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: [email protected]
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
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1400
0 5 10 15 20 25
Cu
mu
lati
ve M
eth
ane
pro
du
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(mL/
L re
acto
r)
Time (days)
Control (15C) Experimet 1 (15C)
0
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0 10 20 30Cu
mu
lati
ve M
eth
ane
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mL/
L re
acto
r)
Time (days)
Control (20C)
Experimet 2 (20C)
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0 5 10 15 20 25
Cu
mu
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eth
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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
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th
e c
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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
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