Problem Statement

1. Figure 1. Schematic diagram of respirometric apparatus Table 1. Experimental sludge-waste physical recipes BA particle size, W:BA Moisture Content, Trial mm (dry mass basis) 1 45 20 – 30 1 / 9.2 2 60 12.5 – 20 1 / 7.9 Figure 4. Predicted cumulative O2 consumption at a constant W/BA ratio of 1/6 Figure 5. Predicted cumulative O2 consumption at a constant MC of 45 % 3 45 8 – 12.5 1 / 6.0 4 45 20 – 30 1 / 2.8 5 70 20 – 30 1 / 6.0 6 30 30 – 40 1 / 7.9 7 30 12.5 – 20 1 / 4.1 8 60 12.5 – 20 1 / 4.1 9 6 0 30 – 40 1 / 4.1 10 45 20 – 30 1 / 6.0 11 45 > 40 1 / 6.0 12 30 12.5 – 20 1 / 7.9 Figure 6. Predicted cumulative O2 consumption at a BA particle size of 25 mm 13 20 20 – 30 1 / 6.0 14 60 30 – 40 1 / 7.9 15 45 20 – 30 1 / 6.0 16 30 30 – 40 1 / 4.1 Note: values were assigned based on the surface response experimental design method 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, 21111 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 (R2=0.84): 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 (R2=0.78) and 28 days (R2=0.57). 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.