What affects the accuracy and applicability of determining wastewater sludge water content via low-field nuclear magnetic resonance?

The accurate determination of waster sludge water content is crucial to sludge dewatering treatment and its disposal management. Though previous studies highlight the great advantages of low-field nuclear magnetic resonance (LF-NMR) in the determination of sludge water content, its accuracy and applicability are not well studied. Herein, this study investigated the settling of operating parameters and the properties of sludge samples on the accuracy and applicability of LF-NMR method in measuring sludge water content. The results showed that the setting of basic parameters such as standard curve, number of scanning times (NS) and sample weight affected the accuracy of sludge water content by LF-NMR. The standard calibration curve constructed by 3 g/L CuSO4, NS = 8 and the sample weight of about 5 g, were suitable for the accurate determination of sludge water content. Furthermore, the existence of magnetic substances in sludge can affect the distribution gradient of main magnetic field, and thus restricted the applicability of LF-NMR. The saturation magnetization of chemical reagents strongly correlated with the measured relative errors of sludge water content (r = 0.995, p < 0.01), the greater the saturation magnetization of the magnetic material, the greater the error of the test results. On the whole, it is necessary to fully consider the influence of process parameters and sludge properties to evaluate the accuracy and applicability of the LF-NMR method, rather than simply copying the parameters in literatures.

Effects of sewage sludge pretreatment methods on its use in agricultural applications.

Pretreatment is widely used in sludge dewatering, however, its potentially impact on the subsequent sludge agricultural applications is often neglected. Here, the potential benefits and risks of the sludge with no pretreatment and with four most commonly used pretreatment methods in sludge agricultural applications were assessed using potted lettuce, an experimental crop. The results show that sewage sludge pretreatment methods can greatly affect its agricultural applications. The application of different pretreatment methods can potentially reduce the harm caused by pathogens. At low dosage (0.2 g kg-1), different sludge fertilizers promoted an increase in crop yield of 14.6% to 49.1%, and the concentrations of heavy metals in the crop and soil were controlled within safe ranges. At high dosage (8 g kg-1), crop yield using pretreated sludge (except anaerobic digestion) decreased by between 32.7% and 57.5%, but heavy metal pollution of both crop and soil increased. In terms of promoting crop growth and reducing heavy metal accumulation, untreated sludge was better than pretreated sludges and sludge with physical pretreatments was better than that with chemical pretreatments. Overall, this study clearly shows that the introduction of pretreatment in sludge dewatering can inevitably impact its agricultural land application.