follows the previously mentioned principles of counter current flow and thereby allows the fresh water to be used to the max¬imum effect. In addition the save-all is followed by an additional DAF unit. DAF 1 is used as a kidney for removal of detrimental substances, treating the make-up water from the paper machine (clear filtrate) together with part of the clear filtrate from the disc filter in loop II. DAF 2 treats the filtrates from reject and sludge dewatering. The clarified water is partly recycled back into loop I, the rest being sent to the effluent treatment.

5.4.3.1.2 Water Circuit System in a DIP-based LWC Mill
Figure 5.3 shows the water circuit system for light weight coated (LWC) paper also using DIP as the fiber source.
There are again three process water loops and the counter current principle is consequently used. Compared to the newsprint, LWC based on DIP requires a higher DIP quality (see 4.3.2.1.1.). The lower ash content required in the final stock and the higher demands on brightness justifying two bleaching stages neces¬sitate bigger efforts also in the water circuit system, i. e. in kidney technologies. Therefore three DAF units are installed. DAF 1 is used for removal of detrimental substances from the paper machine make-up water. DAF 2 provides the necessary ash removal from the filtrates of the screw presses in loops I and II after pre-calibration and treatment of the disc filter filtrate in loop II. DAF 3 clarifies filtrates
 
5.4 Water Circuits
 
from reject and sludge treatment, which are then partly recycled, the rest being sent to the effluent treatment.
5.4.3.1.3 Water Circuit System in a Mill for Packaging Paper
The configuration of water loops in systems for packaging grades is less complex than for graphic paper grades. Older or lower quality packaging paper systems are often single loop systems. Modern systems with an integrated LC-screening in the stock preparation plant use a thickening stage before storage and therefore consist of two water loops, one for the paper machine and one for the stock preparation system. Figure 5.4 shows such a water circuit system in a modern mill producing testliner and fluting.
Again, the fresh water is added only at the paper machine. White water II is used for redilution after thickening in the stock preparation plant. The save-all clear filtrate is used for make-up in the fiber preparation system. Filtrate from the thick¬ening stage in the fiber preparation system is the main source of water for slush¬ing and dilution. Waste water is sent as filtrate from reject handling to the end-of-pipe effluent treatment.
Current Limits on Circuit Closure
The maximum re-utilization of the water employed is limited by the various detri¬mental substances in the process water. Their concentration, measured as COD concentration in the water (in ppm), increases at a disproportionate rate when reducing the specific effluent (in l (kg paper)–1). The higher the water circuit sys¬tem is closed, the more COD (in (kg COD) (t paper)–1) is loaded into the paper instead of being bled into the waste water (Fig. 5.5) [5]. This leads to a dispropor¬tionate increase in various problems due to detrimental substances, as mentioned above. Therefore the state-of-the-art systems are limited to certain specific (l (kg paper)–1) effluent volume rates with respect to certain minimum fresh water con¬sumption as shown in Table 5.2.
 
Consequently, the multi-loop systems for graphic paper grades nowadays follow the counter current principle and partly use circuit water cleaning according to the kidney principle. So it is possible to reduce the specific water emissions down to 8–10 l (kg paper)–1. Further closing would lead to increased amounts of detrimen¬tal substances. This would negatively affect product quality, e. g. decreased optical properties and increased stickies and dirt content as well as runnability problems on the paper machine, such as felt and shower pluggings, reduced retention, scal¬ing and slime formation. The typically simpler board and packaging grade systems with their somehow lower demands on water management can work with specific
Table 5.2 Present limits of system closure.
System application  Specific effluent l (kg paper)–1  volume/Disturbing effects limiting further closure
Graphic paper grades  8–10  decreasing product quality
several water loops   felt/shower plugging
counter current flow   lower particle retention
circuit water cleaning (kidney)   scaling
  slime
Board and packaging grades  3–5  odor problems
  (water and product)
single or two loops   corrosion
water treatment only for showers   deposits

5.4 Water Circuits
 
effluent volume rates of a minimum of 3–5 l (kg paper)–1. Further closing these loops without using additional internal circuit water cleaning could lead to odor problems both in the mill and in the paper itself, as well as corrosion and deposits and the paper machine runnability would be significantly reduced.
Zero Effluent Systems
Reducing the effluent volumes to zero means that fresh water consumption is reduced to approximately 1.5 l (kg paper)–1. Here problems would arise from the extremely high amounts of detrimental substances, which will be bled out of the process only by transferring them into the paper. The only solution to these prob¬lems is to remove the detrimental substances from the process with suitable highly efficient kidney technologies. These include, in addition to circuit water cleaning by coagulation and flocculation with subsequent removal in a DAF unit, anaero-bic/aerobic combinations of biological treatment (see Section 10.1), different membrane filtration technologies like micro-, ultra- or nano-filtration down to re¬verse osmosis or evaporation [6, 7].
Some packaging paper mills in central Europe today run with zero effluent systems (Fig. 5.6 ) using a combination of anaerobic/aerobic biological treatment (see Section 10.1) as kidney technology for COD reduction [6].
The driving forces for reducing the effluent to 0 l (kg paper)–1 for these mills were unique to each mill. In one case there was high cost-saving potential by completely avoiding effluent fees for disposal into a public effluent treatment plant. In another the reason for complete water circuit system closure was the lack of available fresh water due to the local conditions when installing additional pro¬duction capacity [7].
The other mentioned kidney technologies, i. e. membrane filtration and evapora¬tion, are mainly known as pilot scale applications as they are quite cost-intensive and still not proven state-of-the-art