All water treatment plants including the industrial as well as municipal ones produce sludge as a result of water treatment. Even though it is dried and disposed of in landfill zones, water treatment resulting in a high amount of sludge being produced could be harmful at various levels.
How the sludge is harmful
A recent study of the sludge in wastewater treatment plants of Gaziabad in India found that 60% of the sludge consisted of fine grains of sand. Silica, alumina, ferric oxide and lime also make up a major percentage of chemical components of the sludge, and some heavy metals are also present. Disposing of such sludge in freshwater sources such as rivers and streams is harmful for the environment.
In addition, there is the cost factor.
In recent years, industrial production has increased, the population has increased, and the efficiency of wastewater treatment plants has also improved. As a result, many plants are witnessing a large amount of sludge generation. This sludge must be processed further and transported to be dumped into landfill zones. Thus, in many Europan countries, the cost of sludge management is making up approximately 50% of the total running cost of a wastewater treatment plant.
Where does the sludge come from?
The sludge in a wastewater treatment plant can be classified into 3 types based on where it emerges.
Primary sludge: Made up of settable solids from raw wastewater in the primary settling stage. This type of sludge contains 2%-7% of TS is has high purterscibility and can be dewatered more easily than the biological sludge produced in the later stages.
Secondary sludge: Also called biological sludge. This is produced by biological treatment systems such as activated sludge and biofilm systems. It contains microorganisms that have grown on the biodegradable matter, endogenous residue and inert solids that are left unfiltered in the primary stage. This type of sludge contains 0.5% to 1.5% of TS.
Chemical sludge: This type of sludge is produced due to the use of specific substances such as phosphorus in wastewater treatment, and by the presence of suspended solids.
Sludge reduction techniques
Chemical treatment
Chemical oxidation is the most widely used chemical treatment to reduce sludge production. The treatment differs according to the chemical reagent used. Chemical oxidation minimizes the sludge in two ways: One part of the activated sludge is mineralized, while another part is solubilized in biodegradable organic compounds. These compounds are again oxidized in the activated sludge reactor. O3 (ozone) and chlorine are the most commonly used oxidization agents.
The chemical oxidation process improves the ability of the suspended solids in the sludge to settle down, reduces the number of microorganisms in the sludge, and makes it more easily biodegradable.
Biological treatments
MBR system
The Membrane Bioreactor process (MBR) is a mechanism used to remove the sludge that emerges during the filtration of biological waste during wastewater treatment. The MBR system is a combination of biological filtration and microfiltration or ultrafiltration processes.
The MBR system combines activated sludge treatment with slid-liquid separation using microporous membranes instead of traditional separation by secondary clarification. In a standard setup, the MBR module can be introduced into a separate tank after the oxidation tank. The MBR modules can also be introduced directly into the oxidation tank. This setup is called a compact system. The MBR system can be rearranged in a variety of configurations
Explore the working of the MBR system in detail
MMBR system
Mesophilic Membrane Bioreactor technology is being increasingly used in water treatment due to its high-quality effluent, lower volumes and lower sludge generation. MMBR system can be coupled with other treatments for the degeneration of sludge such as ozonation or ultrasonication in to increase the biomass decay rate and ensure a relatively low concentration of TSS in the MMBR system. Lastly, the absence of a secondary clarifier means that MMBR system requires a far lesser amount of space than the Activated Sludge system.
Thermal treatment
Combustion
In combustion, the sludge is oxidezed under high temperatures between 850-1000 degrees C, and in over-stoichiometric conditions of Oxygen. Before undergoing combustion, the sludge is put through mechanical dewatering and thermal drying so that it has the adequate calorific value to enable autothermal combustion.
In case the calorific value is inadequate, there could be problems in the combustion of the sludge. In such situations, the chosen method is co-combustion with materials having a high lower heating value (LHV). In many cases, the sludge is subjected to co-combustion with solid waste. This removes the water from the sludge, and also oxidizes the organic substances, considerably reducing the volume of the sludge produced.
Pyrolysis
The pyrolysis process carried out the degradation of the sludge in an inert atmosphere at a high temperature between 500-1000 degrees C. There are no oxidizing agents present here. In this process, the molecules of the sludge break down due to the effect of the temperature alone, forming 3 by-products: The solid bituminous coal, the liquid pyrolytic oil and the gaseous syngas. Out of these materials, the bituminous coal can then be transformed into activated carbon or used as a solid fuel.
Pyrolysis is especially useful because it reduces the generation of CO2, and dioxins while significantly minimizingthe amount of sludge as well. Additionally, it is a highly flexible process that also has a lower setup footprint.
Gasification
Gasification involves turning the carbon content in the sludge into ash and combustible gas. Gasification is done at temperatures higher than 500 degrees C. The dosing of the oxidation agent in this process is kept lower than the stoichiometric value compared to the amount of organic carbon. Apart from removing water and volatile substance from the sludge, gasification also yields a combustible gas that can be used as a fuel for energy production.
Conclusion
Reducing the amount of sludge generated in wastewater treatment plants is becoming particularly important because of a number of factors. The landfill zones are decreasing, which means the sludge must be transported further away spending precious money. Increasing industrial production and urban populations have also increased the amount of wastewater to be treated. On the other hand, efficient sludge reduction techniques, such as thermal ones are reducing the amount of sludge and creating possibilities to recover the energy produced directly inside the wastewater treatment plants.