RECUPERATIVE THICKENING: A POSSIBLE TOOL TO IMPROVE ANAEROBIC
DIGESTION OF WASTEWATER SLUDGE
Shufan Yang1, Long D. Nghiem1, Heriberto Bustamante2, Derek van Rys2, and Sudhir N. Murthy3
1. University of Wollongong, Wollongong, NSW, Australia
2. Sydney Water, Sydney, NSW, Australia
3. DC Water, Washington DC, USA
ABSTRACT
Recuperative thickening (RT) has been used to
improve anaerobic digestion performance of the
Bondi wastewater treatment plant (WWTP). This
paper describes a laboratory scale study to fill the
knowledge
gaps
identified
during
recent
assessment of the Bondi WWTP. The results
indicate that RT leads to a notable increase in
biogas production. This effect is attributed to the
increase in solids residence time and reduction in
short circuiting achieved by RT. There is also
evidence that polymer addition during sludge
thickening would lead to the sequestration of
biodegradable
colloidal
and
soluble
macromolecules resulting in improved soluble COD
removal. RT could also reduce the concentration of
total volatile sulphur compounds in the biosolids,
resulting in less odorous biosolids.
INTRODUCTION
In conventional anaerobic digestion, both the
hydraulic residence time (HRT) and solids
residence time (SRT) are the same. In the late
1960s, Torpey and Melbinger (1967) introduced the
recuperative thickening (RT) concept, in which a
portion of digested sludge is thickened and returned
to the digester for further digestion. RT allows for
the decoupling of SRT and HRT. Thus, the SRT
value and solids concentration in the digester can
be increased independently of the HRT.
Sydney Water and CH2MHILL have recently
completed a collaborative research project at the
Bondi wastewater treatment plant (WWTP) to
review RT performance and its current status as a
process for biosolids reduction and quality
improvement. This collaborative project identified
several knowledge gaps that needed research to
establish the operational envelop and better
understanding of the factors controlling RT
performance. The results can be used to facilitate
the installation of RT at other WWTPs. Thus, the
study reported in this paper has been formulated to
address the knowledge gaps previously identified
by (Bharambe et al., 2015).
Based on the literature and also on the assessment
at Bondi WWTP by Bharambe et al., (2015) whose
paper is also presented at Ozwater 2015, five main
interlinked mechanisms may occur during the RT
process. These are called 5-S hypotheses whose
elucidation would lead to better anaerobic digestion
performance than by conventional anaerobic
digestion. These are:
(1) The increase in SRT independent of HRT;
(2) Reduction in Short-circuiting;
(3) Microbial Selection or gradual shift to the
methanogen community toward a more resilient
population;
(4) Sequestration of biodegradable and soluble
macromolecules during thickening as a result of
polymer addition; and
(5) The impact of Shearing during thickening that
reduces particle size with the consequent release of
biodegradable substances to the aqueous phase.
It is well established that extended SRT could lead
to further organic conversion to methane and an
incremental increase in volatile solid reduction
(Sieger et al. 2004; 2008). The increased volatile
solid (VS) reduction may also limit volatile organic
sulphur compound generation (directly associated
with biosolids malodour) in downstream digested
sludge processing such as dewatering and disposal
of the biosolids cake. A few WWTPs in North
America have successfully applied the RT process
into their anaerobic digesters. Some positive effects
on the biogas production have been observed at
these plants (Reynolds et al. 2001; Greer 2011).
However, previous data were obtained mostly from
full scale operation where there could be many
factors in play that could also influence anaerobic
digestion performance. None of these previous
studies have attempted to understand the
underlying mechanisms of RT to enhance
anaerobic digestion performance and particularly to
assess and verify the main factors (5-S
hypotheses) described above.
This study aims to experimentally assess the five S
hypotheses that would influence RT. In particular,
this study evaluates the use of RT to increase
treatment capacity and efficiency with respect to a
range of performance parameters including biogas
production, VS and COD reduction, process
stability, and biosolids odour.
MATERIALS AND METHODS
Wastewater sludge
Raw primary sludge and anaerobically digested
sludge from the Wollongong Wastewater Treatment
Plant, NSW, Australia, were used as the feed and
inoculum for the lab scale anaerobic digesters,
respectively. Raw primary sludge was collected
fortnightly and was stored at 4C in a sealed
container.
Laboratory scale anaerobic digesters
Three identical lab scale anaerobic digesters were
used for this study (Figure 1). Each digester
consists of a 28L cone shape stainless steel reactor
(Core Brewing Concepts, Victoria, Australia), a
digester. Part of the withdrawn sludge volume was
then thickened. Thickening polymer (Zetag 8169,
BASF) was added to the digested sludge to obtain
4g/Kg dry sludge. The sludge was stirred at 400
rpm for 2 min and thickened by gravity to about 5%
dry solids content. A pre-determined volume of the
supernatant was wasted. The remaining thickened
sludge and supernatant was mixed and fed back
into the digester together with a pre-determined
volume of raw feed (primary sludge). Mass balance
calculation was used to determine the volume of
wasted supernatant and feed. Using the above
mentioned RT protocol, three experimental
campaigns were conducted to compare AD
performance at different SRTs (Table 1). Each
Campaign ran for approximately 2 months.
Figure 1: Panoramic view of all three anaerobic digesters.
peristaltic hose pump (DULCO®flex from
ProMinent Fluid Controls, Australia), a temperature
control unit (Neslab RTE 7), a thermal couple with
temperature gauge, a digital biogas counter and a
gas trap (1L) for biogas sampling, and a S shape
air lock as gas trap. The digester was heated by hot
water from the temperature control unit flowing
inside plastic tube wrapping around the reactor.
The entire reactor was also covered with insulation
foam to minimise heat loss. The thermal couple
probe was used to monitor the reactor temperature.
A gas line was used to connect the digester to the
gas trap, the S-shape air lock, and the digital gas
meter which was used to record the gas production
rate. When required, the gas trap could be used to
collect 1L of biogas for composition analysis.
Otherwise, the gas trap could be by-passed.
Experimental protocol
All three anaerobic digesters were seeded with 20L
digested sludge individually at the beginning of the
experiment. One digester was operated without RT
as the control for baseline comparison. The
peristaltic hose pump was operated continuously at
a flow rate of 60L/h to ensure adequate circulation
and mixing within the digester. The temperature of
all three anaerobic digesters was maintained at
35±1C throughout the experiment. The anaerobic
digesters were fed once a day. Prior to each feed,
2L of digested sludge was withdrawn from the
Table 1: Hydraulic and solids residence times of
the three parallel digesters (operated for 2 months).
SRT (d)
Experiment
Camp. 1: HRT=20d
D1 (no RTD2 (RT) D3 (RT)
Blank)
20
25
30
Camp. 2: HRT=20d
20
30
60
Camp. 3: HRT=10d
10
15
20
Monitoring and analysis
Sludge samples were collected on a weekly basis
for analysis. The measured sludge characteristics
included total solid (TS), volatile solid (VS), total
COD (CODT), soluble COD (CODS), pH, alkalinity,
as well as NH4+ and PO43- concentration of the
sludge centrate. All other parameters were
measured according to the standard methods
(Eaton et al. 2005). Biogas composition was also
monitored on a fortnightly basis using a portable
gas analyser (GA5000 gas analyser, Geotechnical
Instruments, England) (Nghiem et al. 2014).
Volatile sulphur organic compounds analysis
At the end of Campaign 3, the digested sludge was
dewatered using the modified centrifuge technique
previously reported by Higgins et al. (2006) to
produce biosolids cake. The dry solid content of the
biosolids cake was 20±1%. The biosolids cake was
Average biogas production (mL/min)
20
Experimental Campaign 1
22/01 - 26/03, 2014
HRT = 20 d
Experimental Campaign 2
27/03 - 20/05, 2014
HRT = 20 d
Experimental Campaign 3
23/07 - 13/09, 2014
HRT = 10 d
15
10
5
0
D1
D2
SRT=20 d SRT=25 d
D3
D1
D2
D3
D1
D2
D3
SRT=30 d SRT=20 d SRT=30 d SRT=60 d SRT=10 d SRT=15 d SRT=20 d
CH4 production activity (L CH4/g VS removed)
Figure 2: Average biogas production (SRT and HRT values are as in Table 1).
1.6
Experimental Campaign 1
22/01 - 26/03, 2014
HRT = 20 d
Experimental Campaign 2
27/03 - 20/05, 2014
HRT = 20 d
Experimental Campaign 3
23/07 - 13/09, 2014
HRT = 10 d
1.2
0.8
0.4
0.0
D1
D2
SRT=20 d SRT=25 d
D3
D1
D2
D3
D1
D2
D3
SRT=30 d SRT=20 d SRT=30 d SRT=60 d SRT=10 d SRT=15 d SRT=20 d
Figure 3: Methane production activity at different SRT values.
immediately characterised by its content of total
volatile sulphur organic compounds (TVSOC). This
was conducted using method developed by WERF.
It is based on an incubation technique followed by
Gas Chromatography – Mass Spectrometry (GCMS analysis). In brief, 30g of biosolids was
collected into a 500mL PET bottle. The bottle was
sealed using a rubber cap and incubated at
28±1C. The head space was extracted using a
syringe at a specific time interval for GC-MS
analysis. The results are reported in ppmv in the
incubation bottle head space.
RESULTS AND DISCUSSION
Wastewater sludge characterization
The VS and TS values of the raw primary sludge
are weather dependent and thus can vary. In this
study, TS of the raw primary sludge was
23.7±5.7g/L. Similar temporal variation could also
be observed with the CODT of the raw primary
sludge. Despite this variation in the TS content,
VS/TS ratio of the raw primary sludge remained
relatively constant at 0.90±0.02.
Biogas composition and system stability
Variation in the TS of the raw primary sludge did
not significantly influence the stability (i.e. abnormal
decrease in biogas production or digester alkalinity)
of the anaerobic digestion process in all
experiments conducted with RT.
A major concern associated with RT is the
exposure of the returned sludge to air during the
thickening and sludge recycling process. However,
no deleterious effects due to possible air exposure
were observed in the digesters using RT. In all
experiments, the biogas composition was stable
and consistent with values reported in the literature
(Brisolara and Qi 2013; Nghiem et al. 2014). Biogas
composition obtained from Campaign 1 is
presented in Table 2 as example. In all
experiments, alkalinity of the digested sludge was
consistently higher than 2000 mg CaCO3/L) and pH
was stable at around 7, with only one exception, as
described above, at 10d SRT. The results confirm
that exposure of the thickened sludge to air did not
negatively affect anaerobic performance. This is
consistent with a previous study by Conklin et al.
(2007) who studied the effect of short-term oxygen
exposure to anaerobic digester sludge in batch
mode to simulate RT condition.
Table 2: Composition of biogas from the three
digesters operated at HRT of 20 day (Campaign 1).
Data show mean ± standard deviations of 6-8
measurements over 2 months.
CO2 (%)
CH4 (%)
D1 (SRT=20 d)
56.3±3.1
39.0±4.0
D2 (SRT=25 d)
60.2±1.1
37.2±1.4
D3 (SRT=30 d)
59.2±0.8
37.0±1.0
Biogas production
Decoupling of SRT from HRT and the increase in
SRT, resulted in a notable increase in biogas
production as observed in all three Campaigns
(Figure 2). It is noteworthy that the improvement in
biogas production was most significant when the
12
biogas production (Campaign 2). RT also resulted
in a notable increase in the methane production
activity which is defined as methane production per
gram of VS removed (Figure 3). In agreement with
Hypothesis 3 (see Introduction), it is possible that
RT could enrich methanogenic bacteria in the
digester, allowing for the digestion of more slowly
degradable VS fraction such as fats. It is noted that
detailed microbial characterisation is still required to
conclusively verify Hypothesis 3. A concurrent
study is being conducted to further elucidate the
role of microbial selection due to RT on anaerobic
performance.
TS, VS, and CODT removal
Considerable variation in the removal of VS (Figure
4) and CODT (Figure 5) over time was observed.
The standard deviation of VS and CODT removal
over time was up to 12%. This variation is attributed
to the temporal variation in the raw primary sludge
as discussed above. RT resulted in a notable
increase in biogas production and specific methane
Experiment 1 (HRT = 20 d)
Experiment 2 (HRT = 20 d)
Experiment 3 (HRT = 10 d)
D1=20 d
D2=25 d
D3=30 d
D1=20 d
D2=30 d
D3=60 d
D1=10 d
D2=15 d
D3=20 d
100
80
8
60
6
40
VS removal (%)
VS removal (%)
10
4
D1 removal
2
0
D2 removal
20
D3 removal
0
2
2
3
2
3
1
2
1
3
4
4
4
4
5
5
5 7
7
8
8
8
8
9
9
24/0 29/0 6/0 12/0 20/0 27/0 6/0 13/0 26/0 4/0 11/0 17/0 24/0 2/0 8/0 16/0 23/ 29/ 05/ 12/ 19/ 26/ 02/ 09/
Figure 4: VS removal at different SRT values during the three Campaigns.
Experimental campaign 2 (HRT = 20 d)
Experimental campaign 3 (HRT = 10 d)
D1=20 d
D2=25 d
D3=30 d
D1=20 d
D2=30 d
D3=60 d
D1=10 d
D2=15 d
D3=20 d
100
80
80
60
60
40
40
D1 removal
D2 removal
CODT removal (%)
CODT removal (%)
100
Experimental campaign 1 (HRT = 20 d)
D3 removal
20
20
0
0
2 4 /0
1
2 9 /0
1
6 /0 2 1 2 /0 2 2 0 /0 2 2 7 /0 2
6/03 13/0326/03
4 /0 4 1 1 /0 4 1 7 /0 4 2 4 /0 4
2 /0 5
8/05 16/05 23/7
29/7
0 5 /8
1 2 /8
1 9 /8
26/8
0 2 /9
0 9 /9
Figure 5: CODT removal at different SRT values during the three Campaigns.
SRT was increased from 15 to 20d (Campaign 3).
By contrast, doubling the SRT from 30 to 60d
resulted in a considerably smaller increase in
production activity from 0.4 to 0.7 and 1.4 L CH4/g
VSremoved as the SRT increased from 20 d (no RT)
to 25 and 30 d (with RT) in Campaign 1,
respectively. However, the improvement in biogas
production occurred with minimal increases of both
VS destruction and CODT removal in Campaigns 1
and 2. This occurred for 20 d HRT and the SRT
was in the range of 20 to 60 d. On the other hand,
the increase in both VS and CODT removal was
only conclusively observed in Campaign 3 when the
HRT was 10 d and the SRT was in the range of 10
to 20 d.
Our results provide additional experimental
evidence to previous findings by Reynolds et al.
(2001) and Ostapczuk et al. (2011). These authors
reported that RT would be ineffective to remove VS
in full-scale digesters if adequate SRT value had
been achieved. The finding reported in our study
and those by Reynolds et al. (2001) and Ostapczuk
et al. (2011) can be attributed to several factors.
For instance, RT resulted in an increase in the
organic loading rate. Such an increase in organic
loading rate could decrease the COD removal in
the digester (Rincón et al. 2007; Li 2011). In
addition, the improvement in system stability could
be masked by the variation in the characteristics of
the raw primary sludge and the build-up of less
biodegradable volatile solids in the digester. It is
known that the organic content of raw primary
sludge is heterogeneous. In other words, primary
sludge contains organic fractions with different
degrees of biodegradability. At a sufficiently high
baseline SRT value, the returned thickened sludge
due to RT would contain mostly organic matter that
is rather recalcitrant to further biodegradation. Our
results (Figures 4 and 5) suggest that VS and
CODT destruction was not significantly improved
because the solids returned to the anaerobic
digester have a smaller proportion of readily
biodegradable materials at SRT greater than 20 d.
To
further
elucidate
the
80
70
Experiment 2
SRT=20 d
Experiment 1
SRT=20 d
of
less
COD accumulated (balanced)
COD effluent
COD gas
D1
a
build-up
Experiment 3
SRT=10 d
60
Total COD (g/day)
50
40
30
20
10
0
4/0
-10 2
1
29/0
1
6/02 12/02 20/02 27/02
6 /0 3 1 3 /0 3
26/0
3
4/04 11/04 17/04 24/04
-20
b
2 /0 5
8 /0 5 1 6 /0 5
23/7
2 9 /7
0 5 /8
1 2 /8
1 9 /8
2 6 /8
0 2 /9
09/9
0 5 /8
1 2 /8
1 9 /8
2 6 /8
0 2 /9
09/9
05/8
12/8
19/8
26/8
02/9
0 9 /9
Date
D2
Experiment 3
SRT=15 d
Experiment 2
SRT=30 d
80 Experiment 1
SRT=25 d
70
60
Total COD (g/day)
50
40
30
20
10
0
-10
24/0
1
29/0
1
6 /0 2 1 2 /0 2 2 0 /0 2 2 7 /0 2
6 /0 3 1 3 /0 3
2 6 /0
3
4 /0 4 1 1 /0 4 1 7 /0 4 2 4 /0 4
8/05 16/05
23/7
2 9 /7
Date
-20
c
2/05
D3
80 Experiment 1
SRT=30 d
70
Experiment 3
SRT=20 d
Experiment 2
SRT=60 d
Total COD (g/day)
60
50
40
30
20
10
0
-10
24/0
1
29/0
1
6 /0 2 1 2 /0 2 2 0 /0 2 2 7 /0 2
6 /0 3 1 3 /0 3
2 6 /0
3
4 /0 4 1 1 /0 4 1 7 /0 4 2 4 /0 4
2/05
8/05 16/05
2 3 /7
29/7
Date
-20
Figure 6: COD distribution in the digester, effluent, and conversion into biogas during the three
Campaigns.
1000
900
800
Hydrogen sulfide (L/L)
biodegradable organic matter in the digester at high
SRT and the associated influence on COD removal,
a mass balance with respect to COD was
calculated for all experiments (Figure 6). The
anaerobic digestion process consumes CODT of
raw sludge, which was converted to biogas. The
remaining CODT is discharged via the digested
sludge withdrawn and would built-up in the digester.
Thus mass balance was calculated using the
following equation:
700
Digestion condition
SRT = 10 d (no RT)
SRT = 15 d (with RT)
SRT = 20 d (with RT)
600
500
400
300
200
100
CODT (raw sludge) = COD (gas) + CODT (effluent)
+ CODT (accumulated)
0
1 day
3 day
7 day
Incubation time (d)
CODS removal was improved in all RT experiments
in comparison to the blank (data not shown). This
can also explain the higher biogas production
without discernible improvement in the removal of
CODT with RT. This can be attributed to the
sequestration of soluble and biodegradable
macromolecules and colloidal particles from the
aqueous solution into the solid phase caused by
polymer addition during thickening. In other words,
the addition of polymeric to the sludge prior to
thickening would allow for some soluble
macromolecules and colloidal materials to be
captured and returned to the digester. This is
consistent to Hypothesis 4 described above.
However, further investigation is required to
conclusively confirm Hypothesis 4. Although CODS
(approximately 1000 mg/L) only contributed to less
than 5% of the total COD content of raw primary
sludge, the improvement in CODS removal could
also be a factor attributing to the higher biogas
production due to RT.
Our results demonstrate that RT could be used for
plants with inadequate HRT (i.e. < 15d) to improve
the removal of VS and CODT. Our results
highlight the role of RT in decoupling SRT from
HRT thus allowing for the increase in SRT
(Hypothesis 1) and improve system stability through
the reduction of short circuiting (Hypothesis 2) and
alleviating the impact of feed variation.
Methyl mercaptan (L/L)
200
150
Digestion condition
SRT = 10 d (no RT)
SRT = 15 d (with RT)
SRT = 20 d (with RT)
100
50
0
1 day
3 day
7 day
Incubation time (d)
2000
1800
Digestion condition
SRT = 10 d (no RT)
SRT = 15 d (with RT)
SRT = 20 d (with RT)
1600
Dimethyl sulfide (L/L)
Each bar in Figure 6 presents the total CODT input
of the sampling date. The COD accumulated in the
digested sludge is calculated as the balance of the
COD input and output. Mass balance analysis
showed that RT resulted in higher conversion of
COD to biogas. However, both Digesters 2 and 3
(compared with Digester 1) had more COD
accumulated throughout the experimental period
(Figure 6). This explains the insignificant
improvement in COD removal due to RT when SRT
of the Digester 1 (Blank, ie, no RT) was sufficiently
high (20 d). Thus, RT resulted in an increase in not
only the COD consumed for biogas production but
also COD accumulation in the digester.
1400
1200
1000
800
600
400
200
0
1 day
3 day
7 day
Incubation time (d)
Figure 7: Concentration of specific odour
compounds from biosolids of Campaign 3 as a
function of incubation time.
Hydrogen sulphide and total volatile organic
sulphur compounds concentration in the
biosolids
The reduction in the concentration of total volatile
organic sulphur compound indicated that RT
reduces malodour generation associated with the
biosolids cake. Hydrogen sulfide was prevalent in
all biosolids cake samples. In addition, methyl
mercaptan (which is one of the more important
compounds for malodour reception associated with
biosolids) and dimethyl sulfide could also be
detected from biosolids cake samples. Figure 7
shows the concentration of odour compounds from
Campaign 3 as a function of incubation time.
Biosolids cake from digester 1 (which was operated
at SRT of 10d without RT) exhibited significant
higher concentration of all three odour compounds
compared to both digesters 2 and 3 which were
operated with RT at SRT of 15 and 20d,
respectively. Results reported in this study show,
for the first time, that RT can contribute to a notable
reduction in volatile organic sulphur compounds,
and hence, in biosolids cake odour.
CONCLUSIONS
The increase in biogas production and system
stability due to recuperative thickening (RT)
reported are due to the increase in sludge
residence time (SRT) and a reduction in short
circuiting. There is also evidence that the improved
performance associated with RT would be due to
sequestration of biodegradable and soluble
macromolecules and colloidal particles promoted by
polymer addition used to thicken the sludge as well
as possible enrichment of methanogens in the
digesters. RT led to a reduction in total volatile
organic sulphur compounds. This would result in
the production of less odorous biosolids. The
improvement in VS and COD removal associated
with RT could only be clearly observed when the
reference digester (operated without RT) was
operated at a low SRT (i.e. 10d). Results reported
in this study indicate that RT would be a viable
technique to improve the performance of anaerobic
digesters with inadequate SRT or issues with
system stability. The results obtained in this study
need validation by a full scale investigation.
REFERENCES
Central Kitsap County Wastewater Treatment Plant
Alternatives Development Workshop, Brown
and Caldwell.
Bharambe, G., Kabouris. J., Bustamante. H., van
Rys. D., Cesca. J., Murthy. S., (2015).
“Anaerobic
digestion
with
recuperative
thickening minimises biosolids quantities and
odours in Sydney, Australia.” OZWater:
Adelaide 12 – 14 May 2015.
Brisolara, K. F. and Y. Qi (2013). "Biosolids and
sludge management." Water Environment
Research 85(10): 1283-1297.
Conklin, A., R. Bucher, H. D. Stensel and J.
Ferguson (2007). "Effects of Oxygen Exposure
on Anaerobic Digester Sludge." Water
Environment Research 79(4): 396-405.
Eaton, A. D., L. S. Clesceri and A. E. Greenberg
(2005). Standard Methods for Examination of
Water & Wastewater American Public Health
Association
Greer, D. (2011). Municipal and industry synergies
boost biogas production. BioCycle. Emmaus,
J.G. Press Inc. 52: 43-46.
Higgins M. J., Chen Y. C. and Murthy S. N. (2006).
Understanding factors affecting polymer
demand for conditioning and dewatering, Water
Environment Research Foundation.
Li, J. Z. (2011). "Impact of organic loading rate on
the performance and pollutant removal
efficiency in an ABR." Journal of Harbin
Institute of Technology 43(8): 50-54.
Luo, W., F. I. Hai, J. Kang, W. E. Price, W. Guo, H.
H. Ngo, K. Yamamoto and L. D. Nghiem
(2015). "Effects of salinity build-up on biomass
characteristics and trace organic chemical
removal: Implications on the development of
high
retention
membrane
bioreactors."
Bioresource Technology 177: 274-281.
Nghiem, L. D., P. Manassa, M. Dawson and S. K.
Fitzgerald (2014).
"Oxidation
Reduction
Potential as a Parameter to Regulate MicroOxygen Injection into Anaerobic Digester for
Reducing Hydrogen Sulphide Concentration in
Biogas." Bioresource Technology 173: 443–
447.
Ostapczuk, R. E., P. C. Bassette, C. Dassanayake
and G. Bevington (2011). "Recuperative
Thickening: Decoupling the SRT from the HRT
Reduces Capital Expenditures and Increases
Biogas Production for CHP Utilization."
Proceedings of the Water Environment
Federation 2011(15): 2348-2355.
Reynolds, D. T., M. Cannon and T. Pelton (2001).
Preliminary investigation of recuperative
thickening for anaerobic digestion. Proceedings
of the Water Environment Federation.
Rincón, B., L. Travieso, E. Sánchez, M. de los
Ángeles Martín, A. Martín, F. Raposo and R.
Borja (2007). "The effect of organic loading rate
on the anaerobic digestion of two-phase olive
mill solid residue derived from fruits with low
ripening
index."
Journal
of
Chemical
Technology and Biotechnology 82(3): 259-266.
Sieger, R., P. Brady, J. Donovan and T. Shea
(2004). White Paper on High Performance
Anaerobic Digestion. W. E. Federation, R. a. B.
Committee and B. T. Subcommittee.
Torpey, N. W. and R. N. Melbinger (1967).
"Reduction of Digested Sludge Volume by
Controlled Recirculation." Journal (Water
pollution control federation) 39(9): 1464-1474.