1. Design Considerations for High Strength Wastewater
John R. Blount, P.E.
Harris County Public Infrastructure Department
Assistant County Engineer
Director, Architecture & Engineering Division

2. Topics Define high strength wastewater
Discuss hydraulic vs. organic loading
Explore results of Harris County high strength wastewater studies
Review relevant sections of 30 TAC §285
Develop strategies for moving forward

3. What is high strength wastewater?
Wastewater that has higher amounts of BOD, TSS, or FOG than residential wastewater
Residential
wastewater
Strength < 300 mg/L BOD*
High strength
wastewater
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* Per 30 TAC §285.32(f)2

4. What is high strength wastewater?
It is often highly variable
For example, a restaurant with a lunch rush

5. Why should I be concerned?
Conventional systems will not work well with high strength wastewater due to clogging of the soil interface.
High strength wastewater can cause faster decline of onsite systems without adaptations.
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6. Quantity vs. Quality Hydraulic loading Organic loading
Water volume
Organic loading
Water quality
Know what you’re dealing with- do your detective work!
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7. How strong is strong?
Total Daily Load Wastewater Flow Rates (gal per hour) + Wastewater quality Overall wastewater strength
Agnoli, T The best wastewater systems consider flow rate and waste
strength. Small Flows Quarterly, 1 (2).

8. Texas Restaurant Wastewater Analysis, 2003
Pretreatment is necessary to prevent system failure
Highlighted the need for a design manual for restaurants
Parameter
Typical domestic waste (range, mg/L)
Restaurant waste (average, mg/L)
BOD
1,202
COD
1,717
TSS
318
FOG
16-65
131
Study funded by the Texas Onsite Wastewater Research Council

9. Not all restaurants are created equal
Restaurant type
BOD mg/L, avg
TSS mg/L, avg
FOG mg/L, avg 6
Fast food
2,137
233
102 1
Pizza
1,856
321
183 4
Chinese
1,364
448
241 9
Mexican
1,254
668
190
American
1,063
297
147
American Buffet
792
195 63 2
Steakhouse
601
160 77 3
Seafood
555
229 47
Typical residential waste
16-65

10. Texas Restaurant Wastewater Analysis, 2003
BOD mg/L
COD mg/L
TSS mg/L
FOG mg/L
BOD lbs/day
Hand wash 5
2,617
2,575
366
120
30.5
Commercial Dishwasher 22
1,037
1,912
418
153
36.9
Values shown are averages

11. Texas Restaurant Wastewater Analysis, 2003
BOD mg/L
BOD lbs/day
Conventional fixtures
973 22
Low flow fixtures
1,309 36

12. Harris County Low Flow Fixture Study, 2013
Initiated November 2013
Compare BOD, TSS, and FOG from an older residential neighborhood to a new neighborhood with low flow fixtures (and therefore, high strength wastewater)

13. What about NSF 40? However… Example:
Fast food restaurant with 33 seats, loading factor of 15 gpd/seat
According to TAC, 15 x 33 = It might be assumed a 500 gpd treatment plant would suffice.
However…

14. What about NSF 40?
NSF Standard 40 treatment plants assume BOD= 240 mg/L
Harris County restaurant study
average BOD = 1202 mg/L: 5 times greater!
Reality is this scenario would require a 2,500 gpd plant if you look at it on an organic loading basis.

15. What does the TAC say?
“(1) Tank sizing. Proprietary treatment systems that serve single family residences, combined flows from single family residences, or multi-unit residential developments shall be designed using Table II in §285.91(2) of this title unless there is an equalization tank preceding the aerobic treatment unit. If there is an equalization tank preceding the aerobic treatment unit, the equalization tank shall meet the requirements set forth in §285.34(b)(4) of this title (relating to Other Requirements) and the aerobic treatment units can be sized using the wastewater flows in Table III in §285.91(3) of this title.”
30 TAC §285.32(c)

16. Table III- Wastewater Usage Rate
Table III ONLY applies to hydraulic loading
“This table shall be used for estimating the hydraulic loading rates only. Sizing formulas are based on residential strength BOD5. Commercial/institutional facilities must pretreat their wastewater to 140 BOD5 prior to disposal unless secondary treatment quality is required. For design purposes, restaurant wastewater will be assumed to have a BOD5 of at least 1,200 mg/l after exiting the grease trap or grease interceptor.”
30 TAC §285.91(3)

17. What does the TAC say?
“Proprietary units under this section have been approved to treat flows equal to or less than their rated capacity and with an influent wastewater strength ranging from a 30 day average Carbonaceous Biological Oxygen Demand (CBOD) concentration between 100 milligrams per liter (mg/l) and 300 (mg/l) and a 30 day average TSS concentration between 100 mg/l and 350 mg/l.”
30 TAC §285.32(c)5(A)i

18. What does the TAC say?
“Restaurant/food establishment sewage. When designing for restaurants, food service establishments or similar activities, the minimum design strength value shall be 1,200 mg/L Biochemical Oxygen Demand (BOD) after a properly sized grease trap/interceptor. It is the responsibility of the designer to properly design a system which reduces the wastewater strength to 140 mg/L BOD prior to disposal unless secondary treatment levels are required.”
30 TAC §285.32(f)

19. What does the TAC say?
Defines residential sewage as sewage that has a strength of <300 mg/L BOD
Designer should consider if flow equalization is necessary for system to work properly
30 TAC §285.32(f)2, 3

20. Harris County Onsite Sewage Facility Regulations
“Calculations for hydraulic and organic load for both normal and peak flows on all commercial systems shall be provided showing that both organic and hydraulic overloading of the treatment and/or disposal method is prevented.”
Commercial maintenance frequency- 12 visits a year

21. Conclusion
Systems that are improperly designed for the waste stream may not protect public health and the environment
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22. Ways to manage high strength wastewater
1. Control at the source
Example: scrape plates to reduce FOG
2. Treat to a higher level before discharge
Example: install media filter
3. Adjust loading rates according to strength of wastewater
Source: Hammerlund and Glotfelty, Maryland Department of the Environment Onsite Sewage Disposal of High Strength Wastewater. 17th Annual Maryland Groundwater Symposium 2008.

23. Questions and/or Comments