Diverse Techniques for the On-Site Treatment of Blackwater

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1		Prasaanth Ravi Anusuyadevi	--	5041	2023-02-06 16:49:38	\|
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This entry is adapted from the peer-reviewed paper 10.3390/w15030542

The maintenance of people’s lifestyle against global climate change, exhaustion of groundwater, depletion of minerals, and water scarcity has instigated the recycling and reuse of water from unlikely sources. This situation has motivated researchers to develop effective technologies for treating wastewater, enabling its reuse. Water security has been ensured in myriad, highly populated regions through large-scale centralized treatment facilities. The development and implementation of small-scale, renewable-energy-based, decentralized, on-site treatment methodologies ensure water sustainability in rural areas, where centralized treatment facilities are impractical for application.

Blackwater (BW) Swachh Bharat Mission (SBM) electrochemical (EC)

1. Introduction

Blackwater (BW) generated from the toilet flush system contains a prominent quantity of pathogenic microbes (viruses, bacteria, and unicellular eukaryotes) produced by the products of human excretion. Thus, the untreated BW is always prone to cause diseases like diarrhea, cholera, dysentery, etc. ^[1]. At the same time, the untreated BW also possesses high quantities of nutrients (both micro- and macro-nutrients) as requisites for plant growth. These nutrients are primarily composed of nitrogen-, potassium-, and phosphorus-based materials. Interestingly, a component of human urine consisting of 75–90% nitrogen-based compounds is urea, and the rest comprises amino acids, uric acid, and creatinine, which can be directly taken up by plants ^[2].

Thus, the facile segregation of nutrients and microbial contents or the direct inactivation or removal of microbes, through proper treatment techniques, provides non-potable water for reuse. Usage of such nutrients from BW eliminates the need of hazardous synthetic chemical fertilizers and reduces the cost of current agricultural practices. The BW generated from localized sources, be it single-household toilets or community-based toilets, is mechanically separated into two phases, the overflowing liquid phase (supernatant) and the sediment, a solid phase.

2. Electrochemical Treatment Process

The electrochemical (EC)-based treatment process is the most multifaceted, easily integrable system for the on-site treatment of BW. It is potentially a feasible substitute for decentralized wastewater treatment for lavatories in single households or a group of households in a village. This methodology potentially eliminates the drawbacks associated with the scaling down of conventional treatment processes: (i) chlorination, (ii) ozonation, (iii) membrane filtration, and (iv) ultra-violet (UV) irradiation. The most crucial drawbacks are: (i) high operational and maintenance cost, (ii) generation of byproducts possessing acute side effects and further mineralization of such compounds, and (iii) safety issues associated with the storage of toxic chemicals required for the treatment ^[3]. The EC-based process does not require external chemicals, as it produces the chemicals needed for the treatment of BW in situ within the reactor, where the treatment is carried out. The basic principle of the EC method for the treatment process is the electrolysis of water, basically defined as the EC splitting of water into hydrogen and oxygen. The term EC indicates that the driving force for such splitting of water is the electric current applied to the system. The great advantage of this EC-based water treatment technique in its implementation in upcoming projects of the SBM is that the current required for treating the BW of a household in a village or an entire village’s BW can be powered by solar energy collection systems. The space or the setup in which the above EC process occurs is called an EC reactor, electrolysis cell, or electrolyzer.

In the presence of the environmental pollutants, in case BW, instead of oxygen gas evolution, the direct heterogeneous (on the surface of the anode) or homogeneous elimination of pollutants and disinfection will take place in the BW electrolysis cell ^[4]. Such elimination occurs through mineralization (complete oxidation) of the pollutants. The advanced oxidation processes (AOPs) based on the above EC approach involve the generation of strong oxidizing agents originating from the reactions occurring on the anode side of the EC reactor. The electrolysis settings for BW treatment are controlled in such a way that the in situ direct oxidant generation on the anodic surface predominantly involves the generation of a hydroxyl radical (OH). This radical is a highly powerful oxidant with standard redox potential (E^o (OH/H₂O)) of 2.8 V vs. the SHE (standard hydrogen electrode) ^[5]. OH has a very short lifetime in water in the range of 10⁻⁹ s (nanoseconds), and thus it is easily self-extirpated at the end of the treatment processes ^[6]. Subsequently, the reactive oxygen species (ROS), (i) hydrogen peroxide (H₂O₂), (ii) ozone (O₃), and (iii) superoxide anion radicals (

O_{2}^{-}

), are produced through the reaction between the hydroxyl radicals and molecular oxygen. These ROS-based oxidants play a major role in the purification (oxidation) of oxidizable pollutants and also disinfection of microbes present in the BW ^[7]. However, the primary disinfection is provided by reactive chlorine species (RCS), (i) active chlorine molecules (Cl₂, HOCl, OCl⁻) and (ii) chlorine radical species (Cl). RCS generation is purely in situ within the EC reactor and has no need of external sourcing, as it is achieved through the reaction between natural chloride material in BW and hydroxyl radicals ^[8]. The BW contains higher concentrations of chlorides, as the human urine itself contains a chloride concentration of about 50–150 mM ^[7].

Apart from the aforementioned oxidizing radicals, ROS and RCS, in EC-based AOPs, the pollutants are also directly oxidized on the anode surface by electron transfer; this is termed anodic oxidation or direct oxidation. In the case of an absence of pollutants, it is the simplistic water oxidation occurring on the anodic surface in the water electrolysis cell. From the perspective of degradation of pollutants present in wastewater from myriad sources, the anode chosen for EC-reactor-based purification should be a non-active electrode with very high oxygen (O₂) overpotentials. These non-active anodes possess the potential for O₂ generation in the range of 1.7–2.6 V vs. SHE, as these anodes produce weakly physisorbed (not binding covalently) hydroxyl radicals that have higher chemical reactivity towards pollutant oxidation, resulting in the mineralization of the harmful compounds ^[9]. Electrodes based on materials like tin dioxide (SnO₂), lead oxide (PbO₂), sub-stoichiometric titanium dioxide (TiO₂), and boron-doped diamond (BDD) are the notable non-active anodes. Amongst these options, BDDs are the most powerful anodes, producing a large quantity of hydroxyl radicals and exhibiting an excellent oxidation rate and enhanced current efficiency compared to other conventional electrodes employed for the degradation of pollutants in the regime of anodic oxidation ^[10]. However, the implementation of the above non-active anodes is not recommended for BW generated from single and/or community households, especially in decentralized treatment systems targeting the reuse of treated water or safe disposal of water into natural bodies. This is because the high overpotentials of anodes for O₂ generation during EC operation lead to the synthesis of highly stable perchlorate

({ClO}_{4}^{-})

through the anodic oxidation of its corresponding precursors in the electrolyte: phosphate, sulfate, and carbonate ^[10]. These strong oxidants along with perchlorate, even at very low levels, are very toxic and subsequently hinder the reuse of treated water through EC treatment.

The facile formation of these strongly poisonous oxidants can be circumvented by the implementation of active anodes for the EC treatment of BW through anodic oxidation. Dimensionally stable anodes are materials composed of a titanium metal substrate coated with a thin layer of conducting iridium dioxide (IrO₂) or ruthenium dioxide (RuO₂); platinum (Pt), graphite, and bismuth (Bi)-doped titanium dioxide (TiO₂) [BiO_x/TiO₂] are some of the notable examples of active anodes. These electrodes are termed active anodes, as they exhibit low overpotentials for OER and are highly superior electrocatalysts for OER. Due to their low overpotentials for oxygen generation, the hydroxyl radicals generated on the above anode surfaces strongly interact with the anode’s surface ^[10]. The production of such anode-surface-adsorbed hydroxyl radicals is shown in Equation (1):

{MO}_{x} + H_{2} O \to {MO}_{x} (OH) + H^{+} + e^{-}

(1)

Due to the availability of higher oxidation states on the electrode surface of active anodes, the generated hydroxyl radicals are further oxidized into higher-state oxide. The resulting covalently bound oxygen species, termed “chemisorbed active oxygen (MO_x+1)”, is oxygen present in the oxide lattice of the anode. This higher oxide formation reaction is shown in Equation (2):

{MO}_{x} (\cdot OH) \to {MO}_{x + 1} + H^{+} + e^{-}

(2)

This chemisorbed active oxygen (MO_x+1) along with the anode surface (MO_x) acts as a mediator in oxidation processes leading to the partial oxidation of organics. This prevents the formation of harmful disinfection byproducts (strong oxidants such as percarbonate, persulfate, and perphosphate) and to a certain extent chlorinated organics. Having discussed the nuances of the (electro)chemistry happening on the anodes’ surface depending on the nature of the material chosen for the electrode in EC treatment, this will briefly address the current EC treatment systems studied for the safe recycling of BW. Recently, Xiao Huang et al. reported the practicability of a BW electrolysis cell (BEC) for the application of decentralized mobile toilet wastewater disinfection. The treated BW was aimed for reuse in toilet flushing and irrigation of crops ^[7].

This system was composed of an EC reactor, working inline in batch mode, treating 20 L of BW. The feed to this reactor is the supernatant from the sedimentation tank, which serves as an anaerobic digester. After treatment, the treated water returns to the clean water tank for reuse. However, BW from real toilet systems has a noticeable amount of microorganisms (both bacteria and viruses). The collected BW from the mobile toilet below has a low concentration of microorganisms due to the truncated usage of the toilet, the presence of EC-treated water in toilet flush (which contains residual oxidants responsible for killing the microorganisms), and the long retention time of BW in the anaerobic digester (sedimentation tank). Thus, in their study, EC disinfection optimization was carried out on bench scale, and the collected real BW was seeded with fresh microorganisms. Their bench scale BEC possessed a working volume of 250 mL, composed of a BiO_x/TiO₂ anode with a very low overpotential for OER of about 0.32 V ^[11] and a stainless steel cathode. The distance of separation between the cathode and anodes was about 0.5 cm. The cell is operated in potentiostatic mode at 3 V, 4 V, and 5.5 V, resulting in current density values of 3.9, 1.2, and 2.4 mA/cm², respectively. Here, the inactivation of the microorganisms was not observed under 3 V, but at an applied voltage of 4 V, microbial inactivation was achieved within 60 min of operation. The energy consumption for 1 h of EC reactor functioning was evaluated to be around 2 Wh/L, and for operating the entire setup, it requires around 13–15 Wh/L. This indicates that the BEC can be operated using commercial photovoltaic panels and represents a tremendous potential for a decentralized BW treatment process. However, at both 4 and 5.5 V, the free chlorine quantity (>1 mg/L) in the electrolyte was determined by the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method. Due to the interference of chloramines and other oxidants ([O₃], [H₂O₂], [ClO₂], etc.) in the DPD method, the observed quantification of free chlorine could be an overestimation. It is important to stress that quantification of individual oxidants is also virtually impossible in real BW due to its complexity of composition and swift reactions of oxidants with organic matter present in the BW. Hence, electrolysis studies were also carried out on model BW containing no chlorine [Cl⁻] and at an applied voltage of 4 V; less than 0.5 mg/L of total oxidants was observed. This EC disinfection was compared with the traditional chemical chlorination (CC) technique; here, the EC disinfection outperformed the disinfection by the CC technique, even at the highest dosage level of Cl₂ (36 mg/L). For both treatments, the observed composition of halogenated organics generated was similar. A negligible quantity (<5%) of brominated organics was observed in the treated water samples. Nevertheless, the quantity of disinfection byproducts (like chlorinated organics) was significantly higher in EC-treated BW than in the CC-treated BW. The concentration of the chlorinated organics was well within the permissible range for EC-treated BW and was similar to the range observed in chlorine-disinfected wastewater ^[12]^[13].

Guruprasad et. al. recently reported a synchronized (wetland + EC reactor) system for the localized treatment of BW from single-household toilet (SHH) with quartet usage ^[14]. The EC reactor was composed of an active iridium-oxide (IrO_x)-based mixed-metal oxide (MMO) anode, stainless steel cathode, and cationic exchange membrane (CEM). The distance between the anode and cathode was around 5 mm. The CEM was supported by the frame within the EC reactor. The frame structure extends below the EC reactor into the buffer tank attached with baffles, preventing dead zones for disinfection. The CEM divides the EC reactor into two equivalent chambers (anode and cathode side) with an 8.5 L volume each, and the buffer tank has a volume of 100 L.

The BW from the continuous operation of SHH flows into the septic tank, then into the vertical subsurface of a constructed wetland with a surface area of 0.5 m²/PE (population equivalent) planted with Canna indica plant. The effluent from the wetland is sent to the buffer tank, into the EC reactor, then finally discharged. The wetland was operated continuously for 5 days prior to integration with the EC reactor, with an influent flow rate of 200 L/day. The wetland retains the organic matter in the BW and degrades it aerobically. However, such a wetland was unable to eliminate the bacterial pathogens completely. After the integration with the EC rector, the log reduction of bacterial coliform was 6.0 ± 0.4. Electrolysis of the BW was carried out in constant current mode (4.4 A) with an energy consumption of 16.7 Wh/L. Inside the EC reactor, the BW first flows through the anodic chamber and then through the cathodic chamber. The continuous flow rate of BW from the septic tank to the integrated system was 180 L/day. The wetland reduces the organic and mineral load in the BW, enabling the continuous function of the EC reactor for 60 days, after which the EC reactor was dismantled, and the membrane was washed to remove the accumulated organic layer on its surface ^[14]. In this trial period, the average depletion of total ammoniacal nitrogen (TAN), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), and orthophosphate concentration due to the combined effect (wetland + electrolysis) was 84 ± 8 %, 84 ± 7%, 45 ± 14%, and 98 ± 1%. Within the EC reactor, the COD pruning is due to the direct or indirect oxidation of organic matter in the anode chamber; TAN reduction is due to myriad reasons, one of which is the migration of

{NH}_{4}^{+}

ions to the cathode chamber and the subsequent bubbling of ammonia ^[14].

In view of augmented global population growth accompanied by a growing energy and food demand, the recovery of scare nutrients (for example, phosphorus) from the BW is highly advantageous as compared to its degradation by oxidation ^[15]. Within the EC field, such selective recoveries of nutrients are achieved through electrodialysis (ED) employing the usage of ion-selective membranes. However, such recoveries are not suitable for BW generated from localized sources (such as lavatories built under SBM) and/or wetland due to the low concentration of nutrients ^[16].

Having said that, the composition of the BW and the nature of the pathogens present in such BW vary from different localities. These factors depend heavily on the food habits, lifestyle, and climatic conditions of each locality. Hence, preliminary studies targeting the composition of BW in various locations far away from the centralized treatment systems, as well as optimization studies of EC reactors with highly efficient electrode materials reducing the energy consumption while at the same time maintaining the disinfection efficiency in the upcoming years, would provide pathways for the easy reuse of treated BW in India and also in other parts of the world focusing on the decentralized treatment of BW.

3. Vermicomposting

Vermicomposting is a process in which earthworms convert the solid organic waste into manure rich in highly nutritional content. Vermi-processing toilets have a design in which human excreta are treated by earthworms and redworms. In this process, human waste is transformed into cake-like structures called vermi-cakes by earthworms. Vermi-cakes are used as manure and fertilizers ^[17]. Recently there has been a lot of emphasis on organic fertilizers in lieu of chemical fertilizers. In this method, the organic matter, such as cow dung, kitchen waste, and farm residues, is consumed by earthworms and ejected in a digested form called worm casts and popularly known as black gold. The casts are rich in nutrients, growth-promoting substances, and beneficial soil microflora, and they inhibit pathogenic microbes. Vermicompost is a stable fine granular organic manure which enriches soil quality by improving its physicochemical and biological properties. It is highly useful for raising seedlings and for crop production. Recently, it has become popular as a major component of organic farming. Recent research has indicated that vermicomposting can be useful in other fields, such as wastewater treatment, reduction in BOD and COD, soil remediation, and energy production ^[18]^[19]^[20]. By utilizing vermicompost to produce energy from waste and promoting the three R’s (reduce, recycle, and reuse products and resources), vermicomposting plays an invaluable role in a country’s circular economy. To ensure global sustainability and circular economies without harming the environment, vermicomposting technology must be considered as a viable management technique. The GOI has also promoted the adoption of biological techniques such as composting and vermicomposting for waste recycling ^[21].

There has been a lot of emphasis on treating organic waste and stabilizing sludge from wastewater treatment plants using vermicomposting and vermifiltration ^[22]. This provides information on the design, processes, and uses of vermifiltration technology. Vermicomposting toilets are becoming increasingly popular, in which earthworms break down human feces, urine, and toilet paper. The system includes a conventional flush toilet and handles waste on site. This methodology uses less water and converts dry human feces into humus (organic matter) in a pollution-free manner. The vermicompost toilet comprises two main parts, namely the conventional flush toilet and the worm tank. The worm tank is where BW is treated by worms.

An intermediate bulk storage tank is used to store the sewage coming from the flush toilet before it is sent for treatment in the worm tank. The tank is then encased in insulated housing to prevent the worm tank getting too hot in the summer (cold winters may slow down activity but are less likely to kill the worms than a hot summer). It is critical that the worm tank drains the toilet flush water to maintain a healthy compost worm ecosystem. In order to permit the free flow of the liquid, it is necessary to arrange internal pipes with perforations and a gravel bed at the bottom. Typically, the tank is filled with ¾ coarse organic material and a small layer of kitchen scraps, with partially finished compost or manure at the top. It would take approximately 90 days to convert all the feces into usable decomposed organic matter suitable for agriculture. High-yield worms can be sourced from agricultural universities or online suppliers, and the number of worms required for the biodegradation depends on the amount of the biodegradable material.

The studies indicated that the effluent quality of vermicompost-treated samples showed significant removal of BOD (86.3%), COD (70.2%), and suspended solids (SSs) (45.7%). The significant removal of BOD is due to the enzymatic action of earthworms that degrade the high concentration of organic matter. A removal of 80% suspended solids was observed, which is attributed to the fact that earthworms digest the solids and also improve the adsorption properties of the sands and soils by their grinding action. Similarly, the removal of Total Coliforms (TC) (55.74%), Fecal Coliforms (FC) (53.97%), and Fecal Streptococci (FS) (60.36%) to levels considered acceptable for recreation and irrigation indicates that vermifiltration products are pathogen-free ^[23]. This suggests that the substantial reduction in pathogens in vermifiltration is due to antimicrobial activity in the celeomic fluid of earthworms and in the microflora associated with vermifiltration ^[24].

In essence, the current reports about vermicomposting technology reveal that pathogen removal is a significant benefit in addition to organic fertilizer. Vermifiltration is a cost-effective technology for sewage treatment that is highly efficient, convenient, and has the potential for decentralized treatment. It can be easily integrated with the lavatories constructed under the SBM. Vermifiltration technology is an excellent microbial–geological system and can be improved to offer a greener planet in the future.

4. Microwave (MW)-Based Reactor

Recently, the MW-based thermal treatment method has gained importance in treating BW sludge, owing to its rapid heating ^[25]^[26]. Here, the sludge samples are exposed to microwaves produced by a microwave electron tube known as a magnetron. These microwaves cause the water molecules in the sample to vibrate, producing thermal energy and thereby heating the sample in the least possible time ^[25]^[27]^[28]^[29]. Here, MWs are able to reduce the sludge volume by about 70% while keeping the pathogen concentrations below analytical detection limits under experimental conditions ^[26]^[30]. In a study reported by TU Delft, a domestic MW oven (Samsung, MX245, Samsung Electronics Benelux B.V., The Netherlands) was employed for the treatment of BW collected from the decentralized sanitation site at Sneek city in the Netherlands ^[26].

The MW methodology with the addition of 3 wt.% hydrogen peroxide (H₂O₂) to the sludge ensures all the COD present in the sample is converted to a soluble form, which in turn enhances biodegradability and methane production. This method also offers the advantage of non-contact heating with a high degree of uniformity, rapid reduction in volume, and a compact and portable nature with low thermal energy loss ^[29]^[31]. The efficiency of the MW increases with the power and the contact time of MW radiation. The extra power contributes to the rapid vibrational and rotational movement of water molecules. This movement in turn releases extra heat, resulting in the evaporation of water molecules and the decomposition of the various contaminants in the sludge ^[26]^[29]. Specifically, the MW radiation brings about the intensification of the degradable material in the BW sludge compared to conventional heating, such as with an electric furnace. Owing to its unique capabilities of rapid heating, MW technology appears very promising for situations requiring the rapid treatment of fecal sludge (FS) matter with reduced carbon and reactor footprints ^[25]^[26].

Researchers recently succeeded in using an MW reactor at pilot scale to treat 4 kg of sludge with MW radiation for time durations ranging from 30 to 240 min ^[32]. Here, the sludge with a uniform thickness of 0.5 cm was exposed to MW irradiation at 3.4 kW, and the temperature was raised to 102 °C. The treated samples were analyzed for the reduction in volume, calorific value, organic matter, nitrogen, phosphorous, and Escherichia coli. This showed complete bacterial inactivation and a sludge weight/volume reduction above 60%. Moreover, the dried sludge and condensate had high energy (≥16 MJ/kg) and nutrient contents (solids: TN ≥ 28 mg/g TS and TP ≥ 15 mg/g TS; condensate: TN ≥ 49 mg/L TS and TP ≥ 0.2 mg/L), having the potential to be used as biofuel, soil conditioner, fertilizer, etc. ^[32].

Thus, the MW reactor can be applied for the rapid treatment of sludges generated from areas lacking the capabilities for agriculture and rainfall, like desert areas in the world. Chemically bound heavy metal ions are the most toxic and difficult to remove from waste sludge. MW technology decomposes these metal ions quickly, and the treated sludge can then be used as organic fertilizer ^[29]^[33]. To conclude, MW treatment of BW and municipal wastewater is of significant interest primarily because of the rapid heating. This methodology lacks proven global technologies and requires verification and economical analysis for practical implementation.

5. Biogas-Based Treatment

The term biogas means a mixture of methane and carbon dioxide produced by the bacterial decomposition of organic wastes, and it is used as a fuel. The disintegration of organic waste by anaerobic digestion has gained global attention due to its significant environmental and economic benefits. This technique reduces local waste by recycling. This in turn conserves resources, reduces greenhouse gas emissions, and builds economic resilience in an uncertain future for energy production and waste disposal ^[34]. Biogas would replace fossil fuels to meet the current energy needs of industry and the transportation sector, which in turn contributes to a greener societal ecosystem. CO₂ production is significantly less for biogas when compared to conventional fuels. It can be used as fuel in cooking, a source for heating and lighting, and can be used to generate electricity ^[35]. There are a number of other advantages to biogas plants. They are easy to install and do not require any sophisticated equipment. Furthermore, the costs of installation and handling are cheap. These plants can be installed in villages for individual houses or for a group of houses. The digestate that comes out from the plant can be used as fertilizer in agriculture as a substitute for harmful chemicals.

Water, protein, undigested fats, polysaccharides, bacterial biomass, ash, and undigested dietary remains are the main components of feces in the BW. As a proportion of wet weight, the primary elements in feces are oxygen (74%), hydrogen (10%), carbon (5%), and nitrogen (0.7%), including the oxygen and hydrogen found in the water portion of the feces. Feces are made up of approximately 75% water and 25% solids. Organic materials make up 84–93% of the solid portion. A calorific value of 4115 kcal/kg of dry solids can be used as the design standard for the calorific values of feces ^[36].

Human feces can be a viable source for biogas production. The slurry from BW after coagulation can be used as a substrate for biogas production or can be used along with other household organic waste such as kitchen waste and agricultural waste for the production of biogas.

The production of biogas works on the principle of anaerobic digestion of organic matter in the absence of oxygen by the action of certain anaerobic bacteria, and this process takes place in four steps, namely: (i) hydrolysis, (ii) acidogenesis, (iii) acetogenesis, and (iv) methanogenesis ^[37].

Hydrolysis: Here, proteins, carbohydrates, and lipids (fats) are digested by hydrolyzing bacteria into liquid monomers and polymers in the first stage, after which they are converted back into amino acids, monosaccharides, and fatty acids, respectively. Acidogenesis: The soluble organic monomers of sugars and amino acids are converted by acidogenic bacteria to ammonia, ethanol, acids (such as propionic and butyric acid), acetate, H₂, and CO₂ in the second stage. Acetogenesis: In this step, acetogenic bacteria convert long-chain fatty acids, volatile fatty acids, and alcohols into hydrogen, carbon dioxide, and acetic acid. Methanogenesis: At this stage, methanogenic bacteria transform the hydrogen and acetic acid into methane gas and carbon dioxide. Here, gaseous impurities such as hydrogen sulphide, nitrogen, oxygen, and hydrogen also appear along with methane and carbon dioxide. The higher percentage of methane (>45%) makes the biogas suitable for combustion, i.e., has higher calorific value of the fuel ^[37].

6. Phycoremediation

The term phycoremediation means the usage of algae to remove, degrade, or bio-transform undesirable substances in wastewater ^[38]^[39]. Alternatively, phycoremediation can be defined as the employment of micro- and macroalgae for the removal or biotransformation of toxic pollutants, including nutrients and xenobiotics, from wastewater ^[40]. Recently there has been a lot of attention by researchers on treating wastewater using biological systems, i.e., micro- and macroorganisms, in lieu of traditional expensive chemical treatment methods. This process involves the acceptance of contaminants from polluted bodies of water by algal cells followed by degradation into nontoxic forms and sometimes nutritional sources ^[41]. Here, pollutants are decontaminated by a method known as biosorption, where pollutants simply bind to the cell walls of the algae. In another technique called active uptake, algae have the potential to absorb and use these toxic pollutants in their cellular metabolic processes, thereby rendering the pollutants non-toxic.

The remediation of the pollutants with the aid of algae has proven ideal because it is eco-friendly, cost-effective, and easily managed. Microalgae form peptide bonds during photosynthesis that can bind toxic metal ions from wastewater and form organometallic complexes. These complexes are then further sequestered into the vacuole, in turn neutralizing the toxic effect of the metal ions. For instance, the macroalgae Caulerpa lentillifera have the potential to remove the metals copper, cadmium, lead, and zinc through biosorption by some specific functional groups present on the cell surface ^[41]^[42]^[43].

Algae exist everywhere in aquatic ecosystems and are adapted to diverse conditions. This has enabled algae to develop a wide range of resistance to natural surroundings ^[44]. Due to this benefit, algae have been widely used in the bioremediation of pollutants, producing clean water as well as useful biomass that may be used as feedstock for a variety of valuable goods, including food, feed, fertilizer, pharmaceuticals, and more recently, biofuel. When paired with the production of biofuel and integrated with waste treatment, algae have the potential to be a carbon sink by removing carbon dioxide through photosynthesis ^[45]. A wide range of microalgae, such as Chlorella, Scenedesmus, Phormidium, Botryococcus, Chlamydomonas, and Spirulina, for treating domestic wastewater has been found to be effective and encouraging ^[46]. Specifically, Chlorella vulgaris was found to be effective in treating BW ^[47]. This system has the potential to transform liquid wastes, including BW, into valuable products for agriculture, while reducing pollution levels in water without the need for technical pre-treatment. The results suggest that a 10% diluted raw BW showed the highest growth rate of algae (0.265 per day) and a nutrient removal efficiency of 99.6% for ammonium and 33.7% for phosphate. With a 50% dilution of BW, the highest COD removal (81%) was observed. In summary, the treatment of BW with Chlorella Vulgaris suggested that the dilution factor of 0.5 followed by microalgae cultivation with a hydraulic retention time of 14 days could offer the highest biomass production for the potential use in agriculture and, in parallel, a way to treat raw BW from source-separation sanitation systems ^[47].

Pilot plant studies in high-rate algal ponds (HRAP) are also carried out with a capacity of 3000 L using Chlorella vulgaris with a hydraulic retention time of 5 days. The results demonstrated that the average removal rate of nutrients, such as total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), was found to be 67%, 76%, and 41%, respectively. For contaminants of emerging concern (CEC), the average removal rates of such compounds were 60% for naproxen (NP), 51% for ibuprofen (IB), 92% for methylparaben (MePB), 76% for 2,4-dihydroxybenzophenone (DH-BP), and 80% for oxybenzone (HM- BP) ^[48]. The quality of algal cells that makes them an ideal candidate for remediation is their fast growth rate. They can produce oxygen from sunlight and need only minimal nutrients for growth. During phycoremediation, algae modify the quality of water by exerting effects on various biochemical parameters, including: efficient pH correction, change in smell and color, sludge reduction, reduction in biological and chemical oxygen demand, removal of nutrients, and flue gas/carbon mitigation. To summarize, the rising expense of traditional remediation methods to decrease pollution in aquatic and terrestrial environments has prompted the adoption of innovative techniques such as phycoremediation as cost-effective and environmentally acceptable green alternatives.

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