*2.3.2.3 Anaerobic baffled reactor (ABR)*

McCarty and colleagues created the anaerobic baffled reactor (ABR) at Stanford University in the early 1980s. It is a simple linear reactor with a simple operational design that has widespread use in wastewater treatment. The ABR primarily treats wastewater through sludge and scum retention as well as anaerobic degradation of particulate and dissolvable organic substances. As a result, any factors impacting these processes impact ABR treatment. Baffles guide the flow within the reactor in an ABR reactor under the force of the pressure head at the influent. There is no need for mechanical mixing because the flow directly touches the biomass as it is driven through the sludge bed. As a result, no electricity is required during regular operation for an underground ABR design, while ABR above ground design necessitates pumping energy. In ABR, byproduct sludge is recirculated, discharged, or used as manure.

### *Available Technologies for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.103661*

According to Reynaud and Buckley [53], a long solid retention time is required for anaerobic treatment of low-strength wastewater, and the required reactor capacity is influenced by the hydraulic load instead of the organic load. The upflow velocity of the wastewater inside the reactor compartments containing sludge influences solid retention in the ABR design. Low-strength applications, on the other hand, have negligible solid flotation as well as carry-over due to gas production.

### *2.3.2.4 Anaerobic filter reactors*

In 1969, Young and McCarty invented the upflow anaerobic filter. An anaerobic filter was the first high-rate bioreactor that excluded the separation and effluent recycling requirement. In addition, it offers the advantages of eliminating the mechanical mixing stage, having improved stability even at loading rates higher than 10 kg/m3 day COD, enduring hazardous shock loads, and being inhibitor-resistant. Because the upflow anaerobic filter is loaded with inert support material such as gravel, pebbles, coke, or plastic media, it works similarly to an aerobic trickling filter. As a result, there is no need for biomass separation or sludge recycling in the system. The reactor's designation is to trap particles in the wastewater as it runs through it, while active biomass connected to the surface of the filter material degrades the organic matter [43]. The anaerobic filter reactor can be used as a downflow or upflow filter reactor, with an OLR range from 1 kg/m3 to 15 kg/m3 day COD and separation efficiencies ranging from 75 to 95%. The treatment temperature ranges from 20 to 35.8°C, with HRTs varying from 0.2 to 3 days. The main disadvantage of the upflow anaerobic filter is the possibility of blockage due to undegraded sewage sludge, mineral precipitates, or bacterial biomass [43].

### *2.3.2.5 Anaerobic lagoons*

An anaerobic lagoon is a deep earthen basin with enough volume to allow sedimentation of sedimentable solids, digestion of residual sludge, and anaerobic reduction of some soluble organic substrate [54]. Anaerobic lagoons are typically designed to store and treat wastewater for 20–150 days. They're deep (normally 8–15 feet) and function similarly to septic tanks, where anaerobic microorganisms break down contaminants in the absence of oxygen. Solids in wastewater segregate and settle into strata inside an anaerobic lagoon. Grease, scum, and other floating debris make up the top layer. The layer of sludge that settles at the bottom of an anaerobic lagoon gradually accumulates and must be removed if septic tanks are not used first. The effluent from an anaerobic lagoon will need to be treated further [55].

### *2.3.3 Bioremediation*

Bioremediation is a biological treatment process that uses biological resources to convert environmental pollutants into less hazardous forms. For example, the innate ability of microorganisms, plants, bacteria, algae or fungi to survive, adapt and thrive in unseemingly harsh conditions has been exploited to treat contaminated water bodies or soils. Like any other biological treatment process, bioremediation is preferred because it does not require chemicals or a lot of energy. This technology can be applied both in-situ (on-site) or ex-situ; for example, the wastewater can be treated on-site where the pollution takes place or transported to an external site for proper manipulation of the operating condition if it cannot be achieved at the contaminated

site. Bioremediation can occur in either aerobic or anaerobic environments. Living organisms require ambient oxygen to thrive in aerobic environments. There is no oxygen in anaerobic situations. Microbes in this situation decompose chemical molecules or ions like sulfates in the wastewater to obtain the required energy [56].

Bioremediation is broadly classified into the following;


Microbial remediation and mycoremediation can be classified further based on the strategy used as bioattenuation (natural attenuation), biostimulation (use of organic or inorganic nutrients for remediation), and bioaugmentation (use of genetically engineered microbe).
