Design Considerations for High Strength Wastewater

John R. Blount, P.E.Harris County Public Infrastructure DepartmentAssistant County Engineer Director, Architecture & Engineering Division

TopicsDefine high strength wastewaterDiscuss hydraulic vs. organic loadingExplore results of Harris County high

strength wastewater studiesReview relevant sections of 30 TAC §285Develop strategies for moving forward

What is high strength wastewater?Wastewater that has higher amounts of BOD,

TSS, or FOG than residential wastewater

Photos: http://www.clker.com/clipart-scales-of-justice-3.html, http://www.aediconstruction.com/sustainability/low_flow_water_fixtures.html, http://www.bls.gov/ooh/Production/Food-processing-occupations.htm

Residential wastewater

High strengthwastewater

Strength < 300 mg/L BOD*

* Per 30 TAC §285.32(f)2

What is high strength wastewater?It is often highly variable

For example, a restaurant with a lunch rush

http://www.southpaceblog.com/2012/11/06/what-to-expect-in-todays-restaurant-market/

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: http://bbprof.wordpress.com/2012/03/22/a-d-in-rhetoric/

Quantity vs. QualityHydraulic loading

Water volumeOrganic loading

Water qualityKnow what you’re dealing with- do your

detective work!

Photo: http://www.glogster.com/1jbfan/detective-work/g-6n94f14apu919am1ppmuhdh

How strong is strong? Total Daily Load

Wastewater Flow Rates (gal per hour)

+ Wastewater quality

Overall wastewater strength

Agnoli, T. 2000. The best wastewater systems consider flow rate and wastestrength. Small Flows Quarterly, 1 (2).

Texas Restaurant Wastewater Analysis, 2003

Pretreatment is necessary to prevent system failure

Highlighted the need for a design manual for restaurants

ParameterTypical

domestic waste

(range, mg/L)

Restaurant waste

(average, mg/L)

BOD 100-400 1,202

COD 100-300 1,717

TSS 100-350 318

FOG 16-65 131Study funded by the Texas Onsite Wastewater Research Council

Not all restaurants are created equal

n Restaurant type

BOD mg/L,

avg

TSS mg/L, avg

FOG mg/L, avg

6 Fast food 2,137 233 1021 Pizza 1,856 321 1834 Chinese 1,364 448 2419 Mexican 1,254 668 1901 American 1,063 297 147

1 American Buffet 792 195 632 Steakhouse 601 160 773 Seafood 555 229 47

Typical residential waste 100-400 100-350 16-65

Texas Restaurant Wastewater Analysis, 2003

n

BOD

mg/L

COD

mg/L

TSS

mg/L

FOG mg/

L

BOD lbs/da

y

Hand wash 52,61

72,57

5 366 120 30.5Commercial Dishwasher 22

1,037

1,912 418 153 36.9

Values shown are averages

Texas Restaurant Wastewater Analysis, 2003

BOD mg/L

BOD lbs/day

Conventional fixtures 973 22

Low flow fixtures 1,309 36

Harris County Low Flow Fixture Study, 2013

Initiated November 2013Compare BOD, TSS, and FOG from an older

residential neighborhood to a new neighborhood with low flow fixtures (and therefore, high strength wastewater)

http://stockfresh.com/image/1722713/pending-rubber-stamp

What about NSF 40?Example:

Fast food restaurant with 33 seats, loading factor of 15 gpd/seat

According to TAC, 15 x 33 = 495. It might be assumed a 500 gpd treatment plant would suffice.

However…

What about NSF 40?NSF Standard 40 treatment plants assume

BOD= 240 mg/LHarris 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.

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)

Table III- Wastewater Usage RateTable 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)

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

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)

What does the TAC say?Defines residential sewage as sewage that

has a strength of <300 mg/L BODDesigner should consider if flow equalization

is necessary for system to work properly

30 TAC §285.32(f)2, 3

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

ConclusionSystems that are improperly designed for the

waste stream may not protect public health and the environment

Photo: http://science.kqed.org/quest/2012/04/27/celebrating-earth-day-in-the-age-of-man/hands-on-a-globe/

Ways to manage high strength wastewater

1. Control at the sourceExample: scrape plates to reduce FOG

2. Treat to a higher level before dischargeExample: install media filter

3. Adjust loading rates according to strength of wastewater

Source: Hammerlund and Glotfelty, Maryland Department of the Environment. 2008. Onsite Sewage Disposal of High Strength Wastewater. 17 th Annual Maryland Groundwater Symposium 2008.

Questions and/or Comments