Designing Grease Traps

Transcription

1 Designing Grease Traps

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7 Objective Create a general understanding and knowledge to adequately address high strength grease traps for such places as restaurants High strength wastewater characteristics Oil, fat and grease in wastewater Design of a grease interceptor Use of an effluent filter in a grease interceptor Septic tank performances post grease and pre treatment Maintenance of grease trap

8 Residential or Commercial Residential Average concentrations in domestic raw sewage Parametres Concentrations (mg/l) BOD TSS 350 TKN 60 NH 4 30 TP 12 Note: When we take out the portion representing kitchen wastewater, the BOD concentration of the rest of the sewage drops approximately 225 mg/l

9 Commercial - Kitchen High strength In general, restaurants will produce an effluent with concentrations three to four times higher than residential wastewater Results from a study performed on 28 restaurants in Texas Parametres Average concentration at the grease interceptor outlet (mg/l) BOD SS 358 FOG (fat, oil and grease) Lésikar et al., Food Service Establishments Wastewater Characterization, 2004

10 Characteristics of high strength wastewater Average BOD in different restaurants and percentage that this represents compared with than domestic wastewater 1 Type of Restaurant BOD septic tank influent % comparison to Domestic Wastewater Fast-food 888 mg/l 3,1 Pizza 734 2,5 Chinese 672 2,3 Mexican 641 2,2 American 588 2,0 Buffet 512 1,8 Grill 458 1,6 Seafood 445 1,5 1 Blount, J.R., Texas Restaurant Wastewater Analysis.

11 Characteristics Kitchen wastewater Factors to be considered before designing a high strength wastewater treatment chain Rinse sink Use of a dishwasher: BOD concentration in restaurants that hand wash their dishes is 1,5 times higher than for a restaurant using a commercial dishwasher. (180 degrees at discharge) 12 cycles an hour x 20 Gallons of water per cycle

12 Fat, Oil and Grease in high strength wastewater Problems with Fats, Oils and Greases: 1. Solidified deposit in pipes ( eg. 150 feet of pipe) 2. Solidification in inlet pipe 3. Maintenance and pumping 4. Solidification in the septic field 5. Failure of infiltration systems and surfaces

13 Fat, Oil and Grease in high strength wastewater Type of oil present in wastewater 1. Free oil: most common in wastewater 2. Mechanically emulsified oil 3. Chemically emulsified oil 4. Dissolved oil (Dishwasher and hard water emulsifiers) Free oil: Will float above water Mechanically emulsified oil: Will separate with longer retention time

14 Fat, Oil and Grease in high strength wastewater Chemically emulsified oil: Often use in many restaurant No separation is possible without aeration Dissolved oil: Separation possible but very difficult

15 Fat, Oil and Grease in high strength wastewater How to solve the problem at its origins 1. Change habits such as emptying plate completely in garbage before rinsing it. 2. Use of temperature dishwashers (less hot Residential only) 3. Use shortening instead of oil for cooking

16 Design of a grease interceptor Method one: based on opening hours V = NS x Q x FE x (Hr/2) x FC NS = Q = Number of seats flow rate per seat (kitchen only) = 20 L or deisgner value FE = Storage factor = 2,5 Hr = Opening hours FC = Loading factor (0,5 for restaurant on a secondary road, 1,25 for a restaurant along a highway)

17 Design of a grease interceptor Method two: Based on number of meals served V = NR x Q x FE x 2,5 x FC NR = Q = FE = FC = Number of meals served flow rate per seat (kitchen only) = 20 L or deisgner value Storage factor Loading factor (0,5 without dishwasher, 1 if dishwasher. Add 0,5 if garberator

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19 Use of an effluent filter in a grease interceptor BOD and TSS Removal from effluent filter: (Study performed by Zabel)

20 Use of an effluent filter in a grease interceptor Results from a study performed by Bionest on a family restaurant using a Zabel A-300 effluent filter over 6 months Sampling point Results (CBOD 5 in mg/l) Before filter After filter % removal Average BOD removal attributed to filter only = 64 %

21 Maintenance of a septic tank and grease interceptor Grease interceptor: Pump-outs +/- 4 time per year in first year for sure Pump-out when 50% of water height is occupied by grease. Fill with cold water after pump-out If an effluent filter is used it is recommended to install a clogging alarm. Frequency can be as much as once every 2 weeks (but it is worth it!!!)

22 Questions? Colin Boog