Microbial O2 Uptake During Sludge Biodegradation as Influenced by Material Physical Characteristics

1
Microbial O2 Uptake During Sludge Biodegradation
as Influenced by Material Physical
CharacteristicsA. Mohajer1, A. Tremier2, S.
Barrington1, J. Martinez2, C. Taglia2 , M.
Carone31Department of Bioresource Engineering,
Faculty of Agricultural and Environmental
Sciences, Macdonald Campus of McGill University,
21 111 Lakeshore, Ste-Anne-de-Bellevue, Quebec,
Canada H9X 3V92 Cemagref, Livestock and
Municipal Waste Management Research Unit, 17 av.
de Cucillé, CS 64427 35044 Rennes Cedex, France
3 Department of Biostatistics, Johns Hopkins
Bloomberg School of Public Health, 615 N. Wolfe
St., Baltimore, MD, USA 21205

• Problem Statement
• Increasing urbanization and industrialization
  has led to the production of massive amounts of
  sludge waste from sewage and industrial
  wastewater treatment plants. Composting this
  waste is a cost effective and sustainable
  treatment. However, it requires the use of
  appropriate initial physical recipes in order to
  get optimal microbial growth and activity and
  accordingly, the rapid production of a stable and
  recyclable end-product.
• The moisture content (MC), waste to bulking
  agent (W/BA) ratio and BA particle size are
  important physical characteristics in a mixture
  recipe and their influence and interactions
  during biodegradation need to be fully understood
  for creation of optimal compost mixtures.

A respirometric apparatus was developed to
measure the microbial O2 uptake in the different
physical sludge-waste recipes. (Figure 1).
Table 2. Correlation of physical characteristics
with cumulative O2 consumption and peak OUR
2. Objectives The primary objective of this
study was to monitor O2 uptake in sludge-waste
samples with differing MCs, W/BA ratios and BA
particle sizes, to establish their impact and the
impact of their interactions upon biodegradation.
Additionally, the O2 uptake curves were
examined for any correlations between the
cumulative O2 uptake after 28-days and shorter
measures (the peak O2 uptake rate), to establish
simpler and earlier ways of predicting aeration
requirements and sample biodegradability.
Modeling these associations allows us to predict
an expected cumulative O2 consumption (Y) after
28 days as a function of the significant
variables (R20.84) Y28-days 10572.42
111.42?1 11160.21?2 139.86?3
1853.39?1?2 (2) Where Y O2 consumption,
mmol/kg of DM ?1 (MC-45), ?2 (W/BA
ratio-1/6), dimensionless, and ?3 (BA particle
size-25), mm. Using the model, cumulative O2
consumption response curves can be developed for
a range of physical characteristic values within
the experimental design limits
Approximately 3 kg (wet basis) of substrate was
placed in a 10 L cylindrical airtight reactor,
which received 65 L/h of continuous air via a
glass diffuser located 70 mm above cell bottom.
The reactor was placed in a water-bath maintained
at 40 ?C, which preheated and saturated the inlet
air with moisture.
3. Materials and Methods Sixteen waste mixtures
(Sewage sludge wood residues) were created with
varying levels of moisture, W/BA ratio and BA
particle size (Table 1).

Figure 2. Actual photograph of respirometric
apparatus
4. Results and Discussion The O2 uptake rate
(OUR) profile (Figure 3) obtained in all trials
consisted of an exponential increase in OUR to a
peak value as all easily biodegradable substrates
were consumed. As the remaining complex organic
matter was hydrolyzed, a subsequent drop in O2
uptake was obtained to low and stable values. A
second OUR peak was initiated after mixing of the
samples.

• 5. Conclusions
• The MC, W/BA ratio, BA particle size and the
  interaction of MC and W/BA ratio, significantly
  influence cumulative O2 consumption after 14 and
  28 days of aeration. Only MC and BA particle
  size significantly influence peak OUR.
• Moisture contents outside of the traditional
  50-60 range were found and predicted to result
  in high levels of sludge biodegradation, as long
  as W/BA ratio was adjusted to account for its
  effects.
• o Thus, focus should shift towards establishing,
  not individual optimal physical levels, but
  optimal physical recipes taking into account any
  interaction between the physical characteristics.
• The peak OUR achieved in the first few days of
  aeration is a strong predictor of the aeration
  needs and biodegradability of a sludge-waste
  mixture after 28 days of treatment.

Statistical Analysis Multivariate linear
regression was performed to model the
associations between cumulative O2 consumption
and the physical characteristics at 14 and 28
days, based upon the following equation Y ?0
?1?1 ?2?2 ?3?3 ?12?1?2 ?13?1?3
?23?2?3 (1) Where ?0 the
intercept ?1, ?2, ?3 linear coefficients ?12,
?13, ?23 interaction coefficients ?1 MC,
?2 W/BA ratio, dimensionless ?3 BA particle
size, mm.
Figure 3. O2 Uptake Rate as a function of time
for trials 1, 5 and 13
The peak OUR reached within the first 2 to 6 days
was associated with the cumulative O2 consumption
after both 14 (R20.78) and 28 days (R20.57).
Acknowledgments This project was accomplished
through the collaboration between Cemagref (GERE)
and McGill University (Department of Bioresource
Engineering). This research is part of a larger
project, named ESPACE, financed by the ANR
(French National Research Agency), currently
being carried out in partnership between
Cemagref, Suez-Environment and IMFT. The Natural
Science and Engineering Research Council of
Canada is also acknowledged for its financial
contribution.