Wastewater must be treated before discharge to the environment for reasons of health, aesthetics, and protection of aquatic life. Without treatment, the oxygen-consuming components of waste water compete with aquatic life and thus cause fish kills. Sanitary wastewater (created in homes and restrooms) can contain pathogens that spread disease such as cholera, typhoid, and hepatitis. The decomposition of wastewater can generate smells and scum on water bodies, detracting from the appeal of streams and lakes.
Since the discovery that natural processes of oxidation could be accelerated to treat wastewater, the science and engineering of waste treatment has been dominated by the lessons and experience of treating sanitary waste streams. Industrial waste streams, such as those produced by the food industry, have often been neglected in the development of processes. Yet, these streams present unique challenges and opportunities.
Characterization of Waste Streams
The actual chemical composition of a waste stream is usually complex and difficult to measure. In practice, empirical measures are used. The usual components are suspended solids (SS), biological oxygen demand (BOD), chemical oxygen demand (COD), and fats, oil, and grease (FOG). pH and dissolved solids may be of concern in some circumstances, but normal sanitary or municipal wastes are close to neutral and have relatively low dissolved solids content because they typically are created from potable water. Certain dissolved nutrients, such as phosphorus and nitrogen are of interest because they may support algae blooms in receiving waters. Phosphorus enters the waste stream through the use of household cleaners. Nitrogen enters often as land run-off where fertilizers have been used.
In most sanitary waste streams, SS, BOD, and COD are measured in hundreds of parts per million or milligrams per liter. FOG is a special concern because high levels can lead to clogging of sewers by solid fat deposits. For this reason, generators of high-fat wastes, such as restaurants, are required to have grease traps which prevent FOG from entering the sewer.
Food plant waste streams, in contrast, may have concentrations of these components orders of magnitude higher, that is, in thousands or tens of thousands of ppm. Flows are also often very high, thousands of gallons per day, whereas typical homes may only generate 100–200 gal/day/person, from toilet flushing, laundry, and bathing.
The high strength and volume of food plant waste streams usually require that the plant do some pretreatment, even if the waste stream ultimately is treated by a municipality. Most municipal waste treatment systems are designed primarily for domestic sanitary waste and can achieve 90–95 % removal of BOD, typically leaving less than 10 ppm in the discharge. Discharge streams are usually chlorinated to remove pathogens and may be treated with activated carbon to remove taste and color. The result is a water that is nearly equivalent to drinking water. The other product of municipal wastewater treatment is sludge, a mixture of removed suspended solids and bacterial cells generated by the biological oxidation process. This sludge may be dried, spread on land, or reduced (but not eliminated) by anaerobic digestion.
Food plant waste streams, in addition to being relatively strong, can vary in strength and volume during the day and over a year, because of seasonal changes in production. Most food waste is generated in cleaning and therefore may vary widely in pH. The major component of a food plant waste stream is usually the raw material or product being processed. Thus a dairy waste contains mostly milk, a meat packer will have fats and meat solids, and a vegetable processor will have dissolved and suspended solids from field crops.
Because food wastes are easily degraded, they may be welcome in a municipal plant as a source of nutrients for the biological oxidation process. However, excess FOG is never welcome, and even an oversupply of nutrients can be harmful because of the demand on oxygenators and the production of sludge. Thus food plants need to consider waste generation and treatment in their design and operation.
Strategies for Food Plants
A food plant may be limited in operation by restrictions on its waste and by its water supply. The days of profligate water use and casual discharge of untreated waste are over. Martin Okos, Professor of Biochemical and Food Process Engineering at Purdue University, West Lafayette, Ind. (phone 765-494-1211), is pursuing the ambitious goal of a zero-discharge food plant. He and his group work with various food plants to document every use of water and source of waste, then suggest modifications that reduce water use and waste volume. The approach is similar to that of an energy audit.
A common discovery is that a processor may be using fresh potable water for an initial wash of raw material, where a stream from a previous use might be suitable. Countercurrent flow of wash water may reduce waste discharge to one-third or less of the previous volume.
Cooling water for retorts in canneries is rarely very contaminated, yet it is routinely discharged with wastewater after just one use. Recycling through cooling towers can reduce water consumption and waste volume. Segregating cooling water from more contaminated wastewater can reduce the load on a treatment system.
The first step in most clean-in-place (CIP) procedures is a short rinse with fresh water. This can generate the strongest waste stream, yet the stream is really a dilute source of raw material. It can be saved and reused in the product, reducing the waste discharge and improving yield.
Many food plants use flumes to transport raw materials in high-volume water streams. This water can be recovered and reused, perhaps after simple treatment, rather than discharged.
Hoses are an insidious source of water consumption, because often they are allowed to run long after they have served a purpose. A hose typically runs at about 10 gal/min, so the volume can add up quickly when multiple hoses are in use. It is common for workers to chase spills of liquid or solid food to a drain, using a hose. A better practice is to clean spills with shovels or brooms, dumping them into containers designated for solid waste. Such solid or highly concentrated wastes are often taken by local farmers for animal feed, thus keeping them out of wastewater treatment systems. The number of hose stations should be limited only to those really needed, and they should have shutoff valves on the end so that it takes positive action to initiate flow.
Reusing water within a plant may require some simple treatment such as settling or filtration to remove suspended solids or soil. Some repiping may be necessary to segregate water streams for proper reuse. Each stream should be evaluated to determine if it contains a component worth recovering. For example, Okos has developed a system for fermenting waste lactose in dairy discharges to alcohol.
When Treatment Is Unavoidable
True zero discharge is probably impossible to achieve, though it is a worthy goal. One treatment option, offered by ADI Systems, Inc., Fredericton, New Brunswick, Canada, is anaerobic treatment. Graham Brown, Director of International Marketing & Sales (phone 506-452-7307), points out that conventional aerobic oxidation is a net consumer of energy, because it takes electricity to drive the agitators and pumps needed to contact air with the microorganisms consuming the dissolved solids. Anaerobic digestion produces methane and carbon dioxide, which can be used as an energy source in specially designed boilers or mixed with natural gas for firing in existing equipment, thus reducing gas purchases.
Anaerobic processes are generally slower than aerobic systems, thus requiring larger equipment to treat the same volume of waste. At the highest anaerobic rates, process control is more challenging than at lower rates, so there is a trade-off in stability against size and cost. ADI has found that brewery wastes respond well to its process and can generate almost enough fuel gas to run the brewery. Still bottoms from distilled beverages (brandy and whiskey) are traditionally difficult to treat because the easily fermentable components have been removed, but anaerobic processes can digest this material and generate valuable fuel.
One approach to reduce the capital and operating cost of aerobic digestion is to use pure oxygen, according to Sudhir Brahmbhatt, Senior Group Manager, Applied Technology Group, MG Industries, Malvern, Pa. (phone 610-636-1775). MG Industries, the United States division of Messer, Krefeld, Germany, supplies industrial gases to the food and other industries. Using oxygen instead of air increases the mass transfer to the microorganisms, the limiting step in aerobic treatment, five-fold.
There are many proprietary and generic wastewater treatment processes, including various filters, centrifuges, biological reactors, membranes, and chemical treatments. One source, among many, for much of this type of equipment is Parkson Corp., Ft. Lauderdale, Fla. According to Marcia Sherony, Group Business Manager for Industrial Sales (phone 847-473-3700), the company makes filters, separators, and biological treatment systems, and typically sells through local representatives.
Each treatment system should be tailored to the waste stream, because each plant can have significant differences in composition of its waste. The first step in improving a food plant in this area should always be a careful evaluation of how water is used and how waste is created. Only then should the various treatment options be considered.
by J. PETER CLARK
Consultant to the Process Industries
Oak Park, Ill.
E-mail: [email protected]