1 A BENCH-SCALE STUDY OF FOOD WASTE DEGRADATION IN SEPTIC TANKS Hongjian Lin 1 , Maneewan Sinchai 1 , Carlos Zamalloa 1 , Michael Keleman 2 , and Bo Hu 1,* 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: [email protected]
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1
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.
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: [email protected]