Performance evaluation of novel attached-growth high rate algal pond system with additional artificial illumination for wastewater treatment and nutrient recovery

AIT

Asian Institute of Technology

AG-HRAP

attached-growth high rate algal pond

COD

chemical oxygen demand

CO₂

carbon dioxide

DO

dissolved oxygen

HRAP

high rate algal pond

HRT

hydraulic retention time

LED

light emitting diode

NH₃-N

ammonia nitrogen

NO₂-N

nitrite nitrogen

NO₃-N

nitrate nitrogen

NLR

nitrogen loading rate

OF

optical fiber

OLR

organic loading rate

O₂

oxygen

PLR

phosphorus loading rate

TKN

total Kjeldahl nitrogen

RAB

revolving algal biofilm

TN

total nitrogen

TP

total phosphorus

TSS

total suspended solids

Domestic wastewater generated from households is a major problem cause of water pollution in developing counties. In particular, wastewater discharged from toilets contain high levels of organic matter and nutrients (Polprasert & Koottatep 2017). Direct discharging of untreated toilet wastewater leads to various adverse ecological effects and climate change issues, such as eutrophication, low DO levels in water bodies, changing growth rates of aquatic life and depreciation of water quality (Glibert 2017).

To overcome these issues, due to low investment, operation and maintenance costs compared with other treatment systems, the HRAP system, an algal-based wastewater treatment technology employing microalgal-bacterial symbiotic reactions, has been widely applied to reduce both organic matter and nutrients in wastewater. In addition, biomass is produced, which is a way of recovering valuable bio-products such as protein and nutrients (Polprasert & Koottatep 2017).

In the HRAP system, microalgae and bacteria contribute to the production of O₂ and CO₂, respectively. The O₂ produced from photosynthetic processes of microalgae promotes oxidation of organic matter and ammonia by heterotrophic and autotrophic bacteria (nitrifying bacteria), respectively. The CO₂ released from the bio-degradation of organic matter in wastewater is utilized by the microalgae (Chaiwong et al. 2018).

Recently, many types of HRAP have been used for both wastewater treatment and biomass production (Uggetti et al. 2018). However, HRAP systems have some limitations, such as a large land area requirement, instability of light illumination, low light distribution, low biomass and protein production (Chen et al. 2008; Doma et al. 2016). Therefore, to minimize those limitations, a novel AG-HRAP system was developed in this study by increasing pond depth, employing attached media and additional artificial light sources.

Hence, this study aimed to evaluate the performance of the novel AG-HRAP in various operating conditions to treat domestic wastewater and enhance nutrient recovery from microalgal biomass. Furthermore, observation of microalgal species in the AG-HRAP under the optimum conditions were undertaken in this study.

In this study, microalgal inoculum (Chlorella sp.) was obtained from Chulalongkorn University, Bangkok, Thailand. Untreated domestic wastewater collected from an academic building in the AIT campus, Phathumtani, Thailand, was used as influent wastewater.

Three AG-HRAP units were made out of cement, each with a configuration of 0.46 × 0.92 × 1.00 m (width × length × depth) and a working volume of 400 L (Figure 1(a)). Each AG-HRAP was equipped with a baffle and paddle wheels to prevent short-circuiting and maintain mixing conditions, respectively (Figure 1(a)). Attached-growth media, made of nylon mesh with a surface area of 0.23 m²/piece, were placed into each AG-HRAP, which had a specific surface area of 9.2 m²/m³ (Figure 1(b)). The outdoor AG-HRAPs were equipped with combined OF and red-blue LED lamps (Figure 1(c)) (Bogdan^®, Bogdan LED – J&Y Trading, Thailand) to continuously illuminate the entire mixed liquor of the AG-HRAP units and therefore promote the photosynthetic reactions of the microalgae. The light intensities inside the AG-HRAPs were controlled with dimmer switches. In this study, the light intensities were measured as photosynthetic photon flux density (PPFD).

The untreated AIT domestic wastewater was prepared three times a week by mixing it with blackwater to give an N:P ratio of 7:1 and a COD concentration of about 800 mg/L, corresponding to high-strength domestic wastewater (Metcalf & Eddy et al. 2014). This was fed to the AG-HRAP units. The influent wastewater characteristics are shown in Table 1.

Microalgal-bacterial biofilm was cultured in the AG-HRAP units according to Chaiwong et al. (2018). The three AG-HRAP units were continuously illuminated with light intensities of 40 ± 18, 160 ± 18 and 180 ± 15 μmol/m²/s, respectively, and under ambient temperature (∼30 °C), for 14 days to allow microalgal growth (Lee et al. 2014). After that, the AG-HRAP units were continuously fed with the influent wastewater until steady conditions, based on relatively constant effluent COD concentrations, were obtained.

The AG-HRAP units were continuously fed by a peristaltic pump at the average flow rates of 100, 50, and 27 L/d to achieve HRTs of 4, 8, and 15 days, respectively, corresponding to the OLRs, NLRs, and PLRs of 243, 90, and 55; 15, 12, and 5; and 3, 2, and 1 mg/L/d, respectively (Table 2). During the operating periods, the light intensities in the AG-HRAP units were maintained at 40, 160 and 180 μmol/m²/s to promote growth of algae (Table 2). Furthermore, to prevent biomass settlement and promote better contact between the influent wastewater and the microalgal-bacterial biomass, the rotating speed of the paddle wheel in each AG-HRAP was maintained at 10 rpm, resulting in a mixing velocity of 15 cm/sec and homogeneous water content in the AG-HRAPs (Polprasert & Koottatep 2017).

Temperature, pH, DO and light intensity were measured on-site using a temperature/pH meter (Mettler Toledo AG 2009), DO probe (Mettler Toledo S4-Standard Kit Seven2Go), and quantum flux meter (Apogee MQ-200), respectively. Light intensity was measured at three points (0.05, 0.35 and 0.65 m) down the depth of the AG-HRAP.

Influent and effluent samples were collected twice a week for analysis of COD, TSS, TKN, NH₃-N, NO₂-N, NO₃-N and TP concentrations at AIT's Environmental Engineering and Management laboratory in accordance with the Standard Methods for Examination of Water and Wastewater (APHA 2012).

Both the attached-growth and suspended-growth biomass of the AG-HRAP units were measured three times a week as TSS. A sharp-edged tool was used to manually scrape the attached-growth biomass from the attached media with a certain scraped area (m²) and then diluted in 50 mL deionized water. To sample the suspended biomass 50 mL was taken from three depths of the AG-HRAP units (0.05, 0.35 and 0.65 m from the surface). Both of attached-growth and suspended-growth biomass samples were filtered through glass microfiber filters (GF/C, Whatmann, 0.45 mm). The cells retained on the filter paper were dried at 105 °C for 4 hours (Lee et al. 2014).

Analysis of chlorophyll a concentration in the harvested biomass was done in accordance with the Standard Methods for Examination of Water and Wastewater (APHA 2012). About 25 mL of the harvested biomass was centrifuged at 5,000 rpm for 5 min. Chlorophyll a in the biomass was extracted with 90% aqueous acetone in a tissue grinder. The extracted samples were kept in the dark at 5 °C for 3 hours and then measured at three wavelengths: 664, 647, and 630 nm, using a spectrophotometer (UV-VIS Spectrophotometer, UV-1700 PharmaSpec, SHIMADZU, Japan). The results were used to determine chlorophyll a concentration (mg/L) according to the Standard Methods for Examination of Water and Wastewater (APHA 2012).

Algal species of the attached-growth and suspended-growth biomass samples were identified by using the microscopic method (APHA 2012) at AIT's Environmental Engineering and Management laboratory.

The experimental data of this study were tested for their statistical significance by using analysis of variance (ANOVA, SPSS (V.23.0)).

At the steady-state condition in AG-HRAP#1, AG-HRAP#2 and AG-HRAP#3, the pH values were 7.4 ± 0.5, 7.8 ± 0.5, and 8.2 ± 0.6, respectively, the temperatures were 31 ± 2, 32 ± 2, and 32 ± 2 °C, respectively, and the DO concentrations were 4 ± 1, 6 ± 1, and 7 ± 1 mg/L, respectively.

Light was considered to be a main factor influencing microalgal growth and enhancing biomass production. Light penetration in the AG-HRAP units was measured to express the advantages of using additional artificial illumination to enhance biomass productivity. Due to a combination of sunlight and the artificial light source, and because sunlight could not penetrate below 0.30 m in most raceway ponds (Chisti 2007; Lee et al. 2014), the light intensities of the top layer (water depth 0.05 m) of the AG-HRAPs were observed to be highest (Table 3). The light intensities of the middle and bottom layers (water depth 0.35 and 0.65 m, respectively) of the ponds were lower than those of the top layer. Sforza et al. (2012) reported that a high microalgal growth rate was obtained with light intensities greater than 120 μmol/m²/s. The attached-growth system improved light penetration and promoted microalgal growth at the bottom of the AG-HRAP units. Table 3 shows the average light intensities of AG-HRAP#1, AG-HRAP#2 and AG-HRAP#3 to be 40 ± 18, 160 ± 18 and 180 ± 15 μmol/m²/s, respectively, corresponding to the results of biomass productivity in the AG-HRAP units. The additional artificial light illumination increased light penetration and allowed sufficient light intensity for microalgal cultivation at greater water depths.

The experimental results of COD, TN and TP removal efficiencies, shown in Figure 2, were found to increase with increasing HRT. The highest COD removal of 88 ± 8% (Figure 2(a)) occurred at the HRT of 15 days, resulting in the effluent COD concentration of less than 100 mg/L, meeting Thailand's national discharge standard (PCD 2010). The TN removal efficiencies of 22 ± 12, 52 ± 2 and 62 ± 9% were found in the AG-HRAP#3 at the HRTs of 4, 8, and 15 days, respectively, which were higher than those of AG-HRAP#1 and AG-HRAP#2 (Figure 2(b)). Similarly, the highest TP removal efficiency of 69 ± 8% was found in AG-HRAP#3 at the HRT of 15 days, higher than those of AG-HRAP#1 and AG-HRAP#2 (Figure 2(c)). Due to the relatively neutral pH found in the AG-HRAP units, it could be anticipated that the TN and TP concentrations in the influent were mainly removed via algal-bacterial biomass assimilation and nitrification/denitrification reactions, rather than ammonia volatilization and phosphorus precipitation, respectively (Sawyer et al. 2003), but further studies in this aspect are strongly recommended.

Because lighting conditions could affect algal growth in AG-HRAP units, AG-HRAP#3, using the highest light intensity of 180 μmol/m²/s, gave higher TN and TP removal efficiencies than AG-HRAP#1 and AG-HRAP#2 (Figure 2(b) and 2(c)), corresponding to the results of biomass productivities and abundance of algal species among the AG-HRAP units (Figures 3 and 4, respectively). The data of Figure 2 were used to determine kinetic constants for design and operation of the AG-HRAP system.

The highest biomass and protein productivities of 54 ± 4 and 37 ± 8 g/m²/d, respectively, were found in the AG-HRAP#3, which were higher than those of the other two units (Figure 3), corresponding to the results of the TN and TP removal efficiencies shown in Figure 2(b) and 2(c), respectively. Moreover, the highest attached biomass productivity of this study (54 ± 4 g/m²/d) was higher than those of previous studies, which were 5–41 g/m²/d (Table 4) (Shelef. 1982; Wood et al. 1988; Gross & Wen. 2014; Lee et al. 2014). Therefore, applying additional artificial light illumination in the AG-HRAP system was beneficial for enhancing microalgal growth and biomass productivity. Correspondingly, the highest chlorophyll a concentration of 13 mg/L were found in AG-HRAP#3. The suspended biomass productivities were found to be only about 8% of the total biomass in the three AG-HRAP units (Figure 3). Hence, these results suggest the potential benefits of the novel AG-HRAP in wastewater treatment and biomass and protein productivities. The latter could be reused in animal feeds, fertilizers or some food additives, etc.

The major algal species in all AG-HRAP units was found to be Chlorella sp., the same as the inoculum (Figure 4). Due to the raw wastewater used as the influent, other contaminant algal species i.e. Scenedesmus sp. and Oscillatoria sp. could also be observed in the systems (Polprasert & Koottatep 2017). There were no substantial differences in the algal species among the three AG-HRAP units. The presence of these algal species suggests the applicability of the novel AG-HRAP system for producing useful algal species to perform photosynthesis which produces O₂ for the heterotrophic bacteria to biodegrade organic matter, and the algal biomass could be further reused.

The Stover-Kincannon kinetic model was applied in this study to determine kinetic constants for COD, TN, and TP removal in the AG-HRAP units (Table 5). In this study, the kinetic constants of U_max and K_B for COD removal in the AG-HRAP were found to be 121, 107; 222, 232 and 74, 52 mg/L/d in AG-HRAP#1, AG-HRAP#2 and AG-HRAP#3 (R² = 0.99), respectively, which were higher than the U_max and K_B for COD removal of 19.74–34.68 and 22.4–27.9 mg/L/d, respectively, of the conventional waste stabilization pond (Gratziou & Chalatsi 2016). Probably because COD removals in the AG-HARPs were achieved by biological degradation of the heterotrophic bacteria (Chaiwong et al. 2018), the obtained results of the COD removal constants (Table 5) were not affected by the applied light intensities and the observed biomass productivity in the systems. Additionally, the highest U_max and K_B constants for TN and TP removals of 9, 16 and 1.8, 2.7 mg/L/d were found in AG-HRAP#3, with an R² of more than 0.9 (Table 5). Because there was not only biomass assimilation but also TP removal by other processes, i.e. precipitation or sedimentation (Chaiwong et al. 2018), there were no significant differences among the TP removal constants of the AG-HRAP units. Consequently, the calculated kinetic constants for COD, TN and TP removals could be applicable for the design of AG-HRAPs to achieve desired effluent standards.

The results for energy consumption and cost effectiveness of the AG-HRAP systems are shown in Table 6. The land area requirement for wastewater treatment (m²/m³) of each AG-HRAP unit was 1 m²/m³, which was lower than those of other microalgal systems (1.6–36 m²/m³) (Chauton et al. 2015; Chisti 2016; Asiedu et al. 2018). Additionally, the energy consumption for biomass production of AG-HRAP#1, AG-HRAP#2 and AG-HRAP#3 were found to be 0.51 ± 0.32, 22.78 ± 14.09 and 7.71 ± 4.77 kWh/kg/d, corresponding to the biomass production costs of 0.39 ± 0.03, 2.07 ± 0.82 and 0.81 ± 0.38 USD/kg/d, respectively. The revenues for the produced biomass of AG-HRAP#1, AG-HRAP#2 and AG-HRAP#3 were estimated to be 7.03 ± 0.22, 6.83 ± 1.32 and 10.32 ± 1.82 USD/kg/d, respectively, resulting in the benefit/cost ratios of 14 ± 0.6, 3 ± 0.6 and 11 ± 2.3, respectively. These benefit/cost ratios of the AG-HRAP systems were significantly higher than those of other microalgal systems, which were 0.002–10.04 (Chauton et al. 2015; Chisti 2016; Asiedu et al. 2018). Therefore, it could be suggested that the AG-HRAP system is of value for treating wastewater while simultaneously being highly cost effective in terms of system operation.

The novel AG-HRAP units were found to be effective for wastewater treatment: the treated effluent could meet Thailand's national discharge standards when operated at an HRT of 15 days. Enhanced biomass and protein productivities were achieved in the AG-HRAP units equipped with the additional artificial light sources, and these could be further reused as animal feed, fertilizers, and food additives. The Stover-Kincannon model was found to be applicable for determining the kinetic constants for COD, TN and TP removals, which can be used for the design of the AG-HRAP system treating wastewater. The major algal species in all AG-HRAP units were found to be Chlorella sp., Scenedesmus sp. and Oscillatoria sp., suggesting the applicability of the novel AG-HRAP system in wastewater treatment and biomass production. The operation of the novel AG-HRAP system could be very cost effective when compared to other microalgal systems.

This research was financially supported by Bangchak Initiative and Innovation Center (BIIC), the Royal Thai Government and the Asian Institute of Technology, for which grateful acknowledgements are made.

All relevant data are included in the paper or its Supplementary Information.