How to Size a Grease Interceptor – Simple Guide for Plumbing Engineers

Grease interceptors are very important parts of plumbing systems, specifically in kitchens. However, most individuals find themselves at a loss when it comes to the sizing of these systems. If the interceptor is too small, grease escapes into the drainage system; if it is too large, it increases costs and presents space problems. Therefore, proper sizing is quite necessary.

Now, let’s explain this in an easier and more practical way.

Table of Contents

What Is a Grease Interceptor?

A grease interceptor is a specific plumbing device designed to catch fats, oils, and greases before they reach the sewer line. It works by separating grease from wastewater and lets only the clean water flow out. Due to this purpose, it is required in restaurants, hotels, food courts, and other commercial kitchens.

Grease is less heavy compared to water. This means it floats on top inside the interceptor. On the other hand, solid food particles settle at the bottom. Clean water exits from the middle zone.

Why Correct Sizing Is Important

Proper sizing is important both in terms of performance and compliance. If it’s undersized, grease passes through, resulting in blockages. This leads to drainage backups and foul smells. On the other hand, oversized interceptors occupy more space than necessary at an increased installation cost.

Most importantly, plumbing authorities and health departments follow strict guidelines; hence, sizing errors result in rejection upon inspection.

Factors Used to Size a Grease Interceptor

Sizing of the grease interceptor is based on many practical factors. First of all, you must know the kitchen fixtures. Then you calculate wastewater flow. After that, retention time and grease load must be considered.

1. Kitchen Fixtures Connected

You must enumerate all the fixtures discharging grease-laden water. These include sinks, dishwashers, floor drains, and cooking equipment wash areas. Every fixture adds to the total flow rate.

2. Flow Rate Calculation

Flow rate is usually calculated in GPM, or gallons per minute. In sink-based calculation, the standard method is:

GPM = (Length × Width × Depth × 7.48) ÷ Drain time (minutes)

This gives you the actual wastewater discharge rate.

3. Retention Time

Retention time allows grease to separate properly. Most standards recommend 20 to 30 minutes. Longer retention ensures better grease separation.

4. Grease Load Factor

Different kitchens produce different amounts of grease. For example, fast-food kitchens generate more grease compared to office pantries. Because of this, grease load factors are applied to the final calculation.

Simple Grease Interceptor Sizing Formula

A formula commonly used is:

Size of Grease Interceptor (Gallons) = Flow Rate (GPM) × Retention Time × Safety Factor

Depending on local codes, it usually ranges between 1.25 to 1.75.

This is an easy, practical, and widely accepted method.

Indoor vs Outdoor Grease Interceptors

Indoor grease interceptors are smaller and installed close to the fixtures. They find application in small kitchens with limited grease discharge.

Outdoor grease interceptors, also referred to as grease traps, are much larger. They are installed underground and serve several fixtures. They are very common for restaurants and commercial food facilities.

Sizing is also affected by choosing between indoor and outdoor types.

Common Sizing Mistakes to Avoid

The majority of problems are brought about by simple mistakes. For example, sometimes designers just forget to include all the fixtures producing grease. Sometimes, drain time is assumed instead of being measured. One more big problem is ignoring the local code requirements.

Therefore, all calculations should be verified and local plumbing standards consulted before finalizing the size.

Maintenance Consideration While Sizing

A correctly sized interceptor is easier to maintain. If the interceptor is too small, cleaning will be required very often. However, if it is properly sized, cleaning cycles become predictable and manageable.

This helps reduce operational cost in the long run.

Conclusion

Sizing a grease interceptor does not need to be complicated. Once you understand fixture flow, retention time, and grease load, the calculation becomes straightforward. Most importantly, correct sizing protects the drainage system, avoids legal issues, and improves kitchen hygiene.

With proper planning and calculation, grease interceptor design becomes smooth and reliable.