- Email: [email protected]
Back to basics: Industrial waste liquid treatment
Feature
Filtration+Separation November 2007
Back to basics:
Industrial waste liquid treatment I
n the latest in this series, Ken Sutherland looks at the treatment of industrial waste liquids, intended to raise them to a quality suitable for disposal to the municipal sewer.
An article in the September issue of Filtration+Separation looked at the treatment of municipal sewage, which covered the collection and treatment of waste waters of all kinds, outside the premises where such waters arise, rendering them fit for discharge into the environment, or for re-use. This present article goes back a stage in the factory, dealing with the wastes arising directly from a manufacturing process, and making them fit for discharge to the municipal sewer. Some of the processes used to do so are similar to those employed in the municipal treatment works, but mainly they are of a different kind, able to deal with stronger wastes, with individual treatment requirements because of the varying chemical compositions of the wastes. Municipal waste liquids are characterised by the relatively dilute state, both of dissolved and suspended matter, and by the fact that most dissolved matter and a fair proportion of the suspended matter is organic in nature. The liquid waste is normally neutral or close to it. That proportion of suspended matter that is inorganic (mainly sand and gravel) is usually easily precipitated, while the inorganic solutes tend to be relatively harmless (apart from providing nitrogen and phosphorus as growth promoters for algae). Industrial wastes, on the other hand, are very likely to be quite concentrated, both in suspension and in solution, and the liquid could well be definitely acidic or alkaline. Suspended solids will probably be much finer, and so less easily settled, and will more likely be inorganic in nature, while potentially harmful inorganic solutes will feature in many industrial liquid wastes. The effluents will be much stronger, in terms of BOD or COD, and so, in principle, will be more easily decomposed biologically, but are highly
likely to carry materials that are toxic to the activated sludge bacteria normally used in waste treatment. The on-site treatment of waste liquid flows clearly depends on the composition of the liquid, and could well be an extension of the main manufacturing process that led to the waste. On the other hand, a completely different process may be needed to provide an environmentally acceptable solution.
The waste hierarchy The choice of appropriate processes for industrial waste treatment verges on philosophy as well as practical engineering. Waste processing is usually regarded as an operating cost, making no contribution whatsoever to profit levels. So, the first question to be asked is whether, in an ideal world, the waste material could be considered as a valuable material, either to the producing company or to some other user. The asking of questions such as this generates a hierarchy of possible actions, beginning with the most favourable option, namely to eliminate the occurrence of the waste altogether. (It must be acknowledged that the hierarchy about to be described is primarily concerned with solid wastes, but the philosophy applies just as well to liquids.) Waste Hierarchy Eliminate Minimise Re-use Recycle Recover energy Disposal
Elimination – the prevention of any waste arising – will almost certainly entail a redesign of the manufacturing process, to raise yields
and provide internal recycle of process flows, or by changing operating conditions so that what was once a waste becomes a valuable by-product, with its own refining plant as necessary. If complete prevention of the waste arising is not possible, then it always makes strong economic sense (because waste means a loss of something) to minimise the amount of waste produced – by Industrial wastes are likely to carry materials that are toxic to the activated sludge bacteria normally used in waste treatment.
25
26
Feature
Filtration+Separation November 2007
the same redesign activities that would have eliminated the waste, were that possible.
With liquid waste, suspended and dissolved solids must be treated appropriately before delivery to the municipal sewer, or to a nearby watercourse or lake.
Now we have the irreducible minimum of waste, and some valuable use must be found for it. The first possibility is to see if it can be re-used anywhere else on site. Obviously this will not be within the process that produces it, since that will have already been covered under elimination or minimisation, but there may well be another part of the site that could use the material, either as-is, or after some suitable conversion. Failing on-site re-use, the material may be usable by another company as a raw material, either through the medium of a waste exchange, or by direct negotiation. This recycling is still revenue generating, even if the only solution is the composting of organic material. Once all of these avenues have been explored, and no added-value option found, then it is clear that only destruction or disposal are left. The first choice must then be the recovery of energy values from the waste, probably by incineration with steam generation in the exhaust, or possibly by anaerobic, mesophilic bacterial digestion to generate methane. Both of these processes do, of course, produce some residue – an undigested sludge or an inorganic ash – but these will be in greatly reduced volumes compared to the original waste. Only in extreme circumstances should the last of the options, disposal to landfill, be undertaken (and even then only to a well-managed site, with leachate treatment and fermentation gas recovery and beneficial use). Landfill is becoming an increasingly costly option, as befits the increasing scarcity of suitable sites.
Table 1: Organic or inorganic Equipment end-use sector
Main chemical component
Agriculture, forestry & fishing
Organic
If it is a liquid waste that is primarily of concern, than much of the same reasoning will be required, with suspended and dissolved solids being treated appropriately to the hierarchical levels and the “irreducible minimum” having to be acceptable as a delivery to the municipal sewer, or to a nearby watercourse (or lake). This assumes, of course, that the waste water, from which impurities have been removed, cannot be re-used within the factory. If re-use is possible, even desirable, then the final purification stage will be as stringent in its processes as would have been the initial production process for the feed water.
Automotive manufacture & systems
Neither *
Biotechnical processing
Organic
Bulk chemicals & petrochemicals
Inorganic Organic
The golden rule is thus: if a waste is unavoidable, then look for the most revenue-generating means of treatment.
Basic treatment processes
Municipal waste water
Neither
All industrial liquid wastes can be regarded as having up to four components of concern as regards waste treatment:
Non-metallic mineral products
Inorganic
• the liquid solvent, which will usually be water – if it is not water, then the waste treatment process must treat the whole solvent for recovery, or destruction, as it will
Construction
Neither
Domestic, commercial & institutional
Neither
Electrical & electronics manufacture
Neither *
Energy materials extraction & processing
Organic (Inorganic)*
Fine chemicals & pharmaceuticals
Organic Inorganic
Food & beverage processing
Organic
Fresh water treatment
Neither
Mechanical machinery manufacture
Neither *
Medical & health
Organic
Metal manufacture
Inorganic *
Mining & mineral processing
Inorganic
Power generation
Inorganic *
Pulp, paper & board manufacture
Organic (Inorganic)
Rubber & plastics product manufacture
Organic (Inorganic)
Textiles, clothing & leather
Organic (Inorganic)
Note: * liquid effluents are mainly water, but with suspended oil as well
Feature
Filtration+Separation November 2007
possible, in which case it is unlikely that driven centrifuges will be chosen for the task, although hydrocyclones may have a role to fill. While sedimentation is the most economical way of dealing with a mixed liquid effluent, this may not be the best way of approaching wastes arising from different parts of the plant, where each should be treated as near to its point of arising as possible, to observe the “treat when concentrated” rule. In this situation, filters and even centrifuges may be justified, as the flow rates will be lower, and the equipment sizes correspondingly smaller. If the factory uses “point of arising” treatment, then it is quite likely for there to be a final central treatment plant that takes the effluents from each of the local waste processes, en route to final disposal. The actual nature of the waste is of primary concern in the selection of a treatment process.
certainly not be possible (nor economically sensible) to deliver it to the sewer; • a suspended liquid phase, most often oil – which has a nuisance value, but also a probable recovery value; • solid materials dissolved in the solvent, which must be removed ahead of the sewer, but which are also intrinsically valuable, provided that their recovery cost does not exceed that value (plus their cost of disposal); and • solid materials suspended in the solvent, which must also be removed, but may have value as well. The basic treatment processes suggest themselves from these descriptions: suspended liquid removal by a suitable oil-water separation process; suspended solid removal by a suitable solid-liquid separation process; and precipitation of the dissolved solids from solution as solid particles, followed again by filtration, etc. A number of general principles guide the selection of a suitable waste treatment process: • treat any waste material at the point in the system where it is at its most concentrated (which generally means as soon after its arising as possible), whether the treatment is to be a chemical or physical one; • if concentration of the whole waste is economically feasible, then it should be undertaken before separative treatment is begun; • if there is a suitable process by which to do it, then treat the whole waste in one go (this is especially true of high BOD/COD concentration solutions, where whole-waste anaerobic digestion is possible); • if the dissolved and suspended solids are not the same, then remove the suspended solids as completely as possible before attempting to treat the dissolved material; and • do not undertake a treatment process that will damage or destroy a valuable component
of the waste, unless its destruction is the object of the exercise.
Nature of the waste It will have become apparent by this point that the actual nature of the waste is of primary concern in the selection of a treatment process for it. The prime determinant is between the chemical forms of organic materials and inorganic, and the associated table shows the main nature for each leading end-use sector. In Table 1, the word “organic” against an enduse sector implies that the main raw materials and any processing additives or reagents are organic in nature, while the word “inorganic” implies the same, with inorganic materials. The “Bulk chemicals” and “Fine chemicals” sectors have a very wide range of sub-sectors split between organic and inorganic, so both categories are listed for them. Where the word “inorganic” appears in brackets, the implication is that the main process flows are organic in nature, but that some critical flows are inorganic – where the material may be toxic, for example. For those end-use categories marked as “Neither”, the implication is that processed liquids are mainly handled in such a way as to permit discharge of the processing effluents to the municipal sewer, without treatment, except, perhaps, for oil removal. Fresh and municipal waste water processing, of course, produce significant amounts of sludge (inorganic and organic respectively), but these are not within the compass of this article. Also noted are those sectors where the effluents are likely to be contaminated with oil, some of which is serious, as in the case of machine tool coolant effluents.
Industrial waste treatment processes Most waste treatment processes are going to involve separation of some kind, probably using filters, centrifuges or sedimentation equipment. Where disposal is the intended objective, then this will need to be achieved as economically as
For the removal of oil, an inclined lamella plate separator is a good choice, separating oil into a layer on top of the water in the tank, and allowing solids to settle to the bottom, whence they can be raked out of the tank. If solids are removed in this way, then it is likely that a final polishing stage of filtration will be required. This can either be in a deep bed filter, probably with a mixed-media bed for ease of resettlement after washing, for reasonably large flow rates, or a thick media cartridge or candle filter for lower flows. There is no need for the use of membrane filtration at this point if the water is to be discharged to a sewer, but if it is to be re-used, in a process in which the water cycle is closed, than it is very likely that a stage of membrane filtration (probably ultrafiltration) will be included – to match the treatment given to make-up water for the process. Almost certainly there will be need for pH adjustment of the waste liquid, but this will have to be done in concert with any required precipitation process. Crystallisation following evaporation or cooling of the liquid is a possible means for reducing dissolved salt concentration, but this should have been done as part of the main production process. If the liquid contains organic impurities, then biological treatment is probably called for, preferably by means of whole liquor digestion, using anaerobic mesophilic bacteria. However, if any of the organics are valuable, then membrane separation can be called on to recover large organic molecules, probably by ultrafiltration.
•
Contact: Ken Sutherland. Telephone: +44 (0)1737 218868. Email: [email protected]. Ken Sutherland has managed his process engineering and market research consultancy, Northdoe Limited, for nearly 30 years, a business largely concerned with filtration and other such separation technologies. He was a co-author of Elsevier’s Decanter Centrifuge Handbook, and has also written the second edition of Elsevier’s Handbook of Filter Media. More recently he has written Elsevier’s A to Z of Filtration.
27
Filtration+Separation November 2007
Back to basics:
Industrial waste liquid treatment I
n the latest in this series, Ken Sutherland looks at the treatment of industrial waste liquids, intended to raise them to a quality suitable for disposal to the municipal sewer.
An article in the September issue of Filtration+Separation looked at the treatment of municipal sewage, which covered the collection and treatment of waste waters of all kinds, outside the premises where such waters arise, rendering them fit for discharge into the environment, or for re-use. This present article goes back a stage in the factory, dealing with the wastes arising directly from a manufacturing process, and making them fit for discharge to the municipal sewer. Some of the processes used to do so are similar to those employed in the municipal treatment works, but mainly they are of a different kind, able to deal with stronger wastes, with individual treatment requirements because of the varying chemical compositions of the wastes. Municipal waste liquids are characterised by the relatively dilute state, both of dissolved and suspended matter, and by the fact that most dissolved matter and a fair proportion of the suspended matter is organic in nature. The liquid waste is normally neutral or close to it. That proportion of suspended matter that is inorganic (mainly sand and gravel) is usually easily precipitated, while the inorganic solutes tend to be relatively harmless (apart from providing nitrogen and phosphorus as growth promoters for algae). Industrial wastes, on the other hand, are very likely to be quite concentrated, both in suspension and in solution, and the liquid could well be definitely acidic or alkaline. Suspended solids will probably be much finer, and so less easily settled, and will more likely be inorganic in nature, while potentially harmful inorganic solutes will feature in many industrial liquid wastes. The effluents will be much stronger, in terms of BOD or COD, and so, in principle, will be more easily decomposed biologically, but are highly
likely to carry materials that are toxic to the activated sludge bacteria normally used in waste treatment. The on-site treatment of waste liquid flows clearly depends on the composition of the liquid, and could well be an extension of the main manufacturing process that led to the waste. On the other hand, a completely different process may be needed to provide an environmentally acceptable solution.
The waste hierarchy The choice of appropriate processes for industrial waste treatment verges on philosophy as well as practical engineering. Waste processing is usually regarded as an operating cost, making no contribution whatsoever to profit levels. So, the first question to be asked is whether, in an ideal world, the waste material could be considered as a valuable material, either to the producing company or to some other user. The asking of questions such as this generates a hierarchy of possible actions, beginning with the most favourable option, namely to eliminate the occurrence of the waste altogether. (It must be acknowledged that the hierarchy about to be described is primarily concerned with solid wastes, but the philosophy applies just as well to liquids.) Waste Hierarchy Eliminate Minimise Re-use Recycle Recover energy Disposal
Elimination – the prevention of any waste arising – will almost certainly entail a redesign of the manufacturing process, to raise yields
and provide internal recycle of process flows, or by changing operating conditions so that what was once a waste becomes a valuable by-product, with its own refining plant as necessary. If complete prevention of the waste arising is not possible, then it always makes strong economic sense (because waste means a loss of something) to minimise the amount of waste produced – by Industrial wastes are likely to carry materials that are toxic to the activated sludge bacteria normally used in waste treatment.
25
26
Feature
Filtration+Separation November 2007
the same redesign activities that would have eliminated the waste, were that possible.
With liquid waste, suspended and dissolved solids must be treated appropriately before delivery to the municipal sewer, or to a nearby watercourse or lake.
Now we have the irreducible minimum of waste, and some valuable use must be found for it. The first possibility is to see if it can be re-used anywhere else on site. Obviously this will not be within the process that produces it, since that will have already been covered under elimination or minimisation, but there may well be another part of the site that could use the material, either as-is, or after some suitable conversion. Failing on-site re-use, the material may be usable by another company as a raw material, either through the medium of a waste exchange, or by direct negotiation. This recycling is still revenue generating, even if the only solution is the composting of organic material. Once all of these avenues have been explored, and no added-value option found, then it is clear that only destruction or disposal are left. The first choice must then be the recovery of energy values from the waste, probably by incineration with steam generation in the exhaust, or possibly by anaerobic, mesophilic bacterial digestion to generate methane. Both of these processes do, of course, produce some residue – an undigested sludge or an inorganic ash – but these will be in greatly reduced volumes compared to the original waste. Only in extreme circumstances should the last of the options, disposal to landfill, be undertaken (and even then only to a well-managed site, with leachate treatment and fermentation gas recovery and beneficial use). Landfill is becoming an increasingly costly option, as befits the increasing scarcity of suitable sites.
Table 1: Organic or inorganic Equipment end-use sector
Main chemical component
Agriculture, forestry & fishing
Organic
If it is a liquid waste that is primarily of concern, than much of the same reasoning will be required, with suspended and dissolved solids being treated appropriately to the hierarchical levels and the “irreducible minimum” having to be acceptable as a delivery to the municipal sewer, or to a nearby watercourse (or lake). This assumes, of course, that the waste water, from which impurities have been removed, cannot be re-used within the factory. If re-use is possible, even desirable, then the final purification stage will be as stringent in its processes as would have been the initial production process for the feed water.
Automotive manufacture & systems
Neither *
Biotechnical processing
Organic
Bulk chemicals & petrochemicals
Inorganic Organic
The golden rule is thus: if a waste is unavoidable, then look for the most revenue-generating means of treatment.
Basic treatment processes
Municipal waste water
Neither
All industrial liquid wastes can be regarded as having up to four components of concern as regards waste treatment:
Non-metallic mineral products
Inorganic
• the liquid solvent, which will usually be water – if it is not water, then the waste treatment process must treat the whole solvent for recovery, or destruction, as it will
Construction
Neither
Domestic, commercial & institutional
Neither
Electrical & electronics manufacture
Neither *
Energy materials extraction & processing
Organic (Inorganic)*
Fine chemicals & pharmaceuticals
Organic Inorganic
Food & beverage processing
Organic
Fresh water treatment
Neither
Mechanical machinery manufacture
Neither *
Medical & health
Organic
Metal manufacture
Inorganic *
Mining & mineral processing
Inorganic
Power generation
Inorganic *
Pulp, paper & board manufacture
Organic (Inorganic)
Rubber & plastics product manufacture
Organic (Inorganic)
Textiles, clothing & leather
Organic (Inorganic)
Note: * liquid effluents are mainly water, but with suspended oil as well
Feature
Filtration+Separation November 2007
possible, in which case it is unlikely that driven centrifuges will be chosen for the task, although hydrocyclones may have a role to fill. While sedimentation is the most economical way of dealing with a mixed liquid effluent, this may not be the best way of approaching wastes arising from different parts of the plant, where each should be treated as near to its point of arising as possible, to observe the “treat when concentrated” rule. In this situation, filters and even centrifuges may be justified, as the flow rates will be lower, and the equipment sizes correspondingly smaller. If the factory uses “point of arising” treatment, then it is quite likely for there to be a final central treatment plant that takes the effluents from each of the local waste processes, en route to final disposal. The actual nature of the waste is of primary concern in the selection of a treatment process.
certainly not be possible (nor economically sensible) to deliver it to the sewer; • a suspended liquid phase, most often oil – which has a nuisance value, but also a probable recovery value; • solid materials dissolved in the solvent, which must be removed ahead of the sewer, but which are also intrinsically valuable, provided that their recovery cost does not exceed that value (plus their cost of disposal); and • solid materials suspended in the solvent, which must also be removed, but may have value as well. The basic treatment processes suggest themselves from these descriptions: suspended liquid removal by a suitable oil-water separation process; suspended solid removal by a suitable solid-liquid separation process; and precipitation of the dissolved solids from solution as solid particles, followed again by filtration, etc. A number of general principles guide the selection of a suitable waste treatment process: • treat any waste material at the point in the system where it is at its most concentrated (which generally means as soon after its arising as possible), whether the treatment is to be a chemical or physical one; • if concentration of the whole waste is economically feasible, then it should be undertaken before separative treatment is begun; • if there is a suitable process by which to do it, then treat the whole waste in one go (this is especially true of high BOD/COD concentration solutions, where whole-waste anaerobic digestion is possible); • if the dissolved and suspended solids are not the same, then remove the suspended solids as completely as possible before attempting to treat the dissolved material; and • do not undertake a treatment process that will damage or destroy a valuable component
of the waste, unless its destruction is the object of the exercise.
Nature of the waste It will have become apparent by this point that the actual nature of the waste is of primary concern in the selection of a treatment process for it. The prime determinant is between the chemical forms of organic materials and inorganic, and the associated table shows the main nature for each leading end-use sector. In Table 1, the word “organic” against an enduse sector implies that the main raw materials and any processing additives or reagents are organic in nature, while the word “inorganic” implies the same, with inorganic materials. The “Bulk chemicals” and “Fine chemicals” sectors have a very wide range of sub-sectors split between organic and inorganic, so both categories are listed for them. Where the word “inorganic” appears in brackets, the implication is that the main process flows are organic in nature, but that some critical flows are inorganic – where the material may be toxic, for example. For those end-use categories marked as “Neither”, the implication is that processed liquids are mainly handled in such a way as to permit discharge of the processing effluents to the municipal sewer, without treatment, except, perhaps, for oil removal. Fresh and municipal waste water processing, of course, produce significant amounts of sludge (inorganic and organic respectively), but these are not within the compass of this article. Also noted are those sectors where the effluents are likely to be contaminated with oil, some of which is serious, as in the case of machine tool coolant effluents.
Industrial waste treatment processes Most waste treatment processes are going to involve separation of some kind, probably using filters, centrifuges or sedimentation equipment. Where disposal is the intended objective, then this will need to be achieved as economically as
For the removal of oil, an inclined lamella plate separator is a good choice, separating oil into a layer on top of the water in the tank, and allowing solids to settle to the bottom, whence they can be raked out of the tank. If solids are removed in this way, then it is likely that a final polishing stage of filtration will be required. This can either be in a deep bed filter, probably with a mixed-media bed for ease of resettlement after washing, for reasonably large flow rates, or a thick media cartridge or candle filter for lower flows. There is no need for the use of membrane filtration at this point if the water is to be discharged to a sewer, but if it is to be re-used, in a process in which the water cycle is closed, than it is very likely that a stage of membrane filtration (probably ultrafiltration) will be included – to match the treatment given to make-up water for the process. Almost certainly there will be need for pH adjustment of the waste liquid, but this will have to be done in concert with any required precipitation process. Crystallisation following evaporation or cooling of the liquid is a possible means for reducing dissolved salt concentration, but this should have been done as part of the main production process. If the liquid contains organic impurities, then biological treatment is probably called for, preferably by means of whole liquor digestion, using anaerobic mesophilic bacteria. However, if any of the organics are valuable, then membrane separation can be called on to recover large organic molecules, probably by ultrafiltration.
•
Contact: Ken Sutherland. Telephone: +44 (0)1737 218868. Email: [email protected]. Ken Sutherland has managed his process engineering and market research consultancy, Northdoe Limited, for nearly 30 years, a business largely concerned with filtration and other such separation technologies. He was a co-author of Elsevier’s Decanter Centrifuge Handbook, and has also written the second edition of Elsevier’s Handbook of Filter Media. More recently he has written Elsevier’s A to Z of Filtration.
27