Abstract
The organic fraction of waste is increasingly used for biogas production. However, the fermentation process used for this purpose also produces waste in the form of digestate in addition to biogas. Its liquid fraction can, among other things, be a source of water, but its recovery requires many advanced technological processes. Among the first in the treatment train is usually coagulation/chemical precipitation. Its application changes properties, including the size and zeta potential (ζ) of the fractions that have to be removed in subsequent processes. Changes in particle size distribution and ζ potential occurring in the liquid fraction of municipal waste biogas plant digestate and solutions after coagulation/chemical precipitation with FeCl₃·6H₂O, PIX 112 and CaO were analyzed. The particle size distribution of the raw digestate was wide (0.4–300 µm; up to 900 µm without ultrasound). The median particle diameter was about 12 µm. The ζ potential ranged from −25 to −35 mV in the pH range 5–12, and the isoelectric point (IEP) was at pH 2. The best treatment results obtained with the use of. 10 g FeCl₃∙6H₂O/dm³ shifted particle size distribution towards finer particles (median diameter: 8 and 6 µm, respectively, before and after ultrasound). The ζ potential decreased by about 5–10 mV in the pH range 2.5–12 without changing IEP. An amount of 20 g/dm³ of FeCl₃∙6H₂O caused the disappearance of the finest and largest fraction. d50 was about 21.5 µm (17.3 µm after ultrasound). An amount of 20 g/dm³ of FeCl₃∙6H₂O generated a positive high electrokinetic potential in the range of pH 1.8–5. The IEP appeared at pH 8, and after reaching about −5 mV it again became positive at pH about 11.
Highlights
The constantly growing world population and social and economic growth are increasing energy demand [1]
The energy system is still based on non-renewable energy sources such as coal, oil, and natural gas, and their use for this purpose is associated with a negative impact on the environment [2]
Coagulation/chemical precipitation can be considered as an effective technique of the liquid fraction of digestate from municipal waste plant digestate pre-treatment, but its effectiveness significantly depends on a type and dose of coagulant, as well as the flocculation time
Summary
The constantly growing world population and social and economic growth are increasing energy demand [1]. The energy system is still based on non-renewable energy sources such as coal, oil, and natural gas, and their use for this purpose is associated with a negative impact on the environment [2]. The process of methane fermentation in biogas plants leads to the formation of products such as biogas and digestate. They can be used as a source of heat and electricity (biogas), fuel (pellets and briquettes), an excellent fertilizer for plants and animal bedding or process water [9]
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