RECYCLING OF TREATED WASTEWATER FOR INDUSTRIAL REUSE L Bonomo* * DLLAR- Environmental Section-Politecnico di milano. Piazza leonardo da Vinci, 32-20133 Milano, Italy INDUSTRIAL WATER WITHDRAWALS The extent of the industrial withdrawals with respect to the global water supply is essentially depending on the hydrologic regime and on the country level of industrialisation and income According to the water resources data published by the World Bank, the industrial sector is the second largest water consumer with an average of 23% of the total usage, increasing to 47% in high income countries. Nevertheless different countries offer consistently different situations. Table 1 gives some examples of the importance of the industrial sector on the global water consumption This high variability is also the outcome of a non homogeneous classification of the uses with the frequent inclusion of the energetic sector within the industrial portion Concerning the Italian situation, industry requires around the 19% of the overall water withdrawal Inland thermo-electric plants respond for another 14%. All the sea water usage of coastal thermo electric plants as well as the hydro-electric uses without an effective water consumption, are not included in such figures After a long period of intense exploitation, at present the industrial water consumption is significantly decreasing in the majority of developed countries. This is primarily a consequence of the changing occurred in the industrial structure. Some of the highest water consuming productions (steel, rubber, chemical, metallurgy, petrol-chemical, refinery) have stopped or moved away Sometimes they have converted their activities from primary to manufacturing productions, generally characterised by lower water consumption and pollution loads Table 1. Incidence of the industrial sector on the global water consumption in different countries Country Industrial withdrawals Argentina Denmark Portugal 37% eden 55%
16 RECYCLING OF TREATED WASTEWATER FOR INDUSTRIAL REUSE L. Bonomo* * D.I.I.A.R. - Environmental Section - Politecnico di Milano, Piazza Leonardo da Vinci, 32 - 20133 Milano, Italy INDUSTRIAL WATER WITHDRAWALS The extent of the industrial withdrawals with respect to the global water supply is essentially depending on the hydrologic regime and on the country level of industrialisation and income. According to the water resources data published by the World Bank, the industrial sector is the second largest water consumer with an average of 23% of the total usage, increasing to 47% in high income countries. Nevertheless different countries offer consistently different situations. Table 1 gives some examples of the importance of the industrial sector on the global water consumption. This high variability is also the outcome of a non homogeneous classification of the uses with the frequent inclusion of the energetic sector within the industrial portion. Concerning the Italian situation, industry requires around the 19% of the overall water withdrawal. Inland thermo-electric plants respond for another 14%. All the sea water usage of coastal thermoelectric plants as well as the hydro-electric uses without an effective water consumption, are not included in such figures. After a long period of intense exploitation, at present the industrial water consumption is significantly decreasing in the majority of developed countries. This is primarily a consequence of the changing occurred in the industrial structure. Some of the highest water consuming productions (steel, rubber, chemical, metallurgy, petrol-chemical, refinery) have stopped or moved away. Sometimes they have converted their activities from primary to manufacturing productions, generally characterised by lower water consumption and pollution loads. Table 1. Incidence of the industrial sector on the global water consumption in different countries. Country Industrial withdrawals Argentina 18 % China 7 % Denmark 27 % Portugal 37 % Romania 33 % Sweden 55 %
Table 2. Annual water withdrawals in Italy(km /year) Uses Annual consumption Percentage kmyear' Urban 7.94 18.9% Irrigation 20.14 48.0% Industrial 797 19.0% 14.1 Total fresh water 41.98 100% Sea water(cooling) 17 Moreover the increasing trend in wastewater treatment costs due to the widespread ening of the water quality standards for environmental protection has led to the adoption eaner or advanced technologies in order to minimise the water usage. The Italian case has been a clear example of this evolution after the Water Policy Act was issued in 1976. In the following ten years remarkable efforts were made to reduce the industrial water consumption as shown in Table 3 These data come from investigations performed by IRSA-CNR, the Italian Water Research Institute, in 1972(before the Water Policy Act was issued)and in 1986, 10 years later. This trend is still going on, being cleaner technologies and strategies for water saving still not thoroughly applied by industry Table 3 shows the water reduction per worker, but lacks of analogous estimates referred to the unit product. Therefore greater reductions should be predicted as the industrial productivity has remarkably increased during the last 15 years In temperate regions the increase of wastewater reuse, in terms of direct recycling or in terms of municipal wastewater application, rarely depends on real availability problems of the water resource. More frequently the convenience of the reuse strategy derives from a cost benefit evaluation, since costs for fresh water supply(including wastewater treatment)and costs for water clamation are often comparable. The widespread tightening of the effluent discharge permit limits in developed countries has gradually determined a diffuse need of tertiary treatment steps such as biological removal of nutrients(nitrification/denitrification) final filtration for suspended solid and phosphorus removal chemical oxidation or activated carbon filtration for colour and detergents control enhanced disinfection in order to respect higher microbiological standards INDUSTRIAL WATER RECLAMATION Industry requires water for processing, steam generation, product washing, air conditioning, plant and equipment washing, transport of materials, cooling systems and sanitation. At present, cooling water is the industrial application that consumes most wastewater. It should be stressed that water quality requirements for industrial reuse are necessarily specific, depending strongly on the type of process and on the kind of reclamation strategy. Different water requirements correspond to different uses. Lower quality water may often be employed
17 Table 2. Annual water withdrawals in Italy (km3 /year) Uses Annual consumption km3 year-1 Percentage % Urban 7.94 18.9 % Irrigation 20.14 48.0 % Industrial 7.97 19.0 % Energy 5.92 14.1 % Total fresh water 41.98 100 % Sea water (cooling) 17.00 Moreover the increasing trend in wastewater treatment costs due to the widespread tightening of the water quality standards for environmental protection has led to the adoption of cleaner or advanced technologies in order to minimise the water usage. The Italian case has been a clear example of this evolution after the Water Policy Act was issued in 1976. In the following ten years remarkable efforts were made to reduce the industrial water consumption as shown in Table 3. These data come from investigations performed by IRSA-CNR, the Italian Water Research Institute, in 1972 (before the Water Policy Act was issued) and in 1986, 10 years later. This trend is still going on, being cleaner technologies and strategies for water saving still not thoroughly applied by industry. Table 3 shows the water reduction per worker, but lacks of analogous estimates referred to the unit product. Therefore greater reductions should be predicted as the industrial productivity has remarkably increased during the last 15 years. In temperate regions the increase of wastewater reuse, in terms of direct recycling or in terms of municipal wastewater application, rarely depends on real availability problems of the water resource. More frequently the convenience of the reuse strategy derives from a cost benefit evaluation, since costs for fresh water supply (including wastewater treatment) and costs for water reclamation are often comparable. The widespread tightening of the effluent discharge permit limits in developed countries has gradually determined a diffuse need of tertiary treatment steps such as: – – – – biological removal of nutrients (nitrification/denitrification); final filtration for suspended solid and phosphorus removal; chemical oxidation or activated carbon filtration for colour and detergents control; enhanced disinfection in order to respect higher microbiological standards. INDUSTRIAL WATER RECLAMATION Industry requires water for processing, steam generation, product washing, air conditioning, plant and equipment washing, transport of materials, cooling systems and sanitation. At present, cooling water is the industrial application that consumes most wastewater. It should be stressed that water quality requirements for industrial reuse are necessarily specific, depending strongly on the type of process and on the kind of reclamation strategy. Different water requirements correspond to different uses. Lower quality water may often be employed
Table 3. Industrial water minimisation occurred in Italy, during the 1976-1986 period, after the Water Policy Act was issued. Values are referred to manpower Industry Minimisation Grain mill 28% P 4% Dairy-farming Leather 24% Textile +15% Metallurgic Mechanic Chemical 7% Rubber 80% Synthetic fibres Water requirements for industrial reuse are quite different from agricultural or urban non potable reuse. Microbiological quality is not the main concern for most of the industrial applications albeit when municipal wastewater are employed Additional health protection measures can be adopted to minimise worker exposure to toxic volatile organic compounds or pathogens. Frequently quality concerns include scaling, fouling, corrosion, foaming, biological growth, workers safety The reclaimed water use depends on several factors Availability and cost of alternative water sources Costs of wastewater treatments required to respect regulations on water quality standards Water quality and quantity requirements of the specific industrial processes Possible recovery of heat, process chemicals or by-products(e.g. cellulose fibres in paper industry, sizing agents in textiles, metallic ions in plating industry, chromium salts in Water reclamation alternatives include a)direct reuse of non-contaminated water(e.g. cooling water to general factory use) b) closed loop treatment and recycle of wastewater from a particular source for direct reuse in the process. This is often accompanied by recovery of process chemicals, by-products and heat energy c) cascading of water used on a high quality process to another process requiring lower quality water(e. g. final rinses to first rinse operation). In many application such cascade reuse may require only minimal intermediate treatments, and the lowest quality of water is disposed of as unusable d) treatment and reuse of end-of-pipe factory mixed effluent e) reuse of municipal wastewater for an industrial process. It is normally meant to satisfy the basic water quality requirements for general industrial applications, leaving any further olishing treatment to the specific However, this option is marginal, since, on average it accounts for less than 6-15% of the overall reclaimed wastewater used in the industrial sector Industrial reuse can require a complex array of integrated processes to ensure a safe water supply an affordable cost. This typically implies coupling the most effective wastewater and water treatment technologies in complementary treatment trains
18 Table 3. Industrial water minimisation occurred in Italy, during the 1976-1986 period, after the Water Policy Act was issued. Values are referred to manpower. Industry Minimisation Grain Mills - 28 % Paper - 54 % Dairy-farming - 40 % Sugar - 56 % Leather - 24 % Textile + 15 % Metallurgic - 50 % Mechanic - 23 % Chemical - 7 % Rubber - 80 % Synthetic fibres - 80 % Water requirements for industrial reuse are quite different from agricultural or urban non potable reuse. Microbiological quality is not the main concern for most of the industrial applications albeit when municipal wastewater are employed. Additional health protection measures can be adopted to minimise worker exposure to toxic volatile organic compounds or pathogens. Frequently quality concerns include scaling, fouling, corrosion, foaming, biological growth, workers safety. The reclaimed water use depends on several factors: – – – – Availability and cost of alternative water sources Costs of wastewater treatments required to respect regulations on water quality standards. Water quality and quantity requirements of the specific industrial processes. Possible recovery of heat, process chemicals or by-products (e.g. cellulose fibres in paper industry, sizing agents in textiles, metallic ions in plating industry, chromium salts in tanneries). Water reclamation alternatives include: a) direct reuse of non-contaminated water (e.g. cooling water to general factory use); b) closed loop treatment and recycle of wastewater from a particular source for direct reuse in the process. This is often accompanied by recovery of process chemicals, by-products and heat energy; c) cascading of water used on a high quality process to another process requiring lower quality water (e.g. final rinses to first rinse operation). In many application such cascade reuse may require only minimal intermediate treatments, and the lowest quality of water is disposed of as unusable; d) treatment and reuse of end-of-pipe factory mixed effluent; e) reuse of municipal wastewater for an industrial process. It is normally meant to satisfy the basic water quality requirements for general industrial applications, leaving any further polishing treatment to the specific user. However, this option is marginal, since, on average it accounts for less than 6-15% of the overall reclaimed wastewater used in the industrial sector. Industrial reuse can require a complex array of integrated processes to ensure a safe water supply at an affordable cost. This typically implies coupling the most effective wastewater and water treatment technologies in complementary treatment trains
PINCH ANALYSIS Recently new cost/benefit evaluation strategies have been developed in order to assess the be recycle/reuse alternatives that enable to optimise freshwater usage and the level of water reuse Most of these systematic approaches include elements of Pinch analysis. Pinch"in this context means"bottleneck. Pinch analysis extends the basic concepts of heat recovery to freshwater and wastewater minimisation adopting the model of a thermal mass-transfer unit. Contaminant levels and flow rate in and out of different process units are the equivalent of temperature in a thermal mass/transfer balance Pinch analysis helps to identify potential water use reduction and wastewater minimisation Basically it considers a plant as a whole and provides indication on the critical steps among different process units. On the basis of this critical point(the bottleneck) it's easier to detect which water streams have to be recycled and/or which internal wastewater treatments have to be provided to optimise the water usage. In other words this indication allows to design the optimal scheme for the process units Each operation in a process can be analysed as a single unit characterised by certain ratios between input and output contaminant concentrations. The mass/transfer balance for each operational unit described with a graph( Figure 1)reporting contaminant concentration versus the mass transferred The highest permissible input/output concentrations indicate the operational limit profile. These inlet and outlet concentrations may be fixed by a number of possible factors such as solubility fouling of equipment, minimum flow rate requirements to avoid settling of precipitation of material etc.Any other possible operational profile would lie below the limit profile To represent the overall water consumption, a cumulative curve(Figure 2)has to be built using the limit profiles of each operation unit of the process. The cumulative curve, combined with the limit water supply requirements of the processes(the water requested to guarantee the inlet limit concentrations for each operational unit), allows to identify the bottleneck(the pinch)of the process Figure 1-Mass/transfer balance for a single operational unit and a single contaminant. The bold arrow indicates the operational limit profile mg OUT +M kg h-
19 PINCH ANALYSIS Recently new cost/benefit evaluation strategies have been developed in order to assess the best recycle/reuse alternatives that enable to optimise freshwater usage and the level of water reuse. Most of these systematic approaches include elements of Pinch analysis. “Pinch” in this context means “bottleneck”. Pinch analysis extends the basic concepts of heat recovery to freshwater and wastewater minimisation adopting the model of a thermal mass-transfer unit. Contaminant levels and flow rate in and out of different process units are the equivalent of temperature in a thermal mass/transfer balance. Pinch analysis helps to identify potential water use reduction and wastewater minimisation. Basically it considers a plant as a whole and provides indication on the critical steps among different process units. On the basis of this critical point (the bottleneck) it’s easier to detect which water streams have to be recycled and/or which internal wastewater treatments have to be provided to optimise the water usage. In other words this indication allows to design the optimal scheme for the process units. Each operation in a process can be analysed as a single unit characterised by certain ratios between input and output contaminant concentrations. The mass/transfer balance for each operational unit is described with a graph (Figure 1) reporting contaminant concentration versus the mass transferred. The highest permissible input/output concentrations indicate the operational limit profile. These inlet and outlet concentrations may be fixed by a number of possible factors such as solubility, fouling of equipment, minimum flow rate requirements to avoid settling of precipitation of material, etc. Any other possible operational profile would lie below the limit profile. To represent the overall water consumption, a cumulative curve (Figure 2) has to be built using the limit profiles of each operation unit of the process. The cumulative curve, combined with the limit water supply requirements of the processes (the water requested to guarantee the inlet limit concentrations for each operational unit), allows to identify the bottleneck (the pinch) of the process. Figure 1 – Mass/transfer balance for a single operational unit and a single contaminant. The bold arrow indicates the operational limit profile. M kg h-1 C mg l-1 CwIN max CwOUT max
igure 2-Cumulative water consumption curve. The arrow is minimum water supply required to guarantee the limit concentrations for each operational unit C mg l Cumulative water consumptio Water Supply M kg h On the basis of the knowledge of the pinch, an optimised scheme of the process units can b achieved. An internal recycling placed in an appropriate position would be able to effectively reduce the pinch concentration and therefore the overall fresh-water consumption terative software, specifically developed to return the best operational configuration for a certain set of constraints(maximum inlet/outlet concentration and/or mass load of contaminant transferred and/or minimum flow requirements), can be applied to investigate the best scheme solution WATER RECLAMATION IN SOME INDUSTRIAL SECTORS Textile Textile factories are among the largest industrial consumers of water. Effluents of textile processing plants contain several complex compounds deriving from the major step of sizing, desizing, scouring, dyeing, printing and finishing, and are quite variable because of the frequent changes of atch-type textile processing step Textile wastewater may include many types of recalcitrant and biotoxic chemicals. The variability of these compounds in concentration and discharges makes textile wastewater treatment quite problematic by conventional processes, either physico-chemical or biolog ical This difficulty in treating wastewater has gradually forced industry to adopt additional end-of-pipe treatments(GAC/BAC adsorption, oxidation, evaporation and membrane treatments) in order to match the sewage discharge limits Wet processes in textiles generally require high quality water. Therefore any recycling strategy implies high performance treatments. At present, direct recycling experiences are limited to single wastewater streams: mostly water from final washing steps reused in first rinsing baths. Salinity increase, due to chemicals used in the processes or for the regeneration of ion-exchange resins, is generally the limiting factor for recycling rates Reclaimed water is normally used for specific production steps, not particularly exigent in terms of water quality, such as cooling of dyeing machines, first rinsing and rinsing for dark dyeing. Fresh water is generally essential in the boiler house, for personnel consumption and sanitation, for dyeing and for the last rinsing steps. Such limitations in reclaimed water employment may be overcame ith membrane treatments that now are less expensive than few years ago. Promising results, also
20 Figure 2 – Cumulative water consumption curve. The arrow is minimum water supply required to guarantee the limit concentrations for each operational unit. M kg h-1 C mg l-1 Pinch Water Supply Cumulative water consumption On the basis of the knowledge of the pinch, an optimised scheme of the process units can be achieved. An internal recycling placed in an appropriate position would be able to effectively reduce the pinch concentration and therefore the overall fresh-water consumption. Iterative software, specifically developed to return the best operational configuration for a certain set of constraints (maximum inlet/outlet concentration and/or mass load of contaminant transferred and/or minimum flow requirements), can be applied to investigate the best scheme solution. WATER RECLAMATION IN SOME INDUSTRIAL SECTORS Textile Textile factories are among the largest industrial consumers of water. Effluents of textile processing plants contain several complex compounds deriving from the major step of sizing, desizing, scouring, dyeing, printing and finishing, and are quite variable because of the frequent changes of batch-type textile processing steps. Textile wastewater may include many types of recalcitrant and biotoxic chemicals. The variability of these compounds in concentration and discharges makes textile wastewater treatment quite problematic by conventional processes, either physico-chemical or biological. This difficulty in treating wastewater has gradually forced industry to adopt additional end-of-pipe treatments (GAC/BAC adsorption, oxidation, evaporation and membrane treatments) in order to match the sewage discharge limits. Wet processes in textiles generally require high quality water. Therefore any recycling strategy implies high performance treatments. At present, direct recycling experiences are limited to single wastewater streams: mostly water from final washing steps reused in first rinsing baths. Salinity increase, due to chemicals used in the processes or for the regeneration of ion-exchange resins, is generally the limiting factor for recycling rates. Reclaimed water is normally used for specific production steps, not particularly exigent in terms of water quality, such as cooling of dyeing machines, first rinsing and rinsing for dark dyeing. Fresh water is generally essential in the boiler house, for personnel consumption and sanitation, for dyeing and for the last rinsing steps. Such limitations in reclaimed water employment may be overcame with membrane treatments that now are less expensive than few years ago. Promising results, also