*6.4. Multilateral Approach*

In the hybrid approach, more than one useful product is produced in a single process, for example, integrated hydrogen and methane production, this leads to recover maximum energy from wastewater [96]. MFC coupled with anaerobic membrane bioreactor and integrated photo-bioreactor is a game changer for this energy recovery from the wastewater [66,69]. This technique is capable of treating high levels of wastewater along with polishing treatment for removal of specific pollutant types from the e ffluent.

#### *6.5. Use of Genetically Modified Microorganisms*

Energy generation by wastewater treatment can also be improved by selection of the best microbial species responsible for specific biofuel production and specific pollutant removal; this will lead to reducing the operation costs and energy consumption for the process [62,97].

#### *6.6. Hydrothermal Carbonization of Sewage Sludge*

Hydrothermal carbonization (HTC) is a thermochemical process that can be used as a solid fuel or soil conditioner to transform liquid biomass into so-called biocoal. HTC can be an alternative to anaerobic digestion or a supplement in sewage sludge treatment. In the latter case, digested sludge serves as feedstock to HTC [98]. In 2010, the first industrial plant in Germany (Karlsruhe) was built in a WWTP. HTC's benefit is almost complete recycling of organic matter, very strong dewaterability of the resulting sludge, and an increased energy balance [99]. HTC is a recent, on the market technology, although the process has been known for over a hundred years. It is conceivable that HTC will develop into a standard sludge treatment technology in the future.
