A reader questioned why restaurants and bars are treated differently in terms of system sizing and often use additional and, in the reader’s words, “expensive” pretreatment devices. The straightforward answer to this question is that restaurants and bars almost always have high-strength waste — levels of fats, oils and grease exceeding what are typically found in domestic (residential) wastewater.
As a result, if the sewage generated at taverns and eateries is not treated differently, a more resistant biomat will build up in the treatment area, reducing the soil’s ability to accept the wastewater. If left unchanged, this leads to failure of the final soil treatment unit (trenches, beds, mounds, etc.).
The goal for treatment of these high-strength waste streams is to bring them in line with domestic sewage effluent levels delivered from a septic tank. Each state may have variation in their codes about what constitutes high-strength waste, but in general the numbers revolve around levels no greater than 175 mg/L biochemical oxygen demand, 65 mg/L total suspended solids and 25 mg/L FOG.
NEW INFORMATION
On a personal note, these high-strength wastewater situations were not well understood when I started in the industry. It was recognized that food service and bars impacted soil performance, but it was not known how much. A typical recommendation for restaurants and bars was to simply double septic tank capacity over what would be used for domestic sewage, assuming this would solve the problem. This thinking did not take into account the use of degreasing soaps, very hot water and a variety of other practices, all of which affected waste strength.
We know a lot more now.
For each facility with higher values, the system designer or service provider needs to provide and maintain a system that reduces the values to within these ranges through whatever means necessary. There are practices the restaurant or bar owners can employ to reduce waste strength if they have not already implemented them, which may reduce or eliminate the need for additional pretreatment.
These practices include:
Limiting food particles going down the drain. This means scraping plates and cooking utensils, which is more labor-intensive then having a garbage disposal but can significantly reduce solids entering the system. As indicated earlier, the pre-cleaning reduces reliance on degreasers or cleaners with degreasers added.
Installing a grease interceptor either inside or outside the facility. If one is installed as many codes require, regular maintenance is necessary or the resulting sewage can have higher FOG values than not having one! Also, recognize that at high dishwasher temperatures (at my favorite watering hole, where I sometimes pitch in as a dishwasher, temperatures exceed 125 degrees F and are close to 140), grease will not solidify and can pass through the system, including the grease trap. An important design consideration is cooling the wastewater to allow grease to solidify and be captured. This may require more septic tanks in series to help cool the effluent.
A word of caution: If the line outside to the first tank in series is too long, the sewage can cool in the pipe, causing the grease to congeal. In cold climates like I live in, unclogging those pipes can be a cold, expensive and days-long activity that can shut the business down for a time. This is also why there should be an outside clean-out and regular line cleaning or flushing scheduled to avoid unpleasant situations.
If tanks in series are used to cool the effluent, a set of smaller rather than larger tanks can be more efficient at cooling because more surface area is in contact with soil. There are limits to the size of tanks because the solids also need to settle and be captured in the tanks; there are trade-offs between cooling and settling. Some codes require each individual tank be no less in size than 25 percent of total daily flow.
Tanks in terms of total capacity must have a retention time of 3-4 days for domestic sewage. Retention times for high-strength waste will be longer to allow cooling, floatation and settling to occur. This means the total tank capacity must be much larger, depending on the actual waste strength and temperatures. There are recommendations that tank capacity must be four to seven times than a residence with the same level of daily flow. The tanks may also need to be modified somewhat, fitted with higher baffles, constructed so there is more storage area for scum above the liquid level. The final tank in series should have an effluent screen, and there should be a regular solids removal program.
SAMPLE THE EFFLUENT
One final recommendation for systems using tanks in series to pretreat high-strength waste is to oversize the drainfield or final soil treatment area by 50 percent and to use a dosing resting system to help reduce or eliminate the formation of a more resistant biomat.
The service provider or designer needs to balance all of this versus cost of incorporation of aerobic treatment units, or media filters to provide additional pretreatment. It is important to recognize that to determine if the system is operating as intended, you must sample effluent to make sure BOD, TSS and FOG are within the necessary limits. In any case, as the reader indicated, a system installed and properly maintained for a restaurant or bar will probably have more components and be more expensive than a similarly sized residential system.
