Applications of Microbes in Bioremediation of Point Source Pollutants from Wastewater

Water pollution is the major concern of this era. Industrial developments, agriculture patterns, construction processes and unsustainable mining have contributed to increased level of pollutants in most of the fresh water bodies across the globe. This review categorizes different sources of water pollution and focuses on remediation of wastewater through microbial applications. The point source pollutants can be remediated using ‘at source’ microbial bioreactor installations. This work concludes that hybrid biofilm reactors and membrane bioreactors can be utilize for the removal of micro-pollutants. However, anaerobic bioreactors are more efficient in treating wastewater having high concentration of organic matter.

IQ in children, poor gross and developmental coordination disorder [10][11][12]. Thus, clean water and recycling of polluted water is prime necessity nowadays. In this era of development, urbanization and unsustainable industrialization, toxic substances are spewed into the water bodies polluting both surface as well as groundwater sources [13]. Entry of these pollutant into the water bodies directly impact the aquatic fauna by interfering with their physiological, anatomical or embryogenesis processes producing health abnormalities like birth defects, stunted growth, ion imbalances, endocrine signal disruptions in insects and fishes. In present day and age the pollutants in major fresh water bodies have reached to alarming levels which warrants immediate remediation interventions. There are many technologies available to remove pollutants from water sources but microbial remediation methods is gaining popularity due to the advocacy by the scientific fraternity because of their remediation efficacy, and cost effectiveness [14,15]. Here, in this review, different biological system has been elaborated in details to understand the utility of microbe based technology to treat different organic and inorganic pollutants from contaminated water.

Water pollution
Contaminated water with various pollutants is the major concern of environmentalist in the present scenario. Polluted water not only destroys human health but it also can affect the whole ecosystem. There are two main division between sources of water pollution i.e. point and nonpoint sources. If the source of pollution can be identified to a specific location in a geographical extent it is referred to as point source of pollution [19]. In case of water pollution and water resource management, any industrial effluent discharge, and municipal sewage can be an example of point source pollution. Point sources pollution can be managed 'at the source' by installing water treatment facilities before the waste water is let out into the nearby water bodies such as river stream or sea.
Non-point sources are pollution locations dispersed over a wide area or region and cannot be traced back to a single location. This is possible in case of agricultural run-offs, washing activity along river banks, urban runoff etc. This sort of pollution is hard to tackle because of dispersed sources and 'at source' mitigation measures seldom works. Awareness campaigns, policy level interventions can tackle these issues to certain extent.

General standard limits for wastewater
To tackle with point water pollution, different statutory bodies of different countries have given guidelines to treat wastewater before the discharging in natural water bodies. Now, the challenge is to treat water at point source so that it cannot contaminate into natural ecosystem and impact human health. The major water pollutants are carcinogenic heavy metals, pesticides, micro-plastic (Table1). There are various chemical and biological methods are available to treat polluted water such as adsorption, chemical precipitation, ion exchange, membrane filtration, biological hybrid reactor, activated sludge process, trickling filter. The biological methods are basically based on microbes proven effective and sustainable to remove contaminants at limited extent [21]. As chemical treatment process produced huge waste the biological treatment process is proved more beneficial.

Aerobic Batch and continuous suspended bioreactor
This bioreactor may consist single strain of microbes and mixed microbial culture. In the bioreactor development completely mixed stirred tank reactors were started in the late 1950s

Batch and continuous packed bed bioreactor (PBBR)
The first bed bioreactors are started in the 1970s to treat wastewater

Airlift bioreactors
These advance bioreactors is designed especially for ex situ bioremediation of wastewater.
Airlift bioreactors consists indigenous microbes, which found capable for ex-situ remediation of water [52]. Khongkhaem  The biofilm bioreactor is required regular cleaning to restrict the growth of other microbes to maintain the efficacy of target microbe.

Disposal of sludge
Microbial bioremediation of municipal and industrial wastewater is getting attention, but the only limitation is the huge sludge generation through this process. Researchers have discovered many sustainable techniques for the sludge reuses and disposal. Most used sludge disposal technique is landfill after treatment and pyrolysis. Recent works shed lighted on the reuse of sludge for land application or to improve the degraded soil and energy recovery [63][64]. However, reduction of sludge volume is required before the reuse or disposal. Anaerobic digestion can be the best method for reduction of sludge volume.

Anaerobic digestion
Anaerobic digestion is a sustainable treatment option for the degrading of organic matter from wastewater sludge. The organic matter gets decomposed into biogas i.e. methane and carbon dioxide, with a trace amount of NH3, and H2S  adds on the potential and utility of anaerobic digestion of wastewater. Figure 1 shows the anaerobic digestion of wastewater. There are basically four phases of organic matter digestion through anaerobic digester, these are hydrolysis, acidogenesis, acetogenesis and methanogenesis. Anaerobic digestion reactors have commonly been operated at mesophilic (30-40 °C) and thermophilic (50-60 °C) temperatures. The continuously stirred tank reactor is one of the foremost anaerobic digestion reactor used in various industries can be divided into two stages: In a single-stage digester, all four phases of anaerobic digestion take place in a common environment. A single-stage anaerobic digester is not ideal for all members of the consortia, and one of the possible reasons could be that the bioavailability of the enclosed essential nutrients is not sufficient to maintain enzymatic processing by microbes [66]. However, in a two-stage anaerobic digester, the methanogenesis phase is typically separated from the other three stages of anaerobic digestion, which is reported to provide a higher efficiency and energy recovery, as well as greater biogas production compared to traditional single stage anaerobic digestion [67]. Because of better efficiency, now days most of anaerobic digestion reactors are being constructed as two-stage configurations.\

Conclusion
This review concludes that microbial remediation systems can be utilized for removal of wide range of pollutants such as heavy metals, phenols, herbicide, pesticides, micro plasticspolyethylene and polystyrene from waste water. The batch bioreactors can be modernized to remediate broad spectrum of pollutants with continuous and hybrid reactor systems. The microbial bioreactors are comparatively better than other traditional methods as they do not required large space to install, are easy to use, and require modest maintenance.
Commercialization of microbe base bioremediation process would be effective as well as economically feasible. This work concludes that hybrid biofilm reactors and membrane bioreactors can be utilize for the removal of micro-pollutants. However, anaerobic bioreactors are more efficient in treating wastewater having high concentration of organic matter.
Future research can be focused on fine tuning and scaling up of these lab scale microbial remediation systems for more cost effectiveness and applicability for wider industrial use.
Thus the 'laboratory to industry' transition is mandatory to make microbial remediation system popular, sustainable and cost effective.