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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 58, 2017 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Remigio Berruto, Pietro Catania, Mariangela Vallone
Copyright © 2017, AIDIC Servizi S.r.l. 
ISBN 978-88-95608-52-5; ISSN 2283-9216 

Conceptual Design and Functional Modelling of a Portable 
Thermophilic Biodigester for a High Dry Matter Feedstock 

Erich D. Rössel-Kippinga, Hipólito Ortiz-Laurel*b, Emmanuel E. González-Medinaa, 
Alejandro Amante-Orozcoa  
aColegio de Postgraduados, Campus San Luis Potosí, Iturbide 73, Salinas de Hidalgo, S.L.P., 78600, México. 
bColegio de Postgraduados, Campus Cordoba, km 348 Carr Fed Cordoba-Veracruz, Congregación Manuel León, Amatlan 
de los Reyes, Veracruz, 94946, México 
hlaurel@colpos.mx 

In years to come, water and energy are two issues of concern for trying to keep a continued and sustainable 
existence of mankind on earth. Regarding the generation and reliable supply of energy; it is a vital matter, 
especially for those marginal and inaccessible rural communities, which continually struggle for energy as 
constantly it is in short supply and their only energy sources at hand are crops and forest biomass. When 
there is abundant availability of vegetative biomass and crop residues, consequently processing them for 
biogas generation is a great opportunity for energy diversification which will allow for having a sustainable and 
readily access to other available energy sources. The objective of this work was to undertake a conceptual 
engineering design for a portable biodigester by using sheep manure, whereas its performance is modeled for 
evaluating its descentral operation, especially when it has to handle a previously treated high dry matter 
substrate. Simultaneously, it has to reduce as much as possible the energy utilized for transporting the 
residues and the biofertilizer cake, by carefully planning the location for feedstock loading and biofertilizer 
unloading sites. The biodigester consists of a cylinder with an effective operating capacity of 10 m3, where a 
substrate with 39% of total solids is poured, having a retention time of 30 days, and bearing in mind that it 
should operate within the thermophilic range which is controlled by the container thermal insulation and warm 
water heated by solar energy. During the simulation performance trials for the biodigester running with 
residues to generate biogas, the energy balance for the descentral scheme was more efficient as it got an 
energy saving of 50% when it was compared to a centralized functioning mode. 

1. Introduction 

Mexico is located between 15 and 22 degrees North latitude, covers an area of about two million sq. km. It has 
boundaries, with the United States of America on the North, the Gulf of Mexico on the East, the Pacific Ocean 
on the West, and the Republic of Guatemala on the Southeast. There is a significant geographic mismatch 
between water resources and population of Mexico; only 12% of the nation’s fresh water is on the central 
plateau where 60% of the population and 50% of the basic cropland are located. 
Two thirds of the Mexican Territory is arid or semiarid region, where human activities in agriculture depends 
upon low, seasonal and ill-distributed rainfall, falling often in intense storms and the zones are vulnerable to 
droughts and other adverse climatic variations. Rössel et al. (2013) states that important strategies have to be 
considered in order to secure safe production of food and fibers to feed the population, moving along a 
constant interaction between scientific and technical progress, while energy production is in great need and 
any development has to take into account to highest efficiency and low production costs. 
The production of energy at rural level comes from biomass, indirectly using the solar energy stored up by 
photosynthesis in plants. First perceived inconveniences for developing power plants small enough become big 
advantages in a shaped structure where energy is descentrally produced. Descentrally supply of energy means 
that energy is generated and consumed at the same location. This feature makes energy to be straightforwardly 
consumed by thirsty, marginal and disperse rural communities. Their first bioenergy feedstocks are farming 
residues while, forest and cellulosic biomass are feasible feedstocks to produce other biofuels which rural 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1758078

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Rossel-Kipping E., Ortiz-Laurel H., Gonzalez-Medina E., Amante-Orozco A., 2017, Conceptual design and functional 
modelling of a portable thermophillic biodigestor for a high dry matter feedstock, Chemical Engineering Transactions, 58, 463-468  
DOI: 10.3303/CET1758078 

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population can use directly, mainly for heating and cooking. Furthermore, those readily available biofuels 
contribute to alleviate the energy shortage at rural level.  
Descentral production of energy allows to fully satisfying energy demands for every isolated location by wisely 
using the available and low cost energetic resources at hand, while reducing detrimental effects on 
environment, besides it contributes to diminish energetic losses due to usual transmission and distribution grid 
processes (Gavilán, 2004). Under this scenario, utilization of biodigesters in rural areas is a strategy for 
descentral clean energy generation. Biogas can be locally obtained where livestock manure and other farm 
wastes are readily available. Equally, other renewable energies can be incorporated for aiding to this purpose 
on the same installation which will improve the energy balance and drastically reduce greenhouse gas 
emissions. This proposal is a feasible alternative instead of assisting rural population to continue utilization of 
wood for cooking and heating. 
Thus, the objective of this work is to design a prototype biodigester for field use. The main features for such 
installation are: a) portability, since treatment of organic residues and biogas generation have to be 
descentral, b) other renewable energies can be readily incorporated up its operation and functionality in order 
to increase biogas production and higher methane content, c) capacity for a high load of organic matter that 
includes efficient use of process water. By doing that, there will be a new clean source for energy generation 
for Mexican highlands and at the same time hands-on a technology that allows to care for the environment.  

2. Materials and Methods 

This study was undertaken in the municipality of Salinas, San Luis Potosi, Mexico. Salinas is geographically 
located between coordinate: 23° 11' North and 22º 28’ South latitude North, and 101º 19' East and 101º 57' 
West longitude West, and with a height above sea level of 2099 m (INEGI, 2009). 
Climate in this region is dry medium cold with summer rainfall season and winter precipitation ranking from 
300 to 500 mm annual (INEGI, 2009). Farming and livestock rising take over around 23% of municipal land 
surface, where main crops are kidney beans, maize, onions and red hot pepper. While animal production is 
mainly concentrated on ovine and caprine herds and much less on bovine and porcine heads (INEGI, 2007). 

2.1 Considerations for the conceptual design for the biodigester 

Some aspects were crucial to define the biodigester: transport optimization and safe storage, climatic issues, 
general and effective dimensions of the biofermenter, procedures for functioning, biogas storage, energy 
potential uses and to identify its final destination. 
 
Design of the portable biodigester should fulfill the following features: 

a. Portability which will aid to transport the device from one storage site to the next. There are less 
costs and energy expenditure when the biodigester is moved from one site to the next, instead of 
transporting the organic residues close to a fixed biofermenter. 

b. Energy consumption. Installation for the biodigester will accommodate other renewable energy 
devices for its operation and functioning (thermophillic process), which will aid to optimize energy 
balances and drastically reduce the greenhouse gas emissions. 

c. Should operate with much less water for the process, as sheep manure has a high concentration of 
dry matter. 

2.2 Selection of organic wastes 

One condition for choosing the organic waste was high manure availability in Salinas. Therefore, by using 
information from SINIIGA (2015) allowed to locate communities where sheep are grow. Daily manure 
production in each community was calculated by estimating an average weight per animal of 30 kg, that 
according to Cruz (1986) each generates about 0.7 kg of manure per day. 

2.3 Transport optimization and wastes storage 

For optimization of manure transportation, it was utilized the method applied for a continuous model 
suggested by Rössel et al. (2013). This method is utilized to determine the best location for the establishment 
of the store (D) within a certain region where there are manure production places. The method finds out the 
best place for manure storage, a place where energy expenditure to transport manure from production sites is 
minimum. 
The calculation requires a system of Cartesian coordinate by dividing (North to South and East to West) the 
entire municipality area, where the separation distance can be made at random. While, at each middle point of 
every chosen manure producing areas (Ai) it is attached a respective xi and yi coordinate. Also, it is estimated 

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the average production of organic wastes on each producing area (ai). The distance between the middle point 
of Ai and D is determined by using the Eq(1). 
 

22 )yy()xx(e iDiDi −+−=    (1)
   
Hence, this is an iterative computation where m is the number of manure production sites included in the 
calculation and the selection of site D, is carried out under the condition that organic wastes transportation has 
minimum cost, which can be determined by Eq(2). 
  

!Minimumea
m

i
ii →⋅

=1
   (2)

   
    
When solving Eq(2), each calculated figure sets up an anticipated minimum, which is confirmed when 
coordinate for xd and yd that marks the site for the storage are calculated by Eqs(3) and (4) respectively. 
 

;
a

ax

x

i
i

i
ii

D 

 ⋅
=          (3)

    



 ⋅
=

i
i

i
i

i

D a

ay

y    (4) 

 

3. Results and Discussion 

Using data from SINIIGA (2015) were determined sheep manure production sites in the municipality of Salinas 
(Figure 1). For each site it was calculated the coordinate x and y, while at the same time, was estimated its 
daily manure production. By utilizing those data was calculated the best location for placing the store, under 
the condition that energy use for manure transportation to the site was the minimum (Table 1). 
Since the biodigester has to be portable by considering the handy available resources of farmers, it was 
defined that such device was to be transported over a trailer. This restriction jeopardizes its size and capacity 
for a safe movement on the rural roads. Therefore, a suitable capacity for the fermenter device was 
established as 10 m3. 
However, the usage of a digester of this capacity is not enough to treat around 12.5 tons of daily sheep 
manure produced (Table 1). This restriction can be solved by installing the adequate number of biodigesters 
required to treat that particular amount of manure. It was considered that retention time for the substrate inside 
the biofermenter to be 30 days, so 12 changes of substrate can be realized in a year. Since a 10 m3 effective 
capacity biodigester can treat around 220 tons/year, assuming that maximum load of total solids inside the 
substrate is to be less than 50 %. Thus, the amount of manure moisture content does not affect, as one 
kilogram of sheep manure poured inside the digester has on average 44% of total solids, so added water for 
the process is much less.. 
 
 

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Figure 1: Communities that generate sheep manure in the municipality of Salinas, SLP (yellow color spots) 

Table 1: Communities with sheep herds in the municipality of Salinas, SLP including calculations for 
determination of coordinate xD and yD to find out the storage location 

Community 

Ai 

Sheep 

(heads) 

Coordinate Daily manure 
production 

(kg) ai 
xi * ai yi * ai 

xi yi 

Bajío los Encinos 35 12.7 12.8 24.5 311 314 
Conejillo 2388 13.0 5.5 1671.6 21731 9194 
Diego Martín 41 11.2 10.9 28.7 321 313 
El Estribo 39 7.0 20.3 27.3 191 554 
El Mezquite 36 7.4 19.5 25.2 186 491 
Palma Pegada 4481 13.0 10.1 3136.7 40777 31681 
El Potro 1192 12.8 9.1 834.4 10680 7593 
Punteros 52 12.7 11.5 36.4 462 419 
La Reforma 1173 9.5 11.7 821.1 7800 9607 
Salinas 8112 11.0 8.5 5678.4 62462 48266 
San Cayetano 138 6.2 11.9 96.6 599 1150 
San Tadeo 41 12.0 13.0 28.7 344 373 
Santa María 82 8.4 11.1 57.4 482 637 
Total 17810 12467.0 146349 110591

 
Good stability for the combination of biodigester plus trailer and the motorized vehicle travelling over rural 
roads was a priority, therefore, it was decided to have a height/area ratio equals to 1:3. Because of the 
available trailer characteristics, it was accepted to have two 5 m3 effective capacity cylindrical containers for 
the substrate under fermentation plus an additional one third of this volume for containing the biogas. Thus, 
considering that the trailer has to have an effective legal width of 2.5 m, diameter for each container was 

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chosen as 2.2 m, thus the effective calculated container height is 1.31 m and the free height for containing the 
biogas is 0.42 m. Therefore, the total cylindrical height for each fermenter reaches 1.73 m. 
When organic matter has low concentration of total solids it is necessary to add a large amount of water in 
order to successfully fulfill an adequate fermentation process, as soon as concentration of dry matter 
increases, there is a reduction of quantity of added water. 
According to Figure 2, for an organic waste with low dry matter content, water required for the process is high  
- less saving, while at high concentration of total solids the saving of water is high, i.e. less consumption of 
water. Mandujano and Hernandez, 2001 state that there are advantages for an anaerobic digestion process 
when there is high concentration of solids. Low requirements of water and a high rate of gas generation per 
unit of volume from the biodigester. According to Varnero-Moreno (2011) for a satisfactory operation of an 
anaerobic reactor, the amount of total solid content should not be more than 10%. 

 

Figure 2: Calculation for conserving process water against percentage of total solids. 

Although dry manure holds larger amount of water than wet manure, when fermentation process starts, 
additional water is sprinkled over the substrate until pore saturation, then excess water starts coming out and 
sent back. For a successful fermentation process for the entire mass, it is important to achieve a constant and 
complete mix inside the biodigester as high dry matter concentration complicates movement of substrate. 
Therefore, we solved this issue by modelling hot water (>70°C) spraying inside the biofermenter. This 
procedure creates different temperature spaces which generate a natural mass/liquid movement. In this way 
there is an energy saving because motors and pumps are not used for the mixing. 
Authors Weiland (2001); Postel et al. (2008); Eder & Schulz (2007) state the energetic expenditure according 
to different devices for mixing substrates inside biodigesters. For mechanical mixing data is from 2 to 35 kW 
according to the type and quality of substrate. For pneumatic mixing, energy required is from 0.5 kW to any 
range necessary according to the arranged substrate. On the other hand, hydraulic flow for mixing ranges is 
from 2 to 30 m3/min. For example, it requires 3 kW for a flow of 2 m3/min. 

4. Conclusions 

By utilizing optimization methods for manure transportation and storage it is possible to precisely locate 
adequate capacity storage sites, which minimize energy use and transportation costs for feedstock inside a 
defined region. By dividing a defined waste producing area into same size smaller partial areas, where each 
one will hold a partial manure store will allow to treat these organic wastes inside a portable biodigester. At the 
end, we generate fuel biogas in a descentral way and a valuable biofertilizer as a result from the fermented 
waste cake. 
Increasing the quantity of high dry matter feedstock contributes to save process water. Also, careful sprinkling 
of hot water for wetting the manure inside the biodigester aids to energy saving, as there is no device or 
energy supplied for the substrate mixing. Gravity helps to mix and warm water has lower viscosity so the 
process is more efficient. 
Integration of additional renewable energy devices for the operation and safe functioning of the biodigester 
allows to a more efficient energy balance, saving up to 50% for the descentral scheme, compared to the 
central energy generation, for the whole process. There is also a reduction of the greenhouse gas emission. 

467



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