1. Proceedings of the 3rd
International CEMEPE & SECOTOX Conference
Skiathos, June 19-24, 2011, ISBN 978-960-6865-43-5
449
Energy saving by the application of biosolids to sweet sorghum crop
under deficit irrigation
M. Sakellariou-Makrantonaki1*
and D.Dimakas1
1
Department of Agriculture Crop Production and Rural Environment, University of Thessaly, GR-38446,
N.Ionia, Magnesia, Greece
*Corresponding author: E-mail: msak@uth.gr, Tel +30 24210 93059, Fax: +302421093074
Abstract
The effects of biosolids application to sweet sorghum crop were examined in a field work in 2010 at
the farm of the University of Thessaly, in Velestino, Magnesia, Central Greece. Considering sweet
sorghum as a possible alternative crop for biomass and bioethanol production in Greece in the near
future, the present work focuses on the biomass productivity of this crop as affected by the
application of biosolids, and in possible energy saving by replacing fertilizer application and by
using less water for irrigation. The results of this study showed that biosolids application could
replace the inorganic fertilizers application with the same results in biomass production and that
valuable irrigation water can be saved under deficit irrigation. Using deficit irrigation a significant
amount of irrigation water can be saved.
Keywords: biosolids, deficit irrigation, sweet sorghum, biomass, drip irrigation, water saving.
1. INTRODUCTION
Biosolids are one of the final products of the treatment of sewage at wastewater treatment plants.
Biosolids usually contain high levels of organic matter (40-70%), and are rich in N and P that are
essential for plant growth. The beneficial qualities of biosolids as a soil enhancement are generally
recognized.
When added to soil, biosolids increase the soil organic matter, decrease the soil bulk density and
increase the water holding capacity [1,2]. The increase at water holding capacity at lower tensions,
such as those at field capacity, is primarily the result of an increase in number of small pores.
However, with the addition of organic matter, specific surface area increases resulting in increased
water holding capacity at higher tensions [3,4]. Increase of the soil micro- and macroporosity
improves soil structural conditions, thus preventing the formation of surface-soil crusts, reducing
soil compaction [5] and improving aeration and root penetration [6]. Biosolids application also
contributes to water conservation by reducing water loss from percolation, evaporation, and runoff
[6].
Biosolids can also be an important source of macro- and micronutrients for agricultural crops. The
macronutrients nitrogen and phosphorus can be provided in the required amounts needed by crops,
although potassium, the third most important macronutrient, is low in biosolids. Biosolids are also a
source of iron, boron, copper, nickel and other micronutrients essential for plant growth [7].
Furthermore, the ability of biosolids to fix nitrogen into a form that can be used by plants reduces
the potential for nonpoint-source pollution, such as that associated with applications of commercial
fertilizers [6]. Plants are better able to utilize plant nutrients and water. In many cases, biosolids
increase yield and quality of plants.
During the last decade, the scientific community has also become interested in renewable energy
sources such as biodiesel and bioethanol, in order to replace the depleting traditional fuels and

2. 450
reduce global CO2 emissions. The renewable energy policy of the EU designates that the biofuels
share of total transport fuels consumed in Greece should reach 10% by year 2020.
Agricultural crops can produce oil for biodiesel, fermentable carbohydrates for ethanol, or dry
matter for combustion, pyrolysis, gasification or liquefaction. Among the annual energy crops able
to produce stable amounts of biomass, sorghum has a significant yield potential, even in conditions
of limited water availability [8,9]. Sorghum can have two energy destinations: the production of
bio-oil charcoal gas, gas, bio-oil, and heat, through thermo-chemical conversion, and the production
of ethanol through biological conversion [10]. Sweet sorghum is more suitable for the production of
ethanol because it has a high yield of fermentable carbohydrates [11].
Based on the above and considering sweet sorghum as a possible alternative crop for biomass and
bioethanol production in Greece in the near future, the present work focuses on the biomass
productivity and water use efficiency of this crop as affected by the application of biosolids, and in
possible energy saving by replacing fertilizer application and by using less irrigation water.
2. MATERIALS AND METHODS
The effects of biosolids application in sweet sorghum crop were examined in a field work in 2010 at
the experimental farm station of the University of Thessaly, in Velestino, Magnesia, Central Greece
(latidude 39°23' N, longtitude 22°45' E). The soil is classified as a Typic Xerochrept with a clay
loam texture. Each plot had an area of 32 m2
and a spacing of 1.5 m between the plots. Four
treatments with three replications were organized in a randomized complete block design as
follows:
a) Biosolids application with supplied amount of water equal to the 60% of the daily
evapotranspiration (B60),
b) Inorganic fertilizer application with supplied amount of water equal to the 60% of the daily
evapotranspiration (F60),
c) Untreated Control (no application of fertilizer or biosolids) with supplied amount of water equal
to the 60% of the daily evapotranspiration (C60),
d) Untreated Control (no application of fertilizer or biosolids) with supplied amount of water equal
to the 100% of the daily evapotranspiration (C100).
Biosolids were supplied from the municipal wastewater treatment plant (MWTP) of the city of
Volos. After treatment of sewage in aerobic tanks and belt filter press, the sewage sludge had been
dewatered using an infrared radiation processor (IRP). Infrared Radiation Processing is a treatment
method that achieves direct thermal penetration and has the ability to transport high thermal energy.
Electromagnetic thermal radiation removes moisture and destroys the pathogens that sewage sludge
has.
After treatment with the IRP the produced biosolids had less than 10% moisture and non traceable
pathogens. Biosolids were distributed manually one week before sowing at a rate of 5 Mg ha-1
and
incorporated in the upper 20cm of soil depth by rotovation. Biosolids were applied once a year,
starting in 2009.
The inorganic fertilizer applied, contained the same amount of total nitrogen, phosphorus and
potassium as the biosolids. The whole amount of phosphorus and potassium was applied as basal
one week before sowing, as long as with the 25% of the nitrogen. The fertilizer was sprinkled
uniformly and incorporated in the soil by rotovation. The remained amount of nitrogen was applied
in three equal applications by fertigation, twenty to thirty days after emergence.

3. 451
The crop was sown in early June 2010 with a sweet sorghum hybrid (Sorghum bicolor (L.) Moench
x Sorghum bicolor (L.) Moench 'Sugargraze') at a plant rate of 120.000 plants per hectare. Each plot
had 6 rows with row spacing of 0.78 m. After seed emergence, the same cultivation practices were
applied to all treatments. These practices included three manual weed controls.
During the germination and seedling growth period, six sprinkler irrigations were needed. These
irrigations started just after sowing and stopped when the plants had developed an adequate root
system. Surface drip irrigation was used soon after sprinkler irrigations. Drip irrigation laterals were
made of polyethylene with a 20 mm nominal diameter. Drippers were pressure compensated and
self-flushed (Netafim™) with an emitter spacing of 0.80 m and a flow rate of 2.3 L h-1
.
Evapotranspiration was measured using a class A open evaporation pan. The irrigation water
supplied by both sprinkler and surface drip irrigation was 447.4 mm for the C100 treatment and
305.4 mm for the B60, F60 and C60 treatments.
Crop production was measured by means of aboveground biomass. Data were collected periodically
during the whole growing period. The maximum biomass production was determined by recurrent
harvests of random plants in the middle rows. The total production by hectare was determined by
weighting the collected plant samples in an accurate scale. Statistical analysis was conducted using
Minitab Statistical Software (a=0.05).
The average climatic conditions of the area follow the typical Mediterranean pattern with hot-dry
summers and cool-humid winters. Daily values of mean temperature and precipitation were
recorded in an automatic weather station located at the farm as shown in Figure 1. The precipitation
during the whole growing season was 40 mm, while the average precipitation of the last 25 years
was 87 mm. Temperature in year 2010 did not have a significant deviation from the 25-year
average.
0
20
40
60
80
100
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
Jun Jul Aug Sep Oct
Precipitation,mm
0
5
10
15
20
25
30
35
Temperature,o
C
precipitation 2010
precipitation (25-year average)
temperature 2010
temperature (25-year average)
Figure 1. Mean values (10-day periods) of temperature and precipitation for year 2010 and 25-year
average.

4. 452
3. RESULTS AND DISCUSSION
3.1 Biomass production
The development of dry (aboveground) biomass of sweet sorghum in 2010, under the four
treatments, is presented in Figure 2.
10
15
20
25
30
35
40
70 75 80 85 90 95 100 105
Days after sowing
Drybiomass,Mgha-1
B60
F60
C60
C100
Figure 2. Dry biomass production of sweet sorghum under different treatments.
As shown in Figure 2, the maximum dry (aboveground) biomass production was attained by the
second half of September (106th
day after sowing) for all treatments. The B60 treatment produced
the highest dry biomass, which was 36.3 Mg ha-1
. The F60 treatment produced a dry biomass of
33.6 Mg ha-1
while the C60 treatment produced 28.5 Mg ha-1
. The C100 treatment produced 33.8
Mg ha-1
.
Statistical analysis showed that the maximum dry biomass production between the treatments B60,
F60 and C100 was not statistically different. Statistical difference was observed only between C60
and all the other treatments.
3.2 Water Use Efficiency
Water use efficiency (WUE) has been the most widely used parameter to describe irrigation
effectiveness in terms of crop yield. WUE was defined by Viets [12] as:
ET
Y
WUE
g
E
where WUE is water use efficiency (kg m-3
), Yg is the economic yield (g m-2
), and ET is the crop
water use (mm). Water use efficiency is usually expressed by the economic yield, but it has been
expressed as well in terms of the crop dry matter yield (either total biomass or aboveground dry
matter) [13].
Water use efficiency of sweet sorghum under the four treatments, is shown in Table 1. The water
use efficiency (kg m-3
) for each treatment is expressed as dry biomass (Mg ha-1
) per total water
inputs (mm). The dry biomass values presented in Table 1 are the maximum values. The
evapotranspirated water was estimated using a class A open evaporation pan on a daily basis.
Effective precipitation is the product of precipitation multiplied by a coefficient according to the

5. 453
intention of the rainfall (Effective precipitation = Precipitation x coefficient). The coefficient for the
area of the experiment was found to be 0.8.
Table 1. Water Use Efficiency under different treatments
Treatments
B60 F60 C60 C100
Dry biomass (Mg ha-1
) 36.3 33.6 28.5 33.8
Effective Precipitation (mm) 29.8 29.8 29.8 29.8
Irrigation (mm) 305.4 305.4 305.4 447.4
Total water inputs (mm) 335.2 335.2 335.2 477.2
Water Use Efficiency
(kg dry biomass / m3
total
water inputs)
10.8 10.0 8.5 7.1
As shown in Table 1, the treatments B60, F60 and C60 received 335.2 mm of total water inputs
while the C100 treatment received 477.2 mm . It can be observed that the plants in the B60 and F60
treatment used the water more efficiently than the plants in the other two treatments. Each cubical
meter (m3
) of water received by the crop in the B60 treatment produced 10.8 kg of dry biomass
while for the F60 treatment it produced 10.0 kg. In the C60 treatment one m3
of water produced 8.5
kg of dry biomass while in the C100 treatment it produced 7.1 kg. On the contrary, for the
production of 1 kg of dry biomass, 0.092 m3
and 0.1 m3
of water was used by the crop for the B60
and F60 treatments accordingly, 0.117 m3
for the C60 treatment and 0.14 m3
for the C100
treatment.
4. CONCLUSIONS
Based on this study we can conclude that, under deficit irrigation, the same dry aboveground
biomass of sweet sorghum was produced, using biosolids or inorganic fertilizer. This leads to the
conclusion that biosolids application could replace the fertilizer application. The production of
inorganic fertilizers consumes considerable amounts of energy, and thus by replacing inorganic
fertilizer with biosolids considerable amounts of energy can be saved.
On the other side, water was used more efficiently by the crop under deficit irrigation. Deficit
irrigation (60% of evapotranspiration needs) combined with the application of biosolids or fertilizer
produced the same dry biomass as when full irrigation (100% of evapotranspiration needs) was used
without any application of biosolids or fertilizer. This leads to the conclusion that by using deficit
irrigation a significant amount of irrigation water (40%) can be saved, and thus energy can be
saved.
Sweet sorghum seems to be a very promising alternative crop for biomass and energy production in
Greece in the near future. Biosolids application under deficit irrigation can reduce the application of
fertilizers and also save valuable irrigation water which can lead in energy saving in agriculture.
.
4. ACKNOWLEDGMENTS
This research has been co-financed by the European Union (European Social Fund – ESF) and
Greek national funds through the Operational Program "Education and Lifelong Learning" of the
National Strategic Reference Framework (NSRF) - Research Funding Program: Heraclitus II
Investing in knowledge society through the European Social Fund.

6. 454
References
1. Khaleel, R., Reddy, K.R., Overcash, M.R., 1981. Changes in Soil Physical Properties Due to
Organic Waste Applications: A Review. J. Environ. Qual., 10, 133-141.
2. Powers, W.L., Wallingford, G.W., Murphy, L.S., 1975. Research status on effects of land
application of animal wastes. EPA-660/2-75-010, USEPA, Washington, D.C.
3. Gupta, S.C., Dowdy, R.H., Larson, W.E., 1977. Hydraulic and thermal properties of a sandy soil
as influenced by incorporation of sewage sludge. Soil Sci. Soc. Am. Proc., 41, 601-605.
4. Volk, V.V., Ullery, C.H., 1973. Disposal of municipal wastes on sandy soils. Report to the
Boeing Company. Dep. of Soil Sci., Oregon State Univ., Corvallis. 50p.
5. Pagliai, M., Antisari, L.V., 1993. Influence of waste organic matter on soil micro- and
macrostructure. Bioresource Technology, 43(3), 205-213.
6. USEPA, 1999. Biosolids Generation, Use, and Disposal in the United States. EPA530-R-99-
009, United States Environmental Protection Agency, Washington, D.C.
7. Epstein, E., 2003. Land Application of Sewage Sludge and Biosolids. CRC Press LLC, Boca
Raton, Florida.
8. Cosentino, S.L., 1996. Crop physiology of sweet sorghum. Proceedings of 1st European
Seminar on Sorghum for energy and industry, 1–3 April, Tulose, 30–41.
9. Foti, S., Cosentino, S.L., Copani, V., Patane, C., Guarnaccia, P., 1996. Growth and yield of C4
species for biomass production in Mediterranean environment. Proceedings of 9th European
Bioenergy Conference: Biomass for energy and environment. June 1996, Copenhagen,
Denmark, 616–621.
10. Venturi, P., Venturi, G., 2003. Analysis of energy comparison for crops in European agricultural
systems. Biomass and Bioenergy, 25, 235-255.
11. Dolciotti, I., Mambelli, S., Grandi, S., Venturi, G., 1998. Comparison of two Sorghum
genotypes for sugar and fiber production. Industrial Crops and Products, 7, 265-272.
12. Viets, F.G., 1962. Fertilizers and the Efficient Use of Water. Adv. Agron., 14, 223-264.
13. Howell, T.A., 2001. Enhancing Water Use Efficiency in Irrigated Agriculture. Agron. J., 93,
281-289.