The effect of microwave treatment on the physicochemical characteristics of faecal sludge and implications for sludge treatment.

Abstract

Faecal sludge (FS) is generated from non-sewered sanitation technologies such as pit latrines and septic tanks. Over time, FS needs to be emptied from the pit latrine and processed. Resource recovery from FS is growing because it contains organic matter and other materials that can be valorized to plant nutrients and biofuels. Drying, hydrothermal carbonization (HTC) and anaerobic digestion (AD) are some technologies used to treat FS and recover resources from it. Drying is achieved through solar, thermal, infrared or microwave heating. Microwave (MW) treatment is a technology of interest because of its efficient heating mechanism and versatility. It can efficiently meet several treatment objectives, such as volume reduction, nutrient recovery, or pathogen inactivation in a single operation. It can be incorporated into other treatment processes like pyrolysis, hydrothermal carbonization, and biological degradation to improve process efficiency. This study evaluated the effect of MW treatment on FS characteristics and the subsequent anaerobic digestion. A response surface modelling was used to study the MW operating parameters that influenced FS solubilization. The FS was collected from active ventilated improved pit (VIP) latrines and was treated in a domestic MW oven at 630W, 720W and 810W for 1 to 10 minutes. Changes in the properties of the treated FS were analyzed using ultraviolet-visible (UV/VIS) spectrophotometry, carbon, nitrogen and sulfur (CNS) elemental analyzer, Fourier transform infrared (FTIR), scanning electron microscopy (SEM) imaging, flow cytometry (FCM) and Automatic Methane Potential Test System (AMPTS II). The change in chemical oxygen demand (COD) provided a means of evaluating the effect of MW treatment on FS solubilization. The untreated FS had a high soluble COD (sCOD) to total COD (TCOD) ratio (10%), which indicated that the FS underwent stabilization in the pit latrine and during storage. A high sCOD/TCOD ratio was expected because the average age of VIP latrine FS in eThekwini is approximately five years. Although the FS showed properties of stabilization, MW treatment increased the sCOD, showing an initial sCOD release phase (phase 1), degradation of sCOD (phase 2) and a second slight sCOD release (phase 3). The sCOD release profiles were similar to the temperature profiles. In all the treatments (630W, 720W and 810W), the maximum sCOD release was recorded when the FS reached the boiling temperature (~96°C). The highest sCOD/TCOD achieved was 27% when a microwave power (MP) of 630W was applied for 4 minutes. The highest sCOD release was achieved in all treatments during the first phase. Although there was a second sCOD release (phase 3), it did not result in additional sCOD. Therefore, the microwave operation could be stopped after the first phase if the treatment aims to solubilize organic matter. Soluble proteins (sProt) and soluble carbohydrates (sCarbs) also increased after MWtreatment and followed a similar trend to the sCOD release. A correlation heat map revealed at the centre configuration had the least amplitude at high flowrates. This can be used online to check for particle breakage and the resulting maldistribution in flow in packed bed reactors. A model was developed to analyse pressure fluctuations in packed bed reactors. The model was developed using equations, found from literature, for non-ideal flow in packed beds. MATLAB software was used to solve and analyse the model. The pressure fluctuations obtained from experimental work agreed with the simulated data. The uniform packing had the highest amplitude while the nonuniform (large and small at the centre) had the least amplitude. The model was in agreement with experimental data, as it was seen that the amplitude of pressure fluctuations can be used for diagnosing flow maldistribution in packed bed reactors.