POSITIVE ENERGY BALANCE BY CODIGESTION IN THE
GRANOLLERS WWTP
Àngel Freixó1, Clàudia Turon1, Sandra Rovira2, Joan Canals2, Andreu Calvet3 and Josep Arráez3.
1
Sanejament Intel.ligent S.L. (SISLtech), Av. Sant Julià 241, 08403 Granollers ([email protected])
GS INIMA Environment S.A., C/ Tarragona, 149-157, 08014 Barcelona ([email protected])
3
Consorci per a la Defensa de la Conca del Riu Besòs, Av. Sant Julià, 241, 08403 Granollers ([email protected]).
2
Abstract
The landscape is changing as technologies and concepts are being developed to allow plants to be
energy independent or even net energy producers.
In order to do the Wastewater Treatment energetically sustainable, there is a commitment to
enhance the use of biogas, and therefore, for technologies that increase its production. The
codigestion process is one of the alternatives and compared to other technologies that aim to
increase biogas, the codigestion achieve a 100% increase, while the others range from 20-30%.
The codigestion is based on adding industrial wastes in the digestion process, and depending on its
nature, there is a particular potential of biogas production.
Keywords
WWTP; sludge anaerobic codigestion; industrial waste characteristics; biogas production;
electricity generation.
INTRODUCTION
The Wastewater Treatment Plant (WWTP) Granollers, with a treatment capacity of 30,000 m 3/day,
has had anaerobic digestion for sludge treatment since 1998. This line of treatment processes
around 2,000 tons of annual dry matter and it produced 1 M Nm3 of biogas annually. Since
November 2009 and after the installation of a cogeneration engine of 499 kW, the biogas has been
used in order to produce and export electrical energy.
The potential electricity generated from biogas is estimated between 30% and 50% of the total
energy consumed by the WWTP.
In the case of Granollers WWTP the average energy production before the implementation of
codigestion process was 3,700 kWh/day (40% of total consumption of the WWTP), far from the
12,000 kWh/day that could be provided by the cogeneration engine installed.
Background
For the implementation of the codigestion process there a prior study was performed in which a
series of industrial waste was identified, which are energetically recoverable as coproduct for the
sludge anaerobic codigestion. This industrial waste is characterized by: (1) having a high
concentration of carbonaceous material which is rapidly biodegradable anaerobically, (2) they are
not toxic for the sludge anaerobic digestion process, (3) they are not toxic for the use sludge, (4)
they have a negligible concentration of phosphorous and nitrogenous matter and anaerobically nonbiodegradable compounds, (5) they do not generate an increased production of final sludge and (6)
they are produced in sufficient quantity to ensure continued operation of codigestion anaerobic
processes and, consequently, of production of biogas.
The Codigestion process
In October 2010, after performing the formalities with the “Consorci per a la Defensa de la Conca
del Riu Besòs” and “Agència de Residus de Calalunya”, the feeding of the anaerobic digesters with
these coproducts was initiated in a progressive and stepwise manner, starting with concentrations of
less than 2% of the total COD treated and reaching up to concentrations of 25%. This coproduct
injection increased biogas production from 84,000 Nm3/month to over 150,000 Nm3/month, which
ensures the production and maximizes the energy generation to 12,000 kWh/day. This energy
generation transforms the Granollers WWTP into a facility of positive energy balance: the energy
produced by the process of sludge anaerobic codigestion is greater than the energy consumed by
Granollers WWTP itself (125% of the WWTP total consumption). The results shown in Figure 1,
begins with the addition of coproduct in October 2010.
300.000
250.000
1.400.000
1.200.000
1.000.000
800.000
600.000
400.000
200.000
0
200.000
150.000
100.000
50.000
0
Biogas production (Nm3/month) and Energy
Generation (kWh/month)
350.000
2.000.000
1.800.000
1.600.000
2010/10/01
2010/11/01
2010/12/01
2011/01/01
2011/02/01
2011/03/01
2011/04/01
2011/05/01
2011/06/01
2011/07/01
2011/08/01
2011/09/01
2011/10/01
2011/11/01
2011/12/01
2012/01/01
2012/02/01
2012/03/01
2012/04/01
2012/05/01
2012/06/01
2012/07/01
2012/08/01
2012/09/01
2012/10/01
Wastewater flow (m3/month) and COD sludge and
coproduct to Digesters (kg COD/month)
Figure 1. Biogas production and energy generation before and after the Codigestion.
Wastewater flow (m3/month)
COD sludge to digesters (kg COD/month)
Biogas production (Nm3/month)
kWh potential (kWh/month)
COD coproduct to digesters (kg COD/month)
kWh generated (kWh/month)
To manage and optimize in real time the process of sludge anaerobic codigestion and the biogas
cogeneration, two advanced and intelligent controls were implemented:
which is a system that manages and monitors the co-digestion process, and
(1)
establishes and manages the delivery product to the anaerobic digester based on the process state
and the established biogas production set point, and (2)
a control that optimizes,
manages and monitors the engine cogeneration together with the co-digestion process .
The anaerobic codigestion implementation in Granollers WWTP has been accompanied with a
detailed study of the process, of the biogas produced, of the final sludge generated and its treatment.
The obtained results did not indicate any change in the physical and chemical parameters of the
process, and at the same time has not been observed any change neither in the composition of the
biogas and of the digested and, consequently, nor in the treatment thereof.
Results – economic impact
The Codigestion has allowed to use to the maximum the installed engine and to export all the
generated energy supposing an important economic benefit. These results are shown in Figure 2.
50.000
45.000
40.000
35.000
30.000
25.000
20.000
15.000
10.000
5.000
0
350.000
300.000
250.000
200.000
150.000
100.000
Electricity consumption (kWh/month)
Electricity generation (kWh/month)
2012/08/01
2012/06/01
2012/04/01
2012/02/01
2011/12/01
2011/10/01
2011/08/01
2011/06/01
2011/04/01
2011/02/01
2010/12/01
0
2012/10/01
50.000
Electricity consumption cost (€/month)
Electricity generation income (€/month)
Electricity cost and income
(€/month)
400.000
2010/10/01
Electricity consumption and generation
(kWh/month)
Figure 2. Energy consumed vs energy produced, and energetic cost