Any treatment process that uses anaerobes to remove unwanted organic or inorganic compounds is known as anaerobic treatment. 
Anaerobe is a micro-organism that needs no free oxygen to develop. 
The first anaerobic treatment plant for industrial wastewater was built in 1929 for the treatment of wastewater from yeast production in Slagelse, Denmark. Even if this plant was in operation for almost 30 years, the development went very slowly. The development of this process did not gather momentum until the Dutch UASB plant type was introduced in 1980. 
Anaerobic microorganisms do not require DO in the water to function. They obtain their oxygen requirement from the oxygen chemically contained in organic materials. Anaerobic decomposition involves two separate but interrelated steps. First, the acid-producing bacteria decompose the dissolved organic waste to organic acids, such as acetic, propionic, and butyric acid. The organic acids are then further decomposed by methane producing bacteria to the end products of methane, carbon dioxide, and water. Effective operation requires a balance between acid production and breakdown because methane producers are sensitive to the concentration of volatile acids. 
Anaerobic treatment applies to both wastewater treatment and sludge digestion. 
Anaerobic treatment applied to wastewater treatment:
Anaerobic wastewater treatment is an effective biological method for treating many organic wastes. The microbiology involved in the process includes facultative and anaerobic microorganisms, which, in the absence of oxygen, convert organic materials into gaseous end products such as carbon dioxide and methane. Anaerobic wastewater treatment was discovered in the middle of the last century; however, environmental engineers have only seriously considered it in the last twenty years. Despite intense research in this field in the past few decades, much research is still needed in several areas. These areas include:
- Microbiology: Further research on the biochemistry and genetics related to the anaerobic microbial species is required.
- Startup procedures: Optimal procedures to minimize the lag time between the commissioning of a reactor and its placement into full operation must be investigated.
- Optimization of process engineering: Further optimization of the anaerobic treatment process is required, especially involving ancillary equipment, small-scale reactors, and support media (where applicable). 
The end products of anaerobic degradation are gases, mostly methane (CH4), carbon dioxide (CO2), and small quantities of hydrogen sulfide (H2S) and hydrogen (H2).
The process involves two distinct stages:
- acid fermentation
- methane fermentation
In acid fermentation, the extracellular enzymes of a group of heterogenous and anaerobic bacteria hydrolyze complex organic waste components (proteins, lipids, and carbohydrates) to yield small soluble products. These simple, soluble compounds (e.g., triglycerides, fatty acids, amino acids, and sugars) are further subjected, by the bacteria, to fermentation, b-oxidations, and other metabolic processes that lead to the formation of simple organic compounds, mainly short-chain (volatile) acids (e.g., acetic [CH3COOH], propionic [CH3CH2COOH], butyric [CH3-CH2-CH2-COOH]) and alcohols. In the acid fermentation stage, no COD or BOD reduction is realized since this stage merely converts complex organic molecules to shortchain fatty acids, alcohols, and new bacterial cells, which exert an oxygen demand.
In the second stage, short-chain fatty acids (other than acetate) are converted to acetate, hydrogen gas, and carbon dioxide—a process referred to as acetogenesis. Subsequently, several species of strictly anaerobic bacteria bring about methanogenesis—a process in which hydrogen produces methane from acetate and carbon dioxide reduction. In this stage, the stabilization of the organic material truly occurs. Figure 1 shows the two stages of anaerobic treatment as sequential processes; however, both stages occur simultaneously and synchronously in an active, well-buffered system.
Figure 1: Reaction pathways of anaerobic treatment of complex organic matter. 
The main concern of a wastewater treatment facility in operating an anaerobic system is that the various bacterial species function in a balanced and sequential way. Hence, although other types of microorganisms may be present in the reactors, attention is focused mostly on the bacteria.
The major groupings of bacteria, as numbered in Figure 1, and the reactions they mediate are as follows (Pavlostathis and Giraldo-Gomez 1991):
(1) fermentative bacteria,
(2) hydrogen-producing acetogenic bacteria,
(3) hydrogen-consuming acetogenic bacteria,
(4) carbon-dioxide-reducing methanogens, and
(5) aceticlastic methanogens.
Two common genera of aceticlastic methanogens are Methanothrix and Methanosarcina; and species from the Methanobacterium group are commonly known to produce methane by hydrogen reduction of carbon dioxide.
The anaerobic wastewater treatment processes discussed in this section include the anaerobic contact process, the USB reactor, the anaerobic filter, and the AFBR.
Anaerobic contact process:
The anaerobic contact process is a suspended-growth process, similar in design to the activated-sludge process except that anaerobic conditions prevail in the former process. Figure 2 shows the process schematic. The anaerobic contact process is comprised of two parts. The contact part involves thorough mixing of the wastewater influent with a well-developed anaerobic sludge culture. The separation part involves the settling out of anaerobic sludge from the treated wastewater and recycling back to the contact reactor. The process usually has a vacuum degasifier placed following the aerobic reactor to eliminate gas bubbles that cause SS in the clarifier to float. BIOENERGYand ANAMET are two commercially available, proprietary anaerobic contact processes.
BIOENERGY is a conventional anaerobic contact process that uses a thermal shock procedure to facilitate sludge separation. As the mixed liquor, at 35°C, flows from the contact reactor to the settling unit, a series of heat exchangers rapidly decreases its temperature to 25°C. This temporarily interrupts gasification allowing effective sludge–solids separation by gravity. The temperature of the recycled sludge is increased before it is returned to the contact unit.
In the ANAMET process, an aerobic biological treatment polishing step follows the anaerobic contact process to provide near-complete organics removal. The process recycles the sludge produced in the aerobic treatment process back to the anaerobic reactor to reduce excess sludge production across the entire system and increase biogas yield. Also, recirculation of the nutrient-containing sludge from the aerobic reactor reduces external nutrient requirements in the anaerobic reactor.
Figure 2: Anaerobic contact process 
USB reactor is essentially a suspended-growth reactor, but it is also a fixed-biomass process. Figure 3 shows the process schematic. This USB system is based on the development of a sludge blanket. In this sludge blanket, the component particles are aggregated to withstand the hydraulic shear of the upwardly flowing wastewater without being carried upwards and out of the reactor. The sludge flocs must be structurally stable so that hydraulic shear forces do not break them into smaller portions that can be washed out, and they should also have good settlement properties. The wastewater is fed at the bottom of the reactor, and active anaerobic sludge solids convert the organics into methane and carbon dioxide. The anaerobic biomass is distributed over the sludge blanket and a granular sludge bed. The sludge solids concentration in the sludge bed is high—100,000 mg/l SS—and does not vary over a range of process conditions. The sludge solids concentration in the sludge blanket is lower and depends on process conditions. The reactor can include an internal baffle system, usually referred to as a gas–liquid separator, above the sludge blanket to separate the biogas, sludge, and liquid. A patented USB reactor called the BIOTHANE process was developed by the Biothane Corporation in the United States.
Figure 3: Upflow sludge blanket reactor 
In an anaerobic filter reactor, the growth-supporting media is submerged in the wastewater. Anaerobic microorganisms grow on the media surface as well as inside the void spaces among the media particles. The media entraps the SS present in the influent wastewater that can be fed into the reactor from the bottom (upflow filter) or the top (downflow filter) as shown in the process schematics in Figure 4. Thus, the flow patterns in the filter can be either PF or completely mixed depending on recirculation magnitude. Periodically backwashing the filter solves bed-clogging and high-head-loss problems caused by the accumulation of biological and inert solids. BACARDI and CELROBIC are two proprietary anaerobic filter processes currently available.
Figure 4: Anaerobic filter 
The AFBR is an expanded-bed reactor that retains media suspension from drag forces exerted by upflowing wastewater. Figure 5 shows the process schematic. Fluidization of the media particles provides a large surface area where biofilm formation and growth can occur.
The media particles have a high density resulting in a settling velocity that is high enough so that high-liquid-velocity conditions can be maintained in the reactor. However, the media particles’ overall density decreases as biomass growth accumulates on the surface area. The decrease in density can cause the bioparticles to rise and be washed out of the reactor. To prevent this situation, the reactor controls fluidized-bed height at a required level by wasting a corresponding amount of overgrown bioparticles. The wasted bioparticles can then be received by a mechanical device that separates the biomass from the wasted media particles. The cleaned particles can then be returned to the reactor, while the separated biomass is wasted as sludge.
The AFBR combines a suspended-growth system and an attached-growth system since biomass growth attaches to the media particles which are suspended in the wastewater. The reactor recycles a portion of the effluent flow ensuring uniform bed fluidization and sufficient substrate loading.
Some commercially available AFBR processes include the ANITRON system developed by Dorr-Oliver, Inc.; the BIOJET process, which employs an AFBR with an enlarged top section; and the ENSO-FENOX process, which combines an AFBR with a trickling filter. The AFBR has been applied to a variety of industrial treatment processes with substrates such as molasses, synthetic sucrose, sweet whey, whey permeate, glucose, and acid whey.
Figure 5: Anaerobic fluidized bed reactor 
The major advantages of anaerobic treatment over aerobic treatment are as follows:
- The biomass yield for anaerobic processes is much lower than that for aerobic systems; thus, less biomass is produced per unit of organic material used. This reduced biomass means savings in excess sludge handling and disposal and lower nitrogen and phosphorus requirements.
- Since aeration is not required, capital costs and power consumption are lower.
- Methane gas produced in anaerobic processes provides an economically valuable end product.
- The savings from lower sludge production, electricity conservation, and methane production range from $0.20 to $0.50 per 1000 gal of domestic sewage treatment. The reduction of sludge and aeration energy consumption each result in savings that are greater than the cost of the energy required by the anaerobic process. In addition, a substantial part of the energy requirements for anaerobic processes can be obtained from exhaust gas.
- Higher influent organic loading is possible for anaerobic systems than for aerobic systems because the anaerobic process is not limited by the oxygen transfer capability at high-oxygen utilization rates in aerobic processes. 
Some disadvantages are associated with the anaerobic process as follows:
- Energy is required by elevated reactor temperatures to maintain microbial activity at a practical rate. (Generally, the optimum temperature for anaerobic processes is 35°C.) This disadvantage is not serious if the methane gas produced by the process can supply the heat energy.
- Higher detention times are required for anaerobic processes than aerobic treatment. Thus, an economical treatment time can result in incomplete organic stabilization.
- Undesirable odors are produced in anaerobic processes due to the production of H2S gas and mercaptans. This limitation can be a problem in urban areas.
- Anaerobic biomass settling in the secondary clarifier is more difficult to treat than biomass sedimentation in the activated-sludge process. Therefore, the capital costs associated with clarification are higher.
- Operating anaerobic reactors is not as easy as aerobic units. Moreover, the anaerobic process is more sensitive to shock loads. 
COST INVOLVED IN ANAEROBIC TREATMENT:
Since aeration is not required, capital costs and power consumption are lower. Methane gas produced in anaerobic processes provides an economically valuable end product. The savings from lower sludge production, electricity conservation, and methane production range from $0.20 to $0.50 per 1000 gal of domestic sewage treatment. The reduction of sludge and aeration energy consumption each result in savings that are greater than the cost of the energy required by the anaerobic process. In addition, a substantial part of the energy requirements for anaerobic processes can be obtained from exhaust gas. 
- Dictionary of Water and Waste Management, Elsevier
- Environmental Engineers’ Handbook
- Wastewater treatment: biological and chemical processes By M. Henze
- Environmental Bioengineering By Lawrence K. Wang, Joo-Hwa Tay, Stephen Tiong Lee Tay
- Biological wastewater treatment: principles, modelling and design By M. Henze