Grease Interceptors - materials of construction, does it matter?

Today, as never before, grease interceptors are being offered in a variety of materials including stainless steel, a variety of different thermoplastics and even fiberglass.

The reason is that concrete and metal interceptors inevitably fail over time and have to be replaced - and it ain't cheap to do that!

For those interested in why concrete and metal interceptors keep failing, an understanding of what is going on inside the interceptor may be of importance.

Whats wrong with concrete?

Concrete interceptor with hole
in the bottom

The Portland Cement Association produced an article titled: "Effects of Substances on Concrete and Guide to Protective Treatments" (2001).  In this article they listed a variety of media that will deteriorate concrete ultimately leading to a failure unless a protective coating is used.  Among the types of media listed that are harmful to concrete are: fats, fatty acids, vegetable oils, salts, sugars, acids, bleach and water.  All of these are commonly found in the waste effluent in a normal commercial kitchen.

Even worse is what happens to grease over time as it collects in the interceptor.  As FOG breaks down in an interceptor, anaerobic bacteria convert naturally occurring sulfate molecules into sulfides, which form hydrogen sulfide gas (H₂S) in the free air space of the interceptor.  Airborne aerobic bacteria consume hydrogen sulfide gas and convert it into sulfuric acid (H₂SO₄) in the wastewater of the interceptor. Sulfuric acid reacts with calcium hydroxide (Ca(OH)₂) in concrete forming calcium sulfate (CaSO₄) that in the presence of water (H₂O) creates gypsum (CaSO₄·2H₂O).  Gypsum is essentially a soft coating on the surface of the concrete that is easily washed off during pumping maintenance exposing aggregate over repeated cycles.

Schier GB-250 fitted inside a ruptured 1000 gallon concrete
gravity interceptor

Concrete gravity grease interceptors are guaranteed to fail due to this process and in fact they are being replaced all the time.  On average a concrete gravity interceptor will last approximately 10 years.  When they fail it is very costly to the owner to replace the unit with prices ranging from $10,000 to as high as $50,000 (installed cost).

Products like Schier's Great Basin interceptors are a popular choice for contractors and restaurant owners in replacing failing concrete gravity interceptors because of their compact size, high grease storage capacity, HDPE construction and lifetime warranty. The Great Basin can be retrofitted directly inside of the concrete interceptor reducing the cost of excavation, backfill, and overall installation cost.

Green Turtle's Proceptor series interceptors are also a good choice in replacing failed concrete interceptors as they are made from a very durable and long lasting fiberglass resin and come with a 30 year warranty.

Whats wrong with metal?

Rusted out metal interceptor

Metal is also a poor choice for a grease interceptor. The same process that creates Sulfuric acid in concrete interceptors is at work in metal interceptors.  Sulfuric acid oxidizes metal causing a reaction of iron with water to form iron oxide (rust).  

Many grease interceptor manufacturers still use metal today with a "corrosion resistant" coating on the interior of their units.  Unfortunately the process for pumping out and cleaning these devices invariably scratches the coating leaving bare metal exposed.  The average lifespan of a metal grease interceptor is approximately 5 years.

When you have a choice on what to recommend, specify or approve for a project, consider the materials of construction involved - it really does matter!