Keywords

8.1 Introduction

Due to the growth of the human population, people have begun to live in larger and more densely populated groups. Subsequently, the removal of human waste has become a serious issue. The lack of treatment plants for carbon, ammonia and nitrogen, as well as pathogen removal, is common in recent times due to the scarcity of land for treatment facilities which are mostly centralised with a broad range of consented pollutants. In a highly populated area, it is difficult to install individual treatment systems and this has led to the installation of pumps and pipes to efficiently transport waste to centralised outlets. Currently, European countries have set regulations to maintain the natural quality of wastewater streams without affecting biodiversity.

Domestic and industrial waste sewage discharges contain dissolved and suspended organic and inorganic constituents, faecal and other potentially pathogenic bacteria. Presently, wastewater has to be treated prior to disposal due to a number of reasons. For example, nutrients in wastewater such as nitrogen, phosphorus and sulphur pose a threat to the environment and its ecological sustainability. Besides that, wastewater also emits foul odours due to anaerobic digestion and can also cause potential health risks when it comes into contact with potable water. Reclaimed wastewater is now an important constituent of water supply (5–10%), which can be reused for non-drinking purposes (Wurochekke et al. 2016).

The operational characteristics of a common wastewater treatment plant consist of a few processes such as preliminary, physical, chemical and biological processes. However, the required effluent standard for the discharge of wastewater depends on the flow rate of receiving water. Wastewater preliminary treatment consists of two stages. First, a screening process will be carried out to reduce and remove coarse, medium and fine solids using different screen sizes. Second, the removal of grit and sand will occur in a grit chamber which settles dense materials easily and quickly. The organic matter remains suspended in downstream units for treatment (Al-Gheethi et al. 2015).

The purpose of primary treatment is to remove most of the suspended solids which thus reduces and regulates BOD (typically by 30%) before proceeding with the secondary treatment. This is achieved using gravitational settlement tanks where the residual part of solids is the raw primary sludge. The materials which possess less density than water such as detergents will move up to the surface of the sedimentation tank. They will be collected and removed prior to the next treatment. This process could be improved by using phosphate-precipitating chemicals like Al and Fe ions.

The secondary treatment is purposely designed to remove dissolved organic matter and other fine suspended organic matter. This treatment technique is predominantly controlled and enhances natural decomposition mechanisms in bioreactors. In this situation, the controlled condition ensures shorter treatment time. The organic matter is removed by bacteria that adapt to the environment such as ammonia oxidizers. The secondary process needs a sufficient supply of oxygen so that nitrifying microorganisms are kept in contact with the aerobic process. Besides the process described previously, there is a wide variety of secondary treatment processes available. The main ones include algal stabilisation ponds, land disposal systems, anaerobic reactors, activated sludge systems and aerobic biofilters (Jais et al. 2017).

Lastly, the tertiary treatment is designed to achieve low solid values and to remove pathogenic organisms, heavy metals and nutrients such as nitrogen and phosphorus. This tertiary treatment is a treatment process for wastewater that is expected to achieve the highest degree of effluent quality through the use of sand filters or UV lamps to remove or eliminate pathogenic organisms. These processes are capital intensive especially the backwashing of the sand filter. It requires large amounts of power. However, the chemical precipitation process is widely used for nutrient removal because biological nutrient removal organisms used in absorbing nutrients may not necessarily attain the consistency of chemical treatment even though they are more sustainable (Atiku et al. 2016).

Apparently, wastewater treatment processes are designed to achieve improvement in effluent quality. These processes perform well and meet the required quality standards. However, tertiary treatment such as nutrient removal faces some limitations due to the high costs it incurs. This is especially true for the removal of phosphorus.

For phosphorus, a normal 1 mg/L PO4 is required to meet 95% of the time to meet UWWTD or WFD standards which are challenging and capital intensive compared to traditional chemical processes. Achieving N and P effluent standards is the key to eutrophication control and prevention. In terms of eutrophication, P is considered as the most problematic element in natural waters that causes point source pollution as the average concentration is below 0.5 mg/L. The eutrophic condition of natural waters and microalgae concentration agree with the phosphorus level. An environment that receives more nitrogen than what plants require for growth is called a nitrogen-saturated environment. However, phosphorus in the environment is less soluble in water compared to nitrogen. Therefore, it is considered as a much more significant growth-limiting nutrient in the aquatic system.

Microalgae growth in water bodies can lead to unwanted effects. Though waters with abundant nutrients (N and P) known as eutrophic waters grow algae well, waters with a lower quantity of nutrients limit growth. The eutrophic environment is characterised by cool, dark and deep waters with depleted levels of oxygen, especially in temperate and tropical areas. On the other hand, algae decolourise the water which affects the aesthetic and recreational values of water. Algae also change the taste of water and produce another toxin that is harmful to higher forms of life in the food chain. Yet, they are considered a good bio-treatment technique in the control treatment process. This lack of oxygen in the water makes it difficult for aquatic animals to survive.

The effect of oxygen-depleted water forms a microbial breakdown from the sediment of dead algae and animal waste in environments with excess nutrients such as eutrophic waters. Human activity also affects lakes and rivers through the discharge of P from detergents to surface waters although some waters are already eutrophic in nature. Besides that, the upper layer of water in a stratified lake in autumn cools to a lower temperature less than the deep lower layer of a stratified stagnant lake where mixing occurs in the stratified layers. Thus, the oxygen-depleted water from the bottom rises to the surface replacing the cooler water. Consequently, aquatic organisms that need oxygen sometimes suffocate and die during this process.

Bio-treatment with microalgae is an appropriate method because of their photosynthetic capabilities, changing solar energy into biomasses yields and embracing phosphorus and nitrogen content which inflicts eutrophication (Abdel-Raouf et al. 2012).

This chapter focused on the phycoremediation process as a green technology for nutrient removal from greywater. The selection of the most potent microalgae strain and the process used to enhance removal efficiency are discussed.

8.2 Nutrient Removal Techniques from Wastewater

Biomass is another technique used in wastewater treatment. Microalgae and duckweed are used in lagoon wastewaters, while reed beds are commonly used for non-bacterial options. The presence of nutrients in wastewater makes it an ideal place for the optimal growth of microalgae. Artificial wetlands planted with specific species of reeds are mostly used in rural areas where there are no wastewater treatment facilities. This is a biological process that treats wastewater completely. They do not require chemicals, are inexpensive to run, produce reeds for compost after harvest and are very effective in treatment (See “Reed Beds for the Treatment of Domestic Wastewater, Grant and Griggs”). Wastewater lagoons are also used for the removal of N and P. Typically, wastewater lagoons are involved in the final stage in wastewater treatment system. Wastewater lagoons refer to large ponds or tanks planted with plants which possess properties for nutrient removal like duckweed where the removal efficiency is proportional to plant biomass growth. According to a previous study by Al-Nozaily et al. (2000), N and P absorption rates and the growth rate of duckweed at high ammonia levels were inhibited. However, the tank depth does not affect N removal and P removal is less than that of algae-based pounds (Arceivala 1981). It was noted that duckweed is considerably easier to harvest than algae.

Nitrification is a process whereby reduced forms of inorganic and organic nitrogen, particularly ammonium, are oxidised to nitrate. Ammonia‐oxidizing bacteria first oxidise ammonia to nitrite, followed by the oxidation of nitrite to nitrate by the nitrite‐oxidizing bacteria. The oxidising bacteria Nitrosomonas and Nitrobacter are the main organisms responsible for the reaction, however, the process produces acid and lowers pH. Therefore, alkalinity needs to be added. Hence, gaseous nitrogen is formed through denitrification by the biological conversion of nitrate.

Biological treatment amid other physical and chemical treatments has more advantages. Chlorine is added to the wastewater stream to oxidise ammonia–nitrogen, which later becomes gaseous nitrogen. Ammonia stripping occurs when pH increases up to 11.5 during the treatment process with plant growth when CO2 is reduced. The solution with high pH predominantly has ammonia as dissolved gas while enough air–water contact strips ammonia gas from the solution. Hence, the phycoremediation technique is viable for wastewater treatment.

8.3 Phycoremediation of Wastewater

The phycoremediation technique is defined as a bio-treatment process for wastewater using microalgae species. This process mainly aims to remove nutrients such as nitrogen and phosphorus from wastewater through the assimilation process of microalgae cells. The nutrients absorbed through the cell membrane of microalgae cells are transformed in the cytoplasm to be used in anabolic pathways (Atiku et al. 2016). The phycoremediation process is quite different from phycoremediation in which specific plants are used for the bioremediation of pollutants from wastewater. The potential of several microalgae species in the phycoremediation process of different types of wastewater has been reported in the literature. The phycoremediation process is recommended for the individual users due to the low operation costs, the absence of the toxic by-products, high efficiency in nutrient removal, increase in dissolved oxygen as well as the production of the microalgae biomass with high nutrient values. Besides, this process might also contribute to the inactivation of pathogenic bacteria due to the antibacterial properties of some microalgae species (Al-Gheethi et al. 2017).

Phycoremediation has been shown to exhibit high efficiency in the removal of nutrients from different types of the wastewater such as domestic, brewery and dairy wastewater (Mohamed et al. 2017). Therefore, it has high potential to produce high-quality-treated wastewater compared to those generated during the primary and secondary treatment processes (Abou-Shanab et al. 2013). Microalgae are photoautotrophic as they have the ability to obtain energy and carbon sources. However, these organisms still need some nutrients in form of nitrogen and phosphorus as well as trace elements for the metabolic and anabolic pathways. Hence, during the inoculation of microalgae in the wastewater, nutrients can be absorbed through the assimilation process. Moreover, microalgae growth in wastewater depends on environmental conditions. Therefore, the best option to achieve high efficiency in the removal of nutrients is by using indigenous strains since these strains have been acclimatised to the surrounding conditions and are able to survive and compete with other indigenous bacteria in greywater. Several species of microalgae such as Scenedesmus dimorphic, Botryococcus braunii, Chlorella vulgaris, Spirulina sp., and Phormidium sp., have exhibited high efficiency in the phycoremediation process due to their potential to tolerate harsh environmental conditions. Another effective option is by subjecting the microalgae to the starvation process in which the microalgae are harvested from the culture medium and then dried at room temperature. The deficiency in nutrients would induce the microalgae cells to transform from a vegetative state to form cysts (dormant state). Thereafter, the microalgae inoculum used in the phycoremediation process of greywater added as a dry biomass. The starvation process makes the microalgae biomass more effective in the uptake of nutrients and accelerates the removal process (Mohamed et al. 2017).

8.4 Controlled Eutrophication of Microalgae for Phycoremediation

To control the release of wastewater with high nutrient content especially for P, a standard limit needs to be achieved to prevent effluent from of causing eutrophication in water bodies. The growth of microalgae indicates water pollution since they typically respond to many types of ions and toxins. Green algae play a dual role in the treatment of wastewater as they enable the effective utilisation of different constituents essential for growth which further leads to enhanced biomass production for green products (Rawat et al. 2011).

A controlled environment can be a viable place for microalgae isolated specifically to grow in wastewater in which they consume P and other pollutants for their survival which is termed as phycoremediation. They are used for the removal of pollutants prior to discharge into rivers and lakes. These microalgae are not discharged together with the treated water as they are used for the continuous flow treatment of wastewater and are often harvested to sustain the optimal value of microalgae concentration. The collected biomass has the potential to produce valuable products such as biofuel, biodiesel and fertilisers due to the high content of carbon and nutrients.

Microalgae absorb a large amount of atmospheric CO2 and release oxygen during its growth period. For additional microalgae growth, additional CO2 is required though it affects the condition of bacterial growth. Waters that contain microalgae give out foul odour and may cause a change in how they taste. Nevertheless, freshwater microalgae have absorption abilities where they utilise nutrients and metals found in wastewaters for photosynthesis and thus purifies the water. However, this purified water is not suitable for drinking purposes until it undergoes natural purification after being discharged into rivers and lakes. Furthermore, the water can be drinkable if it undergoes another treatment process using activated carbon to further remove dissolved organic pollutants.

Furthermore, the role of microalgae is to accumulate and convert wastewater nutrients into biomass and lipids. The capability of microalgae to remove or degrade hazardous organic pollutants is well known (Abeliovich 1986). The usage of microalgae is summarised in Table 8.1. The examples given demonstrate that algae are indeed capable of contributing to the degradation of environmental pollutants, either by directly transforming the pollutant in question or by enhancing the degradation potential of the microbial community present. The biomass resulting from the treatment of wastewater can be easily converted into sustainable products. Depending on the species used for this purpose, the resulting biomass can be applied and used in many different ways. For instance, it can be used as an additive for animal feed as well as for the extraction of value-added products like carotenoids, bio-molecules or the production of biofuel. Microalgae biomass is, therefore, useful for both biofuel production as well as wastewater treatment. The successful implementation of this strategy would allow the use of freshwater to be minimised.

Table 8.1 Microalgae contributing to the degradation of environmental pollutants
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8.5 Wastewater Treatment Potential of Microalgae

Microalgae have the potential to survive in a broad range of environmental conditions. Therefore, it represents a good source of biomass (Xin et al. 2010; Xu et al. 2013). The utilisation of wastewater as a production medium for microalgae biomass leads to a reduction in nitrogen and phosphate content (Mata et al. 2010; Xin et al. 2010). In addition, microalgae biomass can be used as feedstock for many industries (Spolaore et al. 2006; Harun et al. 2010).

It has been argued that biodiesel production with wastewater treatment is the most reasonable area to be commercialised in the short term. They provide a way to remove chemical and biological contaminants, heavy metals and pathogens from wastewater while producing biomass for biodiesel production. Wastewater rich in CO2 provides a conductive growth medium for microalgae because CO2 balances the wastewater by allowing higher microalgae production rates, reduced nutrient levels in the treated wastewater, decreased harvesting costs and increased lipid production (Brennan and Owende 2010).

Microalgae have the ability to capture sunlight and use that energy to store carbon. According to Alabi et al. (2009) the amount of carbon found in microalgae biomass is about 45–50%. Algae can have a doubling time of as little as 4 h to accumulate biomass. The microalgae growth will be limited if the culture is being supplied with CO2 from the air because the amount of CO2 found in air is small (0.033%). Therefore, extra CO2 can be mixed with air and later added into the culture to facilitate microalgae growth (Mata et al. 2010). The efficiency of CO2 can be improved by introducing deep level subaquatic through air stones, or through CO2-rich industrial flue gas into the cultures.

In the photosynthesis process, O2 is being generated and released into the air. However, if the O2 gas concentration increases and gets trapped within the cell, it will cause damage to the chlorophyll reaction centres thus leading to a decrease in the production of biomass (Alabi et al. 2009). This problem may occur only in a closed system such as PBRs. In order to avoid such problems, a gas exchanger is needed to help release the O2 gas. In open systems, however, this is not necessary because the O2 gas can be automatically released into the atmosphere (Mata et al. 2010).

According to a previous study by Sawayama et al. (1995), Botryococcus sp. was used to remove nitrate and phosphate from sewage after primary treatment along with the production of hydrocarbon-rich biomass while Martinez et al. (2000) achieved a significant removal of phosphorus and nitrogen from urban wastewater using the microalgae Scenedesmus obliquus. They were able to eliminate 98% of phosphorus and completely remove ammonium (100%) in a stirred culture at 25 ℃ over 94 and 183 h retention time, respectively. To further strengthen the potential of microalgae in wastewater treatment, Hodaifa et al. (2008) recorded a 67.4% reduction in BOD5 with S. obliquus cultured in diluted (25%) industrial wastewater from olive-oil extraction. The percentage of elimination reduced to 35.5% with undiluted wastewater because of low nitrogen content which inhibited microalgae growth during the exponential phase.

In addition, Yun et al. (1997) successfully grew Chlorella vulgaris in wastewater discharged from a steel plant to achieve an ammonia bioremediation rate of 0.022 g NH3 per day. Munoz et al. (2009) found that the use of a biofilm attached to the reactor walls of flat plate and tubular photobioreactors improved BOD5 removal rates by 19 and 40%, respectively, as compared with a control suspended bioreactor for industrial wastewater effluent. The retention of algal biomass showed remarkable potential in maintaining optimum microbial activity while remediating the effluent. To absorb heavy metal ions, Chojnacka et al. (2005) found that Spirulina sp. acted as a biosorbent where the biosorption properties of microalgae depended strongly on cultivation conditions with photoautotrophic species showing greater biosorption characteristics.

Mohamed et al. (2017) used Botryococcus sp. for the removal of BOD5, COD, ammonia (NH3), nitrate (NO3−) and orthophosphate (PO43−) as well as K, Ca and Na ions from bathroom greywater by using a photo-reactor system at village houses. The phycoremediation process was conducted for 21 days. The study revealed that Botryococcus sp. reduced BOD5, by 85.3 to 98%, COD by 71.22 to 85.47%, NH3 by 86.21 to 99% and PO43− by 39.12 to 99.3% after 21 days. The reduction of NO3− was recorded after 18 days of the treatment period at an efficiency of 98%. Moreover, Botryococcus sp. removed 97% of K after 3 days and 95% of Ca at the end of the phycoremediation process. Al-Gheethi et al. (2017) claimed that the kinetic coefficient of Scenedesmus sp. for the removal of NH4− was k = 4.28 mg NH −14 log10 cell mL−1 d−1 with 94% of the coefficient and km = 52.01 mg L−1 (R2 = 0.94) while the removal of NH4− was k = 1.09 mg NH4− log10 cell mL−1 d−1 for PO43− and km = 85.56 mg L−1, with 92% of the efficiency. Both studies by Al-Gheethi et al. (2017) and Mohamed et al. (2017) indicated that microalgae species have high potential in the removal of the nutrients from wastewater. Most of the microalgae species exhibited more effectiveness in the removal of nitrogen compared to phosphorus. This is due to the role of nitrogen in anabolic activities and the building of amino acids. Therefore, nitrogen represents between 7 and 10% of the microalgae cells by dry weight (Richmond 2004). Phosphorus is also necessary for the microalgae genome (DNA and RNA), phospholipids, and ATP (Yao et al. 2015). The removal percentages for different wastewater parameters via the phycoremediation process using microalgae species are presented in Table 8.2.

Table 8.2 Removal of pollutants from different wastewater samples via the phycoremediation process using microalgae species
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Temperature condition has a strong correlation with the biochemical reactions which affect microalgae growth as maximal productivity can only be achieved when nutritional needs are met at correct optimal temperatures (Richmond 2004). Temperature optimal for the growth of microalgae are usually between 20 and 30 ℃ while temperatures lower than 16 ℃ decrease growth rate as many microalgae die at temperatures above 35 ℃. There is a relationship between temperature and light intensity since lamps and sunlight emit heat (Richmond 2004).

The polyunsaturated fatty acids within the membranes and the fluidity of the membrane system increase at low temperature and this is essential for protecting the thylakoids and the photosynthetic machinery of microalgae cells from photoinhibition. Therefore, lipid classes and composition are affected by temperature instead of the total lipid content. However, in any algae media, the value of the pH that is suitable for cultivation ranges from 6 to 8 (Zeng et al. 2011). Although each media may have a different pH value since the values are not fixed during the cultivation process, it can be changed. Whitacre (2010) stated that the general pH value for most freshwater microalgae species ranges between 7 and 9 but the optimum pH is around 7.5. Microalgae species seem to be more tolerant of the broad range of pH values (Lam and Lee 2012). The growth of Botryococcus species has shown its tolerance to different pH conditions ranging from 6 to 8.5. The genus Botryococcus race A strains has been observed to yield biomass with high lipid content (15–35% by dry weight) and when the fatty acids were analysed, the oleic, linoleic, stearic and palmitic acids were the major fatty acids found with traces of pharmaceutically important alkyl substituted fatty acids such as 12-methyl hexadecanoic acid, 14-methyl tetradecanoic acid and 16-methyl heptadecanoic acid (Jin et al 2005). Thus, the algae Botryococcus appears to be a potential organism for lipid-rich biomass production under varying pH conditions. The failure to maintain the correct pH can lead to the slow growth of microalgae or eventual culture collapse because pH can affect the availability and solubility of CO2 and minerals in the medium.

Therefore, one of the most important points in the phycoremediation process is the source of microalgae species. Microalgae species obtained from fresh water might be suitable to be used for the phycoremediation of wastewater. Laboratory observations indicated that most microalgae obtained from freshwater survive and grow in various type of wastewater. Based on the literature, it appears that most microalgae have high efficiency in reducing nitrogen and phosphorous in wastewater during the phycoremediation process. Therefore, phycoremediation as a treatment process could enhance the quality of wastewater which would then be reused for unrestricted irrigation purposes or discharged into surface waters.

8.6 Conclusions

Microalgae species possess high potential to be used in the phycoremediation process of greywater. However, the selection of microalgae strains should be considered based on its ability to survive, grow and remove nutrients from waste. Moreover, the use of indigenous strains might be the best option to achieve high effectiveness for the phycoremediation process. Moreover, the starvation process might enhance the efficiency of the phycoremediation process within a short time.