IMPROVEMENT OF BIOLOGICAL SOLUBILIZATION AND MINERALIZATION PROCESS FOR FOOD WASTE

Biological solubilization and mineralization process was proposed for food wastes. In the process food wastes are mixed with rice hull as biological support medium under wet condition and are solubilized or mineralized. The purposes of the study are to operate the biological solubilization and mineralization process without accumulation of food wastes and to increase mineralization rate for the reduction of organic loading to the sewerage system. Biological solubilization and mineralization process was operated without the accumulation of food waste by aeration in the circulation tank. The process can reduce organic loading to the sewage system to half by aeration in the circulation tank and 80 % by installing biofilm into the circulation tank. The process combined with biofilter would not require further treatment of effluent.

IMPROVEMENT OF BIOLOGICAL SOLUBILIZATION AND MINERALIZATION PROCESS FOR FOOD WASTE

Wataru NISHIJIMA*, Hazel B GONZALES**, Hideki SAKASHITA*, Yoichi

NAKANO* and Mitsumasa OKADA**

* Institute for waste waters treatment, Hiroshima University, 1-5-3 Kagamiyama, Higashi-Hiroshima,

Hiroshima, 739-8513, JAPAN

* * Department of Material Science and Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, JAPAN

ABSTRACT

Biological solubilization and mineralization process was proposed for food wastes In the process food wastes are mixed with rice hull as biological support medium under wet condition and are solubilized or mineralized The purposes of the study are to operate the biological solubilization and mineralization process without accumulation of food wastes and to increase mineralization rate for the reduction of organic loading to the sewerage system Biological solubilization and mineralization process was operated without the accumulation of food waste by aeration in the circulation tank The process can reduce organic loading to the sewage system to half by aeration

in the circulation tank and 80 % by installing biofilm into the circulation tank The process combined with biofilter would not requirefurther treatment of effluent

KEYWORDS

Food waste; solubilization; mineralization; biological treatment

INTRODUCTION

At present, direct landfill and landfill after incineration are the most common technologies being

used to handle food wastes (Ktrith, 1994; Shin et al., 2001) Although food wastes contain more than 80% water aside from oil and biodegradable organic matter (Yun et al., 2000), the incineration

of food wastes requires large amount of energy and direct landfill induces sanitary problems such as the production of flies and other pathogenic microorganisms and putrid odors associated with anaerobic decomposition Some researchers mentioned that treating food wastes in a wastewater

treatment plant might be more effective due to the high water content (Wang et al., 2001; Strutz,

1998; de Koning and van der Graaf, 1996) In some countries such as US, Sweden andAustralia, food wastes are treated by disposers or grinders in the site where food wastes are produced before

discharge to wastewater treatment plants to help the degradation (Karrman et al., 2001; Gallil and

Yaacov, 2001) However, the use of disposers entails additional loading to sewerage systems

(Sankai et al., 1997; Gallil and Yaacov, 2001)

Biological solubilization and mineralization process was proposed for food wastes (Okada and Nishijima, 2001) In the process, food wastes are mixed with rice hull as biological support medium under wet condition and are solubilized or mineralized The process can reduce organic loading to the sewerage system due to mineralization The purposes of the study are to operate the biological solubilization and mineralization process without accumulation of food wastes and to increase mineralization rate for thereduction of organic loading to the sewerage system

MATERIALS AND METHODS

Reactor Specification and Operation

The schematic diagram of the equipment is shown in Figure 1 A 25-l stainless steel reactor was used as solubilization reactor and a 58-l polypropylene container was used as circulation tank The solubilization reactor was equipped with an impeller and a stainless steel rod sprinkler for mixing and water addition, respectively Rice hulls (15 l) were introduced in the solubilization reactor as support media for microorganisms Activated sludge (200 g-dry weight) and 130g activated carbon were also added at the start of operation The reactor has a stainless steel perforated plate (2mm pore-diameter) at the bottom allowing the flow of water containing solubilized wastes to the circulation tank while preventing the loss of solid wastes and support media The circulation tank was filled with water and a pump in the tank intakes surface water with oil and bottom water with sedimentary particles and transfers back to the solubilization reactor Effluent was discharged as overflow water from the mid-depth of the circulation tank

The equipment was operated without and with aeration in the circulation tank (Run 1 and Run 2, respectively) To trap suspended solids (SS) and degrade dissolved and suspended organic substances, coiled filters (polyvinylidene chloride) were installed inside the circulation tank (Run 3) Twice as large as theamount of air in Run 2 was supplied to the circulation tank in Run 3 for the degradation of dissolved and suspended organic substances Biofilter was also installed, instead of coiled filter, outside the circulation tank for the same objectives (Run 4) The biofilter consisted

of a series of 6 columns with expanded/foamed polypropylene as packing material and connected to the circulation tank Air was supplied to the circulation tank and biofilter The total air supplied

in Run 4 was the same as Run 3

circulation tank

support media

perforated plate

solubilization reactor

weighing scale

circulating water

effluent

water supply food waste

impeller sprinkler

circulation tank

support media

perforated plate

solubilization reactor

weighing scale

circulating water

effluent

water supply food waste

impeller sprinkler

Figure 1 Schematic diagram of the biological solubilization equipment

During the operation, 400 g/d of food wastes (food waste loading; 16.0 kg/m3/d) were added once a day to the solubilization reactor Tap water was added to the solubilization reactor once an hour at

a feed rate of 1 l/h Water in the circulation tank was also transferred to the solubilization reactor every 30 minute The solubilization reactor was operated at 35ºC while the circulation tank was at room temperature

Food Wastes

Artificial food waste was used to keep its properties constant The composition of artificial food waste was determined based on a typical composition of food waste from Japanese household

(Aoshima et al., 2001) as listed in Table 1 The average moisture content of the food waste was

82 % and contains 42 % carbon and 2 % nitrogen per dry weight basis The waste was chopped to

1-cm cube size and stored in a refrigerator at 4 ºC before use

Sampling and Analyses

Solid sample was collected from the solubilization reactor and the weight of the reactor was recorded prior to the daily feeding of food wastes The solid sample was subjected to moisture content (oven-drying at 105ºC for 24 hours), carbon and nitrogen (Yanaco MT-CN Corder, Japan) analyses The effluent from the circulation tank was also sampled and analyzed for dissolved organic carbon (DOC) (Shimadzu TOC-500, Japan) and SS (oven-drying at 105ºC) where the latter was further analyzed for its carbon and nitrogen content Twenty-four-hour sampling of the effluent was done to determine the daily variation of DOC and SS Dissolved oxygen (DO) and oxidation-reduction potential (ORP) in the circulation tank weremonitored using DO meter (Horiba

D-21) and ORP meter (Horiba D-21), respectively All analyses were done according to Japan

Industrial Standard (JIS)

Table 1 Composition of food waste used (Standard Garbage – SG)

( kg/m3/d )

Composition (% w/w – wet basis)

Vegetables

Fruits

Others

From the DOC, SS and carbon content data, carbon mass balance was estimated Mineralized

carbon was estimated by deducting the sum of the cumulative amount of carbon in effluent from the

start of experiment and the amount of carbon in the solid sample of the solubilization reactor from

the total amount of carbon in fresh food waste supplied

RESULTS AND DISCUSSION

Effect of aeration

The colors of SS in effluents were black and brown in Run 1 without aeration and Run 2 with

aeration, respectively DO in the circulation tank of Run 1 was almost zero within 24 hours and

the average ORP value was -150mv On the other hand, DO in Run 2 kept increasing within 6

hours after feeding food waste from 0.5 to 4 mg/ l and leveled off Low DO at the beginning of

operation in Run 2 implies that large amount of easily biodegradable organic matter were initially

supplied from the solubilization tank into the circulation tank and much oxygen was used up for the

degradation in the circulation tank

Figure 2 shows DOC concentrations within 24 hours after feeding food waste The large

difference between DOC concentrations in Run 1 and 2 means that DOC was not degraded

sufficiently and was discharged under anaerobic condition (Run 1) even though DOC was easily

biodegradable DOC concentrations in effluent decreased from an average concentration of 148 mg/ l in Run 1 to 26 mg/ l in Run 2

0 50 100 150 200

Time (h)

Figure 2 DOC concentrations within 24 hours after feeding food waste

0 500 1000 1500 2000 2500

Time (h)

Run1 Run2 Run3 Run4

Figure 3 DOC concentrations within 24 hours after feeding food waste

Figure 3 shows SS concentrations within 24 hours after feeding food waste SS concentrations in Run 2 were higher than those in Run 1 Effluent was discharged from the mid-depth of the circulation tank preventing oil and large size of SS from discharging Aeration in the circulation tank caused the large size of SS settling at the bottom to be suspended and discharged

Figure 4 shows the carbon mass balance Approximately 27 % of food waste was solubilized

(converted to DOC and SS – size less than 2mm) and 33 % was mineralized in Run 1, however,

42 % of food waste accumulated in the solubilization reactor When aeration was applied (Run 2), mineralization was slightly improved from 33 % to 47 % and almost negligible accumulation of carbon occurred Therefore, large carbon accumulation in Run 1 was probably due to the occurrence of anaerobic condition in the solubilization tank Tap water and the circulating water are periodically flowed into the solubilization tank and supply oxygen for the solubilization of food waste The circulating water did not contain oxygen when aeration was not carried out in the circulation tank Therefore, oxygen would be deficient for the solubilization of food waste in the solubilization tank

Run4 Run3 Run2 Run1

Carbon (%)

Figure 4 Carbon mass balance

Contribution of biofilm and biofilter on improvement of mineralization

SS concentrations in effluent decreased by installing biofilm inside the circulation tank (Run 3) as shown in Figure 3 However, DOC concentrations increased as shown in Figure 2 Higher DOC

in Run 3 than Run 2 was probably due to the insufficient DO Although twice as large amount of oxygen was supplied in Run 3 than in Run 2 for the biodegradation of SS, DO was not detected just after feeding food waste while DO concentrations just before feeding food waste were less than 2 mg/l On the other hand, DO concentration in Run 2 was 0.5 mg/l just after feeding food waste and were around 4 mg/ l just before feeding food waste

It can be seen in Figure 3 that SS concentrations in effluent were significantly decreased to 17 mg/l

on theaverage and 63 mg/l as maximum by installing biofilter outside the circulation tank (Run 4) DOC concentrations in Run 4 were almost the same as Run 2, that is, 28 mg/l on theaverage and 33 mg/l as maximum Effluent in Run 4 would not require further treatment in the sewage system Run 3 and Run 4 were successfully operated without accumulation of food waste in the

solubilization tank as shown in Figure 4 Around 80 % of food waste supplied was mineralized in Run 3 by installing biofilm into the circulation tank Most of the food waste supplied (98%) was mineralized when biofilter for trapping SS and biodegrading SS and DOC was installed (Run 4)

To compare to the treatment of food waste by disposers or grinders, the biological solubilization and mineralization system can reduce organic loading to the sewage system to half by aeration in the circulation tank and 80 % by installing biofilm into the circulation tank Moreover, further treatment would not be required when biofilter is combined with the system

CONCLUSION

The purposes of the study are to operate the biological solubilization and mineralization process without accumulation of food wastes and to increase mineralization rate for the reduction of organic loading to the sewerage system Biological solubilization and mineralization process was operated without the accumulation of food waste by aeration in the circulation tank The process can reduce organic loading to the sewage system to half by aeration in the circulation tank and 80 % by installing biofilm into the circulation tank The process combined withbiofilter would not require further treatment of effluent

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