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Waste disposal at a salad dressing plant: study offers a solution for wastewater treatment.

Study offers a solution for wastewater treatment

The food processing industry produces many types of effluents, depending on the type of product and the manufacturing procedure.

These effluents can pose disposal problems and each must be considered independently for treatment.

For example, large quantities of sugar and starch in the waste stream can be difficult for local water treatment plants. These facilities often lack sufficient biological activity in their systems to properly treat the waste products.

The basic biological process for treating this type of wastewater uses bacteria to convert sugar and starch into carbon dioxide, water and biomass. The typical procedure used by water treatment facilities is to allow local strains of bacteria to adapt to the conditions present in the wastewater. However, if there is too much material in the wastewater for the bacteria to break down, the system's efficiency is reduced.

Bacterial activity in water is sometimes measured by biological oxygen demand (BOD). As bacteria grow, they consume oxygen. The decrease in oxygen content in the water can be measured as parts per million (ppm) oxygen then used to estimate the amount of bacterial growth. High BOD levels in wastewater show that a large amount of bacterial activity will be needed for proper treatment.

BOD limits are placed on waste streams entering public sewage systems. This creates problems for food manufacturing plants that produce high levels of BOD in their waste streams because they cannot use local sewage systems for disposal.

One example of this problem is at a salad dressing manufacturing plant in Stead, Nevada. The dressmaking process begins with blending dry ingredients. This is followed by mixing with water, egg yolks and remaining ingredients to create dressing "slurry."

The resulting mixture is then blended thoroughly in an emulsifier.

The dressing is processed and bottled in stages including air blasting, filling, capping, labeling and packing. Process piping and equipment are rinsed daily and the resulting wastewater contains appreciable quantities of oils, dissolved and starch.

Oils are separated into one tank, while the aqueous portion containing BOD levels from 1,000 to 4,000 ppm is stored in two separate settling tanks. Figure 1 shows the process in a flow sheet.


Because the wastewater BOD levels are too high for local disposal and treatment, tank trucks must transport the wastewater to a disposal site more than 10 miles from the plant.

The composition of wastewater from the salad dressing plant varies from day to day and week to week as different products are produced. However, the wastewater generally lies within parameters amenable to bacteriological treatment.

Factors affecting biological treatment of wastewater include acidity level (pH), temperature and concentration of organic and inorganic materials. If biological treatment can be demonstrated effective, large-scale BOD reduction in the wastewater could be accomplished simply by converting existing settling tanks into bioreactors. This would eliminate the need for hauling and other disposal expenses.

This potential for biological BOD reduction formed the basis of a series of experiments performed at the University of Nevada, Reno, Mackay School of Mines Chemical and Metallurgical Engineering Department and sponsored in part by First Interstate Bank of Nevada.

Experiments were conducted in a 4,000 ml glass reactor placed in a constant-temperature water bath. The reactor was equipped with a stirring system and an air sparger to control agitation and oxygen availability. Three liters of wastewater taken from the plant settling tanks were placed in the reactor.

Various conditions were maintained during the course of each experiment. Small samples were taken from the reactor at the start of the procedure and at regular time intervals during the course of the experiment. Conductivity and pH readings were noted for each sample at the time of collection.

The samples were diluted using buffered dilution water, measured for dissolved oxygen and incubated at 20 [degrees] C (68 [degrees] F) for five days in a standard BOD test. The results showed that dilutions of 0.05% and 0.1% of the original sample were appropriate for this case.

The dilution water was prepared using magnesium sulfate, calcium chloride and ferric chloride buffers. These were added to de-ionized distilled water and the mixture was saturated with oxygen for two hours. The solution was then stored in a dark place overnight and just before use, an additional phosphate buffer was added.

At each designated collection time, diluted samples were placed in 300 mL bottles equipped with water-seal caps. After five days incubation in a dark place at 20 [degrees] C (68 [degrees] F), the oxygen content of each sample was retested to show the percentage of oxygen consumed by biological activity.

A calibration was performed before every test series to establish the base line of dissolved oxygen. This was performed using pure dilution water that had been incubated at the same time and conditions as the sample dilutions.

The purpose of the calibration was to eliminate any BOD activity reacting arising from the dilution water, providing a comparative result for the sample vs. pure dilution water. Different experiments were performed to establish the effect of airflow, temperature, pH and selected bacteria additions on the kinetics of reduced BOD levels in the samples.

Two experiments were conducted at 20 [degrees] C (68 [degrees] F), continuous stirring, original pH of 4.8 and reaction period of 48 hours. Results show that the supply of air does not significantly change the kinetics of BOD reduction.

Effects of pH adjustment and addition of a commercially available bacteria -- BIO 112 -- were also tested. These experiments show that improved BOD reduction occurred when pH was closer to 7. Even with the addition of air there was less BOD reduction at lower pH levels.

BOD reduction almost doubled with higher BIO 112 concentrations.

Another commercial bacteria -- BIO 104 -- was added for the experiment. This bacteria appears to be the most efficient BOD reducer for the systems observed. With a minimum of media manipulation, about 80% BOD reduction can be achieved in 96 hours. Higher reduction, up to 95%, can be achieved if the temperature is increased to 30 [degrees] C (86 [degrees] F) and at air supply 0.2 L per minute.

A time period was observed when bacteria in the reactor was practically inactive. This possibility must be considered if an industrial application is implemented.

Along with BOD reduction, the conductivity and pH of undiluted samples were also monitored. The change of pH during the experiments with BIO 112 and BIO 104 was recorded. Results show a rapid increase in pH at the beginning of the experiments with a continuing tendency to increase. In both cases, higher values of pH were observed when air was supplied to the system. The conductivity also showed a tendency to increase but with a smaller magnitude compared to changes in pH.

These experiments show that it is possible to reduce BOD levels significantly using methods that could be applied to an industrial scale by transforming existing wastewater holding tanks into bioreactors. The most important factor for implementing this idea is to select suitable bacteria for the process.

The results also showed that there is an incubation time required for the bacteria to effectively reduce BOD levels. This must be considered during large-scale application.

Frank W. Dickson is operations manager, Advanced Specialty Gases, Inc., 28 Enterprise Way, Box 2173, Dayton, NV 89403, USA; 702-246-5333, fax 702-246-5454,

Tze M. Lee is a graduate student, Department of Chemical and Metallurgical Engineering, Mackay School of Mines, University of Nevada, Reno, NV 89557, USA; 702-784-4307, fax 702-784-4764,

Bogidar V. Mihaylov is an associate professor in the Department of Chemical and Metallurgical Engineering at the Mackay School of Mines, University of Nevada, Reno, NV 89557, USA; 702-784-6060. 702-784-4764,
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Author:Dickson, Frank W.; Lee, Tze M.; Mihaylov, Bogidar V.
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Aug 1, 1997
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