1. Introduction
Cellulose is among the most widespread and copious nature of polysaccharides [1], playing a vital role in the plant structure. On average, a dry plant consists of about 40% cellulose. The cellulose microfibrils are insoluble cellulose crystalline, upon which enzymatic saccharification is a demanding process. Consecutive sugars in the cellulose chains are turned around by 180ºC. Cellulose tends to contain a mixed pattern of orderly and non-orderly regions, thus forming crystalline and amorphous regions. Cellulose is among the constituents of biomass, along with hemicellulose, lignin, and some extractives. Cellulose is the largest fraction, whose polymeric structure is linked by glycosidic bonds. According to Zhu and Zhong [2], Naturally produced cellulose has Iα and Iβ structures. Kim et al. [3] suggested that Lignin can be converted catalytically as demonstrated by Ha et al. [4], as well as Sirous-Rezai and Park [5].
Cellulose is mainly found in wastewater. Wastewater is a diverse classification, encompassing all natures of water that have been in contact with humans. It may contain physical, organic or inorganic chemical, biological, or any other impure entities. As water is the driving force of all nature, it may be generated from domesticated and suburban areas, industries, storms and rain water, material processing facilities. Sewage water, in particular, is a subclass of wastewater, which is generated from populous areas. According to the textbook definition, sewage water is any water that contains organic human waste, one of which is toilet paper, which has been regarded as a prime precursor of cellulose that occurs in wastewater.
The per capita consumption of toilet paper per person can be viewed with respect to regional variation in Fig S1. The amount of toilet paper sales varies between countries and the majority of these sold toilet papers meet their fate in municipal wastewater/cellulose and is rather difficult to be rid of, and requires rather extensive physicochemical and biological treatments, resulting in a substantiate amount of capital expenditures (CAPEX) and operating expenditures (OPEX) of wastewater treatment plants. Polluted water contains proteins, lipids, and cellulose, which is a carbohydrate in nature, rich in carbon sources and hence the microbial degradation rate is sluggish. The biologically aided hydrolysis of cellulose is highly susceptible to temperature and the retention time of the sludge. Various research groups have attempted to evaluate several iterations and conclude the optimum conditions and have quoted results which are unfortunately not easy to compare. Principal factors related to the degradation or elimination of cellulose, such as oxygen demand, sludge production, ability to be dewatered, and nutrient removal efficiency have not been thoroughly weighed yet. Cellulose is expected to compose 30% of the total presence of chemical oxygen demand (COD) in the influents of wastewater treatment plant in countries like the Netherlands [6]. Table 1 summarizes the cellulosic content in wastewater, along with the average toiled paper consumption per person in various areas, while Fig. S1. represents the overall usage of TP per person regionally, and the resulting cellulose content, as it can be estimated to be 45–50% of the total TSS occurring in wastewater.
The quantity of sludge grows exponentially with each passing year, as progressively more industries and residences are set up, and existing developments are upsizing. The amount of sludge occupies about 50% of all industrial and inland wastes. There currently exists different routes for waste sludge management and disposal. Domestic sludge is seen as a biomass [7], reserve or resource, due to the huge amounts of constituents such as proteins, carbohydrates, extra polymeric substances, fibrous materials such as cellulose in waste sludge. These constituents, if recovered, are expected to reduce the amount of waste sludge produced and provide a potential to be utilized in industries due to its chemical nature.
Cellulose recovery has not been able to receive dedicated prominence and large scale applicational viability, due to equivocacy in nutrient elimination that has not been evaluated meticulously in recent years, such as the intervention of carbonaceous matter by a mesh sieve, which could hinder the subsequent removal of nutrients. In addition, the influential variables relating to cellulose recovery have not been discussed satisfactorily, which also contributes towards the ineptitude of implementation of cellulose recovery. The present review aims to present a sound rationale for the doubtful understanding of cellulose recovery technologies.
2. Analytical Methods for Quantification of Cellulose
To predict and quantify the fate of cellulose in the wastewater treatment process, reliable methods are required. A quantitative form of analysis predicts the number of fractions present in the material of interest along with their share in the mix, and it is mandatory that an understanding of cellulose quantification is gained to appropriately forecast its destiny in different treatment procedures. Different treatment processes produce varying outcomes and therefore present a need to be measured consistently. Several methods have been developed, which produce trustworthy results. Hurwitz, Beck, Sakellariou and Krup [13] investigated on the cellulose content in wastewater sludge by gravimetrical methods and with (Copper Ammonium Hydroxide) Schweitzer reagent as a cellulose solvent and deduced that cellulose content in primary sludge and wastewater was between 4.5 to 13.5%, which made up 2–10% of the total suspended solids (TSS). The Anthrone method was developed by Hofsten and Edberg [14] in 1972, for the quantification of cellulose and hemicellulose, post hydrolysis with sulfuric acid solution. The phenol-sulfuric acid method was established by Honda, Miyata and Iwahori in 2000, to quantify cellulose in wastewater samples. According to Honda et al. [15] cellulose fraction was 17% in the primary sludge of wastewater and ultimately, 7% of TSS. Honda, Miyata and Iwahori [16] went a step further and developed another process to split up cellulose fractions from the wastewater via aqueous sulfuric acid hydrolysis of sludge and further treatment in autoclave. In their research, Honda et al. [16] applied the method of autoclave in order to recover cellulose from waste sludge. The waste sludge was immersed in 0.3 vol% sulfuric acid for a duration of 2 h. This prepared sample was then autoclaved at 130ºC for 1 h. The outlet was cleansed with distilled water to neutralize the pH. The purity of cellulose was affected by the total percentage of cellulose in the samples, implying that low cellulose content samples had a purity of up to 3.4% while high cellulosic content samples showed a purity higher than 70%. Such are the analytical methods which have been determined by researchers in the period of 70 s and 80 s for the quantitative analysis of cellulose. These are rendered outmoded and have not been updated any further.
In a recent review, Gupta et al. [17] analyzed four different methods of measurement for the recognition of cellulosic components in sludge and wastewater, which were sulfuric acid hydrolysis, National Renewable Energy Laboratory (NREL), enzymatic hydrolysis, and Schweitzer method. It was deduced that the Schweitzer method was the most dependable and produced accurate results. The other hydrolysis methods were time consuming and showed a temperature dependance on conversion efficiency, dependability, and duplicability. Fig. S2. provides a quick insight into the aforementioned cellulosic quantification procedures.
3. Cellulose Degradation
3.1. Cellulose Degradation Mechanisms
As mentioned previously, cellulose aggregates for a cardinal part of the chemical oxygen demand (COD) and the suspended solids (SS) in the inflowing wastewater of municipal treatment facilities. The effectuation of pollutant abatement is prejudiced by the degradational ability and efficacy in activated sludge systems. Various authors have reported diverse ways for cellulose degradation with their efficiencies. Edberg and Hofsten [14] conducted investigation on cellulose degradation in anaerobic conditions with the help of nylon bags and concluded that 70% cellulose was degraded efficiently over the period of 30 d. A similar search was conducted by Verachtert et al. [18] who analyzed the degradation of cellulose in both anaerobic and aerobic conditions but with the similarity of using nylon bags. The results depicted cellulosic degradation of 50% in aerobic setting and 60% deterioration in anaerobic conditions. The Schweitzer method was elaborated by Hurwitz et al. [13] wherein aerobic cellulosic deterioration was studied in a laboratory scale setup. After a duration of 3 days, at a temperature of 12–13ºC, the percentage of cellulose that had been degraded was 6.7%. in another batch the temperature was ramped up to 23 degrees Celsius and a contact time of 4 days was provided which increased the deterioration efficiency to 87%. According to Ahmed et al. [11], increasing the contact time to 4 days from 3 days at a temperature of 12–13 degrees Celsius increased the efficiency of deterioration to 20%, which meant that temperature effect could be offset by the increase in contact time. Secondly, the cellulosic degradation rate increased in appreciable proportion to the mixed liquor suspend solids (MLSS) concentration, thus making it proportional to the Solid Retention Time (SRT).
Ruiken et al. [6] observed that cellulose degradation in anaerobic conditions was reliant on temperature and weather, since 10% of the cellulose degraded in 20 days during winter when the temperature was 9 degrees Celsius compared to a complete annihilation of cellulose was observed at 24 Celsius during the summer in a period of 12 days. Ghasimi et al. [19], [20] shed light on the anaerobic biodegradation of sieved sludge which was cellulose-rich in nature and whose degradation efficiencies were 57% under mesophilic and 62% under thermophilic temperature conditions. Moreover, Alvarez et al. [21] demonstrated cellulosic biodegradation from a tissue paper which provided a disintegration efficiency of 50%. This discussion leads one to opinionate that there are considerable variations in rates of degradation of cellulose contents in sludge and wastewater samples. Therefore, there is an obvious lack of understanding regarding the unavailability data on the mass balance of the conversion of cellulose in water resource recovery facilities [11]. It is important to note that all estimated cellulose degradation efficiencies were determined under controlled conditions (Lab scale setups and nylon bags) apart from Honda et al. [15] and therefore it is an imminent need to conduct large scale studies, backed up by laboratory observations and conducting thorough mass balance calculations in order to explicate the degradation route of cellulose in waste resource recovery facilities. As per Ahmed et al. [11] tremendous benefits pertaining to wastewater treatment plants are to be expected by way of removing fibrous materials from the influent of wastewater, as cellulose forms a major chunk of lethargically biodegradable chemical oxygen demand (COD). Table 2 provides a comprehensive review of the degradation efficiencies of cellulose.
To further elucidate the degradation of cellulose, it is important to first understand its chemical nature. As previously mentioned, cellulose is created from a couple hundred to millenarians of D-glucose subunits connected with 1,4-glycosidic linkages, four hydroxyl groups at one chain end, one half-acetal-hydroxyl group and three free hydroxyl groups at the other. As compared to other polysaccharides, the cellulose chain contains glucose molecules in reversed (beta) orientation, in which the anomeric carbon has a hydroxyl group, which is directed above the plain of the glucose ring, whereas the rest of the carbon atoms are beneath the plane of the ring. The presence of semi-acetyl groups cause cellulose to exhibit properties similar to aldehyde functional group albeit the actual presence of aldehyde groups and the lengthy chain structure protect cellulose’s ability to be reduced. Under acid catalysis, glycoside bonds could be hydrolyzed to glucose, with the added increase in temperature. Golova and Nosova [22] presented that in juxtaposition, cellulose has improved stability in alkaline conditions with the exception of a circumstance where polymerization of cellulose is possible to a certain level. Chaudemanche and Navard [23] further elaborated that the presence of hydroxyl groups in the change give rise to hydrophilicity, up to a certain level.
Cellulose forms a rigid and stacked crystal microfiber structure due to the presence of hydroxyl groups where Van der Waals force causes hydrogen bonds to form between adjacent chains, giving the cellulose a parallel band structure [24]. Cellulose hydrolysis is conducted with the assistance of three kinds of enzymes (Fig. 1.). The initiative step is taken by endo-glucanases by attacking cellulose fibers, particularly at the amorphous ends which are less resistant than the crystalline portion. Following the prior, hydrolysis of long chains to produce shorter chains which undergo further hydrolysis by exo-glucanase enzymes. This results in the previously formed short chain molecules to break down into tetroses and dioses. The decomposition of β-glucosidase takes place in the terminating step, thereby generating monomers. Wilson [25] put forth a concern that although there is limited quantified data available for cellulosic breakdown in aerobic environment, light is yet to be shed upon cellulosic degradation in anaerobic environment.
3.2. Cellulose degrading microorganisms
Fig. 2. summarizes microorganisms capable of cellulase hydrolysis of cellulose. According to Lynd et al. [26], a handful of organisms are well able to digest cellulose, among which the showstoppers are Clostridiales and Actinomycetales. Taking a further look allows one to find bacteria which are capable of cellulosic hydrolysis in anaerobic conditions, of genes particularly Anaerocellum, Caldicellulsiruptor, Eubacterium, Ruminococcus, Spirochaete, Fervidobacterium, Butyryvibrio, Acetivibrio, and Closteridium; and bacteria that perform cellulosic dilapidation in anaerobic environments, with genes namely Bacillus, Cellulomonas, Cytophaga, Erwinia, Pseudomonas, Rhodothermus, Thermobifida, Acidothermus, Caldibacillus, Micromonospora, and Sporocythophaga, to name a few. These bacterial cultures can be readily found in sewage sludge and wastewater.
It has been established that complex cellulase systems, illustrated by thoroughly studied polycellulosome organelles, are responsible for the cellulose breakdown caused by anaerobic bacteria. These cellulolytic enzymes release significant numbers of extracellular enzymes into the surrounding environment and are disseminated in the fluid phase as well as on the exterior of the cell. Enzymes are primarily located on the cell surface of aerobic bacteria. The type of enzymes utilized greatly influences the likelihood of their recovery from response stimuli and application to science.
Various strains of fungi are also functionalized in cellulosic hydrolysis, especially protist similar, primordial Chytridiomycetes and progressive Basidomycetes [26]. Deterioration of traditional cellulose is much more elementary than degradation of cellulosic biomass, due to the presence of hemicelluloses and lignin. Cellulosic biomass contains structural complications which affect the breakdown process and the process of enzymatic diffusion towards the site of attack. The ease of decomposition varies between the plant components; cellulose, hemi cellulose and lignin, with lignin being the toughest to break down. Conforming to Li et al. [27] it takes several weeks for the complete decomposition of cellulose by anaerobic methane assimilation. It was anticipated that a solid retention time of 40 days yielded 83% degradation of toilet paper, a common source of cellulose in wastewater in activated sludge systems. Less than 60% of entering cellulose may be effectively decomposed at a sludge retention time (SRT) of 28–35 days in conventional water treatment facilities. Cellvibrio, which is found in sludge, was the primary cause of the fiber hydrolysis.
4. Cellulose Recovery Technologies
4.1. Clarifier
Cellulose is generally screened or extracted in the pretreating steps of wastewater treatment and is disposed of together with the sludge produced during the decisive step of the wastewater treatment process. Rusten and Ødegaard, [28] proposed fine mesh sieves as a substitute to primary clarifiers to clear the cellulose from wastewater and are employed extensively in the Northern Americas and Europe. Ruiken et al. [6] investigated the usage of fine mesh sieves (<0.35 mm) as a treatment for municipal biological wastewater treatment and found that cellulose fibers that derived from toilet papers were removed efficiently with both high purity and recovery, [28].
It is established that toilet paper is a prime precursor of cellulose in wastewater, wherein they will be reduced to cellulose fibers ranging up to 1 mm in length [6]. Fig 5. shows a typical municipal wastewater treatment plant with the necessary unit operations and processes to treat wastewater, including biological treatment unit, grit chamber, primary and secondary clarifier, and aerated assimilation tank.
Wastewater enters the grit chamber consisting of sand, gravel, and other elements. It helps to retain physical impurities present in wastewater and safeguard downstream units, thereby reducing the frequency of the aerobic digester. The grit chamber can eliminate almost 20% of cellulose, whereas the primary clarifier is responsible for cellulose removal up to 80%. Cellulose has an ability to easily make a bond with other particles in wastewater owing to the presence of multiple functional groups within its chains and is insoluble in nature and these properties aid in removing cellulose effectively. From the principal clarifier, the influent moves towards the aerobic digester tank and the sludge content will be up to 68%. After being treated by the anaerobic digester, almost half of the original sludge content will be degraded by aerobic facultative organisms and the rest shall be treated as residue. Effluent treatment plant comprises of the required operations and processes with the absence of a primary clarifier (Fig. 3.).
The influent enters the grit chamber, as stated previously. Up to 20% sludge can be removed via grit filtration while the rest (up to 80%) is fed into the biological treatment tank where up to 66% cellulosic degradation can be achieved via breakdown facilitated by microbial strains previously mentioned. As a result, only up to 14% sludge moves towards the secondary clarifier.
As reported by Liu et al. [29] Cellulose could put an external strain on cost of operating the wastewater treatment plant. The existence of a primary clarifier may aid in decreasing the cellulosic content as well as sewage sludge quantity available in the effluent.
4.2. Sieving methods
Physical filtration methods such as sieving, and riddling may present an optimal solution for removing cellulose from wastewater. Sieving may be a straight-forward route to removing cellulose, in contrast with sedimentation tanks, or more effective separation techniques. Ghasimi et al. [20] suggested various types of screens suggested for filtering cellulose; fine mesh sieves can be used as an additional separation operation after bar screens to retain cellulosic particles or may even be used independently to retain undesirable solid. Ruiken et al. [6] in their study have recommended fine mesh sieves, of mesh size 0.35–2 mm to filter out cellulose contents. Karia and Christian, [30] in their book have proposed coarse bar screens, of opening size >6 mm, or fine bar screens, of opening size ≥6 mm. Removal efficiencies of various sieving methods are listed in Figure 6.
Li et al. [27], in their study have investigated bar screens and their removal efficiencies with respect to bar openings and discussed the recovery of cellulose fibers in relation to the filtration openings. Coarse screens, with openings of >6 mm was rendered ineffective and unable to trap any particles, regardless of the degree of deterioration. Even in the presence of portly sized flocs, the confining efficiency was only 19.9%. relating to the same degree of disintegration, fine bar screens of 3 mm and 1 mm openings had particularly improved efficiencies of 45.7% and 72.2%, respectively, while fine mesh sieves of size 0.1 mm showed an excellent recovery of 94.5%. If the degree of disintegration were to be evaluated, it will be inferred that, at a high value of disintegration, an insignificant amount of cellulose (0.5%) is recoverable with coarse bar screens. The value of recovery slowly crawled to 1.8% in case of using the fine bar screens of 3mm and it still showed a gradual increase up to 2.9% by 1 mm fine screens. In the case of fine mesh screens (0.1 mm) the highest efficiency was achieved, recorded at 41.8%. Removal efficiencies of various sieves can be perused from Fig. S3.
Nevertheless, using generous sized screens is much more industrially accepted because of their lower manufacturing and maintenance costs, lesser clogging and choking phenomenon. Consequently, it is important to be cognizant of the principal factors such as overall recovery of cellulosic fibers, operation and upkeep costs and the degree of disintegration of cellulose, as these considerations highly impact the effectiveness of prefiltration units designed for cellulose recovery. A notable benefit of prefiltration of cellulose fibers is the substantial decrease in the consumption of energy in the aerobic digester unit. Remarkable work has been done by Honda et al. [8] in regard to a design for simple cellulose recovery from primary sewage sludge.
4.3. Rotating bed filter
Rotating bed filters (RBFs) are a step further solution to sieving cellulose from wastewater (Fig. S4.). The Rotating bed filters comprise of relatively simple equipment, including a belted screen, a screening hopper and a cleaning device pieced together. The belted screen acts as a hinderance and aids in accumulation of sewage sludge into the hopper. RBFs are easily operated and are an economical answer to the plight of cellulose in sewage sludge [6]. These are optimally suited for areas that experience low temperatures for longer durations, as it could potentially reduce the CAPEX of wastewater treatment plants (WWTPs) situated in the regions. The working efficiency of RBFs is heavily dependent upon the perforation size of the belted screen although the cleaning and maintenance device greatly impacts the dependability of operation.
According to Ruiken et al. [6] and Peeters et al. [31], belt screens should always have a mesh size of 350 micrometers to maintain the 30–60% level of sewage sludge and consequently 10–30% of COD. When the rate of loading of suspended solids was expedited between 35 to 58 kg TSS/m2/h, the Sewage sludge efficiency increased from 50 to 78%. Agreeing with the findings of Rusten and Lundar, [32] if a solid loading rate is kept high, the chances of a filter cake formation are much higher, which will consequently increase the conservation of the sewage sludge. Upon the formation of filter cake, the efficiency of holding back sewage sludge depends immensely on hydraulic flowrate and is unbiased of the mesh size. In addition, a comprehensive cleaning mechanism must be designed and implemented as a dense filter may lead to overflow and ultimately, failure. Previously established, the size of cellulose fibrils is about 1 mm, and the RBF which has a mesh size of 350 micrometers is able to retain 40% of influent sewage sludge consisting of cellulose. A comparison of rotating bed filters with their processing and recovery efficiencies by various authors is summarized in Table 3.
Table 3. suggests various mesh sizes for RBFs with are reported to have achieved significant rates of cellulose recovery, although it is not designed solely for the recovery of cellulose. Ruiken et al. [35] reports a retrieval efficiency of up to 40% of Sewage sludge influent by using an RBF of mesh size 350 μm. The recovery of cellulose varies from 35 to 76 mg cellulose/L of wastewater, which is liable for up to 80% of the influent’s total cellulose. A prominent discovery was made by Da Ros et al. [33] wherein two mesh sizes (210 and 350 μm) were analyzed and contrasted, and the results favored towards the use of a lesser mesh size to redeem cellulose. However, it is imperative that, when taking into account the requirement of carbon for biological nutrient removal, the netting size balances cellulose intervention and retention of organic materials. Therefore, the size of the mesh (i.e., 350 μm) is optimally recommended to achieve cellulosic recovery.
RBFs have a lesser footprint, and compared to bar screens and sieves, they can be made to use smaller sieves with lesser possibility of choking. These can be installed in the pre and post operation schemes, meaning it can be installed in the primary treatment unit, if the application requires copious amounts of cellulose to be salvaged. RBFs can also be set up on the sludge discharge line at the secondary clarifier’s output line. The features of this placement contain smaller flow for treatment, lesser operation problems and consequently lesser capital investment and maintenance costs; all of which can be availed at the cost of a significantly lesser amount of cellulose recovery due to the reason that a large amount of cellulose be degraded during the biological treatment phase.
4.4. Ionic liquids
Glińska et al. [36] investigated the recovery of cellulose using an ionic liquid, tetrakis(hydroxymethyl)phosphonium chloride ([P(CH2OH)4]Cl), due to their highly selective nature towards cellulosic and lignocellulosic dissolution. A municipal solid waste sample was acquired and used to precipitate cellulose from it. The sludge is firstly processed and saturated in an ionic liquid solution. It is then subjected to precipitation with the aid of methanol. The product is condensed and purified after being centrifuged and dried. The results were confirmed when the quantity of carbohydrates in the standard paper sludge was measured by traditional methods were almost equal to the total amount of cellulose recovered by ionic liquid method. The results were cited to be 84.6% cellulose, 1.8% proteins and 15.6% ashes, the ashes and proteins were not found in the results of conventional methods. The ionic liquid method warrants significant energy and chemical requirements, which are limitations of this process.
Dissolved air floatation method was proposed to recover organosolv lignin by Macfarlane et al. [37] along with simultaneous precipitation, due to it being lower in energy and maintenance if equated with centrifugation and filtration. The process was optimized by varying highly influential parameters of air saturation, temperature, and regime of mixing. It was noted that lignin recovery decreased above 30ºC, and a higher air saturation allowed for accelerated clarification. Park et al., [38] proposed a route for pyrolysis of Organosolv lignin.
5. Valorization of Recovered Cellulose
There are many benefits that are derived from recovery of cellulose for wastewater treatment plants which can be keyed into revenue, operational ease, performance efficiency and byproduct usage for WWTPs. With the implementation of RBFs for the recovery of cellulose, as discussed above, the recovered cellulose shall consist of extra polymeric substances (EPS) that have been elaborated by Huang et al. [41], which can be pre-filtered out and dried to create sludge with a solids content of 20 to 30 percent, by instigating the embedded thickening module. As investigated by Razafimanantsoa et al. [42] this setting can help with potential cost savings by minimizing secondary practices such as sludge waste management and disposal. Another advantage is that one tenth of a land is required for an RBF with this setup to achieve promising results, as compared to using a conventional clarifier. In their study, Ghasimi et al. [20] referred to RBFs as an encouraging solution to include more carbon into aerobic digestion all the while reducing organic loads decomposition in biological treatment section. If the filtered and retained sludge is used as a feedstock for biogas, the productional efficiency shall be high and a satisfactory product produced, owing to the presence of a high amount volatile suspended solids. Generally, an RBF filtered sludge compared to biogas made from the sludge feedstocks of standard clarifiers, produces 10–20% more methane.
In regards to examining the interaction between cellulose recovery and the required oxygen demand for decomposition of oxygen demand in the biological unit, Pasini et al. [43] analyzed the importance of RBF on the aeration equipment in a sequencing batch reactor (SBR), sequential batch reactor pilot plant and a WWTP and found that the oxygen demand can be reduced by 40% in SBR and 32% in WWTPs, thus confirming that cellulose recovery is indeed beneficial for diminishing oxygen demand in the biological unit by limiting the amount of organic matter inflowing in the biological treatment units. The lesser amount of organic matter in the influent, the lesser requirement of oxygen. In their study, Pasini et al. [43] also stated the oxygen transfer efficiency for cellulose recovery after increasing by 27% and 10–20%, respectively.
Hao et al. [44] backed this up by understanding that the presence of cellulosic content in the influent boosted the concentration of airing in the biological component to keep performance steady. The sludge flocs may be merged and condensed around the cellulose fiber because the presence of cellulose significantly altered the sludge flocs’ linear shape and implanted functional groups of cellulose. The more compact and dense sludge flocs for each cellulosic fiber may prevent oxygen from reaching the flocs, which could be harmful. Therefore, to maintain effective biological therapy, aeration intensity must be increased.
Reijken et al. [45] focused on investigating the concern raised regarding cellulose recovery by RBF will have any effect on the biological nutrient removal occurring in the downriver processes by evaluating the connection between cellulose and removal of nutrients integrating cellulose as a particular COD composition into Activated Sludge Model 1 (ASM1) (based on organic matter reduction modelling in aerobic biological systems). At 20% COD cellulose, nitrogen and phosphorus removal do not illustrate any marked results which suggests that the degradation mostly occurs in the aeration tank. Furthermore, Hao et al. [44] experimentally explained that cellulose broke down initially for a limited period in anoxic conditions and the performance ramped up after supply of oxygen was boosted.
Rusten et al. [46] unearthed the influence of nitrogen removal by employing rotating bed filters with mesh size 33 μm and found the results to be relatively positive, as the nitrogen removal efficiency increased by 10–15%. It was based on the fact clarified by Razafimanantsoa et al. [47] that the retention of organic matter by RBF reduces the conflict pertaining to the oxygen utilization between the organic matter and nitrification. The nitrification is also aided by provision of longer solids retention time (SRT) after cellulose retrieval. According to a study done by Behera et al. [48], if the influent for the RBF is low in C/N ratio, its performance should be closely monitored by looking into the influent carbon and nitrogen content. The design of RBF should be modified in such a way as to avoid retaining a large concentration of organics, thereby promoting denitrification process. RBF design can be optimized by selecting the appropriate mesh size so as to avoid filter cake formation on the surface of rotating bed.
Another benefit of cellulose recovery is the prevention of fouling of membranes in bioreactors. Membrane fouling is the key concern with membrane bioreactor (MBR) design and operation, and frequent membrane fouling warrants recurrent replacement of membranes for operational efficiency. Razafimanantsoa et al. [42] explicated the usage of RBF of mesh size 33 μm, which markedly decelerated the increase in pressure of the membrane, thereby adding to reduced membrane fouling in dead end filtration.
6. Applications of the Recovered and Valorized Cellulose
The recovered cellulose finds its applications in two routes, market export and internal use. The potential markets for recovered cellulose reuse are wide, ranging from building materials to renewable fuels production plants i.e., biogas. Ruiken et al. [6] suggested that paper products, building and construction materials, and bioplastics be produced using the recovered cellulose from wastewater treatment. In another study, Zhou et al. [49] reports sludge cellulose collected from Geestmerambacht WWTP in the Netherlands, wherein this recovered cellulose sludge plastic composites are being developed as a raw material to replace wood flour in wood plastic composite (WPC) in order to create a greener and more sustainable future for the WPC industry and a way for the industry to reduce its carbon footprint and manufacturing costs. The manufacturing facility have accomplished this task by developing compatible sludge plastic composites by using silane coupling and malleated agents and determining crucial factors such as potential of substitution in regard to the properties of materials, environmental influences by life cycle assessment (LCA), eco-efficiency analyses and manufacturing costs. The results showed an auspicious future for sludge cellulose recovered from WWTP in this industry, as sludge plastic composite (SPC) was proven to meet the property requirements needed to replace the conventional raw material, when compared to WPC, SPC was 5.26% more eco-efficient and had 15% lower manufacturing costs. These benefits can be used to intensify the circular economy transition of WWTPs.
Palmieri et al. [50] in their proceeding, have discussed the possibility of using cellulose as reinforcing adhesives in the construction industry and also as a precursor for paper industry. The application within construction industry has further been backed up by evidence from Papa et al. [51] that by the addition of recovered cellulose in a concentration of 20% into the mortar paste, the flexural strength is greatly enhanced, and steam resistance coefficient reduced, comparable to the pure cellulosic fibers.
Recovered cellulose may also be applied in-house, in the plant itself, as illustrated by Grenda et al. [52] Zhang et al. and Jiang et al. [53, 54]. Makinen et al. [55] implied that recovered cellulose may be re-used as a flocculant to improve removal of COD or sludge dewatering, or paper mill sludge. Recovered cellulose show competitiveness and provide a performance that is equivalent to or better than conventionally used polymers. RBFs can also be used for this matter by provision of sludge in cellulose filter cake on the bed, after sludge thickens and is dewatered, polyelectrolyte resin can be used to create a 30% dry sludge cake. This helps the sludge avoid chemical costs and energy input in the aeration tank.
There are a variety of methods to valorize cellulose in a commercially meaningful way, as summarized in Figure 8. Espindola et al. [34] designed a pathway for the conversion of traditional cellulose into a rather high-end product i.e., nanocellulose, which proved to be more economical due to its production from wastewater than by wood pulp and agricultural waste. This so-manufactured cellulose is high grade and meets current commercial standards (chemical structure, morphology, and aspect ratio). Nanocellulose finds diversified applications as discussed by Abouzeid et al., Liang and Hu, and Sayyed et al. [56–58].
Cellulose is currently being utilized extensively for wastewater treatment, elucidated by Jiang et al. [59], Fan and Xue-Meng [60], Alves et al. [61], Grenda et al. [62], Yang et al. [63] and Hamidon et al. [64]. Fig. 4. shows the mentioned applications of valorized cellulose assorted as per their importance.
Other endorsing techniques include the production of fuel grade ethanol from recovered lignocellulosic fibers, which can serve as a practical alternative to fossil fuels, as illuminated by Yuan et al. [65]. The possible production of bio-methane and biofuels are discussed by Gottardo et al. [66] and Rosiek [67], and the process efficiency is debated relating to a co-fermentation unit for bio-methane. Another possible, albeit less economically effective method recommended by Wiśniowska [68] is the direct combustion of cellulose, in place of sewage sludge incineration; as the heating value of cellulose is 17–18 MJ/Kg. Cellulose based hydrogels are also being accentuated lately by Akhter et al. [69] and Sun et al. [70], in addition to cellulose aerogels illuminated by Darabitabar et al. [71], and Hong et al. [72].
7. Techno-Economic Analysis (TEA)
The Techno-economic Analysis (TEA) for the degradation of cellulose is a crucial tool to evaluate the applicability and economic feasibility of processes for the degradation and further recovery of cellulose in wastewater. TEA takes into account the technical and economic variables, such as process efficiency, overall product yield, capital investment and operating costs, and market requirement for the produced products.
The TEA Analysis for the recovery and valorization of cellulose in wastewater involves the initial identification of wastewater sources that contain cellulosic matter, its amount and composition and then the appropriate selection of degradation process based on the acquired data
Mass and energy balances applied after the conjuring of a process flow diagram to estimate the efficiency and yield of products from the degradation process. Finance calculations in the form of CAPEX and OPEX including labor, equipment, and maintenance and finally computation of revenue based on feasible applications of cellulose and determination of the economic viability of the process. TEA for the recovery of cellulose in wastewater involves similar steps, but with a focus on the extraction and purification of cellulose from the wastewater stream, as well as the conversion of cellulose into high-value products.
TEA benefits include the identification of the most encouraging processes for the deterioration and recovery of cellulose enables us to optimize and integrate these processes for maximum efficiency and productivity. It can also influence decision-making for investment in new technologies and the development of new markets for cellulose-derived products.
8. Future Prospects
From the above debate, it can be seen in plain sight that cellulose recovery is a beneficial aspect for WWTPs, by either revenue generation, externally, or reduction in use of the plant’s internal energy consumption (it is ascertained that the biological processing tank usurps up to 70% of the total energy input of a WWTP). Cellulose removal and recovery can reduce load of dissolved organics and total suspended solids. Marcelis and Wessels [73] mentioned that the overall carbon footprint can be reduced as much as 15% by choosing to go via the cellulose recovery by RBF route. Therefore, it is a pristine advantage for water recovery and resource facilities to incorporate cellulose recovery techniques during water treatment. In the case of cellulose recovered by RBF and the use of that cellulose as resource, the total climate impression on that wastewater treatment plant is computed to be 360 tons of CO2e per year, exclusive of the post treatment of cellulose. Cellulosic content in wastewater varies invariably, therefore it is possible to execute plans of recovery in developed and developing countries.
The recovery of cellulose is hampered by lack of appropriate venture capital in recovery facilities and concerns of product acceptance in commercial markets. The technical aspects and design of cellulosic recuperation technologies are entirely sound, nonetheless, not many technical scale installations are operated.
Compared to recovery of other materials such as polyhydroxyalkanoates (PHAs) and volatile fatty acids (VFAs), cellulose recuperation achieves the highest results. It provides all the more reason for the actual implementation of methods to recover cellulose from wastewater. Incentives should be made by rules and regulations authorities for the development of technologies and commercially distribute in the market as valuable resources. Advantages can be derived from recovered cellulosic polymers as mentioned previously.
Removal technologies that have been discussed include Rotating Bed Filters, Ionic Liquids, sieving methods and primary and secondary clarifiers to determine if cellulose ought to be recovered from sludge or wastewater. The importance of grit chamber is highlighted. However, there are no commercial or appropriate technologies to recover microplastics from sludge, which is a clear gap of knowledge and needs to be further looked into, by research.
There is striking prospect for the valorization of cellulose in wastewater, as it is deemed to be a treasured resource used to produce chemicals, biofuels, and other augmented products. Nevertheless, insightful scrutiny is needed to draw a complete picture for the potential capabilities of cellulose in wastewater, and several areas are outlined where future work is essential.
Existing processes should be improved in terms of efficiency, which can be improved through process automation, debottlenecking, and process control and instrumentation. This could employ advanced systems, such as machine learning algorithms artificial and intelligence to optimize the degradation processes and enhance product yields.
Newer technologies ought to be developed that can efficiently extract and convert cellulose from wastewater into beneficial products which may involve the improvement of existing and innovative processes, that can convert cellulose into high-value products.
There is a need to educate industrialists towards sustainable conscience and develop markets for the products originated from recovered cellulose. Consequently, cellulose could be used in the development of new applications such as bioplastics or biofuels, and the creation of supply chains that can capably deliver these products to consumers.
Endorsements have also been proposed by Li et al. [27] that we must modify toilet paper design appropriately, such as lowering the proportion of cork long fiber so that it can break down into relatively large fiber blocks in sewerage.. The life cycle cost investigation found that if the degree of decomposition of toilet paper is diminished by at least 10% and up to 50%, the operational cost of a WWTP may be lessened by 12.1% as much as 18.6% in a timescale of 15 years. Though, if the toilet paper is designed with fragments of size 0.5–1 cm post disintegration, a WWTP’s operating cost will be reduced as abundantly as 46% in 15 years. However, this redesign comes at a cost of unease of use and may cause logging and blocking in pipe networks.
WWTPs can potentially play vital roles in the cycle of circular economy, by installations for recovering substances of importance from wastewater and sewage sludge. Some plants focus on salvaging magnesium ammonium phosphate while elements such as extra polymeric substances and cellulose should be given equal importance, which makes the possibility of recovery and valorization of products other than salts of Nitrogen and Phosphorus entirely possible. Bearing in mind the climate changes and associated contests, circular economy will undoubtedly become a way of life to each sector.
9. Conclusions
Cellulose is undeniably an overlooked plentiful resource available in wastewater and should almost certainly be explored and valorized for the derivation of said benefits. Upon perusal of the aforementioned case, the following conclusions can be drawn:
An assortment of cellulosic content can be seen in different regions across the globe, but developing and developed countries ought to explore their potential in cellulose recovery and valorization.
The quantification of cellulose in different samples was done by analytical methods and its degradation mechanism was unearthed to be carried out by physical, chemical, and bacterial means in aerobic or anaerobic environments.
Cellulose can be recovered from wastewater by mature technologies such as activated sludge digesters, with the assistance of a primary clarifier, conventional methods of sieving, Rotating bed filters, and by chemical assisted technology such as ionic liquid filtration, depending on the applicability and usage of concerned equipment.
Cellulose finds paramount applications in a multitude of dimensions, from which one can assume the importance of cellulose instead of discarding it without any further improvement.
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