Modern approaches to treatment and recovery of secondary sludge of domestic sewage

For today, pollution of the environment, in particular of surface waters, has led to an environmental crisis in many countries of the world. One of the reasons for this is the use of outdated approaches to the treatment and recovery of sewage sludge. The article presents the results of the study of literary sources in order to analyze the proposals for the treatment of secondary sludge of domestic sewage in different countries. To neutralize sewage sludge, scientists consider the possibility of obtaining fertilizes under conditions of biosulfidogenesis during the dissimilation recovery of poorly soluble sulfates or the use of enhanced oxidation technology. A new concept of domestic sewage treatment has also been proposed, which can solve the issue of sewage sludge control at the place of its forming. The main areas of sustainable sludge control are its use in agriculture as fertilizer and for the reclamation of devastated or degraded lands, as well as energy recovery by burning and alternative thermal methods such as pyrolysis, quasi-pyrolysis and gasification. It was established that the applicability of this or that technology of sewage sludge recovery depends on many local factors, in particular: productivity of sewage station; composition and methods of sewage treatment and its sediments; efficiency of sewage treatment plants; climatic zone of the sewage system location; availability of energy and material resources, etc. Today, it is relevant to monitor the qualitative composition of sewage sludge, as well as soils and natural waters regarding pollutants that can be detected in the sewage of the corresponding settlement, in order to make operational decisions to control environmental risks, as well as conduct scientific research to improve recycling and recovery technologies for sewage sludge of various composition in agricultural systems, which will help to protect the environment against pollution and rational use of land.

Secondary Sludge Biodegradation and Electricity Generation in Biocathode Microbial Fuel Cells

The looking for sustainable sewage sludge management technology in the wastewater treatment plants, has brought to light the biocathode microbial fuel cells (bMFCs) which allow simultaneous biological stabilization and direct energy generation, avoiding the production of biogas. In the present study, the performance of bMFCs for the treatment of secondary sludge as anodic substrate was evaluated by analyzing the removal of organic matter, destruction of volatile solids and the generation of electrical energy under different operating conditions and applying two types of cathode chambers. The results indicated that VSS and tCOD removals up to 92% and 87% respectively can be achieved in the anodic chamber generating simultaneously energy. Current and power densities of 1.80 ± 0.09 A∙m−3 and 0.43 ± 0.02 W∙m−3 respectively were reached, showing that bMFCs are a reliable alternative to generate electricity during the sewage sludge stabilization process. It was revealed that the pH value and the type of cathodic zone are statistically significant factors that influenced the performance of the bMFCs. The obtained results demonstrated that the electrochemical performance of the bMFCs was better at pH value of 6 in the anodic chamber and when aerobic cathode zone was used.

Co-Digestion of Extended Aeration Sewage Sludge with Whey, Grease and Septage: Experimental and Modeling Determination

The potential of co-digestion mixing thickened secondary sludge (TS) from extended aeration wastewater treatment plant and locally available substrates (whey, grease and septage) has been studied in this work, using three steps. The first step was a batch test to determine the biological methane potential (BMP) of different mixtures of the three co-substrates with TS. The second step was carried out with lab-scale reactors (20 L), simulating anaerobic continuous stirred tank reactors, fed by three mixtures of co-substrates that were determined according to the previous step results. Modeling was applied in the third step, using ADM1 as a mechanistic model to help understand the co-digestion process. According to the BMP step, septage used as a co-substrate has a negative effect on performance, and the addition of 10–30% grease or whey would lead to a gain of around 60–70% in the production of methane. The results from the reactor tests did not validate the positive effects observed with the BMP assay but confirmed good biodegradation efficiency (> 85%). The main purpose of co-digestion in this scenario is to recover energy from waste and effluents that would require even more energy for their treatment. The protein and lipid percentages of particulate biodegradable COD are important variables for digester stability and methane production, as predicted by modeling. The results of simulations with the ADM1 model, adapted to co-digestion, confirmed that this model is a powerful tool to optimize the process of biogas production.

Co-digestion of Extended Aeration Sewage Sludge With Whey, Grease and Septage: Experimental and Modelling Determination

Potential of co-digestion mixing thickened secondary sludge (TS) from extended aeration wastewater treatment plant and locally available substrates (whey, grease and septage) has been studied using three steps. The first step was a batch test to determine biological methane potential (BMP) of different mixtures of the three co-substrates with TS. The second step has been carried out with lab-scale reactors (20 L) simulating anaerobic continuous stirred tank reactors fed by three mixtures of co-substrates determined according to previous step results. Modelling using ADM1 as a mechanistic model was applied in the third step to help understanding the co-digestion process. According to BMP step, septage used as co-substrate has a negative effect on performance and addition of 10 to 30% grease or 10% whey would lead to a higher production of biogas and with an increase of the methane content. The results from the reactor showed less evi-dence of the positive effects observed with the BMP assay. Protein and lipid fractions of particu-late biodegradable COD are important variables for digester stability and methane production as predicted by modelling. Results of simulations with ADM1 model adapted to co-digestion confirmed that this model is a powerful tool to optimize the process of biogas production.

Heavy Metal Potential of Domestic Biological Wastewater Treatment Plant Sludge

The use of sludge as fertilizers helps to the recycling of nutrients to the environment due to its beginning from wastewater and therefore relatively high content of nutrients and organic matter. However, wastewater does also contain hazardous compounds like heavy metals and micro pollutants which eventually are separated to the sludge during the treatment processes at the wastewater treatment plant. In order to improve the sludge quality, source tracking is a relatively cheap and effective way to find and eliminate hazardous compounds and prevent them from ending up in the sludge. The aim with this thesis was to investigate the fertilizer potential in sludge from wastewater treatment plant. The sludge is analyzed in terms of nutrient and heavy metal contents as well as physiochemical parameters. In order to determine the potential as fertilizer, the quality of the sludge is compared with other fertilizer alternatives, other sludge types and regulations for sludge use in agriculture. The work with sludge quality improvements in Gaborone is also investigated. Sludge from different stages along the treatment processes were collected in order to see differences in quality related to the treatment. The results shows that the samples of primary an aerobically treated sludge tend to have higher heavy metal content than the secondary sludge. The quality of the dry sludge samples indicates lower nutrient content than both the primary and secondary sludge, but similar heavy metal content. The analysis of dry sludge from indicates a low nutrient value and high heavy metal content in comparison to other selected fertilizer options and sludge from other. The processes for removal of nutrients from the wastewater are an important factor for the nutrient content in the sludge. Keywords wastewater; fertilizers; nutrients; sludge treatment

Enhanced Mesophilic Anaerobic Digestion of Primary Sewage Sludge

Processing of the produced primary and secondary sludge during sewage treatment is demanding and requires considerable resources. Most common practices suggest the cotreatment of primary and secondary sludge starting with thickening and anaerobic digestion. The aim of this study is to investigate the anaerobic digestion of the primary sludge only and estimate its impact on sludge treatment and energy recovery. Within this context, the performance of the anaerobic digestion of primary sludge is explored and focused on practices to further enhance the methane production by using additives, e.g., a cationic polyelectrolyte and attapulgite. The results showed that the overall yield in methane production during anaerobic digestion of primary sludge alone was higher than that obtained by the anaerobic digestion of mixed primary and secondary sludge (up to 40%), while the addition of both organic polyelectrolyte and attapulgite enhanced further the production of methane (up to 170%). Attapulgite increased the hydrolysis rate of biosolids and produced relatively stabilized digestate, though of lower dewaterability. Moreover, the results suggest that single digestion of primary sludge may accomplish higher methane production capacities at lower digestors’ volume increasing their overall efficiency and productivity, while the produced digestates are of adequate quality for further utilization mainly in agricultural or energy sectors.