A Case Study on the Anaerobic Treatment of Food by yyy55749


									                                                                             ICCBT 2008

A Case Study on the Anaerobic Treatment of Food Waste and Gas

K.H. Chua, Universiti Tenaga Nasional, MALAYSIA
C. H. Yip, Universiti Tenaga Nasional, MALAYSIA
W. L. S. Nie, Universiti Tenaga Nasional, MALAYSIA


Research has shown that the composition of food waste in Malaysian municipal solid wastes,
MSW ranges from 45 to 49%. The potential of treating food waste anaerobically was
proposed and studied. The results show that the reduction of food waste volume decreased
rapidly at the earlier stage and gradually reach a plateau. The rate of formation of gas
decreased gradually. The adding of microbes in the form of chicken manure enhances the
food waste degradation process and increased gas formation. But the addition of oil retards
the food waste degradation. The maximum volume reduction over 30 days for food waste, oil
laden food waste, and manure added food waste are 32.7%, 39% and 43.7%, respectively.
The biogas collected over 30 days ranging from 9 to 22 ml.

Keywords: Food waste, anaerobic, biogas

*Correspondence Author: K.H. Chua, Universiti Tenaga Nasional, MALAYSIA

ICCBT 2008 - F - (29) – pp311-316
A Case Study on the Anaerobic Treatment of Food Waste and Gas Formation


In a 2006 study carried out by Japan international corporation agency shows that 45% of
Malaysian MSW consists of food waste [7]. Food waste defined as uneaten portion of meals,
leftover and trimmings from food preparation from restaurants, kitchens and cafeterias.
Improper disposal of food waste causes odour and potential vermin and scavengers
infestification. Generally, it was disposed off together with other MSW in landfill. With
capacity of landfill gradually filling up and fewer landfills being commission, it is critical to
look for alternative disposal method. One possible method is composting [2]. Composting is
an aerobic process and it produces humus that can be used as fertilizers or soil conditional,
Incineration is another approach but it is not feasible because the moisture contents of food
waste is very high. The composition of food waste are consists of meat, bones, fats and oils,
greens and fruits, carbohydrates and moisture. The high moisture content is feasible for
composting and anaerobic degradation.

The amount of food waste being generated from population can be reduced by better food
preparation procedures; by removing certain garnishes from food items; by offering smaller
portions and better inventory. The other alternative solution is food donation to charity which
was practice in USA and Japan. But most restaurants do not very supportive of such approach.
Due to high organic and moisture contents, biodegradation using anaerobes is another
potential treatment method. Anaerobic treatment presents the potential of producing biogas
which can be used for cooking, heating and electricity generation. There are three distinct
stages in an anaerobic process [6]. Firstly, the hydrolysis of long chain hydrocarbon into
smaller chain hydrocarbon. The second phase is the conversion of smaller chain hydrocarbon
organic matter to acetic acid, fatty acid and hydrogen by acetogenic bacteria. At this stage, the
pH will drop due formation of acid. The final stage of anaerobic process is the
methanogenesis ie. conversion of acetic acid into methane and carbon dioxide. This process is
pH sensitive and temperature sensitive. The temperature should ranges from 30 to 60 degree C
[1]. It is ideal process for tropical climate due to high daily temperature. The biodegradation
of food waste also produces leachate. Factors that influence the anaerobic process are pH,
temperature and nutrients. Methane producing bacteria require a neutral to slightly alkaline
environment (pH 6.8 to 8.5) in order to produce methane. Acid forming bacteria grow much
faster than methane forming bacteria. If acid-producing bacteria grow too fast, they may
produce more acid than the methane forming bacteria can consume. Then, excess acid will
build up in the system. When the pH drops, and the system may become unbalanced,
inhibiting the activity of methane forming bacteria. Methane production may stop entirely.
The major nutrients required by the bacteria are ammonium and phosphate [2]. There are
normally plenty of proteins present from the decomposition of household waste so there is not
usually a shortage of ammonium ions, but some additional phosphate may be required to
ensure optimum process rates. The micro-nutrients which methane bacteria require in trace
quantities include: iron, manganese, copper, nickel, zinc and cobalt, all of which are likely to
be present in municipal solid waste. The production of biogas is dependent on the optimum
biodegradation process. Generally, the composition of biogas are 40 – 60% methane, 50-40%
carbon dioxide, and other gases such as hydrogen sulfide, ammonia and other tracer gases.

A study by Korean government has successfully converted the food waste into biogas and
compost [3]. The food waste was first shredded and sorted according to metal, sizes and
density. The bones and shells are mixed with acid. The organic waste are then hydrolyzed

312                                                           ICCBT 2008 - F - (29) – pp311-316
                                                                               K.H. Chua et. al.

degraded to organic acid in acid reactor. After that, the solution was transported to methane
reactor to convert it into methane. The heavy, slowly degradable food waste and anaerobic
sludge were then dried and convert into humus. The biogas is stored at gas holder before

This study looks into the potential production of biogas from food scrap from a cafeteria. The
rate of degradation was studied too. A comparison study was conducted on the influence of
grease and addition of microbes/nutrients.


A laboratory scale reactor was constructed using a plastic container with openings for
temperature measurement and gas collection. The gas was measured using invert cylinder
system. Food wqaste were collected from UNITEN food court. Three samples were prepared.
One with food waste FW only, food waste with added oil OFW and food waste with added
chicken manure MFW. The total amount of food waste was about 50 kg. Sample OFW was
added with 1 liter of oil while sample MFW was added with I kg of chicken manure. The
composition of food waste is given below:

                          Type of food              Weight [kg]
                          Rice                           20
                          Fruits                         17
                          Vegetables                      3
                          Chicken meat                   7.5
                          Egg and egg shells             0.8
                          Fish                           1.5
                          Coconut                        0.2
                          Total                          50

The sample was tested for its moisture content, total solid concentration, BOD5, and oil
content. The formation of the biogas and food waste volume reduction was monitored over 30


The food waste volume reductions in the three samples over the 30 days are shown in Figure
1. The percentage reduction for FW, OFW, and MFW over 30 days are 32.7%, 39% and
43.7%, respectively. The rate of reduction is not constant. The initial rate reduction is high
and gradually tapered off. The first 5 days reduction contributed to at least 40% of total
volume reduction.

After that the rate of reduction is averaged 6% of total reduction. The addition of oil into the
food waste had retarded the anaerobic digestion of food waste since oil is slowly degrading
food waste. But the addition of chicken manure [8] introduce extra microorganism into the
reactor and assist the biodegradation process. The biogas formation shows a rapid formation
followed by a slower formation rate as shown in Figure 2.

ICCBT 2008 - F - (29) – pp311-316                                                           313
A Case Study on the Anaerobic Treatment of Food Waste and Gas Formation

During the initial stage, the high percentage of easily biodegradable food waste allows the
digestion process to proceed rapidly. The volume reduction is high. With large amount of
organic acid available, the formation of biogas proceeded in a higher rate. Once the easily
degraded organic waste has been digested, the process slowed down in order to allow long
chain hydrocarbon to be digested. The production of biogas ml per kg food waste for FW,
OFW and MFW were 0.65, 0.55, 1.29 ml/kg food waste respectively. These are very minimal
biogas production.

                                                                                                             Volume of Food Waste (m )



                m 3)


                Volume ( x 10






                                                                          0               5            10          15                 20       25      30      35

                                                                                                                        Tim e (Day)
                                                                              Food w aste only
                                                                              Food w aste w ith oil content
                                                                              Food w aste w ith chicken manure

                                                                                       Figure1: Food waste reduction over time

                                                                                                        Amount of Biogas (ml) Collected

                                A m o u n t o f g as co lle cted (m l)





                                                                              0               5         10         15                 20       25      30      35

                                                                                                                        Time (day)
                                                                              Food w aste only
                                                                              Food w aste w ith oil content
                                                                              Food w aste w ith chicken manure

                                                                                        Figure 2: Biogas production over 30 days

The possible reasons are that there is no mixing occurred in the reactor. There is no constant
temperature and control in the system as it relies on direct sunlight. A slightly high
temperature is desired to optimum the biogas production. There is possibility of lack of
nutrients to encourage the microorganism to thrive in the reactor. Further study is required to

314                                                                                                                                             ICCBT 2008 - F - (29) – pp311-316
                                                                             K.H. Chua et. al.

improve the reactor design and temperature control. This is just a preliminary study. Further
research will look into the construction of reactor, gas measurement and nutrients addition.


A small reactor was constructed to study the anaerobic treatment of food waste from UNITEN
food court.

The results showed that the volume reduction ranged from 32.7% to 43.7%. The production
of biogas ranged from 0.55 to 1.29 ml biogas/kg food waste. The addition of grease had
retarded the anaerobic process whereas the addition of chicken manure had increased the
production rate. Temperature is an important factor, a higher temperature will increase the
degradation rate and hence gas production rate.


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[8].   Barry, G. Chicken manure fuel. 1971.

ICCBT 2008 - F - (29) – pp311-316                                                         315

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