Anaerobic Fermentation Process of Kitchen Waste

The anaerobic treatment process of kitchen waste mainly refers to the process in which the pretreated kitchen waste is fermented in a closed container under the action of anaerobic bacteria at a certain temperature through a mature and stable anaerobic fermentation technology to produce biogas, and the biogas is used to generate electricity and heat through a hot spot cogeneration engine. The biogas slurry and biogas residue produced after fermentation can be reused as fertilizer after harmless and resource treatment, thus realizing the reduction and reuse of garbage. Taking the two-phase anaerobic process as an example, the anaerobic fermentation process of kitchen waste mainly includes:

1. Pretreatment of kitchen wastes
2. Hydrolytic acidification
3. Producing biogas
4. Treatment and reuse of biogas slurry, biogas residue and waste oil

1. Pretreatment of kitchen wastes

After the kitchen waste is transported to the treatment site by the collection and transportation vehicle, it is dumped into the feeding bin. Since plastic bags are used to collect garbage in places where food waste is generated, such as restaurants and restaurants, the feeding garbage is first reduced in particle size and broken in bags by a crusher. The crushed garbage particles are different according to different process requirements, and the particle size is usually about 10 ~ 20mm. Food waste collected and transported usually contains a certain amount of interfering substances, such as paper, metal, bone and so on. These substances can not be degraded in the anaerobic fermentation process, so the impurities should be screened by the food waste depackaging machine in the pretreatment stage, and the organic matter content after separation is as high as 99%.

Finally, the separated impurities or organic matters can be subjected to solid-liquid separation. Through the waste dewatering machine, part of solid substances can be separated from the garbage, and only degradable substances enter the reactor, thereby improving the working efficiency of the anaerobic fermentation tank, ensuring the stability of gas production, and further ensuring the efficient and stable operation of the whole anaerobic device.

2. Hydrolytic acidification

The pretreated kitchen waste enters the hydrolysis acidification tank for hydrolysis acidification. Before that, heat exchange equipment can be arranged, so that the temperature of the waste can be raised in the process of pipeline transportation to reach the temperature required by hydrolysis and acidification, thereby avoiding large fluctuation of the temperature in the reactor.

Under the action of water and hydrolytic acidification bacteria in the reactor, the organic garbage is gradually converted from massive and macromolecular organic substances into micromolecular organic acids, and at the same time, gases such as carbon dioxide, hydrogen, hydrogen sulfide and the like are released. The organic acids produced in the hydrolytic acidification stage are mainly acetic acid, propionic acid and butyric acid.Due to the rapid process of hydrolysis and acidification, an acidic environment is quickly formed in the reactor, that is to say, the pH value is decreasing. Although the acid resistance of hydrolytic acidification bacteria is very good, when the pH value is too low, the bacteria will still be inhibited, resulting in low degradation effect.

In order to solve this problem, alkaline substances can be added to the reactor for neutralization, but the addition of alkaline substances will increase salinity, which will have a negative impact on anaerobic fermentation and biogas slurry treatment. In addition, in order to solve the problem of too low pH value, the circulating reflux water with a higher pH value (about 8) can also be used for neutralization. The use of backflow water can partly solve the problem of biogas slurry treatment after fermentation and realize the recycling of substances in anaerobic fermentation plants. At the same time, the use of backflow water can also supplement part of nutrients and rare metals for anaerobic bacteria to use, so as to avoid the decline of activity or even death of bacteria caused by lack of nutrition.

The gas produced in the hydrolysis acidification stage contains hydrogen sulfide, which cannot be directly discharged into the air. After desulfurization treatment, the gas can be directly discharged or used for other purposes.

The temperature of the hydrolytic acidification stage is generally controlled between 25 deg C and 35 deg C, and does not change with the temperature change of the methanogenic stage. Maintaining the temperature in the reactor can be realized by using the heat generated after the biogas hot spot cogeneration.

3. Methane production

Methane-producing stage, also known as gas-producing stage, is the core stage of anaerobic fermentation, and the main products of anaerobic fermentation come from this stage. Therefore, controlling this stage is the key to control the whole anaerobic treatment.

The products in the hydrolytic acidification stage, such as organic acids, hydrogen dissolved in the liquid and carbon dioxide, are transported into the methane-producing tank through pipelines, and the organic acids and gases are further converted into methane gas and carbon dioxide gas in the reactor. Since the hydrogen sulfide has been released in the hydrolytic acidification stage, the output of hydrogen sulfide in the methane-producing stage is very small and almost negligible.

Because the material entering the methane-producing tank is organic acid after hydrolysis and acidification, the reactor can adapt to higher organic load and shorten the residence time of the material. According to the existing experience abroad, the organic loading of the reactor is usually 3 – 4.5 kg oTS/m3.d. Biogas production can be maintained between 700 – 900 L/kg oTS, and the methane concentration in the biogas is between 60% and 75%.

There are many factors affecting the anaerobic fermentation, such as the temperature, pH value in the reactor, the ratio of carbon to nitrogen in the feed waste, etc. These factors directly affect the stability of anaerobic degradation.

4. Treatment and reuse of biogas slurry, biogas residue and waste oil

The remaining products after anaerobic fermentation still have a high moisture content after coming out of the fermentation tank, and can not be directly landfilled, but need to be dehydrated first. Biogas slurry and biogas residue will be produced after centrifugal dehydration of fermentation residues. Biogas slurry and biogas residue are rich in nitrogen, phosphorus, potassium, trace elements and other nutrients needed by plants, and can be used as organic fertilizers.

The grease in kitchen waste is usually separated from the waste by oil-water separation in the pretreatment stage. These oils and fats can be converted into biodiesel or other chemical industrial raw materials together with recycled “gutter oil” and waste edible oil by chemical or biological methods, which can achieve better economic benefits. Through the separation, treatment and utilization of the grease, the invention not only realizes the reutilization of waste resources and generates better economic returns, but also can eliminate the production of the’gutter oil’from the source, so that the’gutter oil’ does not return to the dining table of people, thereby ensuring the food safety and avoiding the harm to the health of people.