Design of Precast Concrete Grease Interceptors Claude Goguen, PE National Precast Concrete Association 2015 Onsite Wastewater Mega Conference
Terminology
Terminology
Terminology
Grease is the Word If you have been in a commercial kitchen, it is obvious why grease interceptors are needed.
Why are Grease Interceptors Necessary? Problems Downstream • Grease blockages in pipes • Issues in treatment field • Issues in septic tanks (if they’re in series)
Configuration Options
Configuration Options
Factors that influence sizing and design • Wastewater Strength • Wastewater Constituents • Globule Size • Temperature • Flow Rate • Retention Time • Pumping Frequency
Factors that influence sizing and design • Concentration : strength of the influent waste
Wastewater Constituents Chemistry: • Vegetable oils require longer retention times than animal fats. • Emulsified oils require the longest retention times. (Soaps)
Globule Size
Factors that influence sizing and design Grease globule size has an effect
Temperature
Coagulation
Flow Rates
Different Types of flow • Difference exists when draining a filled sink versus some sinks that are never filled and the flow is associated with the associated fixture. • The potential contribution of the filled sinks is determined through the use of Manning’s Equation in conjunction with various plumbing characteristics. • Faucet flow contributions are calculated based on known peak discharge limitations for the various fixtures in a kitchen.
Different Types of flow
Factors that influence sizing and design • Retention Time: Amount of time it takes one particle to travel through the system and discharge out. • Metcaff & Eddy recommend 30 minutes • Stokes Law
Key Factors Affecting Grease Removal Retention Time • Depends on the size and the influent flow rate • Baffles “can” increase retention time and prevent short-circuiting • Size can increase retention time and promote cooling of liquids
Baffle or no Baffle?
WERF Report
WERF Report
WERF Report
WERF Report
WERF Report
Proper design goes beyond sizing…
Proper design goes beyond sizing…
Proper design goes beyond sizing…
Factors that influence sizing and design • Pumping Frequency : Size shall be sufficient to optimize cleaning and pump-outs. - FOG is 25% of liquid level
Interceptor Sizing Sizing Codes GPM EPA WTF? What’s the Formula? DFU UPC
Interceptor Sizing UPC – Appendix H (pre-2006) # Meals flow rate retention time storage factor * . * . * . peak . hour
Interceptor Sizing UPC – Appendix H (pre-2006) # Meals flow rate retention time storage factor * . * . * . peak . hour Example: Restaurant with 100 meals per peak hour. They have a dishwasher connected to the interceptor. They are open from 6:00am to 10:00pm # meals/peak hr. = 100 meals/hr Flow rate (with dishwasher) = 6 gallons / meal Retention time (with dishwasher) = 2.5 Storage factor (16 hours of operation) = 2 Tank Capacity = 100 x 6 x 2.5 x 2 = 3,000 gallons
Interceptor Sizing UPC - (2006-2009)
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains Mop sink Food prep sink 1 commercial dishwasher (8GPM)
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains Mop sink Food prep sink 1 commercial dishwasher
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains Mop sink Food prep sink 1 commercial dishwasher
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains Mop sink Food prep sink 1 commercial dishwasher
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains Mop sink Food prep sink 1 commercial dishwasher
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains Mop sink Food prep sink 1 commercial dishwasher
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains 3 x 2 DFUs = 6 Mop sink 1 x 3 DFU = 3 Food prep sink 1 x 3 DFU = 3 1 commercial dishwasher = 2 Total DFU’s = 14 DFU’s
Interceptor Sizing UPC - (2006-2009) Example: A restaurant with the following fixtures and equipment: 3 floor drains 3 x 2 DFUs = 6 Mop sink 1 x 3 DFU = 3 Food prep sink 1 x 3 DFU = 3 1 commercial dishwasher = 2 Total DFU’s = 14 DFU’s 750 Gallon Tank
Interceptor Sizing U.S. E.P .A. hours . open gal # seats * * storage . factor * * loading . factor meal 2 where: # seats (assume = 100 seats) # gal / meal = 5 Storage Factor = 1.7 to 2.5 (2.5 for onsite disposal) Hours Open = 8 Loading factor = 0.8 Example Tank Capacity = 100 x 5 x 2.5 x 8/2 x 0.8 = 4,000 gallons
Interceptor Sizing
WERF Report “The results of this study seem to suggest that the development of a universal GI design that can be sized solely on the hydraulic loading and the FOG storage capacity is too simplistic. It is possible that analysis of the FOG influent characteristics will have to be assessed prior to the selection of the most appropriate GI size and design.”
Proper design goes beyond sizing… As a result of the analysis of grease interceptor performance, the researchers concluded: ♦ Extending the residence time in a standard GI by a factor of 3 only yielded a 10% improvement in performance, suggesting that FOG loading is only one of the factors affecting performance . ♦ FOG droplet size significantly affected FOG removal performance. ♦ Decrease in GI separation efficiency will result with the use of detergents and mixing while cleaning in-kitchen FOG wastes. ♦ Inlet/out configurations must be designed to distribute the flow . ♦ More effective FOG separation was achieved when fluid velocities near the inlet and outlet were kept below 0.015 m/s. ♦ Only include baffle walls with specific inlet/outlet configurations. ♦ Design baffle wall to distribute the flow and minimize the occurrence of high local fluid velocities.
All Specifications should include.. ASTM ASTM C1613 C1613 - “ Standard Specification for Precast Concrete Grease Interceptor Tanks ” o Materials and Manufacture o Structural Design Requirements o Physical Design Requirements o Quality Control o Watertightness Requirements
Difference? Septic Tank – ASTM C1227 Grease Interceptor – ASTM C1613
Summary • Retention time important factor, but not only factor • Smooth slow laminar flow • Many sizing formulas – in absence of one, use UPC 2003 Appendix H • Specify ASTM C1613 • Septic Tank is not a Grease Interceptor • Work with your precaster to find solutions
Additional Information Sources www.precast.org Claude Goguen : cgoguen@precast.org
QUE QUESTIONS? STIONS?
Design of Precast Concrete Grease Interceptors Claude Goguen, PE National Precast Concrete Association
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