Understanding Clean Technology Part 1

Consequently the industry generates considerable amounts of solid, liquid and gaseous wastes. Well planned clean technology practices through the use of minimising, re-cycling, and re-use of water and solids following the best available technologies (BAT), can allow tanners to comply with tough environmental pressures. The modern leather industry has a responsibility to operate in a manner that is compatible with the best ecological and environmental practices. Worldwide there is a lot of emphasis on these aspects, which requires many tanneries and supply industries to have a better understanding of the whole environmental picture from the start to the end. Consequently many new developments in the field of application processes, chemicals and dyes, as well as the process control and mechanical developments are concerned with clean technology. This means ecological factors like reducing the load in the wastewater, exhaust emissions and lowering solid waste production have become the focus. Often it is asked if the new environment-friendly processes and products are also feasible for the tannery from the point of view of economy. There are many very positive and impressive examples which prove that economical and ecological considerations are not irreconcilable; to mention but a few: * safe and reliable hair-save processes * ammonium-free deliming * dust-free enzymes * high-exhaust chrome tannage * low salt liquid syntans and dyes * solvent-free aqueous finishes Clean technology processes can have up-front additional costs but these have to be balanced against large savings from lower wastewater charges and reduced sludge disposal costs. The regulations relating to our environment will become stricter in the coming years and clean technology has to be understood and implemented. The idea of this paper is to help the lay person understand the factors involved and provide in a clear and uncomplicated way some of the options that one has 3. Solid wastes * chrome shavings/gluestock and fleshings/fat * solids in effluent * sludge The solid wastes like shavings, fleshing and trimming wastes and natural fat are normally reasonably efficiently removed in mechanical operations. They can be readily collected for further processing and a number of possible uses for these wastes are available, such as converting to gelatine etc. 3.1 Solid wastes in effluent Gross solids are those larger than effluent sampling devices can handle, hence they are not measured. The waste components that give rise to this problem are: * trimmings and gross shavings * fleshing residues * solid hair debris * large pieces of leather cuttings * other solid contaminants like paper/plastic bags They can be removed by means of coarse bar screens set in the wastewater flow. If not removed these materials can block the pipe work causing severe problems. 3.2 Suspended solids in effluent Main source: Most of the suspended solids are protein residues from the beamhouse operations ?mainly from the liming process. However, large quantities are also produced owing to non-exhausted vegetable tannins, another source being poor uptake during retanning. Problems: If the wastewater is to be treated on-site, the main problems that arise are due to the large volume of sludge that forms as the solids settle. Sludge often contains up to 97% water, giving rise to very large quantities of ?ight? sludge. Even viscous sludge has a water content of around 93% and can easily block sludge pumps and pipes. All this sludge has to be removed, transported, de-watered, dried and disposed of. Analysis: The suspended solids component of an effluent is defined as the quantity of insoluble matter contained in the wastewater and can be determined by filtration or by centrifugation. The majority of these solids settle within 5 to 10 minutes, although some fine solids require more than an hour to settle. Semi-colloidal solids are very fine solids that will not settle even after a considerable period of time. They can, however, be filtered from solutions. Together with the more readily settleable solids, they thus comprise the suspended solids of an effluent that can be measured. 4. Liquid wastes 4.1 Wastewater treatment The major part of the waste in a tannery is diluted in the effluent. This sewage water cannot be discharged into surface waters or even communal wastewater without intensive treatment in purification plants. Legal limits are given as concentrations of the relevant pollutant in the wastewater, usually in milligrams/litre (mg/l), sometimes as parts per million (ppm). Peaks of concentrations of certain toxic pollutants above these limits must be avoided to protect the environment. Most of the communal fees are calculated according to the sum parameter; where the total amount of a discharged pollutant per day or per month is determined. However, the disposal charges can also include penalty fees if the peak limits are exceeded. Typically holding tanks are used to even out fluctuations over a 24 hour period. Primary treatment of waste water A simple overview the process is shown in the figure below: Primary treatment (precipitation and flocculation treatments): * removed 50 ?90% total suspended solids 40 ?70% COD ~60% BOD * not removed inorganic salts Many tanneries undertake this primary treatment process either on-site or in a collective water treatment facility. Through flocculation and precipitation it removes most of the suspended organic matter and anionic organic products like syntans, fatliquors and dyes. Prior to this the concentrated sulfide waste stream is separated and converted by aeration to sulfates. The chromium waste stream can be separated as well and in modern treatment plants the chromium salts can be relatively easily precipitated and recycled. Secondary (biological) wastewater treatment A simple overview of the secondary or biological process that follows the primary treatment can be seen in the figure, top right: Secondary treatment (biological treatment): * removed 80% total suspended solids 70% COD 99% BOD * not removed inorganic salts The standard aerobic biological treatment plant will readily degrade the typical effluents of leather processing. Once the bacteria in the active sludge have digested the organic components they will settle out and be removed as sludge. To avoid depleting the bacteria in the active sludge it is important that some 40% of the liquid flow-through is returned to the digestion tank. The nitrogenous compounds can be broken down by combining intensive aerobic and anoxic biological treatments. The oxygen demand is very high, thus leading to correspondingly high operational and energy costs, (40% of the oxygen demand). An efficiently run secondary treatment plant can meet relatively stringent limits for COD and BOD and allow discharge to surface water. Tanners are often reluctant to re-use water as they are not certain what contaminants remain in it. However, this water, which will contain residual salts, can be considered for re-use, for example in the pickle float. 4.2 Improvements possible using optimised processes As has been indicated it is difficult to remove inorganic salts from wastewater using treatment plants. So logically it is best to try and minimise the amount used in producing leather. A number of reduced salt processes have been developed and the results of one study using an optimised (low-salt) procedure are presented. If we take a specific parameter such as total dissolved salt (TDS), which is a considerable problem in locations where the water supplies are limited and are not close to the sea, there can be significant reductions in the amount of TDS in the wastewater. Naturally, the major effect of implementing an optimised process is to be had in the beamhouse. Change in TDS parameter when using a low-salt optimised process The individual components in the effluents were also measured in order to have a better overall idea of how the effluent parameters have changed. As expected the salts like sulfate and chloride are noticeably reduced. 5. Contaminants in wastewater The effect on the environment of excessive pollutant levels commonly found in untreated tannery effluents can be severe. So it is important that wastewater parameters can be measured and monitored. The individual components that are typically measured in effluents and their impact is described below in a simplified manner for guidance. The main problems presented by those components are summarised together with an outline of quantitative analytical methods 5.1 pH value Main source: Most discharged floats. Problem: Acceptable limits for the discharge of wastewaters to both surface waters and sewers vary, ranging between from pH 5.5-10. If the surface water pH shifts too far away from the pH range of 6.5-7.5, sensitive fish and plant life are susceptible to loss. Municipal and common treatment plants prefer discharges to be slightly alkaline as it reduces the corrosive effect on concrete and helps compensate for the domestic wastewater that tends to be slightly acid. When biological processes are included as part of the treatment, the pH is lowered to more neutral conditions by the carbon dioxide so evolved. Analysis: pH-meter; titration with acid/ alkali. 5.2 Oxygen demand Main source: Surfactants, all non-exhausted organic auxiliaries, hide substance from the liming process, ammonium and sulfide. Problem: All these components in effluents are broken down by bacterial action into more simple components. Oxygen is required for both the survival of these bacteria (aerobic bacteria) and the breakdown of the components. Depending on their composition, this breakdown can be quite rapid or may take a very long time. If effluent with a high oxygen demand is discharged directly into surface water, the sensitive balance maintained in the water becomes overloaded. Oxygen is stripped from the water causing oxygen dependent plants, bacteria and fish, to die. The outcome is an environment populated by non-oxygen dependent (anaerobic) bacteria leading to toxic water conditions. A healthy river can tolerate substances with low levels of oxygen demand. The load created by tanneries can be excessive, so the effluent requires treatment prior to discharge. Analysis: This can be achieved in two different ways: 5.2.1 Chemical oxygen demand (COD) This method measures the oxygen required to oxidise the effluent sample completely. It gives a value for all the contaminants. This means the materials that would normally be digested in the BOD5 analysis (within 5 days), the longer term biodegradable products, as well as the chemicals that remain unaffected by bacterial activity. The method is fast and very aggressive. A suitable volume of effluent is boiled with an oxidising agent (potassium dichromate) and sulfuric acid. As the effluent components oxidise, they use oxygen from the potassium dichromate. The amount used is determined by titration. 5.2.2 Biochemical oxygen demand (BOD5) This method is more complex. Essentially, the effluent sample is diluted in water, the pH is adjusted and it is seeded with bacteria (often settled sewage effluent). The samples are then incubated in the dark for five days at 20¡C. Bacteria use the oxygen dissolved in the water while the organic matter in the sample is broken down. The oxygen remaining is determined and the BOD5 can be calculated by comparison to the oxygen in the effluent-free sample. The results of the BOD5 are always lower than those obtained using the COD analysis. As a rule of thumb, the ratio between COD: BOD is 2.5:1, although in untreated effluent samples variations can be found as great as 2:1 and 3:1. This depends on the chemicals used in the different leather making processes and their rate of biodegradability. 5.3 Nitrogen Main source: The most common sources are ammonia (from deliming materials) and the nitrogen contained in proteinaceous materials (from liming/unhairing operations). Problem: Nitrogen is a key nutrition factor for plants. High levels released by substances containing nitrogen over-stimulate growth. Water-based plants and algae grow too rapidly, thereby waterways become clogged and flows are impaired. The nitrogen released through protein breakdown and the deliming process is in the form of ammonia. Large volumes of oxygen are needed so bacteria can convert it into water and nitrogen gas. If oxygen demand is greater than the level supplied naturally by the water stream, toxic anaerobic conditions can rapidly develop. Both processes lead to an eutrophication of the surface water. Analysis: Ammonia is released from the wastewater by boiling it with sodium hydroxide, and subsequently trapping it in a boric acid solution. The level of ammonia released is determined by titration and its value calculated as ammoniacal nitrogen. The other method, the Total Kjeldahl Nitrogen (TKN) method, analyses all the nitrogenous matter including proteins. This is broken down first by boiling the sample with sulphuric acid to form ammonium compounds. These are then analysed according to the method above. 5.4 Sulfide (S2-) Main source: Sodium sulfide, sodium hydrosulfide and the breakdown of hair in the unhairing process. Problems: The problem here arises when the pH of the effluent drops below 9.5, very toxic hydrogen sulfide gas, H2S, evolves from the effluent; characterised by a smell of rotten eggs. Even a low concentration causes headaches, nausea and eye irritation. At higher levels it is lethal. Hydrogen sulfide is readily soluble in water and causes rapid corrosion of metal pipes, fittings and building materials. If discharged to surface water, even low concentrations can be a toxicological hazards. Analysis: The most accurate methods rely on the acidification of effluent to generate hydrogen sulfide. It is trapped and converted into zinc sulfide. The amount of sulfide is determined by titration. 5.5 Sulfates (SO42-) Main source: Sulfuric acid, chromium sulfate tanning and retanning agents and sodium sulfate, used as a standardising salt in many powdered products like bating enzymes, synthetic retanning agents and dyes. An additional source is created by oxidation of sulfide from the effluent treatment process. Problem: Problems arise with soluble sulfates for two main reasons: 1. Sulfates cannot be removed completely from a solution by chemical means. Under certain biological conditions, it is possible to remove the sulfate from a solution and bind the sulphur into micro-organisms. Generally, however, the sulfate either remains as sulfate or is broken down by anaerobic bacteria to produce malodorous hydrogen sulfide. If effluents remain static this bacterial conversion process occurs very rapidly in effluent treatment plants, sewage systems and watercourses. This results in the corrosion of metal parts and concrete. 2. The total concentration of salts (TDS) in the surface water is increased. Analysis: Adding barium chloride solution to a sample of filtered effluent. 5.6 Chlorides (Cl- ) Main source: Common salt (sodium chloride) used in hide and skin preservation or the pickling process. Problem: Chlorides are highly soluble and stable and cannot be removed by effluent treatment and nature. They remain as a problem in surface waters since chlorides inhibit the growth of plants, bacteria and fish. If the effluent water is used for irrigation purposes, surface salinity increases through evaporation. High salt contents are only acceptable if the effluents are discharged into tidal/marine environments. Analysis: Titrating of effluent with a silver nitrate solution. 5.7 Oils and grease Main source: Natural oils and grease released from the skin structure, non-exhausted fatliquors. Problem: Grease and fatty particles tend to float and agglomerate; they bind to other materials causing potential blockage problems especially in effluent treatment systems. Grease or thin layers of oil on the water surface can reduce the oxygen transfer from the atmosphere. If these fatty substances are in emulsions, they can create a very high oxygen demand on account of their slow bio-degradability. Analysis: Extraction of the effluent sample with a suitable solvent and evaporation of the organic phase. The residual grease can be weighed and calculated. 5.8 Metals from the tannage Metal compounds are not biodegradable. They can thus be regarded as long-term environmental features. Heavy metals are the subjects of close attention since they can also have accumulative properties. 5.8.1 Chromium salts (chromium III, trivalent chrome) Main source: Non-exhausted floats from the tanning and retanning. Problem: Chrome tanning is carried out in the form of greenish chromium (III) sulfate salts. (It should be clearly differentiated from the highly toxic and oxidising chromium (VI), chromate salts which are not used for tanning.) Excess chromium sulfate from the tanning process float is typically discharged to a separate tank, where it can be easily precipitated under alkaline conditions and collected by filtration for re-use. This procedure is a common practice worldwide and efficiently removes most of the soluble chromium salts from the effluent. A small amount of the chromium salts can also be washed out from the leather during retanning, dyeing and acidification processes, so together with proteins it finishes up in the sludge. Depending on the amount of chromium in the sludge it may be required to be disposed of separately as a hazardous waste. Analysis: Oxidation of the sample by nitric acid to form the soluble chromate. Several analytical techniques are possible, for example, atomic absorption; titration as barium chromate or colorimetric measurement at a wavelength of 670 nm. 5.8.2 Other metals Main source: Non-exhausted floats from chrome-free aluminium or zirconium based tanning and retanning processes. Problem: Depending on the chemical species, these metals have differing toxicity that is also affected by the presence of other organic matter, complexing agents and the pH of the water. Aluminium, in particular, appears to inhibit the growth of green algae and crustaceans are sensitive to it in low concentrations. Analysis: Complexometric titration methods using chelating ligands like EDTA and specific indicators. 5.9 AOX chemicals and APEO surfactants Main source: Degreasing of small skins and finishing operations. Problems: Organic halogen containing chemicals (AOX) and alkyl phenol ethoxylated surfactants (APEO), eg nonylphenyl ethoxylate (NPE), can be difficult to break down. Thus they can remain in the eco-system for extended periods of time and can accumulate in the food chain. These substances with low bio-degradability in effluents discharged to surface waters can contribute significantly to the COD/BOD load. Analysis: Highly specialised methods often based on HPLC (high performance liquid chromatography) or GC (gas chromatography). 5.10 Toxicity of effluent components Main source: Non-exhausted bactericides and fungicides, tanning agents. Problems: Biocides by their very nature are toxic or they would not function. They are used to protect the partially processed hides from bacterial and fungal damage. Often processes using these substances cannot be avoided. If they are insufficiently exhausted during the process or spilled by accident, they end up in the wastewater and can cause problems in the sewage plant and in surface waters. Analysis: A measure of toxicity of a chemical in water can be expressed as LD50, representing the dose, which will kill 50% of a sample species. Not every species reacts to the same degree to a given exposure, and the type of response to an equal dose of a chemical may differ widely. When values are given, the species under test should be stated and the time period taken for evaluation should normally be either 24 hours, 96 hours or 14 days. Highly specialised analytical methods often based on HPLC (high performance liquid chromatography) or GC (gas chromatography). 6. Air wastes In the water treatment processes like tanning and post-tanning it is unlikely any significant air emissions will be released if good operating procedures are followed. With sulfides from the liming/unhairing process as long as the pH is kept above 8.5 then the formation of H2S gas is avoided. In the finishing process the introduction of water-based technologies has reduced the amount of VOC emissions to the air. The exhaust air emissions from spray finish applications are typically passed through a wet scrubbing procedure to keep this from becoming a problem. 7. Finished leather: unwanted contaminants What the consumer buys is the end product, finished leather. With a growing demand for more information about consumer products and their preparation it is logical that the end article leather is also subject to an array of comments about its ecological and toxicological properties. The media is quick to highlight comments like ?ome 20% of leather consists of chemicals and therefore it could be harmful? without considering the real truth of the comment in more depth. The overwhelming majority of products found in leather are harmless and offer no danger to users of the natural product, leather. The development of analytical methods to determine very low levels of contaminants in consumer products like textiles has also been applied to leather. So now a list of unwanted substances found at trace levels have been developed and are widely circulated. A full risk analysis to determine the real toxic limits has in most cases not been undertaken. Analyses for trace levels of the following substances are now common: * formaldehyde * chromium (VI) * certain organic amines derived from azo dyes * organotin compounds * nickel, cadmium, lead and other heavy metals * pentachlorophenol and chlorinated phenols Additionally tests for NPE, extractable chromium (III), extractable organic substances, biocides, organohalogen compounds are requested by some ecological labels. This list can look very daunting at first when you are asked to ensure none of them are your leather. In most cases they have been taken from lists for other purposes and products, therefore some of the listed contaminants are not found in leather. Some of the more important ones are detailed here in order to give a good background to the problem and the methods used for testing them. 7.1 Formaldehyde in leather Introduction Formaldehyde is widely used in the manufacture of chemicals. In the case of leather chemicals it is used to join together molecules to form larger molecular structures, the so-called condensation products. Once this chemical reaction has occurred the formaldehyde is no longer available and cannot be detected. The only free-formaldehyde that is detectable originates from the small excesses used in manufacture that have not reacted completely. Some of this formaldehyde is released (also called reversibly bound) by water extraction. Also some formaldehyde is enclosed in the matrix of the leather and can be emitted in gas form as free formaldehyde. In our environment the main sources of formaldehyde are from incomplete combustion processes. So it can be found outside in the streets from motor exhaust fumes and inside probably from cigarette smoke, which can contain levels of 100 ppm. Trace levels of formaldehyde are commonly found as emissions from some building materials, but also in many natural products, for example, fruit like apples can contain 10-20 ppm. Additionally up to 2,000 ppm in cosmetic items and 1,000 ppm in toothpaste is allowed in the EU. Under mild alkaline conditions formaldehyde reacts readily with protein and collagen and this has been used in the past for formaldehyde tannages of leather. In combined tannery effluents formaldehyde can normally not be detected even in trace levels. It rapidly reacts with other compounds of the effluent and is readily degradable in wastewater treatment plants. Test Methods There are several methods for quantitative analysis of formaldehyde in leather and care must be taken in determining which method is being used, what limits are required and in interpreting the results. The two main methods used are as follows: 1) Free formaldehyde and releasable formaldehyde by water extraction method (Test Methods: CEN ISO/TS 17226, IUC 19, DIN 53315) This is the traditional method of analysis, extracting the leather at 40¡C for 1 hour in a deionised water solution containing a small amount of wetting agent. The extracted solution is analysed either by liquid chromatography (preferred method) or colorimetrically for formaldehyde (or more correctly aldehyde) content. Since the analytical conditions for preparing these samples for the two procedures are not the same they will give different results. Additionally the colorimetric method is subject to interference from extracted dyes and additionally measures all aldehyde substances, so it is not specific for formaldehyde. BLC has reported that using this colorimetric method, some 80% of results obtained from coloured leathers must be considered incorrect. 2) Free-formaldehyde by gas phase method (Test Methods: VDA 275, PV 3925 - VW/Audi, EN 717-3) A gas phase method was originally developed for building materials and is now used especially by the automotive industry. This method determines just the free-formaldehyde content in leather. A leather sample is suspended over a deionised water solution in a closed bottle. This container is heated at 60¡C for three hours and after cooling for one hour the amount of formaldehyde transferred via the gas phase into the water solution is measured colorimetrically. The leather sample is not in direct contact with the water, so the method is only measuring the formaldehyde that can be released into the air by warming the sample. Limits allowed In Europe there are currently no legal limits for formaldehyde in leather. In leather the limit values normally requested are set by commercial companies promoting their ecological label, for example Oeko-Tex and SG. For shoes an additional ecological label covering the whole life cycle of the shoe is available from the EU. Typically requested eco-label limits for ?ree-formaldehyde?in leather using the traditional water extraction method are: l for leather with indirect skin contact, such as shoes, limits of 150 or 300 ppm, are required depending on the ecological label l for leather items with direct skin contact a level of 75 ppm is required * for children? shoes the level is lower at 50 ppm The automotive industry normally recommends the gas phase method of analysis, but note carefully, the limits for this different method are much lower, typically 10 ppm. What differences are there between the water extraction and gas phase methods? It is not possible to compare results between the two methods. The traditional water extraction method will typically give considerably higher results compared with the gas phase method. Some comparisons indicate that values 5 to 10 times higher could be expected. The water extraction is measuring both types of formaldehyde, namely the free-formaldehyde as well as the releasable formaldehyde. It should be noted that changes in the extraction temperature and times could have a considerable influence on the result, so these parameters in the test method must be followed closely. What level of formaldehyde in leather is achievable? Modern chemical products and application processes enable a limit of 300 ppm (water extraction method) to be complied with. With appropriate selection of products the lower levels can also be complied with. Where does the formaldehyde in leather come from? The most common and, therefore, the most critical sources are formaldehyde-releasing products such as: * organic tanning agents * resin-type retanning agents * fixing agents used at the end of the drum application process There are several other possible sources such as: syntans, fatliquors and retanning/dyeing auxiliaries, but generally they only become a problem if one uses high amounts or has to reach very low limits. How can you avoid having formaldehyde in leather? Select only those products that are either formaldehyde-free or have low formaldehyde content. Basically, in the normal tannery situation this means avoid using those few products such as resin-based retanning agents, fixing agents and organic tanning agents which could release high levels of formaldehyde. It is very difficult to try reducing the formaldehyde level during or after leather production. For example, processes that include the use of products to react with formaldehyde can reduce the level of formaldehyde in leather but cannot eliminate it. 7.2 Chromium (VI) in leather Introduction Chromium can exist in several oxidation states. For leather tanning the most important chemical used is basic chromium sulfate, it has the oxidation state chromium (III). This oxidation state is a natural trace mineral that we need in our bodies for our everyday life. Chromium (III) can be oxidised in some special situations to the much more toxic form, namely chromium (VI). It should be clearly stated that this Cr (VI) oxidation state does not tan leather and does not form organic complexes, eg organic chromium complex dyes cannot be chromium (VI) based. Test Method The standard method for chromium (VI) in leather is: 1) Cr (VI) in leather (Test Methods, CEN/TS 14495, IUC 18, DIN 53314) The traditional method of analysis is to shake the leather in a deaerated pH 8 buffer solution at room temperature for three hours. The solution is analysed colorimetrically for chromium (VI) content. The analytical conditions require it to be made under an inert atmosphere to avoid the oxidation of the Cr (III) during the analytical procedure. The colorimetric method can be subject to interference from organic substances such as extracted dyes and possibly vegetable tanning agents. Therefore, it is important that these components are removed by using small chromatographic cartridge clean-up procedures before adding the diphenylcarbizide colour-developing agent. a) Limits allowed In Europe since Cr (VI) is listed as a carcinogen the legal limit is quite simple: it must not be detectable. Hence the actual limit is de facto the detection limit for the analytical procedures. The DIN 53314 method stated 3 ppm as the detection limit but inter lab trials showed wide variations in results when making analyses on leather samples. Subsequently it was shown the DIN method is unsuitable for many coloured leathers as any extracted dye caused an interference to the analytical result. Therefore, the DIN method is only suitable for analysing for Cr(VI) in undyed leathers. The new CEN/TS 14495 method has a clean-up procedure to remove the extracted dye. So this method and various other eco-labels have recognised the complex matrix that leather is in terms of analysis and they set 10 ppm as a level, which can be detected with reliability over a wide range of leather samples.