Food Industry

Food industry can be classified into a number of industries such as fruit and vegetable industry, vegetable oil industry, dairy industry, canning industry, beverage industry, meat industry, and so on. Most of these industries typically generate large volumes of effluents and solid waste. The main solid wastes generated from food industry are organic wastes, including discarded seeds, fruits, and vegetables. Odor problems can occur with poor management of solid wastes and effluents, when onions are processed, and when ready-to-serve meals are prepared, for example. So, it is very important to deal with such organic waste with full understanding of the processes, requirements, and hygiene.

Food waste recycling can take place through aerobic fermentation (composting) or anaerobic fermentation (biogas) processing. Composting is a process that involves biological decomposition of organic matter, under controlled conditions, into soil conditioner or organic fertilizer through aerobic fermentation.10 While anaerobic fermentation process involves biological decomposition of organic waste under controlled conditions to produce fertilizer and biogas.11

Aerobic Fermentation Process

Aerobic fermentation is the decomposition of organic material in the presence of air. During the composting process, microorganisms consume oxygen, while CO2, water, and heat are released as result of microbial activity, as shown in Figure 1.21. Four main factors control the composting process: moisture content, nutrition (carbon: nitrogen ratio), temperature, and oxygen (aeration).

FIGURE 1.21 Composting process.

1. Moisture content. The ideal percentage of the moisture content is 60 percent.10 The initial moisture content should range from 40 to 60 percent, depending on the components of the mixture. If the moisture content decreases less than 40 percent, microbial activity slows down and became dormant. If the moisture content increases above 60 percent, decomposition slows down and odor from anaerobic decomposition is emitted.

2. Carbon to nitrogen ratio. Microorganisms responsible for the decomposition of organic matter require carbon and nitrogen as a nutrient to grow and reproduce. Microbes work actively if carbon to nitrogen ratio is 30:1. If the carbon to nitrogen ratio exceeds 30, the rate of composting decreases. Decomposition of the organic waste material will slow down if C:N ratios are as low as 10:1 or as high as 50:1.

However, in order to compensate the low nitrogen content of organic waste, nitrogen should be added to obtain more effective compost. To increase the C:N ratio in rice straw as an agricultural waste, for example, these techniques may be implemented:

• Chemical additives: This method could be done by the addition of either uric acid, or urea or ammonia.

• Natural additives (manure): In this technique, nitrogen is obtained from animal or poultry manure.

3. Temperature. The activity of bacteria and other microorganisms produce heat while decomposing (oxidize) organic material. The ideal temperature range within the compost to be efficient varies from 32°C to 60°C. If the temperature is outside this range, the activity of the microorganisms slows down, or might be destroyed. A temperature above 55°C while composting kills the weeds, ailing microbes, and diseases, including shengella and salmonella; this help to reduce the risk of diseases' transmission from infected and contaminated organic wastes.

4. Oxygen (aeration). A continuous supply of oxygen through aeration is a must to guarantee aerobic fermentation (decomposition). Proper aeration is needed to control the environment required for biological reactions and to achieve the optimum efficiency. Different techniques can be used to perform the required aeration according to the composting techniques. The most common types of composting techniques are natural composting, passive composting, forced composting, and vermi composting.

Natural Composting Piles of compost are formed along parallel rows, as shown in Figure 1.22 and are continuously moisturized and turned. The distance between rows can be determined according to the type and dimension of the turning machine.12 Piles should be turned about three times a week at summer and once a week at winter to aerate the pile and achieve homogenous temperature and aeration throughout the pile. This method needs large surfaces of lands, many workers, and running cost.

FIGURE 1.22 Natural composting process. (Source: F.R. Gouin et al., "On-Farm Composting," Northeast Regional Agricultural Engineering Services, NRAES-54, Cooperative Extension, 1993.)

Passive Composting Parallel rows of perforated high-pressure PVC piping are placed at the bottom, on which compost is added above it. The pipes are perforated with 10 cm holes to allow air to enter the composting piles, as shown in Figure 1.23. The pipe manifold helps in distributing the air uniformly. Air flows through the ends of the pipes to the compost. This system is better than the natural system because of the limited flow rates induced by the natural ventilation. This method needs limited surfaces of lands, less running cost, and does not need skilled workers. This method is recommended for cost effectiveness; it is the most economic aeration method. Therefore, it is the most suitable method for developing communities that want to achieve maximum benefit from the food recycling with the minimum capital investment and a good-quality soil conditioner. The soil conditioner can be converted into organic fertilizer by adjusting the NPK ratio (nitrogen: phosphorous: potassium) through additives.1314

Forced Aeration Forced aeration works like the previous system except that the ends of plastic pipes are connected to blowers that force (or suck) the air through the compost with a specific rate and velocity. Otherwise, if the air rate exceeded a certain limit, the temperature inside the compost pile would decrease, affecting the microbial activity. Also, the air velocity during the day should always be higher than at night. This system needs higher technology with air velocity control and more energy consumption. That is why it is less economic compared to the other two systems and it is not recommended for rural or developing countries that want to make profit out of all recycling processes. This method needs capital investment, skilled workers, and running cost.

FIGURE 1.23 Passive aeration process. (Source: F.R. Gouin et al., "On-Farm Composting," Northeast Regional Agricultural Engineering Services, NRAES-54, Cooperative Extension, 1993.)

Vermi Composting It is an ecologically safe and economic method that depends on the worms' characteristic of transforming the organic wastes to fertilizers that are extremely beneficial to earth. There are two types of earthworms that are used due to their insensitivity to environmental changes:

1. The red wiggler (Eisensia Foetida)

2. The red worm (Lumbricus Rebellus)

Under suitable aeration, humidity, and temperature, worms feed on organic wastes and expel their manure (worm castings) that breaks up the soil, providing it with aeration and drainage. It also creates an organic soil conditioner as well as a natural fertilizer. Worm castings have more nutrients than soil conditioner in terms of nitrogen and phosphorous.

A mature worm will produce a cocoon every 8 to 10 days that contains an average of eight baby worms that mature in approximately 70 days, and in one year each 1,000 worms produce 1,000,000 worms.15

Vermi composting can be used in houses easily by using a special container (worm bin) that can be placed anywhere that is not subjected to light such as kitchen, garage, and basement. The organic waste is put in this container, and the worms with them. The worms are odorless and free from disease.

Anaerobic Fermentation Process

Biogas conversion is the anaerobic fermentation of organic matter (organic waste) by microbiological organisms under controlled conditions. The aim of fermentation is to produce methane (biogas) that can be used as an energy source. The fermentation process is done anaerobically—that is, without the presence of air—to allow the bacteria to perform the breakdown. The byproduct of fermentation consists of about 60 percent CH4 and 40 percent CO2, along with traces of H2, N2, and H2S. Biogas is produced by means of a digester, which is a device used to process organic waste and produce methane. There are many types of digesters available; however, the two most famous designs are the Chinese fixed dome (constant volume) and the Indian floating cover (constant pressure). A combination of both could also be designed.

Chinese Fixed Dome The Chinese fixed-dome design is one of the oldest digester designs dating back to the 1930s. It consists of an underground fermentation chamber made of bricks and a dome-shaped tank on top of the chamber. The biomass mixture is entered through the inlet and fermentation occurs, with the gas rising to fill the tank and the slurry exiting through the outlet. This design combines the digester with the holding tank where the gas is stored. The gas then passes through an outlet pipe at the top, as shown in Figure 1.24.

FIGURE 1.24 Chinese fixed-dome digester. (Source: Matthias Plochl and Monika Heiermann, "Biogas Farming in Central and Northern Europe: A Strategy for Developing Countries?" 2006,

FIGURE 1.25 Indian floating cover. (Source: Matthias Plochl and Monika Heiermann, "Biogas Farming in Central and Northern Europe: A Strategy for Developing Countries?" 2006,

FIGURE 1.25 Indian floating cover. (Source: Matthias Plochl and Monika Heiermann, "Biogas Farming in Central and Northern Europe: A Strategy for Developing Countries?" 2006,

Indian Floating Cover The Indian floating-cover design is shown in Figure 1.25. This design was first presented as the Janata design but was further developed in 1984 to the Deenbandhu model, with both models based on the Chinese dome design. This model employs the same principles as the Chinese fixed-dome digester, with the biomass mix entering through the inlet and decomposed in the underground brick chamber. However, in this design the cylindrical gas tank is a floating cover that is separate from the mixing tank. The slurry left from the fermentation process is used as a fertilizer. The mixing process should occur at a high temperature range (30° to 40° or 50° to 60°C), and could take up to two months, depending on the quantity of biomass processed.16

TABLE 1.4 Uses of Biogas as Energy Sources


1 m3 Biogas Equivalent


Equal to 60 to 100 W bulb for 6 hours


Can cook 3 meals for a family of 5 to 6

Fuel replacement

0.7 kg of petrol

Shaft power

Can run a 1 horsepower motor for 2 hours

Electricity generation

Can generate 1.25 kilowatt hours of electricity

Source: Practical Action. "Biogas And Liquid Biofuels," 2006. information_service/biogas_liquid_fuels.pdf.

Source: Practical Action. "Biogas And Liquid Biofuels," 2006. information_service/biogas_liquid_fuels.pdf.

Biogas can be used in several applications as an energy source, ranging from light bulbs to internal combustion engines. It has an energy content of about 5,000 Kcal/m3. Some of the uses of biogas as an energy source are shown in Table 1.4.

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