Food Waste vs. Sewage Degradation in Septic Tanks: Better Biodegradability and Less Sludge Accumulation

Hongjian Lin1, Maneewan Sinchai1, Carlos Zamalloa1, Michael Keleman2, and Bo Hu1,*

1 Department of Bioproducts and Biosystems Engineering, University of Minnesota, St Paul, MN 551082 InSinkErator, Emerson Commercial & Residential Solutions, 4700 21st Street, Racine, WI 53406-5031

* Bo Hu: phone: 612-625-4215; e-mail: bhu@umn.edu

October 23rd, 2017Ballroom B-C

2017 NOWRA Onsite Wastewater Mega-ConferenceOctober 22-25, 2017

Dover Downs Hotel & CasinoDover, Delaware

• Meta-analysis indicated an average disposal rate for food waste was 0.615 pounds (0.279 kg) per person per day (only including streams after retail and wholesale) in U.S.

• 32.2 million metric ton of food waste disposed annually

• Disposal:

Landfill: organic fraction of municipal solid wastes

Ground by food waste disposer and subjected to wastewater treatment

Discrepant observations about economic gains and emission reduction of the two disposal methods

Food waste generation and disposal/treatment in US

Tucker, C. A., & Farrelly, T. (2016). Household food waste: The implications of consumer choice in food from purchase to disposal. Local Environment, 21(6), 682-706.

Maalouf, A., & El-Fadel, M. (2017). Effect of a food waste disposer policy on solid waste and wastewater management with economic implications of environmental

externalities. Waste Management.

Diggelman, C., & Ham, R. K. (2003). Household food waste to wastewater or to solid waste? That is the question. Waste management & research, 21(6), 501-514.

Septic Systems

• Serving ~20% of U.S. homes• System components (illustrated in the left figure)• Functions: pathogens; solids; BOD; FOG• Important roles in environmental protection and

conservation• Advantages and disadvantages over centralized

WWTP

http://www.extension.umn.edu/environment/Personal communication with Dr. Sara HagerBooklet Country and Cottage Water Systems

Impact of food waste disposal on septic systems

Maalouf, A., & El-Fadel, M. (2017). Effect of a food waste disposer policy on

solid waste and wastewater management with economic implications of

environmental externalities. Waste Management.

Michael Keleman. Food Waste Disposer Impacts on Septic Systems & the

Current U.S. Regulatory Landscape. NOWRA On-Site Mega-Conference, 2016

Septic systems

SewageEffluent to drain-field

Without Effluent Filter With Effluent Filter

Constituent

Typical

without

disposers

Typical

with

disposers

%

Increase

Typical

without

disposers

Typical

with

disposers

%

Increase

BOD5, mg/L 180 190 5.6 130 140 7.7

TSS, mg/L 80 85 6.3 30 30 0.0

Effluent water characteristic change

1. Biodegradability of composite food waste in anaerobic condition (short-term)

2. Effect of FWD on ST water quality (long-term)

3. Effect of FWD on ST solids accumulation (long-term)

Acronyms:

ST: septic tanks

FW: food waste

FWD: food waste disposal

Research goals

Methods: FWD on organic loading in sewage

Michael Keleman. Food Waste Disposer Impacts on Septic Systems & the

Current U.S. Regulatory Landscape. NOWRA On-Site Mega-Conference, 2016

Burton, F. L., Stensel, H. D., & Tchobanoglous, G. (Eds.). (2014). Wastewater

engineering: treatment and Resource recovery. McGraw-Hill.

~ 30% of BOD/COD/TSS increase induced by food waste

Methods: setup1. Short-term experiment: 900 mL bottles at three temperatures (15 ±2oC, 20 ±2oC and

35 ±2oC) for a period of 21 days, inoculated with anaerobic sludge2. Long-term experiment: running two different wastes (sewage for control tank;

sewage+FW for study tank) in simulated septic tanks at 15oC

Control tank: sewage

Study tank: ~30% FW COD increase

Discharge lines

Feeding lines

Cooler for temperature control at 15 oC

Short-term exp Long-term- exp

Short-term results: degradation at 15 oC

Day 0 21-day degradation at 15 oC

Parameters Control Experiment Control Experiment

Total solids, TS (g/L) 9.3 ±0.7 11.2 ±1.2 8.2 ±3.1 10.5 ±0.9

Volatile solids, VS (g TS/L) 5.8 ±0.5 6.5 ±0.3 5.1 ±0.4 6.7 ±0.5

Ash (g TS/L) 3.5 ±0.3 3.6 ±0.4 3.0 ±0.9 4.0 ±0.6

Total chemical oxygen demand, tCOD (g O2/L) 15.5 ±2.1 20.2 ±1.2 12.1 ±0.2 19.4 ±0.6

Soluble COD, sCOD (g O2/L) 0.56 ±0.04 1.30 ±0.11 0.4 ±0.1 1.3±0.2

Total ammonium nitrogen, TAN (mg NH4-N/L) 328.9 ±10.7 341.1 ±10.0 337.6 ±8.0 371.0 ±4.6

Total Kjeldahl Nitrogen, TKN (mg-N/L) 784.7 ±49.5 839.9 ±37.4 742.3 ±67.4 876.6 ±81.4

pH 7.48 ±0.05 7.45±0.05 7.15 ±0.01 6.55 ±0.02

The addition of FW increased the total solid content in about 20% and the total COD in about 30% compared with the control bottles before the experimentation

Short-term results: biogas

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25

Cu

mu

lati

ve M

eth

ane

pro

du

ctio

n

(mL/

L re

acto

r)

Time (days)

Control (15C) Experimet 1 (15C)

0

200

400

600

800

1000

1200

1400

0 10 20 30Cu

mu

lati

ve M

eth

ane

pro

du

ctio

n (

mL/

L re

acto

r)

Time (days)

Control (20C)

Experimet 2 (20C)

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25

Cu

mu

lati

ve M

eth

ane

pro

du

ctio

n

(mL/

L re

acto

r)

Time (days)

Control (35C)

Experimet 3 (35C)

• The lower the temperature, the lower the biodegradation of organic matter• The bottles with kitchen waste produced about 3 times more than the control

bottles at 15 oC• At 20 oC and 35 oC, bottles with kitchen waste produced similar amount of

methane, and the breakdown of kitchen waste was both about 20%

Long-term: tank maturity improved over time (control as an example)

y = 0.1705x + 32.495R² = 0.4405p<0.0001-100

-50

0

50

100

0 50 100 150 200

Tota

l CO

D r

em

ova

l ef

fici

en

cy e

volu

tio

n in

th

e c

on

tro

l ST,

%

Operating time, days

y = 0.2686x - 37.269R² = 0.147p = 0.0076

-100

-50

0

50

100

0 50 100 150 200

Solu

ble

CO

D r

em

ova

l ef

fici

en

cy e

volu

tio

n in

th

e c

on

tro

l ST,

%

Operating time, days

ST matured gradually: 1. Sludge volume is accumulated, so the overall capacity of nutrients degradability

including carbon is increased2. Sludge adaptation or enriched with suitable microorganisms

0

500

1000

1500

2000

2500

0 50 100 150 200

Tota

l CO

D, m

g/L

Operating time, days

Influent: Sewage

Influent: Sewage+Food Waste

Effluent: Sewage

Effluent: Sewage+Food Waste

Long-term test: total COD (n=48)

Control tank effluent

Study tank effluent

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Solu

ble

l CO

D, m

g/L

Operating time, days

Influent: Sewage

Influent: Sewage+Food Waste

Effluent: Sewage

Effluent: Sewage+Food Waste

Long-term test: soluble COD (n=48)

Control tank effluent

Study tank effluent

Summary on effluent water quality: COD

Influent Effluent Average removal efficiencyAve SE Ave SE %

tCOD (n=48), mg/LControl tank 639 36 335 23 47.6Study tank 861 45 394 27 54.3Difference due to FW 223 59

sCOD (n=48), mg/LControl tank 300 23 332 29 -11.0Study tank 404 26 366 32 9.6Difference due to FW 105 33

With 34.8% of tCOD increase induced by FW supplementation, this study found in the setting of 1-L simulated septic tank operation, that:• In the influent, sCOD was increased by 35.0%• Comparing the effluents of the two tanks, tCOD was increased by 17.7%, sCOD by

10.0%.

Summary on effluent water quality: solids

Influent Effluent Average removal efficiencyAve SE Ave SE %

TS (n=17), g/LControl tank 1.912 0.038 1.699 0.032 11.2Study tank 1.991 0.026 1.736 0.030 12.8Difference due to FW 0.079 0.037

VS (n=17), g/LControl tank 0.445 0.031 0.250 0.032 43.8Study tank 0.518 0.024 0.278 0.031 46.4Difference due to FW 0.074 0.028

TSS (n=17), g/LControl tank 0.202 0.017 0.007 0.010 96.5Study tank 0.278 0.023 0.022 0.010 92.0Difference due to FW 0.076 0.015

• FW increased TSS content in effluent compared with the control tank effluent, 22 mg/L vs. 7 mg/L

Summary on effluent water quality: other parameters

Influent Effluent Average removal efficiencyAve SE Ave SE %

pH (n=27)Control tank 6.74 0.02 6.78 0.02Study tank 6.73 0.01 6.80 0.01Difference due to FW -0.01 0.02

Sulfide (n=37), mg/LControl tank 1.83 0.31 5.16 0.68Study tank 3.73 0.62 6.76 0.86Difference due to FW 1.90 1.61

TP (n=53), mg/LControl tank 9.17 0.59 9.35 0.60 -2.0Study tank 9.88 0.66 9.70 0.65 1.8Difference due to FW 0.71 0.35

TN (n=51), mg/LControl tank 66.45 0.82 61.47 0.84 7.5Study tank 69.75 0.80 64.37 1.11 7.7Difference due to FW 3.30 2.90

• In the influent, TP increased by 7.8%, and TN by 5.0%• When compared between the effluents of the control tank and the treatment

tank, TP was increased by 3.8%, TN by 4.7%. All were smaller than or comparable to the corresponding increase in the influent

Septage solids (tank sludge) accumulation: mass accumulation

Control tank Study tank

FW increased TSS content in influent by 37.9%, but the sludge accumulation (ML-TSS mass) in tank was only increased by 12.6% due to FW addition

Septage (tank sludge) accumulation: depth (volume) accumulation

Control tank Study tank

15.6% 16.5%

Again, FW increased TSS content in influent by 37.9%, but the sludge volume increase was 5.8% (denser sludge)

Septic tank: accumulated suspended solids, TSSa, g

Substrate, TSSi, g

*Calculation based on total suspended solids Days of operation: 209 daysTSSi ≈ (=) TSSe + TSSa + TSSd

Effluent, TSSe, g

TSS degraded: solubilized or gas release, TSSd, g

Mass balance of suspended solids

Mass balance of suspended solids

Fed to tanksDischarged in effluent

Accumulated in tanks

Degraded

Sewage TSS, g 6.61 0.19 3.63 2.79

Fraction in sewage TSS 100% 2.9% 54.9% 42.2%

FW TSS, g 2.28 0.41 0.46 1.41

Fraction in FW TSS 100% 18.1% 20.0% 61.8%

Conclusions

Add FW to sewage @34.8% of tCOD increase:

• Influent: sCOD↑35.0%, TP↑7.8%, and TN↑5.0%• Effluent: tCOD↑17.7%, sCOD↑10.0%, TP↑3.8%, TN ↑4.7%. All smaller

than the corresponding increase in the influent• Effluent TSS: 22 mg/L vs. 7 mg/L, but within MN limit (60 mg-TSS/L)• TSS degradation: 61.8% FW-TSS, and 42.2% sewage-TSS• Solids accumulation: 20.0% of FW-TSS and 54.9% of sewage-TSS

Conclusions

Overall, compared with sewage TSS, FW TSS tends to be biodegradedby a larger proportion, and to be accumulated by a smallerproportion, and to form denser sludge. The effluent hascorrespondingly slightly higher strength, but the TSS is well within MNlimit of 60 mg-TSS/L. Larger-scale tank and cruder FW particles will betested in future study.

Acknowledgement

The authors very appreciate the assistance received from Mr. ScottJoseph from Blue Lake Wastewater Treatment Plant in Shakopee, MN