Energy-consuming operations in anaerobic digestion process and cost-benefit analysis

The article discusses the energy-consuming operations in anaerobic digestion process, cost-benefit analysis, and environmental benefits. It also provides calculations for diluted manure production and digester volume requirement.

QUESTION 1
1) List all major energy-consuming operations (for example transport, mixing etc.) in an
anaerobic digestion process.
2) Then consider a) a high-tech digester in northern Europe and b) a low-tech digester in a
tropical climate.
Name which of the energy-consuming operations (that you listed in 1)) is likely to be the
most energy-consuming and why?
The list of energy-consuming operations performed within an Anaerobic digestion process are:
Mixing: The collected materials are mixed in the mixing tank that consumes energy
Transportation: From the collection site to processing site, transportation also consumes energy
Mechanical Separation: During separation of solid phase and liquid phase fertilizer high amount
energy is consumed (Abbasi, Tauseef and Abbasi 2012).
Heating: During the heating process large amount of thermal and electrical energy are consumed.
Fermenting: At the initial and mediatory stage in both Main –Fermenter and Secondary
Fermenter high amount of electricity is consumed.
A high tech Anaerobic digestion plant in Northern Europe: SSE Barkip Anaerobic Digestion
Plant, B707, Dalry KA24 4JJ, UK (Sitorus and Panjaitan 2013).
A low tech digestion plant in tropical climate: Grid-connect anaerobic digester plant at Gorge
Farm Energy park, Naivasha, Kenya (Kwietniewska and Tys 2014).
In High tech Anaerobic Digestion Heating is the most energy consuming process followed by the
mechanical separation process. For Heat chamber of digestion process as well as for the heat

recycling process the required energy amount is considerably higher than any other operation in
the Anaerobic digestion process. In the Low tech Anaerobic Digestionthe mechanical separation
also needs lots of electrical and mechanical energy for execution of the separation process
(Stronach, Rudd and Lester 2012). However the energy requirement of external transportation is
the major cause of high energy consumption. If the only the internal energy consumption of Low
tech digestion plant has to be considered, Mechanical separation process is the most energy
consuming process.
QUESTION 2
1) Describe what a cost-benefit analysis is. 2) State 3 examples each for costs and benefits
when planning an anaerobic digester.
Cost benefit analysis (CBA) is an analytical process through which the operational cost and
potential benefit can be measured by numerical calculation and financial data. Through cost
benefit analysis the overall operational benefit of a particular process can be determined.
Effective cost benefit analysis helps to plan the process by maintaining the opportunity for
adequate amount of profit. Like any other operation in an Anaerobic Digestion process planning
the cost benefit analysis is highly important (Yenigün and Demirel, 2013). If the cost benefit
analysis shows that the operational benefit is lower than expected the operating management has
to change the entire process module to ensure the profitability level and the cost efficiency of the
project.
Examples of cost:

Feeding or input cost: The feeding input cost refers all the economical investment required to
maintain the supply of raw material with adequate source availability and transportation.
Electrical cost: In this era most of the industrial equipments use the electricity as their power
source, The Mixer, Fermenter, Separator, Energy generator needs lots of external electrical
energy.
Repair and maintenance: In a digestion plant the operational equipments and machineries need
sufficient maintenance and repairmen cost to maintain the operational efficiency. Apart from that
regular checking and confirmation system also require some amount off additional cost.
Examples of benefits:
Biogas production: The produced biogas from the Anaerobic Digestion is highly environment
friendly and beneficial from financial aspect. Therefore selling the biogas after adequate
processing can help to earn huge benefit.
Electricity production or Output: CHP motors are used to generate electricity from the heat of the
biogas. This produced electricity is can be stored in a high powered electrical storage unit.
Therefore, this electricity causes benefit (Zhang, Banks and Heaven, 2012).
Heat production and recycling: The heat produced by the biogas processing is also used as the
energy source of the digestion processing. This feedback or recycling module helps to decrease
the operational cost and increase the benefit level.
QUESTION 3
1. Consider a farm-based anaerobic digestion process where the biogas is used for electricity
production and the digested residue is utilised as field amendment to reduce the
application of artificial fertiliser. 1) Which ENVIRONMENTAL benefit of this complete

process - if calculated as money savings - is likely to represent the largest fraction of all
environmental benefits and why? 2) Just state shortly how payments for environmental
benefits could be implemented in future?
Two types of environmental friendly output are produced from the Anaerobic Digestion. The
produced biogas from the Anaerobic Digestion is highly environment friendly and beneficial
from financial aspect. From the fundamental point of view this high calorie natural Biogas
production substitutes the polluted usage of fossil fuel. On the other hans, from monetary
beneficial aspect, after adequate processing this preserved biogas can help to earn huge benefit.
On the other hand, CHP motors are used to generate electricity from the heat of the biogas. This
produced electricity is can be stored in a high powered electrical storage unit. Therefore, through
this procedure the conventional process of electricity production can be replaced that would be
highly beneficial for environment (Kwietniewska and Tys 2014). Apart from that, this produced
and stored electricity also causes financial benefit.
Currently, both conventional power sources and alternative environment friendly power
sources are available from various public or private service suppliers. However, to produce these
physical or energy sources namely fuel gases, liquid fuel and electricity requires huge amount of
money. Therefore, the financial profit of these companies depends on the purchasing power of
the consumers. Therefore, the purchasing choice of the consumers regulates the profitability of
both kinds of power industries. The profit gained by supplying environment friendly components
and power source is called payment of environmental benefit (Sitorus and Panjaitan 2013). The
increase of this payment will also enhance the productivity and efficiency of the alternative fuel
producing system. Therefore, the increased efficiency will be also beneficial for our

environment. From this analysis it is clearly understandable that this implemented payment
practice will be highly beneficial for our future.
QUESTION 4
Consider a farm-based one-stage digester. The farm has 237 cows which produce 36 liter
manure (density 1020 kg/m3) each every day. The manure contains 13 weight-% of total solids
(before dilution) and the volatile solids make up 84 % of the total solids. For cleaning the
stables and the milking parlor 20 liter of water (density 1000 kg/m3) per cow and day are
needed and this water is collected together with the manure. NOTE: Only answer with
numbers and units. This is a 3 part question.
________________________________________
a) How much diluted manure (in m3!) is produced per day? Round your result to the next
full cubic meter!
Daily Manure from Milking parlor and cleaning= 20 litre per cow
Daily Manure from per cow= 36 litre
Total number of cow = 237
Total Manure per day= 237 x (20+30) litre
= 13272 litre
= 13.27 m3

b) Assume an organic loading rate of 2,5 kg VS/(m3*day). What volume does the digester
need to have (in m3)? Round your result to the next full 10 m3!
(examples: 123,45 m3 ==> 120 m3; 176,45 m3 ==> 180 m3). )
Manure from cow= (36 x 237) litre
= 8532 litre
= 8.53 m3
Weight of Manure from cow = (8.53 x 1020) Kg
= 8700 Kg
Manure from Milking parlor and cleaning = (20 x 237) litre
= 4740 litre
= 4.74 m3
Weight of Manure from Milking parlor and cleaning = 4.74 x 1000 Kg
= 4740Kg
Total weight = (8700 + 4740)Kg
= 13440 Kg
Total solid = 13440 x 13%
= 1747 Kg
Volatile Solid = 1760 x 84%
= 1467.5 Kg
Loading requirement = 1467.5 ÷ loading rate
= 1467.5 ÷ 2.5

= 587m3 (approx)
= 590 m3
c) How long (in days) will the resulting HRT be? Round to the next full day.
Digester Capacity = 590m3
Loading requirement per day = 587m3
HRT = 1 Day

Reference:
Abbasi, T., Tauseef, S.M. and Abbasi, S.A., 2012. Anaerobic digestion for global warming
control and energy generation—an overview. Renewable and Sustainable Energy
Reviews, 16(5), pp.3228-3242.
Kwietniewska, E. and Tys, J., 2014. Process characteristics, inhibition factors and methane
yields of anaerobic digestion process, with particular focus on microalgal biomass
fermentation. Renewable and Sustainable Energy Reviews, 34, pp.491-500.
Sitorus, B. and Panjaitan, S.D., 2013. Biogas recovery from anaerobic digestion process of
mixed fruit-vegetable wastes. Energy Procedia, 32, pp.176-182.
Stronach, S.M., Rudd, T. and Lester, J.N., 2012. Anaerobic digestion processes in industrial
wastewater treatment (Vol. 2). Springer Science & Business Media.
Yenigün, O. and Demirel, B., 2013. Ammonia inhibition in anaerobic digestion: a
review. Process Biochemistry, 48(5-6), pp.901-911.
Zhang, Y., Banks, C.J. and Heaven, S., 2012. Anaerobic digestion of two biodegradable
municipal waste streams. Journal of environmental management, 104, pp.166-174.