The waste management hierarchy suggests that reduce, reuse and recycling should always be given preference in a typical waste management system. However, these options cannot be applied uniformly for all kinds of wastes. For examples, organic waste is quite difficult to deal with using the conventional 3R strategy.
Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer.
There are numerous places which are the sources of large amounts of food waste and hence a proper food-waste management strategy needs to be devised for them to make sure that either they are disposed off in a safe manner or utilized efficiently. These places include hotels, restaurants, malls, residential societies, college/school/office canteens, religious mass cooking places, airline caterers, food and meat processing industries and vegetable markets which generate organic waste of considerable quantum on a daily basis.
Feedstock for the food waste-based biogas plant includes leftover food, vegetable refuse, stale cooked and uncooked food, meat, teabags, napkins, extracted tea powder, milk products etc. Raw waste is shredded to reduce to its particle size to less than 12 mm. The primary aim of shredding is to produce a uniform feed and reduce plant “down-time” due to pipe blockages by large food particles. It also improves mechanical action and digestibility and enables easy removal of any plastic bags or cling-film from waste.
Fresh waste and re-circulated digestate (or digested food waste) are mixed in a mixing tank. The homogenized waste stream is pumped into the feeding tank, from which the anaerobic digestion system is continuously fed. Feeding tank also acts as a pre-digester and subjected to heat at 55-600C to eliminate pathogens and to facilitate the growth of thermophilic microbes for faster degradation of waste.
From the predigestor tank, the slurry enters the main digester where it undergoes anaerobic degradation by a consortium of Archaebacteria belonging to Methanococcus group. The anaerobic digester is a CSTR reactor having average retention time of 15 – 20 days. The digester is operated in the mesophilic temperature range (33 – 38°C), with heating carried out within the digester.
As per conservative estimates, each ton of food waste produces 150 – 200 m3 of biogas, depending on reactor design, process conditions, waste composition etc. Biogas contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its corrosive nature.
Biogas is eventually utilized in a combined heat and power (CHP) unit for its conversion into thermal and electrical energy in a cogeneration power station of suitable capacity. The exhaust gases from the CHP unit are used for meeting process heat requirements. The digested substrate leaving the reactor is rich in nutrients like nitrogen, potassium and phosphorus which are beneficial for plants as well as soil.