3.1. Municipality wastewater
Municipal or domestic wastewater due to the presence of large amounts of pollutants like nitrogenous wastes, phosphates, medium levels of COD and other suspended and dissolved solids is a major cause of concern and needs to be treated effectively. Around 75% of municipal wastewater generated is not treated properly due to the lack of proper infrastructure (Peccia and Westerhoff 2015). Hence a large number of treatment methods have been developed over the years, use of biological agents is one such development in this area as municipal wastewater has an ample amount of N and P that can support algal growth without the use of synthetic fertilizers. Such algae will have high biomass content and can, later on, be used for other purposes (Kumar, Singh et al. 2019). Figure III shows the number of papers on treatment of municipality wastewater by phycoremediation, published per year. Li et al., (Li, Zhou et al. 2011) carried out experiments to find out the robust strains of microalgae that can be used to treat municipal wastewater and to know about the effects of environmental factors like light and dark cycle and CO2 concentration. Fourteen different algal species, collected from the Culture Collection of Algae at the University of Texas from the following genus Haematococcus, Chlorella, Scenedesmus, Chloroccum and Chlamydomonas were studied. The highest accumulation was shown by Chlorella kessleri (2.01 g/L) followed by Chlorella protothecoides at 1.31g/L. Higher light intensity, photoperiods and exogenous carbon dioxide promoted biomass accumulation and nutrients (N) and COD removal, but low exogenous CO2 promoted phosphorous removal. The lowest net biomass accumulation (0.23 g/L) was observed with algal strain Chlamydomonas noctigama, UTEX 1338. Apart from studying single strains many researchers experimented with algal consortia for wastewater treatment and got favourable results. Su et al., (Su, Mennerich et al. 2011) used an algae-bacteria culture in a stirred tank photobioreactor to treat water collected from the secondary clarifier of Suderburg municipal wastewater treatment plant (Germany). The main algal species used was cyanobacteria while the bacterial species used were Gammaproteobacteria, Betaproteobacteria, Flavobacteria and Bacteroides. The consortia settled down in 20 minutes only and reduced suspended solids from 1.84 to 0.016 g/L. At the end of 8 days, 98.2 ± 1.3% COD, 88.3 ± 1.6%TKN, 64.8 ± 1% P were removed and the average biomass productivity was 10.9 ± 1.1 g/m2day. This biomass accumulation can be attributed to nutrient removal. Nannochloropsis salina was found to remove P and N at the rate of 3.8 mg /L per day and 35.3 mg /L per day, respectively and achieved the highest lipid productivity of 38.7 mg/L per day with 2 days of harvesting (Cai, Park et al. 2013). Zhao et al., (Zhao, Kumar et al. 2018) compared the efficiencies of a revolving algal biofilm reactor (RAB) and control raceway ponds in treatment of wastewater collected from the supernatant after sludge sedimentation at Metropolitan Water Reclamation District of Greater Chicago for nutrients and heavy metals removal. Two RAB reactors of height 0.9 m and 1.8 m tall were taken. The RAB removed 30 times more nutrients as compared to raceway ponds, also the taller RAB produced more biomass as compared to the shorter one. At the end of the 7th day 80% total P and 87% total Kjeldal N, and 100% ammonia was removed. The RAB reactors also showed some metal removing capacities. The RAB reactors were hence recommended for the treatment of municipal wastewater. Sarmah et al., (Sarmah, Das et al. 2019) investigated the wastewater treatment potential of two different microalgal consortia (MAC1 and MAC2) on sewage water collected from Dabra (Hisar) at 4 different dilution conditions i.e. 25%, 50%, 75%and 100% wastewater. MAC1 had mostly species like- Nannochloropsis sp., Chlamydomonas reinhardtii, Chlorella sp., Scenedesmus bijugatus, and Oscillatoria. while MAC2 had Nannochloropsis sp., Kirchnella, Chlorella sp., Scenedesmus dimorphus, and Microcoleus. Best growths were seen with wastewater at 75% dilution and MAC1 showed a lipid content of 31.33% of dry cell weight at the end of 10 days. Total protein (0.12–0.16 mg/mL) and chlorophyll (19.17–25.17 µg/mL) were also observed in the 75% diluted wastewater, however, maximum biomass was obtained at 50% diluted wastewater at 1.53 and 1.04 g/L for MAC1 and MAC2, respectively. MAC1 performed better and removed 86.27% organic carbon, 87.6% COD, 75.2% Cr, 99.6% Cd, 85.06% Cu and 98.2% Pb. Since conventional treatment requires huge energy costs for aeration of the water, often an algae-bacteria consortia were used for the treatment of municipal wastewater, so that the algae can provide the required oxygen for the bacteria. Amini et al., (Amini, Babaei et al. 2020) performed a group of experiments in semi-continuous photo-bioreactors using different Chlorella vulgaris and activated sludge (AS) ratios (5:1, 1:1 and 1:5). For all inoculum ratios, higher than 93% COD removal was achieved. Algae: AS ratio of 5:1 gave the highest ammonium and phosphate removal of 88% and 84% removal, respectively and biomass concentration from 0.3 g/L to 1.96 g/L. Thus, at the end of 10 days, the 5:1 ratio was chosen and taken for further studies to check nutrient removal in a membrane bioreactor which showed 98% and 89% removal of ammonium and phosphate, respectively. Although the algae bacteria consortia is an attractive wastewater treatment approach yet full-scale development is limited due to various environmental complexities affecting the growth and sustenance of the culture in the wastewater. Xu et al., (Xu, Zou et al. 2021), studied the effects of photoperiod and temperature variations on the algae-bacteria systems in a lab-scale municipal wastewater treatment system. Scenedesmus obliquus, Chlorella vulgaris, Spirulina platensis and AS were used for the treatment of raw municipal wastewater, collected from the grit chamber in a sewage treatment plant at Xian, China, to study the biomass growth, nutrient removal, and settling ability. Seasonal variations were observed with the highest biomass production and removal rates during the summer and autumn. The highest Total Nitrogen (TN) removal rate was 2.34 mg/L per day, the highest specific growth rate was 0.46 mg/L per day and the (Total Suspended Solids) TSS removal efficiency was 96.3 ± 2.1% during summer under aerated conditions. The highest Phosphorous removal rate of 1.67/day was obtained during autumn. Scenedesmus obliquus and Chlorella vulgaris, collected from the Institute of Hydrobiology, Wuhan, China, combined with bacteria (Bacteroidetes, Proteobacteria, Firmicutes, Chloroflexi) were found to be good microbial agents for removal of nutrients from municipal wastewater. Works of some researchers in treatment of municipality wastewater using phycoremediation is given in Table II. Over time a large number of algal species has been checked for their municipal wastewater treatment capacities. Some prominent species of microalgae recognised by researchers to treat municipal wastewater include strains like Chlorella, Scenedesmus; Phormidium; Botryococcus; Chlamydomonas. Scenedesmus obliquus, Chlorella Vulgaris and C. kessleri (Gupta and Dhandayuthapani 2019) ,(Eladel, Esakkimuthu et al. 2019).
Table II: Table for algal treatment of municipality wastewater
Algal strain(s) used
|
Wastewater (simulated/ real) and operatin
|
Reactor
|
% Removal and days of culture
|
Biomass/Biomolecules
|
Remarks*
|
Reference
|
Chlorella kessleri
|
|
Batch reactor
|
|
Biomass growth 2.01 g/L
|
|
(Li, Zhou et al. 2011)
|
Algae-bacteria culture
|
Real
|
Stirred tank photobioreactor
|
8 days COD-98.2 ± 1.3%, TKN-88.3 ± 1.6%, P -64.8 ± 1%
|
|
|
(Su, Mennerich et al. 2011)
|
Nannochloropsis salina
|
|
Batch reactor
|
2 days
P -3.8 mg /L per day
N- 35.3 mg /L per day
|
Lipid-38.7 mg/L per day
|
|
(Cai, Park et al. 2013)
|
|
Real
|
Revolving algal biofilm reactor (RAB) and control raceway ponds
|
7th day
total P – 80%
total Kjeldal N – 87%, ammonia- 100%
|
|
Removal efficiency of RAB 30 times than raceway ponds
|
(Zhao, Kumar et al. 2018)
|
Two different microalgal consortia (MAC1 and MAC2)
|
Real
|
Batch reactor
|
10 days,
organic carbon − 86.27%, COD- 87.6%,
Cr-75.2% Cd- 99.6%, Cu -85.06%
|
lipid content of 31.33% of dry cell in mac1
|
|
(Sharma, Kumar et al. 2020)
|
Chlorella vulgaris
|
Simulated
|
Semi-continuous photo-bioreactors
|
10 days
ammonium – 88%
phosphate- 84%
|
|
|
(Amini, Babaei et al. 2020)
|
Scenedesmus obliquus, Chlorella vulgaris, Spirulina platensis
|
Real wastewater from China
|
Batch reactor
|
Total Nitrogen-2.34 mg/L per day
Posphorous-1.67mg/L/day
|
|
|
(Xu, Zou et al. 2021)
|
A consortium of 15 native algal isolates
|
85–90% carpet industry effluents
|
Batch reactor
|
> 96% nutrient removal
|
Biomass- 9.2–17.8 tons ha_1 year_1
Lipid content- 6.82%,
|
|
(Chinnasamy, Bhatnagar et al. 2010)
|
3.2. Food Industry
In comparison to other industrial sectors, the food industry consumes significantly more water per tonne of product. Food waste has special properties that distinguish it from normal municipal wastewater treated by governmental or private water treatment plants around the world: it is biodegradable and harmless, but it contains high levels of biochemical oxygen demand and suspended particulates (Sepúlveda-Muñoz, Ángeles et al. 2020). Food processing wastewater, such as that produced by the food and dairy sectors, is characterised by high BOD (442–523.5 mg/L) and COD (8960–11900 mg/L), high levels of fats, oil and grease, as well as other nutrients like nitrogen. Wastewater was also high in nutrients such as phosphorus (108 mg/L), nitrogen (1385 mg/L), aluminium (316.4 mg/L), iron (24.7 mg/L), and varying amount of total organic carbon, phosphates, and potassium (Ji, Yun et al. 2015). Several research works have been carried out in this field to check for potential algal species that can treat such wastewater from food processing units. Figure IV shows the number of papers on treatment of food wastewater by phycoremediation, published per year. Dual step biological approach were applied for the purification of food industry wastewater (Chi, Zheng et al. 2011). In the first step different yeast strains like Yarrowia lipolytica, Cryptococcus curvatus, and Rhodotorula glutinis were used with wastewater and a solution of glucose water was used as a control along with it. In media created from primary wastewater, R. glutinis and C. curvatus produced more biomass than Y. lipolytica, both with and without glucose supplementation. In the next step, a procedure for growing C. curvatus and R. glutinis in food waste and municipal wastewater media was investigated, and the effluents from these operations were treated using yeast culture and phototrophic algae culture. Chlorella sorokiniana biomass in phototrophic cultures reached 1.53g/L and 0.58g/L in the food and municipal wastewater, respectively. Final effluents were having residual nitrogen and phosphorous concentrations of 33 mg/L and 1.5mg/L in food wastewater and 34 mg/L, and 0.6 mg/L in municipal wastewater, respectively. The lipid percentage of the biomass generated ranged between 18.7% and 28.6%. Gani (Paran Gani 2016) collected water samples from a food processing factory effluent in Johor, Malaysia. A pure sample of wastewater and 50% diluted wastewater were taken for testing the waste removal ability of Botryococcus sp. At the end of the twelfth day, the highest growth rate of 3.72×106 cell/mL was observed in the pure sample. It also showed 86.62% removal of nitrate, 78.23% phosphate and 76.66% total organic carbon (TOC). However, 50% diluted wastewater showed a greater growth rate of algae. On the thirteenth day, it attained a maximum growth rate of 9.7×105 cell/mL while removing 78.78% of nitrate, 69.03% of BOD and 67.93% of TOC. This work suggested that Botryococcus sp. can be used for the treatment of food processing wastewater.
Pork consumption has increased globally in recent years. This expansion has resulted in a large rise in waste generated by the industry, particularly wastewater. Luo et al., (Luo, He et al. 2016) has used Coelastrella sp. QY01 to treat piggery wastewater. The species was able to reduce 90% and 78% of nitrogen and inorganic carbon, respectively, while the lipid content increased to 24.8% from 22.4%, thus indicating a possibility of obtaining value-added products. Liu et al., (Liu, Lv et al. 2019) had studied the treatment of aquaculture wastewater with low biomass concentrations (50–100 mg/L) of P. kessleri TY, it showed good growth and pollutants removal. With inoculation of 100 mg/L, on the 3rd day, it removed 94.4%, 96.2%, 99%, 94.3%, 95.6% of ammonium, nitrite, nitrate, and phosphorus, respectively. Despite a rise in wastewater treatment, there is still a scarcity of effective technology for treating wastewater from the pork industry (Lopez-Pacheco, Silva-Nunez et al. 2021). In present times pre-treatment lagoon and anaerobic lagoon are commonly used for treatment of pork wastewater (Lopez-Pacheco, Silva-Nunez et al. 2021). According to the works of Ansari et al (Ansari, Gupta et al. 2019), the aquaculture industry is a fast-growing food sector industry growing at 10% per annum providing feed for 47% of worldwide human fish consumption and hence new and better wastewater treatment is the necessity of the hour. There are a variety of food processing plants each with its unique wastewater characteristics and hence requires a slightly different treatment process from one another.
Milk processing industries produces disposable water-carried or liquid waste which is known as dairy wastewater. Large volumes of wastewater approximately 0.2–10 L of waste per litre of processed milk is released by dairy plants (Karadag, Köroğlu et al. 2015). According to Prazerres et al., (Prazeres, Carvalho et al. 2012) dairy wastewater is characterized by a high organic load which is mostly due to milk proteins and carbohydrates. Such wastewater mostly contains huge amounts of oxygen-demanding waste, milk solids, pathogenic organisms, oil and grease, nitrogen, phosphorus, carbohydrates (lactose), surfactants and sanitiser. Such wastewater has a high amount of COD (80k – 90k mg/L) and BOD (40k -48k mg/L) depending on the source of dairy wastewater (Pandey, Srivastava et al. 2019). Although there are many physicochemical methods in practice, with the advancement of biological treatment methods and increased popularity of algae as a potential treatment agent for wastewaters, many types of researches were carried out to find the most suitable algal strain for algae based secondary treatment of dairy wastewater (Kotteswari M. 2012).
A study by Kothari et al., (Kothari, Prasad et al. 2013) by using Chlamydomous polypyrenoideum on dairy wastewater collected from a plant located at Lucknow, India showed that it could reduce nitrite, nitrate, phosphate, ammonia, chloride and fluoride by 74%, 47%, 90%, 61%, and 58% respectively in just 10 days. The lipid content of algae grown on dairy wastewater was 1.6 g in just 10 days while that grown in BG-11 solution was 1.27g. Kotteswari (Kotteswari M. 2012) studied the removal rates of TDS, TSS, BOD, COD by a cyanobacteria Nostoc sp. on dairy waste water. Nostoc sp. amended with dairy effluent wastewater showed maximum growth of 330 mg/L (dry weight) on the 15th day. The following removal percentages were obtained at the end of 15th day: TSS- 53.93%, TDS − 20.21%, Phosphate-21.08%, COD- 40.25%, BOD- 40.44%. Research showed that when Chlorella vulgaris grown in a mixotrophic condition showed a triple growth rate in comparison to that when grown in an autotrophic condition, although the lactose present in the dairy wastewater must be hydrolysed so that glucose and galactose can be effectively absorbed by Chlorella. The autotrophic and mixotrophic growth with hydrolysed and non-hydrolysed effluent showed growth rates of 0.13, 0.43 and 0.12 g/L/day, respectively (Abreu, Fernandes et al. 2012). Ding et al,. (Ding, Zhao et al. 2014) also checked the viability of algal growth and removal rates in dairy wastewater diluted to different concentrations (5, 10, and 20 times). The growth rate in 5 times diluted wastewater was 0.86g/L while the growth rates in 10 and 20 times diluted wastewater were 0.74g/L and 0.59 g/L, respectively. The removal efficiency for the 5-times diluted wastewater was ammonia: 83%, phosphorous: 92%, COD: 90%, 10 times diluted sample had ammonia: 93%, phosphorous: 91%, COD: 88% removal while the 20 times diluted sample removed ammonia: 99.62%, phosphorous: 89.92%, COD: 84.18%. Lu et al., (Lu, Wang et al. 2015) studied the removal capacities and biodiesel production efficiencies of Chlorella sp. in raw dairy wastewater in both outdoor pilot-scale and indoor bench-scale photobioreactors. The maximum productivity on an indoor scale reached 260 mg/L per day while for the outdoor setup it was 110 mg/L per day. COD, nitrogen, phosphorous removal rates were 88.38, 37.28, and 2.03 mg·L− 1·day− 1, respectively for indoor and 41.31, 6.58, and 2.74 mg·L− 1·day− 1, respectively, for the outdoor systems. Although the removal rates were less for the outdoor system, the domination by C16/C18 fatty acid groups in the biomass obtained from outdoor systems showed that Chlorella can be a potential source for biodiesel production using dairy wastewater. Gupta et al., (Gupta, Rani et al. 2019) showed that algae not only treated dairy wastewater but also showed high growth rates and lipid production, thus serving a dual purpose. Raw dairy wastewater was found to be a much better cultivation medium than synthetic BG11 solution for the cultivation of Scenedesmus sp, Chlorella sp., and C. zofingiensis., along with 100% NH3 removal (Gupta, Rani et al. 2019). In another study, it was found that although phosphate consumption by microalgae Acutodesmus dimorphus was slower than nitrate consumption, however, it was found to be completely consumed from 5.96 to 0.00 mg/L after 8 days in unsterilised dairy wastewater (Pandey, Srivastava et al. 2019). Mehar et al., (2019) showed that the lipid productivity by a consortium of algae was observed at 10% CO2 supply. Lipid content of 16.89% was obtained (Mehar, Shekh et al. 2019). Nannochloropsis salina, when grown at a 3% effluent rate, showed lipid storage of 35% of the dry cell weight, however, when effluent loading was increased to 24%, lipid productivity declined from 29.2 to 14 mg/L per day (Pandey, Srivastava et al. 2019). Works of some researchers in treatment of food industry wastewater using phycoremediation is given in Table III. Although there are research works to verify the claim that microalgae can be used for dairy wastewater treatment, large scale commercial application still needs a lot of work.
Table III. Table for algal treatment of food-industry wastewater
Algal strain(s) used
|
Wastewater (simulated/ real) and operating conditions
|
Reactor
|
% Removal and days of culture
|
Value added product (if any)
|
Ref.
|
Botryococcus sp
|
Real dairy wastewater from Malaysia
|
Batch reactor
|
13 days
Nitrate- 78.78%
BOD-69.03%
total organic carbon - 67.93%
|
|
(Paran Gani 2016)
|
Coelastrella sp
|
Real piggery wastewater
|
Batch reactor
|
Nitrogen- 90%
inorganic carbon- 78%
|
24.8% lipid.
|
(Luo, He et al. 2016)
|
P. kessleri TY
|
Real aquaculture wastewater
|
Batch reactor
|
3 days
Ammonium- 94.4%
nitrite- 96.2%
nitrate- 99%
phosphorus-94.3%
|
|
(Liu, Lv et al. 2019)
|
Chlamydomous polypyrenoideum
|
Real dairy wastewater at Lucknow, India
|
Batch reactor
|
10 days
Nitrite- 74%, nitrate- 47%, phosphate- 90%, ammonia-61%,
chloride − 58%
|
|
(Kothari, Prasad et al. 2013)
|
Nostoc sp
|
Real dairy wastewater from India
|
Batch reactor
|
15 days
TSS- 53.93%, TDS − 20.21%,
Phosphate-21.08%, COD- 40.25%, BOD- 40.44%.
|
|
(Kotteswari M. 2012)
|
|
Real dairy wastewater
|
Batch reactor
|
ammonia: 99.62%, phosphorous: 89.92%, COD: 84.18%.
|
|
(Ding, Zhao et al. 2014)
|
Diplosphaera sp
|
Real dairy wastewater
|
Batch reactor
|
14 days
|
Biomass up to 2.3g/L
|
(Liu, Wang et al. 2012)
|
Acutodesmus dimorphus.
|
Real dairy wastewater from Gujarat,
Inoculation- 10%
of actively growing culture,
Temp-35ºC
LI- 60 µmol m-2 s-1 light:dark period − 12:12
|
Batch reactor
|
COD − 90% (4 days), ammobiacal nitrogn − 100% (6 days)
|
1 kg biomass gives 195 g of biodiesel and 78 g of bioethanol,.
|
(Chokshi, Pancha et al. 2016)
|
3.3. Heavy metals in petrochemical and organics Industry
Around 84 million barrels of crude oil is yielded which results in 33.6 million barrels of wastewater in the petroleum industry per day globally (El-Naas, Alhaija et al. 2014). Wastewater from the petrochemical industry contains compounds like nitrogen, cyanide, phenols, polycyclic aromatic hydrocarbons and heavy metals like vanadium, cadmium, nickel, copper, chromium, iron, molybdenum, lead, selenium, mercury, zinc and silver (Asatekin and Mayes 2009). He and Chen, (He and Chen 2014) pointed out that these heavy metals are carcinogenic and mostly non-biodegradable and thus, needs to be taken care of. The presence of large amounts of aliphatic hydrocarbon in petrochemical wastewater makes it difficult to purify. For better treatment of petrochemical wastewater sometimes the wastewater is subjected to algae based secondary treatment after it has once passed through the conventional treatment. The activated sludge process is the most commonly used secondary treatment for the treatment of organic waste effluents (Su, Mennerich et al. 2012, Kumar, Singh et al. 2019). Figure V shows the mechanism of uptake of heavy metals by an algal cell through bioaccumulation and bioabsorption. Even though certain microalgal species have the ability to utilize heterotrophically or mixotrophically organic acids such as fermentative butyrate or acetate as carbon and/or energy source, these could show some inhibitory action when their concentration becomes higher than tolerance limits (Turon, Baroukh et al. 2015).
Figure VI shows the number of papers on treatment of petrochemical wastewater by phycoremediation, published per year. Anastopoulos and Kyas (Anastopoulos and Kyzas 2015) also studied the removal of cations from such wastewater utilizing red algaes cation exchange potential by virtue of sulfated polysaccharide that is present in their cell wall. Studies by several authors (Christenson and Sims 2011),(Ellis, Hengge et al. 2012), (Sathish, Glaittli et al. 2014), also demonstrated that such a Rotating Algal Biofilm Reactor system can be a potential source of algal biomass. Madadi and Pourbabaee (Madadi and Pourbabaee 2016) studied the efficacy of Chlorella vulgaris for the treatment of petrochemical wastewater obtained from an Iranian petrochemical plant. The wastewater was diluted up to different concentrations and the various removal rates and growths were measured. Phosphorous and nitrogen were removed up to 100% in almost all experimental setups. In another study (Mazur, Pozdniakova et al. 2016) used L. hyperborea. in a fixed bed column to observe the zinc removal rate of such wastewater. A six times reduction was observed in wastewater with zinc in comparison to pure zinc solution. A similar effect was also observed (Cechinel, Mayer et al. 2016). Huo et al., (Huo, Chen et al. 2018) had used Tribonema sp. in open photobioreactors for the advanced treatment of petrochemical wastewater. Around 97.8% of COD removal rates were obtained and Phosphorous, NH3–N were completely removed within 5–7 days. Using Tribonema sp for such a post-treatment process showed greater removal and biomass production. Cechinal et al., (Cechinel, Mayer et al. 2016), studied four species of algae namely, Fucus spiralis, Pelvetia canaliculata, Ascophyllum nodosum and Pelvetia canaliculata for their effectiveness in transition metal removal in petrochemical wastewater. These brown microalgae are found to be effective biosorbents, just like ion-exchange resins. As per the study of Liu et al., (Liu, Wu et al. 2017), mixotrophic systems are efficient compared to single-species systems and show better removal. In a dual bacteria - algae system, the algae produce oxygen through photosynthesis and assimilate nutrients (Su, Mennerich et al. 2012), while the bacteria helps in breaking down organic substances, and produces other promoting factors for algal growth (Jauffrais, Agogué et al. 2017). Hodges et al, (Hodges, Fica et al. 2017) demonstrated that when a mixed algal culture of microalgae was used in a rotating algae biofilm reactor with petrochemical wastewater, it showed higher productivity and removal of phosphorous, nitrogen and other suspended solids as compared to pond lagoon systems. Again, Cechinel et al., (Cechinel, Mayer et al. 2016) took two red macro-algae, Gracilaria cervicornis and Gracilaria caudate to study heavy metals removal in the synthetic petrochemical wastewater systems. While both the algae showed around 1.2 mEq/g ion-exchange capacities, the orders of affinity was nickel < zinc ≪ copper. G. cervicornis favoured transition metals in the synthetic solution. When loaded with calcium, copper was favoured in comparison to nickel or zinc. Huo et al, (Huo, Chen et al. 2019) used Tribonema sp. in wastewater obtained from Zhenjiang Petrochemical Corporation, Jiangsu, China. This species was cultured in the effluents obtained from different stages of the wastewater treatment process, in which the anaerobic stage showed the ideal growth rates. Organic contaminants and total phosphorous were removed almost completely, also the highest biomass productivity was observed. The catalogued and identified tolerant algal species that can be used against different harmful wastes were found to be Scenedesmus, Euglena, Chlamydomonas, Nitzschia, Oscillatoria, Chlorella, Stigeoclonium and Navicula (Arora, Tripathi et al. 2019). Madadi et al., (Madadi, Zahed et al. 2021) used Chlorella vulgaris on 50% diluted watstewater for 30 days, maximum removal was obtained as follows: 30.36%, 69.89%, 10.89%, and 92.59%, for BOD, total nitrogen, COD, and total phosphorous respectively. Works of some researchers in treatment of petrochem and organic industry wastewater using phycoremediation is given in Table IV. Microalgae as organic compound removal agents in petrochemical wastewater is thus a promising research domain calling for further investigations.
Table IV. Table for algal treatment of petrochemical and organic wastewater
Algal strain(s) used
|
Wastewater
(simulated/real) and operating conditions
|
Reactor
|
% Removal and days of culture
|
Value added product (if any)
|
Remarks*
|
Ref.
|
Chlorella vulgaris
|
Iranian petrochemical plant.
|
Batch reactor
|
phosphorous and nitrogen – almost 100%
|
|
|
(Madadi and Pourbabaee 2016)
|
Tribonema sp.
|
Real wastewater
|
open photobioreactors
|
5-7days
COD- 97.8%
|
|
|
(Huo, Chen et al. 2018)
|
Chlorella vulgaris
|
Real wastewater
|
Batch reactor
|
30 days
BOD − 30.36%,
total nitrogen- 69.89%, COD- 10.89%,
total phosphor- 92.59%
|
|
|
(Madadi, Zahed et al. 2021)
|
Tribonema sp.
|
Real dairy wastewater with light intensity:
300 µmolm − 2 s-1,, temp-25°C.
|
open vertical tubular
photobioreactors
|
COD- 98.4%, TN- 96.8%
|
biomass
concentration − 4.4 g/L
|
|
(Huo, Chen et al. 2019)
|
C. vulgaris
|
artificial wastewater prepared
from organic substances,NH4Cl, KH2PO4 and trace elements
Temp- 25 ∘C,
Light − 120 µmolm − 2 s − 1
|
Batch reactor
|
12 days COD-88.56 ± 2.59% (in glucose )84.08 ± 2.31% (in NaNC)
|
dry biomass concentrations of 0.652 (in glucose)0.572 g L − 1 (in NaNC)
|
|
(Peng, Gao et al. 2019)
|
Scenedesmus intermedius Chod. and Nannochloris sp.
|
Simulated water
L:D cycle-14:10
temp- 20 ± 2 ◦C. pH – 8–9
|
Batch reactor
|
S. intermedius –(free) 0.014 mg P h − 1 and 0.022 mgNh − 1
Nannochloris sp.(free) − 0.006 mg P h − 1 and 0.011 mgNh − 1
|
|
Species isolated from piggery wastewater performed better than commercial species
|
(Jiménez-Pérez, Sánchez-Castillo et al. 2004)
|
Chlorella
vulgaris and Chlorella VT-1
|
Simulated wastewater Temp − 25°C
light intensity-76 µmol m − 2 s − 1 phenol conc − 100–400 mg/L (light)and 100 mg/L (dark)
|
Batch reactor
|
20 days, under light – little removal,
Dark-100% removal
|
|
No growth of algae in 400 mg/L phenol culture
|
(Zhang, Yang et al. 2019)
|
3.4. Tannery Industry
Reports on how tanneries turn animal hides into leather through a series of complicated processes that use a lot of water and chemicals like lime, ammonium sulphate, sodium sulphide, sodium chloride, and chromium salts are aplenty in the literature (Mukherjee, Okolie et al. 2019). After China and Italy, India is the 3rd highest producer of leather products in the world, with Tamil Nadu, Uttar Pradesh and West Bengal having 88% of the total tannery units in the country. Most of these being located near the river basins, all the toxic wastewater is generally dumped into the river causing serious pollution (Ajayan, Selvaraju et al. 2015). These tannery effluents having pollutants such as salts, dyes, nitrogen, phosphorous, detergents and heavy metals like aluminium, iron, titanium and zircon cause serious threat of pollution when released to the nearby water sources. Most of these effluents have high concentrations of total suspended and dissolved solids magnesium, phosphorous, sulfate, fluoride, phenols and grease, exceeding acceptable limits, hence proper treatment is essential. Bioremediation is preferred by many scientists as an environmentally friendly and cost-effective treatment method for such wastewaters (Letry, Castro et al. 2019), (Gupta, Rani et al. 2019).
The high levels of organic content and large amounts of nitrogenous compounds present in tannery wastewater clubbed with the nutrient absorption capacity of microalgae serve as an excellent option for bioremediation of pollutants from tannery wastewater. Ajayan et al., (Ajayan, Selvaraju et al. 2015) studied the growth of Scenedesmus sp. in tannery wastewater. The test species after having grown under laboratory conditions for 12 days reduced the heavy metal concentrations significantly. Chromium, copper, lead, zinc were reduced by 81.2–96%, 73.2–98%, 75–98% and 65–98%, respectively. Nutrients like nitrate and phosphate were removed by more than 44.3% and 95%, respectively. Das et al., (Das, Naseera et al. 2016) studied the pollutant removal efficiency of salt-tolerant Chlorella vulgaris NIOCCV in tannery wastewater. This study showed remarkable results in terms of pollutant removal. NO3-N and Cr were completely removed (100%) at the end of the 6th and 12th days, respectively. About 91.73% phosphate was removed by the 6th day which increased to 99% by the 21st day, the sulphate removal was slightly less up to 67.4% by the 21st day. COD and BOD were removed upto 95.75% and 95.93% respectively by the end of the experiment. The study suggested that Chlorella vulgaris NIOCCV can be used for the treatment of tannery wastewater. Da Fortoura et al., (da Fontoura, Rolim et al. 2017) studied the ability of Scenedesmus sp. in removing phosphorous, ammoniacal nitrogen and COD from tannery wastewater. Studies were carried out under different parameters like concentrations of wastewater (20–100%), light intensity (80–200 µmol/m2s) while keeping aeration and temperature constant (25°C). Maximum growth (biomass concentration of 0.90g/L) and removal of 85.63% ammoniacal nitrogen, 96.78% phosphorous, 80.33% COD were achieved when wastewater was kept at a concentration of 88.4%, light intensity of 182.5 µmol/m2s. The ability of two marine microalgae, Chlorella sp. and Phormidium sp., to decrease different contaminants in tannery wastewater, both separately and in combination, was assessed by Das et al., (Das, Naseera et al. 2016). Raw tannery wastewater was taken for the survey for 20 days. Both the species could remove > 90% of BOD and COD while in a consortium and slightly less (80%) when individually used. Total Phosphorous (TP) and Total Nitrigen (TN), Total Dissolved Solids (TDS) and chromium were reduced by 88%, 91.16%, > 50% and 90.17–94.45%, respectively by the consortium. The consortium was proven to be better removal agents. Jahan et al., (M. A. A. Jahan, N. Akhtar et al. 2014) studied the effects of macrophytes and algae on tannery wastewater having TSS, Dissolved Oxygen (DO), BOD and COD concentrations of 250 mg/L, 21300 mg/L, 4464 mg/L and 12840 mg/L, respectively. Eichhornia crassipes emerged as the best removal agent for reducing TDS, TSS and COD. Although both algae and macrophytes reduced the heavy metals present in the wastewater, Eichhornia crassipes was most effective because of its extensive root system that could easily uptake pollutants from the wastewater. For the first time, influence of photoperiod on tannery wastewater treatment by Tetraselmis sp. was studied by Pena et al.,(Pena, Agustini et al. 2020).Turbidity of the wastewater affected light penetration. The maximum biomass concentration was observed by using the 24-hour light culture at 1.40 g/L and 1.04 g/L in the 50% and 75% wastewater concentration, respectively. Maximum removal observed was 71.74% of nitrogen, 97.64% of phosphorous, 100% of ammonia, 56.7% of COD, and 20.68% of BOD. Successful pollutants removal and high biomass production statistics shown by the works of various researchers hence suggest the use of microalgae as a treatment agent for tannery wastewater. Figure VII shows the number of papers on treatment of tannery wastewater by phycoremediation, published per year and works of some researchers in treatment of tannery wastewater using phycoremediation is given in Table V.
Table V. Table for algal treatment of tannery wastewater
Algal strain(s) used
|
Wastewater
(simulated/real) and operating conditions
|
Reactor
|
% Removal and days of culture
|
Value added product (if any)
|
Ref.
|
Scenedesmus sp
|
Real tannery wastewater
|
Batch reactor
|
12 days
Cr: 96%
Cu: 98%
Pb: 98%
Zn: 98%
|
|
(Ajayan, Selvaraju et al. 2015)
|
Chlorella vulgaris NIOCCV
|
Real tannery wastewater
|
Batch reactor
|
NO3-N-100% 6th day
Cr- 100% 6th day
|
|
(Das, Naseera et al. 2016)
|
Scenedesmus sp
|
Real tannery wastewater with conc – 88.4%
light intensity of 182.5 µmol/m2s
|
Batch reactor
|
ammoniacal nitrogen- 85.63%, phosphorous − 96.78%, COD-80.33%
|
biomass concentration of 0.90 g/L
|
(da Fontoura, Rolim et al. 2017)
|
Chlorella sp. and Phormidium sp
|
Real tannery wastewater
|
Batch reactor
|
20 days
COD &BOD- >90%
TP- 88%,
TN-91.16%, TDS->50% chromium-90.17-94.45%
|
|
(Das, Ramaiah et al. 2018)
|
Tetraselmis sp.
|
Real wastewater with 24 hrs light supply
|
Batch reactor
|
Nnitrogen 97-71.74%.phosphorous- 64% ammonia- 100%, COD-56.7%, BOD-20.68%.
|
Biomass- at 1.40g/L
|
(Pena, Agustini et al. 2020)
|
3.5. Textile Industry
Khan and Malik (Khan and Malik 2014) stated that textile industries are technologically most complex industries and the various wet processing techniques produce an enormous amount of wastewater with high TOC value. High salinity, colour, high temperature and variable pH along with high COD are typical characteristics of textile wastewater. Paul et al., (S. A. Paul 2012) stated that the effluent from textile mills contains alkalis, acids, starch, dyes, surfactants, soaps of metals, hydrogen peroxides and dispersing agents, which upon mixing with water bodies enter into the food chain and can cause serious diseases in animals and humans. Out of all the chemical wastes generated by textile industries, the dyes pose a very serious threat due to their complicated and stable structure and non-biodegradable nature (Ding, Li et al. 2010). Presently aromatic and heterocyclic dyes are used in general. Several researchers have worked with various algae to remove the colouring substance from wastewater before discharging it into water bodies. Some common dyes used are Indigo Carmine, Coomassie Brilliant G-25 and Remazol Brilliant Blue R (Paz, Carballo et al. 2017). In 2025, the global textile and clothing trade is anticipated to exceed US$1600 billion (Sarkar, Banerjee et al. 2017).
Many research works have been carried out to check the colour removing potential of microalgae from textile wastewater. Figure VIII shows the number of papers on treatment of textile wastewater by phycoremediation, published per year. One of the very early experiments was carried out by Acuner and Dilek, (Acuner and Dilek 2004) which removed mono-azodye, tectilion yellow 2G (TY2G) with the help of Chlorella vulgaris in real wastewater emitted from Samur carpet industry, Turkey. Experiments were carried out with both unacclimated (cultured in growth medium not previously exposed to TY2G) and acclimated (exposed to TY2G) to find the difference in removal capacities of the two. Unacclimated algae removed 69, 66 and 63% of COD from the setups having 50, 200 and 400 mg/L of TY2G initially, while upon acclimatization the percentage removal increased to 88, 87 and 88 respectively. Chu et al.,(Chu, See et al. 2008), studied the comparative remediation efficiencies of immobilized Chlorella vulgaris algae for treatment of textile wastewater. The algae–alginate solution was withdrawn using a sterile syringe and the beads were formed by extruding the mixture drop-wise into 0.2 M CaCl2 solution for curing. On the other hand, Lim et al., (Lim, Chu et al. 2010) studied the removal of a single dye (supranol red 3BW) from textile wastewater by 10 different types of algae. Real wastewater from a garment factory at Senawang Industrial Estate, Negeri Sembilan was used for such purpose. C. vulgaris UMACC 001 was chosen from the set of 10 microalgae for further test, owing to their better ability to grow in dye laden wastewater. The biomass growth range varied from 0.17 to 2.26 mg chlorophyll a/L while removal was 41.8–50.0%. El-Kassas and Mohammed (El-Kassas and Mohamed 2014) carried out batch experiments to reduce the COD from wastewater collected from Alexendria, Egypt. The maximum growth rate was shown at 5% concentration of dye in wastewater while the highest specific growth rate was shown at 17.5% and subsequently highest colour and COD removal occurred at this concentration. Pathak et al., (Pathak, Kothari et al. 2015) carried out a comprehensive study on the capacities of Chlorella pyrenoidosa in the treatment of textile wastewater collected from Kundan Nagar Unnao, Lucknow (India). Experiments were carried out at different concentrations of wastewater, which showed that the algae can grow in distilled water having up to 73% of wastewater and can reduce BOD, nitrate and phosphate up to 63%, 82% and 87% respectively. Works of some researchers in treatment of textile industry wastewater using phycoremediation is given in Table VI. With the emerging variety of dyes, the treatment methods used for such wastewater need to be updated regularly.
Table VI. Table for algal treatment of textile wastewater
Algal strain(s) used
|
Wastewater
(simulated/real) and operating conditions
|
Reactor
|
% Removal and days of culture
|
Value added product (if any)
|
Reference
|
Chlorella vulgaris
|
Wastewater from Samur carpet industry
400mg/L mono-azodye, tectilion yellow 2G
|
Batch reactor
|
COD- 88%
|
|
(Acuner and Dilek 2004)
|
C. vulgaris UMACC 001
|
garment factory at Senawang Industrial Estate, Negeri Sembilan
|
Batch reactor
|
COD- 41.8–50.0%.
|
0.17 to 2.26 mg chlorophyll a/L
|
(Lim, Chu et al. 2010)
|
Chlorella pyrenoidosa
|
Real wastewater (73%)
from Lucknow, India
|
Batch reactor
|
BOD – 63%, nitrate- 8%,
phosphate − 87%
|
|
(Pathak, Kothari et al. 2015)
|
3.6. Steel Industry and Coke Oven Plant
Coke, which is created by carbonising coal at extremely high temperatures, such as 1000°C to 1200°C, in a coke-oven facility, is used in the steel industry to extract iron from iron ore in a blast furnace. There are 540 coke-oven facilities worldwide, with Asia hosting the majority of them (Rychlewska, Kwiecinska et al. 2018). China leads this with more than 400 pants (Ksepko, Klimontko et al. 2019) During the coke production process, a lot of effluent is produced. Coke-oven wastewater is typically created by quenching the coke, washing the gas, and cleaning with ammonia (Kumar, Sengupta et al. 2017). According to the literature, coke-oven wastewater contains large concentrations of inorganic pollutants such ammoniacal-N, nitrate, heavy metals, and so forth, as well as organic pollutants like phenol, cyanide, polyaromatic hydrocarbons (PAHs), and so forth. The overall pollution ranges from 0.3 to 4 tonnes of coke per cubic metre (Mishra, Paul et al. 2018, Mierzwiński, Łach et al. 2019). A large number of scientists have tried different means to treat this wastewater generated from coke-oven plants. Of the different methods tried by scientist worldwide, phycoremediation is often sought after. Figure IX shows the number of papers on treatment of coke-oven plant wastewater by phycoremediation, published per year. A study by Saha et al., (Saha, Banerjee et al. 2015) showed that duckweed (Lemna minor L.) has the phytoremediation potential to remove chloride and sulphate from the coke oven plants biological oxygen treatment (BOT) wastewater. The physico-biochemical characteristics of pH, BOD, COD, total dissolved solids (TDS), and elemental content were examined in order to evaluate the BOT water quality. An increase in pH value was shown to be a sign of better water quality. The experimental findings indicated that duck weed is capable of phytoremediation for the removal of chloride and sulphate from BOT wastewater, as it effectively removed 30% chloride, 16% sulphate, and 14% TDS. After 21 days of experimentation, duck weeds relative growth rate increased up to a maximum of 30%. Due to its lower operating costs and environmental friendliness, the use of cyanobacteria for the removal of heavy metals from wastewater is attracting interest. Chromium (Cr(VI)) removal from water and wastewater is necessary to prevent water pollution because it may be harmful and carcinogenic for humans. In a study by Sen et al., (Sen, Dutta et al. 2017), the ability of a living cyanobacterial consortium made up of Limnococcus limneticus and Leptolyngbya subtilis to remove Cr(VI) under various operating conditions was investigated. The consortium was collected from the East Kolkata Wetland, a wetland of international significance. One factor at a time (OFAT) analysis was used to vary input factors such as initial Cr(VI) concentration, pH, and inoculum size in the range of 5–30 mg/L, 7–11, and 2–10%, respectively. With an initial concentration of 15 mg/L Cr(VI) at pH 9 and an inoculum size of 10%, an optimal clearance of 50% was attained after 12 days of inoculation. Studies using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) have determined how much Cr(VI) the living consortium is absorbing. According to a study using Fourier Transform Infrared Spectroscopy (FTIR), the amine, phosphate, and carbonyl groups play a role in the binding of chromium as opposed to its biosorption (VI). The proportion of chromium removed was improved by the larger inoculum. The cells were taken out and their dry biomass and lipid content was measured to determine whether biofuel generation is feasible. When living consortium was grown on simulated wastewater that has been contaminated with Cr(VI) rather than BG-11 media, a rise in both dry biomass and lipid content was seen. In order to forecast the interaction between four input variables—the initial concentration of Cr(VI), the initial solution pH, the inoculum size, and the time—and three output variables—dry biomass, lipid content, and percentage removal of Cr(VI)a regression model is created. The final step is to use Response Surface Methodology (RSM) to improve the process conditions for removing Cr(VI). Initial Cr(VI) concentration: 10 mg/L, pH: 9, inoculum size: 4%, and time: 9 days were the ideal conditions, and the anticipated percentage elimination (51%) closely matches the experimental one (52.7 percent ). In another study by Sen et al., (Sen, Nandi et al. 2018) with Dinophysis acuminata and Dinophysis caudata, a cyanobacterial consortia, was used to treat both synthetic cyanide solution and actual coke-oven wastewater. They were taken from East Kolkata Wetland. When analysing the growth kinetics, nitrate was used as the substrate. Since the consortium grew in cyanide solution, a model that included both nitrate and cyanide as substrates was suggested. The simulated data and experimental data agree fairly well. Two samples of coke-oven wastewater were taken: one from the equalisation tanks untreated effluent and the other from the secondary clarifiers effluent, both of which were treated independently with consortium. In addition to treating raw coke-oven wastewater and secondary clarifier effluents, lipid was also recovered from the biomass of cyanobacterial consortium. Using a gas chromatograph, the fatty acid methyl ester of these lipid samples was examined. In 2020 a study by Rai et al., (Rai, Wadhwa et al. 2020), the results of phycoremediating contaminants from synthetic coke-oven effluent utilising a cyanobacterial consortia of Leptolyngbya sp. and Planktothrix sp. was observed. For test strains with different levels of pollutants, lethal dose analyses were conducted. The highest amounts of biomass were 322.7 ± 22.54, 322.3 ± 12.06, and 352 ± 12.53 mg L–1 at concentrations of 2 mg L–1 phenol, 175 mg L–1 ammoniacal-N, and 30 mg L–1 nitrate, respectively. By adjusting the pH (8–10), inoculum size (IS) (5–10 percent), initial concentrations (ICs) of phenol (2–3 mg L–1), ammoniacal–N (150–200 mg L–1), and nitrate (30–40 mg L–1), the optimal operating conditions for maximal elimination were found. According to OFAT analysis, the ideal conditions were pH 8; initial concentrations of phenol 2.5 mg/L, ammoniacal-N 175 mg/L, and nitrate 30 mg/L. The best conditions for pollution removal and biomass production were found using the response surface methodology (RSM). With synthetic treated coke-oven wastewater (STCW) containing mixed contaminants, additional tests were carried out under the ideal circumstances as determined by OFAT and RSM, and the outcomes were compared. Although the results of the OFAT and RSM analyses were equally useful for actual wastewater treatment, the OFAT analysis was more appropriate economically. To evaluate the possibility of creating value-added products, biomolecules, specifically protein, carbohydrate and lipid molecules, were isolated from treated cyanobacterial biomass. In both BG-11 and synthetic coke-oven effluent, three cyanobacterial and microalgal cultures, including a consortium of Leptolyngbya sp. and Planktothrix sp. (Type I), (Rai et al., 2019) Tetraspora sp. NITD 18 (Type II) (Rai et al, 2020), and a consortium of Chlorella sp. and Synechococcus sp. (Type III) (Rai et al., 2021), were grown in synthetic coke oven wastewater by Rai et al., (Rai, Kamila et al. 2022). In BG-11 medium for Type II culture, maximum productivities of protein (75.63 (mg/L)/day), fat (13.96 (mg/L)/day) were obtained and for carbohydrate (86.25 (mg/L)/day) was attained for Type I. However, in STCW, maximal productivities were obtained as follows: lipid: 12.51 mg/L/day (Type I), protein: 57.67 mg/L/day, and carbohydrate: 75.55 mg/L/day (Type III). The highest yields were achieved for the following substances: lipid (Type I) − 111.45 mg/g and carbohydrate (Type II) − 146.47 mg/g and 122.26 mg/g in BG-11 and STCW solutions, respectively. The modelling and optimization process used the Artificial Neural Network-Genetic Algorithm (ANN-GA) to achieve the highest possible results. In another work by Aratrika Ghosh et al., (Aratrika Ghosh, Ganta Upendar et al. 2018), cyanobacterial strain Leptolyngbya foveolauram was used in the bioremediation of thiocyanate from coke-oven wastewater. By changing the initial concentration of thiocyanate from 10 to 250 mg/L while maintaining the same inoculum size and pH, a kinetic study for the bioremediation of thiocyanate was conducted. During thiocyanate remediation, concentrations of biomolecules such as protein, lipid, and carbohydrates in the cyanobacterial strain are also examined. With a starting thiocyanate content of 10 mg/L for synthetic wastewater and 50 mg/L for real wastewater, respectively, the results showed that 100% and 78.20% of thiocyanate had been remedied. It is thus observed by a number of researchers that coke-oven wastewater can be treated for purification effectively by cyanobacteria consortia, however more research needs to be carried out for making the treatment process economically efficient.