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How Does Anaerobic Digestion Work?
These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant biogas and digestate (the solid and liquid material end-products of the AD process) which is discharged from the digester.
Anaerobic digestion is a process through which bacteria break down organic matter—such as animal manure, wastewater biosolids, and food wastes—in the absence of oxygen
The following figure illustrates the flow of feedstocks through the AD system to produce biogas and digestate
Composed of methane (CH4), which is the primary component of natural gas, at a relatively high percentage, carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor, and trace amounts of other gases
The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems, among other uses
Biogas can also be purified by removing the inert or low-value constituents (CO2, water, H2S, etc.) to generate renewable natural gas (RNG)
Can be processed further to generate alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.
The residual material left after the digestion process. It is composed of liquid and solid portions. These are often separated and handled independently, as each have value that can be realized with varying degrees of post processing.
Can be used in many beneficial applications, such as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), a foundation material for bio-based products (e.g., bioplastics)
Anaerobic System Design and Technology
The digester is designed to provide the optimal conditions for converting the organic waste into biogas.
A batch digester is the simplest form of digestion, where manure is added to the reactor at the beginning of the process in a batch and the reactor remains closed for the duration of the process.
induced Blanket Reactors are digesters in which a blanket of sludge develops and retains anaerobic bacteria, providing a bacteria-rich environment through which the feedstock must pass.
Fixed film digesters contain plastic media (e.g., pellets) on which bacteria attach and grow, instead of relying solely on suspended bacteria to break down the digester feedstock.
(wet / dry systems)
Biogas is most often used to generate electricity.
Thermal energy in the form of waste heat, produced during electricity generation, can be recovered to heat digesters or adjacent buildings.
Biogas can be fired directly in boilers or heaters as a replacement for propane
Biogas can be processed to pipeline quality and sold to the local gas utility.
Biogas can be converted to compressed natural gas (CNG) which can be used in applications such as vehicle fuel.
Must be UV-, temperature- and weather-proof
Must be gas
tight and pressure-resistant
Biogas generation varies during the day according to feeding patterns and ambient temperature changes
In addition, gas production continues during the night
Biogas generation and consumption often do not happen at the same
time. It is thus necessary to collect the produced biogas temporarily in appropriate storage facilities.
Biogas can be stored in a gastight container for long periods of time
without losing its energy content
The size of the gas storage container is determined by the rate of gas production
and rate of biogas usage. The easiest way to store biogas is in low-pressure systems
such as a floating-drum, a fixed-dome or in a gas storage bag (balloon), all of which
are used in developing countries.
Fixed-dome
The upper part of a fixed-dome digester serves as a gas storage facility and it is therefore essential that it is gas-tight to prevent gas leaks. When the exit
valve is closed and gas accumulates in the dome (e.g. during night), the gas pressure will increase and push the slurry in the digester downwards and out into the compensation tank
Figure 13: Fixed-dome digester without (left) and with (right) gas pressure [2].
Gas tank in India with 10 bar
pressure (photo: Sandec).
A System Template defines a suite of compatible
technology combinations from which a system can be designed.
STEPS:
Identify the products that are locally generated and/or available THEN the system templates that process the defined products
For each template, select a technology from each
functional group where there is a technology
choice presented
Compare the systems and iteratively change individual technologies or use a different system template based on user priorities, the demand
for specific end-products
Different designs for squatters, sitter, washers,wipers
Water based and waterless models
The choice influence the subsequent technology
An inclusive design of user interface allows barrier-free access for all users
Function in collecting, storing, and sometimes treating the products generated at the user interface
The treatment provided by these technologies
is often the function of storage, and is usually passive, without requiring energy input.
Products that emanate from these technologies
often require subsequent treatment before use or disposal.
Hole in the ground
Three or more meters deep.
It normally has a lining to stabilize
the walls of the pit.
The user interface can then be
placed on top of it
PROS
Can be at very low cost
Most suited to rural area
The sludge needs to be further treated.
Emptied sludge is still very pathogenic
leakage can contaminate groundwater resources.
Odors are normally noticeable
Longer life span
easier emptying
significant reduction in pathogens
product can be used as soil conditioner
However
Manuel removal of humus is required
possible contamination of groundwater
higher costs
The pits are dug to a maximum depth of 1.5 meters,
and therefore, often do not require a lining on the entire side walls allowing for soil organisms to enter and do their job.
Greywater should not be added because it fills the pore spaces
and deprives the aerobic bacteria of the oxygen that is required for degradation.
The word 'anaerobic' describes biological processes that occur in the
absence of oxygen.
These processes naturally take place in swamps, marshes, ponds and other
standing water bodies.
Blackwater and greywater enter the tank through an inlet T.Settleable solids fall down to the bottom, where they accumulate as sludge.
With time, the sludge undergoes some stabilization by anaerobic digestion.
It has to be removed every 3-5 years.
Floatable substances move up to the surface, forming a scum layer.
The effluent from the clearwater zone, in the middle, flows out through an outlet T and is then typically are infiltrated
The blackwater from a toilet can be directly connected to the digestor
In order to produce significant amounts of biogas, additional inputs like cow dung or
organic solid waste, are normally mixed in at the inlet.
The substrate flows into the reactor, where it has a retention time of, typically, 15-to-25 days.
The biogas collects in the dome at the top of the reactor, and is transported to the point of use, through a gas pipe. When new substrate is added to the reactor
or when the gas pressure rises, the digestate flows into the expansion chamber from where it can be removed
What happens when the pit is full?
The sewered systems which rely on on pipe networks
The non-sewered systems, relying on different kinds of container-based solutions
remove suspended solids by sedimentation.
The low flow velocity in the settler
allows sediment particles
to sink to the bottom,
while the lighter constituents
will float to the surface,
forming the scum.
Settlers can achieve an initial reduction of up to 50% to 70% of suspended solids,
and up to 20% to 40% of organic material.
Settlers can be independent units,
or merged into other technologies.
the flow tank,
or settling compartment,
is designed with a "V" shape,
which prevents gas bubbles to come up
and fold them to the sides, as well as the scum.
Wastewater enters the reactor from the bottom,
and flows upwards.
A suspended sludge blanket
filters and treats the wastewater
as it flows through it.
The sludge blanket is comprised of
small agglomerations of microorganisms
with a diameter of about 1 to 3 mm,
which because of their weight, resist being washed out in the upflow.
Aim to replicate the naturally occurring processes of a natural wetland, marsh or swamp.
Be Aware !!
Risk of clogging
Management of the plants,
High ammonia levels
The water slowly flows through the wetland particles settle, and pathogens are eliminated through the combined action of sun, settling, adsorption, and predation from higher organisms.
It is only appropriate for low strength wastewater.
The water flows down and is collected by a drainage system.
The wastewater is applied intermittently,
four to ten times a day.
Thus, the filter goes through stages of being saturated and unsaturated,
Organics are degraded by the biofilm covering the filter material.
Small land requirement;
however, flies and odors are often problematic.
This technology is relatively low-cost
but requires a large land area,
if possible, far from habitations,
as it generates odors and flies.
The plants and their root systems maintain the porosity
of the filter, which means that sludge can be added
layers after layers, which imply also a much longer retention time.
Effluent is the general term
for a liquid that leaves the technology,
typically after blackwater or sludge has undergone solid-liquid separation
or some other type of treatment.
Drip irrigation where the water is slowly dripped on or near the root area
Surface irrigation where water is routed over land
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