Team:CIEI-China/Background
There is one thing people are longing for: a greener world. However, ever since industrial revolution, our environment has been facing an increasingly overwhelming situation: air pollution, energy shortage, global warming…
Among all the environmental problems, management of waste is a big issue, especially food waste. According to FAO, nearly 1.3 billion tons of food including fresh vegetables, fruits, meat, bakery, and dairy products were lost along the food supply chain in 2012 (FAO, 2012).
Because of its complex composition, food waste is very hard to be disposed or utilized. Furthermore, without proper management, it will lend to tremendous harms to our environment and health. Countries all over the world are making great efforts to solve this problem; however, there is still a long way to go.
Food waste (precooked and leftover) comes from various sources, particularly from food production industries and daily household. Annually, developed and developing countries produce massive amounts of food waste (Fig. 1). For example, in China, the most densely populated country, the food waste reaches a striking amount of 195 million tons each year (Fig. 1b), which equals to the food supply for 200,000,000 people each year. Therefore, effectively dispose these huge amounts of food waste has become a must.
Fig. 1 Worldwide Generation of Food Waste in Developed and Developing Countries
Because of its complicated composition, Food waste disposal is much more difficult than ordinary waste management. Unlike other kinds of waste, food waste is composed mostly of water (70%-80%); it contains rich amounts of organic substances, such as sugar, protein, and lipid; salt concentration is also especially high. Although in different parts of the world, the proportion of different components may differ in values, the main components are the same (Fig. 2,3) (Tang et al., 2008; Pan, 2006). These characteristics of food waste make it more likely to decay, produce repulsive smell and corrosive solution, and thus deteriorate the environment.
Fig. 2 Composition of Food Waste in the United States Fig. 3 Composition of Food Waste in China
If not treated appropriately, food waste will give rise to terrible consequences. Such as……
Reckless disposal of food waste will pollute the water and soil.
The rich organic substances in the food waste, once poured into the waterbody, may cause eutrophication (Evangelist et al., 2014; Salemdeeb and Al-Tabbaa, 2015; Whiting and Azapagic, 2014). Rampant algae will destroy the balance of the ecosystem Fish and other animals who originally live in the area will be unable to adapt themselves to the new environment. In the worst-case scenario, the whole population of some species may go extinct.
If the food waste is land-filled, its high salinity may influence the fertility of the soil. The salt in the food waste will accumulate. In the long term, the land may even become alkaline and infertile. Even with careful preservation and treatment, the soil will not recover in the next several decades.
After exposed in air for a long time, the nourishing food waste will attract numerous insects, and harmful bacteria may thus grow inside the food waste. Some bacteria can spread over the air, so when people pass by the pile of food waste, they may unconsciously breathe in the bacteria into respiratory system and get infected.
Moreover, some farmers feed their animals with untreated food waste. In this case, the situation is even worse, because once the animals get infected, they can spread the disease through the food chain. Avian influenza, foot-and-mouth disease, and mad cow disease all partly resulted from the inconsiderate use of animal forage.
As the food waste is rich in organic substances, there is a chance that we can recycle it to produce energy and other resources. If we simply dump or land fill our food waste, we are in fact wasting a huge amount of resources. In our world where energy and resources are at the edge of running out, any nutrition loss is unforgivable (Fig. 4) (Paritosh et al., 2017).
Fig. 4 Worldwide Bioenergy Potential from Food Waste in Developed and Developing Countries
Undoubtedly, it is extremely urgent to figure out a way to properly manage the troublesome food waste!
Focusing on the issue of food waste, our team analyses the current disposal methods, trying to look for the best solution.
Incineration, the combustion of organic substances in the food waste, is a traditional disposal method of the waste. This method can produce electricity by the heat. However, there are many disadvantages.
First of all, the high moisture in food waste makes its calorific value far less than the general calorific value in other kinds of waste (3100kJ/kg compared with 7500kJ/kg) (Min et al., 2016); also, during incineration, the temperature of the combustion has to be over 850℃, to avoid producing cancerogenic substances. Overall, it is not cost-effective.
Furthermore, the incineration will cause air pollution, because some squalid substances like ammonia and hydrogen sulfide are produced.
Landfill operation condenses the waste in a hollow, and then covers the waste by a layer of soils or woodchips. Though this method is easy and cheap, some countries forbid landfill because of its problem. For example, the limited landfill area cannot satisfy the unlimited food waste, and the leachate of the food waste may cause the secondary pollution of soil and water.
Aerobic composting is a process that the microbes absorb, oxidize and decompose the waste in a ventilated and aerobic enough environment. The principle of the aerobic composting is to transfer complex organics into simpler forms which animals can absorb.
However, aerobic composting has strict requirements of moisture and carbon/nitrogen ratio, so it needs large amounts of straws and wood filings to adjust the contents, which are hard to decompose. Furthermore, high osmotic pressure in the food waste affects the efficiency of the composting. Also, the leachate may result in secondary pollutions.
Anaerobic composting is the opposite. Microbes decomposes the organics in food waste in an anaerobic atmosphere. It generally can be divided into three steps: hydrolysis, acid production and methane production. The final product of this method is methane which can be used as an energy resource.
Although the efficiency of the microbes is still limited by the high osmotic pressure, anaerobic composting is in many ways a satisfying approach: it does not have high requirements for moisture or carbon/nitrogen ratio; it produces methane to serve as energy; the space it required is less than its aerobic counterpart required, and the period is also shorter (Table 1)
Table 1 Comparisons among Different Methods of Food Waste Disposal
According to Table 1, we can easily conclude that anaerobic composting is the most ideal solution compared with incineration, landfill, and aerobic composting.
In order to further our research about food waste disposal, we try to analyze the details of anaerobic composting and see what we can do to improve it.
Generally in the waste disposal plants, several types of microbes, for example, yeasts, Lactobaillus, and Bacillus subtilis, function together to exert the most powerful effect.
During our research, we discovered that yeasts have distinct advantages over other microbes.
Anaerobic can be divided into three steps: hydrolysis, fermentation, and methane production (Fig. 5) (Zhao Z., 2009). In the four steps, hydrolysis usually takes the longest time. If we can successfully increase the speed of hydrolysis, we will be able to improve the efficiency of anaerobic composting. Yeasts can serve this function well.
Fig.5 The Process of Anaerobic Composting
From the essay Influence of Yeasts on Anaerobic Fermentation of Kitchen Waste for Producing Acetic Acid written by Zhao Zhenhuan, we discovered that yeasts are able to hydrolyze cellulose, lignin, and other macromolecular substances effectively, and thus improve the efficiency. However, the real problem now occurs for us to solve.
As mentioned before, the salt and sugar concentration in food waste is higher than in other kinds of waste, which causes hypertonic environment for the microbes. The hypertonicity is significant in food waste of Asian countries, with more salt and sugar added to flavor dishes.
During the human practice, our team visited the canteen in China Agriculture University. After receiving the admission from the manager, we collected samples of both solid and liquid leftovers in the canteen. Then we analyzed the salt concentration (one factor of osmotic pressure) in these samples as shown in the following tables.
| Restaurant Sample | 1 | 2 | 3 | 4 | 5 | 6 | average |
| Salinity (%) | 0.64 | 0.32 | 0.51 | 1.43 | 0.32 | 1.68 | 0.82 |
| Cateen Sample | 1 | 2 | 3 | 4 | 5 | average |
| Salinity (%) | 0.58 | 0.72 | 0.51 | 0.72 | 0.84 | 0.67 |
| Family Sample | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | average |
| Salinity (%) | 1.66 | 1.30 | 1.79 | 0.13 | 2.05 | 2.46 | 0.72 | 1.03 | 0.65 | 1.51 | 1.39 |
From the literatures, we discovered that hypertonicity will influence the survivability and decompose efficiency of the microbes, including yeasts. The microbes may die of dehydration, which will definitely decrease the decomposition efficiency, increase the cost and lengthen the duration of food waste disposal. However, during our human practice, we realized that refuse processing plants seldom consider the troublesome issue of osmotic pressure.
Therefore, our team aims to use synthetic biological method to transfer osmotic pressure tolerant gene into the plasmid of yeasts to enhance their resistance to hypertonicity and thus improve the efficiency of food waste disposal.
