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A BENCH-SCALE STUDY OF FOOD WASTE DEGRADATION IN SEPTIC TANKS

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

ABSTRACT

Solids in food waste (FW), when disposed in sink via food waste disposers, have a potential to

settle at the bottom of septic tanks and may contribute to an increased sludge accumulation rate

and therefore increase the sludge pump-out frequency. FW may also decrease water quality of tank

effluent with an increased nutrient loading. However, to the best of our knowledge, there has no

empirical peer-reviewed study of assessing the contribution of FW on the septic tank performance.

This study was designed based on the assumption that on average, FW increased the total chemical

oxygen demands (tCOD) by ca. 30% in influent, and was conducted in simulated bench-scale 1-L

tanks (boxes). Two tanks, the control tank fed with sewage and the treatment tank fed with sewage

plus additional FW suspension, were operated at 15 °C with an average hydraulic retention time

of 7.5 day for 209 days. In summary, it was found that FW solids were better degraded than sewage

solids (61.8% vs. 42.2%), and less proportion of FW solids than sewage solids contributed to

sludge accumulation (20.0% vs. 54.9%). Total suspended solids content in the effluent was

increased from 7 mg/L to 22 mg/L due to FW addition, but was substantially smaller than 60 mg/L

required by Minnesota Rules Part 7080.2150. In future study, larger scale operation with tank fed

with FW directly will need to confirm the better biodegradability of FW in septic tanks.

KEYWORDS

Food waste, Food waste disposers, Septic tanks, Water quality, Sludge accumulation

INTRODUCTION

A recent study conducted by Food and Agricultural Organization (FAO) estimated that roughly

one third of the global food production for human consumption is getting lost or wasted every year

(Gustavsson et al. 2011). Food waste (FW) is high in organic matter content, energy content and

nutrient contents, with an average dry weight composition of 30-60% starch, 5-10% proteins and

10-40% lipids (Pleissner and Lin 2013). Although it is suggested by some researchers to collect

and utilize FW as an enormous resource for energy, nutrient, and chemical production (Kiran et al.

2014, Lin et al. 2013, Pham et al. 2015), the mainstream method of FW handling is through the

landfill of solid wastes or wastewater/sewage treatment because of technical and economic

hindrance of recovery technologies. Food waste disposer (FWD, or garbage disposal/grinder)

grinds raw FW and other organic materials into smaller particle sizes of mostly between 0.01” to

0.5” by design as specified by the American Society of Sanitary Engineering Standard (ASSE)

#1008. The ground waste is then flushed with water and transports through plumbing or sewer

pipes to treatment facilities, either centralized wastewater treatment systems or in decentralized

(on-site) sewage treatment systems, commonly known as septic systems. Over 50% of US

1 Department of Bioproducts and Biosystems Engineering, University of Minnesota, St Paul, MN 55108 2 InSinkErator, Emerson Commercial & Residential Solutions, 4700 21st Street, Racine, WI 53406-5031

* Corresponding author. Department of Bioproducts & Biosystems Engineering, 1390 Eckles Ave., BAE Bldg,

University of Minnesota, St. Paul, MN 55108-6005; phone: 612-625-4215; e-mail: bhu@umn.edu

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households installed FWD, and when installed, FWD unit is considered an cost-effective

component of waste management system over the landfill of solid waste (Iacovidou et al. 2012,

Marashlian and El-Fadel 2005).

Previous studies concluded that marginal (less than 3.5%) or no increase in household water

consumption and in the subsequent hydraulic load (Iacovidou et al. 2012, Keleman 2016) caused

by FWD installation, but many studies have shown the resulting sewage composition generally

goes up to a higher strength and the range of increase depends on the water characteristics under

discussion. This has raised concerns of stratifying FW from solids disposal to wastewater treatment

systems, and has probably affected the use of FWD in European countries. For example, the

country with the highest FWD installation rate in Europe is UK and the rate is estimated only

around 5% (Iacovidou et al. 2012). An important pattern of FW is that FW induces a higher

proportion of chemical oxygen demands (COD) and total suspended solids (TSS) concentration

increase over nitrogen (N) and phosphorus (P), e.g., 7.5% to 62% for COD, by 2% to 60% for

TSS, 1.4% to 19% for total N, and 1.2% to 14% for total P, compared with the domestic wastewater

without FW. This fact of strength increase led to experimentation on the treatment cost estimate in

centralized systems of WWTP due to additional aeration and nutrient control measures (NYCDEP

1997, Thomas 2011). But it was also suggested by some other studies that the higher loading of

organic carbon would improve the biological nutrient removal performance and therefore reduce

the associated cost of chemical nutrient removal methods (Battistoni et al. 2007).

Among all the households in US, nearly 25% of them are relying on septic systems, either at

individual or community cluster scales, for sewage treatment according to USEPA’s estimation

(EPA NA). However, to the best knowledge of the authors’, it is rarely seen any single piece of

peer-reviewed empirical study that assesses the impact of FWD on the treatment performance of a

septic system, or on septic tank effluent. An anecdotal study (Crites and Technobanoglous 1998)

stated that due to FWD installation, the septic tank effluent had a slight or no increase in terms of

COD (from 345 to 400 mg/L), TSS (from 80 to 85 mg/L), total ammoniacal N (from 40 to 44

mg/L), organic N (from 24 to 31 mg/L), and total P (no change, 16 mg/L). However, no influent

change or types and doses of the wastes were monitored in that study and therefore it was unknown

if the FW compounds and loading were typical or not in this particular work.

This study aimed to fill the knowledge gap of how a typical FW loading from FWD changed the

influent and effluent water qualities of septic tanks and how it affected the sludge accumulation in

septic tanks. As a starting point, the experiment was designed based on the assumption that FWD

induced a 30% of COD increase in the influent sewage according to previous estimation and results

(Abu-Orf et al. 2014, Iacovidou et al. 2012). Then both wastewater and FW supplemented

wastewater were individually fed to bench-scale 1-L simulated septic tanks (one control tank and

one study tank) and operated under the same conditions. The major water characteristics of influent

and effluent were analyzed. At the end of the operation, the sludge volume accumulation were

measured. Through the performance comparison between the control tank and the study tank, the

effect of FWD on septic tank effluent water quality and sludge accumulation can be assessed.

MATERIALS AND METHODS

Inoculum, sewage, and food waste preparation

Anaerobic sludge collected from Blue Lake Wastewater Treatment Plant (Shakopee, MN) was

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processed to remove dissolved solids by centrifuging at 5k×g, decanting supernatant, washing

with deionized water, centrifuging again at 5k×g, and decanting supernatant. Two batches of

representative FW were formulated and provided by Emerson Commercial & Residential

Solutions, Racine, WI. 66.7 g of the provided FW was diluted with tap water to 1 L, and filtered

through 2 mm drum screen. The rejects were ground for 2 min, washed by FW filtrate and filtered

through the screen again. The grinding-washing cycle was repeated for another three times so that

there was only little amount of remaining fiber rejects, and the filtrate was in a form of FW

suspension. Raw sewage was collected from the same wastewater treatment plant and was filtered

with a 2 mm-pore drum screen to remove large debris, and FW suspension was added to sewage

at ratio that increased chemical oxygen demand (COD) by ~30%.

Setup and operation of simulated septic tanks

Simulated 1-liter septic tanks (made from LDPE plastics box, internal W×L×H as 110 mm × 165

mm × 55 mm) were inoculated with the processed anaerobic sludge, fed with sewage (the control

tank) or sewage plus FW (the treatment tank), with an initial COD ratio of the inoculum and

sewage at 3:1. The simulated septic tanks were then operated at an average hydraulic retention

time (HRT) of 7.5 days with two feeding/withdrawal events per HRT, and kept in a cooler set at

15°C for 209 days. The setup was shown in Figure 1.

Figure 1. Setup of simulated septic tanks (Note that the symbols of ST5 and ST6 indicate the

septic tanks fed with sewage and sewage plus FW suspension, respectively).

Water quality analysis of influent, effluent, mixed-liquor, and sludge accumulation

For each feeding and withdrawal event, influent and effluent samples were collected. Water

characteristics analysis on total COD (tCOD), soluble COD (sCOD), total phosphorus (TP), total

nitrogen (TN), pH, and total sulfide were performed using commercial colorimetric methods with

a UV-vis spectrophotometer or electrode probes. Total suspended solids (TSS) were calculated

from subtracting total dissolved solids from total solids (TS) obtained by drying filtered samples

and nonfiltered samples overnight at 105 °C, respectively. Volatile solids (VS) were obtained by

subtracting the ash content (combusted at 550 °C in muffle furnace for 30 min) from TS. After 209

days’ operation, the mixed liquor was obtained from the tank for solids analysis. The height of

sludge accumulated in each tank was indirectly measured after the mixed-liquor was transferred

to a transparent HDPE bottles and settled for three and half a day before reading the sludge depth.

RESULTS AND DISCUSSION

COD removal

This study aimed to add FW to induce a 30% of tCOD increase in sewage. The tCOD removal

efficiencies of both tanks gradually improved over operation (

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Figure 2), indicating the maturity of the tanks due to the microbial community evolution and sludge

accumulation. The real tCOD increase in influent due to FW addition was 34.8% (from 639 to 861

mg/L, Table 1), and that of the sCOD increase was 35.0% (from 300 to 404 mg/L). The

corresponding tCOD and sCOD change in effluent due to FW addition were 17.7% and 10.0%,

respectively. This decreased impact of FW on effluent COD concentrations clearly suggest that

septic tank holds a better pretreatment on FW than on the other portion of sewage COD, either by

better degradability or better settleability, and will be further discussed in later section about TSS

and VS removal.

A

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Figure 2. Time course of total COD removal efficiency: the control tank (A) and the study tank

(B).

Table 1. Major water quality data summary and the corresponding removal efficiency(SE:

standard error)

Influent Effluent

Average

removal

efficiency

Ave SE Ave SE %

pH (n=27)

Control tank 6.74 0.02 6.78 0.02 -

Treatment tank 6.73 0.01 6.80 0.01 -

Difference due to FW - - - - -

Sulfide

(n=38), mg/L

Control tank 1.83 0.31 5.16 0.68 -

Treatment tank 3.73 0.62 6.76 0.86 -

Difference due to FW 1.90 1.61 -

tCOD (n=48),

mg/L

Control tank 639 36 335 23 47.6

Treatment tank 861 45 394 27 54.3

Difference due to FW 223 - 59 -

B

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sCOD

(n=48), mg/L

Control tank 300 23 332 29 -11.0

Treatment tank 404 26 366 32 9.6

Difference due to FW 105 - 33 - -

TP (n=53),

mg/L

Control tank 9.17 0.59 9.35 0.60 -2.0

Treatment tank 9.88 0.66 9.70 0.65 1.8

Difference due to FW 0.71 - 0.35 - -

TN (n=51),

mg/L

Control tank 66.45 0.82 61.47 0.84 7.5

Treatment tank 69.75 0.80 64.37 1.11 7.7

Difference due to FW 3.30 - 2.90 - -

TS (n=17),

mg/L

Control tank 1.912 0.038 1.699 0.032 11.2

Treatment tank 1.991 0.026 1.736 0.030 12.8

Difference due to FW 0.079 - 0.037 - -

VS (n=17),

mg/L

Control tank 0.445 0.031 0.250 0.032 43.8

Treatment tank 0.518 0.024 0.278 0.031 46.4

Difference due to FW 0.074 - 0.028 - -

TSS (n=17),

mg/L

Control tank 0.202 0.017 0.007 0.010 96.5

Treatment tank 0.278 0.023 0.022 0.010 92.0

Difference due to FW 0.076 - 0.015 - -

Total phosphorus and nitrogen removal

Data summary were given in Table 1 and the time course data were displayed in

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Figure 3. The sewage and FW supplemented sewage had average TP content of 9.17 and 9.88

mg/L, respectively. The corresponding septic tank effluent had average TP content of 9.35 and

9.70 mg/L. The slight increases of TP in effluent of both the control and treatment tank were most

like released from the initially inoculated sewage, which had an initial TP content of 25.6 mg/L.

Generally, septic tank did not show noticeable TP removal efficiency. Influent TP was increased

by FW by 0.71 mg/L, while in the effluent, the corresponding TP increase was 0.35 mg/L,

suggesting that half of the TP induced by FW was removed by septic tank.

A

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Figure 3. Time course profiles of major influent and effluent water characteristics: tCOD (A),

sCOD (B), total phosphorus (C), and total nitrogen (D).

Septic tanks showed a slight TN removal in both sewage and FW supplemented sewage, by 8%

and 8.7%, respectively.

B

D

C

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Solids removal

Septic tank had TS removal of 11.1% and 12.8% for sewage and FW supplemented sewage,

respectively. It had better VS removal as 45.4% and 47.4%, respectively. TSS was a deciding water

quality in septic tank, because it had an impact on the hydraulic performance of the following leach

field. In this study, septic tank was observed to remove 98.4% of TSS from sewage, decreased

from 202 to 7 mg/L. FW addition decreased the TSS removal efficiency to 91.2%. Among the 22

mg-TSS/L in the tank effluent, 15 mg/L was therefore contributed by FW supplementation, which

was about 20% of the initial TSS contained in FW.

Sludge accumulation and solids balances

In the total tank depth of 55 mm, the control tank had a sludge depth of 8.6 mm, while the tank fed

with FW supplemented sewage had a sludge depth of 9.1 mm (Figure 4). This corresponded to a

6.4% sludge increase caused by FW. Concerning that there was a 37.9% of TSS increase in the

influent induced by FW addition, FW solids were either better degraded or more discharged to

effluent. A mass balance was needed to address the fate of TSS to see if there was a better

degradation.

Figure 4. Sludge depth in the control tank and treatment tank.

Figure 5 showed the TS, VS, and TSS in the mixed-liquor after 209 days operation. Since the

volume of the mixed-liquor was 1 L, it can be calculated that the tanks held 5.577 and 5.773 g of

TS, 3.127 and 3.423 g of VS, and 3.627 and 4.083 g of TSS, in the control and treatment tanks,

respectively. Further calculation results were summarized in Table 2. It can be seen that FW TSS

was better degraded than sewage TSS, 61.8% vs. 42.2%. The FW TSS had smaller effect on sludge

accumulation, since only 20.0% of FW TSS was accumulated in tank while 54.9% of sewage TSS

was accumulated. However, FW TSS had an adverse effect on water discharge TSS because a

higher proportion (18.1%) of this TSS went to effluent compared with sewage (2.9%).

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A

B

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Figure 5. Sludge depth in the control tank and treatment tank.

Table 2. Mass balance of total suspended solids

Fed to tanks Discharged 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

With 34.8% of tCOD increase induced by FW supplementation, this study found in the setting of

1-L simulated septic tank during 209 days’ operation, that:

In the influent, sCOD was increased by 35.0%, TP by 7.8%, and TN by 5.0%

When compared between the effluents of the control tank (fed with sewage) and the

treatment tank (fed with sewage plus FW suspension), tCOD was increased by 17.7%,

sCOD by 10.0%, TP by 3.8%, TN by 4.7%. All were smaller than the corresponding

increase in the influent

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

7 mg/L

FW increased TSS content in influent by 37.9%, but the sludge accumulation in tank was

only increased by 6.4% due to FW addition

61.8% of FW TSS was degraded, while 42.2% of sewage TSS was degraded

20.0% of FW TSS and 54.9% of sewage TSS were accumulated in tank, respectively

Future experiment based on larger-scale septic tanks will be designed and conducted to

confirm the effect of FW on sludge accumulation and characteristics of septic tank effluent

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