twinning project si04/en/01 reference document on the

Twinning Project SI04/EN/01 Integrated Pollution Prevention and Control (IPPC)

Reference Document on the application of

Best Available Techniques to

Industrial Cooling Systems

December 2001

Activity 6 Mission 6.1

Erstellt von:

Dr. Kurt Müller Bavarian Environment Agency

Ljubljana im März 2006

A project supported by the EU Phare Programme - Implemented by the Slovenian Ministry of the Environment and Spatial Planning, the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety and the Flemish Institute for Technological Research

Ljubljana, March 2006 BREF Industrial Cooling Systems – Activity 6; Mission 6.1 from 2

Reference Document on the application

of Best Available Techniques

to Industrial Cooling Systems

December 2001

Dr. Kurt Müller

Bavarian Environment Agency



Contents 1. Scope of the BREF-document 2. Contents of the BREF-document (overview) 3. General BAT Concept 4. Elements of the primary BAT-Approach and general BAT 5. Waste Water in Power Plant Facilities



1. Scope of the BREF-document The scope of the BREF covers all types of cooling systems that are used in connection with activities enlisted in annex I of the IPPC directive, i.e., both industrial cooling systems, like the systems used in chemical industry, pulp and paper, steel production or textile industries, and in power plants. There are significant differences between industrial and power plant cooling systems with respect two the processes to be cooled. These differences have to be regarded in developing and applying BAT. The BREF presents a thorough description of different types of cooling systems. There are two major variants concerning the use of cooling water: In the once-through system, the water is used only once and is then discharged to the receiving water body, either with or without cooling in a cooling tower. In recirculation systems the water is used multiple times in a circuit and cooled in a cooling tower before reuse. Depending on the cooling medium used in the cooling tower we can differentiate between wet, hybrid and dry cooling systems. In the case of wet and hybrid systems, waste water is discharged as blow down from the circuit system. We can further differentiate between direct cooling and indirect cooling. In direct systems, there is only one level of heat exchangers, where the cooling water and the process medium meet in the cooling process. Indirect systems contain two levels of heat exchangers: a first one, where the primary cooling water and the secondary cooling water meet, and a second one, where the secondary cooling water and the process medium meet in the cooling process. Leakages of the process heat exchanger release hazardous substances into the cooling medium. Indirect systems prevent discharges of the released substances into the receiving water body by confining them to the secondary cooling system. Table 3.1 of the BREF comprises the basic differences between the major types of cooling systems concerning environmental aspects. The main differences are characterised by the degree of energy consumption, i.e., the over-all energy efficiency, and the emissions to the receiving water, i.e., in particular the load of heat and additives. Therefore, in most cases these aspects will be the basis for decision making when designing a new system or checking an existing system with respect to the implementation of BAT. There are also more complex systems in operation, that can switch between a once-through and a recirculation type of operation, and in addition can apply a combined process, in which part of the heated cooling water is discharged to the river without cooling and the remaining part is reused after having passed through the cooling tower. To sum up:

• The scope of the BREF covers both industrial and power plant systems • The BREF deals with different types of cooling systems, which show significant differences

in terms of environmental aspects (esp. energy efficiency and emissions of heat and chemical substances).

• Complex industrial and power plant sites may be equipped with different types of cooling systems that may be operated in a combined fashion.



2. Contents of the BREF-document (overview) The BREF-document consists of the main document and 12 annexes. The main document comprises the following 5 chapters, the most important being chapter 4, which describes BAT:

1. General BAT Concept 2. Technological Aspects 3. Environmental Aspects 4. BAT 5. Conclusions and Recommendations

In the 12 annexes listed below, additional technical background information and examples are given, e.g. concerning the selection of material for cooling systems or the selection of additives.

I. Thermodynamic principles

II. Energy saving by optimised cooling III. Shell and tube exchangers IV. Selection of material for cooling systems V. Additives for cooling systems

VI. Example of legislation VII. Safety Concept (VCI-Concept)

VIII. Assessment of additives for cooling systems IX. Model for estimating emissions of biocides in the blow down X. Investment cost and operational costs

XI. Examples for techniques to be considered within the primary BAT approach XII. Special Application: Power Industry



3. General BAT Concept In chapter 1 of the BREF-document the general BAT concept is described. This information is very important when it comes to implementing the BAT in the permission procedure. The general concept is based on three kinds of approaches:

• Horizontal approach • Integrated approach • Primary approach.

Horizontal approach means, that the BREF deals with basic aspects of BAT for cooling systems used within industrial and power plants, independent of the process, in which the cooling is applied. Thus, the BREF does not discuss specific aspects of industrial production or power generation processes. Integrated approach means, that the BREF does not contain ready-to-use model solutions that could be applied for every single case situation without modifications. On the contrary, many aspects and influence factors have to be considered to arrive at an adequate site-specific solution. The most important factors are shown in this scheme:

• If a new plant is to be built, both design and operation can be controlled • In existing plants, modifications often are restricted to operational aspects • The performance of a cooling system with respect to environmental aspects is far more

determined by design (ca. 80 %) than by operation (ca. 20%). • The design features of a cooling system are strongly determined by the process

requirements (e.g. required cooling capacity, water demand and energy efficiency). • On the other hand, the site limitations have to be regarded, esp. concerning climatic

conditions, water availability and discharge limitations (tolerance of the receiving water body).

• Design and operation of a cooling system have to meet the relevant environmental requirements, in particular concerning emissions of heat, chemical substances, noise.

• Last but not least, the investment and operation costs will influence the final solution in a decisive way.

The BREF intends to provide guidance for integrating these different aspects to reach an adequate site-specific solution. This guidance is called the primary approach. The primary approach is a general approach to deduce adequate site-specific BAT requirements by identifying the essential influence factors. It is based on a sort of worksheet that is summarised in Fig. 1. In Tab. 4.3 – 4.12 techniques are compiled, that have been identified as general BAT. These techniques have to be considered, when applying the primary approach to deduce the site-specific BAT. The aim of the primary approach is to reach an optimised technical solution with respect to four elements of decision-making. The first three elements comprise the requirements that are essential for maximum overall energy efficiency:

• First, the heat management has to be optimised to reduce the required cooling capacity to a minimum.

• Second, the process requirements have to be analysed to provide a framework concerning the required performance of the cooling system.



• Third, the site characteristics, e.g. the availability of water, are important with respect to energy efficiency, but they are also important with respect to environmental impacts, e.g., concerning the relevance of noise reduction measures.

• The fourth element of the primary approach comprises techniques identified as general BAT. It has to be decided, whether these techniques make sense for the activity under consideration and, thus, are to be implemented in the permission procedure.

Thus, the decision making process is based on two counterparts: maximum overall energy efficiency and minimum environmental impact. The requirements deduced from both have to be balanced to find an adequate site-specific solution with respect to choice of cooling technology, location of the activity, plant design and plant operation. The requirements defined in this way represent the site-specific BAT.



4. Elements of the primary BAT-Approach and general BAT

4.1 Preventive Approach: Heat Management

4.2 Process Requirements

4.3 Energy Efficiency

4.4 Water Requirements

4.5 Fish Protection at Water Intake Devices

4.6 Minimisation of Emissions to Water

4.7 Minimisation of Emissions to Air

4.8 Minimisation of Biological Risks

4.9 Minimisation of Noise

The list above presents the main elements of the primary approach and of the general BAT. This structure roughly corresponds to the structure of chapter 4 of the BREF document.

4.1 Preventive Approach: Heat Management Before planning the cooling system for an industrial facility or a power plant, the heat management should be optimised as described in chapter 4.2.1 of the BREF-document. This particularly means an analysis of heat reuse options, either internal or external. Internal reuse could be heating of process streams or the use of heated cooling water as process water, e.g. for dye baths in textile finishing (as required by the BREF document textiles industry). In external reuse, excess heat is used in adjacent installations or for district heating. Heat reuse reduces the required cooling capacity, but depends very much on site-specific and process-specific factors.

4.2 Process Requirements In Tab. 4.1 of the BREF-document, important factors for cooling system design according to the process requirements are presented. The cooling requirements of the process essentially determine the design data for the cooling system, in particular the required end temperature, the cooling capacity and amount of heat to be dissipated. The temperature level of the excess heat is important for deciding, whether air cooling, or at least air precooling, can be applied. If hazardous substances are used in the process, an indirect cooling system may be required to prevent damage by leakage.



4.3 Energy Efficiency In Tab. 4.3 of the BREF-document general techniques for increasing the overall energy efficiency are presented. The aim is to reduce direct and indirect energy consumption for cooling system operation as far as possible. The lowest energy consumption is linked to once-through systems (10 kWe/MWth), whereas recirculation systems with wet cooling towers have a significantly higher energy demand (27 kWe/MWth). Once-through systems with cooling tower lie in between. Consequently, the use of a once-through system is considered BAT, if supported by site-specific factors. Energy consumption can be minimised by choosing energy efficient design, e.g. for pumps, ventilators, pipes a.s.o.. The cooling efficiency is significantly reduced, if a short circuit between the outlet of the heated cooling water and the intake from the water body is generated. This situation has to be prevented by adequate location and design of the intake and outlet devices. 4.4 Water Requirements Basic differences between types of cooling systems not only exist with respect to energy efficiency, but also with respect to water requirement, as shown in Tab. 3.3. Once-through systems have the highest demand of ca. 86 m³/h/MWth, whereas recirculation systems require a significantly lower amount of water (2 m³/h/MWth when a wet cooling tower is used, 0,5 m³/h/MWth in the case of a hybrid tower). Systems with air-cooling have a zero water requirement. The general BAT for reduction of water requirement is presented in Tab. 4.4 of the BREF-document. It has to be stated that there is a conflict between maximisation of energy efficiency and reduction of water requirement (and of heat emission). Once-through systems provide maximum energy efficiency, but depend on sufficient water supply and deliver high heat loads. Thus, recirculation systems have to be used, if there is a limitation of water availability or of the receiving capacity of the water body, in particular concerning the heat load. When applying a recirculation system, choosing an elevated number of cycles of concentration can further reduce the water requirement. If water availability is insufficient for wet cooling systems or if plume formation has to be controlled, hybrid systems or air-cooling can be applied. This may especially be relevant, if the use of ground water has to be reduced. If water availability or the receiving capacity of the water body is limited only during certain periods of the year, combined systems should be used, that can be operated in the once-through or recirculation mode or in a combination of both. Cooling towers can be built in a cell-type fashion and the number of towers in action can be adjusted to the cooling capacity actually needed.



4.5 Fish Protection at Water Intake Devices This BAT-element is not linked to the performance of a cooling system, but to how the water is taken from the water body. Water intake currents can entrain fish, which are then injured or killed upon contact with the intake structures or the water treatment devices (e.g. screens, sieves). Tab. 4.5 of the BREF-document presents some rather vague ideas how to minimise these harmful effects. Generally, the location and design of intake (and outlet) devices should be optimised, based on an analysis of the site-specific biotope. Experience shows, that most fish species have a good chance to escape entrainment, if the cross current generated by the intake is lower than 0,3 m/s. Repellent devices at the intake, using both power pulse current and light barriers, further support this effect. It is important to change the amplitude and frequency of the electric current in a random fashion to avoid adaptation of the fish. Washing water from screens and especially sieves may contain small fish and fry and therefore should be returned to the receiving water body without further treatment.

Kurt Müller, Dr., STEBavarian Environment AgencyLjubljana, 06 -08 March 2006

Activity 6: Training on the application procedureMission 6.1: Using BREF in application procedure 25

EU-Twinning Project SL04/EN/01Integrated Pollution Prevention and Control (IPPC)

Example for optimised design of water intake

cooling water intake

Repellent device Intake from bay

Intake from water body

Cross current > 0,3 m/s Cross current < 0,3 m/s

4.5 Fish Protection at Water Intake Devices (Tab. 4.5)

The figure above shows an example for optimised design of the water intake of a large power plant location on the river Danube. If the water intake device is located directly at the border of the river, a strong cross current exceeding 0,3 m/s is generated leading to fish entrainment, even if a repellent device is applied. But if a bay is constructed and the intake device is located at the root of the bay, the cross current generated at the intake into the bay will be safely below 0,3 m/s, because of the much larger cross section of the water body, that is subject to the intake current. A repellent device covering this cross section will be very effective.



4.6 Minimisation of Emissions to Water The most important elements of BAT besides overall energy efficiency are in most cases the measures required to minimise emissions to water. These measures refer to

• reduction of heat emissions • reduction of emissions by optimised design and maintenance • reduction of emissions by optimised cooling water treatment • reduction of emissions by minimising the risk of leakages.

4.6.1 Reduction of Heat Emissions Concerning reduction of heat emissions, no general BAT has been identified, because the necessity for measures has to be assessed predominantly on a site-specific basis. Two aspects have to be considered:

1. The environmental quality standards set by the directive 78/659/EWG have to be met 2. Apart from the requirements of the directive, the tolerable heat emission should be

assessed using biotope analysis and simulation studies provided by an environmental impact assessment.

The essential factors, which have to be assessed, are: • The tolerable maximum temperature in the water body after mixing (summer and winter

conditions have to be regarded). • The tolerable maximum range of heating in the mixing zone (delta Tmax). • The possible effects of heating with respect to the oxygen concentration in the water body.

The requirements of the directive 78/659/EWG are also based on these essential factors.




Cooling water discharge from thermal power plantinto backwater of hydroel.power plant

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Example: Impact of cooling water discharge from a thermal power plant upstream of a hydroelectric power plant

Hydroelectric power plant

Upstream of thermal power plant



The figure above shows an example for the assessment of the impact of heat emissions from a thermal power plant located upstream of a hydroelectric power plant. Number 1 marks the reference situation upstream of the thermal power plant, number 2 the cooling water discharge of the thermal plant into the backwater of the hydroelectric plant and number 3 the situation downstream of the turbines of the hydroelectric plants, where the cooling water should be completely mixed with the water body.




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Temperature profiledownstream of cooling water discharge

Temperature profiledownstream of hydroelectric power plant

Temperature profileupstream of cooling water discharge

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Temperature Profile of Cross Sections of the Water Body

The figure above shows the temperature profiles of cross sections of the water body as they have been analysed by measurement at the three locations shown before. In the reference location there is a uniform profile. Downstream of the cooling water outlet one can see a top layer of warm cooling water, that does hardly affect the cold bottom zone. Further downstream, behind the turbines of the hydroelectric plant, there is again a rather uniform profile, but at an elevated level compared to the reference situation.

Obviously, environmental impact analysis should concentrate on the situation downstream of the hydroelectric plant, where complete mixing is given. The figure below shows the result of a numerical simulation of the maximum temperature after complete mixing of cooling water with the water body. It refers to cooling water to be discharged from a once-through system of two power plant units, that are planned for enlarging an existing power plant site already comprising two units. The simulation was performed in order to decide, whether once-through cooling could be applied for the new units as for the existing units, or whether a recirculation system would be necessary. The green area represents the uninfluenced temperature course at the reference point upstream of the site. The yellow line shows the heating effect of the cooling water from the existing units. The red line represents the anticipated effect of the planned units.






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Example: Numerical simulation of max. temperature after mixing caused by cooling water discharge from two new power plant units planned at an existing site.

The blue line shows the permitted maximum temperature of the water body (28°C) and the black bars within the blue line indicate the periods, in which the permit would not be met according to this simulation. The result of this simulation was, that if a once-through system is used, the requirements of the permit couldn’t be safely met at every day of the year. On the other hand, exceeding values are to expect only for a few days of the year. Therefore, it makes more sense to apply once-through cooling and avoid violations of the permit by reducing the power performance (and heat generation) of the plant during the critical days, than to apply a recirculation system, which would lead to a much higher energy penalty. In Tab. 3.1 of the BREF-document, the environmental aspects of cooling system types are summarised. Here again we can see, that once-through cooling and recirculation systems behave in a counter-current way with respect to energy efficiency and heat emission on the one hand and emission of additives and costs on the other. Thus, the adequate solution has to be obtained for each site by identifying the crucial factors. 4.6.2 Reduction of emissions by optimised design and maintenance In Table 4.6 of the BREF-document, measures for reduction of emissions by design and maintenance are presented. It is the aim of these techniques to minimise corrosion by selecting corrosion resistant material and smooth surfaces (heat exchangers, condensers, pipes, conduits), by avoiding stagnant zones and by providing a minimum water velocity to prevent depositions. Thus, emission of corrosion products (heavy metals) and of additives can be minimised. Besides selecting proper material, periodical cleaning of surfaces helps to reduce emissions. Condensers, e.g., can be equipped with an automated cleaning system based on foam balls, which are added to the cooling water stream before it enters the condenser. The balls are removed and cleaned downstream of the condenser and then returned to the system.



4.6.3 Reduction of emissions by optimised cooling water treatment To reduce emissions, the treatment of the feed water has to be optimised. The corresponding BAT is presented in Tab. 4.7 of the BREF-document. There are three main steps of treatment applied for the preparation of feed water for recirculation systems: mechanical treatment by screens, sieves and filters, chemical treatment by flocculation and decarbonisation, and the conditioning using additives required to maintain the chemical quality of the circuit water with respect to inhibition of scaling, of corrosion and of the growth of bacteria and for the removal of suspended solids. The treatment scheme is supplemented by side-stream filtration applied to remove particles and suspended solids from the circuit. Optimised treatment means: providing the required water quality by applying adequate and cost-effective treatment techniques and by selecting additives according to their effect and to their environmental properties. Tab. 3.7 of the BREF-document presents additives, that are frequently used in cooling water systems. Additives are almost exclusively used in recirculation systems. Once-through systems can be often operated without any additives, depending on the feed water quality.

Biocide treatment is necessary in recirculation systems, because without biocides, the surfaces of the system get covered by biofilm, in particular the surfaces of heat exchangers, condensers and of stagnant zones. The growth of the biofilm is accompanied by an increase of frictional resistance and by a decrease of heat transfer (1 mm of biofilm reduces the heat transfer by 50 %). Biofilms also promote microbial induced corrosion and can even lead to the formation of plugs in heat exchangers or conduits. As already mentioned, the required treatment of feed water depends very much on the type of the cooling system. The general BAT are presented in Tab. 4.7 of the BREF-document For once-through cooling using fresh water, mechanical treatment is sufficient in most cases. Biocide treatment is applied only in some cases, e.g., when a wet cooling tower is used. In such






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cases oxidising agents are used for shock treatment. The necessity for treatment should be determined by periodical observation of surfaces susceptible to biofilm growth. Continuous treatment of fresh water by chlorination is not considered as BAT. In most cases, treatment of the cooling water is not necessary before it is discharged, if the use of additives is performed according to the principles outlined above. For open circuit cooling systems, mechanical and chemical water treatment is necessary in most cases, especially if surface water is used for feed. Intensive use of additives may be necessary, depending on the number of concentration cycles. Both oxidising and non-oxidising biocides are used to prevent biofilm growth. The blowdown path should be closed during application of biocides until tolerable concentrations are reached. The required period can be deduced from reduction curves provided by the supplier or manufacturer of the biocide product. Side-stream filtration removes contaminations that promote bacterial growth and thus minimises the required quantity of biocides. In most cases, treatment of the cooling water is not necessary before it is discharged, if the use of additives is performed according to the principles outlined above. The figure below shows an example of a biocide reduction curve. The biocide DBNPA is applied with a start concentration of 10 mg/l. Within 15 hours the inhibition of a G12 dilution, as measured by toxicity to chemoluminescent bacteria, drops below 20 % and thus reaches a tolerable level. The blow down path therefore should be closed for at least 15 hours following the dosage of the biocide. Tab. 4.7 of the BREF-document contains some general requirements for biocide treatment. It is not BAT to use hazardous compounds based on chromium, mercury, organometallic substances and mercaptobenzothiazole.






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Monitoring of Biofilm Growth by Differential Pressure Measurement

Biocide treatment should be performed using a defined program. The optimum dosage time and quantity should be periodically determined based on monitoring procedures and the dosage should be performed in a controlled way according to the actual demand. Generally, biocides should be selected not only with respect to effect and costs, but also to their environmental impact. Annex VIII contains a systematic approach for selecting biocides based on the PEC/PNEC-ratios for the receiving water body. The directive 98/8/EG (Biocide Directive) also contains regulations for biocides used in cooling systems (see below). In many installations, the additive treatment and the related monitoring is performed not by the operator, but by external experts. The figure below shows a device, which can be used for monitoring of biofilm growth. It consist of a box with defined sample surfaces exhibited to the cooling water stream. The surfaces can be removed periodically from the box and the biofilm can be analysed.

In the figure below an alternative way of monitoring biofilm growth is shown. The cooling water flows through a capillary system and the differential pressure between inlet and outlet of the capillary is measured. Biofilm growth leads to plugging of the capillary and increase of the differential pressure. This triggers the signal for biocide dosage.




Probestromauslaß

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Monitoring of Biofilm Growth Using Test Surfaces



The selection of biocides will be strongly influenced by the directive 98/8/EG, as only biocides meeting the requirements of the directive will be available in the long run. Basic features of the directive are:

• All biocidal products must be subject to registration • Only products containing active substances listed in annex I can be authorised. • Products with new active substances may not be placed on the market, if they are not

authorised.

Products that have been on the market before 14.05.2000 are subject to an interim solution: • They shall be evaluated during a 10-year review program (terminated by 13.05.2010) • All products have to be either identified or notified • Substances, which have been identified, but not notified, can stay on the market not longer

than 01.09.2006 • For substances, which have been notified, evaluation dossiers have to be submitted. They

can stay on the market until a decision is made with respect to authorisation • Substances, which have been neither identified nor notified had to be removed from the

market by 24.11.2003 the latest. Therefore, after September of this year, only notified products should be available and after 2010 only authorised products, which exclusively contain active substances of the positive list, should be on the market.

4.6.4 Reduction of the Risk of Leakage Besides corrosion products and additives, chemical emissions can also occur because of leakages of the process containment. Therefore, reduction of the risk of leakage is considered as general BAT and the respective measures are presented in Tab. 4.10 of the BREF-document. They are mainly based on the VCI concept, which is described in annex VII and which should be applied as a safety concept for industrial once-through systems for cooling of hazardous substances. This concept leads to graduated safety measures in a two-step process:

1. The process substances are evaluated on the basis of their risk capacities (R-phrases) and their relevance concerning water, soil and human health. The evaluation results in a numerical score.

2. Based on the numerical score, the adequate set of measures for leakage prevention is deduced. The measures refer to the design of the cooling system, maintenance measures, analytical monitoring of the discharge and the availability of retention systems.

Tab. 4.10 10 of the BREF-document presents VCI score ranges and the corresponding possible prevention techniques.

E.g., if the evaluation results in a score of 5 – 8, two variants can be considered:

• For solution 1, the pressure of the cooling water is maintained at a higher level compared to the process pressure level. In this case, monitoring of the cooling water without automatic analytical measurements is sufficient.



• When, as in solution 2, the cooling water pressure and the process pressure are on the same level, automated analytical control is required to compensate for the higher risk of leakage of process substances into the cooling system.

4.7 Reduction of Emissions to Air The main concern with respect to emissions is directed towards emission into water. But general BAT also includes techniques to reduce emissions to air, which are presented in Tab. 4.8.10 of the BREF-document. The main aspects are:

• Reduction of drift losses in wet cooling towers by application of drift eliminators. They

should be able to reduce drift losses to less than 0,01 % of the total recirculation flow. • Reduction of plume formation, if this is required by the site-specific limitations, in

particular when living districts or major traffic routes are located in the neighbourhood. Plume formation can be reduced or avoided by the use of hybrid towers or air-cooling.

4.8 Reduction of Biological Risks In connection with emissions to air from cooling towers, one more element of BAT has to be addressed: the reduction of biological risks. Biological risks can be caused by the growth of pathogenic bacteria on surfaces of the cooling system, in particular of the species Legionella. Bacteria can be spread into the surroundings of the cooling tower. Tab. 4.11.10 of the BREF-document presents techniques to reduce the biological risks. They comprise two kinds of actions:

• The prevention of bacterial growth in the cooling system. The system should be designed so that it can be cleaned easily and every part of it is constantly flushed, i.e., providing smooth surfaces and avoiding stagnant zones. A periodic monitoring should be performed to identify and quantify pathogenic bacteria in the cooling system. An adequate biocide treatment should be performed to control bacterial growth.

• If the monitoring reveals elevated concentrations of pathogens, the system has to be clean and disinfected.

It has to be stated, that there is no fixed limit level for concentrations of Legionella. Legal requirements are in effect only in some states; in other states the authorities may give recommendations. Therefore, limit levels and the operator of the cooling system should establish action programs after consulting the competent local authority.

4.9 Reduction of Noise Emissions An element of BAT, which can be very important due to site-specific limitations, is the reduction of noise emissions. The corresponding BAT approach is presented in Tab. 4.9.10 of the BREF-document, both for natural draft and mechanical draft cooling towers. The constructive measures for noise reduction concentrate on the situation given at the tower base in the case of natural draft towers. For mechanical draft towers, the fans are the main subjects of reduction measures.






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Waste Water Streams in Power Plants

5. Waste Water in Power Plant Facilities The BAT relevant for the waste water situation in power plant installations is defined not only by the cooling systems BREF, but also by the BREF for large combustion plants. To describe the waste water situation, one has to consider the sources of waste water, the pollutants which may be in the waste water, the treatment used for elimination of the pollutants and the parameters used for characterising the quality of the waste water and for setting of the requirements in the permission procedure, both in the sense of limit levels and monitoring parameters. The following statements are based on the legal regulations that are in effect in Germany. They are laid down in the appendices 31 and 47 of the waste water ordinance and represent general binding rules for the permitting authority according to Art. 9 (8) of the IPPC directive. The figure below shows a schematic representation of the waste water streams to be found in a power plant installation (here: recirculation system with wet cooling tower). There are two major sets of streams:

• The discharge from the cooling system (here: blow down) and the waste water from the feed water treatment for the cooling system are subject to the BREF cooling systems.

• The waste water generated in connection with steam generation and flue gas scrubbing is subject to the LCP BREF.

The main sources of waste water in power plants are:

• The cooling system (heated cooling water). • Feed water treatment (waste water from mechanical and chemical treatment). • Boiler house (waste water from steam generation processes). • Flue gas desulphurisation. • Rain water from sealed surfaces.



5.1 Waste water from Cooling Systems

To limit the impacts of the discharged heat on the receiving water, it is necessary to regulate the following parameters, based on a site-specific analysis:

• With respect to the tolerable hydraulic impact of the receiving water body and to confine the tolerable heat load, the water flow has to be limited.

• In addition, a limit level has to be set for the cooling water temperature at the outlet (TE). • There has to be a limit for the max. temperature in the water body (TGmax) and for the

max. heating range in the water body (delta TG). In both cases the limit refers to the point of complete mixing. The limit levels must comply with the requirement according to the directive 78/659/EWG. The aim is to avoid temperatures exceeding the level tolerable for fish and to avoid over-sudden temperature variations.

• Additionally, the max. heating range of the cooling water (delta TE) may be regulated. This is necessary in the case of high heat loads leading to intensive plume formation in the water body, to prevent over-sudden temperature changes around the outlet zone.

• In the case of high heat loads to be discharged, usually a limit level is explicitly set for the tolerable heat load.

For monitoring, the cooling water flow and the cooling water temperature at the intake and outlet are actually measured. The temperature of the water body after mixing and the heating range of the water body after mixing usually are not actually analysed, but calculated, as in most cases it is hardly possible to define a sampling point representing the zone of complete mixing.

The limit levels usually stipulated in permits for different types of cooling systems correspond to the differences in cooling water flow and heat load at the outlet. The limit level for the max. temperature at the outlet usually is 30°C for once-through systems without cooling tower and 33°C for the same system with cooling tower. In both cases the max. heating range is usually set at 10 K, but may be as high as 15 K (e.g. during winter period). For recirculation systems, the limit for the max. cooling water temperature usually is 35°C. The max. heating range is only limited, if high heat load and low water flow in the receiving water body coincide. For limitation of the max. heating range and the max. temperature in the water body, the requirements of the directive 78/659/EEC are implemented. Depending on local conditions, more stringent limits than described above, can be stipulated. Apart from heat emission, the emission of chemicals has to be limited. The concept is not to eliminate additives by end-of-pipe technology, but to select additives, that are less harmful, and to confine their consumption to levels absolutely necessary for optimised operation of the cooling system. In German regulations, there are general requirements to support this concept: There is a matter-of-fact prohibition of hazardous substances, which have been in use as additives before: poorly degradable organic complexing agents, chromium and mercury compounds, nitrite, organometallic compounds and mercaptobenzothiazole; zinc compounds may not be used in main circuits of power plants. Usually, there is no analytical monitoring performed to prove compliance with this requirements. But the operator has to document the additives he uses in an operating journal and he has to provide data, which show that these additives comply with the requirements. General requirements concerning biocide treatment are:



• biocide treatment of once-through systems without cooling tower is not allowed, because bacterial growth is no problem, if adequate maintenance and cleaning is performed.

• in all other cooling systems, permanent biocide treatment is allowed only, if hydrogen peroxide or ozone are used.

• Other biocides are only to be used by periodic treatment (shock treatment). Apart from general requirements, the scopes of parameters relevant for the permit and monitoring have to be considered. In the case of once-through systems, the use of additives usually is restricted to oxidising biocides for the treatment of the cooling tower surfaces. Consequently, the parameters AOX (generated by halogen-based biocides) and oxidising agents are limited. In the case of once-through cooling without cooling tower, these parameters usually are not regarded, because, as mentioned above, the use of biocides is prohibited by general requirements. In the case of recirculation systems, a wide range of additives is used, among them oxidising and non-oxidising agents. Consequently, there are three sets of parameters:

• The parameters COD, P-compounds and zinc compounds are applied to generally limit the consumption of additives

• AOX and oxidising agents cover the use of oxidising biocides • The use of non-oxidising agents is regulated by the parameter toxicity to luminescent

bacteria. The requirements linked to the parameters mentioned above can be met by operational measures both in the case of once-through and recirculation systems. Thus, in most cases treatment of the cooling water is not required before discharge. 5.2 Waste Water from Feed Water Treatment Feed water treatment is a main source of waste water within a power plant installation. At the mechanical step of feed treatment, waste water is generated as washing water from sieves. It should be discharged without further treatment to avoid damages to small fish and fry. If filters are used, waste water is generated as back flushing water, that contains suspended solids and, if chemical treatment is applied, flocculants. It has to be treated by sedimentation. Back flushing water usually is returned to the process system after sedimentation. If feed water treatment extends to demineralisation by ion exchange, the regenerates have to be neutralised to obtain a tolerable range of pH. Parameters relevant for permit and monitoring are:

• suspended solids • AOX • pH.

5.3 Waste Water from Steam Generation The processes linked to steam generation also generate a number of waste water streams. The main sources are:

• Steam generator blow down containing the additives used for boiler feed treatment (e.g. hydrazine); usually, no specific treatment is required; in the case of hydrazine, oxidation may be required..






Lime milk

coagulation aids

Thickener

Organicsulphide

FeCl3Lamellaseparator

waste wateroutlet

Waste water input

HydroxidePrecipitation Floccul. 1

Sludge

Organic sulphidePrecipitation

Coagulation Floccul. 2Sludge

Example of a waste water treatment plant

Waste Water in Power Plants Flue Gas Scrubbing

Lime milk

coagulationaids

• Regenerates from condensate polishing by ion exchange, which, in addition to salts, may contain significant loads of nitrogen, when nitrogen compounds are used for boiler feed conditioning; the regenerates have to be neutralised to obtain a tolerable range of pH; nitrogen elimination may be necessary in the case of big power plants (stripping or biological elimination).

• If wet ash or slag removing systems is applied, the waste water generated has to be treated by sedimentation and in some cases by filtration to eliminate the solids. Usually the treated waste water is reused after cooling.

• If water-based devices clean plant components contaminated by flue gas, the waste water contains solids and heavy metals from the fuel. It has to be treated by neutralisation, precipitation/flocculation and sedimentation. If SCR-technology is used to reduce NOx, the waste water may contain significant loads of nitrogen.

The parameter scope is: COD, P-compounds, N-compounds, AOX, hydrazine, chlorine, heavy metals (Zn, Cr, Cd, Ni, V, Cu), pH and temperature. 5.4 Waste Water from Flue Gas Desulphurisation A main source of waste water is flue gas desulphurisation using wet limestone scrubbers. The scrubber water usually is reused after separation of the gypsum generated by desulphurisation. Depending on the chloride content of the fuel, a certain purge has to be discharged from the closed loop to maintain a tolerable chloride concentration. The purge contains sulphur compounds, chlorine hydrogen, fluorine hydrogen, organic compounds and heavy metals. It has to be treated by precipitation, flocculation, sedimentation and neutralisation. The treatment is in most cases performed in two-step treatment plants. The figure above shows an example of a two-step waste water treatment plant. In the first step, hydroxide precipitation with lime milk and flocculation with coagulation aids is performed. After crude separation of sludge in a thickener, organic sulphide precipitation is performed, followed by a coagulation step using ferric chloride (which eliminates excess organosulphides) and a coagulation unit using coagulation aids. Sedimentation and sludge separation is performed in a lamella separator. The parameter scope for waste water from flue gas desulphurisation is:






< 1Zn< 0,1Pb< 0,5Ni0,01 – 0,02Hg< 0,5Cu< 0,5Cr< 0,05Cd1 – 30Fluoride< 0,2Sulphide0,5 - 20Sulphite1000 – 2000Sulphate< 50Total Nitrogen< 150COD5 - 30Suspended solids Tab. 4.71 LCP-BREF:

Emission levels of FGD wastewater treatment plant associated with BAT (mg/l in 24 h composite sample)

Suspended solids, COD, sulphate, sulphite, sulphide, fluoride, toxicity to fish, heavy metals (Cd, Cr, Ni, Hg, Cu, Pb, Zn), pH Tab. 4.71 of the LCP BREF presents emission levels of treatment plants for waste water from flue gas desulphurisation, that are associated with BAT (see below). These levels can be obtained using the two-step treatment scheme described above. 5.5 Rainwater Rainwater from sealed surfaces of a power plant installation may be polluted, so that it has to be collected and treated before discharge to a water body. Rainwater streams therefore should be characterised according to their potential content of pollutants, and treated respectively. There are four main categories:

• Rain water from surfaces, where pollution can be excluded; this rainwater should be collected separately and discharged without treatment.

• Rain water potentially containing mineral oils/hydrocarbons: treatment by oil separator. • Rain water polluted by solids, but not by dissolved substances; treatment by sedimentation/filtration • Rain water polluted by dissolved substances: treatment by precipitation / flocculation /

sedimentation. Options for reuse of rainwater (as process medium, cooling water, fire fighting water) should be assessed on a site-specific basis.

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