Engineering To Beat Grease

Today’s good news is that there are a record number of restaurants and food processors in the United States. The bad news is that these operations are generating and discharging enormous amounts of fats, oils, grease (FOG), and coarse solids into already overloaded sewage collection lines and treatment plants. But, there really is good news. More and more engineers are designing and installing new kitchen drain water systems which utilize the latest, most cost-effective equipment to win the battle against grease buildup and line clogging.

Engineers have found the solution in automatic units that remove as much as 98% of these FOG elements from kitchen flow before it reaches the collection line. This efficient equipment is also part of a developing trend toward municipal-mandated pretreatment programs for commercial users in sanitary districts.

Long-term trends, such as the progressive tightening of the Clean Water Act regulations and diminishment of funding for replacing/maintaining municipal collection systems are putting a serious financial squeeze on the nation’s sewer districts. As cities encounter increasing difficulty dealing with the cost of servicing food-processing facilities, there has been a correlated increase of mandatory pretreatment programs to coerce restaurants, hospitals, prisons, casinos, schools and other grease discharging sites to do a better job or pay for service costs incurred. Thus, a properly engineered drainage system, which already incorporates efficient pretreatment, gets a free ride. But, to reach this point, it takes the talent and education of a plumbing engineer to evaluate contributing factors.

There are four major effluent sources affecting commercial foodservice kitchens involving grease and oils. First, the menu establishes the amount and types of nutrients to be washed off the cooking vessels and utensils. Fried chicken-type fast food restaurants wash a lot of cooking grease, flour and breading down the drain. Cafeterias wash large quantities of foodstuffs down the drain.

Second, the greatest factor affecting effluent quality is the management and operating practices of a restaurant. The care given to scraping and cooking pans into a grease rendering collection vessel and the discarding of solids into waste containers is a major consideration. The emptying of high BOD (bio-chemical oxygen demand) waste, such as leftover milk products or beverages into proper waste containers, instead of down the drain, have a great affect on effluent nutrient concentration. They vary from day to day and restaurant to restaurant, even within chains. Because there is such variance, it is necessary to engineer in control measures that:
Automatically hold effluent discharge to a minimum, or
Provide a greater degree of onsite sewage treatment system over capacity.

Thirdly, contrary to popular conception, floor drains receive comparatively little waste. The entry points for the greatest amount of effluent nutrients is the multi-compartment pot wash sink and the pre-rinse sink at the dishwashing station.

Fourth, the types of cooking and food production equipment influence the amount and types of nutrient by-products. For example, fast food hamburger restaurants utilize griddles for cooking and fryers for french fries, apple pies and such. The utensils and fryer baskets are washed out in a three-compartment pot sink. The same is true of a chicken or pizza restaurant. Nearly 90% of all effluent nutrients, fats, oils, grease, and other foodstuffs pass into the three-compartment sink (75%) and pre-rinse sink at dishwashing (15%). No matter what changes might occur in the menu, if you can control that which goes into the three-compartment sink and the pre-rinse sink, you have controlled the effluent quality.

Here’s a design tip: To minimize FOG values, do not route the dishwasher discharge into a grease interceptor or automatic grease removal system. Dishwasher flows are rich in detergents and emulsify a high percentage of the separated fats held in a trap or separator. A sewer district sampling may show high level FOG, but visual inspection shows little free-floating grease in the trap.

Institutional and large commercial kitchens utilize a wide range of warewashing and food preparation equipment. The increased number of grease-bearing point sources generally mandates centralized automatic grease removal systems. For example, an automatic 100-gpm system servicing a 300-bed hospital kitchen typically removes a 55-gallon barrel of grease every 7-10 days.

Another design tip: Rarely seen in full service kitchens, tilt kettles (used to cook large volumes of cooked meats, soups, sauces and other foods) are installed in more than 70% of institutional kitchens. These put large volumes of greases and foodstuffs into the drainage system. Kitchens in prisons, hospitals, and colleges frequently incorporate potato peelers, pulpers, wet hoods, bakery sinks, and other equipment that also can discharge large quantities of grease and foodstuffs. When specifying central removal systems for institutional kitchens, determine the maximum drainage flow value and provide at least 100 pounds (45 kg) of grease removal capacity per day.

Tip: When selecting an automatic grease removal system, determine the peak flow volume. The GPD (gallons per day) figure is not as important as the peak flow value.

PRETREATMENT SYSTEMS: UNDERSTANDING GREASE TRAPS AND AUTOMATIC REMOVAL SYSTEMS

In the United States, the three predominant pretreatment systems for foodservice operations are small passive grease traps, large pre-cast concrete grease traps, and automatic grease/oils removal systems. Passive grease traps date back to 1885 when a US patent was issued. Today’s large and small grease traps use the same basic operating design as the 1885 model. Grease and oils are 10 to 15% lighter than water and immediately float to the surface inside a grease separator. This displaces water out of the bottom of the separator and is held on the surface. This is why you see a grease “mat” when observing a separator. Since grease and oil fill from the “top down”, it is frequently hard to measure the depth or fullness of the grease trap. The single most important aspect to understand is that, as a grease trap fills, its separation efficiency diminishes. When a trap is filled to capacity with retained grease/oils, there is no separation occurring in the flow stream. This creates a considerable volume of business for pipe cleaners.

OPTIMIZING PRETREATMENT FOR ON-SITE SYSTEMS

Effluent nutrient levels flowing to a septic field can be minimized and controlled by the following design and operation decisions:
Install a grease/oils interceptor at the primary point sources in the kitchen, i.e., the three-compartment sink and the pre-rinse sink. Have personnel empty system strainers and grease collectors every day.
Cease pouring nutrient rich liquids down the drain (beer, milk, buttermilk, yogurt and such).
Avoid leaking detergent dispensers. Dishwashing detergent has a high BOD value.
For full service restaurants with high levels of flour, cornmeal, and other particulate-type solids, install a 750-gallon pre-cast tank (or larger) to act as a suspended solids clarifier. Plumb outlet approximately 12” from tank bottom. Pump the clarifier when solids are 12” deep on the bottom.

It is indeed good news for restaurateurs that business is booming. But the restaurant boom is taking its toll on the collection systems of the local sewer authorities. Fines and surcharges from these authorities are beginning to mount in areas that are strictly enforcing Clean Water Act regulations.

However, there is a growing trend towards aggressive engineering to confront these issues. Design engineers are becoming proactive by specifying innovative technology like automatic grease interceptors to help deal with this problem. By identifying and controlling the specific effluent sources in the kitchen, properly engineered drainage systems are controlling sticky situations caused by grease.