In this article we will discuss about the methods used to treat and control waste water in chemical and allied industries.
Waste-Water Treatment in Petroleum Refinery:
Removal of pollutants from oil refinery can be classified in three types of treatments:
Waste-water may contain coarse, suspended and floating solids, grease, etc. These need to be removed before waste-water is subjected either to chemical or biological treatment. Common unit operations of physical treatments are bar screens, grinders, grit chambers, grease traps, flocculation, sedimentation, flotation, chemical precipitation, sludge pumping. This treatment basically removes inert material which may hinder the subsequent treatments.
After removal of grit and floating matter, suspended and dissolved organic matter are removed. Important unit operations and processes involved in the chemical treatment of waste-water are: Chemical coagulation, flocculation and sedimentation.
Biological treatment unit is primary meant for removal of pollutants like phenol, residual sulphide and BOD and also the non-recoverable oil present in the secondary effluent. Bacterial seeding and fertilisation of the oily waste with appropriate bacterial species will accelerate biological degradation provided that dissolved oxygen and sufficient time are available.
Petrochemical and Allied Products:
It is difficult to make any general statements on the petrochemical industry owing to the fact that product mix, raw materials and production technology vary from one installation to another. The inputs to the petrochemical industry are almost all petroleum-based products.
The primary inputs are ethylene, propylene, butadiene, benzene, toluene, xylenes, ammonia and methanol. These are produced from the following raw materials: petroleum distillates, propane, ethane, natural gas, coal, shale oil and biomass.
The variation in inputs, processes and products corresponds to a wide range of water uses both in quantity and quality and even more so in a wide range of waste streams. The variety means that such indicators as specific water use depend strongly on the various processes and water-treatment technologies, so that an overall analysis is difficult.
However, a general perspective of the alternative technologies and the resulting water use and waste streams will be provided. Possible recycling and reuse of waste-water and effluent treatment practices will also be considered.
The three basic uses for water in the petrochemical industry are: cooling water, which accounts for 80 per cent of intake water; steam generation, which uses 5 per cent of intake water; and process water, which uses 15 per cent of intake water.
A large amount of heat is generated in the petrochemical industry. This heat is removed from the processes through contact or non-contact cooling with water. The exact amount of water needed is a function of the particular process employed. In closed (non-contact) systems, the quality of the water is an important factor for the maintenance of the cooling system. In contact cooling the water quality is very important to prevent contamination of the processes outputs.
There are three major uses of steam generated within a petrochemical plant:
1. Non-contact process heating.
2. Power for a variety of purposes.
3. As a diluent, stripping medium or source of vacuum using steam jet injectors.
Although process water accounts for only 15 per cent of total water use, it is vital to the production of petrochemical products. As more of the cooling systems become closed cycles, the percentage of process water used will increase. Petrochemical processes have been categorised by the USEPA into four groups on the basis of water use within the process.
Water in the petrochemical industry is used in a variety of ways; the resulting raw waste-water streams are just as varied.
The major sources of waste-water in the petrochemical industry are:
1. Raw materials themselves.
2. Products remaining in the solution after separation.
3. By-products produced during reactions.
4. Spills, washdowns and vessel clean-outs.
5. Cooling tower and boiler blow-down, steam condensate and water-treatment processes.
6. Storm water run-off.
7. Sanitary systems.
Wastes from these sources have a variety of characteristics that include high COD concentrations, high total dissolved solids, high COD/BOD ratios and contamination by heavy metals or compounds inhibiting biological treatment.
The three main components of petrochemical wastes are biological substances, other chemical substances that are only slightly biodegradable or not at all, and suspended matter.
Water Pollution Control in Petrochemical Industries:
Huge volume of water is required for various purposes in manufacture of petrochemicals. Water is used for cooling, process operations, firefighting, drinking, housekeeping, steam raising, etc. Whole water after use reappears as effluents which require treatment and disposal.
Treatment of Phenolic Waste-Water:
Increasingly stringent regulation for the quality of drinking water has resulted in an enhanced interest in the decontamination of water, waste-waters and polluted industrial effluents for phenolics. Phenolic compounds contain hydroxyl groups attached to aromatic nucleus.
They may contain other functional groups such as alkyl, methoxy, halo, carbonyl groups. At very low concentrations, they impart taste and odours in drinking water and may taint the flavour of fishes grown in contaminated waters. Aquatic life is adversely affected by phenolics at ppm levels.
These effects have led to the specifications of acceptable limits for phenolics by Bureau of Indian Standards as following: Public water supply, 0.005 mg/l, industrial effluents to be discharged to surface waters, 1.0 mg/l; and industrial/trade effluents to be discharged to public sewers, 5.0 mg/l.
Sources of Phenolic Waste-Water:
Generally the level of phenolics in domestic waste-waters are low. Many industries produce phenolic waste-waters. These include petrochemical industries petroleum refineries, coal cooking and coal gasification, resin manufacturing, dye synthesis, wood preserving plants, pulp and paper mills and aircraft manufacture.
Commonly used methods for the treatment of phenol bearing waste-waters include solvent extraction, physical adsorption, chemical oxidation and aerobic biological processes. Alternate biological treatment methods such as anaerobic and anoxic processes and enzymatic approach for the removal of phenolic are receiving wide attention only recently. These methods are discussed briefly with their merits and demerits.
Solvent extraction procedure is practiced for the recovery of phenols from concentrated industrial wastewaters.
The criteria for choosing the appropriate solvent for extraction are:
(i) Low water solubility,
(ii) No emulsion formation with water, and
(iii) Must be easily regenerable.
Among many solvents which include octane, mixtures of benzene with butylacetate and isopropyl, isobutyl acetate and isopropyl ether two are extensively used for dephenolising waste-waters. Solvent extraction process can typically reduce phenol concentrations from as high as 17,000 to 920 mg/l.
Dephenolised waste-water will have to be treated by other suitable methods for further removal of phenols. Adsorption method can be used for the removal of phenols from contaminated drinking water sources as well as for waste-waters that contain moderately low phenol concentrations.
It is a separation process in which phenols are transferred from aqueous phase to the surface of adsorbent. Extensively used adsorbent is activated carbon. Commercially available activated carbons are derived from natural materials such as coconut shell, wood charcoal, which are carbonised at controlled conditions so as to have precise surface properties and high surface area.
These materials are costly and need to be regenerated and reactivated for further repeated use. Generally used-procedure for the regeneration of activated carbon is burning at 600°C under controlled conditions of moisture and oxygen, which is highly energy intensive.
About 5-10 per cent material is lost during regeneration. Other source materials, such as straw, used rubber tyres, fertiliser waste slurry have been tried in an attempt to produce low cost, disposable activated carbons.
These materials have exhibited adsorptive properties similar to commercially available activated carbons. Activated carbon derived from fertiliser waste slurry has been successfully used for the removal of phenolics from synthetic as well as actual waste-waters from oil refineries.
Many oxidants like ozone, H2O2 and permanganate have been used for the chemical oxidation of phenols. It has been reported that ozonisation can result in structural modifications of pollutants, which make them amenable for biodegradation.
Requirement of around 7.5 kg of ozone per kg of phenol present is reported for the oil refineries waste-waters, if effluent quality of 0.2 mg/l is to be achieved. Reaction between phenol and hydrogen peroxide is slow under non-catalysed conditions.
However in the presence of Fe(II) ions H2O2 oxidises phenol to hydroquinone and catechol, which are further oxidised to quinones, carboxylic acid and finally to CO2. Fenton’s Reagent [H2O2 = Fe (II) Catalyst, pH 2-4] is reported to be the cheapest oxidation system as compared to ozone, chlorine dioxide and potassium permanganate.
If the waste-water contains phosphates, catalytic action is reduced and the oxidation of phenols cannot be achieved. Phenolic effluents from paint and, pharmaceutical manufacturing have been treated with Fenton’s Reagent.
Enzymatic approach for the treatment of phenolic waste-waters is now attracting wide attention. Use of enzymes which include peroxidases, tyrosinase and laccases are now being explored to convert soluble phenolics to insoluble polyphenolic precipitates, which can then be removed by filtration.
Biological treatment processes are preferred over abiotic methods as they lead to the complete mineralisation of phenol to CO2 and other inorganic components. Physical and enzymatic methods convert phenol from one form/phase to another, which still requires further treatment.
Biological methods can be classified as:
(ii) Anoxic, and
(iii) Anaerobic depending on the environment present in the treatment system.
Generally biological processes are used after dephenolising the concentrated phenolic waste-waters by solvent extraction. However, there are also a few reports on the application of anaerobic and anoxic biological treatment processes for treating concentrated waste-water.
Tannery effluents if disposed off without any treatment either on land or in inland surface waters, may create severe problems leading to damage of the environment. Tannin, trivalent chromium, proteinous matter, sulphides and high BOD/COD of the waste-water call for proper treatment.
Tannery wastes in general, can be treated to get the desired end results in the following main stages:
1. In-plant measures.
2. Primary treatment.
3. Chemical treatment.
4. Secondary treatment
In-plant measures include reduction of water consumption in the tannery, process modifications to reduce pollutional load of the waste being generated, segregation of soak liquor and lastly, recovery of the by-products and reuse of various process liquors.
Reduction of Water Usage in a Tannery:
To reduce the volume of the effluent, the water usage in tanneries can be considerably reduced by:
1. Better housekeeping.
2. Alteration of processes and low float systems to use less water.
3. Separation of cleaner fractions of the waste for direct reuse without treatment.
4. Recycle after complete or partial treatment.
Process Modifications to Reduce Pollutional Load:
The quantities of pollutants which are inevitable in tannery waste-water are as follows:
Hide salt – 150 g/kg
Hair protein – 40 g/kg
Hide protein – 25 g/kg
Hide fat and carbohydrate – 15 g/kg
Dirt and manure – 5 g/kg
Organic solids from cleaning and premises – 3 g/kg
Recovery and Utilisation of By-Products:
The various wastes generated by the tannery have great potential for their reuse. Tannery unhairing effluent and acid are the two waste liquids individually difficult to dispose of or to utilise, but if combined together, useful products can be recovered from this combination. These precipitated products have high contents of essential amino acids such as cysteine, lysine, valine, leucine, etc.
Most of the tanneries do not have any treatment facilities. In tanneries where treatment of effluents is carried out, it is limited to the mixing of the various effluents, followed by sedimentation. Even those operations are not carried out satisfactorily.
Screening for removal of coarser impurities, hair and fleshing followed by settling for atleast 4 hours in a continuous flow settling tank form the essential primary treatment of tannery waste-waters.
Low cost technology and conventional treatment systems (both aerobic and anaerobic or in combination) can be employed for the secondary biological treatment of equalised and settled tannery wastes. Tannery effluent can also be satisfactorily treated in admixture with sewage in a sewage treatment plant provided the proportion of tannery effluents is not high.
Some of the tanneries are using anaerobic lagoons with success. Experiments on anaerobic lagoon with the settled waste-water gave percentage reduction varied from 42.4 per cent at a BOD loading of 0.41 kg/m3/day to 85.5 per cent at a BOD loading of 0.14 kg/m3/day.
It was found that ten days detention time would be sufficient to bring BOD reduction of about 85 per cent. If the final effluent has to be discharged on land, it would be sufficient to treat the settled waste in anaerobic lagoons for 10 days.
Anaerobic lagoon treatment could reduce BOD by 88.5 per cent but colour removal could not be obtained. Anaerobic treatment of vegetable tannery waste-water is practically feasible at shorter time using fixed film reactors.
The success in treatment is strongly dependent on the high concentration of active biomass retained within the reactor and on provisions of sufficient contact time between active biomass and waste-water. The increased sludge retention enhances the rate of conversion of organic matter to methane and carbon dioxide. The reactors, having different methods of sludge retention investigated for the treatment of vegetable tannery waste-water corroborate the above facts.
For complete treatment to meet the standards of discharge into river, anaerobic treatment followed by aerobic process is necessary. The treatment of raw tannery waste-water having BOD concentrations of 2000-4000 mg/l is not economically feasible to treat by aerobic method due to oxygen transfer limitations.
The effluent produced after anaerobic treatment of tannery waste-water has COD and BOD concentrations permissible for discharge into public sewer. The anaerobic pre-treatment with faster conversion rate at shorter time with added advantage of biogas generation will be a better approach to opt before aerobic polishing treatment.
Anaerobic-aerobic reactor system exhibited the ability to provide 98 per cent removal of suspended solids, 86 per cent removal of COD and 70 per cent removal of sulphides from raw combined tannery beamhouse waste-water without pre-treatment.
The primary treatment units principally comprise coarse screens, two numbers of equalisation-cum- settling tanks (used alternately) and sludge drying beds. The settling tanks, are of about 1-2 days capacity each, thereby acting also as equalisation tanks.
Alternatively separate equalisation tank and settling tank/clarifier have been provided by a few tanneries. Depending on the quality of composite effluent, addition of neutralising chemicals and coagulants like lime, alum, ferric chloride, etc. would be required for effective precipitation of chromium and removal of suspended solids in the sedimentation process.
The sludge from the settling tank/clarifier is removed and dried on sludge drying beds made up of filtering media like gravel, sand with supporting masonry structure. For operational reasons, sludge drying beds are divided into four or more compartments.
The sludge drying period varies from 4 to 8 days depending upon the type of sludge, atmospheric conditions, etc. The dried sludge from the sludge drying beds can be used as manure or for landfill if it is from vegetable tannery waste. In case of chrome tannery wastes, the dried sludge should be buried or disposed off suitably as per the directions of regulatory and local bodies.
The pretreated effluent needs suitable secondary biological treatment to meet the pollution control standards. The biological treatment units generally adopted by the Indian tanneries are anaerobic lagoon, aerated lagoon, extended aeration systems like oxidation ditch, etc.
Anaerobic lagoon is a simple anaerobic treatment unit adopted by some tanneries. The system is suitable for tanneries located outside town limits and having sufficient land. The depth of the lagoon may be 3-5 metres and the detention time may be 10-20 days depending upon the pollutional load and atmospheric conditions.
The lagoon is provided impervious lining to prevent any subsurface infilteration of waste-water. The anaerobic lagoon is an open type digester with no provision for mixing and gas collection. No power is required for this system and its performance has proved to be efficient.
pH control, sulphate reduction and atmospheric temperature are important factors in the performance of the system. pH in the range of 7.0 to 8.5, sulphates—amounts less than 500 mg/1 and atmospheric temperature of 25°-40°C are favourable for anaerobic lagoon treatment.
Anaerobic contact filter and Upflow Anaerobic Sludge Blanket (UASB) are other types of anaerobic treatment system for tannery wastes under pilot scale study. These are closed type units made up of RCC or steel. These units occupy less land area since the detention time is about 1-2 days in case of contact filter and about 8 to 10 hours in case of UASB.
These systems are found to be efficient for treating tannery effluent combined with domestic sewage. Though the capital cost would be high as compared to anaerobic lagoon, this system can be adopted for partial treatment of tannery wastes when adequate admixture of sewage is possible.
About 60 per cent reduction in BOD is reported to be achieved by this system. Aerated lagoon is a shallow water tight pond of about 2-3 metres depth with a detention time of about 4-6 days.
Fixed or floating type surface aerators are provided to transfer oxygen from atmospheric air to the effluent for biological treatment using micro-organisms under aerobic conditions. Many tanneries have provided aerated lagoons as a second stage biological treatment unit and the system is found suitable for treating low organic loads.
As in all waste control programs, good housekeeping is the first step to prevent wastage of water and materials in tannery. Economical use and reuse of water are necessary to reduce the volume of the waste-water.
The water usage in tanneries can be considerably reduced by:
1. Better housekeeping.
2. Alteration of processes and low float systems to use less water.
3. Segregation of cleaner fractions of the waste-water for direct reuse without treatment.
4. Recycle after complete or partial treatment.
The adoption of batch washing as an alternative to continuous rinsing using the lattice door can reduce water consumption. It has been observed that paddles may have some advantages over drums for certain types of production from the point of view of water usage.
Direct reuse is possible in a tannery since some wash waters are relatively clean, such as from washing after bating, pickling, neutralising and dyeing which can be used for less important tasks such as washing after soaking and liming and floor washing.
Segregation of chrome waste stream, chrome recovery and reuse is proved to be one of the viable procedures. The chrome recovery and reuse in medium and large size tanneries should be practised because it makes good economical sense and prevents pollution due to chromium. The spent chrome liquor is captured, filtered, precipitated, reactified, strengthened and reused as the tanning liquor.
Process Changes for Reducing Pollution Load:
Water can be saved in the deliming, bating and pickling stages either by recycling of water or regeneration of liquors. Replacement of salt by some other biodegradable chemicals could solve the major pollutional problem of tanning industry. Conveyance by refrigerator containers may also be tried to avoid salt problem. Processing fresh hides and skins upto wet blue near the slaughtering place can also be considered.
The quantity of waste-water depends on the different processes and products made by different tanneries. The water and waste-water quantity varies from process to process and product to product as already shown in Table 17.1.
It is evident from the data that average figures cannot be taken for evolving the quantum limits. Because the industries which are using less water will be encouraged to use more water at the same time, industries which are consuming more water will find it difficult to reduce the water consumption.
In the light of the above, at this stage only a guideline on quantum limits are prescribed. In evolving quantum limits, waste-water generation has been considered as 40 m3/MT of raw hide processed.
Based on the above considerations the pollutant limits are as follows:
The experience gained during the implementation of quantum limits, could be reviewed to firmly establish the quantum limits.
Pulp and Paper Industry:
The unit process involved in the treatment of waste-water from agricultural residue-based paper mills are:
1. Equalisation of flow from pulp wash section.
2. Primary clarification for combined waste-water.
3. Secondary biological treatment.
4. Sludge drying beds or lagoons for primary sludge depending on the availability of land.
Equalisation for Pulp Wash Waste-Water:
Pulp washing section accounts for about 20-25 per cent of total waste-water and contributes around 70-80 per cent of pollution load from small paper mills using agricultural residues as raw materials. These waste-waters are discharged intermittently since, in most of the mills, washing of pulp is done in batches using pouchers. Normally two pouchers are employed and operated in series and thus generate two washes.
The period of washing is more or less same in both but the quantity of water used varies appreciably and thus the first wash water is more concentrated than the second wash water. The flow variation has been observed to be fairly wide since the minimum and maximum values, respectively are 0.15 and 2.3 times the average flow.
Discharge of these washes to the main waste-water stream (on an intermittent basis), as is being practised will alter the composition of the combined waste-water appreciably. This necessitates provision of equalisation to the pulp washes and discharge at a constant rate into the main sewer. Two alternatives can be considered for this purpose.
It envisages flow equalisation for the first pulp wash waste-water. This waste-water is generated at a rate of 12.5 cu.m./T of paper made and is discharged at a rate of 1.56 cu.m./batch in 8 batches of 2 hours each per day. The discharge rate is fairly uniform during the 2 hours of washing period.
Capacity of equalisation tank to be provided works out to – 1.0 cu.m. /T paper
Adding 25 per cent extra volume including free board to meet any surge discharge – 0.25 cu.m./T paper
Total volume of equalisation tank – 1.25 cu.m./T paper
Rate of pumping of waste-water from the equalisation tank into sewer – 0.52 cu m./hr for 24 hours
Therefore, two equalisation tanks are to be provided to facilitate cleaning and maintenance.
It envisages flow equalisation of the entire pulp wash waste-water (2 washes). The total pulp wash waste-water is generated at a rate of 50 cu.m./T of paper and is discharged at a rate of 6.25 cu.m./batch in 8 batches of 2 hours each per day. As stated above, the flow rate of waste-water is fairly uniform during the 2 hours washing period.
Capacity of equalisation tank to be provided works out to – 4.0 cu.m./T paper
Adding 25 per cent extra volume including free board to meet any surge discharge – 1.0 cu.m./T paper
Total volume of equalisation tank – 5.00 cu.m./T paper
Rate of pumping of waste-water from the equalisation tank into sewer – 2.08 cu.m./hr for 24 hours
Therefore, two equalisation tanks are to be provided to facilitate cleaning and maintenance. Pumps of suitable capacity have to be provided one each for the two tanks and one as a stand-by.
Primary Clarifier and Sludge Drying:
If secondary treatment like activated sludge, oxidation ditch or rotating biological disc is used as suggested, the excess secondary biological sludge should be added to the combined waste-water before primary settling such that the settled sludge can be filtered on drying bed. The combined primary and secondary sludges do not require biological stabilisation. The dried sludge can be disposed off by burning in an incinerator or dumping in open pits in a controlled manner.
Any pulp and paper mill with best internal measures will find it difficult to discharge effluents meeting standards without recourse to end of the pipeline treatment. Though, through internal measures, it is possible to reduce the pollution load by about 40 per cent, yet to meet MINAS levels, the waste-water needs external treatment.
Soap and Detergent Industry:
Significant in-plant control of both waste quantity and quality is possible particularly in the soap manufacturing subcategories where maximum flows may be 100 times the minimum. Considerably less in-plant water conservation and recycle are possible in the detergent industry, where flows per unit of product are smaller.
The largest in-plant modification that can be made is the changing or replacement of the barometric condensers. The waste-water quantity discharged from these processes can be significantly reduced by recycling the barometric cooling water through fat skimmers, from which valuable fats and oils can be recovered and then through the cooling towers.
The only waste with this type of cooling would be the continuous small blow down from the skimmer. Replacement with surface condensers has been used in several plants to reduce both the waste flow and quantity of organics wasted.
Significant reduction of water usage is possible in the manufacture of liquid detergents by the installation of water recycle piping and tankage and by the use of air rather than water to blow down filling lines.
In the production of bar soaps the volume of discharge and the level of contamination can be reduced materially by installation of an atmospheric flash evaporator ahead of the vacuum drier. Finally, pollutant carry-over from distillation columns such as those used in glycerine concentration or fatty acid separation can be reduced by the use of two additional special trays.
In a recent document presenting techniques adopted by the French for pollution prevention, a new process of detergent manufacturing effluent recycle is described in which washout effluents from reaction and/or mixing vessels and washwater leaks from the paste preparation and pulverisation pump operations are collected and recycled for use in the paste preparation process.
The claim was that pollution generation at such a plant is significantly reduced and although the savings on water and raw materials are small, the capital and operating costs are less than those for building a waste-water treatment facility.
Besselievre reported in a review of water reuse and recycling by the industry that soap and detergent manufacturing facilities showed an average ratio of reused and recycled water to total waste-water effluent of about 2:1.
That is, over two-thirds of the generated waste-water stream in an average plant was being reused and recycled. Of this volume, about 66 per cent was used as cooling water and the remaining 34 per cent for the process or other purposes.
The soap and detergent manufacturing industry makes routine use of various physico-chemical and biological pre-treatment methods to control the quality of its discharges. A survey of these treatment processes is presented in Table 17.5 which also shows the usual removal efficiencies of each unit process on the various pollutants of concern.
According to Nemerow the origin of major wastes is in washing and purifying soaps and detergents and the resulting major pollutants are high BOD and saponified soaps (oily and greasy, alkali and high-temperature wastes), which are removed primarily through air- flotation and skimming and precipitation with the use of CaCl2 as a coagulant.
Figure 17.7 presents a composite flow diagram describing a complete treatment train of the unit processes that may be used in a large soap and detergent manufacturing plant to treat its wastes. As a minimum requirement, flow equalisation to smooth out peak discharges should be utilised even at a production facility that has a small-volume batch operation. Larger plants with integrated product lines may require additional treatment of their waste-waters for both suspended solids and organic materials reduction.
Coagulation and sedimentation are used by the industry for removing the greater portion of the large solid particles in its waste. On the other hand, sand or mixed-bed filters used after biological treatment can be utilised to eliminate fine particles.
One of the biological treatment processes or, alternatively, granular or powdered activated carbon is the usual method employed for the removal of particulate or soluble organics from the waste streams. Finally, as a tertiary step for removing particular ionised pollutants or Total Dissolved Solids (TDS), a few manufacturing facilities have employed either ion exchange or the reverse osmosis process.
Flotation or Foam Fractionation:
One of the principal applications of vacuum and pressure (air) flotation is in commercial installations with colloidal wastes from soap and detergent factories. Waste-waters from soap production are collected in traps on skimming tanks, with subsequent recovery floating of fatty acids.
Foam separation or fractionation can be used to an extra advantage. Not only do surfactants congregate at the air/liquid interfaces, but other colloidal materials and ionised compounds that form a complex with the surfactants tend to also be concentrated by this method.
An incidental, but often important, advantage of air flotation processes is the aerobic condition developed, which tends to stabilise the sludge and skimmings so that they are less likely to turn septic.
However, disposal means for the foamate can be a serious problem in the use of this procedure. It has been reported that foam separation has been able to remove 70 to 80 per cent of synthetic detergents, at a wide range of costs.
Gibbs reported the successful use of fine bubble flotation and 40 minutes detention in treating soap manufacture wastes, where the skimmed sludge was periodically returned to the soap factory for reprocessing. According to Wang the dissolved air flotation process is also feasible for the removal of detergents and soaps from water.
Activated Carbon Adsorption:
Colloidal and soluble organic materials can be removed from solution through adsorption onto granular or powdered activated carbon, such as the particularly troublesome hard surfactants. Refractory substances resistant to biodegradation, such as ABS, are difficult or impossible to remove by conventional biological treatment and so they are frequently removed by activated carbon adsorption.
The activated carbon application is made either in mixed-batch contact tanks with subsequent settling or filtration or in flow- through GAC columns or contact beds. Obviously, because it is an expensive process, adsorption is being used as a polishing step of pretreated waste effluents. Nevertheless, according to Kucharski much better results of surfactant removal have been achieved with adsorption than coagulation/settling.
The coagulation/flocculation process was found to be affected by the presence of surfactants in the raw water or waste-water. Such interference was observed for both alum and ferric sulphate coagulant, but the use of certain organic polymer flocculants was shown to overcome this problem.
However, chemical coagulation and flocculation for settling may not prove to be very efficient for such waste-waters. Wastes containing emulsified oils can be clarified by coagulation, if the emulsion is broken through the addition of salts such as CaCl2 the coagulant of choice for soap and detergent manufacture wastewaters.
Also lime or other calcium chemicals have been used in the treatment of such wastes whose soapy constituents are precipitated as insoluble calcium soaps of fairly satisfactory flocculating (‘hardness’ scales) and settling properties.
Treatment with each can be used to remove practically all grease and suspended solids and a major part of the suspended BOD. Using carbon dioxide (carbonation) as an auxiliary precipitant reduces the amount of calcium chloride required and improves treatment efficiency. The sludge from CaCl2 treatment can be removed either by sedimentation or by air or vacuum flotation.
Ion Exchange and Exclusion:
The ion-exchange process has been used effectively in the field of waste disposal. The use of continuous ion-exchange and resin regeneration systems has further improved the economic feasibility of the applications over the fixed-bed systems.
One of the reported special applications of the ion-exchange resins has been the removal of ABS by the use porous anion exchanger that is a strong base and depends on a chloride cycle. This resin system is regenerated by removing a great part of the ABS absorbed on the resin beads with the help of a mixture of HCl and acetone.
Other organic pollutants can also be removed by ion-exchange resins and the main problem is whether the organic material can be eluted from the resin using normal regeneration or it is economically advisable to simply discard the used resin. Wang and Wood successfully used the ion-exchange process for the removal of cationic surfactant from water.
The separation of ionic from non-ionic substances can be effected by the use of ion exclusion. Ion- exchange can be used to purify glycerine for the final product of chemically pure glycerine and reduce losses to waste, but the concentration of dissolved ionisable solids or salts (ash) largely impacts on the overall operating costs.
Economically, when the crude or sweet water contains under 1.5 per cent ash, straight ion exchange using a cation-anion mixed bed can be used, whereas for higher percentages of dissolved solids, it is economically feasible to follow the ion-exchange with an ion-exclusion system.
For instance, waste streams containing 0.2 to 0.5 per cent ash and 3 to 5 per cent glycerine may be economically treated by straight ion-exchange, while waste streams containing 5 to 10 per cent ash and 3 to 5 per cent glycerine have to be treated by the combined ion-exchange and ion-exclusion processes.
Regarding biological destruction, surfactants are known to cause a great deal of trouble due to foaming and toxicity in municipal treatment plants. The behaviour of these substances depends on their type, i.e. anionic and non-ionic detergents increase the amount of activated sludge, whereas cationic detergents reduce it and also the various compounds decompose to a different degree.
The activated sludge process is feasible for the treatment of soap and detergent industry wastes but, in general, not as satisfactory as trickling filters. The turbulence in the aeration tank induces frothing to occur and also the presence of soaps and detergents reduces the absorption efficiency from air bubbles to liquid aeration by increasing the resistance of the liquid film.
On the other hand, detergent production waste-waters have been treated with appreciable success on fixed-film process units such as trickling filters. Also, processes such as lagoons, oxidation or stabilisation ponds and aerated lagoons have all been used successfully in treating soap and detergent manufacturing waste-waters. Finally, Vath demonstrated that both linear anionic and nonionic ethoxylated surfactants underwent degradation, as shown by a loss of surfactant properties, under anaerobic treatment.
Many pesticide products are formulated by mixing active pesticide ingredients with inert materials (e.g. surfactants, emulsifiers, petroleum hydrocarbons), to achieve specific application characteristics. When these ‘inerts’ mix with water, emulsions may form.
These emulsions reduce the performance efficiency of many treatment unit operations, such as chemical oxidation and activated carbon adsorption. In many situations, emulsion breaking is a necessary pretreatment step to facilitate the removal of pollutants from Pesticide Formulating, Packaging and Repackaging (PFPR) waste-waters.
Although emulsion breaking is a pretreatment step, its importance in the treatment of PFPR waste-waters can make it a major part of the technology train for treating PFPR waste-waters. Temperature control and acid addition are simple, inexpensive methods of breaking emulsions in a variety of PFPR waste-waters.
Acid (e.g. sulphuric acid) added to emulsified waste-water dissolves the solid materials that hold the emulsions together. The de-emulsified oil floats because of its lower specific gravity and can be skimmed off the surface, leaving the waste-water ready for subsequent treatment.
The de-emulsification also causes suspended solids with a higher specific gravity to settle out of the waste-water. Heating the emulsion lowers the viscosities of the oil and water, and increases their apparent specific gravity differential. The oil, with a significantly lower apparent specific gravity, rises to the surface of the waste-water.
Heating the waste-water also increases the kinetic energy of the individual molecules in the waste-water, causing the molecules to collide with each other more frequently. The increased number of molecule collisions aid in breaking the film present between the oil and the water. Once freed from the water, the oil rises, where it can be skimmed from the surface of the waste-water.
At secondary treatment plants, the primary effluent is further treated in a biological process, such as the activated sludge process. The activated sludge process and its variants are the most commonly used secondary processes for medium-to-large treatment plants.
In the process, a special consortium of bacteria and other micro-organisms are grown. They metabolise the pollutants as they grow. This removes the BOD and TSS and can detoxify many pollutants and adsorb others. The secondary process removes at least 85 per cent of the BOD and TSS, and well-designed and operated plants may remove 95 per cent of the BOD and TSS.
The effluent is clear with only a few pinhead sized suspended solids. It is not potable and still contains pathogens. It may contain trace metals and hard-to-treat organics compounds, such as pesticides. Nevertheless, it is usually suitable to be discharged into many receiving waters.
Activated Carbon Adsorption:
Activated carbon effectively removes organic constituents from waste-water through the process of adsorption. The term ‘activated carbon’ refers to carbon materials, such as coal or wood, that are processed through dehydration, carbonisation, and oxidation to yield a material that is highly adsorbent, due to a large surface area and high number of internal pores per unit mass.
As waste-water flows through a bed of carbon materials, molecules that are dissolved in the water may become trapped in these pores. In general, organic constituents (including many pesticide active ingredients) with certain chemical structures (such as aromatic functional groups), high molecular weights, and low water solubility are amenable to activated carbon adsorption.
These constituents adhere to the stationary carbon material, so the wastewater leaving the carbon bed has a lower concentration of pesticide than the waste-water entering the carbon bed. Eventually, as the pore spaces in the carbon become filled, the carbon becomes exhausted and ceases to adsorb contaminants.
Spent carbon may be regenerated or disposed of; cost and/or other regulatory factors (e.g. Resource Conservation and Recovery Act or RCRA) generally determine the choice. Carbon adsorption depends on process conditions, such as temperature and pH and process design factors such as carbon/waste-water contact time and the number of the carbon columns.
If performed under the right conditions, activated carbon adsorption can be an effective treatment technology for PFPR industry waste-waters. Carbon adsorption capacity depends on the characteristics of the adsorbed compounds, the types of compounds competing for adsorption, and the characteristics of the carbon itself.
If several constituents that are amenable to activated carbon adsorption are present in the waste-water, they may compete with each other for carbon adsorption capacity. This competition may result in low adsorption or even desorption of some constituents.
Activated carbon comes in two sizes: powdered carbon which has a diameter of less than 200 mesh, and granular carbon which has a diameter greater than 0.1 millimetre. While granular carbon is more commonly used in waste-water treatment; powdered carbon is used less frequently because the small particle size creates regeneration and design problems.
Activate carbon is obtained from vendors in bulk or in a variety of container sizes. At smaller facilities, the container in which the carbon is sold is intended to be used as the carbon bed, with influent waste-water passing into one end of the container and treated effluent water passing out of the opposite end. At larger facilities, carbon is purchased and added to a column that is installed at the facility.
Carbon is regenerated by removing the adsorbed organic compounds through steam, thermal, or physical/chemical methods. Thermal and steam regeneration are the most common methods to regenerate carbon used for waste-water treatment.
These methods volatilise the organic compounds that have adsorbed onto the carbon. Afterburners are required to ensure the destruction of the organic vapours; a scrubber may also be necessary to remove particulates from the air stream.
Physical/chemical regeneration uses a solvent, which can be a water solution, to remove the organic compounds. Carbon is usually shipped back to the vendor for regeneration, although some facilities with larger carbon beds may find it economical to regenerate carbon on-site.
Chemical oxidation modifies the structure of pollutants in waste-water to similar, but less harmful, compounds, through the addition of an oxidising agent. During chemical oxidation, one or more electrons transfer from the oxidant to the targeted pollutant, causing its destruction.
One common method of chemical oxidation, referred to as alkaline chlorination, uses chlorine (usually in the form of sodium hypochlorite) under alkaline conditions to destroy pollutants, such as cyanide and some pesticide active ingredients.
However, facilities treating waste-water, using alkaline chlorination should be aware that the chemical oxidation reaction may generate toxic chlorinated organic compounds, including chloroform, bromodichloromethane, and dibromochloromethane, as by-products.
Adjustments to the design and operating parameters may alleviate this problem, or an additional treatment step (e.g. steam stripping, air stripping, or activated carbon adsorption) may be required to remove these by-products.
Chemical oxidation can also be performed with other oxidants (e.g. hydrogen peroxide, ozone, and potassium permanganate) or with the use of ultraviolet light. Although these other methods of chemical oxidation can effectively treat PFPR waste-waters, they typically entail higher capital and/or operating and maintenance costs, greater operator expertise, and/or more extensive waste-water pretreatment than alkaline chlorination.
Chemical precipitation is a treatment technology in which chemicals (e.g. sulphides, hydroxides, and carbonates) react with organic and inorganic pollutants present in waste-water to form insoluble precipitates.
This separation treatment technology is generally earned out in the following four phases:
1. Addition of the chemical to the waste-water;
2. Rapid (flash) mixing to distribute the chemical homogeneously throughout the waste-water.
3. Slow mixing to encourage flocculation (formation of the insoluble solid precipitate).
4. Filtration, settling, or decanting to remove the flocculated solid particles.
These four steps can be performed at ambient conditions and are well suited to automat control. Hydrogen sulphide or soluble sulphide salts (e.g. sodium sulphate) are chemicals commonly used in the PFPR industry during chemical precipitation.
These sulphides are particularly effective in removing complexed and heavy metals (e.g. mercury, lead, and silver) from industrial waste-waters. Hydroxide and carbonate precipitation can also be used to remove metals from PFPR waste-waters, but these technologies tend to be effective on a narrower range of contaminants.
Hydrolysis is a chemical reaction in which organic constituents react with water and break into smaller (and less toxic) compounds. Basically, hydrolysis is a destructive technology in which the original molecule forms two or more new molecules.
In some cases, the reaction continues and other products are formed. Because some pesticide and active ingredients react through this mechanism, hydrolysis can be an effective treatment technology for PFPR waste-water.
The primary design parameter considered for hydrolysis is the half-life, which is the time required to react 50 per cent of the original compound. The half-life of a reaction generally depends on the reaction, pH and temperature and the reactant molecule (e.g. the pesticide active ingredient).
Hydrolysis reactions can be catalysed at low pH, high pH or both, depending on the reactant molecule. In general, increasing the temperature increases the rate of hydrolysis. Identifying the best conditions for the hydrolysis reaction results in a shorter half- life, thereby reducing both the size of the reaction vessel required and the treatment time required.
The pharmaceutical industry, although a strong and important entity in itself, is frequently considered to be part of the chemical industry. The pollution problems of the pharmaceutical industry, particularly those of the major companies, parallel those of the chemical industry.
Because the problems of the industry are not unique, will be concerned with emphasising the problems, where they should be looked for, and some of the more interesting solutions to solve these. The pharmaceutical companies facing major problems in connection with both air and water pollution are the ones that operate fermentation plants, principally in the production of antibiotics (penicillin, streptomycin, etc.).
Waste generation from the basic drug manufacturing houses are higher than the formulating units. The nature and the quantity of waste generated vary depending upon the processes involved.
In general, the waste and emissions generated during manufacturing of pharmaceuticals depend on the raw materials and equipment used, as well as the manufacturing, compounding and formulation process employed.
In designing bulk manufacturing processes, consideration is given to the availability of the starting materials and their toxicity, as well as the wastes (e.g. mother liquors, filter residues, and other by-products) and the emissions generated. When bulk manufacturing reactions are complete, the solvents are physically separated from the resulting product.
Due to purity concerns, solvents are not often reused in a pharmaceutical process. They may be sold for non-pharmaceutical uses, utilised for fuel blending operations, recycled, or destroyed through incineration.
Pharmaceutical manufacturers use water for process operations, as well as for other non-process purposes. However, the use and discharge practices and the characteristics of the waste-water will vary, depending on the operations conducted at the facility. Additionally, in some cases, water may be formed as part of a chemical reaction.
Several cyanide destruction treatment technologies are currently used in the pharmaceutical manufacturing industry, including alkaline chlorination, hydrogen peroxide oxidation, and basic hydrolysis.
1. The alkaline chlorination treatment process involves reacting free cyanide with hypochlorite (formed by reacting chlorine gas with an aqueous sodium hydroxide solution) to form nitrogen and carbon dioxide. The reaction is a two-step process and is normally performed separately in two reactor vessels.
Because treatment is normally performed in batches, it is necessary to use an additional equalisation tank to store accumulated waste-water during treatment. The reactors need to be equipped with agitators, and both reaction steps require close monitoring of pH and Oxidation Reduction Potential (ORP). These reactions are normally performed at ambient temperatures.
2. Hydrogen peroxide treatment involves adding hydrogen peroxide to cyanide-bearing wastewater to convert free cyanide to ammonia and carbonate ions. This treatment is normally performed batch-wise, in a reaction vessel.
The treatment process consists of heating the wastewater to approximately 125°C and adjusting the pH in the reaction vessel to approximately 11. Hydrogen peroxide is added to the vessel and is allowed to react for approximately one hour.
Equipment required for this process includes reaction vessels, storage vessels for hydrogen peroxide and a pH adjustment compound (typically sodium hydroxide), an equalisation tank, and feed systems for hydrogen peroxide and sodium hydroxide.
3. Hydrolysis treatment involves reacting free cyanide with water under basic conditions to produce ammonia. This process requires approximately one hour and is typically performed at a temperature between 170° and 250°C, at a pH of 9 and 12. Hydrolysis is normally performed in a reactor vessel equipped with a heat exchanger and a system to store and deliver sodium hydroxide (or other basic compounds).
Oxidising Agents Like Ozone, UV Rays and Hydrogen Peroxide:
Ozone gas is highly reactive and is used in the treatment of waste-water. Ozone is effective in oxidising antibiotics, betablockers, antiphlogistics, lipid regulator metabolites, the antiepileptic drug, carbamazepine and the natural estrogen, estrone from STP. Also, the combined treatment of hydrogen peroxide and UV radiations reduces the amount of diclofenac present in waste-water.
Waste-waters from a fermentation unit emanate mainly from the recovery and purification of the final product and also from washings of floor, vessels, equipment, etc. About 5 m3/kg of final product is the waste-water generated.
A general flow diagram for the treatment of waste-waters from various types of pharmaceutical products is presented in Fig. 17.15. Depending upon the nature of the waste-water, selection/omission of the specific treatment units can be made.
Before selection of a particular treatment system for effluents of pharmaceutical industries the following aspects are required to be considered:
1. Good housekeeping practices.
2. Segregation of certain waste-water streams.
3. Process and equipment modifications.
4. Recovery of by-products and recycle possibilities.
Certain waste-waters containing pathogenic micro-organisms from fermentation particularly vaccine section needs autoclaving before discharge into common treatment plant.
Various treatment processes are discussed below:
The common physical treatments are plain settling, dissolved air flotation, adsorption, solar evaporation. These methods can be adopted individually or in combination depending upon the waste-water quality and quantity.
Water insoluble compounds can be removed from the waste-water by plain settling. The settling characteristics can be improved with the addition of suitable coagulants. The system consists of a settling basin with adequate volume to provide sufficient settling time for the impurities.
Suspended, colloidal and emulsified impurities can be removed from waste-water by air flotation technique. Air dissolved in the waste-water under pressure when released in the flotation tank forms bubbles which trap the lighter impurities and lift them to the surface. The materials form a floating layer which is removed by a skimmer mechanism.
Segregation of different waste streams is an important step for economic design of a treatment plant.
The following streams may be segregated for this purpose:
1. Strong process liquors.
2. Streams containing cyanide, heavy metals, toxic chemicals.
3. Condensate and cooling waters.
4. Acidic and alkaline streams.
Various treatment alternatives for strong process liquors are incineration, solar evaporation and treatment by anaerobic filter. The toxic effluents either can be incinerated or treated by suitable technologies like carbon adsorption, ion-exchange, chemical precipitation, reverse osmosis, etc.
Condensate and cooling waters can be recycled and reused. The acidic and alkaline waste streams can be either treated separately with acid/alkali for pH correction or may be combined suitably with other waste streams.
The effluent containing chemical sludge, settleable solids and high oil concentration (over 50 mg/l) can be treated by coagulation, flocculation and settling after neutralisation. Coagulants like alum, FeSO4, FeCl3, etc. with/without polyelectrolytes can be used. The coagulation process also breaks oil emulsions and nullifying the zeta potential.
Pre-aeration for 2 to 3 hours by means of diffused air may help to bring down the BOD load of about 30 to 40 per cent. Diffused aeration is known to bring oily and fatty matters in suspension form in the waste-water.
Various factors are responsible to select a suitable treatment system, i.e. quality and quantity of influent to be treated, desired degree of treatment, site conditions, change of products and overall economics. Secondary biological treatments employed are mainly aerobic and in some cases anaerobic followed by aerobic treatment.
Trickling filter, extended aeration and conventional activated sludge systems are generally practised. Anaerobic filter and anaerobic lagooning are also being used for treatment of pharmaceutical industry waste-waters.
Desired effluent quality is the basis for the selection of any treatment scheme. In most cases, pharmaceutical industry effluents are not suitable for land disposal for farming due to the presence of high concentration of dissolved salts.
Tertiary treatments are required to kill virus and bacteria and to remove other impurities like colour, bad smell, etc. Chlorination and sand filtration may generally be practised for tertiary treatment.
Various factors are responsible for the selection of biological treatment system, i.e. quality and quantity of waste-water to be treated, desired degree of treatment, site conditions and overall economics.
Biological treatment systems employed for pharmaceutical wastes are mainly aerobic in nature. In some cases where the organic loading is high, anaerobic treatment units are used prior to aerobic system.
The widely used anaerobic treatment unit for pharmaceutical wastes is anaerobic lagoon and for aerobic treatment, trickling filter, aerated lagoon or aeration tank followed by clarifier are used.
Food Processing Industry:
Agro-industries or food-processing industries, are concerned primarily with the production of edible goods for human and animal consumption from raw agricultural products.
These industries include:
(iii) Brewing and distilling,
(iv) Meat packing and rendering (including poultry and feedlots),
(v) Sugar refining,
(vi) Soft drinks and beverages, and
(vii) Miscellaneous, including coffee, seafood, rice, grains, and bakeries.
Variety is the one word to describe water use in the agro-industries. Not only are there thousands of products, but agro-industries are well established in many countries with different availabilities of water resources.
The industry is not as heavily concentrated in the developed countries as other industries since food processing is nearly a universal need.
Sources and characteristics of waste-water – Water is in contact with raw or finished products in most processes in the agro-industries. This contact results in wastes containing organic matter, in dissolved or colloidal states and in varying degrees of concentration.
The sources of these waste-waters are water that contacts spoiled raw material or finished products, rinsing or washing water, transporting water, process water, cooling water, spills and water used for cleaning equipment.
The waste streams exhibit extreme ranges among the industries. The effluent BOD can range from 100 to 1,00,000 ppm. Suspended solids can range from nearly zero to 1,20,000 ppm and pH values range from 3.5 to 11.0. The volumes of the waste also vary greatly. Table 17.18 shows data that provide a brief summary of waste-water problems facing the food industries.
Waste-water treatment practices – Since the primary component of agro-industries’ effluents is organic wastes, biological forms of treatment are effective. The alternatives for the treatment of agro-industries’ waste are in-plant treatment with the effluent discharged directly to the water environment or the effluent discharge to a municipal waste treatment plant with or without in-plant pretreatment.
Discharge to municipal treatment facilities usually requires pretreatment owing to higher concentrations of organics. In-plant treatment for direct discharge or as pretreatment will most likely use aerobic or anaerobic biological treatment, with the most effective methods being activated sludge, biological filtration, anaerobic digestion, oxidation ponds, lagoons and spray irrigation. The selection of a method will depend upon the degree of treatment required, nature of the organic wastes, concentration of organic matter, volume of wastes and local costs.
Table 17.19 suggests treatment methods for various types of waste that may be encountered in the agro-industries.
A new system has been developed that can treat the caustic waste separately installed. The system is composed of land treatment for the noncaustic waste and an aerobic/anaerobic pond system with a sand filter and pH adjustment system.
The rapid expansion of dairy industry is one of the significant developments in the food processing field during this period. The subject dairy and food engineering occupies a major importance in the food science curriculum and this emphasis is likely to continue.
In dairy industry, water has a multipurpose use. Water used for the purpose of processing, cleaning and other general uses should be of potable standard and absolutely free from microbial contaminants. Use of contaminated water in the plant, results in milk and milk products to be unsafe for human consumption.
Waste disposal in the milk industry may be divided into two programs, first, waste prevention or saving, and second, waste treatment. The utilisation of by-products and a waste-saving program will materially reduce the loss of milk solids and simplify the requirements for treatment. Such a program should always precede the design of treatment facilities.
The first step in the program is to segregate all possible clean water from the water containing milk solids. Segregation necessitates changes in the drain system of the plant in order to provide a separate line for cooling water, ice machine water, boiler blow-down, roof drains and vacuum pan water. The condenser water from the vacuum pan will contain entrained solids, but because of its large volume it must be segregated from the plant wastes.
All floor wastes should pass through a simple combination sand trap, fat trap, and screen tank. This is desirable for all plants even if the waste is discharged to a large stream or a municipal sewer.
The simplest means of waste disposal for the milk plant is the discharge of the waste to a municipal sewer system. However, a sewer service charge can be expected which should be adequate to permit the municipality to construct the additional waste treatment facilities and pay for their operation.
Some of the effluent can be used directly for the irrigation or to spread them over an area of grassland or cultivated land. The composition of the effluent does, however, impose limits of a maximum of 200 litres per m2 per year.
Furthermore, the irrigation must be done at least 200 metres away from an inhabited area. Return of by-products waste to farmers is the most economical method of disposal. Farmers use such by-products as whey for feeding purposes. Effluent may be treated by mechanical, chemical and biological means. A combination of these methods is often used.
Mechanical treatments are screening, filtration, sedimentation, or flotation. The latter process is aided by gas bubbles to which the particles of impurities becomes attached. Only insoluble constituents can be removed by these methods.
Reverse osmosis would also remove dissolved constituents, but this method is not suitable for large scale use at the present time because large volume of effluent produced makes the process too costly to run.
Chemical treatment consists of the precipitation of dissolved substances by means of suitable precipitating agents such as iron sulphate, iron chloride, aluminium sulphate, and lime, etc. A sedimentable coagulum is formed which also contains suspended solids.
The coagulum may be subsequently separated from the water by mechanical means. Chemical purification is not sufficient for dairy effluents because it does not remove the dissolved lactose.
The most suitable method of treating dairy effluents is biological purification. Dissolved or colloidally suspended organic compounds are decomposed by oxidation with the aid of aerobic bacteria. Oxygen must be supplied by means of artificial ventilation. Organic substances can also be decomposed by reduction of anaerobic bacteria in a septic tank.
The bakery industry is one of the world’s major food industries and varies widely in terms of production scale and process. Traditionally, bakery products may be categorised as bread and bread roll products, pastry products (e.g. pies and pasties) and speciality products (e.g. cake, biscuits, donuts and speciality breads).
The major equipment includes miller, mixer/kneading machine, bun and bread former, fermentor, bake ovens, cold stage and boilers. The main processes are milling, mixing, fermentation, baking and storage. Fermentation and baking are normally operated at 40°C and 160°-260°C, respectively. Depending on logistics and the market, the products can be stored at 4°-20°C.
Waste-water in bakeries is primarily generated from cleaning operations including equipment cleaning and floor washing. It can be characterised as high loading, fluctuating flow contains rich oil and grease. Flour, sugar, oil, grease and yeast are the major components in the waste.
The ratio of water consumed to products is about 10 in common food industry, much higher than that of 5 in the chemical industry and 2 in the paper and textiles industry. Normally, half of the water is used in the process, while the remainder is used for washing purposes (e.g. of equipment, floor and container’s).
Typical values for waste-water production are summarised in Tables 17.21-17.22. Different products can lead to different amounts of waste-water produced. As shown in Table 17.21, pastry production can result in much more waste-water than the others.
The values of each item can vary significantly as demonstrated in Table 17.22. The waste-water from cake plants has higher strength than that from bread plants. The pH is in acidic to neutral ranges, while the 5-day biochemical oxygen demand (BOD5) is from a few hundred to a few thousand mg/l, which is much higher than that from the domestic wastewater. The suspended solids (SS) from cake plants is very high. Grease from the bakery industry is generally high, which results from the production operations.
The waste strength and flow rate are very much dependent on the operations, the size of the plants and the number of workers. Generally speaking, in the plants with products of bread, bun and roll, which are termed as dry baking, production equipment (e.g. mixing vats and baking pans) are cleaned dry and floors are swept before washing down.
The waste-water from cleanup has low strength and mainly contains flour and grease (Table 17.22). On the other hand, cake production generates higher strength waste, which contains grease, sugar, flour, filling ingredients and detergents.
Due to the nature of the operation, the waste-water strength changes at different operational times. As demonstrated in Table 17.22, higher BOD5, SS, Total Solids (TS) and grease are observed from 1 to 3 am which results from lower waste-water flow rate after midnight.
Bakery waste-water lacks nutrients; the low nutrient value gives BOD5:N:P of 284:1:2. This indicates that to obtain better biological treatment results, extra nutrients must be added to the system. The existence of oil and grease also retards the mass transfer of oxygen. The toxicity of excess detergent used in cleaning operations can decrease the biological treatment efficiency. Therefore, the pretreatment of waste-water is always needed.
Generally, bakery industry waste is nontoxic. It can be divided into liquid waste, solid waste and gaseous waste. In the liquid phase, there are high contents of organic pollutants including Chemical Oxygen Demand (COD), BOD5, as well as Fats, Oils and Greases (FOG) and SS. Waste-water is normally treated by physical, chemical and biological processes.
Pretreatment or primary treatment is a series of physical and chemical operations, which precondition the waste-water as well as remove some of the wastes. The treatment is normally arranged in the following order- screening, flow equalisation and neutralisation, optional FOG separation, optional acidification, coagulation-sedimentation and dissolved air flotation.
In the bakery industry, pretreatment is always required because the waste contains high SS and floatable FOG. Pre-treatment can reduce the pollutant loading in the subsequent biological and/or chemical treatment processes; it can also protect process equipment. In addition, pretreatment is economically preferable in the total process view as compared to biological and chemical treatment.
The objective of biological treatment is to remove the dissolved and particulate biodegradable components in the waste-water. It is a core part of the secondary biological treatment system. Micro-organisms are used to decompose the organic wastes.
With regard to different growth types, biological systems can be classified as suspended growth or attached growth systems. Biological treatment can also be classified by oxygen utilisation: aerobic, anaerobic and facultative. In an aerobic system, the organic matter is decomposed to carbon dioxide, water and a series of simple compounds. If the system is anaerobic, the final products are carbon dioxide and methane.
Compared to anaerobic treatment, the aerobic biological process has better quality effluent, easier operation, shorter solid retention time, but higher cost for aeration and more excess sludge. When treating high-load influent (COD > 4000 mg/l), the aerobic biological treatment becomes less economic than the anaerobic system.
To maintain good system performance, the anaerobic biological system requires more complex operations. In most cases, the anaerobic system is used as a pretreatment process. Suspended growth systems (e.g. activated sludge process) and attached growth systems (e.g. trickling filter) are two of the main biological waste-water treatment processes. The activated sludge process is most commonly used in treatment of waste-water.
The trickling filter is easy to control and has less excess sludge. It has higher resistance loading and low energy cost. However, high operational cost is its major disadvantage. In addition, it is more sensitive to temperature and has odour problems. Comprehensive considerations must be taken into account when selecting a suitable system.
In the activated sludge process, suspended growth micro-organisms are employed. A typical activated sludge process consists of a pretreatment process (mainly screening and clarification), aeration tank (bioreactor), final sedimentation and excess sludge treatment (anaerobic treatment and dewatering process).
The final sedimentation separates micro-organisms from the water solution. In order to enhance the performance result, most of the sludge from the sedimentation is recycled back to the aeration tank(s), while the remaining is sent to anaerobic sludge treatment.