What is Anaerobic Digesters?
Anaerobic digesters all serve the same basic purpose. They store manure in the absence of oxygen and provide the ideal environment for methane-forming microorganisms to grow. There are many different types of anaerobic digesters, each performing the same basic function in slightly different ways.
This article describes the most common digesters. Within the major categories, construction and material handling techniques can vary greatly.
Where is the process of anaerobic digestion used?
Anaerobic digestion is used in the processes and systems for biowaste treatment, animal manure treatment, sewage treatment, and biogas generation.
There are several ways to classify anaerobic digesters: by whether the biomass is attached to a surface (attached growth) or can freely mix with the reactor liquid (suspended growth); by the organic loading rate (the influent mass rate of chemical oxygen demand per unit volume); and by whether they are centralized or decentralized.
The majority of anaerobic digesters installed throughout the world use wet-type anaerobic digestion, in which biomass (typically animal dung) and water are mixed in equal volumes to generate a slurry with a total solids (TS) percentage of about 10-15%.
To simplify things, digesters are classified into two types:
Low-Rate Systems: The main source of methane-forming microorganisms is manure flowing through the digester.
High-Rate Systems: To increase efficiency, methane-forming microorganisms are trapped in the digester.
1: Low Rate Systems
Anaerobic digestion is becoming a more popular wastewater treatment technology. Netsol Water Solution specializes in the turnkey design and construction of anaerobic wastewater treatment plants. We have installed a variety of low-rate anaerobic digester systems for various industries including distilleries, breweries, pharmaceuticals, pulp and paper, citric acid, yeast, chemicals, and so on.
These digesters have a large volume (hydraulic retention time: 15-45 days), which improves process stability while producing biogas. Depending on the nature of the wastewater, BOD reductions of 80 percent to 95 percent and COD reductions of 60 percent to 80 percent are possible. These digesters can be built above or below ground in rectangular or circular shapes.
Bacteria decompose organic pollutants (BOD, COD, etc.) to carbon dioxide, methane, and water in the absence of oxygen, i.e. in an anaerobic environment. The large amounts of biogas produced can be used to replace fuel in boilers and to generate electricity in generators, providing our clients with quick returns.
The advantages of this anaerobic system include significant reductions in organic load, resulting in less loading and power consumption in the downstream treatment process. Furthermore, biogas can be used as a renewable non-conventional energy source.
2: High Rate Systems
The high rate anaerobic hybrid digester combines an Up-flow Anaerobic Sludge Blanket (UASB) digester and an Up flow Filter (UFF) digester – the open design plastic media provides a high specific surface area and maximum void space. An acclimatized sludge bed is used to maintain the bottom portion of the anaerobic hybrid digester. The effluent is first passed through this sludge bed, where it is intimately mixed with highly active microorganisms. As a result, a significant portion of the effluent is treated in the sludge bed zone.
The other half of the anaerobic hybrid digester is filled with fixed media, so that after passing through the sludge blanket, the effluent passes through this media, where largely soluble organics are removed. It also ensures that biological solids are retained in the reactor. Effluent recycle pumps are provided to return some of the anaerobic hybrid digester effluent to the digester. Dedicated sludge wasting pumps waste excess sludge from the bottom of the anaerobic hybrid digester. Biogas blowers can be used to remove the biogas produced by the anaerobic hybrid digester.