The treatment of wastewater for reuse is a potential solution to meet ever increasing urban, industrial, agricultural, and environmental demands across the world, where clean water availability is scarce. There are several traditional wastewater treatment processes that offer varying degrees of effectiveness in addition to presenting environmental, economic, and social disadvantages. Development of promising and inexpensive technologies to provide the reusable water in needful amounts using wastewaters as a cheap source of key nutrients and organic matter is required. Wastewater treatment by biological methods is becoming more important in the light of recovering value-added plant nutrients, heavy metals, biosolids, and bioenergy resources. Different types of solid contaminants in effluents can be removed simultaneously by pure cultures or mixed microbial consortia. Based on the structural organization of microbial biomass, biological treatment systems are classified into two types: dispersed growth system and attached growth system. Biological treatment methods associated with fixed-film growth have been recognized as highly effective and more energy efficient than suspended growth systems. This review discusses the recent breakthroughs in advanced biological wastewater treatment using both the systems, and also focuses on key energetic resources recovery driven by biological technologies.
Water is the most useful and crucial natural resource on the Earth. Even though most parts of the planet are occupied with water, clean water availability is becoming scarcer in recent years. It has been predicted that in less than 20 years, two-thirds of the world's population might face a fresh water shortage. The present scenario of increasing water usage and disposal on a global scale will be the main cause to this undesirable future (Cosgrove & Loucks 2015). To decrease the future fresh water crisis, recycling of wastewater effluents from human activities and agricultural, and industrial sites and reuse of the same is the best way to salvage the dwindling natural source (Khajuria 2015; Vasyutina 2018).
Domestic sewage is mainly comprised of 99.9% water and 0.1% suspended and dissolved organic and inorganic solids, including macronutrients as well as essential micro-nutrients (Puyol et al. 2017a). Among the solid substances present in sewage is a mixture of detergents, food leftovers, fats and oils, grease, heavy metals, various biomolecules and their decomposition products, sands and grits, excrements, paper products, and various natural and synthetic organic chemicals from the process industries (Qu et al. 2013; Stamatelatou 2017). In addition, different kinds of pathogenic microbial strains can be found in wastewater, which produce odorous gases and a bad smell. Wastewater which is rich with carbohydrates, lipids, phosphorus, and nitrogen ultimately results in the growth of large amounts of algal biomass and other aquatic plants, which leads to the deterioration of the aquatic environment quality, termed as eutrophication (Arimoro et al. 2008).
According to the World Health Organization, nearly 30% of human diseases and 40% of morbidity across the globe are attributed to polluted water. In order to maintain the essential levels of clean water as high as possible and separate the solids that are significantly hazardous to healthy life or the natural environment, it is required for wastewater to be recycled prior to being reused or discharged directly into waterways (Mojiri 2011). Furthermore the principle motivation for wastewater treatment is to reduce soil and groundwater pollution, protect the sea shore and marine life, reuse the treated effluent for industries, agriculture and groundwater recharge, and also solve social problems caused by the accumulation of sewage.
The removal of pollutants during effluent treatment in order to reach the necessary quality or required discharge standards for further use is associated with the concept of treatment level and treatment efficiency (Naidoo & Olaniran 2013). Wastewater treatment primarily consists of four levels, called preliminary, primary, secondary, and finally tertiary treatment (Mostafa 2015; Samer 2015). In the preliminary approach, first, coarse material, grit, and suspended solids can be removed by gravitational setting in primary settling or sedimentation tanks, while primary treatment aims for the removal of bulky suspended organic solids before discharge, known as primary effluent. A report of the American Chemical Society states that approximately 65% of total suspended solids (TSS), 40% of the incoming biochemical oxygen demand (BOD), 30% of chemical oxygen demand (COD), 60% of grease and oil, 20% N and 10% P are removed during preliminary and primary treatments (Shelley et al. 1976; Rivas et al. 2010). Physical or mechanical pollutant removal processes are predominant before primary treatment (Rivas et al. 2010). Secondary treatment of primary effluent, associated with biological mechanisms, mainly involves the consumption of major elements of organic compounds and suspended solids by different microorganisms for their growth under controlled conditions. The degraded component settles out in secondary settling tanks and later settled sludge is removed by sedimentation. This stage removes 90% of BOD and TS, and the maximum portion of N, P, and heavy metals (DNR 2016). The treated wastewater can be discharged into the outfalls of surface waterways and can be used for non-potable purposed such as toilet flushing and gardening. Finally, tertiary treatment is sometimes called the final or advanced treatment, which is aimed at further purification of wastewater particularly by removing the hazardous materials or the nutrients that are still in sewage after secondary treatment and specifically to eradicate or disinfect the pathogenic bacteria. Tertiary treated water can be reused for drinking purpose. In addition to the above, chlorination may be used at all stages during treatment to enrich the quality of water (Mittal 2011; Samer 2015; DNR 2016; Quach-Cu et al. 2018).
During wastewater treatment, a parallel focus is on utilization of effluent sludge, particularly exploring recovery of nutrients, which are present in far higher concentrations compared to those in the raw sewage. Nutrient recovery is the process of extracting essential resources such as polyhydroxyalkanoates (PHAs) composite (Albuquerque et al. 2010; Morgan-Sagastume et al. 2015; Basset et al. 2016); production of long-chain microbial exo-polysaccharides, including alginates (Lin et al. 2010), particularly through the use of aerobic granular sludge; and even direct recovery of ubiquitous fibers such as cellulose in wastewater (van Loosdrecht & Brdjanovic 2014), bio-gas (Miranda et al. 2017), biofuels (Wang et al. 2009; Zhao et al. 2018), heavy metals (Medina et al. 2015; Mosa et al. 2016), and purified plant nutrients (Medina et al. 2015). Approximately 50–80% of energy resources are lost in sewage. Sewage treatment plants aided with valuable product recovery can mitigate global pollution while enhancing water quality and meeting stringent nutrient discharge limits. In addition, purified nutrients such as N and P from sewage effluent that would otherwise be discarded can be recovered and transformed into organic fertilizer used for ecological and agricultural purposes.
Among many physiochemical technologies applied to the challenge of wastewater treatment and nutrient recovery, biological technology is a promising, simple and cost-effective alternative technology applied for both urban and industrial wastewater. Many researchers have successfully made significant contribution to bioremediation of wastewater (Costley & Wallis 2001; Henze et al. 2008; Abou-Elela et al. 2010; Mononen et al. 2010; Hossain et al. 2016; Mosa et al. 2016; De Beer et al. 2017; Ghimire & Wang 2018). Apart from needing organic matter, microorganisms involved in the bioprocess also require water containing nutrients, basically C, N and P, in order to proliferate by adapting continuously to changing external conditions (Henze et al. 2008). On the basis of structural organization of biomass, biological wastewater treatment methods can be classified into two types: suspended and attached growth systems. This review discusses the recent breakthrough in biological wastewater treatment using both the systems, and also focuses on key energetic resources recovery driven by biological technologies.
SOURCES AND TYPES OF WASTEWATER
There are mainly five types of wastewater or sewage, namely, domestic sewage, industrial effluents, mining, commercial, and agricultural sewage (Figure 1). Domestic wastewater also known as sanitary sewage carries used water from bathrooms, sinks, and kitchens of residential and commercial dwelling units. Although domestic municipal sewage contains quite a small percent (<1%) of a wide range of dissolved and suspended impurities by weight, the toxicity of these pollutants and the very large volumes of sewage make recycling of domestic wastewater a significant challenge. Domestic wastewater is not only a major source of impurities such as highly putrescible food materials, detergents, and plant nutrients, but also very likely to contain disease-causing microbes (Vasyutina 2018). Industrial wastewater is an undesired by-product from various product manufacturing industries, particularly from chemical and pharmaceutical units, and usually contains hazardous contents that have to be treated rather than directly released into water bodies. In general, contaminants in industrial effluents often include specific and easily distinguishable pollutants depending on the nature of the processing industries (Sun et al. 2016). Agricultural wastewater, also referred to as irrigation tailwater, is the water that leaves through the low end of furrows, borders, basins, and flooded areas in the course of land preparation to grow agricultural crops (Vasyutina 2018).
In general various pollutants contaminate water by means of two different sources, namely, point source and non-point or dispersed source. A point-source contamination is an identifiable and confined source of pollution discharged into water originating from a single pipeline or channel, such as operational wastes from industries and municipal sewage treatment plants. Some of the major industries having the need for processing of effluents include electronic equipment manufacturers, heavy metal processing systems, textile, pulp and paper mills, food production units, and leather tanners. By contrast, non-point or dispersed source water pollution occurs via broadly distributed and unconfined sources of pollutants, such as spills, leaks and acid drainage from abandoned mines; sediment erosion from improperly managed construction sites and forest lands; and storm water and snowmelt runoff. Municipal storm water sewage, which brings grit and sand residues and fuel components from automobile service stations, is also known as a non-point source pollution because of the various places from which it enters local water streams. In addition surface runoff from farms, carrying nitrogen fertilizers, pesticides, and bacteria and nutrients from livestock, is a large part of the overall water pollution problem. Point-source contaminants are easier to control than non-point source contaminants, since they flow to a single location where treatment processes can eliminate them from the polluted water. It has been reported that nonpoint source pollution is a leading cause of water quality problems, as the impacts of dispersed pollutant sources vary and may not be fully analyzed. Non-point source water contamination is best decreased by enforcing the most effective land-use plans and development standards (Henze et al. 2008).
KEY PARAMETERS TO ASSESS WATER QUALITY
It has been found that the efficiency of wastewater treatment technology and quality of discharged water may be influenced by several key parameters. They include BOD, dissolved oxygen (DO), TSS, and microorganisms (Mostafa 2015; Farooq et al. 2017; Puyol et al. 2017b; Wijaya & Soedjono 2018).
One of the most significant parameters to analyze treated water quality based on putrescible organic material in sewage is BOD. The more of organic impurities, the higher the BOD- which is the amount of oxygen needed by microbial consortium to breakdown the organic contaminants in sewage. The BOD value in combination with COD and total organic carbon (TOC) is widely considered as an indication of contamination of water samples by measuring organic matter sewage. Industrial waste effluents may have BOD levels many times that of domestic sewage (Choi et al. 2017).
Oxygen that is dissolved in water is termed as DO, is also an important water quality parameter for surface waters. The higher the concentration of non-compound oxygen, better the water quality. When waste streams entered in to water bodies, biodegradation of the organic pollutants initiates. Oxygen is consumed by microorganism for their respiration, metabolic activities and proliferation. This situation drastically reduces the level of free oxygen available in the water. When the amount of oxygen drops too low for breathe, soon distinct aquatic species, pike, bass, caddisfly larvae and trout perish. Indeed, if the oxygen concentration in water body reaches to zero, it will become septic. Thereby, biodegradation of organic pollutants in the absence of oxygen leads to unwanted gases generally associated with putrid conditions. Domestic wastewater decreases the concentration of oxygen and enhances the BOD in surface waters due to presence of higher amount of organic materials (Holenda et al. 2008; Henze et al. 2008; Bo & Zhang 2018).
Total suspended solids are other important factor in assessing and monitoring water clarity. Other important key parameter need to consider is total suspended solids. The volume of sewage sludge generated during treatment process is directly proportional with the TSS present in the wastewater. It is generally accepted that industrial and agricultural sewage generally contains higher amounts of suspended solids compared to domestic effluents. The extent, to which a treatment system eliminates BOD, as well as TSS, governs the effectiveness of treatment process (Henze et al. 2008). Moreover, sewage has many millions of microorganisms per gallon. Faecal coliform bacteria are most commonly used as indicators of pollution level. A high coliform count usually demonstrates recent wastewater contamination. While coliform is not a reason of illness, their count indicate the level of other pathogenic organisms of faecal origin, be it bacteria, viruses, or protozoa may exist. Furthermore, the removal of two major essential nutrients such as nitrogen and phosphorus from the waste effluents has become an emerging concern because these components results in eutrophication of waterbodies through excessive growth of algal blooms (Srivastava & Srivastava 2011).
Wastewater is commonly prepared for the treatment process by the removal of floating wood pieces or branches, plastics, papers, dead animal debris and also heavy settleable inorganic solids during preliminary treatment. The preliminary effluent must be further treated using primary and secondary treatments. By the nature of the treatment process, wastewater treatment methods can be organized into three types, namely, physical, chemical, and biological, in order to improve the water quality so that humans and animals can consume it and make use of it for other purposes (Sen 2015; Solon et al. 2019) (Table 1).
Several conventional physical unit operations include adsorption, vibrating and rotary screens, sedimentation, comminution, flotation, granular filtration, and membranes by use of naturally occurring forces such as gravity, electrical attraction, van der Waal forces, and energy gradients as the driving force to separate pollutants during the primary treatment step and facilitate subsequent treatment processes (Kesari et al. 2011). In general, physical mechanisms accomplish removal of pollutants without changing the chemical structure of the target substances. The most widely used adsorbents such as activated carbon, silica gel, and artificial pumice have been extensively used for the adsorption of phenols, pesticides, mixture of detergents, and toxic compounds from petroleum sewage (Grégorio 2005). Several membrane separation processes, including nanofiltration (NF), ultrafiltration (UF), diffusion-dialysis and reverse osmosis, have been used for slaughterhouse wastewater treatment to remove contaminants with efficiencies of up to 90% (Kabuk et al. 2015). Unlike physical methods, different chemical wastewater treatment processes take advantage of the nature of pollutants' reactivity with treatment chemicals, such as oxidation or reduction reactions to form insoluble solids, conversion of non-biodegradable to biodegradable materials, or reduction of surface charge to produce coagulation of colloidal suspension, thereby making it easy to separate target pollutants (Lin & Chen 1997). These chemical methods include ozonation, photocatalytic oxidation (Oppong et al. 2019), ion exchange (Chou et al. 2011), hydrodynamic cavitation (Musmarra et al. 2016), and ozone oxidation (Yin et al. 2019). Advanced oxidation processes have emerged as a promising sewage treatment technology for oxidation of various contaminants such as recalcitrant organic dyes, toxic chemicals derived from fixing agents, detergents, and salts. Zhu and co-workers showed that an ultrasound-enhanced electrochemical oxidation process is efficient for the thorough removal of dyes and demineralization of textile wastewater (Zhu et al. 2018).
In addition, combinations of physico-chemical processes and operations have been also demonstrated for decontamination of wastewater (Dimoglo et al. 2004). For example, the combination of volume coagulation and advanced oxidation with Fenton's reagent was used to remove fine suspensions, heterocyclic compound, tar substances and inorganic pollutants such as cyanides, sulfides, sulfates, thiosulphate, ammonia and heavy metal ions from coking plant wastewater (Mittal 2011). Although these methods have a multitude of advantages in treating sewage, there are a couple of disadvantages. For instance, formation of biofouling layers on membrane modules very often during physical wastewater treatment limits membrane service life, and the enormous burden of disposing of chemical sludge in terms of both economic and environmental aspects, the difficulty of recovering common catalysts, and high power fluctuations, and turbidity levels caused significant ineffectiveness in system performance (Chun et al. 2017). It is also important to mention that chemical adsorbents, catalysts, and chemical additives such as Al2(SO4)3, FeSO4 and FeCl3 are generally more expensive (Bayramoğlu et al. 2004).
It is noticed from literature that biological wastewater treatment technologies not only eliminate abovementioned disadvantages, but also offer low construction and operational costs, and mainly have less harmful effects on the environment with low sludge production. Some of these technologies include aerobic lagoons, oxidation ponds, aerated lagoons, trickling filters or biofilters, activated sludge systems, rotating biological contactors, anaerobic lagoons, facultative ponds, and biofilm reactors. The choice of treatment system is largely dependent on the nature of the sewage effluent, and the economic viability and technological feasibility of the proposed treatment plant (Mittal 2011; Delgadillo et al. 2016; Sehar & Naz 2016).
This section outlines some of the commonly available biological wastewater treatment systems which are also applicable for valuable resources extraction. Based on growth mode or structural organization of microbes, biological processes are classified into two groups: suspended growth and established attached growth (Figure 2). In suspended growth systems, microbial cells grow as planktonic form in bulk liquid medium without any support to the substratum.
Several research groups have reported promising contributions to biological treatment of all types of sewages. For example, Lefebvre and colleagues made an attempt to treat hypersaline effluents from a tannery using an aerobic sequencing batch reactor inoculated with halophilic bacteria, and estimated the performance of treatment at different organic loading rates and salt concentrations. Despite the variations in the characteristics of the effluent, the treatment system achieved optimum removal efficiency with 5 days of hydraulic retention time (Lefebvre et al. 2005). Likewise, a real case study by Abou-Elela et al. (2010) used an activated sludge reactor with single Staphylococcus xylosus inoculum to treat pickled-vegetable plant sewage, and showed nearly 90% COD removal efficiency. Recently, a membrane bioreactor configuration which couples a UF or NF membrane with a bioreactor has been considered as an effective technology for the treatment of various industrial sewages including from pharmaceutical process, food processing, pulp and paper, and textile industries. For instance, a submerged membrane reactor (MBR) configuration was proved very efficient in removal of toxic pollutants, namely, hydrocarbons, benzene, phenol, and ester derivates from petrochemical wastewater, and facilitated reuse of treated water for various industrial processes (Llop et al. 2009). A similar configuration was used to eliminate antibiotics, lipid regulators, analgesics, and anti-inflammatory pharmaceuticals from sewage and achieved removal rate higher than 80%, and obtained high quality sludge' called biosolids – as a by-product. It was mentioned that about 70% of nutrient-rich sewage sludge or biosolids was applied to land as a fertilizer for soil enrichment for land cultivation (Medina et al. 2015; Radjenovic et al. 2007). In the last years, pulp and paper industry sewage has been facing firm restrictions through the discharge standards. Wastewater discharged from the paper industry was treated by an integrated MBR and continuous membrane filtration process to meet the treated water quality requirements for the paper manufacturing process (Zhang et al. 2009).
Recovery of nutrients or resources from biosolids and liquid effluents in the field of wastewater treatment has paved the way for utilities to better manage sewage and optimize treatment operations. For example, N and P are the bio-essential macronutrients that are used as fertilizers for agricultural purposes, and may be recovered from wastewater treatment. At present, synthetic fertilizers are produced with the help of nonrenewable resources such as natural gas and phosphate rock through conventional energy-intensive processes. Untreated waste streams contain abundant N- and P-based nutrients and conversion of these nutrients into more useful form has gained much attention in order to manage global nutrient demand through sustainable approaches (Yuan et al. 2012; Kodera et al. 2013; Mayer et al. 2016). Moreover, recovery of resources from wastewaters has the potential to decrease energy consumption and enhance overall efficiency of treatment plants for pollutants removal. The developments in resources recovery embrace the ‘fit-for-purpose’ approach, where integrated nutrient harvesting configurations would allow present and future energy demands driven by a growing society to be met.
Concentrating phosphorus in polyphosphate accumulating microbes such as Candidatus Accumulibacter and Candidatus Competibacter was a key idea for enhanced biological phosphorus removal from domestic waste streams. Thereafter biosolids which were rich in P can be either directly applied to land as fertilizer or solubilized and the P recovered as a mineral form (Yuan et al. 2012). Recently, a variety of microalgae species have been reported as promising ecofriendly microorganisms to treat wastewater from the dairy industry, aquaculture, and municipality due to their ability to remove CO2, N, P, C, and toxic metals. Wang and co-workers demonstrated the potential removal efficiency of essential nutrients, such as 83% of N as ammonium and 90% of P as phosphate, in municipal sewage with use of the microalgae Chlorella sp. (Wang et al. 2009). A microbial consortium along with a microalgae system was able to diminish eutrophication by assimilating N, P, and C from municipal wastewater; additionally proliferating biomass transformed assimilated resources into useful fertilizer (Delgadillo et al. 2016). In another work, Chlorella vulgaris was cultivated in aquaculture wastewater for bioremediation purposes followed by protein production (Blanco-Carvajal et al. 2018).
During the last decade, advanced research is exploring the recovery of high value nanometals and polymers that could be used for the production of bioplastic from biosolids (Koller et al. 2013; Fernández-Dacosta et al. 2015; Morgan-Sagastume et al. 2015; Basset et al. 2016). For example, PHAs are sustainable ‘green plastics’ that, can be used to manufacture thermoplastics, which are regarded as outstanding substitutes for petroleum-derived polymers. Bioplastic can be decomposed within a couple of months, instead of the centuries currently required to break down conventional petroleum-based plastics. Subsequently, many researchers are focusing on value-added conversion of low-value and renewable waste streams from the sugar industry, petrochemical industry, and agricultural systems towards microbial biopolyesters. Jiang and co-workers used activated sludge and enriched it in sequencing batch reactors for PHA production by Plasticicumulans acidivorans a novel gammaproteobacterium, and obtained up to 90% PHA (Jiang et al. 2011). Koller et al. (2013) presented strategies for metabolization of wastes from dairy and biofuel plants to feedstocks for eco-friendly next-generation bio-based polymer production.
Pollution of water source by metal accumulation due to untreated sewage discharges from cement industries, manufacturing units such as for photovoltaic devices, and Ni-Cd batteries, mining and metallurgical operations, petrochemical effluents and electroplating is of major concern. Recovery of high value metals such as gold, silver, iridium, palladium, gallium, and cadmium by biological methods has been investigated for decades, but investigations dealt with metal extraction from mining sources (Johnson 2014), and their bioremediation (Mosa et al. 2016) rather than recovery from wastewater sources. Microbial biomass has the potential to sequester heavy metals from polluted water. For instance, it was found that 60% of the lead and 65% of the cadmium were removed by Chlorella vulgaris at a retention time of 60 minutes (Kikuchi & Tanaka 2012). The unicellular red alga Galdieria sulphuraria has shown 90% of gold and palladium recovery by biosorption from aqua regia-based metal wastewater (Ju et al. 2016). The filamentous fungus Aspergillus niger exhibited 70% zinc (Zn) and 91% copper (Cu) removal efficiency in a wastewater (Price et al. 2001). Similarly, Goncalves and colleagues evaluated the heavy metal removal efficiency of sulfate reducing bacteria (SRB) from metallurgical industry drainage using an upflow anaerobic sludge blanket reactor, and obtained high percentages of removal (>99%) of Cd and Zn (Goncalves et al. 2007). Anaerobic biological treatment of acid mine drainage from abandoned mines using SRB has shown nearly 99.2% effective removal of copper, zinc, and nickel (Sierra-Alvarez et al. 2006).
The use of suspended biological methods for bioremediation of wastewater and resources recovery is an eco-friendly and cost-efficient way. However, there are a couple of major issues associated with submerged or dispersed growth processes, such as microbes can easily be ‘washed out’ from the reactor due to shorter retention time for their proliferation. Moreover the density of free-floating biomass is low, close to the effluent stream, which could result in migration of cells in the effluent direction with the same velocity. This ultimately leads to loss of active biomass at times and influences the efficiency of the wastewater treatment process using suspended biomass. More importantly, suspended growth systems may use pure cultures which would be beneficial for the removal of specific contaminants only, and would not get used to or adapt to external stresses quickly. Thus development of a multifunctional attached growth system known as biofilms is of practical significance for simultaneous separation of different pollutants and value-added resources recovery from wastewater (Kesaano & Sims 2014; Sehar & Naz 2016). Different types of contaminants in sewage sludge would be removed simultaneously by mixed microbial consortia. In the past few years the biofilm technology has gained much attention and been widely applied to the breakdown of pollutants from waste sludge (Miranda et al. 2017).
Most of the microorganisms in moist environments have a greater tendency to proliferate on a biotic or abiotic substratum and develop as a complex, three dimensional, resilient community, called a biofilm. The varying species in biofilms, such as bacteria, fungi, algae, and yeast, form micro colony clusters enclosed within a self-produced glue-like extracellular polymeric substances (EPS) matrix. EPS, as the ‘house of the biofilm cells’, have many roles including promoting biofilm mechanical and chemical stability, acting as a natural absorbent and sequester of metals, and protecting against distinct physiochemical stresses. By including distinct phenotypes in single multi-organism communities, biofilm growth mode can acquire a multitude of different advantages, such as metabolic cooperation, nutrient gradients, inorganic and organic materials and voids filled with water, cell–cell communication, enlarged gene pool with more efficient DNA sharing, and many other synergies which give them a competitive advantage (Dang & Lovell 2016; Machineni et al. 2017; Machineni et al. 2018). For example, strongest biofilm formation was observed when a combination of bacteria, namely, Acinetobacter calcoaceticus, Comamonas denitrificans and Pseudomonas aeruginosa with distinct properties were grown together and complemented each other, providing synergistic cooperation unlike pure culture biofilms (Land et al. 2008). In contrast to suspended growth mode, microorganisms are adhering on carriers such as rocks, sand, plastic, peat, textile, polyethylene, or plant materials to create biofilm (Limoli et al. 2015). There are two common fixed-film systems that are used today for wastewater treatment; they are trickling filters, and rotating biological contactors (RBCs).
Numerous investigations have used biofilms for the treatment of wastewater, and recovery of nutrient sources, heavy metals, and other pollutants (Costley & Wallis 2001; Andersson et al. 2008; Chavan & Mukherji 2008; Schneider & Topalova 2013; Sehar & Naz 2016). For instance, Costley & Wallis (2001) investigated the efficiency of an RBC biofilm, in terms of ability to recover sorbed metals from high-strength metal-contaminated industrial effluents, and its reusability. Metals, namely, Cu, Cd, and Zn were recovered rapidly from the biofilm due to its extracellular sorption and subsequent high metal removal percentages were recorded on microbial consortia re-exposure to fresh effluent. A study demonstrated that microalgal biofilms can be applied to remove both nitrogen and phosphorus simultaneously from municipal wastewater effluent (Boelee et al. 2011). The effective removal of organic matter and nitrates from dairy wastewater by biofilm and planktonic biomass from an anaerobic sequencing batch biofilm reactor was compared. The obtained data demonstrated that total dehydrogenase activity for COD reduction and nitrate reductase activity for denitrification of biofilm was four times and two times higher than their free planktonic counterparts respectively (Schneider & Topalova 2013).
It is well known that biofilms provide extremely robust environments to their colonizers against external physiochemical and biological stresses due to EPS presence. For instance, it was demonstrated that a rotating-disk biofilm reactor was up to 600 times more resilient to heavy metal toxicity compared to suspended Pseudomonas aeruginosa. By binding metal ions to the EPS matrix, the biofilm was capable of removing pollutant metals such as copper, lead, and zinc from the effluent (Teitzel & Parsek 2003). It has been concluded that wastewater treatment technology based on microbial biofilms is an efficient inexpensive alternative, at least in smaller units suitable for single family homes (Mononen et al. 2010). In the last years, two different aerobic biofilm reactors, namely, moving bed biofilm reactors and aerobic submerged fixed bed reactors have found a niche in the treatment of municipal and agricultural wastewaters due to their promising advantages such as stability, reusability, low maintenance, and increased reaction rates (Ghimire & Wang 2018).
An integrated fixed-film activated sludge configuration used for denitrification of municipal wastewater resulted in excellent effluent quality under steady state conditions and showed good resilience to extreme organic loading conditions (Vergine et al. 2018). A modified trickling filter in which a biofilm of polyphosphate accumulating organisms was allowed to grow was used for phosphorus recovery from effluents containing phosphate (Kodera et al. 2013). Zhao and colleagues investigated the treatment efficiency of pilot-scale revolving algal biofilm reactors for the removal of plant nutrients and metals from municipal wastewater. Interestingly, decreasing hydraulic retention time (HRT) resulted in improved nutrient removal capacity, and also facilitated metals removal from wastewater of a revolving algal biofilm reactor (Zhao et al. 2018). Recently a metagenomics approach confirmed that the wastewater treatment efficiency of microbial biofilm was improved by augmenting the metabolic diversity of biological systems. Fluorescent in situ hybridization coupled with microscopy presented the heterogeneity of growth rates across biofilm depth due to localized nutrient gradients (De Beer et al. 2017).
Treatment of animal, municipal and mining wastewaters with natural biofilm which were isolated from saline lakes and marine habitats presented a greater removal rate of ammonia, selenium and other heavy metals than suspended biological treatment systems. Structurally, natural microbial consortia represent complex combination of various bacteria, algae, and photosynthetic heterotrophs etc. It has been found that individual algal biofilms, as the next generation of bioenergy and biofuels feedstocks, can grow using wastewaters as a cheap source of key nutrients and organic matter (Miranda et al. 2017). For instance, algal biofilms are becoming increasingly popular for effective consumption of micropollutants and primary nutrients, and efficient removal of heavy metals from wastewater. These photosynthetic microbes can grow in various effluents by utilizing inorganic carbon from carbon dioxide (CO2) as carbon source in the presence of light energy (Guzzon et al. 2019). In addition to CO2 mitigation, it has been shown that algal biomass has multifaceted advantages in production of bioenergy, such as bioplastics, biofuels, biofertilizers, and biogas upgrading (Cheng et al. 2017; Miranda et al. 2017; Igiri et al. 2018).
Currently, more research is focusing on development of environmentally friendly approaches to replace energy-demanding conventional treatment techniques (Wollmann et al. 2019). Several algal strains, namely, Chlorella sp., Scenedesmus sp., Chlamydomonas sp., and Spirulina sp. were used to treat industrial wastewater, and successful removal of nitrate, phosphate, ammonium, and sulfate was achieved within a week of inoculation (Mohamad et al. 2017; Kumar et al. 2019). In addition, these microalgal strains were recovered and dewatered for extraction of products of interest, such as oils, proteins, polymers, and fertilizers. These bioproducts are determined to be sustainable and biodegradable (Rawat et al. 2016; Pal et al. 2019). Therefore, the biological method by means of an algal biorefinery is a cost-effective, planet-safe, and alternative green approach for wastewater treatment (Figure 3). Also, a bench-scale submerged microfiltration membrane bioreactor, where the surface of the membrane had an attached sticky layer of white rot fungi, has been successfully applied for bioremediation of textile wastewater effluents. Interestingly, it was found that treated wastewater met the water quality standards for reuse (Hossain et al. 2016).
The development of efficient, sustainable, and cheaper methods of wastewater treatment and resource recovery techniques is an important field of research. However, there are numerous publications on a national and international level appearing each year, which are demonstrating process and economic feasibility of biological processes over conventional treatment processes (Zia et al. 2013; Rajasulochana & Preethy 2016; Ioannou-Ttofa et al. 2017; Goffi et al. 2019). Unlike conventional methods, bioremediation and biosorption by biofilm technology does not entail high maintenance and operational cost; many potential biofilm carriers are cheaply and readily available. In addition, a common issue faced with traditional approaches is the difficulties encountered in discarding the waste sludge subsequently produced, usually containing high concentration of chemicals accumulated during treatment. In contrast, the active sludge existing in biological treatment concentrated the wastes into smaller volumes which are subsequently easier to recover or dispose of appropriately. Even though biofilms sometimes have some health issues, they actually offer a wide range of potential for significant applications, such as bioremediation of hazardous pollutants from various municipal, industrial and agricultural effluents, biosorption of heavy metals, and formation of biobarriers to protect soil and groundwater from contamination (Proto et al. 2016).
The major advantages of engineered biofilm technologies include low space requirements, optimal HRT tolerance, quick adaptation to changes in the environment, highly active heterogeneous biomass concentration with improved ability to break down high-strength pollutants mixtures resulting in lower sludge production, and easy separation and sequential reuse of attached biomass. Once contaminants are removed, treated wastewater is either discharged into the environment or utilized for agriculture and other recreational purposes (Sehar & Naz 2016). Therefore it is important to understand the growth dynamics of heterogeneous populations growing in biofilm during wastewater treatment to optimize the residual treatment process, and also recover value-added components and bioenergy from wastewater. A systematic investigation is also required to identify key parameters for the controlled treatment process and production of high quality treated sludge. Overall this article summarizes the literature on biological wastewater treatment processes along with their application in resources recovery from waste residues.
Limitations in renewable resource availability is motivating an upgrading of the present societal situation, changing the aim from pollution removal, such as wastewater treatment, towards utilization of residues to harvest resources to meet current and future demands of our growing society. The major disadvantages associated with conventional physical, chemical and combined methods are that they generally cannot remove microbial flora, and are associated with high installation, maintenance and operational cost, and production of toxic by-products which cause several harmful effects on humans. It has been found that utilizing microbial flora in biofilm form is an important biological system for most of the engineering applications, such as wastewater treatment, biocatalysis, bioremediation, bioleaching, biomineralization, and phytoremediation. The complex heterogeneous microbial consortia in the biofilm consume different nutrients and carbonaceous materials as well as absorb metals from the wastewater by secreting a wide range of enzymes. Biofilms offer low operational costs, and have less harmful effects on the corresponding environment compared to conventional physiochemical wastewater treatment approaches. In addition, biofilm associated processes facilitate high removal efficiencies of BOD and COD, reduced footprints and no further polishing requirement for clarification, and disposal of treated effluent, and produce treated wastewater that can meet the quality regulations for reuse. The problems associated with anaerobic and aerobic biological treatments are CO2 release and nutrient-rich biosolids. However, these major challenges can be avoided in parallel by using algal biofilms for the wastewater treatment, because an algal biorefinery needs CO2 and light energy along with nutrients available in wastewater for photosynthesis, and generates high-value algal biomass. This can be further used for the production of biofuels, biodegradable polymers, biofertilizers, and enrichment of biogas simultaneously. However, there is much more to investigate regarding the efficiency of biofilm for treating all kinds of sewages to overcome future water crises. From a practical point of view, in the future, there should be more evaluation on the biofilm potential during longer treatment, carried out on repetitive cycles, in order to mimic the industrial or desired conditions the biofilm would work in. Furthermore, the process should be scaled-up to larger volume, in order to confirm the flexibility, robustness and the parallel applicability of the system for resources recovery. Therefore it is important to identify and understand the dynamics of biofilm-associated communities to explore their relative contribution to the treatment of wastewater and value-added energy recovery.