Most food plants produce copious quantities of wastewater, primarily from the frequent cleaning and sanitation operations. It is common to dispose of wastewater through municipal sewers and waste treatment plants, but often these systems impose additional charges because of the volume and strength of the wastewater, as compared with household wastewater. In many locations, an adequate municipal system is not available, and so food plants frequently must provide some preliminary or even complete treatment before being allowed to discharge to local streams or lakes.
Whether to reduce surcharges, to accommodate limitations of a municipal system, or to be more environmentally conscious, it is almost always productive to scrutinize water consumption in a food plant. Opportunities to reduce water use and resulting waste flows frequently exist because water is often relatively inexpensive compared to other utilities and has often been neglected in favor of energy conservation, for instance.
In a dairy products plant that I helped to design and build, we faced a severe limitation of permissible wastewater discharge volume and so were challenged to reduce water consumption by design. Some of our innovations may have applications elsewhere.
We found that a major source of dissolved solids was the initial rinse in the clean-in-place (CIP) operation. Rather than discharge this stream to waste, we recovered it to a storage tank from which it could be used as recipe water in future batches of ice cream mix. No credit was assumed for the recovered milk fat and solids because these were presumed to be relatively minor and variable, but this practice kept them and the water out of the waste stream.
We also deliberately reduced the number of water hoses in the plant and equipped them with shut-off nozzles. Later we learned that the plant added hose stations at the request of operators and then complained that the water supply lines were inadequate. We explained our reasoning and motivation. One lesson from that experience is to communicate clearly the reason for conservation measures that may depart from traditional practices.
We also evaluated packaging equipment for water consumption and found wide variations. One model filler used a continuously running water stream to clean a conveyor belt that experienced spills and overflows, while a competitor used intermittent flows with significantly lower consumption. Considering water consumption in evaluating equipment can result in significant savings.
--- PAGE BREAK ---
In another plant, a small meat slaughter operation with edible rendering, the stick water from rendering was discarded. With the addition of some simple equipment—an evaporator and storage tank—this water could be concentrated as a source of natural meat flavor.
Finally, many plants use water in cooling and then discharge it even though it is usually relatively clean. Often it makes sense to recover this water and store it for use in cleaning, produce washing, or fluming. Water to transport raw materials in flumes can usually be recycled with simple screening to remove dirt and leaves. With some imagination, most plants can reduce water consumption, wastewater volume, and costs.
An Edifying Example
Hilmar Cheese Co. in Hilmar, Calif., is the world’s largest single-site cheese and whey company, according to Burt Fleischer, Environmental Director (phone 209-656-2271). It may also have one of the largest water reclamation facilities in the world.
Hilmar Cheese Company (HCC) was founded in 1984 by 12 dairy farm families and is still owned by them. They raised Jersey cattle and were dissatisfied when their milk did not receive a price premium for its high solids content—about 14% compared with an average of 12.5% for milk from other breeds. High-solids milk is a good raw material for cheese because it results in higher yield. Today, the HCC California facility makes in excess of one million pounds of cheese per day. The company draws upon milk from a 200-mile radius in the Central Valley of California and has another facility in Texas. HCC employs a total of about 1,000 people. The HCC California water reclamation facility is interesting for its combination of technologies.
Most of the water going to the water reclamation facility results from cleaning, as is true for most food plants. However, Fleischer points out that even Hilmar’s high-solids milk still has 87% water, which is rejected when the protein and fat are precipitated in the first cheese-making step. This water, called condensate of whey (COW water), contains lactose, some soluble protein, and salts (analyzed as ash). It has long been common for cheese manufacturers to fractionate and concentrate the protein and lactose from whey as co-products. Typically, this is done using a series of polymer membranes. Whey protein is a valuable food ingredient. Usually, the water from whey processing is discarded, but HCC uses much of it in cleaning after the co-products are recovered.
The water reclamation facility process is a combination of physical and biological treatment units employing 33 people and treating 2.1 to 2.3 million gal/day. The process begins with equalization tanks in which the sporadic flow during the day (as process equipment is cleaned and restarted) is stored and fed more steadily to the balance of the process. Equalization also helps to balance pH fluctuations as CIP uses both acid and basic cleaning agents and some cheese whey is naturally acidic.
--- PAGE BREAK ---
From equalization, the flow goes to dissolved air flotation (DAF) in which small air bubbles are injected into the wastewater as it is held in a vessel. The air bubbles help float suspended milk fat to the surface, from which it is skimmed. Fats, oil, and grease (FOG) typically have stringent limits on waste discharge because they can block sewers and interfere with subsequent treatment steps. The FOG from HCC’s DAF units is removed by truck to an off-site facility, where it is used for energy recovery.
The next step is anaerobic digestion, which produces methane gas that is used as fuel in the cheese plant. Anaerobic digestion involves consumption of dissolved solids by microbes in the absence of oxygen. In municipal sewage treatment plants, it is usually applied to sludge after activated sludge digestion using air and other microbes, but at HCC it is the first biological step.
Following anaerobic digestion, there are three sequential batch aerobic reactors (SBRs), which provide 95% removal of dissolved and suspended solids. A fourth SBR is currently under construction and will be operational in the first quarter of 2011. In an SBR, microbes oxidize organics to carbon dioxide and water. This is similar to the activated sludge process previously mentioned except that in a conventional sewage treatment plant, biological solids—the microbes, primarily—are removed by settling and returned to the aeration vessel. In SBRs, the mixture is agitated by introducing air and the mixture is transferred to the next SBR after a set interval.
The biological solids and some precipitated minerals (milk contains calcium, which can combine with carbon dioxide to form calcium carbonate) are removed in a second DAF unit. These solids are removed by truck for off-site disposal. What remains is a clear solution of residual dissolved solids. This is passed through an ultrafiltration system, consisting of four similar membrane units.
The concentrate, which is mostly organic, while the dissolved minerals flow with the permeate through the membrane, is recycled to the second DAF unit. The permeate passes to the first of two reverse osmosis membrane systems.
Reverse osmosis (RO) is a separation technology that can remove low molecular weight solutes, such as salt and other minerals. It requires a tighter polymer than is used in ultrafiltration and uses pressures up to about 900 psi. The permeate from RO is almost pure water and is used for irrigation of the arid farm land around the plant and for cooling and boilers in the nonfood portions of the plant. The concentrate is a mineral solution that is injected into a highly saline formation by a series of three 4,000-foot-deep wells, which operate under Environmental Protection Agency permit.
--- PAGE BREAK ---
Some food plants can use direct land application for wastewater disposal, but HCC’s permit limits the biological oxygen demand (BOD) of water directed to irrigation. High BOD or high mineral content can create odors and damage soils.
The combination of processes used by Hilmar Cheese Co. in its water reclamation facility is unique and may not be applicable to every situation, but it demonstrates how relatively conventional and available unit operations can be combined to treat a specific waste. There are multiple suppliers of each unit operation, so choosing a particular combination could be daunting. It probably helps that HCC is privately held, as the water reclamation facility represents a significant capital investment, but given the location and scale, there probably is no choice but to do something this extensive. The achievement of turning wastewater into a useful resource is commendable.
J. Peter Clark, Contributing Editor, Consultant to the Process Industries, Oak Park, Ill.