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Design Considerations for High Strength Wastewater

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Presentation on theme: "Design Considerations for High Strength Wastewater"— Presentation transcript:

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 Photos: * 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. Photo:

6 Quantity vs. Quality Hydraulic loading Organic loading
Water volume Organic loading Water quality Know what you’re dealing with- do your detective work! Photo:

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 Photo:

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

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