Biocarriers for biofilm immobilization in wastewater treatments: a review

Abstract

Biofilm carriers, named ‘biocarriers’, are solids that allow the good attachment of microbes during wastewater treatment. Biocarriers also act as redox mediators to speed up the biotransformation of contaminants in industrial effluents, yet it is challenging to choose a biocarrier material that achieves strong biofilm adhesion and high degradation rates. Here we review insoluble polymeric biocarriers with focus on selecting a biocarrier, biofilm growth, metabolic pathways, applications, surface modification, and composites. According to the efficiency to decrease the chemical oxygen demand, conventional and modified composite biocarriers are rated as following: polyvinyl alcohol > polyurethane > polyethylene > polypropylene, and polyvinyl alcohol > waste tire > polyurethane > polyethylene, respectively. We also discuss biological and physical fouling.

Introduction

Biocarrier (biofilm carrier) system has been presented to colonize microbes without biomass recycling to increase solid retention and reduce the required space area (Sajjad et al. 2018). Traditionally, biocarriers have a high specific surface area, durability, porosity, and roughness (Feng et al. 2012). Each piece of biocarriers serves as an active platform enhancing mass transfer and sustaining biofilm growth (Al-Amshawee et al. 2020a). It results in the formation of a sheltered and stable biofilm, defending microbes from an alteration in wastewater parameters such as influent characteristics and operating procedures.

The biocarriers could be manufactured from different materials. To date, various carriers have been presented for supporting microbial adhesion, including powdered and granular activated carbon carriers, plastic carriers, wood chips, ceramic carriers, non-woven carriers, and naturally occurring materials (Peng et al. 2018; Wang et al. 2018; Bouabidi et al. 2019). However, only a few are commercially applied in full-scale systems. In general, the most used carriers for biofilm attachment are made of plastic due to its low density and high mechanical resistance (see Fig. 1) (Dong et al. 2011). The dominating type is kaldnes K1 carrier that is characterized by an active surface area of up to 350 m²/m³ reactor volume (Rusten et al. 2006). The carrier material has been reported to influence the growth and distribution of the biofilm (Chu et al. 2014). However, most studies focused on optimizing the operating conditions like packing rate (Zhang et al. 2016b). Thus, it is still crucial to determine how the carrier material impacts biofilm activities and process efficacy.

Fig. 1

Fabricated biofilm carriers inside fixed and moving bed biofilm reactors. Reprinted from Le Noir et al. 2009 with permission from John Wiley and Sons

Müller (1998) defined three quantities that have to be determined for designing a biocarrier, including biocarrier material, the volume percentage of biocarrier to total reactor volume, and amount of biofilm per unit of biocarrier. Researchers reviewed natural materials as biocarriers such as zeolite (Montalvo et al. 2012) and fibrous materials (Dzionek et al. 2016). The study of Rusten et al. (2006) is one of the first papers reviewed artificial biocarriers, but it investigated only kaldnes.

This review study investigates scientific reports about the use of artificial solid polymers as biocarriers for wastewater bioremediation, including the impact of carrier properties, like material, coating, and composites, for an enhanced biofilm attachment. Besides, it does answer frequently asked questions including (1) How to select a suitable biocarrier? and (2) What is the best polymeric material to fabricate biocarriers?. Furthermore, this review study discusses advanced studies of using biocarriers and paves the steps to scale up laboratory experiments. In summary, this review aims to be a substantial reference to support different studies in choosing a biocarrier material.

Choosing a biofilm carrier

The selection of a biocarrier is a critical action to be made, as it determines process cost-effectiveness, biofilm sustainability, and treatment performance. It significantly determines the optimum biofilm thickness and pollutant removal rate (Chang et al. 2009). It is a critical process to maintain an adequate growth of biofilm for successful bioremediation (Luo 2001). The sort of operating bioremediation influences the selection approach because each biocarrier suits specific circumstances of working microorganisms and targeted contaminants (Al-Amshawee et al. 2020b). The source and concentration of targeted contaminants should be taken into account while selecting a biocarrier (Al-Amshawee et al. 2020c). It is also essential to consider the type of process, in situ or ex situ.

Choosing a biocarrier is difficult because the disadvantages are not well recognized (Rauch 2014). Media designs come with efficiencies like removal rates of chemical oxygen demand and biochemical oxygen demand, and service life, at different conditions that can guide researchers and buyers to select a proper biocarrier (Al-Amshawee et al. 2020a). Elias et al. (2002) recommended a biocarrier having a high surface area and high air and water permeability to obtain maximum performance. However, not every biocarrier is suitable for biomass immobilization. A good biocarrier should be inexpensive, easily accessible, easy to handle and regenerate, insoluble, non-biodegradable, and non-toxic for the immobilized biofilms and environment. Further, a good biocarrier should have low density, a suitable surface for quick biofilm immobilization, and good structure to protect biomass against shear and sloughing (Hirai et al. 2001; Chaudhary et al. 2003). In wastewater bioremediations, the biocarrier is recommended to have high mechanical resistance because it will face different kinds of physical forces.

Simple and harmless immobilization requires biocarriers with distinct characteristics. Biocarriers used for biomass adsorption should be as large as possible with high porosity to deliver an effective contact area. Further, it must possess a high loading capacity for biomass attachment and deliver optimum diffusion of nutrients from flowing material to the center of the carrier. Most importantly, it should provide an inert habitat for the immobilized cells to retain the inherent harmony of enzymes without comprising their bioactivities.

With respect to biocarrier specifications, it is crucial to determine the biodegradation rates because a number of properties have not been explained and possibly not even figured out. Therefore, researchers, as of today, find an immense curiosity to inspect different biofilm supports in order to identify the most suitable biocarrier in terms of material and satisfactory specifications.

Solid polymeric biofilm carriers

The core of the attached growth processes is the supporting media for microbial adhesion. In principle, a wide range of porous and non-porous polymeric solids could be used as biofilm supports, which are characterized by different structures and diversified surface areas (Falletti and Conte 2007; Landy et al. 2012). They can be fabricated into various shapes, and they are merely available and comparatively cheap. However, the poor hydrophilicity and biocompatibility lead to deficiencies in biofilm culturing and attachment. Also, these biocarriers have some essential problems presenting a negative surface charge, which leads to easy detachment of biofilm and a long start-up time of systems (Mao et al. 2017). Thereby, biomass accumulation and reactor head-loss may take place due to the fast biofouling and jamming of the plastic carriers below the sieve layer (Dong et al. 2011). Physical and chemical modifications of the biocarrier surface are feasible options to accelerate and produce stiff biofilm. The following discusses applications and developments of various solid biocarriers categorized based on their polymeric fabrication material.

Polyurethane sponge

Polyurethane consists of a matrix of support material, forming a complex network of continuous voids. It provides a high porosity of 97% and a high specific surface area that can reach 2400 m²/m³ (Prabhavathi et al. 2014; Zheng et al. 2017). The flexible and porous polyurethane carrier is considered the most often applied as an ideal growth medium due to its sizeable specific surface area, rough surface, high mechanical strength, and good adhesion to microorganisms (Ivanovic and Leiknes 2012; Zhang et al. 2016b; Sultan 2017; Song et al. 2019). It can deliver simultaneous nitrification and dentrification as a result of the highly dissolved oxygen gradient in the cubic biofilm (Zhang et al. 2016b). Also, it is a low cost and can promote rapid and stable biofilm growth (Ngo et al. 2008).

Polyurethane sponge has been extensively investigated as a support media to entrap biomass and decompose hazardous materials than releasing them into the environment. The first experimental application of polyurethane sponge in a separation process was reported by Bowen (1970). Researchers have been examining polyurethane sponge to immobilize microbial communities, like denitrifying anaerobic microorganisms (Hatamoto et al. 2017), and to remove a variety of substances such as organics (de Oliveira et al. 2009), cadmium and copper (Pakshirajan and Swaminathan 2009), oil biodegradation (Wan et al. 2013), azo dye (Lu et al. 2015) and naphthalene (Sonwani et al. 2019). History of the polyurethane utilization as a sorbent for organic dyes goes back to the’90s (Chow et al. 1990). The use of polyurethane sponge as a surface for biomass immobilization has been primarily utilized in fixed bed biofilm reactors delivering solids retention and providing adequate environmental conditions (Mockaitis et al. 2014).

Carvalhinha et al. (2010) implemented a similar study of Siman et al. (2004) to investigate 5 mm inert polyurethane foam with 95% porosity inside a mechanically stirred anaerobic sequencing batch biofilm reactor to remove organic solids and produce methane. In a batch mode, it effectively removed organic solids of 87%, 86%, and 80% at influent’s chemical oxygen demand concentrations of 500, 1000, and 2000 mg/L, respectively, and generated 5.20 mmolCH₄, which corresponded to biogas composition of 78%. Though, the inert media could not achieve stable operation at an influent concentration of 3000 mg/L chemical oxygen demand and removed only 72% of chemical oxygen demand. In the study of Siman et al. (2004), high organic loading of 2000 mg/L chemical oxygen demand led to total volatile acids accumulation and lowered organic matter removal to 55%. Unlike Siman et al. (2004) and Carvalhinha et al. (2010), studies by Damasceno et al. (2007), Moreira et al. (2008) and de Oliveira et al. (2009) did not observe unstable performance during different influent’s chemical oxygen demand concentrations of 976, 2005, 4020, and 5969 mg/L or organic shock loads up to 12 g/L d of chemical oxygen demand (Damasceno et al. 2007; Moreira et al. 2008; de Oliveira et al. 2009). Though, all the studies resulted in a lower quality of effluents at a higher loading rate. Carvalhinha et al. (2010) attempted to implement a gradual and longer filling period of 4 h for the sake of a higher removal rate, but unfortunately, it did not enhance the removal efficiency. Therefore, it was concluded that the support media could not exhibit satisfactory results for chemical oxygen demand higher than 2000 mg/L, which appears to be the threshold value for this biocarrier (Al-Amshawee and Yunus 2019).

A study explained the limited performance of polyurethane sponge as satisfactory due to possessing anaerobically non-biodegradable chemical oxygen demand of about 35% in every influent (Kim et al. 1992). On the other hand, Zheng et al. (2017) proposed further dilutions to bypass all potential problems that inhibit biodegradation and cause bed clogging. Despite the findings of Carvalhinha et al. (2010) of 2000 mg/L chemical oxygen demand as a limit value, Banerjee and Ghoshal (2017) packed a bioreactor with polyurethane foam to biodegrade actual petroleum wastewater with initial concentrations of 9200 mg/L chemical oxygen demand, 4548 mg/L total organic carbon, 3561 mg phenolics/L, 121.1 mg PO⁻³₄ P/L, and 121.09 mg NH₄⁺–N/L. The process successfully removed 90.9% of chemical oxygen demand, 88% of total organic carbon, 91.4% of phenolics, 25.8% of PO⁻³₄ P, and 42.76% of NH₄⁺–N. The substantial difference between the research results of Carvalhinha et al. (2010) and Banerjee and Ghoshal (2017) is due to the influence of bioparticle size on the degradation kinetics, which can largely limit the performance of biodegradation process. Small particle size could contribute to greater hydrodynamic conditions and higher dissolution rates. Therefore, polyurethane is an adequate biocarrier for the treatment of low suspended solids rich influents. However, the removal of chemical oxygen demand was undoubtedly affected by more complex factors.

Modifications of polyurethane biocarrier

Polyurethane has been utilized as biofilm support because of its low cost, high specific surface area, and known sorbent capacity of hydrophobic compounds (Li et al. 2018). However, biofilms and immobilized cells are at risk of being washed out due to the poor hydrophilic properties, which result in an extremely cloudy reactor medium (Dai et al. 2019). Thus, scholars proposed new ways such as incorporation with the number of additives and hydrophilic or charged modification to advance the polyurethane physical properties, inhibit electrostatic repulsion between the carrier and the bacteria, and enhance nutrients removal and effluent quality (Mao et al. 2017).

Li et al. (2012) investigated a low-temperature sintering method, called the O method, to adhere TiO₂ on the polyurethane surface. The adhering photocatalyst supports cell immobilization on the exterior surfaces because biofilms mainly grow at the polyurethane interior. The O method coated the polyurethane surface with seven times higher TiO₂ density than sol–gel method. Zhang et al. (2016a, b, c) coated macroporous polyurethane foam with high thermostable SiO₂–TiO₂ via ultrasonic vibration and deposition approach inside a photocatalytic circulating-bed biofilm reactor (see Fig. 2) (Zhang et al. 2016a). TiO₂ or SiO₂–TiO₂ did not enhance the biocarrier properties towards more significant biodegradation. However, it leads to new studies about hybridizing polyurethane biocarriers.

Fig. 2

Images of polyurethane biofilm carrier. a Polyurethane sponge (Reprinted from Li et al. 2012 with permission from Elsevier); b coated polyurethane (Reprinted from Zhang et al. 2016a with permission from Elsevier); c uncoated polyurethane in 100 μm scale (Reprinted from Li et al. 2012 with permission from Elsevier); d coated polyurethane in 100 μm scale (Reprinted from Li et al. 2012 with permission from Elsevier)

Chu et al. (2014) reported a short communication about developing a hydrophilic cationic polyurethane by coating a conventional polyurethane material with N-methyl diethanolamine. The coated biocarrier delivered faster water immersion, a higher affinity for biofilm attachment, more complex community, and 1.3 times more biofilm amount than the conventional polyurethane carriers. Li et al. (2018) hydrophilized polyurethane biocarriers with hyperbranched polymeric diazonium salts via the layer-by-layer self-assembly. Just like Chu et al. (2014), the modified biocarriers showed excellent adsorption for microbes and shortened the cultivation time according to the images of scanning electron microscope. Besides, the scholars pointed out to seek for direct practical synthesis of hydrophilic polyurethane with cheap coating material towards the best possible physical properties.

Song et al. (2019) filled the pores of polyurethane with zeolite to remove nitrogen from municipal wastewater by simultaneous nitrification and denitrification. Tests showed that the enhanced biocarrier increased simultaneous nitrification and denitrification and removal efficiency by only 10%. Though, the modified and the conventional biocarriers removed a similar range of NH₄⁺–N and total organic carbon. Similar to the N-methyl diethanolamine-based polyurethane of Chu et al. (2014), zeolite-based polyurethane delivered 1.3 times a biofilm amount than the conventional polyurethane sponge. However, the researchers have not demonstrated the mechanisms behind the increased biofilm attachment.

Cells immobilization and development

Biomass attachment and growth are important factors for bioreactor start-up and stability. The polyurethane foam is conducive to microbial attachment and development. Since the’80s, the polyurethane foam has been indicated as an effective biocarrier having a high capacity for microbial colonization (Fynn and Whitmore 1982; Fukuzaki et al. 1990). Entrapped cells on the porous structure of polyurethane are semi-protected from external shear forces, but they are not confined within the structures by any barrier. Morphological examinations showed a rapid and dense adhesion of microorganisms on the polyurethane surface, and it highly attributed to stabilizing the process under shaking conditions (see Fig. 3) (Fukuzaki et al. 1990; Dizge et al. 2011). Microbiological analyses demonstrated the existence of a diversified community such as Methanosaeta sp., Vibrio, Vibriones, Bacilli, non-fluorescent Cocobacilli, Bacillus cereus, Methanosarcina sp., Methanogens, Hydrogeno-trophic bacilli, Proteobacteria, Firmicutes, Clostridia, Flavobacteria, and Bacteroidia (Siman et al. 2004). Therefore, optimal conditions should be provided for the cultivated bacteria, particularly during the start-up.

Fig. 3

Biofilm growth within 1 cm scale in polyurethane support media (Reprinted from Dizge et al. 2011 with permission from Elsevier)

Other studies have precisely designed their experiments for the attachment of specific biofilm-forming microbes and specialized chemical oxygen demand degraders. Silva et al. (2006) reported the attachment of methanogenic archaea and sulfate-reducing bacteria on various biocarriers of polyurethane, low-density polyethylene, alumina-based ceramics, and vegetal carbon. Remarkably, the polyurethane exhibited more than ten times specific biomass adhesion of the maximum value obtained for the other materials. Additionally, Wan et al. (2013) investigated Pseudomonas aeruginosa attachment on porous ceramic and polyurethane biocarriers for cooking oil-rich wastewater treatment. Attached P. aeruginosa to the polyurethane needed a short duration to deliver the maximum cooking oil removal of 3.1 kg/m³ days. Similar to Wan et al. (2013) and Nie et al. (2016) immobilized biofilms of P. aeruginosa NY3 to remove hydrocarbons from highly concentrated oil-containing wastewater. It resulted in a quick biofilm formation that could reach up to 488.32 mg dry cell/g dry polyurethane and could be recovered for reuse. Further, Zhang et al. (2016a, b, c) reported maximum adhesion of Actinobacteria, Cyanobacteria, Nitrospirae, Bacteroidetes, and Proteobacteria on the polyurethane sponge compared to ceramsite and zeolite (Zhang et al. 2016c).

Recently, Dai et al. (2019) and Sonwani et al. (2019) researched different biocarriers. Measurements of dry biomass showed that polyurethane possessed the most excellent range of specific biomass attachment and colonization. Moreover, it had better qualitative and quantitative results of aerobic ammonia-oxidizing bacteria, anammox bacteria, and denitrifying bacteria than other biocarriers. Eventually, it showed a substantial potential for adhesion and development of nitrification–denitrification–anammox bacteria.

Duarte et al. (2008) immobilized Clostridiales sp. on 5 mm cubic polyurethane particles for 313 days to degrade linear alkylbenzene sulfonate inside a horizontal-flow anaerobic immobilized biomass reactor. In the end, the process delivered only a 35% biodegradation rate. Despite that, Park et al. (2013) and Zhang et al. (2019) suggested a periodical injection of carbon dioxide as a carbon source. It successfully tripled the denitrification rate. Miqueleto et al. (2010) examined the influence of different carbon sources in an anaerobic sequencing batch biofilm reactor with immobilized biomass in polyurethane foam. It yielded extracellular polymers of 1.4 mg/g carbon, 9.0 mg/g carbon, 13.3 mg/g carbon, and 23.6 mg/g carbon when the reactor was fed with meat extract, fat acids, soybean oil, and glucose, respectively. Also, the extracellular polymeric substances decreased from 23.6 to 2.6 mg/g carbon when the carbon source was reduced. It is all driven by the immobilized cells that primarily favor breaking down and consuming a certain element over others. The results propose having high extracellular polymers to create stronger initial adhesion of microorganisms to the polyurethane surfaces. However, high production of insoluble extracellular polymeric substances might lead to bed clogging, or problems with biomass granulation or flocculation, causing a gradual deterioration of the process performance that eventually collapses the system.

Among hundreds of polyurethane studies, studies of Park et al. (1996, 2010, 2011a, b) are the only investigations that examined pilot-scale studies of polyurethane effectiveness as biocarrier with 60L, three 15 L reactors, and two 15 L reactors, respectively (Park and Lee 1996; Park et al. 2010, 2011a). The need for studies of large pilot scale with more than 250 L active volume is essential to bring the polyurethane as an effective biocarrier to the wastewater industry.

Drawbacks of polyurethane biocarrier

There are several operational challenges with utilizing the polyurethane biocarrier. As the biofilm ages, their adhesion becomes loose. So, the biofilms need to be removed from time to time either with backwash or by restarting the bioreactor to allow biomass detachment from the packing medium. Else, the excessive biomass growth, extracellular polymers, and continuous flow of suspended solids will accumulate in the interstices of the packed bed. It narrows the pores and causes physical and biological fouling, and channeling. The probability of clogging increases along with the operation time (Al-Amshawee et al. 2020d). Therefore, the growth of the biofilm should be controlled to avoid clogging of the media. Low buoyancy and high compressibility of the polyurethane require special measures during filling of the columns. Otherwise, large biomass accumulation will restrict oxygen and nutrients diffusion towards inner biofilm zones, which leads to low metabolically active biofilms.

Different studies proposed various ways to prevent biofouling like (1) replace the industrial wastewater by synthetic wastewater until efficiency is regained (de Oliveira et al. 2009), (2) redesign the filtration system (Yuan et al. 2013), and (3) having a high porosity (Chen et al. 2015). Unfortunately, not a single report among the reviewed studies discussed designs of the polyurethane biocarrier or placed recommendations for future studies to improve the design. Further, using mechanical stirring might have the potential to scale up the polyurethane biocarrier-based bioreactor.

Polyethylene

Polyethylene is one of the common plastic (Nerland et al. 2014). Despite polyethylene price is only 7, 9, 20, and 29% of the value of polyvinyl chloride, polyacrylic, polystyrene, propylene, and hard polyethylene, respectively, it is essential to review its performance for different wastewater applications (Tabraiz et al. 2016).

Applications of polyethylene biocarrier

Polyethylene has been used in different applications such as decolorization of dye-containing textile wastewater (Khelifi et al. 2008), treatment of wastewater contaminated with terephthalic acid (Liu et al. 2019), biogas generation with anaerobic digestion (Martí-Herrero et al. 2014), petroleum hydrocarbon removal (Qaderi et al. 2018). However, researchers preferred investigating nitrification and denitrification thoroughly for organics and nitrogen removal.

Pastorelli et al. (1997) studied pilot-scale moving bed biofilm reactor packed with polyethylene biocarriers characterized by 10 mm diameter, 8 mm height, and 1.0 g/cm³ density for nitrification and organic carbon removal. The polyethylene biocarriers rapidly started the nitrification with no clogging problems. Despite that the highest biofilm growth was noticed in the protected internal surfaces. Celis-García et al. (2007) examined the use of a down-flow fluidized bed reactor for the treatment of sulfate-rich synthetic wastewater. Commercially available polyethylene plastic pellets were grounded into fine low-density polyethylene particles with density of 267 kg/m³. About 150 g of it were used as plastic supports. Inspections identified methanogenesis as the predominant cells, but sulfate reduction became the superior process after 187 days. Eventually, it removed sulfate of 75% and chemical oxygen demand of 93% after 369 days. However, the high sulfide load about 1215 mg ${\text{SO}}^{ - 2}_{4} / {\text{L}}$ stopped the sulfate-reducing bacteria activity, resulting in approximately 30% performance decrease. In contrast, another study delivered 93% removal of chemical oxygen demand and 75% reduction of sulfate using polyurethane foam in a batch packed-bed horizontal flow reactor at sulfide concentration of 1000 mg/L during 187 days (Silva et al. 2002).

One more study delivered 65.68 and 67.89% chemical oxygen demand removal, 79.17% and 83.43% nitrate-nitrogen removal, and 91.64 and 94.35% phosphate–phosphorus removal at high and low organic loading rate, respectively (Cabije et al. 2009). It demonstrates that raising the organic loading rate reduces carbon, nitrogen, and phosphorus removal rate, but it turns microcolonies thickness into twice as much in 24 h. Most importantly, all the studies were operated under comparable circumstances but ended up with different removal rates due to the attachment of different microorganisms. Analysis indicated that methanogenesis, sulfate reduction bacteria, and coliform bacteria were the leading parameters in the studies of Celis-García et al. (2007), Silva et al. (2002), and Cabije et al. (2009), accordingly.

Alvarado-Lassman et al. (2008) and Ma et al. (2009) opposed that by proving that carrier shape is the driving force. Ma et al. (2009) validated that porous polyethylene balls had a better simultaneous removal of chemical oxygen demand and nitrogen than polyethylene fibers hung on plastic rings. Furthermore, Alvarado-Lassman et al. (2008) concluded that spherical shape polyethylene promoted a uniformed growth around the particle, reached 100% superficial colonization, and managed seven times organic load than the triturated polyethylene support. Shortly, studies agreed that the spherical shape is the foremost for the polyethylene biocarrier.

Developments of polyethylene biocarrier

The main drawback of biofilm-based treatments is the long start-up duration. Hence, a rapid biofilm growth on the biocarrier surface is one of the fundamental considerations. Samuelsson and Kirchman (1990) as one of the baseline studies, prepared a biocarrier composite constructed from borosilicate glass and polyethylene to entrap Pseudomonas sp. strain S9 supplied with ribulose-1,5-bisphosphate carboxylase as a protein. It demonstrated that an enhanced biocarrier material could attract more cells to attach, and the biofilm growth rate consequently ascends. Besides, it can preserve a low biofilm thickness, which is right for higher activities during immobilization. Furthermore, Hadjiev et al. (2007) found that polyethylene powdered activated carbon composite tripled the cell immobilization rate of a coated polyethylene vinyl acetate. At a chemical oxygen demand concentration below 1000 mg/L, both biocarriers delivered insignificant and alike performance, but when the chemical oxygen demand concentration was increased to more than 1000 mg/L, the coated polyethylene vinyl acetate all at once delivered 1.4 times more significant removal of chemical oxygen demand than the polyethylene powdered activated carbon composite.

Mao et al. (2017) sought moving bed biofilm reactor start-up acceleration with using electrophilic biocarriers. The conventional negatively charged polyethylene was adjusted by two kinds of positively charged polymers noted as polyquaternium-10 and cationic polyacrylamides, where polyquaternium-10 has a higher positive charge than cationic polyacrylamides. Accordingly, the polyquaternium-10-based polyethylene biocarriers resulted in more significant biofilm attachment, faster start-up, and higher removal of total nitrogen and chemical oxygen demand than conventional polyethylene biocarriers and cationic polyacrylamides-based polyethylene biocarriers. The biofilm development was exponential with higher activity during the early period of the reactor start-up.

Other studies compared the polyethylene biocarrier to polymer-based biocarriers in terms of bioreactor performance and effluent quality. For instance, Mijaylova Nacheva et al. (2008) studied seven types of biocarriers, including polyurethane, polypropylene, cubes of polyethylene, grains of polyethylene, crushed tezontle, and ceramic spheres placed inside continuous downflow reactors. Polyurethane biocarriers produced the best phosphorus and nitrogen removal, while polyethylene delivered the best removal of chemical oxygen demand. In another study, polyethylene and polyvinyl chloride were packed for total acid extractable organics removal using endogenous microorganisms (Hwang et al. 2013). The initial adhesion of biofilm was favored on the polyvinyl chloride surface, so polyethylene biocarrier possessed a lower range of bacterial growth, biofilm thickness, and microbial diversity. Moreover, the polyvinyl chloride biocarrier reached steady state faster than the polyethylene. All of that because polyethylene biocarriers have higher hydrophilicity and surface negativity charge than polyvinyl chloride. Khan et al. (2013) compared between acetal polymeric plastic, polyethylene, melamine, and nylon in terms of surface chemistry and rates of nitrification. The study found surface energy and contact angles as following nylon > melamine > polyethylene > acetal polymeric plastic, and acetal polymeric plastic > polyethylene > melamine > nylon, respectively.

Recently, Peng et al. (2018) investigated the influence of N-Octanoyl-L-homoserine lactone (C₈-HSL), N-Hexanoyl-L-homoserine lactone (C₆-HSL), and N-acyl homoserine lactones (AHLs) on biofilm adhesion on the surface of polyethylene terephthalate, polyamide, and polystyrene. Polyamide presented the greatest adhesion properties, while the largest thickness and highest deposition were found on polyethylene and polystyrene, respectively. On the other hand, the polyethylene biocarrier showed the slowest biofilm deposition after polyamide, and polystyrene, respectively. Opposing a lot of reports, Hwang et al. (2013) mentioned that higher roughness is one of the factors make the polyethylene biocarriers possessing lower bacterial growth and biofilm thickness, and Sonwani et al. (2019) brought attention to the micropores of the biocarrier surface as the main factor presenting the active surface area. Indeed, contact angle and hydrophilicity are not the only controlling factors of the biocarrier performance.

In terms of design, Sreeda et al. (2018) utilized a polyethylene mesh with a 2-mm pore size for 135 days for wastewater treatment inside an aerobic bioreactor. A space was made between the mesh layers to inhibit dense cake formation. The system delivered stability over the long duration, with 82.16% ± 6.47 chemical oxygen demand removal and 97.21% ± 0.62 ammonia removal at optimized conditions of organic loading rate and hydraulic retention time. The study aimed to find whether a continuous biofilm formation could effectively be utilized for pollutants biodegradation, or it would clog the mesh pores. There were great considerations towards mesh clogging, and the 2-mm pore size was carefully selected to inhibit mesh fouling. Figure 4 demonstrates a specially designed polyethylene biocarrier with biofilm grown on its surfaces.

Fig. 4

Images of polyethylene biocarrier (Reprinted from Paul et al. 2007 with permission from John Wiley and Sons). a Polyethylene biocarriers at 10 mm; b polyethylene surface at 3 mm; c cells growth on polyethylene surface; d and e scanning electron microscope images of polyethylene surface

Drawbacks of polyethylene biocarrier

Studies gave attention to operating conditions to inhibit biofilm sloughing and washout. Studies have substantially neglected optimizing biofilm thickness and still have not been decided, such as 95 µm (Alvarado-Lassman et al. 2008), 13.3 µm (Hall Stoodley et al. 1999), and 7.71 µm (Cabije et al. 2009). Thick biofilm limits the diffusional mass transfer of substrates and microbial metabolic products, while thin monolayer biofilm is suitable for better activities. Practically, biofilm thickness is the conclusion of the operating conditions, hydrophobicity, substrate, and biocarrier geometry and material/coat or composite. Hence, biofilm thickness is of essential importance to the biodegradation process. As the robustness is essential in wastewater treatment, there is a necessity to avoid substrate diffusivity limitations. Positively charged polyethylene biocarriers have been frequently introduced as an enhanced way to trap biofilm bacteria and possibly control the growth.

On the other hand, the manufacturing of charged carriers can be pretty much costly for long-term industrial-scale processes. Therefore, future studies have to take into account cost-effectiveness, optimize operating circumstances, improve biocarrier geometry, and coat polyethylene surfaces if it is necessary. Finally, coated polyethylene biocarriers or composites need to get examined to verify the polyethylene stability against decomposition for long-term wastewater treatment, where 200 species of pseudomonas strains can cause approximately 15–20% mass loss of polyethylene after 30 days of incubation (McCormick et al. 2016).

Polyvinyl alcohol

Polyvinyl alcohol gel beads belong to synthetic polymers, which have been successfully employed in many reactors such as moving bed biofilm reactor, anaerobic fixed bed reactor, anaerobic fluidized bed reactor,. The beads are typical polymers like polyethylene and polystyrene made up of carbon–carbon bonds. They possess useful properties including excellent mechanical and physical stability to overcome high shear strength, large surface area, resistant to biodegradation, hydrophilicity, non-toxic and harmless to biofilms, resistance to oxidation, high inner porosity (see Fig. 5), easy and inexpensive in commercial and industrial applications and less excess sludge generation compared to other polymers (Lee and Cho 2010; Sarayu and Sandhya 2012; Dolejš et al. 2014; Qiao et al. 2014; Premarathna and Visvanathan 2019). It has a superior performance in housing biofilm-forming microorganisms in its pores for encapsulation and cultivation. The strong adhesion to the beads reduces the detachment rate (Singh et al. 2016).

Fig. 5

Scanning electron microscope photograph showing the porosity of polyvinyl alcohol biofilm carrier at 200 μm scale (Reprinted from Su et al. 2020 with permission from Elsevier)

Applications of polyvinyl alcohol

The beads have been successfully packed for nitrification and denitrification (Guerrero et al. 2019), pollutants biodegradation (El-Naas et al. 2013), and enhancing attached growth membrane bioreactor performance (Chaikasem et al. 2014). Razavi-Shirazi and Veenstra (2000) selected and developed a polyvinyl alcohol matrix to remove 2,4,6-trichlorophenol from groundwater. During 166 days of continuous treatment, the immobilized cells removed 91–99.9% 2,4,6-trichlorophenol without producing intermediates at a loading rate as high as 600 mg/L d. However, the immobilized cells faced a high loading rate and insufficient dissolved oxygen. Therefore, biodegradation was resumed once sufficient dissolved oxygen was available. On the other side, Hsieh et al. (2002) and Cao et al. (2002) reported a stable nitrogen removal performance that resisted shear stress over 5 months.

Ho et al. (2002) immobilized biofilms on the polyvinyl alcohol surface to investigate simultaneous nitrification and denitrification. The process was supplied with air and hydrogen to deliver oxygen for the nitrification, and hydrogen for the autotrophic denitrification. After a short acclimation period, the study resulted in an effective simultaneous nitrification and denitrification, and the biofilm inhibited dissolved oxygen penetration which led to a high denitrification rate. The biocarriers largely contributed to sustaining biofilm thickness and adhesion, which resulted in a stable simultaneous nitrification and denitrification. Though, when the liquid medium was saturated with dissolved oxygen, biofilms could not sustain the denitrification rate, and it was reduced by 8%. Likewise, Premarathna and Visvanathan (2019) reported a low denitrification rate and variations in the removal of chemical oxygen demand due to the saturation at the biofilms. On the other hand, insufficient dissolved oxygen can primarily reduce the efficiency of immobilized cells. This problem requires more critical studies with the help of kinetics to pave the way to estimate and deliver dissolved oxygen without reducing or inhibiting nitrification and denitrification.

Unlike Ho et al. (2002) and Chung et al. (2007) preferred developing separate reactors, including an anoxic tank followed by an aerobic tank and a settling tank called hybrid shortcut biological nitrogen removal. The system purpose is the same as simultaneous nitrification and denitrification to convert ammonium through nitrite to nitrogen gas. Only the aeration tank was 20% packed of 0.75 mm polyvinyl alcohol media to immobilize ammonium oxidizers. The aeration tank marvelously delivered a stable nitrite treatment for 1.5 years and preserved < 1 mg/L dissolved oxygen concentration and 20–25 mg/L ammonia concentration. Obviously, the carriers had contained thick and active biofilms giving the solids high retention time to be decomposed. Ho et al. (2002) and Chung et al. (2007) used different reactors, but both studies reached high simultaneous nitrification and denitrification rate, thanks to the polyvinyl alcohol carrier properties towards housing the biofilms. Though, both studies did not report specific details about organic and inorganic carbons removal.

As the carrier proved to be ready to process wastewater and applicable in different reactors, researchers decided to explore its ability to decompose different influents. Chang et al. (2003), as one of the first studies, acclimated polyvinyl biocarriers for 4 months to immobilize anaerobic sludge so that it would form a biofilm for 2-chlorophenol dechlorination. The researchers supplied the bioreactor with hydrogen gas as an electron donor, as Ho et al. (2002) did in their study. It removed 92.8% 2-chlorophenol, and unlike the study of Razavi-Shirazi and Veenstra (2000) that did not produce intermediates, this study produced phenol as a dechlorinating intermediate. Like the study of Guerrero et al. (2019), the process favored sulfate and nitrate as the primary substrates, which largely reduced the dechlorination performance (Chang et al. 2003).

If the bioreactor contains only nitrate and sulfate, the microbes would prefer consuming nitrate because it is more efficient than sulfate as an electron acceptor. Besides, nitrogen compounds penetrate faster because of their small molecular size and greater concentrations. It can make the diffusion profile occupied with sufficient nitrification capacity but negligible organic utilizing ability. However, the packing rate of polyvinyl alcohol can be increased to immobilize more suspended cells and grow further biofilms. Consequently, the available nutrients will be insufficient to feed the attached cells, so it will largely consume the rest of nutrients. Guerrero et al. (2019) and Chang et al. (2003) reported the competition between the primary substrate and nitrogen compounds and sulfate. Therefore, new studies are recommended to find methods to control or amend cell preference towards substrates.

Few studies compared between the polyvinyl alcohol performance to other biocarriers. For instance, Buchtmann et al. (1997) studied and compared between modified cellulose (Aquacel^®), polyurethane foam (Bayvitec^®), and polyvinyl alcohol with a view to their effectiveness to immobilize cells (see Table 1). Unexpectedly, all the biocarriers delivered no significant difference under stable conditions. Nevertheless, the polyvinyl alcohol biocarrier had regular biofilm growth and demonstrated the densest and smooth biofilm. Another study compared carbon fibers, polyvinyl alcohol, and polyethylene (An et al. 2018). Polyvinyl alcohol removed a higher rate of chemical oxygen demand, total nitrogen, and total phosphorus than polyethylene. Guerrero et al. (2019) concluded that polyvinyl alcohol carriers are fully retrievable, unlike polyurethane foam.

Table 1 Properties of Aquacel^®, Bayvitec^®, Siran^®, and polyvinyl alcohol biocarriers (Reprinted from Buchtmann et al. 1997 with permission from John Wiley and Sons)

Surface adjustment and composites

Physical biofilm fixation around the polyvinyl alcohol beads might result in weak bonds which induce leaking of live cells. According to the literature, different ways have been proposed to adjust the biocarrier surface and increase the cells persistence against detachment and eruption like biocarrier modification with magnetic particles, Fe₃O₄, and sodium alginate (Khowala 2012; Siddiqui et al. 2018; Brião et al. 2020). Besides that, it can increase the biocarrier mechanical stability, flexibility, eco-friendly characteristics, and possibly economize the operational cost (Lee and Cho 2010). Reliable biocarrier coating results in easy management of the attached biofilm, and the chances of biofilm contamination are significantly reduced.

Guo et al. (2007) set calcium alginate, polyvinyl alcohol-H₃BO₃ and agar composite bead for azo dyes removal at anaerobic circumstances. With no trouble, it removed 90% color and continued delivering a stable treatment even after four cycles. Accordingly, Jiang et al. (2018) reported the ability of ZnNPs/polyvinyl alcohol composite to maintain steady performance after several times of reusability, while it preserved performance even after ten consecutive cycles in Su et al. (2020) study, and up to 20 cycles in another study (Liu et al. 2009; Jiang et al. 2018; Su et al. 2020). In contrast to Guo et al. (2007) and Jiang et al. (2018), Toh et al. (2013) found the beads recovery could be obtained up to three cycles of use, but with only at lower initial loading rate. Habiba et al. (2017) found that the addition of zeolite can improve the beads recovery. On the other hand, Choi and Hu (2008) found some materials coated on the polyvinyl surface that can significantly inhibit the biological treatment. For instance, the presence of Ag inhibits 50% of nitrifiers growth.

In 2009, a study developed novel macroporous polyvinyl alcohol cryogel with molecularly imprinted polymer particles embedded in it (Le Noir et al. 2009). The composites were placed inside open-ended protective shells, known as kaldnes carriers (see Fig. 6). The biocarriers were placed inside a 120 mL moving bed biofilm reactor and 70-mL packed-bed reactor to remove endocrine-disrupting compounds such as atrazine, 4-nonylphenol, and 17b-estradiol. The systems removed 86% of atrazine and 100% of 17b-estradiol within 4 min of short retention time. Another study by Abigail and Das (2015) aimed to remove atrazine from aqueous environment by immobilizing Pichia kudriavzevii Atz-EN-01 cells on a sodium alginate–polyvinyl alcohol composite. At a flow rate of 60–480 mL/h, it delivered a higher atrazine removal rate of 94.3–100% than the study of Le Noir et al. (2009). However, substrate diffusion and cell leakage from the beads were highly dependent on the pore size, which significantly affected atrazine degradation.

Fig. 6

A macroporous gel particle comprised of polyvinyl alcohol gel inside a kaldnes carrier (Reprinted from Le Noir et al. 2009 with permission from John Wiley and Sons). a Kaldnes carrier; b polyvinyl alcohol gel inside the kaldnes carrier; c scanning electron microscope image of the macroporous gel particle

Cells immobilization

Other studies investigated various microbes immobilization to remove nitrogen and carbon simultaneously, like Ho et al. (2002) entrapped nitrifying and denitrifying bacteria, and Al-Zuhair and El-Naas (2011) immobilized Pseudomonas putida. Lee and Cho (2010) immobilized 14 species of useful microorganisms in modified polyvinyl alcohol hydrogel beads with calcium alginate. The process stably removed 93% of chemical oxygen demand and 73% of total nitrogen at a 3:1 ratio of aerobic to anoxic time. Chou et al. (2012) utilized a similar composite of Lee and Cho (2010) study to entrap nitrifying bacteria for partial nitrification. At various ammonium concentrations, the immobilized cells delivered a rapid start-up and delivered stable and efficient partial nitrification. Almost all the ammonium was converted into nitrite. Fluorescence in situ hybridization analysis indicated that ammonium-oxidizing bacteria represented 96% of the total microbial population in the biofilms. Thereby, all the ammonium was biodegraded to nitrite. However, when Zheng et al. (2017) added cyclodextrin to the same composite, it delivered lower nitrogen removal of 85.4% than Chou et al. (2012). The experiments showed that cyclodextrin had increased the bead’s porosity structure, which decided the process performance, as El-Naas et al. (2013) pointed out. However, the high-throughput sequencing revealed that the critical factor of having a lower nitrogen removal rate was due to entrapping a different cell, known as Comamonadaceae.

Without a doubt, researchers are required to put more efforts to conclude the real trigger to high and low efficiency as porosity or live cells. If the immobilized biofilm is the leading cause, then the carbon source has to be well investigated towards the microbial activity. On the other hand, if the porosity is the leading cause, then preparation and polymerization of the polyvinyl carrier have to be optimized like what Krasňan et al. (2016) highlighted in their mini-review study.

Miyake-Nakayama et al. (2006) noted that substrate accumulation could decrease the activity, and prolonged incubation is necessary to construct a dense cell community, while the addition of nutrients does not influence the immobilized cells. Above all, the construction of a dense cell community is identified to assure the superior characteristics lacking in the free-living cells. Su et al. (2020) synthesized PPy@Fe₃O₄/polyvinyl alcohol to enhance denitrification and consequently remove Mn(II) and Cd(II). The morphological analysis confirmed that the biocarrier features rough surface and well-developed porous structures. During the operation, the bioprocess reached 100% maximum denitrification efficiency. Eventually, it delivered 96.19% and 91.16% removal of Cd (II) and Mn(II) at 10 h hydraulic retention time.

Studies have primarily reported standard results such as chemical oxygen demand, total nitrogen, and total phosphorus for various applications of polyvinyl alcohol in wastewater applications. Though, they did not report specific details about organic and inorganic ions removal. For example, Guerrero et al. (2019) found that their polyvinyl alcohol biocarrier did not raise organic carbon removal. Besides, the biocarrier can be subjected to biodegradation during long-term operation once the polyvinyl alcohol degraders are developed within the biofilms such as fungi Fusarium lini, Alcaligenes sp., Alcaligenes faecalis, Bacillus sp., Bacillus megaterium, Pseudomonas sp., Sphingopyxis sp. PVA3, Pseudomonas vesicularis PD, and Pseudomonas O-3 (Raghul et al. 2014). Finally, further experiments on the composition and behavior of attached biomass on polyvinyl alcohol beads during steady-state and shock operation regimes shall assist in better understanding of the mechanism and advantage of using the polyvinyl alcohol as a biofilm carrier (Sajjad and Yunus 2019).

Polypropylene

Polypropylene biocarrier has been used in different applications such as phenolics removal, saline wastewater treatment, and applied in membrane bioreactor to reduce fouling and deliver a higher effluent quality. For 2 months, Zellner et al. (1994) used five curler-shaped polypropylene fixed bed reactors in parallel for treating synthetic wastewater characterized by chemical oxygen demand of 20 g/L and composed of 3.66 g/L butyric acid, 3.66 g/J propionic acid, and 7.32 g/J acetic acid. The hydrophobic polypropylene surface developed biofilms of 20–200 µm and was predominated by Methanothrix sp.

In 1997, a 9.5 L rotating bio-disc contactor was developed by Kargi and Uygur to immobilize halophilic as biofilm for saline wastewater treatment. The discs were made of polypropylene with a total surface area of 2.512 m² and a diameter of 20 cm, where 40% of the discs were immersed in a wastewater medium. The researchers expected difficulties because the presence of salt can broadly impact the plasmolysis of microorganisms. As was postulated, the process delivered low chemical oxygen demand removal of 67%, but surprisingly, the lack of forced aeration was found responsible for the low performance. Consequently, the removal range of chemical oxygen demand was increased to 90% after the aeration rate was raised. Unfortunately, the study did not investigate the polypropylene effect on process performance.

In 2004, a submerged artificial substratum involving cubic carriers made of polypropylene was developed for phosphorus removal from eutrophic lakes using periphyton (Jöbgen et al. 2004). Polypropylene was selected as a carrier because of its coarse texture made of fibers, which dramatically increased the surface area and resulted in higher microhabitat heterogeneity compared to glass carrier. The Fühlinger See Lake involved ciliates, nematodes, rotifers, copepods, chironomids, trichopterans, ephemer-opterans, and oligochaetes. With the help of polypropylene carriers and periphyton, the process removed 100 mg/m² total phosphorus. However, not all the microorganisms were available in the biofilm because they competed for substrates and attachment on the polypropylene surface. A different study was reported by Sokół and Woldeyes (2011) employed low-density propylene biocarrier to mitigate excessive biomass growth that led to bioparticles channeling over the fluidized bed. The media significantly controlled biomass growth and retained sufficient dissolved oxygen concentration. Also, it removed hydrocarbons of 90%, phenolics of over 95%, and chemical oxygen demand of 96%.

A 450 m³/d pilot plant was employed by Kim et al. (2013) as advanced water treatment. It involved a mesh made of synthetic polypropylene with size of 3–20 mm to remove soluble organic carbon and suspended solids of secondary settling tank effluent. During 45 h, it was observed that the process was limited up to suspended solids loading rate of 3.8 kg SS/m³ d at maximum linear velocity of 16 m³/m² h. However, the polypropylene mesh did remove not only suspended solids but also soluble organic carbon. Hence, the study of Kim et al. (2013) recommended the process to be used as a direct filtration because of its high capacity and fast screening, unlike polyvinyl chloride, where Healy et al. (2010) found an additional process is needed.

Felföldi et al. (2015) examined three similar polypropylene and one polyester fiber biocarrier. The biocarriers developed different structures of biofilm. The polypropylene possessed more than 2.5-fold biomass weight value to the other carriers due to having the highest wetting force. Moreover, similar microbial diversity was found on all the polypropylene but had a different population. Biocarriers having different surface areas, roughness,...etc., are frequently reported in their significant impact on the mass transfer (Welander et al. 1998), but the study of Felföldi et al. (2015) added new facts that wettability and microbial diversity influence biomass amount and activity.

Novel studies to augment the polypropylene surface with charges are highly needed. The new studies can help to establish an equal comparison between the insoluble biocarriers for a stable biofilm thickness. Also, pilot-scale studies can expose the processability and technical needs towards establishing a practical industrial scale (Sajjad et al. 2020). Finally, it will help to define the process limitations and drawbacks against influent characteristics such as high strength wastewater with chemical oxygen demand of 10,000 mg/L.

Polyvinyl chloride

Polyvinyl chloride was examined by Xue et al. (2012) and Li et al. (2011) for the biodegradation and removal of geosmin and microcystins. It could not stabilize the removal rate during various circumstances. A contact oxidation biofilm reactor was 10% packed of 150 mm average length elastic polyvinyl chloride fibers for high strength ammonia removal (Qiao et al. 2008). Gradually, biofilms were developed on the polyvinyl chloride surfaces and accomplished complete ammonia oxidation of 150–360 mg/L in a short duration. Qiao et al. (2008) concluded that polyvinyl chloride is suitable for treating concentrated ammonia-rich wastewater.

Healy et al. (2010) used sheets of polyvinyl chloride, allowing the influent to flow over and back along with it. The attached biofilm reduced nutrients, suspended solids, and organic carbon. Unlike the study of Healy et al. (2010) and Hassard et al. (2014) applied polyvinyl chloride in the shape of mesh media. The process effectively removed substrates and protected the biofilms from high shock loadings. Then, Hassard et al. (2016) investigated the influence of two mesh rotating reactors containing polyvinyl chloride and polypropylene for microbial performance and viability (see Fig. 7). High organic loading rate resulted in low biofilm growth and a twofold decrease in the microbial activity on all mesh, i.e., the microbial activity negatively correlates with the organic loading rate. Also, it was found that low surface area mesh is the best to process a very high organic loading rate with a soluble chemical oxygen demand of 160 g/m² d.

Fig. 7

Polymeric mesh media for biofilm development (Reprinted from Hassard et al. 2016). a Polyvinyl chloride low surface area; b polyvinyl chloride high surface area; c polypropylene low surface area; d polypropylene high surface area

It was concluded that high surface area is preferred to deliver high nitrification rates, and low surface area is needed for influent pretreatment having a high organic loading rate. The polyvinyl chloride mesh demonstrated an enhanced ability to sustain biofilm development compared to the polypropylene media studied. Also, it delivered the best removal of soluble chemical oxygen demand. In the end, the polyvinyl chloride media was recommended for wastewater pretreatment application at high and very high organic loading rates.

Habouzit et al. (2011) observed polyvinyl chloride and polycarbonate having high affinities to develop biofilm, but its composition is primarily affected by the influent composition. After 2 years, Stephenson et al. tested polyvinyl chloride, polycarbonate, polypropylene, polyethylene, acrylonitrile butadiene styrene, tufnol, polyte-trafluoroethylene, and nylon as biocarriers to enhance the nitrification process in the presence of heterotrophs inside aerobic batch reactor receiving domestic settled wastewater (Stephenson et al. 2013). The polyvinyl chloride had the least biomass accumulation over the experimental period, but it had evenly thick distributed biofilm compared to the rest of the carriers mentioned. The studies of Sousa et al. (1997) and Stephenson et al. (2013) agreed to rank the media as polypropylene > polyethylene > polyvinyl chloride in terms of nitrification efficiency.

About modifying polyvinyl chloride, there is only one study that suggested to coat the polyvinyl chloride disks of rotating biological contactors with hydroxyapatite for large-scale wastewater treatments (Pesciaroli et al. 2013). The few available studies have not widely enhanced polyvinyl chloride as a biocarrier to process different influents under shocking operating conditions. Thus, extensive studies are needed to adjust the surface of polyvinyl chloride and compare its performance to other biocarriers.

Waste tires

The current scrap tire recycling market is too small compared to the annual number of waste tires generated globally (Herrera-Sosa et al. 2015). The global demand for tires is estimated to rise 4.1% per year to 3.123 billion units by 2020, which corresponds to approximately 34 million tons of waste tires production annually (Sharafat et al. 2018). Approximately 4 billion used tires were ended in landfills and stockpiles worldwide (Mathews 2006; WBCSD 2008). Waste tires are nearly non-biodegradable and take up sizeable landfill space. The principal chemical component of tires is a blend of natural and synthetic rubbers and other components, including carbon black, softeners, oils, fabrics, pigments, antioxidants, paraffin waxes, polymers, cobalt salts, sulfur, silica, bead and belt materials (Warith et al. 2004; Mondal and Warith 2008). The widely differing chemical compositions and the cross-linked structures of rubber in these tires largely account for its high resistance to biodegradation, photochemical decomposition, chemical reagents, and high temperatures.

Most of the industrial countries have banned scrap tires from landfills, and regulations have been legislated to manage the end of tires’ life (Serumgard 2014). Therefore, finding feasible options with low financial and technical requirements for scrap tire use cannot be ignored, and it is essential to promote new markets to reuse waste tires. Waste tires have been introduced as a novel media and biofilm carrier in constructed wetlands (Chyan et al. 2013), up-flow biological aerated filter (Han et al. 2012), trickling filter system (Mondal and Warith 2008), fixed bed biofilm carriers (Derakhshan et al. 2017), and fixed bed sequence batch reactor (Derakhshan et al. 2017) for wastewater treatment.

Waste tires seem to be a valuable option for wastewater treatment since they can be obtained easily at low costs and in large quantities. They are comparatively cheaper than any other usable packing material (Mondal and Warith 2008). Toledo et al. (2019) examined waste tires by scanning electron microscope and found it having significant roughness (de Toledo et al. 2019). Fourier transform infrared spectroscopy and scanning electron microscope analysis of Naz et al. (2014) study confirmed tire-derived rubber having large surface area and durability under aerobic conditions which qualify it as a biofilter for wastewater treatment (Naz et al. 2014). The durability depends upon its durable polymeric nature combined with different adhesives, retardants, and vulcanizing agents. Limited biotransformation of surface composition of the waste tire indicates that it could be valuable support material for long-term use in wastewater treatment.

The term tire shred is used for 50–305-mm large tire particles with no proper geometrical shape. The product of tire-shredding is usually referred to as tire chips, where they are between 12 and 50 mm in size with geometrical shape and have most of the steel beltings removed (Kaushik et al. 2015). Further, tire chips can be granulated into 1.5–6.5 mm tire crumbs to remove loose steels and fibers entirely (Mondal and Warith 2008). Finally, any combination of tire chips and tire shreds is called tire-derived aggregates.

Applications of shredded tires

Recent applications of shredded tires in treatment systems such as drain fields, septic tanks, and constructed wetlands indicated potential opportunities for recycling scrap tires. Shredded tires are reasonably durable, insoluble, permeable and able to provide more surface area than that of crushed stone/rock (Mondal and Warith 2008). They can adsorb volatile organic compounds up to granular activated carbon sorption capacity of 1% to 6% on a volume basis (Park et al. 1996). Desorption tests indicated that 94.5% of the contaminant mass stays within the polymers under fully saturated conditions (Edil et al. 2004). Park et al. (1996) introduced shredded tires as an alternative for a leachate collection system medium to remove volatile organic compounds in landfill leachate. They affirmed that a 300-mm-thick shredded tire could eliminate 90% volatile organic compounds for 20 years. Park and Jhung (1993) claimed that lifetime could be prolonged if the biological degradation occurred in the shredded-tire layer.

One characteristic of tire chips is that the void is far less than that of the tire shreds, resulting in compressibility among tire chips (Yang et al. 2002). Hence, it can be indicated that the total surface area of tire chips is higher than that of an equal volume of tire shreds. The permeability of tire chips is approximately 0.03 cm/s, which is much higher than the standards of 0.01 cm/s. It demonstrates that tire chips are convenient as packing material in wastewater treatment systems (Cecich et al. 1996). The experiment of Mondal and Warith (2008) found that the bioreactor packed with tire crumbs was slightly higher in temperature than the bioreactor packed with tire chips. Taking into account the higher surface area of the tire crumbs compared to that of tire chips, it can be pointed out that the tire crumbs bed reactor can have more sites for biofilm immobilization than the tire chips bed reactor, resulting in better and more consistent nutrients and organic carbon removal. Besides, tire chips packing media is more exposed to clogging than the tire crumbs, which possibly reduces NH₃-N and total suspended solids removal efficiency.

An experimental study was carried out in six stages trickling filter system with 0.91 m height and 0.1 m width total packing of tire chips and tire crumbs to determine the efficiency of tire chips against tire crumbs for landfill leachate treatment (Mondal and Warith 2008). Due to the high surface area of tire crumbs and chips, a film of 1–2-mm-thick biomass was attached to them and detached at an interval of 21 days. Tire chips removed two times chemical oxygen demand concentration than the crumbs, likely due to possessing different void space, which is greater in the tire chips packing media. In contrast, tire crumbs packing media showed more consistent removal of total suspended solids, NH₃-N, and biochemical oxygen demand than tire chips. Though, both media showed similar rapid removal of NH₃-N and suspended solids. Their high porosity extended the detention time of wastewater inside the reactor. It also can be credited to an initial rapid organic adsorption on the surface of tire chips and crumbs because tires are fabricated from polymeric materials, which can adsorb and absorb chemical compounds. For example, Knocke and Hemphill (1981) and Rowley et al. (1984) showed that tires were effective sorbents for lead (ll), mercury, and cadmium (ll). However, they are not recommended to be used alone due to their highly compressible nature.

Studies and developments

Different studies established a vital role for the waste tire in the attached growth batch reactors. Certain important properties of tire chips, such as surface area, hydraulic conductivity, sorption capacity and compressibility can impact wastewater treatment (Warith et al. 2004).

Han et al. (2012) packed up-flow biological aerated filter with tire chips for treating 1,4-dioxane rich wastewater. In contrast to a lot of studies that noticed treatment performance since the first days (Shin et al. 1999; Lee et al. 2007) and reached maximum biofilm growth after 6 weeks (Naz et al. 2014), Han et al. (2012) reported their bioreactor efficiency starting from the first hours. The process delivered chemical oxygen demand and 1,4-dioxane removal of 69.2% and 59.8% during the first hour of the operation and consequently increased to 88.6 and 95.8 between the 8th to 12th h of operation, respectively. Eventually, it successfully removed 99.5% 1,4-dioxane. The small size of the tire chips was the leading cause of the initial high concentration removal (Kaushik et al. 2015). Though, the packing media was found incapable of sustaining the bioreactor performance against higher ratio of chemical oxygen demand/1,4-dioxane within a range of 3–46. Kaushik et al. (2015) had pointed to that issue and called to monitor the packing bed porosity using hydraulic conductivity reduction profile tests with time.

Krayzelova et al. (2014) hybridized sulfur-oxidizing denitrification with scrap tire chips to take place inside an up-flow packed-bed bioreactor for treating a synthetic nitrified wastewater. The reactor bed contained waste products involving scrap tire chips, crushed oyster shells, and elemental sulfur pellets. The hybridization enabled the tire chips to adsorb NO₃⁻ during the process even it does exceed the denitrification capacity. The modified scrap tire chips resulted in an adsorption capacity of 0.66 g NO₃⁻N/kg of scrap tires and removed 90–94% NO₃-N. Shin et al. (1999) recommended acidic conditions to have a higher adsorption capacity. Though, Krayzelova et al. (2014) did not demonstrate reasons for using a waste combination involving tire chips. Besides, studies compared gravel to tire chips as a biofilter and investigated their mixes. It finally concluded that no material had been reported can enhance waste tire performance inside the bioreactor. Further, Han et al. (2012) used a similar bioreactor packed only with tire chips, and it delivered a 99.5% higher removal rate compared to Krayzelova et al. (2014) which removed only 90–94% pollutants. It proves that waste tires at any conditions have the potential to support biological activities for a variety of wastewater treatment applications as a biofilm carrier towards reaching a better performance in chemical oxygen demand and total suspended solids removal.

Cells immobilization

Adsorption is an essential feature of scrap tires due to having rubber polymers and carbon black (Naz et al. 2014). The surface of the shredded tire can adsorb contaminants from water, and consequently, a biofilm can be developed. Besides the adsorption process, diverse microorganisms colonize the surface of scrap tire, forming a biofilm composed of a mixture of bacterial layers and embedded particles. A large variety of bacteria was observed attached to waste tire surfaces (Krayzelova et al. 2014; Derakhshan et al. 2017). The rise in density is a result of cell immobilization and development. Scanning electron microscope analysis showed homogeneous and abundant microorganisms on every surface of the scrap tire inside a biofilm reactor (Park et al. 2011b). An increase in the biofilm extracellular protein and polysaccharide concentrations was observed with time to reach a stable value. Scrap tires can potentially retain biofilm growth, which thereby delivers a stable performance of wastewater treatment (Park et al. 2011b).

For the sake of verifying microbial fixation, Naz et al. (2014) cut bus radial tire into cubical pieces with each having 21.95 cm² surface area and used activated sludge as seed for bacterial community colonization on the tire-derived rubber media. Against all expectations, P. aeruginosa had been unable to form biofilm on the shredded tires. However, Sharafat et al. (2018) observed P. aeruginosa’ ability to grow during the availability of Fe₃⁺. Biochemical testing and 16S rDNA phylogenetic analysis showed higher growth of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria during the availability of Fe₃⁺, and denser biofilms when the Fe₃⁺ concentration was increased. In a study by Chyan et al. (2013), the steel belt connected to tire chips was entirely dissolved after 300 days of denitrification process. It had been demonstrated that the iron ions of the shredded tires were naturally released into the bioreactor medium and accordingly consumed during the wastewater treatment. Due to iron ions decomposition, the bioreactor packed with shredded tires effectively removed total phosphorus and delivered a 62.2% nitrate removal rate, which is higher than that 13.1% nitrate removal rate of gravel-bed bioreactor. Finally, the count and types of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria on the surfaces of the shredded tire had proven the ongoing nitrification and pollutants digestion (Sharafat et al. 2018).

Recently, de Toledo et al. (2019) evaluated the re-utilization of motorcycle waste tires, sewage sludge, and wood chips to remove volatile organic compounds from artificially contaminated water. Pseudomonas sp. and Acinetobacter sp. adapted to the tire efficiently within 1–3 days of acclimation, resulting in the removal of volatile organic compounds. Everyday, the removal efficacy increased and optimally when tires and inoculum were simultaneously added to deliver biosorption process involving physical and biological treatment.

Drawbacks of waste tires

One vital concern about tire usage is long-term stability. The main conditions that cause rubber degradation are heat, light, oxidation, and mechanical conditions (Richter and Weaver 2003). Inside a wastewater treatment reactor, the shredded tires are not exposed to heat, or light, but experience mechanical impact inside the bed bioreactor. The diverse chemical compounds and cross-linked structures of rubber in these tires much account for its high resistance against biodegradation, high temperatures, chemical reagents, and photochemical decomposition. Though, the pollutant affinity towards the chemical groups present in the tire matrix decides the degree of biodegradability and strength of biofilm attachment.

Despite the shredded tires being regeneratable with using only clean water compared to activated carbon, their adsorption is only 0.04–0.3% of that of activated carbon (Shin et al. 1999). Besides, shredded tire promotes a high accumulation rate of biomass, leading to bed clogging, where shape and size of the void determine the level of clogging. During the clogging period, the microbial community can be changed into fungi, protozoa and filamentous.

Upcoming investigations are needed to present uniformed voids to deliver fewer small-sized voids and less tortuous flow paths. Besides, surface interface plays a significant role in determining the level of clogging in shredded tire containing bioreactors. Studies are required to adjust the scrap tire surfaces to inhibit biofilm sloughing and present strong adhesion for various contaminants that possibly have different affinities. Also, optimized operational conditions are needed to keep the biofilm structure intact and to achieve the desired wastewater treatment efficiency. Further research is needed in order to investigate media replacement, and the costs of regeneration must be seriously taken into account when using these materials. It is worth noting that scrap tires must be cleaned from zinc, mercury, lead, chromium, barium, phenol, toluene, methyl ethyl ketone, and carbon disulfide before employing it inside a bioreactor.

Conclusion

Despite polymeric biocarriers are inexpensive and can develop biofilms up to 500 times more resistant to extreme influent properties than suspended microorganisms, wrong selection of biocarrier material can significantly increase operating costs, and negatively affect biofilm sustainability. Along with the operation time, excessive biomass growth will accumulate in the interstices of the packed bed, narrowing the pores, and restricting oxygen and nutrients diffusion. Consequently, it leads to low metabolically active biofilms, physical and biological fouling, and channeling. Unfortunately, past studies gave substantial attention to kinetics and operating conditions to inhibit biofilm sloughing and washout.

The discussed materials could be ranked based on their performance of chemical oxygen demand removal as following: polyvinyl alcohol > polyurethane > polyethylene > polypropylene, and polyvinyl alcohol > waste tire > polyurethane > polyethylene for conventional and modified/composites polymers, respectively. The polyvinyl alcohol biocarrier is the most excellent material that could promote a uniformed growth around the particle and could reach 100% superficial colonization. Thanks to the properties and shape of polyvinyl alcohol that have been the main drivers in maintaining steady formation of sheltered and stable biofilm even after several times of biocarrier reusability. Therefore, this conclusion finds biocarrier specifications and material properties are the right information to be used for choosing a biocarrier, where selecting a biocarrier based on its commercial name/material is impractical.

The polymeric material of biocarriers is subjected to biodegradation during long-term operation once biological degraders are developed within the biofilms and suspended biomass. The disposal of these polymeric biocarriers is technically not feasible and expected to be costly. Therefore, it is essential to review any selected biocarrier material in terms of performance and long-term stability for different strength of influents compared to other biocarriers. Moreover, physical and chemical modification of the surface is a necessary option to accelerate biofilm formation process.

Further research is needed to investigate polymeric biocarriers replacement and regeneration costs, which seem pretty much costly for long-term industrial-scale processes. Also, studies are recommended to investigate the removal of specific organic and inorganic ions instead of general analysis like chemical oxygen demand. Further experiments on the composition and behavior of attached biomass on polymeric biocarriers during steady-state and shock operation regimes shall assist in better understanding of the mechanism and advantage of using these polymers as biofilm carrier. Finally, the need for studies of large pilot scale with more than 250 L active volume is essential to bring these biocarriers to the wastewater industry.