The following article will guide you about how to treat waste in bakery industry.
Introduction:
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).
ADVERTISEMENTS:
The major equipment includes miller, mixer/kneading machine, bun and bread former, fermenter, 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:
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 and 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 containers).
Typical values for waste-water production are summarised in Tables 27.1-27.3. Different products can lead to different amounts of waste-water produced. As shown in Table 27.1, pastry production can result in much more waste-water than the others. The values of each item can vary significantly as demonstrated in Table 27.2. 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 27.2). 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 27.2, higher BOD5, SS, total solids (TS) and grease are observed from 1 to 3 a.m., 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.
Bakery Waste Treatment:
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.
ADVERTISEMENTS:
Pre-treatment 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. The pretreatment of bakery waste-water is presented in Fig. 27.1.
In the bakery industry, pretreatment is always required because the waste contains high SS and floatable FOG.
ADVERTISEMENTS:
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.
i. Flow Equalisation and Neutralisation:
In bakery plants, the waste-water flow rate and loading vary significantly with the time as illustrated in Table 27.3. It is usually economical to use a flow equalisation tank to meet the peak discharge demand. However, too long a retention time may result in an anaerobic environment. A decrease in pH and bad odours are common problems during the operations.
Screening is used to remove coarse particles in the influent. There are different screen openings ranging from a few µm (termed as microscreen) to more than 100 mm (termed as coarse screen). Coarse screen openings range from 6-150 mm; fine screen openings are less than 6 mm. Smaller opening can have a better removal efficiency; however, operational problems such as clogging and higher head lost are always observed.
Fine screens made of stainless material are often used. The main design parameters include velocity, selection of screen openings and head loss through the screens. Clean operations and waste disposal must be considered. Design capacity of fine screens can be as high as 0.13 m3/sec; the head loss ranges from 0.8-1.4 metre. Depending on the design and operation, BOD5 and SS removal efficiencies are 5-50 per cent and 5-45 per cent, respectively.
As waste-water may contain high amount of FOG, a FOG separator is thus recommended for installation. The FOG can be separated and recovered for possible reuse, as well as reduce difficulties in the subsequent biological treatment.
Acidification is optional, depending on the characteristics of the waste. Owing to the presence of FOG, acid (e.g. concentrated H2SO4) is added into the acidification tank; hydrolysis of organics can occur, which enhances the biotreatability. Grove designed a treatment system using nitric acid to break the grease emulsions followed by an activated sludge process. A BOD5 reduction of 99 per cent and an effluent BOD5 of less than 12 mg/l were obtained at a loading of 40 lb BOD5/1000 ft3 and detention time of 87 hours. The nitric acid also furnished nitrogen for proper nutrient balance for the biodegradation.
Coagulation is used to destabilise the stable fine SS, while flocculation is used to grow the destabilised SS, so that the SS become heavier and larger enough to settle down. The Coagulation-flocculation process can be used to remove fine SS from bakery waste-water. It normally acts as a preconditioning process for sedimentation and/or dissolved air flotation.
The waste-water is preconditioned by coagulants such as alum. The pH and coagulant dosage are important in the treatment results. Liu and Lien reported that 90-100 mg/l of alum and ferric chloride were used to treat waste-water from a bakery that produced bread, cake and other desserts. The wastewater had pH of 4.5, SS of 240 mg/l and COD of 1307 mg/l.
Values of 55 per cent and 95-100 per cent for removal of COD and SS, respectively, were achieved. The optimum pH for removal of SS was 6.0, while that for removal of COD was 6.0-8.0. It was also found that FeCl3 was relatively more effective than alum. Yim used coagulation-flocculation to treat a waste-water with much higher waste strength. Table 27.4 gives the treatment results.
Owing to the higher organic content, SS and FOG, coagulants with high dosage of 1300 mg/l were applied. The optimal pH was 8.0. As shown, removal for the above three items was fairly high, suggesting that the process can also be used for high-strength bakery waste. However, the balance between the cost of chemical dosage and treatment efficiency should be justified.
Sedimentation, also called clarification, has a working mechanism based on the density difference between SS and the water, allowing SS with larger particle sizes to more easily settle down. Rectangular tanks, circular tanks, combination flocculator-clarifiers and stacked multilevel clarifiers can be used.
vii. Dissolved Air Flotation (DAF):
Dissolved air flotation (DAF) is usually implemented by pumping compressed air bubbles to remove fine SS and FOG in the bakery waste-water. The waste-water is first stored in an air pressured, closed tank. Through the pressure-reduction valves, it enters the flotation tank. Due to the sudden reduction in pressure, air bubbles form and rise to the surface in the tank. The SS and FOG adhere to the fine air bubbles and are carried upwards. Dosages of coagulant and control of pH are important in the removal of BOD5, COD, FOG and SS.
Other influential factors include the solids content and air/solids ratio. Optimal operation conditions should be determined through the pilot-scale experiments. Liu and Lien used a DAF to treat a waste-water from a large-scale bakery. The waste-water was preconditioned by alum and ferric chloride. With the DAF treatment, 48.6 per cent of COD and 69.8 per cent of SS were removed in 10 minutes at a pressure of 4 kg/cm2 and pH 6.0. Mulligan used DAF as a pretreatment approach for bakery waste. At operating pressures of 40-60 psi, grease reductions of 90-97 per cent were achieved. The BOD5 and SS removal efficiencies were 33-62 per cent and 59-90 per cent, respectively.
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.
i. Activated Sludge Process:
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. A recommended complete activated sludge process is given in Fig. 27.2.
The activated sludge process can be a plug-flow reactor (PFR), completely stirred tank reactor (CSTR), or sequencing batch reactor (SBR). For a typical PFR, length-width ratio should be above 10 to ensure the plug flow. The CSTR has higher buffer capacity due to its nature of complete mixing, which is a critical benefit when treating toxic influent from industries. Compared to the CSTR, the PFR needs a smaller volume to gain the same quality of effluent. Most large activated sludge sewage treatment plants use a few CSTRs operated in series. Such configurations can have the advantages of both CSTR and PFR.
The SBR is suitable for treating non-continuous and small-flow waste-water. It can save space, because all five primary steps of fill, react, settle, draw and idle are completed in one tank. Its operation is more complex than the CSTR and PFR; in most cases, auto operation is adopted.
The performance of activated sludge processes is affected by influent characteristics, bioreactor configuration and operational parameters. The influent characteristics are waste-water flow rate, organic concentration (BOD5 and COD), nutrient compositions (nitrogen and phosphorus), FOG, alkalinity, heavy metals, toxins, pH and temperature. Configurations of the bioreactor include PFR, CSTR, SBR, membrane bioreactor (MBR) and so on.
Operational parameters in the treatment are biomass concentration [mixed liquor volatile suspended solids concentration (MLVSS) and volatile suspended solids (VSS)], organic load, food to micro-organisms (F/M), dissolved oxygen (DO), sludge retention time (SRT), hydraulic retention time (HRT), sludge return ratio and surface hydraulic flow load. Among them, SRT and DO are the most important control parameters and can significantly affect the treatment results. A suitable SRT can be achieved by judicious sludge wasting from the final clarifier. The DO in the aeration tank should be maintained at a level slightly above 2 mg/l.
Owing to the high organic content, it is not recommended that bakery waste-water be directly treated by aerobic treatment processes.