Hi again! In this last module on anaerobic digestion, we'll talk about using the

product of AD. The gas and the digestate. I will try to explain what needs

to be considered so that a biogas facility can be sustained and maintained.

Before we do this however, let me quickly sum up what we have heard about AD until

now. So, we have looked at some issues of the substrate chain, we have talked about

particle size, we had talked about volatile solids of different waste

materials I also described the biochemical process stages and the main

parameters, which are contained in here and some typical digester technologies.

So, what now is left to do, is to cover the product chain. The gas and the

digestate. We will also discuss the issues around sustaining the whole

system and answering the question, when can anaerobic digestion be an

appropriate treatment option for waste management. And when not. So, now with an

appropriate digester and in good operating conditions the reactor will

produce biogas. This biogas is saturated with water vapor and in the

first instance the water vapor moving through the pipes will condensate and

moisture will collect as water in the pipes. We will need to remove this

moisture. We call this dewatering. We can remove the water at the lowest point in

the pipes. If this water is not removed, it can block the pipes, hindering flow of

gas. That is why you will need to install a water trap at the lowest point in the

gas lines. There are different types of water traps you can install. Here are two

examples: an automatic one and a manual one. Both have T-joints at the lowest

point in the gas pipes and a water storage unit below the T-joint, where the

condensate can flow into. The automatic trap

empties itself, when full, by using a U-bend overflow, whereas the manual water trap

is emptied by opening a valve. The biogas produced in the reactor will vary during

the day according to feeding patterns and also ambient temperature changes.

Biogas production also continues during night time when less gas is used and

therefore there are periods when the gas has to be stored. This can either be in

the digester itself as in the top part of the fixed dome digesters, as we had seen

before, or in bags or balloons like in this unit in Indonesia. Gas can also be

stored with medium or high pressure in gas tanks.This can be done with

compressors, but of course this requires energy. The Indian Institute of

Technology in Delhi is engaged in research on bottling of biogas. For use

as fuel in three wheeled vehicles. They combine conditioning and cleaning of biogas

and compression. In an uncompressed form, the energy content of one cubic meter of

biogas corresponds to around a half a litre of diesel. As a rough rule of thumb,

about 10 kilograms of biowaste are needed to produce this one cubic meter of

biogas. For all use in machines, engines or generators, biogas needs to be

conditioned. We could also call this cleaned. The goal is to remove water

vapor, which we've already talked about, but more importantly also hydrogen sulfide, which

is very corrosive and CO2, which has no energy value. Removing hydrogen sulfide

is what we call desulphurisation. We can do this with ferrous oxide, which

then transform the hydrogen sulfide into iron sulfide. We can also do this

with water scrubbers which at the same time also can remove CO2.

If we think about biogas application and appliances, actually the most simple

way is to use the biogas close to the anaerobic digestion facility for direct

combustion. For this purpose,

conditioning of biogas is not necessary. A household cooking stove only needs

20 to around 450 litres per hour.

Frequently, when no direct use of gas is obvious, methane is fed into gas

generators to produce electricity. However, with the conversion losses we

can only retain about 30 percent of the energy as electricity, the remaining is

lost as heat. Besides the gas, another product from anaerobic digestion is the

digestate. In wet digestion systems, this is a nutritious slurry with

nitrogen, phosphorus, potassium, as well as trace elements. 50% of this nitrogen is

available as ammonium nitrogen that can be directly assimilated by plants. So, all in

all, it's a good fertilizer, that can be used in agriculture. But remember if it's

in the mesophilic range, we're not having high temperatures, so if we have

pathogenic substances these are not hygienisized. If it is not possible to use

this digestate in agriculture, then the digestate has to be treated before

disposal, as it still has a high organic load and based on regulations would not

be allowed for discharge into surface waters. Remember no system works without

maintenance and care. What does that mean for biogas systems? We need to regularly

check for leaks.

We can do this with a pressure test, looking if the gas pipes are ok. We also

need to check and unblock pipes if required. We don't need to forget, to

empty water traps and we need to regularly observe the slurry level in

the outlet chamber.

It gives us a lot of information on the processes. Remember to not overload the

digester, otherwise we will have acidification. And now and then we need to

desludge the digester. Typically recommended is every 5 to 10 years

and this will be mostly sand and grit which accumulates at the bottom of the

digester. So, now we've heard a lot about anaerobic digestion as an option for

waste management. But when should we actually choose this option?

Well, one aspect is that we need access to a well-sorted organic waste source.

What we also need to think about is that we need local skills and experience in

construction and operation. Then need and demand for gas is a requirement. Or we

need to have a small power grid which we can feed into. We also need a competitive

chance to compete with other energy sources. And this is an economic issue.

Finally, we need an institutional environment allowing feeding into the

grid, especially for converting gas to electricity. And ideally we need a demand

for the digestate for reuse in agriculture. So let me summarize what we

covered in this module. We looked at biogas dewatering and conditioning. We

looked at biogas storage options, we looked at gas use and energy content, we

discussed what issues are necessary to maintain the system. And finally, we also

looked at criteria favoring the choice of using anaerobic digestion for organic

solid waste treatment. Thanks for listening

3.8 Using the Products of Anaerobic Digestion

Municipal Solid Waste Management in Developing Countries

Meet the Instructors