Hence, the advancement in manufacturing industry along with WWT blueprints will enhance the application of these technologies for the sustainable management and conservation of water. Water and Wastewater Treatment. Wastewater, which is the biggest waste stream from municipalities, petrochemical, pharmaceuticals, food, textile, agricultural, polymer and paper industries and so on contain high contaminants of oil and salts of organic and inorganic compounds [ 1 , 2 , 3 , 4 , 5 ].
This strikes as a major ecological problem with high environmental impacts when discharged into the ecosystem without proper treatment. Furthermore, the industrial revolution associated with demographic growth have increased the demand for freshwater supply, which is depleting the natural fresh water supply sources [ 3 , 5 ], although wastewater can be treated through various physical, chemical and biological strategies [ 1 , 2 , 3 ].
Unfortunately, the current conventional wastewater treatment methods cannot eliminate the contaminants. In addition conventional wastewater treatment can be expensive. Therefore, the quest for clean water and clean environment has resulted in various environmental protection agencies setting stringent discharge limits [ 3 , 4 ]. Conversely, there are always variations in wastewater qualities which have different impacts on the environment [ 2 , 3 , 4 ], where a proper wastewater treatment incorporated with primary, secondary and advanced treatment strategies seems to more viable [ 1 , 3 , 4 ].
The primary treatment involves separating the solids from the liquids via filtration or sedimentation, whereas the secondary treatment removes the dissolved solids and other contaminants through chemical precipitation and biological process [ 4 , 6 , 7 ]. Then UV light or membranes are used for further treatment [ 1 , 2 , 5 ].
After which, the treated water can be profitable to farmers as well as the environment positively in sustainable manner viz. In this study, evaluating the streamline flow of innovative wastewater treatment technologies for reuse and subsequent sludge generation as an energy source is being addressed. The biological treatment is presented in section one, followed by membrane technology and lastly the advanced oxidation process. The current limitations and future prospects of each technology are also presented.
Municipal solid wastes are attracting more obstructive legislation with respect to landfill disposal of the biodegradable fraction [ 4 , 8 ]. The treatment process for these organic fractions is biological wastewater treatment. These technologies maximises the recycling and recovery processes of waste components. The biological treatment is regarded as an important and vital aspect of wastewater treatment and is a technique employed for municipal or industrial use for soluble organic components [ 9 ].
Among all, the most widely employed method for sludge treatment is anaerobic digestion [ 9 , 10 ]. In this process, a large fraction of the organic matter cells is broken down into carbon dioxide CO 2 and methane CH 4 , and this is accomplished in the absence of oxygen. About half of the amount is then converted into gases, while the remainder is dried and becomes a residual soil-like material.
Kougias and Angelidaki [ 11 ] reported that the end products of organic assimilation in anaerobic treatment of waste are CH 4 and CO 2 as depicted in Figure 1. The AD technology has encountered significant recognition in the last few decades with the applications of separately configured high rate treatment processes for industrial wastewater streams. In the wastewater treatment settings, the AD has been employed in several instances throughout the world for bioremediation and biogas production [ 8 , 12 , 13 ].
Biogas, a well-known and common renewable source of energy, is produced via the AD process, consisting largely of CH 4 and CO 2. As an alternative source of energy, the AD process produces biogas that can be chiefly used as fuel in combined heat and power gas engines [ 11 , 12 , 14 ]. There has also been a rapid adoption of anaerobic co-digestion, where two or more different feed stocks are digested together in anaerobic biodigesters with the core aim of improving the biogas yield [ 8 , 11 , 12 , 13 , 15 , 16 , 17 ].
Demerits include longer hydraulic retention times, pretreatment requirements for delignification of lignocellulosic biomass, odour built-up in bioreactors, costs associated with CO 2 upgrading, no nutrient recovery and high energy requirements [ 8 , 12 , 17 ]. It presents advantages such as a lower consumption of energy, low chemical consumption, low sludge production, its enormous potential for the recovery of resources, simplicity of the operation and the requirement of less equipment. Some advantages of the biological treatment method over other treatment techniques such as thermal and chemical oxidations are capital investments required and costs in operation of the processes [ 8 , 11 , 12 , 14 ].
Schematic diagrams of a aerobic treatment principle and b anaerobic treatment principle, adapted from [11, 12]. Some operating parameters which are usually monitored and optimised to maximise the performance and operation of AD include organic loading rate, pH, hydraulic retention time, temperature, carbon to nitrogen ratio and many more [ 15 , 20 ]. As a result, any sharp variation in these parameters could adversely affect the substrate concentration in the biodigesters. Some of the operating parameters are discussed in Sections 2.
This prediction becomes viable during the selection of the reactor-type and other process parameters such as pH control. OLR has been found to increase with decreasing biodegradation of the volatile solid and the subsequent bioenergy produced.
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The performances of bioreactors decrease when the OLRs increase with energy production [ 10 , 15 , 17 ]. Furthermore, the pH range suitable for AD is reported to be within the range of 6. There is usually a variation in pH during AD especially during acidogenesis where volatile fatty acids such as propionate, butyrate and acetates are produced [ 19 ].
The growth of microorganisms in AD is largely dependent on the pH of the substrates undergoing the overall biodegradation [ 8 ]. In the treatment of wastewaters, the observed pH range of 6. The anaerobes are found to be more active under both mesophilic and thermophilic temperatures as compared to psychrophilic temperatures. Comparatively, thermophilic temperatures are considered suitable for the enhancement of biomethanation by accelerating the hydrolysis of the polymeric feedstock and other metabolic pathways [ 12 , 13 ].
However, several studies have shown that thermophilic digesters suffer from poor process stability due to volatile fatty acid accumulation during the acidogenesis process, most especially propionate [ 13 , 17 ]. This is the measure of the time required to achieve the complete biodegradation of an organic matter associated with process parameters such as the temperature of the medium and the waste composition [ 12 ]. The full-scale application of the AD technique in the treatment of industrial wastewater depends on the hydrodynamic configuration of the AD reactor.
For instance, Figure 3 depicts schematic cross section view of the upflow hybrid anaerobic sludge-filter bed and the upflow anaerobic sludge blanket reactor. The integrated anaerobic-aerobic bioreactors have been most preferred in the past few decades due to its ability to meet stringent constraints in terms of mitigating odorant compound release and minimising sludge production.
Research has shown that among the various reactors used in the performances for the treatment of wastewaters, the UASBR configuration is the most widely used with a high-rate anaerobic reactor for the treatment of high-strength wastewater [ 8 , 12 ]. Several modifications have been carried out in the design of bioreactors to enhance both the consistency and the efficiency of the reactors. The AD process does encounter failures causing serious environmental hazards [ 8 ].
In addition, some of the aforementioned operating parameters as previously discussed Section 2. Further drawbacks observed in AD large scale operation include microbial shift, process instability, low yield of biogas production and poor water quality [ 8 , 10 , 12 , 14 , 19 ]. For instance, monodigestion of energy crops still struggle to meet the reduction targets concerned with the drawbacks in AD compared to anaerobic co-digestion AcoD such as a mixture of slurry and energy crops [ 5 , 20 ].
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In response, pretreatment techniques for cellulose enhancement and the use of energy crops as feed stocks have been found to increase the efficacy of biogas production via AD [ 12 ]. Some of the improvement techniques which have gained attention in terms of research for the betterment of AD process design and the optimisation includes evaluating the AD process kinetics and dynamics, nitrification-denitrification, recycling of the centrate back to the AD reactor, wastewater characterisation, optimisation of operational and environmental parameters, and microbial community shift. The seventh goal focuses on the production of affordable and clean energy globally which is environmentally friendly [ 21 , 22 ].
Renewable energy has gained attention to cater for the ever-increasing use and over-reliance of non-renewable forms of energy. This arises because of the emission of greenhouse gases compelling researchers in the past decades to search for an alternative means of sustainable energy production [ 17 , 23 , 24 , 25 ].
The reserve for energy has become necessary for global concern in maintaining a sustainable way in lieu of the resources available especially at WWTPs.
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Aside the protection of the environment, wastewater treatment plants WWTPs also serve as a source of generating renewable forms of energy such as biogas. Also, wastewaters with a high content of nitrogen can be treated with the nitrification and denitrification technique form of AD generally known as Anammox [ 26 ]. Constructive government policies have shown Germany as being the dominant global biogas energy generation country globally for the future [ 23 , 25 ]. Latest reports predict that biogas production could increase from 18, Gigawatt hours GWh in to 28, GWh in , indicating a compound annual growth rate CAGR of 3.
Membranes, as a thin layer barrier for size differential separation, are usually integrated with chemical and biological treatment or standalone systems in secondary treatment of wastewater settings [ 28 , 29 , 30 ]. In a typical membrane mechanism, there is usually a driving force, such as a semi-permeable barrier which controls the rate of movement of components by fractional permeation and rejection through pores of different sizes as depicted in Figure 4 [ 32 ]. The permeation and selective rejection is a function of the membrane pore size and chemical affinity, which helps to have a product stream devoid of target components [ 33 ].
Due to the relatively low energy requirement and wastewater treatability efficiency, membrane technology has tremendously improved by the development of new materials and configurations for industrial applications. Some of these applications include microbial fuel cells, removal of organic and inorganic components, disinfection, pathogen removal and desalination [ 30 , 33 , 34 ].
Generally, the major driving force for selective filtration is a potential gradient of variables such as hydrostatic pressure, electrical voltage, temperature, concentration or a combination of these driving forces [ 29 , 32 ]. These variables including nature natural and synthetic and structure porous or non-porous and heterogeneous or homogenous have been used in the classification of membranes [ 28 , 31 , 32 , 34 ]. However, most commercially available and industrially used membranes are pressure-driven and energy driven electrodialysis and electrodialysis reversal membranes [ 35 ].
These are also classified by their pore sizes or molecular weight cut-off MWCO.
It should be noted that as the pore size of these membranes decrease, the driving force for the operation increases. Microfiltration utilises a sieving mechanism to retain macromolecules or particles more than 0. As indicated, the larger pore sizes of MF membranes limit removal to suspended solids, bacteria, some viruses up to 2-log , protozoan cyst, turbidity and on a lesser extent, organic colloids within the region [ 28 , 29 , 32 ].
The role of UF is increasing as a pretreatment for desalination and membrane bioreactors. Ultrafiltration UF like MF utilises physical sieving as a separation mechanism. As such, UF with a definite MWCO are impermeable to compounds with molecular weights exceeding the MWCO and have shown a 3—6 log removal of chlorine resistant protozoan cysts, active Giardia lamblia , colloids, viruses and coliform bacteria.
Both are often used as pretreatment for NF and RO processes to reduce membrane fouling and is also applied as a post treatment to chemical precipitation for organic chemical removal and pH adjustment, phosphorus, hardness and metals [ 29 , 30 , 31 , 33 , 34 ]. Fouling is highly eminent in UF due to the high molecular weight of fractions retained in relations with the small osmotic pressure differentials and liquid phase diffusivity.
The configuration for application is influenced by the mechanical stability, hydrodynamic requirement and economic limitations. Nano filtration is a pressure related process where the mechanism of separation is based on molecular size for the removal of dissolved micro pollutants and multivalent ions.
The NF is a complex process characterised by solvent diffusion, transport and electrostatic repulsion effects at the membrane surface and within the nanopores [ 29 , 30 ]. The difference between the pore diameter and particle size forms the basis of the sieve effect. Nano filtration is often used as a post treatment or polishing step in conventional treatment processes. Although it is not advisable to be used in desalination processes, it is used to reduce the salt content of slightly saline water.
Recent applications have used NF as a pretreatment to RO reducing the operating pressure in RO providing savings in operational and maintenance costs [ 31 , 32 , 35 ]. Second stage fouling is usually reduced in NF systems through ozone pretreatment and non-thermal crystallisation while cleaning is done using suitable alternatives that also exist for MF and UF [ 30 , 33 ].
Reverse osmosis, often referred to as tight membrane has been widely used in brackish water and wastewater treatment with its effectiveness in desalination against conventional thermal Multi stage flash. Using concentration gradient as the driving force, separation and concentration in forward osmosis FO occurs as the concentrated solution e. Characteristic advantages include low energy consumption, simple configuration and operation, low membrane fouling tendency and high rejection of a wide range of contaminants. The use of FO operates at ambient conditions, hence irreversible fouling is low [ 37 , 39 ].
However, FO technology is faced with the lack of recyclable and economical drawing solutions, internal and external concentration polarisation and a difficulty of developing effective large scale plants [ 37 ].
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To achieve desired process flow and optimum configuration, ROs are arranged in stages and passes. The sequence of the stages has the concentrate stream of the first stage as the feed inlet of the second stage. Permeate streams from both streams are summed into one discharge channel. However, passes involve either a one path recovery of permeate or the rechannelling of permeates from the first RO into the second RO to improve quality [ 40 , 41 ] as summarised in Table 1. Summary of pressure-driven membranes [ 39 , 40 , 41 ]. Low-pressure driven MF and UF used for critical solid-liquid separation has been integrated with biological treatment into a hybrid activated sludge process termed as membrane bioreactors MBR for wastewater treatment.
However, together with other plastics, for example from tyre rubber in road run-off, they end up in the water-treatment plants.
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Last year the World Health Organisation WHO announced it would launch a review into the potential risks of plastics in drinking water after a study by the State University of New York in Fredonia found 93 percent of popular water brands tested included microplastics. Elsewhere, research in Finland found that microplastic removal in certain wastewater treatment plants was as high as 99 percent, depending on solids removal. In addition, it was unclear, except in models, how much nanoplastic was retained in water treatment plants and how much entered the environment. Researchers said the nanoplastic particles bind very quickly to the sludge flocculate, resulting in an ultimate elimination level of more than 98 percent.
Even if only small percentages make it into surface waters, these can add up to higher concentrations downstream. Humans could ingest microplastics per year through salt. More data needed on microplastics and wastewater treatment. Share your water technology stories with us Do you have an innovation, research results or an other interesting topic you would like to share with the international water technology industry?