CompostingAfter the dry fermentation process whichtakes place over a period of 28 days, the end reject called the digestate isbrought in for composting process. The solid content of this digestate isfurther broken down adding few amendments and composting it. Coupling thiscomposting with a Dry Anaerobic Digestion will increase the financial gain byadding more value to the end product. The compost obtained at the end has widevariety of applications which also means that the waste discharge from theplant is decreased considerably. The use of digestate for the composting makesthe process more efficient and quick as the organic material in it is alreadybroken down to some extent. Process DescriptionComposting degrades the organic substancesin the input under aerobic conditions with the help of microbes and bacteria.In this process evolution of heat water and carbon dioxide takes place.
In theinitial phase called the Mesophilic phase at a temperature of 42 degreescentigrade, bacteria degrades the easily degradable substances. The subsequentthermophilic phase experiences an increase in temperature due to the release ofheat. The tempertaure may range from 52 to 65 degrees centigrade in this phase.Thermophillic fungi and Actinomycets degrade Hemicellulose, Cellulose and thedegradable compounds further.
In the next phase the microbial activitydecreases and so subsequently the temperature also decreases. Thi sis calledthe cooling phase. In the Maturing phase further formation of humic substancesand the stabalization of the process takes place.
This phase is also called thecuring phase. This phase is important for pathogen destruction. TechnologyA Rotary drum system is chosen for the composting system to ensure a maximum quality yield. Th erotarydrums for this are takren from XACT Systems. These XACT BioReactors are largein – vessel composters which fasten the process and shorten the compostingperiod to 4-7 days. At the end of the 7 days almost 75% mature compost isobtained.
It is followed by windrow composting for complete maturation. EngineeringThese Rotary drums are made of carbon steelwith a thickness of 5-8 inches. To facilitate the process of rotating, thedrums are fitted with Solid Steel Wheels. To withstand the rigors of turningtons and tons of material 24 hours per day, the are coupled with Heavy DutyMounting Frames.To ensure proper aeration to the processand to regulate the odor emissions, Negative Ventilation Systems are in place.The rotary drums have a foam insulation for 24*7*365 temperature control. Thedischarge of the composted material is made easier with Automatic Dischargedoor.
The drum rotates at 10 rph and is driven by 10hp motor. There are notemperamental interior plunges, fins or paddles. The input material simply movesalong the vessel as it decomposes nad becomes lighter and fluffier. It is alsoinstalled with Soft Touch Screen Control panel. Which allows us to optimizeprocess controlling. It also helps in controlling ventilation, drum rotationand temperature inside the drums. These BioReactors are available in variousdimensions. The drums we have chosen have a dimension of 10 ´ * 60.
That isapproximately 3meters in diameter and 18 meters long. Each drum can handle avolume 120 cubic yards of organic waste, that is approximately 91 cubic meters.It can rotate 75 tons of waste for about 240 times. http://xactsystemscomposting.
com/wp-content/uploads/2014/06/In-vessel-composter-infeed-conveyor.jpgThis BioReactor is 10? x 60? long and iscomposting 120 cubic yards of solid organic waste per day. The waste isresident in the composter for 4 days; therefore, the daily throughput is 30cubic yards per day. 4 Friction Drives Rotate the BioReactor For better moisture and odor control,this composter has duct work exhausting air out of the vessel through thedischarge end of the composter. The exhausted air / duct work is directed to abiofilter.http://xactsystemscomposting.com/wp-content/uploads/2014/06/In-vessel-composter-infeed-conveyor.jpg The major factors affecting the process of composting are the C/Nratio, moisture content, PH of the substrate and aeration.
The uniform airdistribution regulated by the ventilation system available in the rotary drums.The even mixing of the content throughout the process is facilitated by therotating motion of the drums. The remaining factors can be controlled by addingrequired amount and type of amendments to the digestate to ensure the highquality product at the end.
We can easily calculate the desired composition ofthe input substrate with the Learn2 compost web application. Feed The substrate is mainly composed of thedigestate and garden waste. The digestate is properly dewatered before beingsent in for the composting process. It should contain 40% and remaining 60% ofwater. The digestate is compressed using a screw compressor and excess water issqueezed out. In the garden waste available to us only we use only 60% of it.This 60 % consists of the small stuff like the leaves and the short branches.
The bigger fraction that is the wood content of the garden waste goes to thepyrolysis plant. Other substances like Newspaper and Corrugated paper are addedin small quantities to balance the mentioned factors. To calculate the numberof rotary drums for the composting for the whole year we consider the maximumamount of possible substrate to betreated. This value will help us tocalculate the sufficient number of rotary drums required. From the given datain the contract, the maximum amount is treated in the month of October.Including the amendments the amount of substrate is around 3,700 tons.
considering the bulk density for each input amount, the total volume will be4,959 cubic metre . To get the totalnumber of boxes, boxes required per workday has to be considered. No. of boxes per day = = = 2.55 ? 3 boxes per dayThe retention time for the compost is one week.So, with a work shiftof 5 days in a week and with a peak flow of 4959 cubic meter for the month ofoctober we need a total of 15 rotary drums and 1 additional one for maintenancepurpose. We have taken the highest amount in the whole year to calculate thenumber of rotary drums to accommodate the maximum amount of input given to us.The rotary drums are equipped with automatic feeding-in and unloading system.
MATURATION PHASEFor the first two phases Rotary drums are used for composting thereject from the digestate. The third phase, maturation phase also known as thecuring phase takes place in a closed box system. To get high quality compost itis crucial to give enough curing time to the compost. So, we let the compostfrom the rotary drums to mature for the next 4 weeks.
A closed box system ischosen for this curing period of the compost.BOX SYSTEMWe initially thought of using the boxes from BIODEGMA box composting system. They use a membrane coveringsystem on the top of the boxes. Since these membranes are made of ePTFE(extended PolyTetraFlouroEthylene ) it is non- environmental friendly. So, we decidednot to use this technology. We are using normal box composting system for thematuration step. Assuming, that there is no reduction in volume in the initialone week in the rotary drum, each week there will be a volume of 1149 cubicmeter. Each box with a volume of 400 cubic meter and three boxes per week, wewill need 12 boxes for a retention time of 4 weeks.
With dimensions of2m*20m*20m Each box will be equipped with duct system to suck in the odorous airand send it to the biofilter to be treated. The holes at the bottom of thefloor will help in the aeration. The rigid wall system of the boxes will retainthe heat in the boxes.
At the end of the 4 weeksthe compost will drastically reduce in amount compared to the initial input.The water content will reduce to 50% from initial 60%. The organic dry solidswill reduce by half of its original amount. So, a total of 60% reduction in theamount of initial compost will be there.
The amount of substrate generated forcomposting per year is approximately 32795 tons. So, 0.4 fraction of thisamount will be 13118 tons.
With a density of 0.75 the volume of compostgenerated will be 16,616 cubic meters.To ensure high quality of thecompost, it s sent through a screening step. We assume a loss of 5% in thescreening process. The total compost after screening per year will be 12462tons. StorageWe need a storage space for storing thecompost for 3 months at least.
So, with a peak volume of around 1875 cubicmetres produced in the month of October after screening, we have a storagespace triple of this volume. So, the volume of the storage is approximately6000 cubic meters. By adapting innovative techniques for marketing for sellingthe compost, we try to maintain a continuous sale of the compost.