Composting content of this digestate is further broken


After the dry fermentation process which
takes place over a period of 28 days, the end reject called the digestate is
brought in for composting process. The solid content of this digestate is
further broken down adding few amendments and composting it. Coupling this
composting with a Dry Anaerobic Digestion will increase the financial gain by
adding more value to the end product. The compost obtained at the end has wide
variety of applications which also means that the waste discharge from the
plant is decreased considerably. The use of digestate for the composting makes
the process more efficient and quick as the organic material in it is already
broken down to some extent.  

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Process Description

Composting degrades the organic substances
in 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 the
initial phase called the Mesophilic phase at a temperature of 42 degrees
centigrade, bacteria degrades the easily degradable substances. The subsequent
thermophilic phase experiences an increase in temperature due to the release of
heat. The tempertaure may range from 52 to 65 degrees centigrade in this phase.
Thermophillic fungi and Actinomycets degrade Hemicellulose, Cellulose and the
degradable compounds further. In the next phase the microbial activity
decreases and so subsequently the temperature also decreases. Thi sis called
the cooling phase. In the Maturing phase further formation of humic substances
and the stabalization of the process takes place. This phase is also called the
curing phase. This phase is important for pathogen destruction.



A Rotary drum system  is chosen for the composting system  to ensure a maximum quality yield. Th erotary
drums for this are takren from XACT Systems. These XACT BioReactors are large
in – vessel composters which fasten the process and shorten the composting
period to 4-7 days. At the end of the 7 days almost 75% mature compost is
obtained. It is followed by windrow composting for complete maturation.


These Rotary drums are made of carbon steel
with a thickness of 5-8 inches. To facilitate the process of rotating, the
drums are fitted with Solid Steel Wheels. To withstand the rigors of turning
tons and tons of material 24 hours per day, the are coupled with Heavy Duty
Mounting Frames.

To ensure proper aeration to the process
and to regulate the odor emissions, Negative Ventilation Systems are in place.
The rotary drums have a foam insulation for 24*7*365 temperature control. Th
edischarge of the composted material is made easier with Automatic Discharge
door. The drum rotates at 10 rph and is driven by 10hp motor. There are no
temperamental interior plunges, fins or paddles. The input material simply moves
along the vessel as it decomposes nad becomes lighter and fluffier. It is also
installed with Soft Touch Screen Control panel. Which allows us to optimize
process controlling. It also helps in controlling ventilation, drum rotation
and temperature inside the drums. These BioReactors are available in various
dimensions. The drums we have chosen have a dimension of 10 ´ * 60. That is
approximately 3meters in diameter and 18 meters long. Each drum can handle a
volume 120 cubic yards of organic waste, that is approximately 91 cubic meters.
It can rotate 75 tons of waste for about 240 times.

This BioReactor is 10? x 60? long and is
composting 120 cubic yards of solid organic waste per day. The waste is
resident in the composter for 4 days; therefore, the daily throughput is 30
cubic 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 the
discharge end of the composter. The exhausted air / duct work is directed to a


The major factors affecting the process of composting are the C/N
ratio, moisture content, PH of the substrate and aeration. The uniform air
distribution regulated by the ventilation system available in the rotary drums.
The even mixing of the content throughout the process is facilitated by the
rotating motion of the drums. The remaining factors can be controlled by adding
required amount and type of amendments to the digestate to ensure the high
quality product at the end. We can easily calculate the desired composition of
the input substrate with the Learn2 compost web application.


The substrate is mainly composed of the
digestate and garden waste. The digestate is properly dewatered before being
sent in for the composting process. It should contain 40% and remaining 60% of
water. The digestate is compressed using a screw compressor and excess water is
squeezed 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 the
pyrolysis plant. Other substances like Newspaper and Corrugated paper are added
in small quantities to balance the mentioned factors. To calculate the number
of rotary drums for the composting for the whole year we consider the maximum
amount of possible  substrate to be
treated.  This value will help us to
calculate the sufficient number of rotary drums required. From the given data
in 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 be
4,959 cubic metre .  To get the total
number of boxes, boxes required per workday has to be considered.




                No. of boxes per day =


= 2.55 ? 3 boxes per day

The retention time for the compost is one week.So, with a work shift
of 5 days in a week and with a peak flow of 4959 cubic meter for the month of
october we need a total of 15 rotary drums and 1 additional one for maintenance
purpose. We have taken the highest amount in the whole year to calculate the
number 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.



For the first two phases Rotary drums are used for composting the
reject from the digestate. The third phase, maturation phase also known as the
curing phase takes place in a closed box system. To get high quality compost it
is crucial to give enough curing time to the compost. So, we let the compost
from the rotary drums to mature for the next 4 weeks. A closed box system is
chosen for this curing period of the compost.


We initially thought of using the boxes from BIODEGMA box composting system. They use a membrane covering
system on the top of the boxes. Since these membranes are made of ePTFE
(extended PolyTetraFlouroEthylene ) it is  non- environmental friendly. So, we decided
not to use this technology. We are using normal box composting system for the
maturation step. Assuming, that there is no reduction in volume in the initial
one week in the rotary drum, each week there will be a volume of 1149 cubic
meter. Each box with a volume of 400 cubic meter and three boxes per week, we
will need 12 boxes for a retention time of 4 weeks. With dimensions of
2m*20m*20m Each box will be equipped with duct system to suck in the odorous air
and send it to the biofilter to be treated. The holes at the bottom of the
floor will help in the aeration. The rigid wall system of the boxes will retain
the heat in the boxes.


At the end of the 4 weeks
the compost will drastically reduce in amount compared to the initial input.
The water content will reduce to 50% from initial 60%. The organic dry solids
will reduce by half of its original amount. So, a total of 60% reduction in the
amount of initial compost will be there. The amount of substrate generated for
composting per year is approximately 32795 tons. So, 0.4 fraction of this
amount will be 13118 tons. With a density of 0.75 the volume of compost
generated will be 16,616 cubic meters.

To ensure high quality of the
compost, it s sent through a screening step. We assume a loss of 5% in the
screening process. The total compost after screening per year will be 12462







We need a storage space for storing the
compost for 3 months at least. So, with a peak volume of around 1875 cubic
metres produced in the month of October after screening, we have a storage
space triple of this volume. So, the volume of the storage is approximately
6000 cubic meters. By adapting innovative techniques for marketing for selling
the compost, we try to maintain a continuous sale of the compost.