DESIGN OF HIGH STRENGTH WASTE TREATMENT SYSTEMS FOR FOOD SERVICE FACILITIES. By Allison Blodig 1

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1 Abstact DESIGN OF HIGH STRENGTH WASTE TREATMENT SYSTEMS FOR FOOD SERVICE FACILITIES By Allison Blodig 1 For many current or potential restaurant owners, onsite wastewater treatment is their only available option to treat the wastewater that their facility produces. Depending on the size of the restaurant, the menu, and the practices in the kitchen; the wastewater strength from these facilities can range between two (2) and ten (10) times higher than the wastewater produced at a home. This difference is due to the food (organic material) that goes into the system from food preparation activities, service and cleaning of dishes and prep areas. Also other flow producing fixtures, such as showers and clothes washing are usually absent from the waste stream causing the influent strength to be higher. Due to lack of adequate experience in designing onsite high strength waste systems, some designers design systems that are sized based solely on flow and do not take into account the strength of the wastewater or the sustainability of the treatment prescribed. With all of the other issues a restaurateur has to face in a highly competitive and risky business, having to deal with a system that is designed improperly and will eventually fail is yet another stress that is unnecessary and could be avoided with properly designed systems. To achieve this, training of both the designer and the regulatory authorities is necessary. This paper will discuss the necessary design steps for proper high strength wastewater treatment systems. These steps could also provide guidance for evaluation of onsite systems for regulatory permitting agencies. Introduction According to the USEPA (2005), one of the major contributors to failing systems when it comes to state programs without a comprehensive management program is the Failure to consider the special characteristics and requirements of commercial, industrial, and large residential systems. In that same document they cite that one of the critical elements of a successful program is Technical guidelines for site evaluation, design, construction and operation/maintenance. While all of these areas are important, this paper focuses on technical guidelines for designing and reviewing commercially available wastewater treatment systems for high strength wastewater. In the absence of this knowledge, there is the propensity to use the exact same types of systems that are used for residential homes and simply add a grease trap; or to rely solely on engineers or designers to understand the nuances of these systems and design something appropriate for the site. Unfortunately, in many areas of the country, neither of these approaches will produce reliable or sustainable systems. There are technologies on the market that can treat high strength wastewater and do it in a sustainable way. They utilize media in most instances. However they are larger systems that provide the appropriate air and the costs of the systems that are designed based on organic loading versus flow are a lot higher than what a restaurateur may be expecting. 1 Allison Blodig is Director of Regulatory Affairs at Bio-Microbics, Inc., Shawnee, Kansas

2 This paper focuses on Biochemical Oxygen Demand (BOD), the amount of organic material that requires aerobic microorganisms to break down the waste. It is assumed other parameters have been accounted for in the design but the practitioner, in practice, will need to assure this is the case. For example a full service restaurant should have a standard requirement to have adequate oil and grease management with a grease trap or in some cases a mechanical grease removal system. A system with abnormally high total suspended solids (TSS) may require additional settling or screening. If nitrogen treatment is required even more air will be required. However when it comes to BOD, which actually requires oxygen for treatment, all practitioners need to understand three main design components of wastewater systems critical to designing a functioning and sustainable onsite high strength wastewater system. They will be discussed herein: Determining the expected BOD loading for a facility in pounds per day; determining the aeration capacity of a pre-engineered system; and sludge production/management. BOD Loading According to a study done by Lesikar et al. (2004) in Texas, approximately 75% of 284 wastewater samples taken from 28 different kinds of restaurants for BOD were 1400 mg/l (after removal of outliers) or less with an average of about 1000 mg/l. The samples were taken during the busiest times of operation and were taken after the grease trap. The demographics of the types of restaurants and their average BOD s are shown in Table 1. Table 1. Demographics and average BOD s of restaurants sampled in Lesikar et al. (2004) study. Type of Restaurants Number of Systems in Group Average BOD mg/l Fast Food/Burgers Pizza Chinese Mexican American American Buffet Steakhouse Seafood When sizing a new restaurant, based on the numbers above one could size most restaurants using mg/l. Some different kinds of foods like pizza, Chinese and Mexican appear to produce a higher BOD so one could rate those types of restaurants higher. One of the more helpful things to do is to look at the menu. Those restaurants that have a large menu and serve a lot of alcohol, or use a lot of grease or sauces that are harder to remove from plates before washing would likely be closer to the 1000 mg/l range, maybe more. Also understanding the tableware (washable glasses plates and silverware versus single service items) will affect the BOD. One could consider using a lower BOD if the restaurant uses single service items that are thrown away versus being washed.

3 One could also sample a similar type of restaurant that exists. Taking more than one effluent sample is certainly prudent and taking four or even more during high flow periods is ideal. It takes studies like the one cited above, sampling, and considerable experience in sizing high strength waste systems to make any kind of educated assumption about the wastewater strength. If practitioners do not have that type of experience or information available, they should contact someone who does and learn from them. The next factor for determining the BOD loading is to determine the flows. Flows are often dictated by regulations that are usually quite conservative. Sizing done on conservative design flow may be able to assume a 10-30% lower influent BOD so that the customer is not paying for more treatment than is needed. Most restaurants are not going to use the design flow on a daily basis and may never come close to it, so the design flow becomes a safety factor in the design. This is about the only area that the author feels is an appropriate area to make exceptions. Actual flows are very helpful if the facility already exists or if one similar can be evaluated. If the regulation allows for the use of actual flows then actual BOD s are invaluable. Also an option could be utilized where the average flow (takes into account the hours and days of operation) can be used. This would require flow equalization and in that case it is suggested to assume a BOD on the higher end. ALL designs should characterize the flow and BOD levels used to size the system. The two together are used to determine the total BOD load (expressed in pounds per day) on the system and represents what needs to be treated with oxygen. The formula to determine the total pounds of BOD per day is as follows: Flow (gpd) * Influent BOD (mg/l) * 8.34 = lbs. of BOD/day 1,000,000 An example would be an American restaurant with an actual peak flow of 1200 gpd and a sampled influent value of 825 mg/l BOD. Putting that information into the equation looks like this: 1200gpd * 825 mg/l * ,000,000 = 8.26 lbs. of BOD per day that needs to be treated. To aerobically treat 1 pound of BOD it takes pounds of dissolved oxygen with an assumed average of 1.2 pounds. This is a well-known and accepted standard in the industry that has been proven repeatedly. So for the example above, the treatment system provided would need to provide 9.9 pounds of oxygen per day. This is known as the Actual Oxygen Transfer Rate or AOR (Metcalf and Eddy, 2003) which will be discussed further in the next section. The next step is to design the treatment system to provide the necessary oxygen. Aeration Capacity of Pre-engineered Systems The aeration capacity of any pre-engineered system can be determined by obtaining the specification sheet and evaluating the blower aeration curve or table. This will provide the scfm (standard Cubic Feet per Minute). The drawing of the unit is also helpful with dimensions

4 of what depth the blower releases the air. If the drawing does not specify this then the treatment system manufacturer should be consulted. The type of aeration device must be known. It will either be fine bubble or coarse bubble diffusion and this information is also available from the manufacturer. The Oxygen Transfer Efficiency or OTE is measured in percent per foot of liquid depth of the aerator. The accepted numbers in the wastewater industry for the OTE of course bubble diffusion is 0.75%/ft and 3.0%/ft for fine bubble. With the information gathered the Standard Oxygen Transfer Rate (SOR) in pounds per hour (pph) can be calculated (Metcalf and Eddy, 2003), which is how much oxygen is transferred into clean water at the standard conditions (zero dissolved oxygen and at a temperature equal to 20 degrees Celsius) by the aeration device being evaluated. The calculation for that is as follows: SOR in pph = * scfm * OTE * air release depth in feet The constant is an engineering conversion factor. Taking this answer times 24 will get the SOR in pounds per day (ppd). The SOR can then be used to determine the Actual Oxygen Transfer Rate or AOR of the aeration device (Metcalf and Eddy 2003). The AOR is = k*sor where k is a constant that is a function of the aerator s efficiency in sewage versus clean water. The k for fine bubble diffusion is between 0.4 and 0.45 and the k for coarse bubble diffusion is between 0.5 and 0.6. Using the AOR the pounds of BOD that can be removed per day can be calculated by dividing the AOR by 1.2, which is the amount of oxygen needed in pounds to remove 1 pound of BOD/day. Going back to the example above of an American restaurant producing 8.26 lbs. per day of BOD, what is often times designed would use gallon per day fine bubble diffusion systems for treatment. At first glance one can see that it exceeds the flow rate of the project since 1500 gallons of treatment per day is being provided for 1200 gallons per day. Looking at the specification sheet/aeration curve for the aeration device and the drawings of the system shows the blower produces.49 scfm at 66 inches (5.5 feet) of air release depth. The manufacturer confirms it is fine bubble diffusion; therefore the calculations above would be used to estimate the BOD removal capacity of this unit: SOR in pph = 0.49 (scfm) * * 2.75%/ft * 5.5 ft SOR in pph =.077 pph * 24 h/d = 1.85 ppd AOR = ksor = 0.4 * 1.85 =.74 ppd BOD removal capacity =.74 ppd/1.2 =.62 ppd So removal capability of one of the 500 gpd system is only.62 ppd and with three of them the system has the capacity to remove 1.86 pounds of BOD per day. Assuming that the system will have settling to remove about 25% of the BOD that still leaves 6.19 pounds of BOD per day to

5 be removed. To remove 6.19 pounds would require 10 of these units which would not be practical. This proves that the system design is not adequate and the design needs to be redone. Sludge Production/Management The final piece to the puzzle is sludge production and storage. It is helpful to start by using the same facility and design example as above. It can be assumed that for every pound BOD coming into a system after settling, about pounds of sludge produced on average per day (Metcalf and Eddy, 2003). Because the assumption was the aerator was very efficient, 0.7 is the value for sludge production per day used lbs of BOD/day * 0.7 lbs sludge/lb BOD = 4.33 pounds of sludge/day This value needs to be converted to mg/l of sludge accumulation/day because in a suspended growth system the sludge will be suspended and mixed in the reaction chamber. This reading, Mixed Liquor Suspended Solids (MLSS), is what is used to determine treatment efficiency and when sludge needs to be wasted. The conversion is back calculation from ppd to mg/l, which was the formula used above to determine the BOD loading above. However the gallons used is a function of the reaction chamber size which should be calculated by looking at the drawings or talking to the manufacturer. In this example the reaction chamber size is approximately 475 gallons/unit ppd * 1,000,000 = 364 mg/l of sludge is accumulating in the mixed liquor every day 8.34 * 1425 gallons In 30 days that would equal 10,920 mg/l. This is too much for a suspended growth plant to operate well. The MLSS needs to be between 3,000 and 5,000 mg/l to operate efficiently (Metcalf and Eddy, 2003) or the sludge will start bulking up, not settling, and leaving in the effluent. So based on these calculations the system being modeled here would have to be pumped about every 11 days in order to maintain good treatment. Therefore the operational costs of this type of treatment plant would be a burden on the owner of the facility. It obviously adds a tremendous amount of cost for pumping but it also adds operational costs to have an operator watch over it. While the sludge accumulation numbers we have done here assume full loading, even at half the loading operation and maintenance costs are too high to be sustainable. More units could be added, but this goes back to the issue of being practical. The result of this evaluation is again that the designer needs to revisit what treatment system to use for this application. Conclusion Determining an accurate influent BOD loading per day is critical to the design of an onsite treatment system designed to treat high strength waste from a food service facility. Determining if the system specified or proposed can treat the expected loading in a sustainable way is critical to the long term performance of the system.

6 If the owner accepts the costs involved with a system that can treat the wastewater; knows the system was designed and installed correctly; and maintains the service on the system then it is something that should not cause them significant losses in the future and it will last a long time. However, if the prospective food service facility owner is not prepared to accept the costs to install the correct system as the cost of doing business, then they should not be allowed to put something substandard in and they should not own a restaurant on an onsite system. The cost of doing it wrong the first time could be extensive by possibly causing the loss of a food service license; clientele due to odors and other nuisances created by a failing system; and/or ultimately the entire business. Finally, as a profession that is in the business of protecting the environment, no one should ignore the damage a failing system can do to the environment. Citations Office of Water, Office of Research and Development, US EPA. Onsite wastewater treatment systemsmanual, February 2002 Lesikar, B.J., O.A. Garza, R.A. Persyn, M.T. Anderson, A.L. Kenimer. Food service establishments wastewater characterization. In the Proceedings of the ASAE Conference Meeting, ASABE, St. Joseph, MI. Metcalf and Eddy (2003). Wastewater engineering; Treatment and Reuse, 4 th Edition. New York, NY: McGraw-Hill.