Methane production from anaerobic digestion (AD) of sludge is commonly
practiced, providing a residual carbon content of 35%. AD process is naturally
present in many ecosystems such as the digestive tract of insects (e.g., termite) and
mammalians (e.g., cows, pigs, sheep, gazelle) and humans as well as in natural and
cultivated ecosystems like wetlands, marine sediments, and rice fields where it is
actively involved in biogeochemical cycles of matter. Indeed, It has been applied
since the end of the nineteenth century for the treatment of household WWs in septic
tanks of slurries in digesters. AD not only enables to reduce and stabilize the volume
of sludge to be disposed, but it is also a way to recover energy from WW process.
This leads at least to reduce the energy demand for the aeration on the water line to
reach energy self-sufficiency for WW treatment plants. Because of these different
aspects, there is no doubt that AD is a mature and exciting process that gathers many
advantages that is worth to optimize and to promote. As for incineration, it produces
CO 2 and energy (12–20 MJ/kg of sludge’s calorific value) [23]. Nurrokhmah et al.
showed that direct application to agricultural soil presents the lowest cost compared
to the other options, whereas co-incineration had the highest cost [92].
Biomineralization represents a novel route of sludge valorization based on
microbial-induced calcite precipitation. Biomineralization is a physiological process
that allows living organisms to develop a mineral structure, called the biomineral,
which is distinguished from its purely mineral equivalent by the presence of organic
molecules that give it specific properties such as better resistance to fracture. This
phenomenon occurs naturally and chemically but over several thousand years to give
rise to rocks such as sandstone or stromatolites. Bio-calcification occurs when
microorganism metabolic activities lead to the precipitation of calcium carbonate
(CaCO 3 ). This process has been utilized in broad spectrum of applications using
different bacterial communities, such as calcium removal in WW, carbon sequestration, soil stabilization, and concrete durability improvement as well as cement
manufacturing alternative fuel [93, 94]. Bacterial precipitation of calcium carbonate
can be accomplished quickly (day-hours) by the hydrolysis of urea via a biological
enzyme catalyst, the urease. This leads to the alkalization of the microenvironment,
allowing the precipitation of carbonate ions in the presence of calcium ions. The
alkalineophilic bacterium Sporosarcina pasteurii, with high intracellular concentrations of urease, is often used to catalyze the biomineralization process. This process
is carried out without energy supply and releases ammonium, a source of valuable
nitrogen fertilizer. Biomineralization may be an efficient alternative for recycling
WW sludge. Calcium carbonate precipitation by sludge microorganisms would be
used in applications for environmental protection, material technology, and other
applications. Recently a strain called Microbacterium sp. GM-1, isolated from active
sludge, was investigated for its ability to produce urease and induce calcium
carbonate precipitation in a metabolic process. Xu et al. evidenced that
Microbacterium sp. GM-1 can biologically induce calcification, and they suggested
that this strain may play a potential role in the synthesis of new biominerals and in
bioremediation or biorecovery [95].
Due to the sludge biochemical characteristics, it has been suggested that biomineralization could potentially provide an eco-friendly cost-effective alternative for
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E. Ammar et al.
practiced, providing a residual carbon content of 35%. AD process is naturally
present in many ecosystems such as the digestive tract of insects (e.g., termite) and
mammalians (e.g., cows, pigs, sheep, gazelle) and humans as well as in natural and
cultivated ecosystems like wetlands, marine sediments, and rice fields where it is
actively involved in biogeochemical cycles of matter. Indeed, It has been applied
since the end of the nineteenth century for the treatment of household WWs in septic
tanks of slurries in digesters. AD not only enables to reduce and stabilize the volume
of sludge to be disposed, but it is also a way to recover energy from WW process.
This leads at least to reduce the energy demand for the aeration on the water line to
reach energy self-sufficiency for WW treatment plants. Because of these different
aspects, there is no doubt that AD is a mature and exciting process that gathers many
advantages that is worth to optimize and to promote. As for incineration, it produces
CO 2 and energy (12–20 MJ/kg of sludge’s calorific value) [23]. Nurrokhmah et al.
showed that direct application to agricultural soil presents the lowest cost compared
to the other options, whereas co-incineration had the highest cost [92].
Biomineralization represents a novel route of sludge valorization based on
microbial-induced calcite precipitation. Biomineralization is a physiological process
that allows living organisms to develop a mineral structure, called the biomineral,
which is distinguished from its purely mineral equivalent by the presence of organic
molecules that give it specific properties such as better resistance to fracture. This
phenomenon occurs naturally and chemically but over several thousand years to give
rise to rocks such as sandstone or stromatolites. Bio-calcification occurs when
microorganism metabolic activities lead to the precipitation of calcium carbonate
(CaCO 3 ). This process has been utilized in broad spectrum of applications using
different bacterial communities, such as calcium removal in WW, carbon sequestration, soil stabilization, and concrete durability improvement as well as cement
manufacturing alternative fuel [93, 94]. Bacterial precipitation of calcium carbonate
can be accomplished quickly (day-hours) by the hydrolysis of urea via a biological
enzyme catalyst, the urease. This leads to the alkalization of the microenvironment,
allowing the precipitation of carbonate ions in the presence of calcium ions. The
alkalineophilic bacterium Sporosarcina pasteurii, with high intracellular concentrations of urease, is often used to catalyze the biomineralization process. This process
is carried out without energy supply and releases ammonium, a source of valuable
nitrogen fertilizer. Biomineralization may be an efficient alternative for recycling
WW sludge. Calcium carbonate precipitation by sludge microorganisms would be
used in applications for environmental protection, material technology, and other
applications. Recently a strain called Microbacterium sp. GM-1, isolated from active
sludge, was investigated for its ability to produce urease and induce calcium
carbonate precipitation in a metabolic process. Xu et al. evidenced that
Microbacterium sp. GM-1 can biologically induce calcification, and they suggested
that this strain may play a potential role in the synthesis of new biominerals and in
bioremediation or biorecovery [95].
Due to the sludge biochemical characteristics, it has been suggested that biomineralization could potentially provide an eco-friendly cost-effective alternative for
68
E. Ammar et al.