DAUDA Sada Abba， YU Feng， ZHOU Yun

(1.University of Maiduguri，Maiduguri 1069，Nigeria;2.Shenyang University of Chemical Technology，Shenyang 110142，China;3.Shenyang Research＆ Design Institute of Environmental Protection Engineering，Shenyang 110003，China;4.College of Natural Resource and Environment Engineering，Wuhan University of Science and Technology，Wuhan 430081，China)

The dry fermentation of wastes is a process that has already been implemented successfully for a number of years.Particularly since the amendment of the Renewable Energy Sources Act(EEG)in Germany in August 2004 and the resulting granting of the technology bonus for dry fermentation plants，the established techniques used by the waste disposal industry have been adapted to suit the numerous applications connected with the fermentation of energy crops［1］.Although the technology bonus for dry fermentation plants is being dropped due to the latestamendment(2009)，the advantages with regard to reactor volume，the choice of substrate and susceptibility to impurities still remain.

The dry fermentation process is a single stage batch process for biogas generation from biomass with high dry matter content.The organic matter is inoculated with substrate that has already been fermented.It is then filled into the digester and fermented under airtight conditions.Continual inoculation with bacterial matter occurs through the recirculation of percolation liquid，which is sprayed over the organic matter in the digestion.Stirring of the organic matter is not necessary during the dry fermentation process，which is essential in conventional wet fermentation systems［2-3］.

The temperatures of the process and of the percolation liquid are regulated by a built-in floor heating system in the digestion and a heat exchanger in the tank that acts as a reservoir for the percolation liquid.

The different stages of degradation(i.e.hydrolysis，acid genesis and methane genesis process)take place in the same digester，which has a lot of advantages in comparison to other systems.Usually considerably more expense is involved in the respective processes concerning the mechanical technology required，which in turn has an adverse effect on managing the energy consumption and maintenance costs and makes the a lot cheaper.During the fermentation process，no further mixing，pumping or stirring is necessary inside the digester.Therefore，dry fermentation technology requires only a low input with respect to the process and the mechanical demands.Contrastingly，conventional liquid fermentation systems require a large amount of processing the energy during operation.The highly specialized and unique process technology concerning dry fermentation requires an extremely low energy input.

Anaerobic digestion is an attractive waste treatment strategy by microbial consortia in an oxygen free environment to produce methane and carbon dioxide and reclaim nutrient rich fertilizer［4-5］.During the digestion process，complex organic macromolecules are first hydrolyzed and fermented in simpler soluble molecules which are then converted by acid forming bacteria into volatile fatty acids，carbon dioxide and hydrogen.Biogas usually contains 50%～70%methane and 30%～40%carbon dioxide along with trace amounts of hydrogen sulfide，ammonia and hydrogen.

The rapid increase and accumulation of solid waste caused serious health and environmental problems.As uncontrolled decomposition of wastes generate epidemic diseases such as cholera，diarrhea and bad odor which can cause serious stomachache.Land filling also constitutes very huge environmental problems such as greenhouse effect，underground and surface water pollution，and bad odor which also pollute the air.Incineration for energy recovery can be a costly capital investment and pose societal and environmental health risks.With regards to composing to obtain high quality compost，would produce greenhouse gases and no energy recovery.

The treatment of manure and other organic wastes by wet fermentation for biogas［6］production has been widely used but the high dilution rate would increase digester size and also lead handling problems of effluent.In other hand，the dry fermentation process stabilizes the manure and wastes in its produced form.Therefore，the production of biogas through dry digestion offers significant advantages over other forms of wastes treatment processes.The study was carried out to review optimization techniques of dry fermentation process for potential energy recovery and sustainable waste management and suggested possible areas where improvements could be made，including the reactor configuration，mixing，substrates，pretreatment and environmental conditions within the diseases.

1 Dry Fermentation

AD systems are quickly arising as a method to efficiently utilize and manage non-liquid organic wastes［7］.Dry systems can use input organic material that has much higher total solids content of up to 50%(i.e.，if the material is stackable).The organic fraction of MSW makes an ideal input for a dry fermentation system.

Unlike wet systems，dry fermentation plants are designed around the principle that microorganisms are more easily moved than a large amount of material.To facilitate digestion，a solution containing the necessary microorganisms is percolated through the mass of waste by the forces of gravity.This allows the organic input to remain stationary for the digestion retention time while the needed biochemical interactions still occur.

Because the mass says stationary，the overall structure of a dry fermentation plant is very different than a wet plant.BIOFerm dry fermentation plants，for example，are modular and consist of garage-like fermentation chambers into which organic material is filled by front loaders.There are no moving parts inside the fermentation chamber.Solution is sprayed over the organic material and collected as it seeps through to be recycled within the system.After digestion，remaining material is removed from the fermentation chamber and can be used as a soil amendment or further aerobically composted.

Dry fermentation offers many advantages for the processing of the organic fraction of MSW.Because material does not require movement or pumping in a dry AD plant，less pre-processing of the input materials is necessary［8］.Input material does not need to be ground，diluted with water，nor even have the contaminants removed.Expensive water input needs and waste water treatment requirements do not have to be dealt with in the sensitive municipality atmosphere.Because there are no moving parts inside the fermentation chamber of a dry system，non-organic pollutants，like plastic bags，do not pose a hazard.Contrastingly，a plastic bag in a wet fermentation system can catch on the agitation mechanism and cause failure.The absence of mechanical parts in the fermentation chamber，and fewer mechanical parts in the dry system overall，reduces overall maintenance time and costs.These characteristics also lead to lower parasitic energy values of the system when compared to wet fermentation plants.A dry system also requires less processing of the digestive after digestion，generating an output than can directly be taken to a composting operation.All of these aspects present great savings of energy，resources and money and create an optimal situation for processing the organic fraction of municipal solid waste.

When considering the use of an AD system for the processing ofthe organic fraction of MSW，selection of the appropriate type is a major concern.Using a dry fermentation system minimizes processing costs，both prior to and after digestion，and the use of water and other resources within the system itself.This allows for the most efficient and productive recovery ofresources within the organic material.

Many private companies and some government agencies from different parts of the world are initiated with a view to converting solid waste to energy through dry fermentation process.An example of these companies or agencies is Berkon Energy Technologies GmbH ＆ Co.KG［9］.

2 The Dy Frmentation Pant

Easily decomposable bio-waste is particularly suitable for fermentation.Therefore，only the material from the organic waste bin is used in the dry fermentation plant.Environmentally friendly bioenergy is generated from these raw materials.Unloaded bio-waste is mixed with an equal amount of previously fermented material in the mixing hall using a wheel loader.The digestion is used to inoculate the new material with bioactive microorganisms［2，10］.The kompogas high-solid dry plugflow anaerobic reactor treating municipal organic waste is described in fig.1.

Fig.1 Schematic description of the Kompogas high-solid dry plug-flow anaerobic reactor treating municipal organic waste

2.1 Using Available Raw Materials

The initial material——household bio-waste and green waste is collected in the organic waste bin.All households throughout the rural district of Diepholz can dispose of their organic waste in these 120 or 240 litre bins.The waste is collected every fortnight［11］.In addition，AWG also operates a large number of green waste collection facilities，where private householders can bring tree and shrub trimmings.These materials，however，do not play any role in biomass fermentation.Once it has arrived at the waste treatment centre，some of the bio-waste is treated in the dry fermentation plant and some of it is sent directly to the composting plant with the shredded green waste and(aerated)digestate from the biogas production process［12］.

2.2 Raw Materials for the Organic Waste Bin

Gathered organic waste:

(1)Fruit and vegetable waste.

(2)Stale bread.

(3)Coffee grounds and coffee filters，tea bags，tea leaves.

(4)Egg shells，nut shells.

(5)Hair and feathers.

(6)Green waste from the garden.

Easily decomposable bio-waste is particularly suitable for fermentation.Therefore，only the material from the organic waste bin is used in the dry fermentation plant.Environmentally friendly bioenergy is generated from these raw materials.Unloaded bio-waste is mixed with an equal amount of previously fermented material in the mixing hall using a wheel loader.The digestate is used to inoculate the new material with bioactive microorganisms［13-14］.The purpose of a very different biological process，the composting of waste，is to produce a high-quality soil conditioner——BassHum Qualit?tskompost.The treatment begins in the delivery hall of the composting plant.Biowaste and(aerated)digestate from the fermentation process are mixed and shredded in a screw mill.To improve the structure，shredded tree and shrub trimmings are also added.A magnet removes any metal that is present.A drum screen filters out all other foreign substances.The material is then transported on conveyor belts to the composting hall.

3 Fermentation Process

The term fermentation is often used interchangeably with anaerobic digestion when describing the physical decomposition of organic material(typically when discussing foods and beverages).In reality，fermentation is a distinct biological reaction that makes up one step in the greater process of anaerobic digestion.It is responsible for acid genesis，the forming of acids.Fermentation is a metabolic pathway for certain microbial organisms in anoxic environments［15-16］.During fermentation，larger organic molecules，like sugars，are converted into a mixture of reduced end products(products that have gained electrons).The process occurs in two steps(see fig.2).First，energy(in the form of ATP molecules)is produced by the reactions of glycolsis，a process that breaks down sugars and converts them into pyruvate molecules.NAD+molecules are used up in this step and are transformed into NADH.In the second step，NAD+is recreated from NADH via oxidation and reduction reactions(which involve repositioning electrons).NADH molecules donate an electron to an acceptor.Because a typical substance that normally receives the electron，like oxygen，is not available，endogenous electron acceptors are utilized in this cycle.Pyruvate molecules(created during glycolysis)accept the electron and are subsequently converted into substances such as acids and alcohols through further molecular rearrangement.Specific fermentation reactions［17］differ according to themicroorganism performing the process as well as the original substrates(sugars)being used.The result is the creation of varying end products.In the case of fermentation within anaerobic digestion，the productions of a mixture of organic acids drive the decomposition process to create biogas.Municipal solid wastes(MSW)are being generated in increasing quantities and is a major source of organic waste，that emits biogas or greenhouse gases that contribute to the global warming effect.If the biogas can be harvested and used for generating electricity，it would be a valuable source of renewable energy.It would not only help reduce greenhouse gas emission but also help reduce the use of fossil fuels for electricity generation.Our dry fermentation anaerobic digestion plants are particularly suitable and efficient for converting the organic fraction of MSW to biogas，and thence forelectricity generation.In the process，the residual digestate can be processed into valuable fertilizer or soil conditioner.Furthermore，by utilizing the organic fraction of MSW in this way，this fraction need not go to the landfill，thus enabling the lifespan of the landfill to be extended.

Fig.2 Diagram of simplified fermentation process

3.1 Discovering Bio-waste as a Valuable Resource

BIOFerm technology enables the generation of primary energy from biowaste and waste material from landscape gardening.It is environmentally friendly，sustainable and CO2-neutral in production.Only the most rudimentarydemands are made of the source material.Food and feedstuffs are not used to generate biogas.

3.2 Process Operational Data

3.2.1 Area

An aerobic digestion facility must be accommodated in a sufficient space for fermenter and power unit installation.Some additional space for refining and storage is also necessary，while traffic and turning areas，a buffer zone for landscaping，wastewater treatment infrastructure and a small office must also be taken into account.According to data from various existing facilities land requirements range from 0.15～0.40 m2per tonne input per year.

3.2.2 Energy

The energy requirements of dry fermentation are assumed to be supplied by the process itself with energy exploitation of biogas produced.The amount of produced biogas，which methane content is 55% ～65%，depends on the composition of feedstock.Comparatively little information is available on the amount of biogas produced by different feed stocks.The only pure research is from bench-scale studies carried for the USEPA，where the relative contributions of different materials to landfill gas generation(through anaerobic digestion)were measured.Results of this work，according to RIS et al.2005，are presented in the table 1.

Table 1 Various waste

3.2.3 Biogas Creates Independence

The utilization of bio-waste and home-grown raw materials［9］has a long-term positive effect on the handling of fossil fuels.Short transport distances and independence from expensive energy imports，and make their contribution felt.Biogas is a fuel that can be stored，enabling it to be used where and when it is required.Today，approximately 3.5 million German households cover their electricity demand with biogas.In rural areas with poor infrastructure，building biogas plants also contributes to local job creation —— currently some 16，000 are employed in the German biogas sector.

3.2.4 Highest System Safety

All BIOFerm plants have been TüV-certified and meet the highest criteria for operational reliability.The system with its modular design can be extended at any time if demand，or the amount of available biomass increases.Fig.3 Shows the EU’s on primary energy demand.

Fig.3 Renewable energy and the EU’s on primary energy demand

Mtoe can be achieved from biomass cultivated on 20%of arable land in EU-27.This corresponds to more than 10%of primary energy demand in 2030，equals 50% ～60%of the RES share.

4 The Future of Biogas in Europe

4.1 Continue a Real Movement

(1)Biogas upgrading＆utilization for.

(2)Biogas for combined heat and power production.

(3)Biogas＆natural gas;integration in the European gas grid(combining and synergies of renewable gas and fossil gas).

(4)Biogas as transportation fuel.

(5)Biogas for high performing gas power plants.

(6)Biogas as fuel for micro CHP，gas grid.

(7)Biogas as fuel in fuel cells，gas grid.

(8)Biogas for multiple purposes;bio-industrial processing.

4.2 Comparison between Dry Fermentation and Wet Fermentation

4.2.1 Dry Fermentation

Organic input remains stationary throughout process，eliminating moving parts and resulting in low system maintenance and repair costs.Batch process and stationary system allow precise control over input removal ensuring maximum energy yield.Closed loop liquid cycle——no additional liquid required following start-up，eliminating post-process waste water treatment needs.No pretreatment or sorting of inputs required prior to system loading，saving time and money for system operators.Almost no limitations to inputs—over 3 000 tons inputs have been identified and researched.BIOFermTMsystem has low energy consumption，using only 5%of the energy generated for plant operation.Organic input volume reduced by minimum of 40%，a significant additional cost benefit，and waste water is eliminated，removing risk of groundwater contamination.

4.2.2 Wet Fermentation

System requires mechanical parts to circulate biomass in liquid holding tank，leading to increased maintenance and repair costs.Liquid mixture causes premature removal of input before all organic matter has been digested，resulting in a loss of energy.System requires additional liquid to allow fermentation，greatly increasing the amount of system waste water and costly post-process treatments.Inputs require pre-treatment to prevent breakdown of mechanical parts as input is agitated and moved through system.Input limited to“wet”waste streams.Typical systems consume 10%～30%of the energy generated for plant operation，and treatment of waste water requires additional energy.Waste water volume increased up to 70%，requiring high energy input for treatment and increasing risk of groundwater contamination.

4.3 Composting Technologies

Composting is the aerobic，or oxygen requiring，decomposition of organic materials by microorganisms undercontrolled conditions.During composting the microorganisms consume oxygen while feeding on organic matter.This generates heat，carbon dioxide and water vapor，which are released into the atmosphere.Composting reduces both the volume and mass of the raw materials while transforming them into a composted organic material(fig.4).

Composting can occur at a rapid rate when optimum conditions that encourage the growth of micro-organisms are established and maintained.Composting technologies come in a range of designs(fig.5).All systems are designed and engineered to control and optimize the biological stabilization，sanitization，or in some cases，drying of biodegradable materials.These processes can last anywhere from a few days to 8 or more weeks depending on the degree to which the material is to be stabilized.

Fig.5 Categorization of composting systems

4.4 Advantages of Dry Fermentation Processing

The main advantages of organic waste dry fermentation are follows:

(1)Reducing greenhouse gas emissions

By monitoring decomposition of organic waste in a controlled environment，the bio-digester system is able to capture generated methane that would otherwise be released into the earth’s atmosphere in open，uncontrolled environment.According to van Haaren et al.(2010)total energy requirements for in-vessel composting facilities are 55 kWh per tonne of feedstock.This figure includes only electricity and compares to 30 kWh per metric tonne of organic feedstock based on EPA(2003).Diaz et al.from US suggests average energy requirements in a MSW composting facility are 34.4 kWh/t of MSW.The measured values include energy consumed directly within the facility，including extensive pre-processing prior to composting(e.g.，size reduction，screening)，but no odor control system.Komilis and Ham(2004)suggest values of 97 kWh/t and 167 kWh/t for high quality(HQCF)and low-quality facilities(LQCF)，respectively，including consumptions for feedstock preparation，odor control，building etc.Arcadis et al.(2010)proposes fuel consumption of 3 kWh(0.3 liter of diesel)of and 40 kWh power consumption per tonne of organic waste.

(2)Turning waste into a resource

Food scraps，yard waste，food processing waste and agriculture waste are all considered organic waste added to landfills and compost piles every day.Instead of dumping and ignoring this waste，the bio-digester is turning organic waste into a resource producing up large amount of electricity in a community.

(3)It is more efficient

The dry fermentation anaerobic digestion(DFAD)process is more energy and labor efficient.Dry fermentation does not require the substrate to be diluted with large amounts of water and the resulting digestate is therefore dry and does not require dewatering or heat for drying.

(4)Technically perfected systems

The fermentation residue can be used in agriculture as a high grade organic fertilizer or can be further composted.With the exception of filling and removal by wheeled loaders or similar，the system operates under fully automatic control.

4.5 Disadvantages of Dry Fermentation Processing

Here are some disadvantages of dry fermentation in the modern world especially in developing and underdeveloped nations.

(1)Acceptance in the market

When a new technology is introduced，it is very difficult to be accepted by individuals as such，this require more time for people to get acquainted to it.

(2)Inadequate organic waste

Wherever there is inadequate of waste，the process tends to be very slow and time wasting as such，this requires adequate supply of waste that can facilitate and stretched the process.

(3)Insufficient space

Enough space is for the activities occurring at a material recover facility are decisive.Site land should be big enough to contain the physical structure，pre-processing materials and post-processing products storage and orderly vehicle movement.

5 Conclusions

In conclusion，this type of composting technology belongs in the relatives of contained systems.Contained systems include technologies that are all with this，either in buildings and/or specifically designed vessels(e.g.tunnels，drums，towers and boxes)and are typically known as in-vessel composting(IVC).The techniques used to control the supply of oxygen required by the process are the mechanical agitation of waste(turning)and/or blowing or sucking air through the waste(forced aeration)offering differing levels of process control and automation.Two methods of material flow are offered:batch or continuous input.Fuel is used for the operation of windrow turners，for materials loading and transportation，while electricity is consumed in the operation of machinery used for pre and post-treatment operations.

［1］ Bechberger M，Reiche D.Renewable Energy Policy in Germany:Pioneering and Exemplary Regulations［J］.Energy for Sustainable Development，2004，8(1):47-57.

［2］ Ghanem I I，GU G W，ZHU J F.Leachate Production and Disposal of Kitchen Food Solid Waste by Dry Fermentation for Biogas Generation［J］.Renewable Energy，2001，23(3/4):673-684.

［3］ Pleissner D，Kwan T H，Lin C S K.Fungal Hydrolysis in Submerged Fermentation for Food Waste Treatment and Fermentation Feedstock Preparation［J］.Bioresource Technology，2014，158:48-54.

［4］ Mata-Alvarez J，Macé S，Llabrés P.Anaerobic Digestion of Organic Solid Wastes.An Overview of Research Achievements and Perspectives［J］.Bioresource Technology，2000，74(1):3-16.

［5］ Weiland P.Anaerobic Waste Digestion in Germany-Status and Recent Developments［J］.Biodegradation，2000，11(6):415-421.

［6］ Zheng Y H，Wei J G，Li J，et al.Anaerobic Fermen-tation Technology Increases Biomass Energy Use Efficiency in Crop Residue Utilization and Biogas Production［J］.Renewable and Sustainable Energy Reviews，2012，16(7):4588-4596.

［7］ Abouelenien F，Nakashimada Y，Nishio N.Dry Mesophilic Fermentation of Chicken Manure for Production of Methane by Repeated Batch Culture［J］.Journal of bioscience and Bioengineering，2009，107(3):293-295.

［8］ Singhania R R，Patel A K，Soccol C R，et al.Recent Advances in Solid-state Fermentation［J］.Biochemical Engineering Journal，2009，44(1):13-18.

［9］ Haque M O，Sharif A.Utilization of Open Pit Burned Household Waste Ash-a Feasibility Study in Dhaka［J］.Waste Management ＆ Research，2014，32(5):18-22.

［10］ Poincelot R P，Day P R.Simple Dye Release Assay for Determining Cellulolytic Activity of Fungi［J］.Applied and Environmental Microbiology，1972，23(5):875-879.

［11］ Kang D H，F(xiàn)ung D Y.Stimulation of Starter Culture for Further Reduction of Foodborne Pathogens During Salami Fermentation［J］.Journal of Food Protection，2000，63(11):1492-1495.

［12］ Greco M，Mazzette R，De Santis E P，et al.Evolution and Identification of Lactic Acid Bacteria IsolatedDuring the Ripening of Sardinian Sausages［J］.Meat Sci.，2005，69(4):733-739.

［13］ Ramesh V，Biswal M，Mohanty S，et al.Recycling of Engineering Plastics from Waste Electrical and Electronic Equipments:Influence of Virgin Polycarbonate and Impact Modifier on the Final Performance of Blends［J］.Waste Manag Res.，2014，32(5):379-388.

［14］ Zirkler D，Peters A，Kaupenjohann M.Elemental Composition of Biogas Residues:Variability and Alteration During Anaerobic Digestion［J］.Biomass and Bioenergy，2014，67:89-98.

［15］ Tomicka A，Chen J，Barbut S，et al.Survival of Bioluminescent Escherichia coli O157:H7 in a Model System Representing Fermented Sausage Production［J］.Journal of Food Protection，1997，60(12):1487-1492.

［16］ Beneduce L，Spano G，Massa S.Escherichia coli O157:H7 General Characteristics，Isolation and I-dentification Techniques［J］.Annals of Microbiology，2003，53(4):511-527.

［17］ Dohrmann A B，Baumert S，Klingebiel L，et al.Bacterial Community Structure in Experimental Methanogenic Bioreactors and Search for Pathogenic Clostridia as Community Members［J］.Appl Microbiol Biotechnol.，2011，89(6):1991-2004.