*2.3.2 Anaerobic processes or anaerobic digestion*

Anaerobic treatment of waste is a biological process in which microorganisms degrade organic pollutants without oxygen. When there is no free dissolved oxygen in the wastewater, anaerobic breakdown or putrefaction takes place where anaerobic and facultative bacteria break down complex organic substances into sulfur-based organic molecules, carbon, and nitrogen. This sequence of biochemical events produces biogas such as methane, hydrogen sulfide, ammonia, and nitrogen. This approach minimizes the number of bacteria in wastewater [45–47]. Anaerobic technologies are generally used before aerobic treatment for streams with high organic material (measured as high BOD, COD, or TSS). Anaerobic treatment is a tried-and-tested low-energy way of treating industrial effluent. The following Eq. (4) represents the anaerobic process.

$$\text{Organic matter} + \text{Batteria} \rightarrow \text{CO}\_2 + \text{CH}\_4 + \text{Batteria} + \text{Bypprodants} \tag{4}$$

The anaerobic digestion (AD) approach is appealing because it treats wastewater, provides renewable energy, and generates byproducts that may be utilized as farm fertilizers, making it an environmentally benign process [48]. When compared to the aerobic wastewater treatment process, the AD process offers the following advantages: fewer nutrients required and the creation of less biological sludge, which requires simply drying as further treatment [49]. It also necessitates a small reactor capacity and no oxygen, reducing the power needed to deliver oxygen in the aerobic approach, and the organic loading on the system is not restricted to an oxygen supply. Thus, a higher loading rate can be used in AD, allowing for a faster response to substrate addition after long periods without feeding and semi-feed strategies for a few months. This benefits the system, making AD a viable option for seasonal industrial wastewater treatment and off-gas elimination that causes air pollution. Examples of anaerobic treatment systems

are upflow anaerobic sludge bed (UASB) reactor, expanded granular sludge bed (EGSB), anaerobic baffled reactor (ABR), anaerobic filter reactors and anaerobic Lagoons

### *2.3.2.1 Upflow anaerobic sludge bed (UASB) reactor*

The Upflow anaerobic sludge blanket (UASB) technology is particularly effective for treating wastewater with a high carbohydrate content. As a result, the UASB reactor has become one of the most common designs for treating wastewater from agroindustrial processing companies because it can endure fluctuations in effluent quality and complete reactor shut down during the season [50]. In addition, wastewater containing carbohydrates are readily degraded by bacteria and acts as a nutrient-rich precursor for the anaerobic process. Because of its minimal sludge production and low energy and space requirements, the UASB technique has become well-known for treating wastewater. However, the most significant benefit of this technology is that it can generate energy rather than consume it while treating wastewater [51].

The treated wastewater enters the reactor from the bottom and runs upward through a blanket of biologically activated sludge, typically in granular aggregates. The anaerobic bacteria digest (degrade) the wastewater as it moves upward through the blanket. Under realistic conditions, the blanket is held by the upward flow coupled with gravity's settling action with the support of flocculants and does not wash off, resulting in better treatment efficiency. Intrinsic mixing is facilitated by anaerobic gas production, which aids in the creation and enhancement of biological granules. However, because some of the gas created in the sludge blanket is connected to the granules, a gasliquid-solid separator (GLSS) is added to the reactor's top for effective gas, liquid, and granule separation. In GLSS, gas-enclosed particles collide with the bottom of degassing baffles, fall back into the sludge blanket, and treated water exits the reactor [52].

### *2.3.2.2 Expanded granular sludge bed (EGSB)*

An improved anaerobic treatment system based on an up-flow anaerobic sludge blanket is the expanded granular sludge bed (EGSB). The differentiating feature is that the wastewater passing through the sludge bed has a faster rate of upward flow velocity. In addition, the enhanced flux allows for partial expansion (fluidisation) of the granular sludge bed, boosting wastewater-sludge interaction and enhancing sludge bed segregation of small inactive, suspended particles
