2 Literature Review
2.7 Microalgae-based advanced wastewater treatment
Oxygen is required for methane conversion. Through the coupled activities of eukaryotic algae or photosynthetic bacteria and methanotrophs in syntrophic bio-aggregates, i.e., photogranule, oxygen may be provided by direct, or at least local transfer. The produced oxygen is then immediately utilized by the methanotrophs to convert organic matter to CO2, which is in turn used by phototrophs for autotrophic CO2 fixation. Oxygenic photogranules are a light-driven consortium of phototrophic and non-phototrophic microorganisms which are embedded in a matrix of extracellular polymeric substances (EPS). The phototrophic microorganisms in photogranules are a mix of algae and/or mostly filamentous cyanobacteria (Milferstedt et al., 2017).
The methane conversion relies on syntrophic interactions between phototrophic cyanobacteria and methanotrophic bacteria aggregated in oxygenic photogranules. These interactions are found in natural systems, for example, at the chemocline between anaerobic and aerobic water layers in freshwater lakes (Milucka et al., 2015), and are also utilized in engineered systems, e.g., by van der Ha et al. (2012) to produce lipids or polyhydroxy butyrate using co-cultured eukaryotic algae and methanotrophs. Rasoulie et al. (2018) also investigated a co-culture of green microalgae and methanotrophs for removing methane and recovering nutrients (Rasouli et al., 2018).
colonial cells are easier to harvest (Chisti, 2007). Ideal microalgal strains for wastewater treatment have properties such as a high nutrient requirement and high uptake affinity (Torres-Franco et al., 2021; Wang et al., 2017). Several applications of microalgae in wastewater treatments using several types of photobioreactor (PBR), such as flat-plate membrane and bubble column PBR, are summarized in Table 2.5.
Microalgae-based systems removing nutrients in wastewater treatment rather than conventional treatment methods have several advantages.
These include cost-effectiveness, low-energy use, decreased greenhouse gas emissions, and production of high-value microalgal biomass, for example, fatty acids for nutraceutical productions (Huy et al., 2022).
Microalgae-based systems leave low residual nutrient concentrations without adding extra chemicals. Some limitations of microalgae-based treatment of wastewaters, specifically anaerobic wastewater treatment effluent, have been addressed by Torres-Franco et al. (2021). Besides operational and practical issues, such as high energy demand for light and large area requirement for open ponds, ammonia inhibition, light blockage, and unbalanced macronutrients ratio are the main challenges that directly affect the ability of microalgae to grow in anaerobic effluent (Torres-Franco et al. 2021). Furthermore, drawbacks include a relatively long hydraulic retention time (HRT), complicated processes for separating microalgae within treated wastewater and reduced performance under contamination and predation (Wang et al., 2017).
Table 2.5 Microalgae-based treatment in different types of wastewater, adapted from Kong et al. (2021) Feed Microalgae Reactor CO2
(%)
RCO2
(g∙l-1∙d-1) CN
(mg∙l-1) RN
(mg∙l-1) CP
(mg∙l-1) RP
(mg∙l-1∙d-1) CC
(mg∙l-1) RC
(mg∙l-1∙d-1)
Ref.(s)
Synthetic wastewater
Spirulina platensis UTEX 2340
Bench-scale PBR
0.038 n.a. TN: 412 23-49# TP: 90 64-81# n.a. n.a. Yuan et al.
(2011) Spirulina
platensis
Hollow fiber membrane PBR
2
0.912-1.44
NO3- 82# n.a. n.a. n.a. n.a. Kumar et
al. (2010) Treated
domestic wastewater
Chlorella vulgaris Botryococcus braunii Spirulina platensis
Flat-plate membrane PBR
1% CO2
20% O2
79% N2
1.5-22.4 g∙m⁻3∙d-1
0.62-2.4 1.4-6.9 0.08-0.89 0-0.071 TOC:
1.7-3.6
n.a. Honda et al. (2012)
Municipal wastewater
Chlorella strains (10 strains)
Erlenmeyer flasks
10
26.14%-35.51%
TN: 29.32 NH4+ -N:26.13
n.a. TP: 3.62 n.a. COD:
52.42
n.a. Hu et al.
(2016) PBR, photobioreactor; RCO2, CO2 fixation rate; CN, initial nitrogen concentrations; RN, nitrogen removal rate; CP, initial phosphorus concentrations; RP, phosphorus removal rate; CC, initial organic carbon concentrations; RC, organic carbon removal rate; n.a., not available; # represents the unit of nitrogen, phosphorus and organic carbon removal rate is %.
Table 2.5 Microalgae-based treatment in different types of wastewater, adapted from Kong et al. (2021) (continued) Feed Microalgae Reactor CO2
(%)
RCO2
(g∙l-1∙d-1) CN
(mg∙l-1)
RN
(mg∙l-1) CP
(mg∙l-1) RP
(mg∙l-1∙d-1) CC
(mg∙l-1) RC
(mg∙l-1∙d-1)
Ref.(s)
Domestic wastewater
Scenedesums sp. Bubble column PBR
0.03-10 0.239-0.368
NH4+-N:
38.6 NO3--N:
17.1
5.16-5.42 1.38-1.71
9.24 0.96-1.08 COD:
142.2
14-19.5 Nayak et al.
(2016)
Chlorella sp.
Scenedesmus sp.
Sphaerocystis sp.
Spirulina sp.
Flask 20-50 150-291 mg∙g⁻¹
NH4+-N 39# PO43-P 59# n.a. n.a. Bhakta et
al. (2015)
Chlorella minutissima
Assembly based on fish aquarium
5.36-86.4%
Biogas
486.2-210.76 g∙m⁻3∙d-1
n.a. n.a. n.a. n.a. n.a. n.a. Khan et al.
(2018)
Spirulina platensis mixed indigenous microalgae
Pilot plant 2.5-20 0.05-0.60
NH4+-N: 42 NO2-: 0.81 NO3-: 10.5
50-95# TP:
9.3±0.3
50-90# 52.0±0.5 50-100# Almomani et al. (2019)
PBR, photobioreactor; RCO2, CO2 fixation rate; CN, initial nitrogen concentrations; RN, nitrogen removal rate; CP, initial phosphorus concentrations; RP, phosphorus removal rate; CC, initial organic carbon concentrations; RC, organic carbon removal rate; n.a., not available; # represents the unit of nitrogen, phosphorus and organic carbon removal rate is %.
Table 2.5 Microalgae-based treatment in different types of wastewater, adapted from Kong et al. (2021) (continued)
Feed Microalgae Reactor CO2
(%)
RCO2
(g∙l-1∙d-1) CN
(mg∙l-1) RN
(mg∙l-1) CP
(mg∙l-1) RP
(mg∙l-1∙d-1) CC
(mg∙l-1) RC
(mg∙l-1∙d-1)
Ref.(s)
Synthetic domestic wastewater
Chlorella vulgaris Pseudokirchneriella subcapitata
Synechocystis salina Microcystis
aeruginosa
Flask 0.038
0.075-0.471
NaNO3: 250
6.75-18.18 6.79-17.82 7.04-22.86 8.85-19.63
KH2PO4: 45
0.68-2.67 0.55-2.22 0.38-1.92 0.50-1.67
n.a. n.a. Gonçalves et al. (2014)
Synthetic municipal wastewater
Scenedesmus obliquus
Flasks;
Cylindrical plexiglass PBR
0.03-15 0.257 11-14 97.8# 1-1.5 95.6# TOC:
20-120
59.1-93.3# Shen et al.
(2015)
Scenedesmus obliquus
Erlenmeyer flasks
0.038-10
n.a. NH4+-N:
30-70
20-100# PO43-P:
13
n.a. 50 n.a. Liu et al.
(2019) Aquaculture
wastewater
Chlorella sp. GD Glass-fabricated PBR
2-8%
Flue gas
2.333 TN: 60 40-90# TP: 6.8 87-99# COD:
112
61-80# Kuo et al.
(2016) PBR, photobioreactor; RCO2, CO2 fixation rate; CN, initial nitrogen concentrations; RN, nitrogen removal rate; CP, initial phosphorus concentrations; RP, phosphorus removal rate; CC, initial organic carbon concentrations; RC, organic carbon removal rate; n.a., not available; # represents the unit of nitrogen, phosphorus and organic carbon removal rate is %.
Table 2.5 Microalgae-based treatment in different types of wastewater, adapted from Kong et al. (2021) (continued) Feed Microalgae Reactor CO2
(%)
RCO2
(g∙l-1∙d-1) CN
(mg∙l-1) RN
(mg∙l-1) CP
(mg∙l-1) RP
(mg∙l-1∙d-1) CC
(mg∙l-1) RC
(mg∙l-1∙d-1)
Ref.(s)
Tequila vinasses and culture media
Chlorella vulgaris U162 Chlorella sp.
Scenesdesmus obliquus U169 Scenedesmis sp.
Flasks 25%
CO2
75%
CH4
(Biogas) 0.15-0.91
39-50 for tequila vinasses;
0.075-5 for media
n.a. 80.04-83.3
n.a. TOC:
12.9-9024
n.a. Choix et al.
(2018)
Effluent of cattle farm and manure leachate
Coelastrum sp.
SM
Airlift PBR: 6-16 0.153-0.302 g∙l-1
TKN:
about 62
7.548-9.471
TP: 4-9 3.45-6.90 sCOD:
n.a.
71.749-98.192
Mousavi et al. (2018)
Palm oil mill effluent
Chlorella sp. Transparent glass bottles
10-25 0.02-0.14
TN:
330±30 28-92.11
PO43--:
273±17
n.a. COD
2900±1 10
n.a. Hariz et al.
(2018) Seafood
processing industry wastewater
Chlorella vulgaris NIOCCV
Tubular PBR 5-20 0.149-0.430 mg∙l-1∙d
-1
n.a. 79.68-82.42#
n.a. 63.64# TOC:
n.a.
23.46# Jain et al.
(2019)
PBR, photobioreactor; RCO2, CO2 fixation rate; CN, initial nitrogen concentrations; RN, nitrogen removal rate; CP, initial phosphorus concentrations; RP, phosphorus removal rate; CC, initial organic carbon concentrations; RC, organic carbon removal rate; n.a., not available; # represents the unit of nitrogen, phosphorus and
Table 2.5 Microalgae-based treatment in different types of wastewater, adapted from Kong et al. (2021) (continued) Feed Microalgae Reactor CO2
(%)
RCO2
(g∙l-1∙d-1) CN
(mg∙l-1) RN
(mg∙l-1) CP
(mg∙l-1) RP
(mg∙l-1∙d-1) CC
(mg∙l-1) RC
(mg∙l-1∙d-1)
Ref.(s)
Textile and food processing wastewater
Chlorella vulgaris Chlorococcum infusionum
Bubble column PBR
0.03-10%
Coal-fired flue gas
0.103-0.187 0.543-0.947
NH4+-N:
153.1
85.3-95.9# 65.3-75.5#
PO43-P 11
89.5-98.8# 80.4-85.8#
850 71.4-91.9# 65.1-85.2#
Yadav et al.
(2019)
Ossein effluent
Phormidium valderianum BDU 20041
Open tank 15%
Coal burning flue gas
0.0564-0.0658
60.24 66.35# 56.67 35.66# n.a. n.a. Dineshbabu
et al. (2017)
Steel-making facility wastewater
Chlorella vulgaris UTEX 259
PBR 0.03-15 0.624 NH3: 50 0.86-0.92 gNH3∙ m-³∙h-1
PO43-P:
400 g∙m-3
n.a. n.a. n.a. Yun et al.
(2018)
Petroleum wastewater
Spongiochloris sp.
Airlift bioreactor
0.038 2.9205 63.5 n.a. 17 n.a. COD:
285
97# Abid et al.
(2017) Oil sands
tailings water
Chlorella pyrenoidosa CCCM 7066
Erlenmeyer flask
n.a. 0.11 NH3: 23.9-68
n.a. PO43-P:
0.02-0.4
n.a. n.a. n.a. Yewalkar et
al. (2011) PBR, photobioreactor; RCO2, CO2 fixation rate; CN, initial nitrogen concentrations; RN, nitrogen removal rate; CP, initial phosphorus concentrations; RP, phosphorus removal rate; CC, initial organic carbon concentrations; RC, organic carbon removal rate; n.a., not available; # represents the unit of nitrogen, phosphorus and
Microalgal-based wastewater treatment is often not only performed by microalgae but also by natural consortia of microalgae and bacteria, naturally developed or specifically inoculated from cultures (Muñoz &
Guieysse, 2006). Photosynthetic activity of microalgae can provide oxygen to support bacterial needs and therefore avoid the energy consumption associated with external aeration (Casagli et al., 2021).
There are three mechanisms for nutrient removal, specifically nitrogen and phosphorous, in microalgal-bacterial systems: Assimilatory nutrient recovery into biomass production; abiotic nutrient removal by elevated pH during microalgal photosynthesis, removing NH3-N and enhancing phosphate precipitation; and dissimilatory nutrient removal by nitrification-denitrification (Posadas et al., 2017). Hence, complex interactions between microalgae and bacteria during wastewater treatment can support an efficient removal of nutrients and/or other pollutants.
The type of wastewater and its characteristics affect the microalgal-based wastewater treatment performance. The initial carbon (C), nitrogen (N), and phosphorus (P) ratio in wastewater is often correlated with its biodegradability by microalgal-based treatment. For this, the Redfield ratio of 106C:16N:1P is widely used to quantify possible carbon and/or nutrient limitations (Tyrrell, 2019), though many studies have found the evidence of deviations from this ratio (Acién et al., 2016; Posadas et al., 2017).
Carbon is present in wastewater as inorganic carbonates and as organic molecules. Carbon dioxide can be assimilated from the atmosphere and from industrial exhaust gas by microalgae carbon fixation (Richmond, 2003). Some microalgae are also capable of using organic carbon through heterotrophic assimilation, while others are mixotrophic using both inorganic and organic carbon sources (Cai et al., 2013). Additional carbon supply in the form of CO2 or bicarbonate is needed to allow for the complete assimilation of N and P contained within the wastewater.
Moreover, even if the wastewater composition does not necessitate CO2
injection to supply carbon, it is still important to do so in order to control the pH (Acién et al., 2016).
Nitrogen is present in wastewater in various forms, including ammonium (NH4+), nitrate (NO3-), nitrite (NO2-) and organically bound nitrogen (Wang et al., 2017). Microalgal dry mass contains around 7% nitrogen, distributed among proteins, enzymes, peptides, chlorophylls, genetic material (DNA, RNA) and energy transfer molecules (ATP, ADP) (Richmond, 2003). Nitrate, ammonia and urea are widely applied nitrogen sources for microalgal cultivation. Changes in nitrogen supply can potentially influence metabolic pathways, leading to altered composition of the microalgae. Bacterial nitrification-denitrification leads to some nitrogen being lost as nitrogen gas (N2) and minute amounts of NH3 may also escape by volatilization at elevated pH, temperature and mixing intensities (Wang et al., 2017).
Autotrophs assimilate dissolved phosphorous into intracellular energy transfer molecules, nucleic acids and nucleotides, cell membrane phospholipids, proteins (Richmond, 2003), and phosphorylated metabolic intermediates. Some cyanobacteria and eukaryotic coccal green microalgae can accumulate phosphate as polyphosphate granules.
Phosphorous is commonly removed from wastewater by trivalent metallic cation precipitation or by Ca2+, Mg2+, precipitation at high pH, as struvite and apatite (Wang et al., 2017). Dry microalgal biomass contains approximately 1% phosphorous (Becker, 2007).
Beside, wastewater chemical composition, microalgal growth can be affected by biotic factors, such as the presence of pathogens and competition by other microalgal species, and abiotic factors, such as temperature, light, pH, salinity and mixing (Gonçalves et al., 2017).
Optimal culture temperature will vary with type of media and microalgal strains used for culturing. The most common cultured species tolerates temperatures from 16 - 27 °C, where 18 - 20 °C is commonly utilized for culturing (Richmond, 2003). Light is essential for cultivation of
microalgae as it is the main source of energy. The intensity of illumination varies with depth and density of the microalgal culture.
High depth and cell density increase attenuation, however, if the light intensity is too high this can cause photo-inhibition or even overheating.
Typical light intensity used in laboratory studies range from 100 to 200 µE∙s-1∙m-2 (5-10% of full daylight) and diurnal cycles are often applied as many microalgal species do not grow well under constant illumination (Richmond, 2003).
While some microalgal species grow in acidic environments, the optimal pH for the cultivation of most species ranges from 7 to 9 (Posadas et al., 2015). Aeration and/or addition of CO2 can be used to control pH in cultures (Richmond, 2003). High partial pressure of CO2 can lead to acidification of culture conditions, inhibiting microalgal growth. A sufficient supply of carbon is vital for microalgal growth due to microalgal biomass carbon content of 50% (w/w) (Becker, 2007). pH affects microalgal carbon uptake. At pH values ranging from 5 to 7, CO2
is taken up through diffusion, while bicarbonate is taken up by active transport at pH values above 7 (Gonçalves et al., 2017). Agitation of microalgal cultures is essential to avoid sedimentation of microalgae.
Proper mixing provides illumination and enhances gas transfer between culture medium and the gaseous headspace (Richmond, 2003).