Improved Algal Sludge Methane Production and Dewaterability by Zerovalent Iron-Assisted Fermentation.

This study investigated the methane production improvement of algal sludge by zerovalent iron (ZVI)-assisted anaerobic digestion. The zerovalent iron added were 0.5, 2, 5, 10, and 20 g·ZVI/g·TS (total solid). The results indicated that the addition of ZVI at 2, 5, 10, and 20 g·ZVI/g·TS has improved the methane production 1.07, 1.24, 1.41, and 1.46 times as compared with no ZVI added. The dewaterability of treated algal sludge has improved 1.06, 1.08, 1.08, and 1.11 times as compared with no ZVI addition. The biochemical methane production test results fitted to both one-substrate and two-substrate models. The one-substrate model indicated that the hydrolysis rate k has increased 8.21, 7.07, 9.39, 3.50, and 5.07 times as compared with R1 where no ZVI was added. The two-substrate model implied that the rapid hydrolysis rate k rapid values were 5.23, 4.5, 5.98, 2.23, and 3.23 times as compared with R1. The one-substrate model predicted that the value of methane production was in high correlation with the actual value (R 2 > 0.98). The addition of ZVI in algal sludge for methane production without an extra pretreatment process has improved the hydrolysis rate and methane production. This has the potential to be developed as an effective and economic technology in resource recovery from algal sludge.

Introduction

Algal sludge collected from the algal bloom lakes is an important environmental pollution problem where the treatment and resource recovery of this waste algal sludge has aroused wide concern.[1−3] Anaerobic digestion is a promising process in using algae biomass for producing methane biogas; however, the methane production rate is relatively low due to the low biodegradability of algae.[4,5] Pretreatment technologies such as mechanical, ultrasound, microwave, thermal, chemical, biological, and combined processes have been investigated in improving the methane production from algae anaerobic digestion.[6,7] Bai et al. reported that using free nitrous acid (2.31 mg HNO2-N·L–1) as a pretreatment process has improved the algae methane production yield from 161 to 250 L·CH4/kg·VS added.[8] Wang et al. found that free ammonia at 60–530 mg NH3-N/L has significantly improved the algae solubilization and the methane generation during anaerobic digestion.[5] Keymer et al. demonstrated that a high-pressure thermal hydrolysis process has increased the algae yield by 81%.[9] Marsolek et al. achieved an increase of biogas production from 0.28 to 0.39 L biogas per g volatile solids by thermal pretreatment at 90 °C.[10] All those pretreatment processes were used to improve the biodegradability or hydrolysis rate of the algae by disrupting the algae cell wall.

Zhen et al. reported that zerovalent scrap iron (ZVSI) stimulated the anaerobic digestion of sludge with the methane yield increased by 38.3% where the ZVSI has enhanced the methanogenesis as electron donors and accelerated the hydrolysis–acidification and methanation steps of the wasted activated sludge (WAS).[11] It is also reported that the addition of zerovalent iron (ZVI) could accelerate the anaerobic digestion of sludge. Feng et al. achieved a methane production increase of 43.5% by using ZVI where the degradation of protein and cellulose was enhanced.[12] Yang et al. investigated the nano ZVI on methanogenic activity during the anaerobic digestion, and the results indicated that ZVI at 30 mM increased methane production, while nano ZVI inhibited the methanogenic growth and methane production at concentration of 1 mM and above.[13] Suanon et al. reported that the nanoscale ZVI and iron powder addition in sludge anaerobic digestion has enhanced the methane yield up to 25 and 40%, respectively.[14] Zhang et al. indicated that the ZVI could enhance the methanogenic activity in the anaerobic sludge digestion.[15] The total solid (TS) and total chemical oxygen demand (TCOD) of blue algae harvested from the algal bloom lakes have quite similar values as compared with the activated sludge that was used for anaerobic digestion. Microalgae contains a high portion of organic components including ash (5–17%), carbohydrate (18–46%), crude protein (18–46%), crude lipid (12–48%), and energy (19–27 MJ/kg).[16] This implied that the ZVI, which was effectively applied in enhancing the methane production from WAS anaerobic digestion, could also be effective in the algal sludge methane production.

This study innovatively investigated the potential of methane production improvement for algal sludge anaerobic digestion by the assistance of ZVI. Biochemical methane production tests were used for accumulative methane production analysis for the ZVI-assisted algae fermentation under different ZVI dosages. The methane production potential and hydrolysis rate of ZVI-assisted algal sludge digestion were analyzed by one-substrate and two-substrate mathematical models. Economic analysis was conducted to assess the economic potential of the proposed algae anaerobic digestion process.

Materials and Methods

Algal Sludge and Inoculum Sludge

The algal sludge used in this study was Microcystis sp. It was harvested from Guanqiao Base of Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China. Over 98% of the algal sludge was Microcystis by the microscopic examination where the TS was 11.28 ± 0.24 g/L and the volatile solid (VS) was 9.23 ± 0.23 g/L. The inoculum sludge used was collected from the Sanjintan wastewater treatment plant (WWTP) anaerobic fermenter, Wuhan, China. A mesophilic anaerobic digestion process is used in the Sanjintan WWTP for wasted sludge digestion. The characteristics of inoculum sludge, such as TS, VS, TCOD, soluble chemical oxygen demand (SCOD), and pH, were shown in Table .

Table 1

Characteristics of the Algal Sludge and Seed Sludge Used in this Study

parameters	algal sludge	seed sludge
TS (g/L)	11.28 ± 0.24	32.05 ± 1.01
VS (g/L)	9.23 ± 0.23	12.71 ± 0.01
TCOD (g/L)	11.28 ± 0.64	15.3 ± 0.19
SCOD (mg/L)	323.58 ± 48.58	400.25 ± 32.75
pH	7.19 ± 0.01	6.36 ± 0.02

Zerovalent Iron Addition

The algal sludge was added with ZVI at concentrations of 0, 0.5, 2, 5, 10, and 20 g·ZVI/g·TS of the algal sludge before anaerobic digestion. The ZVI powder applied is of analytical grade with purity of 98% and size of 0.147 mm (Kefeng Ltd., Shanghai, China). The biochemical methane potential (BMP) tests were conducted immediately after addition of ZVI into the algal sludge.

Biochemical Methane Potential Test

BMP tests were conducted to assay ZVI-assisted algal sludge anaerobic digestion methane production. The algae and inoculum were transferred to corresponding reactors at an initial VS ratio of 1:2. The reactor total volume is 310 mL in which 105 mL of inoculum and 70 mL of algal sludge were added. The ZVI powder, seed sludge, and algae were totally mixed and flushed with pure N2 gas for 5 min where anaerobic conditions were created. A blank test was also conducted where the seed sludge (105 mL) and MilliQ water (70 mL) instead of the algae were added in the reactor. The BMP test was thus started with the reactors sealed by a rubber stopper and shaken at a 37 ± 1 °C constant-temperature incubator. The batch tests were conducted in triplicate in each of the incubators for 57 days until the pressure increase in the reactors dropped to a negligible level. The pressure in each reactor was measured, and the biogas production from the reactors was collected every 2–4 days. The net biogas generation from the algae in each reactor was determined by subtracting the biogas produced by the blank reactor from each reactor. The methane production was calculated based on the multiplication of net biogas pressure increment in the reactor and the methane concentration produced. The methane production was recorded as the volume of methane produced per kg of the TS of the total algae and inoculum added in the reactor (L·CH4/kg·TS). The accumulative methane production was the summation of methane produced per day in the corresponding reactor. The BMP test experimental conditions were shown in Table .

Table 2

Experimental Design with Different ZVI Dosages

reactor	function	experimental conditions
R0	blank	105 mL of seed sludge + 70 mL of MilliQ water
R1	ZVI-0	105 mL of seed sludge + 70 mL of algal sludge
R2	ZVI-0.5	105 mL of seed sludge + 70 mL of algal sludge + 0.5 g·ZVI/g·TS
R3	ZVI-2	105 mL of seed sludge + 70 mL of algal sludge + 2 g·ZVI/g·TS
R4	ZVI-5	105 mL of seed sludge + 70 mL of algal sludge + 5 g·ZVI/g·TS
R5	ZVI-10	105 mL of seed sludge + 70 mL of algal sludge + 10 g·ZVI/g·TS
R6	ZVI-20	105 mL of seed sludge + 70 mL of algal sludge + 20 g·ZVI/g·TS

Mathematical Modeling Analysis of BMP Test Results

The hydrolysis rate (k) and biochemical methane potential (B0) are two key parameters associated with the methane generation. Two models including one-substrate model and two-substrate model were used to simulate the BMP test results. The models were as shown in eqs and 2.

As shown in eqs and 2, the B(t) is the cumulative methane production at day t (L·CH4/kg·TS, t = time (day)). Yt (L·CH4/kg·TS) is the cumulative methane production in day t by the corresponding simulated equations and 2. In the one-substrate model, B0 is the biochemical methane potential (L·CH4/ kg·TS), and k is the hydrolysis rate (day–1). In the two-substrate model, the algae were considered as consisted of rapidly biodegradable components and slowly biodegradable substrates. B0,rapid is the rapidly biodegradable substrates’ biochemical methane potential (L·CH4/ kg·TS), krapid is the hydrolysis rate of the rapidly biodegradable substrates (day–1), B0,slow is the biochemical methane potential of the slowly biodegradable substrates, and kslow is the slowly biodegradable substrates’ hydrolysis rate (day–1). The two-substrate model is able to give information of rapidly and slowly biodegradable components of the ZVI-assisted algal sludge anaerobic digestion. The changes in the parameters B0, k, B0,rapid, B0,slow, krapid, and kslow under various ZVI concentrations in each reactor can thus be compared.

Basic Parameters Analysis

The basic parameters of the inoculum and algae were tested in triplicate following standard methods (APHA, 2016). The TCOD, SCOD, pH, NH4-N, and fluorescence excitation emission matrix (FEEM) of the algae and inoculum mixture were analyzed after the BMP test was stopped. FEEM was analyzed by a fluorescence spectrometer at emission wavelengths of 200–400 nm and excitation wavelengths at 280–540 nm (QM-4CW, PTI, USA). The dewaterability of the digested algal sludge was tested by measuring the free-water volume of sludge after being centrifuged at 3000 rpm for 30 min. The generated biogas pressure was measured by a manometer before sampling, and the actual volume was calculated from the pressure increase in the headspace volume (135 mL) and expressed at standard atmospheric pressure (25 °C, 1 atm). The methane concentration was measured by a GC analyzer (GC7890, Agilent, USA).

Results and Discussion

ZVI-Assisted Algal Sludge Methane Production Improvement

The accumulative methane production of the reactors with different ZVI concentrations added was shown in Figure . The accumulative methane production of algae kept increasing with time and approached a stable value at day 57. The final accumulative methane yield was 225.11, 227.36, 241.11, 279.69, 317.78, and 328.22 L·CH4/kg·TS for ZVI addition at 0, 0.5, 2, 5, 10, and 20 g·ZVI/g·TS, respectively. The accumulative methane yield has enhanced 1.01, 1.07, 1.24, 1.41, and 1.46 times with the addition of ZVI at 0.5, 2, 5, 10, and 20 g·ZVI/g·TS as compared with no ZVI added, respectively. The algal sludge methane yield was rarely increased with ZVI concentration at 0.5 g·ZVI/g·TS while kept increasing with the ZVI dosage increased and achieved the highest value at a ZVI dosage of 20 g·ZVI/g·TS. The ZVI dosage and the accumulative methane production value were in linear correlation (Figure , R2 = 0.8524). This implied that the addition of ZVI has enhanced the accumulative methane production with ZVI dosage at 2 g·ZVI/g·TS and above. Zhen et al.[11] reported that the ZVSI concentration at 1.0 g/g·VSS has increased the WAS methane yield by 38.3% to a value of 174.9 mL/g·VSSfeed. The dosage used of ZVSI was a bit lower than the ZVI used in this study, which indicated that the algal sludge was more recalcitrant as compared with the WAS. The biogas generated in this study was much higher than the WAS as reported by earlier work, which indicated higher methane production potential of the algae.[11−13,17] The ZVI added could have improved the algae solubilization during the fermentation process and thus improved biodegradability of the algae.[1,8] The existence of ZVI powder also could provide physical effects in improving the algae cell disruption during the shaking process.[14,18] The iron ions also could be reacted as an electron donor and change the pH in the reactor, thus causing the algae cell disintegration.[11] Based on which, the algal sludge fermentation process was accelerated with the existence of ZVI, and the accumulated methane production was also improved.

Figure 1

. Accumulative methane production of ZVI-assisted algae digestion.

Figure 2

. Correlation between the concentration of ZVI added and the corresponding accumulative methane production.

Biochemical Methane Potential Analysis

To understand the effect of ZVI addition in the enhancement of methane production for algal sludge, both one-substrate model and two-substrate model were used to simulate the methane production. Both one-substrate model and two-substrate model were fitted well with the methane production results in this study (Table , R2 > 0.96 and Table , R2 > 0.96). The predicted biochemical methane production by the one-substrate model and the actual methane production value during the BMP test were in linear correlation (Figure , R2 > 0.98). As shown in Table , results of the one-substrate model shows that the hydrolysis rate k values of the algal sludge were 0.0028, 0.023, 0.0198, 0.0263, 0.0098, and 0.0142 day–1 for reactors with ZVI dosage at 0, 0.5, 2, 5, 10, and 20 g·ZVI/g·TS added. The hydrolysis rate of the reactors with ZVI added has improved 8.21, 7.07, 9.39, 3.50, and 5.07 times as compared with the control reactor where no ZVI was added. The k value is much lower than the earlier reported work of WAS, while the k value enhanced with the addition of ZVI was much higher.[19−21] This implied that the ZVI has largely improved the hydrolysis rate of algae, not only promoted the algae cell disruption but also improved the hydrolyzed organic components’ biodegradability.[11,18] This implied that with the existence of ZVI in the reactor, the hydrolysis rate of algae anaerobic digestion was improved. The predicted biochemical methane production by the one-substrate model was 247.50, 239.19, 257.42, 289.50, 338.19, and 344.48 L·CH4/kg·TS in reactors R1 to R6, respectively. It is also shown in Table that the pure algae without ZVI added has an extremely high B0 and a low k value. This implied that the pure algae have a high methane potential but a low hydrolysis rate, which means that the methane production from pure algae might need a very long period and assistance methods are necessary.

Table 3

Determined Hydrolysis Rate (k) and Biochemical Methane Potential (B0) of Algae at Different Reactors Using the One-Substrate Model

reactors	R1	R2	R3	R4	R5	R6
k (day–1)	0.0028	0.0230	0.0198	0.0263	0.0098	0.0142
B0 (L·CH4/kg·TS)	1677.80	327.46	380.51	372.74	790.17	620.82
Y57 (L·CH4/kg·TS)a	247.50	239.19	257.42	289.50	338.19	344.48
R2	0.9659	0.9853	0.9776	0.9882	0.9917	0.9904

The predicted accumulative methane production at day 57 in each reactor by corresponding equation simulated.

Table 4

Determined Hydrolysis Rate (k) and Biochemical Methane Potential (B0) of Algae at Different Reactors Using the Two-Substrate Model

reactors	R1	R2	R3	R4	R5	R6
krapid (day–1)	0.0044	0.023	0.0198	0.0263	0.0098	0.0142
B0,rapid (L·CH4/kg·TS)	1129.76	160.30	189.21	180.28	395.25	308.72
kslow (day–1)	1914.85	0.023	0.0198	0.0263	0.0098	0.0142
B0,slow (L·CH4/kg·TS)	–3.6383	167.16	191.299	192.46	394.92	312.11
R2	0.9662	0.9853	0.9776	0.9882	0.9917	0.9904
Y57 (L·CH4/kg·TS)a	246.97	239.19	257.42	289.50	338.19	344.48

The predicted accumulative methane production at day 57 in each reactor by corresponding equation simulated.

Figure 3

. Actual and predicted biochemical methane potential by the one-substrate model (R2 > 0.98).

The two-substrate model results show that the rapid hydrolysis rate krapid values for R1 to R6 were 0.0044, 0.023, 0.0198, 0.0263, 0.098, and 0.0142 day–1 (Table ). The rapid hydrolysis rates for algal sludge with ZVI added at R2 to R6 were 5.23, 4.50, 5.98, 2.23, and 3.23 times as compared with the pure algae. The corresponding rapid biochemical methane potential B0,rapid values were 160.3, 189.21, 180.28, 395.25, and 308.72 for R2 to R6. All the reactors with ZVI added show higher rapid hydrolysis rates where higher ZVI dosage results in a higher B0,rapid. The rapid hydrolysis rate for pure algae was quite low, and the B0,rapid was quite high. This was similar with the one-substrate model results. The slow hydrolysis rate kslow for pure algae was quite high with a value of 1914.85 day–1, and the slow biochemical methane potential B0,slow has a negative value. This could summarize that the methane production rate for pure algae was much lower than that of the ZVI-assisted algae anaerobic digestion. The kslow for R2 to R6 were 0.023, 0.020, 0.026, 0.010, and 0.014 day–1. The B0,slow for R2 to R6 were 167.16, 191.30, 192.46, 394.92, and 312.11 L·CH4/kg·TS. The B0,slow of reactors with higher ZVI added has a lower value as compared with R2 where ZVI was added at 0.5 g·ZVI/g·TS. This summarized that both rapidly and slowly biodegradable components were improved with the assistance of ZVI addition.[22,23]

Characteristics of the Digested Algal Sludge

The TCOD and SCOD of the digested algal sludge were shown in Figure . The SCOD/ TCOD values of the digested algal sludge were 0.02, 0.03, 0.21, 0.35, 0.26, and 0.24 for ZVI added at 0, 0.5, 2, 5, 10, and 20 g·ZVI/g·TS. The highest TCOD was shown in the pure algae reactor R1. The results indicated that the reactors with ZVI added above 0.5 g·ZVI/ g·TS has a much higher SCOD/TCOD even for the digested sludge. The ZVI added at 0.5 g·ZVI/g·TS only slightly improved the SCOD/TCOD value, while the TCOD was also lower than the pure algae reactor. This implied that the ZVI addition has improved the biodegradability of the algal sludge and thus improved the methane production rate; this was in accordance with the BMP test results. It is also obvious that a high SCOD of around 3000 mg/L was retained in R3 to R6 for the digested algal sludge. The BMP test was stopped when the pressure in the reactor stopped increasing in this study. The SCOD value of R3 to R6 implied that the algal sludge still has potential to generate more methane gas.[24,25] This also can be seen from the FEEM results shown in Figure . There were still fluorescence-responsible organic components in R4 to R6 even though the intensity is much lower than initial activated sludge.[26] This once again could conclude that ZVI addition has enhanced the SCOD release from the algae, improved the biodegradability of the algae, and results in improvement of methane production in the same period as compared with the pure algae.

Figure 4

. TCOD and SCOD values of digested algal sludge.

Figure 5

. FEEM of digested algal sludge supernatants after 57 days of anaerobic digestion. (a) R1 - 0 ZVI, (b) R2 - 0.5 g·ZVI/g·TS, (c) R3 - 2 g·ZVI/g·TS, (d) R4 - 5 g·ZVI/g·TS, (e) R5 - 10 g·ZVI/ g·TS, and (f) R6 - 20 g·ZVI/g·TS.

Dewaterability and Economics of the Treated Algal Sludge

The algal sludge has similar characteristics with wasted activated sludge and also shares the same problem of dewaterability, which finally affects the treatment costs. The dewaterability of the digested algal sludge was as shown in Figure . The reactors with ZVI added at 2 to 20 g·ZVI/g·TS have much higher dewaterability as compared with R1 and R2 where ZVI added were 0 and 0.5 g·ZVI/g·TS. This could conclude that the addition of ZVI also enhanced the dewaterability of the algal sludge. The ZVI could have promoted the algae cell disintegration and the release of free water, and this improved the digested algae biodegradability and dewaterability. The particle size of algal sludge also could be decreased with ZVI or iron ions as a conditioner and thus facilitated the filtration process and improved the dewaterability.[27]

Figure 6

. Dewaterability of the algal sludge after ZVI-assisted digestion stopped.

The economic analysis of the ZVI-assisted algal sludge, ZVI+hydrogen peroxide, and conventional Fenton conditioning methods for sludge dewaterability improvement were conducted and compared by a desktop scaling-up study. In the economic analysis, the improvement in algal sludge dewaterability was assumed to be same for the three conditioning methods. As shown in Table , the ZVI-assisted process without pretreatment compared with other conditioning processes has saved up to 74.72% ($50,181) for the digested algal sludge treatment. The dry sludge amount in the economic analysis in Table cited from Zhou et al.[27] was information from a wastewater treatment plant with population equivalent to 300,000. Thus, the cost saved for digested algal sludge was not counted as money saved per year but per ton. The algal sludge dewaterability enhancement with ZVI only assistance could save $21.44/ton algae to be treated. This preliminary lab experiment results in this study indicated that the ZVI-assisted algae digestion process has the potential to deliver substantial savings in the industrial area. However, further investigation, especially for full-scale application, requires future verification of the results.

Table 5

Economic Analysis of ZVI-Assisted Algal Sludge Dewaterability Enhancement

parameters	ZVI conditioning (this study)	ZVI+H2O2 conditioninga	classical Fenton conditioning (Fe(II)+H2O2)a
dry sludge amount (ton/year)	2340	2340	2340
ZVI powder ($/year)	16,977	16,977
ferrous chloride ($/year)			58,171
H2O2 ($/year)		8488	8488
sulfuric acid ($/year)		499	499
total cost ($/year)	16,977	25,964	67,158
total saving with only ZVI ($/year)	50,181 = 67,158 – 16,977 (74.72%, $21.44/ton)

Cited from Zhou et al. (2014).[27]

Perspectives of Algal Sludge Resource Recovery

The algal sludge collected from the algal bloom lakes has been an environmental problem for decades, and the algae biomass resource recovery is an important issue. This study indicated that adding ZVI at a dosage of 2 g·ZVI/g·TS and above could enhance the methane production from algal sludge, enhance the digested algal sludge dewaterability, and save cost. This study added ZVI directly into the fresh algae and started the anaerobic digestion without a complicated pretreatment process. This is also more convenient and energy-saving as compared with other studies, which used complicated pretreatment before the anaerobic digestion process.[19,28,29] It is also reported that the ZVI was used for soil or groundwater remediation;[30,31] thus, the environmental risk of reuse of the digestate with ZVI from this study could be ignored. This makes the ZVI-assisted algal sludge digestion more attractive for future industrial application. At the meantime, the ZVI recovery and reuse or ZVI-based materials to be integrated into an anaerobic digester should be further investigated.

Conclusions

This study investigated the methane production potential of algal sludge by the assistance of zerovalent iron addition. The results indicated that the addition of ZVI has improved the methane production and the dewaterability of the digested algal sludge. The addition of ZVI at 2, 5, 10, and 20 g·ZVI/g·TS has improved the methane production 1.07, 1.24, 1.41, and 1.46 times as compared with no ZVI addition. Both one-substrate model and two-substrate model fitted well with the biochemical methane potential test results. The addition of ZVI has enhanced the hydrolysis rate 3.5 to 9.39 times as model results of the one-substrate model and improved the rapid hydrolysis rate 2.23 to 5.98 times in the two-substrate model as compared with pure algae anaerobic digestion. The economic analysis results implied that the algal sludge dewatering has saved 74.72% of cost by using ZVI as compared with the conventional process in activated sludge treatment. The ZVI addition has the potential to be developed as an effective and economically friendly technology for algae resource recovery.