Anaerobic Digestion of Activated Sludges from Malting Wastewaters
Industrial-Organic Agriculture cooperation - 1º part: Laboratory stage
Ing. Horacio Campaña, Ing. Aldo A. Basaglia
GEIA - Universidad Tecnológica Nacional, Facultad Regional Bahía Blanca
11 de Abril 461 8000 Bahía Blanca Argentina
Land application of excess sludges from barley malting process is a common practice, and GEIA have developed
knowledge and experience, with this treatment and disposal methodology. Campaña H., et al have presented (1998) soil
improvements results on the ecosystems with several application rates used. Just now the farmer using malting sludges
as fertilizer is developing a certification process of land, for organic production. So the sludge from the malting
wastewater should be stabilized in order to make the treated sludge less odorous and putrescible (according national
and international requirements of organic fertilizer quality, as usual defined by IFOAM , USDA, CEE, etc). The
proposal of Sludge Stabilization Process (SSP) is an anaerobic digestion of excess aerobic sludges from the wastewater
treatment plant, recycling energy (biogas) and organic fertilizer for an agroecological production of food (organic
production of meat and cereals). As first stage, evolutionary design involving laboratory assays is presented.
Anaerobic treatment of wastes has been practiced for many years, in many different configurations
and operating schemes. The final stage, methanogenesis, (terminal electron accepting process)
involves a complex microbial community growing in the absence of air. The high-rate systems in
use today usually operate at mesophilic (25-35ºC) or thermophilic (45-65ºC) temperatures, and
often have hydraulic retention times (HRT) less than 20 hours (Boardman G.D. et al, 1995). These
systems contain 30-50 kg of volatile suspended solids (VSS)/m3, in comparison with 2-5 kg
VSS/m3 in typical aerobic suspended systems. One such high-rate system is upflow anaerobic
sludge blanket (UASB) technology (Rebac S. et al 1997). The UASB has been demonstrated to
reliably wastewater treatment with moderate to high chemical oxygen demands (COD) at
temperatures ranging from 15-40ºC (Lettinga G et al 1999).
This project is developed to evaluate and design an anaerobic digester to stabilize aerobic
sludge from a malting wastewater treatment plant, with main design criteria as follow: batch
digestor, simple and easy operation, reduce operating time and associated costs (maintenance),
integrated and compact design, minimized cost of setup, start-up and control, integrated to waste
heat source of malting process.
The production of biogas under different conditions of temperature, mixing, volume and
composition, was the main parameter for control and evaluation in a laboratory-scale reactor, the
actual project stage under development. At the very beginning, first approach process was obtained
with SuperPro Designer (1995 version). Simulated methane production was about 1.35 kg/h, for the
typical flowrate and VSS. Selected residence time was 105 hours with an operating temperature
about 45ºC and the reactor volume about 100 m3. Also a preliminary economic evaluation was
performed and adjusted with actual coefficients.
Sludge description, sampling and chemical analysis
Sludges for this study were collected from a barley malting plant, located Bahia Blanca, Argentina.
Samples were frozen at a 5ºC, and tested with storage time lower than 12 h. Sludge quality was
highly dependent on operating wastewater treatment conditions like: pH, solids contents, ambient
temperature changes, etc. In most assays chemical analysis performed were: pH, Solid Matter (SM)
Complete characterization test were made several time before (GEIA is working with this
type of agroindustrial residues from 1996). The pH was measure by a combined electrode, and in
many cases, bicarbonate was added (malting sludges are lightly acid) to correct values near 7. The
characterization of activated sludges from malting wastewater can be observed in Table 1.
Estimated composition of unstabilized malting sludge Organic matter fractionation
pH 6.9 Soluble carbohydrates 21.15 %
MS (g/l) 24 (total solids) Fats / oil / grease 10.26 %
VSS (% MS) 82 (organic matter) Cellulose / hemicellulose 52.05 %
Ashes (% MS) 18 Lignin 16.54 %
OC (% MS) 48 (organic carbon)
N tot (% MS) 6
P tot (% MS) 0.15
Table 1: Characterization of excess activated sludges.
With the initial approach different experimental parameters were tested: heating method, way
of sludge loading, mixing (time and energy involved). And so, operating temperature, C/N
relationship, nutrients dosification, pH adjusting (w/o control). The samples were collected from the
facility sludge thickener (activated sludge treatment plant). A heat bath was used, maintaining the
temperature between 37 and 38ºC. Thus the reactor temperature was maintained 35ºC +/- 1ºC.
Flasks bottles of 500 ml were used to simulate the digesters, and the gas volume produced was
measured by displacement of water. At this stage of the Project, biogas composition was only
qualitatively tested (ignition quality).
Results and Discussion
As a measure of performance efficiency, produced biogas volume was increasing in all assays. That
means evolutionary laboratory test search to optimal operating conditions. Sludge tested from
malting plant has light quality changes. Some nutrients and inoculums (like sunflower seeds,
sugar, powdered milk, anaerobic sewage sludge, rumen liquor, and the own stabilized malting
sludges etc.) were tested. The residence time was also tested. Operating temperature range was 50ºC
to ambient (20ºC). Final selection was 35ºC, because higher biogas production, that means lowest
ammonia concentration. The kinetic constants of the process seem to be increased when the initial
pH of the sludge was adjusted. The results were optimal from an initial pH of 7 to 7.5.
Figure 2 shows the results after 7 days. The inoculum in the VIII-3 assay was different to the
XI-2. The first one with higher VSS, therefore the gas production was greater. Anyway, the XI-2
assay continued more days and the final volume was 591 ml (Table 2). Powdered milk and sugar
were tested in order to evaluate granulation .
Assay Loading Time pH-i pH-f % SM final % VSS final Gas [ml]
VIII-3 Sludge: 170ml + Inoculum: 80ml 5 days 7 5.34 4.67 N/A 521.9
IX-3 Sludge: 500ml 14 days 8.09 6.98 2.68 N/A 950.61
XI-2 Sludge: 360ml + Inoculum: 40ml 8 days 7.02 6.78 1.55 76.55 591
XII-3 Sludge: 500ml + Powdered milk: 12g 7 days 7.54 5.92 N/A N/A 531.12
Table 2: Assays results 35ºC and short residence time.
Figure 1 Figure 2
0 24 48 72 96 120 144 168
0 96 192 288 384 480 576
Assay VIII-3 Assay IX-3
Assay XII-3 Assay XI-2
Assay IX-2 Assay X-1 Assay X-3
Figure 1: Biogas [ml] vs. time [h] in four assays (35ºC) and short residence time.
Figure 2: Biogas [ml] vs. time [h] in four assays (35ºC) and long residence time.
As can be seen in Figure 2, assay X-1 biogas production was greater than assay IX-2 at the
beginning. Again the influence of an inoculum is important as starting the process.
Assay Loading Time pH-i pH-f % MS final % VSS final Gas [ml]
IX-2 Sludge: 500ml 36 days 7.58 7.07 2.5 N/A 3842.03
X-1 Sludge: 320ml + Inoculum: 80ml 37 days 7.45 7.31 2.43 81.84 4161.28
Sludge: 320ml + Inoculum: 80ml +
22 days 7.45 7.07 5.84 31.40 2817.76
X-3 Fine sand: 45g
Table 3: Assays results 35ºC and long residence time.
Dry matter (VSS related) sludge concentration and pH were found to be the most sensitive process
parameters in the model. The production of biogas increased when the initial pH was adjusted about
7.0. In most of assays the pH dropped considerably within the 24-48 h, with a slight increase from
the third day onwards until the pH stabilized to around 6.8 to 7.3. For the tests with long residence
time, the pH dropped from about 7.5 to 7.0. This shows that the optimal pH could be between 6.8
and 7.3. The temperature was another important parameter. Best results were observed working at
mesophilic temperature, 35ºC.
Batch experiments indicated that nutrient supplementation (adjusting C/N) did not enhance
anaerobic biodegradation of the sludge, high degradable nutrients like sugar or milk, gave
promissory results, and probably could be recommended for start up operations. Biogas production
cycles were tested and contrasted with published values (acetogenic methanogenic degradation
stages). Specific biogas production were similar to published data.
Granulation, (just based on visual observations), was developed and remains most easy, at
quiet experiments (partial or non-mixing), and the removal efficiency (VSS reduction) of the reactor
increased with the development of granulation. This suggests that mixing conditions should also be
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