**2.1 Anaerobic digestion (AD)**

Anaerobic digestion (AD) is a familiar method in which biodegradable constituents of Sewage are broken down with the help of micro-organisms in the absence of oxygen. This can be carried out in a single stage or multiple stages. This not only leads to decreased organic loads but also will simultaneously produce biogas. The process takes place under temperatures of about 30 to 38°C, where mesophilic digestion can

#### *Sewage Treatment Using Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.109407*

take place or about 49–57°C where thermophilic digestion takes place [1]. Anaerobic Digestion is usually carried out in three stages [2]:


Sewage s, as well as the Sludge coming out from the last stage generally is found to be rich in organic matter and thus can be used to produce energy in the form of biogas. Biogas obtained from anaerobic digestion plants can be utilized for various purposes like heating, production of electricity etc. This energy utilization makes the anaerobic digesters a versatile piece of equipment as it sustains their energy and heat requirements without the need for looking out for other sources of external energy requirements [3].

#### **2.2 Photo-fermentation**

In this method anaerobic micro-organisms which are photosynthetic like Rhodobacter and *Rhodopseudomonas* act as catalysts and convert organic acids, such as acetic and butyric acids, and sugars into glucose, fructose and sucrose in the presence of sunlight. There is a similar called as dark fermentation which is mostly carried out in the absence of sunlight where anaerobic bacteria, such as *Clostridium* and *Enterobacter*, convert glucose, sucrose, starch and cellulosic materials to H2 [4]. But for the above-mentioned processes Sewage s containing huge amounts of carbohydrate content is required to produce adequate amounts of hydrogen typically 15% of the maximum theoretical potential. Due to this constraint, these processes have yet to emerge on a large industrial scale.

#### **2.3 Microbial fuel cells**

Microbial fuel cells (MFCs) are showing promising results in Sewage treatment in recent years as these are sustainable technologies which can accomplish the removal of organic pollutants with the help of micro-organisms and also produce electricity, water, CO2 and other inorganic residues as by-products [5–7]. MFCs come under the category of bioreactors and are usually operated under anaerobic conditions. A positively charged ion membrane separates both the anode and cathode. Reactions at the anode include oxidation of organic materials present in Sewage by microorganisms, therefore generating CO2, electrons and protons. These electrons are then relocated to the cathode compartment via an external electric circuit, producing electricity, whereas protons are moved to the cathode through the membrane. During this process, water is also formed as electrons and protons combine with oxygen in the cathode [8, 9]. MFCs offer the benefits of low cost, as they utilize inexpensive catalysts, that is, microorganisms present in Sewage s, huge energy efficiencies, and low volume of solid disposal.
