Abstract
Chemical Oxygen Demand (COD) solubilisation was used to evaluate the impact of thermal pretreatment on the transfer of sewage sludge from particulate to soluble phase. It was gathering the experimental data needed for building of empirical mathematical model describing the relation between applied temperature and time and rate of COD solubilisation and degradation. In view of repeated measurements, in order to describe the relationship between changes in the fraction of dissolved COD and the time and temperature, mixed models have been adopted where by fixed factor measurement conditions have been adopted: time and temperature, while the random factor changes the characteristics of waste activated sludge. Linear and logistic nonlinear mixed models were analyzed. The tests demonstrated that all variables are statistically significant in assessing their impact on the efficiency of liquefaction of sludge. On the basis of the estimated model, the temperature rise of 10°C increases degree of disintegration 1.7% above the average treatment time for 0.5h, by 2.6% for 1 hour, and by 3.9% for 2h. COD values decrease between 3 to 23% at temperatures in the range of 55 to 115°C. At higher temperatures COD was reduced in the range of 32 to 44%. Disintegration time did not have the significant impact on the degradation effect.
Highlights
The processing and disposal of sewage sludge is one of the most important and complex problems in the operation of municipal wastewater treatment plants
In view of repeated measurements, in order to describe the relationship between changes in the fraction of dissolved Chemical Oxygen Demand (COD) (SCOD) in hydrolysates and the loss of organic matter (ΔTCOD) and the time and temperature, mixed models have been adopted where by fixed factor measurement conditions have been adopted: time and temperature, while the random factor changes the characteristics of waste activated sludge
The lowest value for soluble COD” (SCOD) (281 gm-3) was observed in the sample disintegrated at 55°C for 0.5 hour, and the highest value – 3,793 gm-3 for the sample treated at 175°C for 2 hours
Summary
The processing and disposal of sewage sludge is one of the most important and complex problems in the operation of municipal wastewater treatment plants. Use of pre-treatment of sewage sludge is aimed at breaking microorganisms' cells and release of intracellular organic matter to the liquid phase. The positive impact of the disintegration of sludge is considered in the following aspects, too: - the higher degree of substrates transformation of volatile fatty acids (VFA), - the decrease in the final waste mass through greater effectiveness of the dewatering processes and the required volumes of waste treatment and sludge treatment facilities, - the increase in the biogas production, - the increase in the degree of mineralisation of the organic matter of sludge, - the increase in the use of conditioning agents before the dewatering and mechanical thickening processes, - the recovery of certain desirable components from the sludge, for instance, plant-available nitrogen [2,3,4]
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