1. Microbial O 2 Uptake During Sludge Biodegradation as Influenced by Material Physical Characteristics A. Mohajer 1, A. Tremier 2, S. Barrington 1, J. Martinez 2, C. Taglia 2, M. Carone 3 1 Department of Bioresource Engineering, Faculty of Agricultural and Environmental Sciences, Macdonald Campus of McGill University, 21 111 Lakeshore, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9 2 Cemagref, Livestock and Municipal Waste Management Research Unit, 17 av. de Cucill é, 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 1.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. 2. Objectives The primary objective of this study was to monitor O 2 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 O 2 uptake curves were examined for any correlations between the cumulative O 2 uptake after 28-days and shorter measures (the peak O 2 uptake rate), to establish simpler and earlier ways of predicting aeration requirements and sample biodegradability. A respirometric apparatus was developed to measure the microbial O 2 uptake in the different physical sludge-waste recipes. (Figure 1). Figure 2. Actual photograph of respirometric apparatus 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. Statistical Analysis Multivariate linear regression was performed to model the associations between cumulative O 2 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. 4. Results and Discussion The O 2 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 O 2 uptake was obtained to low and stable values. A second OUR peak was initiated after mixing of the samples. Modeling these associations allows us to predict an expected cumulative O 2 consumption (Y) after 28 days as a function of the significant variables (R 2 =0.84): Y 28-days = 10572.42 + 111.42  1 + 11160.21  2 – 139.86  3 – 1853.39  1  2 (2) Where Y = O 2 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 O 2 consumption response curves can be developed for a range of physical characteristic values within the experimental design limits: The peak OUR reached within the first 2 to 6 days was associated with the cumulative O 2 consumption after both 14 (R 2 =0.78) and 28 days (R 2 =0.57). 5. Conclusions The MC, W/BA ratio, BA particle size and the interaction of MC and W/BA ratio, significantly influence cumulative O 2 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. 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. Physical Paramaters Cumulative O 2 consumption at 28 days Peak OUR Moisture Content, % ++ W/BA Ratio, dry mass + None BA Particle Size, mm —— Interaction of MC and W/BA ratio — None Interaction of MC and BA particle size None Interaction of W/BA ratio and BA Particle Size None P value<0.05 Table 2. Correlation of physical characteristics with cumulative O 2 consumption and peak OUR: Figure 1. Schematic diagram of respirometric apparatus 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 4. Predicted cumulative O 2 consumption at a constant W/BA ratio of 1/6 Figure 5. Predicted cumulative O 2 consumption at a constant MC of 45 % Figure 6. Predicted cumulative O 2 consumption at a BA particle size of 25 mm Figure 3. O 2 Uptake Rate as a function of time for trials 1, 5 and 13 %