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Renewable Energy from Food Wastes

Natural Gas Word Cluster

Introduction

Food waste is an untapped energy source which mostly ends up rotting in landfills thereby releasing greenhouse gases into the atmosphere. Food waste is difficult to treat or recycle since it contains high levels of sodium salt and moisture, and is mixed with other waste during collection. Major generators of food wastes include hotels, restaurants, supermarkets, residential blocks, cafeterias, airline caterers, food processing industries etc.

In United States, food waste is the third largest waste stream after paper and yard waste. Around 12.7 percent of the total municipal solid waste (MSW) generated in the year 2008 was food scraps which amounted to about 32 million tons. According to EPA, about 31 million tons of food waste was thrown away into landfills or incinerators in 2008. As far as United Kingdom is concerned, each year 8.3 million tonnes of food is thrown away by households. These statistics are an indication of tremendous amount of food waste generated all over the world.

The proportion of food waste in municipal waste stream is gradually increasing and hence a proper food waste management strategy needs to be devised to ensure its eco-friendly and sustainable disposal. Currently, only about 3% of food waste is recycled throughout USA, mainly through composting. Composting provides an alternative to landfill disposal of food waste, however it requires large areas of land, produces volatile organic compounds and consumes energy. Consequently, there is an urgent need to explore better recycling alternatives. Anaerobic digestion has been successfully used in several European and Asian countries to stabilize food wastes, and to provide beneficial end-products. Sweden, Austria, Denmark, Germany and England have led the way in developing new advanced biogas technologies and setting up new projects for conversion of food waste into energy.

Anaerobic Digestion of Food Waster

Anaerobic digestion is the most important method for the treatment of organic waste because of its techno-economic viability and environmental sustainability. The use of anaerobic digestion technology generates biogas and preserves the nutrients which are recycled back to the agricultural land in the form of slurry or solid fertilizer. The relevance of biogas technology lies in the fact that it makes the best possible utilization of various organic wastes as a renewable source of clean energy. A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. Thus, anaerobic digestion of food waste can lead to climate change mitigation, economic benefits and landfill diversion opportunities.

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. Food waste can either be utilized as a single substrate in a biogas plant, or can be co-digested with organic wastes like cow manure, poultry litter, sewage, crop residues, abattoir wastes etc.

Description of a Typical Energy Conversion Plant

The feedstock for the food waste-to-energy 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 digestate is added to adjust the solids content of the incoming waste stream from 20-25% (in the incoming waste) to the desired solids content of the waste stream entering the digestion system (10 to 12 percent total solids).

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 55-60°C 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. Food waste is highly biodegradable and has much higher volatile solids destruction rate (86-90%) than biosolids or livestock manure. 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. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H2S into elemental sulfur.

The biogas produced as a result of anaerobic digestion of waste is sent to a gas holder for temporary storage. Biogas is eventually utilized in a combined heat and power (CHP) unit for its conversion into thermal and electrical energy in a co-generation 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. The digested slurry is dewatered in a series of screw presses to remove the moisture from slurry. Solar drying and additives are used to enhance the market value and handling characteristics of the fertilizer.

Conclusions

Food waste is one of the single largest constituent of municipal solid waste stream. Diversion of food waste from landfills can provide significant contribution towards climate change mitigation, apart from generating revenues and creating employment opportunities. Rising energy prices and increasing environmental pollution makes it more important to harness renewable energy from food wastes. Anaerobic digestion technology is widely available worldwide and successful projects are already in place in several European as well as Asian countries which makes it imperative on waste generators and environmental agencies to root for a sustainable food waste management system.

Discussions

dennis baker's picture
dennis baker on Feb 28, 2012
The solution to climate change

The primary source of GHG is fossil fuel burning electrical generating facilities. http://dingo.care2.com/pictures/causes/uploads/2012/01/GHG-emitters-2010... 7 Billion humans generate vast quantities of excrement. I believe this excrement is capable of providing all human electrical demands. http://en.wikipedia.org/wiki/Radiolysis Right now hydrogen is perceived as a negative by product, of Nuclear Energy, when it should be the product, as the Pentagon has considered. reference info Request for Information (RFI) on Deployable Reactor Technologies ... DARPA-SN-10-37@darpa.mil https://www.fbo.gov/index?s=opportunity&mode=form&id=d0792af88a6a4484b3a...... Large scale conversions sites are intended to replace fossil fuel powered electrical facilities the Primary Source of Carbon Emissions. http://www.populist.com/99.12.krebs.blob.html In what officials now say was a mistaken strategy to reduce the waste's volume, organic chemicals were added years ago which were being bombarded by radiation fields, resulting in unwanted hydrogen. The hydrogen was then emitted in huge releases that official studies call burps, causing "waste-bergs," chunks of waste floating on the surface, to roll over.

Dennis Baker 106-998 Creston Avenue Penticton BC V2A1P9 cell phone 250-462-3796 Phone / Fax 778-476-2633

Don Hirschberg's picture
Don Hirschberg on Mar 2, 2012
Of course restaurants and food stores should recycle and minimize food wastage. As for me, a mouse could not survive on my wastage. But then I am a child of the Great Depression. My view will soon be archaic and irrelevant. Unknown.

What I observe in restaurants often spoils my meal. I see a family of five. Each orders French Fried potatoes. When they leave there are five helpings of French fries half-eaten. Same at breakfast with sausage or bacon or ham.

The nutritionists say “wonderful” They shouldn’t be eating fired potatoes anyway. The nutritionist scientists have changed their minds so many times in my lifetime that I have little respect for them. I am old. There was almost no obesity among my peers. Yet we had no low calorie options. All soft drinks were loaded with calories and we cooked wit lard.

Amber Rhodes's picture
Amber Rhodes on Aug 14, 2014
I wonder if, through a combined effort of individuals making an effort to waste less food (through their grocery purchases, how much they cook, and what they order at restaurants), and making an effort to reuse what food waste we can't simply reduce (both on an individual and commercial level) - if we could all but eliminate this particular waste stream into our landfills.
Salman Zafar's picture

Thank Salman for the Post!

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