Keywords

1 Introduction

Wastewater that has been treated and refined from the wastewater treatment plant (typically consisting of preliminary, primary and secondary treatment) has the potential to be valorized into valuable bioproducts such as bioenergy and biomaterials. The end waste at the treatment site is in the form of dewatered sludge (biosolid) which is composed of a lot of nutrient composition and tonnes of microbes. Table 1 tabulates the differences between sludge, sewage sludge, activated sludge and leachate.

Table 1 Comparison of sludge, activated sludge, sewage sludge, and leachate
Full size table

2 Overall Reaction and Type of Microbe Degraders

2.1 Sugar Degrader

The source of sugar came from lignocellulosic material which can be found in plant-derived residue and waste such as paper mill sludge [1]. Research conducted by Ducan and team [2], found that the conversion of mill sludge to sugar later can be used as either isoprene or ethanol. Based on Yildiz et al. [3, 6], microorganisms are used due to their ability to remediate the sugar industry effluent. The application of microorganisms is eco-friendly because they do not require any chemicals during the sludge treatment. Basically, lactic acid bacteria (LAB) that are used as sugar degrader reacted can be monitored by the reduction of pH. There are several types of LAB strains that are used as sugar degraders, for example, Lactobacillus plantarum, Lactobacillus casei, and Streptoccuslactis. The production of lactic acid in the early fermentation stage suppressing the growth of putrefying bacteria while enhancing the availability of inorganic compounds which are being used by these lactic acid bacteria for growth and reproduction.

Besides, Saccharomyces cerevisiae is the most useful microorganism for ethanol production through alcoholic fermentation by metabolizing sugar in the absence of oxygen which leads to the production of ethanol and carbon dioxide [9].

The metabolic reaction of sugar degradation is further described below:

$${\text{C}}_{6} {\text{H}}_{12} {\text{O}}_{6} \to 2{\text{CH}}_{3} {\text{CH}}_{2} {\text{OH}} + 2{\text{CO}}_{2}$$
(1)

where \({\mathrm{C}}_{6}{\mathrm{H}}_{12}{\mathrm{O}}_{6}\) is glucose, \({\mathrm{CH}}_{3}{\mathrm{CH}}_{2}\mathrm{OH}\) is ethanol and \({\mathrm{CO}}_{2}\) is carbon dioxide.

2.2 Amino Acid Degrader

Amino acid is a soluble monomer from a breakdown of complex organic matter dependent upon syntropic interaction of a consortium of microorganism in anaerobic digestion [10]. Amino acids vary significantly in size and structure and are fermented via different pathways to a range of products where these products are built up by amphoteric substances that contain amino and carboxyl groups. These amino acids are comprised of a four-step process of hydrolysis; amino acid fermentation, acid production and methanation of the anaerobic degradation process of proteins.

The degradation of amino acids produces organic compounds such as ammonia, carbon dioxide and small amounts of hydrogen and sulphur compounds. Amino acids are degraded in two ways that include deamination through a Stickland reaction; injection of two types of amino acids. One side of the amino acid (containing the majority of the carbon atoms) acts as an electron accepter, while the other (containing one or only a few carbon atoms) acts as an electron donor.

The reaction that takes place is the deamination by bacteria within the Clostridium species (obligatory species). The second type of amino acid decomposition occurs through the general fermentation process of single amino acids that requires the presence of hydrogen-utilizing bacteria. The fermentation of amino acids by the Stickland reaction; a chemical reaction that involves the coupled oxidation and reduction of amino acids to organic acids, is known to be the dominant reaction among these two types [11, 12].

Based on Table 2, there are five classifications of the bacteria based on their involvement in Stickland reactions and the amino acids that they typically utilize [11]. Group I bacteria are organisms that carry out the Stickland reactions. Fermentation process intermediately utilize proline and produce δ-aminovalerate, α-aminobutyrate or γ-aminobutyrate by these enzymes were accumulated with Clostridial species. While Groups II, III, IV and V do not carry Stickland reactions but ferment amino acids. These classifications mainly form obligate spore-formers (Clostridial species) and some non-sporing obligate anaerobes, for example, Peptostreptococcus (Micrococcus) spp.

Table 2 Classification of anaerobic bacteria which degrade amino acids [11]
Full size table

Table 3 summarizes the amino acid metabolic degradation. All of the reactions are described either as Stickland or non-Stickland where there are five amino acids involved in Stickland reaction. These reactions can act either as an electron donor or electron acceptor.

Table 3 Stoichiometry for amino acid fermentation (catholic reactions only) [11]
Full size table

2.3 Long-Chain Fatty Acid (LCFA) Degrader

Long-chain fatty acid (LCFA) is generated from the hydrolysis of lipids in sludge [13]. Fatty acids are organic molecules composed of a hydrophilic head, a carboxyl group and a hydrophobic aliphatic tail. The absence or presence of double bonds in the fatty acid aliphatic chain makes them saturated or unsaturated. Saturated and unsaturated LCFA are palmitate and oleate, respectively, thus they become the most abundant constituents [14]. The Prime way to identify the differences between saturated and unsaturated LCFAs are the presence of double bond in the fatty acid aliphatic chain, respectively. Table 4 showed the common unsaturated and saturated LCFA found in wastewater.

Table 4 Saturated and unsaturated LCFA commonly found in wastewaters (shown as % of total LCFA) [14]
Full size table

Hydrogen transfer between microorganisms plays a central role in LCFA degradation in methanogenic environments. This degradation through obligation syntrophic communities of proton-reducing acetogenic bacteria, converting LCFA to acetate and hydrogen/formate, acetoclastic methanogenic archaea, and hydrogen/formate-consuming methanogenic archaea as shown in Table 5.

Table 5 Gibbs free energy changes for some of the acetogenic and methanogenic reactions (presumably) involved in syntrophic conversion of different fatty acids [14]
Full size table

The degradation of saturated LCFA follows the classic ꞵ-oxidation pathway while the unsaturated LCFA may require a preliminary step of hydrogenation or an alternative degradation pathway. The coculture of Syntrophomonas and Methanospirillumhungatei can degrade palmitate in LCFA [15, 16]. There are 14 fatty-acid-degrading syntrophic bacteria that have been obtained in pure culture and coculture with hydrogen-consuming microorganisms, all belong to Syntrophomonadaceae and Syntrophaceae within the phyla Firmicutes and Deltaproteobacteria, respectively. During fatty acid degradation, these syntrophic bacteria are working together with hydrogenotrophicarchaea or hydrogen-consuming sulphate-reducing bacteria [14].

2.4 Valerate and Butyrade Degrader

Butyrate and valerate are two compositions which can be found in a typical volatile fatty acid of an acidic anaerobic digestion reactor of sludge [17]. The degradation kinetics of normal and branched chain butyrate and valerate are important in protein-fed anaerobic systems, as a number of amino acids degrade to these organic acids.

Based on Table 6, the degradation for both n-buytrate and n-valerate is via ꞵ-oxidation to acetate and acetate + propionate, respectively. The organisms that are capable to degrade butyrate are Syntrophaceae sp, Tepidanaerobacter sp. and Clostridium spp. Typically, if one of these substrates can be degraded by these organisms then it may potentially degrade the others. I-butyrate is also oxidized by the same organisms, and reciprocal isomerism between the two forms of butyrate has been well established [18, 19]. Both neo-valerate and i-valerate are more complex and difficult to access in environmental situations, as they are lumped in gas chromatography measurements.

Table 6 Butyrate and valerate degradation reactions [18]
Full size table

Clostridium bryantiisp. can oxidize neo-valerate to acetate and propionate via ꞵ-oxidation while i-valerate degrades to acetate as the only organic acid product [18].

.

2.5 Propionate Degrader

Abundance of Smithella spp. among Syntrophotbacterales indicates syntrophic degradation of propionate and butyrate. The syntrophy of bacteria (illustrated in Fig. 1) is responsible for carrying out degradation of amino acids, aromatic compounds and propionate and butyrate which ultimately leads to the formation of \({\text{CH}}_{4}\) [20].

Fig. 1
A flow diagram explains how the syntrophy of bacteria degrades amino acids, aromatic compounds, and propionate and butyrate which leads to the formation of methane.

Adapted from [20] (Created with Biorender)

Schematic representation of anaerobic carbon mineralization in sewage sludge with the microbial communities.

Full size image

2.6 Acetate Degrader

The source of acetate in sludge is originated from the conversion of volatile fatty acid in dark fermentation: acetogenesis [17]. Acetotrophic is a condition in which methyl groups are reduced by Methanosarcinales genus which uses simple compounds (acetate) for their growth. Acetotrophic methanogens are obligatory anerobes that transform acetate to methane and carbon dioxide. It was found that, during the anaerobic processing of sewage sludge and manure, the number of Methanosaeta genus increased with decreasing acetate in environment, simultaneously intensive growth of bacteria which are acetotrophic methanogens [21, 22]. Research conducted by Detman et al. [23] highlighted that Methanosaeta genus can be evaluated based on MAGs phylogenetic tree which shows Methanothrix soehngenii had the most abundant (12.1%) [23].

Stoichiometry reaction degradation of acetate:

$${\text{Acetate}} + {\text{H}}_{2} {\text{O}} \to {\text{HCO}}_{3}^{ - } + {\text{CH}}_{4}$$
(2)

where \({\text{H}}_{2}{\text{O}}\) is water, \({\text{HCO}}_{3}^{-}\) is bicarbonate and \({\text{C{H}}}_{4}\) is methane.

$${\text{CH}}_{3} {\text{COO}}^{ - } + {\text{SO}}_{4}^{2 - } \to 2{\text{HCO}}_{3}^{ - } + {\text{HS}}^{ - }$$
(3)

where \(\mathrm{\text{C}}{\text{H}}_{3}{\mathrm{COO}}^{-}\) is acetate and \(\mathrm{S}{\text{O}}_{4}^{2-}\) is sulphate.

2.7 Hydrogen Degrader

The anaerobic microorganisms produce hydrogenase enzyme which is capable to evolve and taking up hydrogen (\({\text{H}}_{2}\) [24]. Hydrogen production by fermentative microorganisms is an expectable method compared with photosynthetic bacteria due to its high utilization of organic compounds or wastes as substrate to produce hydrogen day and night. The production of molecular hydrogen (fermentation process) is generally associated with intracellular iron–sulphur protein, ferredoxin, which is an electronegative electron carrier [24]. The electrons transfer from ferredoxin to \({{\text{H}}}^{+}\) is catalyzed by hydrogenase enzyme. Two classes of fermentative bacteria are capable of producing hydrogen at a high rate and yield, including strictly anaerobic and facultative anaerobic bacteria. First Clostridium butyricum largely utilized in the biotechnological hydrogen production and secondly Klebsiella pneumonia typically a facultative anaerobic bacteria as nitrogen fixing [24].

Reaction 4summarized stoichiometry for both Sporomusasphaeroides and Woliniela for reduction of \({\mathrm{CO}}_{2}\) to acetate.

Metabolism degradation of hydrogen:

$${\text{H}}_{2} + {\text{fumarate}} \leftrightarrow {\text{Succinate}}$$
(4)

Clostridium butyricum strict anaerobic bacterium, is known as a classical acid producer and usually ferments carbohydrates to butyrate, acetate, carbon dioxide, and molecular hydrogen [25, 26]. Based on Fig. 2, there are two pathways to produce hydrogen, one is via the cleavage of pyruvate to acetyl-CoA and the other to \(\text{NAD}^{+}\) to generate \(\text{NADH}_{2}\).

Fig. 2
The diagram presents the two pathways to produce hydrogen, one is via the cleavage of pyruvate to acetyl-C o A and the other to N A D plus to generate N A D H 2.

Adapted from 24 (Created with Biorender)

Metabolic pathway of glucose by Clostridiumbutyricum under anaerobic conditions. (1) Pyruvate: ferredoxinoxidoreductase (PFOR); (2) Hydrogenase; (3) NADH: ferredoxinoxidoreductase.

Full size image

The production of 2,3-butaediol, ethanol and lactate from pyruvate by \(\text{NADH}_{2}\) as a reductant, but not for \(\text{H}_{2}\) [27].While Klebsiella pneumonia; a facultative anaerobic and nitrogen-fixing bacteria also has the ability to produce hydrogen in high quantities. Nitrogen is mainly associated for hydrogen production by K. pneumonia.

2.8 Sulphate Degrader

Sulphate ion (\({\mathrm{S}0}_{4}^{2-})\) is one of the most universal anions occurring in rainfall, especially in air masses that have encountered metropolitan areas (During anaerobic conditions, sulphate is reduced to sulphide by sulphate-reducing bacteria [SRB]). This SRB play a fundamental role as sulphate bioremediator through the conversion of sulphate to sulphide in the stabilization process [5]. Additionally they can compete with other anaerobic bacteria for a wide range of carbon sources and electron donors such as glucose, lactate, propionate, acetate, butyrate and ethanol. SRB found famously to grow at pH range 6–8 or called as neutrophilic condition [28]. Sulphate reducers that degrade carbon can be divided into two groups: (i) bacterial group that can completely degrade the carbon to carbon dioxide and (ii) bacterial group that catalyze partial carbon degradation to acetate which can be clearly figure in Table 7. The SRB can generate twice as much energy during the incomplete oxidation of lactate compared with its complete oxidation [29].

Table 7 Reduction of sulphate only partially oxidized [30]
Full size table

3 Conclusions

This book chapter provides an insight into the fundamental components of sewage sludge, including the natural microbe degraders present in the sludge. The knowledge of the microbial community in the sludge allows for the exploitation of the sludge and the isolation of suitable microbes for bioremediation purposes. The microbiological approach is a greener method for solving environmental pollution and has the potential to provide a sustainable solution.In addition to bioremediation, the chapter highlights the potential for the purification of useful chemical compounds from sewage sludge, such as expensive fatty acids that can be obtained through the isolation of certain species found in the sludge. This demonstrates the potential for the valorization of sludge in new emerging green technologies.One such technology is the microbial fuel cell (MFC), which requires a comprehensive and effective microbial degrader to accelerate the degradation process and increase the oxidation process, resulting in higher current density for energy recovery. The understanding of the microbial degraders in sewage sludge is essential for the development of effective and sustainable approaches to sludge management.Overall, this book chapter provides an insight into the components and typical mechanisms of sludge degrader microbes in dewatered sludge and highlights the potential for the exploitation of sludge valorization in sustainable approaches to sludge management. The utilization of natural microbe degraders can provide solutions to environmental pollution, produce valuable chemical compounds, and contribute to the development of new emerging green technologies such as the microbial fuel cell.