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Septic Systems: Rumors, Rules, and Research Questions

By Eberhard Roeder, Ph.D., P.E.Bureau of Onsite Sewage Programs

FL Dept. of Health, Division of Environmental Health

WSE Seminar FAMU/FSU College of EngineeringFebruary 25, 2005

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Black Box Rumors

• If you don’t know where the pollution is coming from, it is the septic system

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Black Box Rumors

• (Failing) septic systems are responsible for pathogen indicators in surface water

• Watershed Management Model Version 4.1 (1998)

– 10% of septic systems are failing

– Failing septic systems contribute nitrogen and phosphorus, but no pathogen indicators and oxygen demand to stormwater

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• How many septic systems are there in Florida?

• What are septic systems and how do they work?

• What about nitrogen?

• What is the Karst Study?

• How can septic systems be managed?

Outline of the Presentation

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Florida’s Onsite Wastewater Treatment Systems

• > 2. 3 million septic systems (2001)

• 7.3 million housing units (2000)

• 31% served by septic systems

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Septic Systems treat Sewage Onsite– wastewater that is treated and disposed of at the

location where it is generated (on your property)

– In contrast to central sewer– Treatment and Disposal is achieved by an

“onsite sewage treatment and disposal system” (OSTDS)

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What is coming into Septic Systems?

• oxygen-consuming material (~300 mg/L measured in carbonaceous oxygen consumption)

• suspended solids (~250 mg/L)• Nutrients

– nitrogen ~50 mg/L (~23 lbs/year and household of three at 50 gal/cap day )

– phosphorus ~25 mg/L

• Pathogens among the Bacteria (~1 billion/L) and virus (~50-7k PFU/L)

• Traces of organics, and other elements

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What is a standard septic system?

2 feet between bottom of drainfield and seasonal high water tableGroundwater

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What’s happening in the septic tank?• Septic tank (anaerobic):

– Collects solids (~60lbs/year TSS)-> must be pumped regularly

– Consumes a third of biodegradable material anaerobically (without oxygen)

– Nitrogen from protein is converted into ammonia

• Rules: Approval testing for water tightness and structural integrity required since mid-1990s

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What’s happening in the drainfield?

Groundwater

• Drainfield (aerobic): – Consumes biodegradable material (cBOD5, TSS) using oxygen– Removes/filters pathogens, cBOD, suspended solids in the unsaturated

zone above groundwater (2 feet minimum separation to water table is foundation of system design)

– Converts ammonia to nitrate– Disposes of water

• Rules: Built from gravel or alternative materials (chambers, pipes, artificial gravel)

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What’s happening in an ATU?

• ATU=aerobic treatment unit– Brings sewage (usually pretreated by septic

tank) mechanically into contact with air for aerobic treatment, before discharging to drainfield

– Reduces cBOD and TSS– Can be used for drainfield size reduction,

drainfield repair

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Standard Septic Systems and Aerobic Treatment Units as secondary wastewater treatment plants

• Primary treatment=settling of solids

• Secondary treatment= removal of oxygen-consuming material

• Tertiary treatment= removal of nutrients

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(Average) Treatment Expectations

cBOD5 (mg/L)

TSS (mg/L) TN (mg/L) TP (mg/L)

Below drainfield at groundwater interface

<5 <5 25-40 <5

Secondary Treatment

<20 <20

Florida Keys <10 <10 <10 <1

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What is failure?

Failures per 1000 SystemsGoal: <15

56789

101112131415

Fai

lure

s pe

r T

hous

and

• System does not function in a sanitary manner: Loss of flush, or discharge of untreated or partially treated wastewater onto ground surface, into ground water, into surface water (64E-6.001, FAC)

• Tracked as number of repair permits divided by all systems accounted for

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What’s happening to nitrogen?

• 1) Ammonification in the septic tank

Organic N + microorganisms -> NH3/NH4

+ + microorganisms

• 2) Nitrification in the presence of oxygen (drainfield)

NH4+ + 2O2 -> NO3

- + 2 H+ + 2 H2O

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• 3) The removal step: Denitrification

NO3- + organic matter -> N2 +CO2 +OH- +H2O

• Problem: little organic matter left after drainfield(need extra carbon for denitrification)

• Solutions:– Recycle nutrients to vegetation via drip-irrigation (generally in conjunction

with an ATU, effectiveness unclear)

– Tertiary treatment at onsite scale. Recirculate aerated effluent to septic tank or add carbon.

• Overall: about 20-40% of nitrogen is removed from when sewage reaches the septic tank to when effluent reaches the groundwater

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Nitrogen Management Approaches: technically

• No sewage

• Limit flow and/or number of OSTDS per acre. This approach has been in Florida OSTDS rules for at least 30 years.

• Increased Treatment: – drip-irrigation (generally in conjunction with an ATU)– tertiary treatment at onsite scale (tested in Keys Demonstration Study,

proposed for Wekiva).

• Natural attenuation: – In some areas, nitrogen and phosphorus decrease subsequent to the

drainfield in the groundwater and don’t appear to affect surface water (St George Island Study, Indian River Lagoon Study).

– In some areas this natural attenuation process appears to be less important (Lake Okeechobee Study, Karst Study)

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What is the (OSTDS in) Karst Study• Observations:

– karst is widespread and allows rapid groundwater transport (Karst =landscape of dissolving limestone)

– Springs experience increasing nitrate-nitrogen concentrations (eutrophication)• Question: What are the impacts of OSTDS on groundwater in karst areas?

Wakulla Springs N

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Karst Study

• Project: University Project

– Monitor Groundwater downstream of OSTDS for chemical tracers, nutrients and fecal coliforms

River Front On top of Cave System

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Magnolia II Tracer Experiment

Well M3

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

Days After Injection

nM S

F6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

mg/

L F

luor

esce

in

M3 SF6

M3 Fl

Tracers at Magnolia II

Magnolia II Tracer ExperimentWell M1

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

Days After Injection

nM S

F6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

mg/

L F

luor

esce

in

M1 SF6

M1 Fl

M-1: 75 feet from injection pointFl arrival in 2.5 days

M-3: 135 feet from injection pointFl arrival in 1.4 days

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Conceptual Flow Model: Upland

??????

???

???

???

???

Drainfield

Cave

Sue Sink

Manatee Spring

0.4 0.6 12 21 0.6 0.1Average Nitrate Concentration (mg/L)

~60 feet

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M-4 Core

poor recovery, washout30 medium sand

washout, poor recovery15 medium sand

34.4 packstone

Screen 16.2’-26.2’

M-2 Core

25.5 packstone

Screen 7’-17’8.1 7.0

6.0 fine, medium sand6.5 wackestone

12.5 packstone

3 ft of washout 6.5’-11.5’

19 medium sand

4.5 fine, medium sand

M-1 Core

25.7 packstone26.5 wackestone

31.5 packstone

6.6 poor recoveryrubbly limestone

???

???

???

???

???

???

???K=4.4E-4cm/s

K=3.4E-4cm/s

K=1.7E-4 cm/s

K=9.6E-5 cm/s

K=5.7E-5 cm/s

K=4.0E-4 cm/s

K=5.2E-7cm/s

K=2.7E-4cm/s

K=1.3E-4cm/s

K=5.2E-6cm/s

K=1.9E-4cm/s

M-3 Core 8.0 medium sand

8.6 packstoneScreen 10’-20’ 8.4 fine, medium sand

poor recovery 22.5 packstone

35 wackestone

Screen 19.3’-29.3’

4.55.9

10 medium sand

30.0 packstone

M-4 Core

11.7 packstone

17 wackestone

25 wackestone

K=1.0E-7cm/s

K=2.8E-4 cm/s

K=1.5E-5 cm/s

K=5.0E-6 cm/sK=failed due to channeling

K=1.4E-5 cm/s

K=4.4E-4 cm/s

K=3.3E-5 cm/s

K=3.2E-5 cm/s

K=6.0E-3cm/s

???????????????

Conceptual Flow Model: River Front

M-2 CoreM-1 Core???

???

M-3 Core

???????????????

Drainfield

Groundwater

~30 feet

Average Nitrate Concentration (mg/L) 29 23 15 0.3

Suwannee River

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http://www.state.fl.us/nwfwmd/pubs/nitrate/lowspeed/nitrate_fig53.pdf

OSTDS and Wakulla

• Estimates for number of systems in 2000

• 28,400 in Leon

• 8,900 in Wakulla

• Estimate for Nitrate loading (mid-range of septic tank effluent, without accounting for losses in groundwater)

• 4kg /year and capita

• ~25 lbs/year and household

NW Florida Water Management District Study (Chellette, Pratt and Katz, 2002)

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Residual Disposal 12%

Commercial Fertilizer

27%WWTF

29%

OSTDS 22%

Livestock 12%

Estimated Contributors of anthropogenic Nitrogen to Wakulla Karst Plain

(yearly average 1990-1999: 1.3 million kg –N)Data from Chellette, Pratt and Katz, 2002

Missing?

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Delta N-15 as indicator of N-source in monitoring wells in the Wakulla Springs area<3 artifical fertilizer>10 animals/sewage

Data from Chellette, Pratt and Katz, 2002

Wakulla Springs

Delta N-15 as indicator of N-Source

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• EPA March 2003 Voluntary Guidelines provide a framework for discussion

http://www.epa.gov/owm/septic/pubs/septic_guidelines.pdf

How to Manage Onsite Systems?

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How to Manage Onsite Systems?

• Levels of Management will depend on:– severety of expected impacts (protection zones) – technical complexity of onsite systems, – amount and type of available funding– enforcement capabilities

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EPA’s Management Models

• Homeowner Awareness

• Maintenance Contracts

• Operating Permits

• Responsible Management Entity Operation and Maintenance

• Responsible Management Entity Ownership

http://www.epa.gov/owm/septic/pubs/septic_guidelines.pdf

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How does Florida manage?

• Standard Septic Systems: – Level 1 homeowner awareness; – also: design, construction and training certification and standards,

(e.g. water tightness)

• Aerobic treatment units: – Level 2/3 operating permit to homeowner requires contract with

qualified maintenance entity. Regular inspection required

• Performance-based systems:– Level 2/3 engineer-designed; operating permit to homeowner

requires contract with qualified maintenance entity. More frequent inspection and sampling required.

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Summary

• About 30% of households in Florida are served by onsite systems• The standard septic system consists of a septic tank and a drainfield.

The purpose of such a system is to remove solids, oxygen-consuming material and pathogens from the sewage, and dispose of the water without contacting people.

• Standard septic systems and aerobic treatment units remove only some phosphorus and nitrogen. Some additional removal can occur in the groundwater depending on site conditions. Additional technical or institutional measures can further reduce nutrient loads.

• The OSTDS Karst Study suggests that nutrients from OSTDS travel quickly and far (~100 feet). Direction depends on local conditions.

• Levels of OSTDS management range from homeowner ownership and control to utility ownership and control. More complicated systems and higher standards require higher levels of management.