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Anaerobic digestion is the most widely used sludge stabilisation process. AD stabilises the sludge biologically in the absence of air. As with most sludge and wastewater unit processes, the design and performance of AD depends on the feed characteristics.
In sludge stabilisation, the amount of volatile solids is reduced by converting them to a biogas (methane CH4, carbon dioxide CO2 and water H2O). The biogas then needs further processing for recovery and reuse of its methane content.
AD can be operated at 'mesophilic' temperatures, with 30−39°C the optimum range, or at 'thermophilic' temperatures, optimally 49−57°C. The precise optimum temperature within these ranges depends upon the desired outcomes in terms of biogas production and composition, solids reduction and energy efficiency.
Operation below ~20°C is referred to as 'psychrophilic'. Operation at such low temperatures can be more energy efficient.
AD is essentially a multi-step process comprising 'hydrolysis' (reaction with water to convert large organic molecules to smaller ones), 'acidogenesis' (conversion of small organic molecules to volatile fatty acids), 'acetogenesis' (conversion of volatile fatty acids to acetic acid and mineral products), and finally 'methanogenesis' (conversion of organic molecules to methane).
The first three steps concern the breakdown of large organic molecules to smaller ones, and specifically acetic acid. In the final step, methane is generated.
Anaerobic digestion systems can be designed as a single or multiple tank process, a stirred or unstirred system, and operational at either moderate or elevated temperatures.
There are also various reactor configurations, including a continuous stirred tank reactor (CSTR) and upflow anaerobic sludge blanket (UASB). The sludge can be fed either as a slurry, or as granules or a powder.
Pretreating sludge upstream of the anaerobic digestion is sometimes carried out to improve the sludge biological treatability. This is usually achieved through heat treatment, and referred to as thermal conditioning.
Thermal conditioning is especially favoured for waste activated sludge (WAS) sources. WAS is less readily hydrolysed – the first step of the anaerobic degradation mechanism – than primary sludge, the sludge produced from the initial sedimentation step of conventional municipal wastewater treatment.
Anaerobic digesters can be operated at different conditions of mixing and temperature. The temperature conditions are normally defined by the types of micro-organisms that are sustained within the temperature range applied.
A further key operating parameter is the pH. The pH must be kept within a specific optimal range to ensure that the process works effectively, as governed by the biochemical mechanism.
There are two widely recognised challenges to anaerobic digestion operation − foaming, and over-acidification.
These two factors are often inter-related since they both relate largely to control of the microbiology through balancing of the organic and nutrient loads. They also both lead to a decrease in the methane yield, which can then be diagnosed through monitoring the biogas methane and carbon dioxide concentrations.
A key aspect of the anaerobic digestion process is the generation of a biogas product which contains around 50−60% methane. The methane is a useful fuel, and must be removed from the gas stream prior to discharge since it is a highly active greenhouse gas.
The biogas can either be used directly on site in combined heat and power (CHP) co-generation or upgraded for export to the national gas grid.
The digestate is the residual slurry, containing up to 20% solids, generated from the anaerobic digestion process. It is usually rich in nutrients such as ammonia and phosphates as well as micronutrient metals. Its primary use is as a soil conditioner, subject to its meeting regulatory standards regarding toxic metals content.
If the digestate metals content is too high to allow it to be applied to soil then further treatment is required, or else alternative end disposal options, such as landfilling, explored.
The supernatant is the liquid stream generated from the anaerobic digestion process. The supernatant has a high nutrient content (nitrogen and phosphorus): nitrogen and phosphorus are not removed by AD and are instead concentrated and partitioned between the digestate and supernatant.
For municipal wastewater treatment applications, this stream is normally returned to the head of works and blended with the incoming wastewater. If this is not an option, then the supernatant must be treated prior to discharge.
