Speaker: Principal Mdolo
A ventilated improved pit (VIP) latrine is the most basic form of sanitation in South Africa. VIP latrines generate large volumes of faecal sludge (FS) which require safe treatment. The slow hydrolysis of FS limits biological treatment. Microwave treatment disintegrates sludge flocs and cell walls, releasing intracellular materials. It is a promising FS treatment technology. A domestic microwave treated FS at several power and time intervals. Soluble chemical oxygen demand (sCOD) and hydrolysis increased with microwave treatment. The disintegration of sludge flocs and cell walls released intracellular biopolymers. Microwave treatment prepared the sludge for downstream biological treatment.
Faecal sludge (FS), if not adequately managed, pose serious public health and environmental risks. Biological treatment is a low-cost FS treatment option. It relies on microorganisms degrading the FS in an anaerobic or aerobic environment. Anaerobic digestion of FS is an exciting option because it recovers energy and offers other benefits. However, anaerobic digestion is a slow process and requires large reactor volumes. Hydrolysis is the rate-limiting step in anaerobic digestion. Hydrolysis is slow because of the complex nature of FS. Hence pretreatment is necessary to break the complex materials and make the intracellular organics available for microbial attack (Zhen et al., 2017). Microwave is a promising FS treatment method. Microwave treatment is efficient in heating the substrate because the microwaves are supplied directly to it. Heat is generated within the substrate and distributed throughout its volume by the friction of polar molecules generating heat as they try to align with the oscillating electromagnetic field. This paper presents the evaluation of the effect of microwave treatment on the hydrolysis of FS.
Samples were collected from VIP latrines in eThekwini municipality during routine manual pit emptying. The samples were mixed and stored at 4℃. Aliquots of the homogenised samples (50g) were treated in a domestic microwave with a total power of 900 W. Samples were treated at 50% (450W), 70% (630W) and 90% (810W) power levels for 60, 120 and 180 seconds. Total solids (TS), volatile solids (VS), total and soluble COD were analysed on treated and untreated samples. TS and VS were determined through loss on drying at 105 ℃ and 550 ℃, respectively. Chemical oxygen demand was determined using a portable Hach spectrophotometer.
The TS of the raw sample was 21%, which was within the range of VIP FS and higher than the TS of thickened waste activated sludge (Kocbek et al., 2020). The TS content of VIP sludge is beneficial for microwave treatment. For high TS substrates, the heat generated efficiently treat the target substrate without wasting it on raising the temperature of moisture (Tang et al., 2010). The sCOD in untreated FS was 208 mg/L. The highest sCOD was 325mg/L, observed when the sample was treated at 90% microwave power for 180 seconds. The lowest was 215 mg/L at 50% microwave power for 60 seconds. These findings agree with the results of other authors, who observed an increase in sCOD after microwave treatment (Yu et al., 2010). Solubilisation of COD is a metric of hydrolysis, an essential step in anaerobic digestion. Microwave irradiation disintegrated cell walls and extracellular polymeric substances (EPS) releasing the intracellular organic compounds into the aqueous phase (Yu et al., 2010). This process transferred COD from the solid phase to the aqueous phase.
The sCOD/tCOD ratio in the untreated FS was 45%, which increased to 70% after treatment at 90% microwave power for 180 seconds. Microwave treatment increased the hydrolysis of FS, making the complex organic matter accessible to microorganisms.
Improving hydrolysis has a positive effect on the biological treatment of FS. For example, anaerobic digestion could be faster; requiring smaller digester volume and increased methane production (Gil et al., 2018). Microwave treatment helped to convert complex organic matter to easily accessible form hence adding a carbon source. Therefore, microwave pretreatment can reduce the overall anaerobic digestion process time by making the organic matter readily available by the microorganisms. Further, the results indicated that microwave treatment reduced FS mass by 53.4%. This characteristic is beneficial for FS transport and disposal.
F. S. & TORO-BAPTISTA
E. 2018. Effect of microwave pretreatment on semi-continuous anaerobic digestion of sewage sludge. Renewable Energy
B. & BRDJANOVIC
D. 2020. Microwave treatment of municipal sewage sludge: Evaluation of the drying performance and energy demand of a pilot-scale microwave drying system. Science of The Total Environment
J. & ZHUO
Y. 2010. Energy efficiency of pre-treating excess sewage sludge with microwave irradiation. Bioresour Technol
X. & LIANG
R. 2010. Physical and chemical properties of waste-activated sludge after microwave treatment. Water Res
Y. & LI
Y.-Y. 2017. Overview of pretreatment strategies for enhancing sewage sludge disintegration and subsequent anaerobic digestion: Current advances
full-scale application and future perspectives. Renewable and Sustainable Energy Reviews