The U.S. produces an estimated 79 million dry tons of sustainably collectable livestock manure and food processing waste annually [1
]. This has encouraged a growing interest in hydrothermal liquefaction (HTL), a process by which organic matter, such as sewage or manure, is converted to bio-crude oil under high temperatures and pressures [2
]. Furthermore, this process produces an aqueous byproduct that contains crop nutrients including nitrogen, phosphorus, and potassium [3
]. The high temperatures and pressures needed for the HTL process destroy biological and organic contaminants, such as pharmaceuticals, pathogens, and genetic material, which are present in the HTL feedstock [4
]. Therefore, there is high potential for post-hydrothermal liquefaction wastewater (PHW) for food crop production.
Wastewater reuse for irrigation is a current topic of study, in particular on the effect of treated wastewater on crops and soil [6
]. Some concerns with the use of wastewater as irrigation are metal(loid) contamination, drugs, and pathogens [8
]. However, the high temperatures and pressures in HTL are hypothesized to eliminate the drugs and pathogens from the PHW, which leaves a potential risk for metal(loid)s in PHW. Studies have shown the benefits and limitations of non-PHW wastewater sources in the hydroponic production. Hydroponic crop production entails plant production in the absence of soil in which roots are directly exposed to nutrient solution. Compared to non-hydroponic agriculture, hydroponic production requires 82 times more energy but 92% less water and produces 11 times more yield per area [10
Treatment of wastewater has been shown to be possible over 7 days with a Pistia stratiotes
phytofiltration lagoon, with an average biomass production of 5.8 g of dry weight per square meter per day [11
]. Lettuce (Lactuca sativa
) can successfully be produced with treated greywater without posing a substantial health risk for pathogens or metal(loid)s, while also providing a 5.1 log10 reduction of E. coli
in the final effluent [12
]. Hydroponic barley (Hordeum vulgare
) production using wastewater has been investigated [13
] and found a decrease in yields of 47% and 17% for conventional activated sludge and lagoon system water compared to half-strength Hoagland Solution, concluding that further fertilization with wastewater would be necessary and that metal(loid) concentrations in plant tissue increased, but did not exceed, toxic levels. Al-Karaki [14
] explored using tertiary-treated sewage wastewater to grow barley in controlled environment vertical hydroponic growing towers. Compared to treatments of tap water alone, which was used without fertilizer for hydroponic green fodder due to short growth period, wastewater, and wastewater mixed with municipal tap water improved both yields and water efficiency. Furthermore, no pathogens were found in any hydroponic treatment, compared to previous research in conventional agriculture.
Primary and secondary treated municipal wastewater was investigated for hydroponic cucumber (Cucumis sativus
) irrigation with a nutrient film technique [15
]. It was found that primary treated wastewater, with or without nutrient supplementation, decreased plant height and reduced leaf and flower number, but this was not seen with secondary treated wastewater. However, the increased fruit number and weight resulted in increased yields, but it was also found to spread plant disease in plant roots and fruit. Chow et al. [16
] cultivated lettuce and Chinese cabbage (Brassica rapa
) with primary and secondary municipal wastewater effluents and half-strength Cooper nutrient solution. Chinese cabbage showed signs of phytotoxicity with both effluents, but lettuce did not; both crops had lower yields than the control. However, da Silva Cuba Carvalho et al. [17
] sought to evaluate nutrient absorption and yields of lettuce cultivated hydroponically with wastewater from a domestic sewage treatment station. Using nutrient film technique with treatments of tap water and fertilizer, wastewater supplemented with fertilizers, and wastewater alone, they found that wastewater supplemented with fertilizers had lettuce yields and nutrient uptake comparable to tap water with fertilizers. However, treated sewage effluent alone did not have the nutrients necessary for lettuce production and had a quarter of the mass compared to supplemented wastewater. Cui et al. [18
] treated septic tank effluent using vertical-flow wetlands followed by hydroponically growing spinach (Spinacia oleracea
) and lettuce, which resulted in removal efficiencies sufficient for a secondary biological treatment plant discharge, with yields less than nutrient solution hydroponic control, but higher than in field production. Norström et al. [19
] investigated using hydroponics and microalgae production in conjunction with conventional biological treatment to treat domestic wastewater. The system was able to achieve a 90% chemical oxygen demand (COD) removal, 72% total nitrogen reduction, and 47% phosphorus reduction, but was not sufficient for Swedish effluent standards.
It has been demonstrated that wastewater spiked with PHW can augment the growth of mixed culture algae and rice. Phototrophic algae growth was quantified as Chlorophyll a
concentration, which was increased by over 30% when using a small percentage of PHW (0.5%). However, larger concentrations of PHW had an inhibitory effect, perhaps due to high ammonia concentrations or organic inhibitors [20
]. In past work, phenol, toluene, benzene and a variety of nitrogenous heteroatomic compounds have been identified in PHW [24
] and are known to be toxic to some algae [25
]. It is also known that acclimated algae can tolerate up to 15% HTL aqueous product. In comparison to conventional wastewater treatment processes like activated sludge, it has been shown that phototrophic algae can enhance removal of wastewater nutrients, N and P, by 86% and 95%, respectively [27
Average rice yields from plants grown using a blend of nutrients from swine manure and PHW was approximately 18.5% higher than control plants grown in municipal water containing synthetic nutrients [28
]. However, one problem noticed in this study was that the rice irrigated with PHW had higher levels of certain metals (i.e., Hg, Cd, Pb, with only Hg exceeding the current FDA standards for rice), which is a topic of this proposed study.
The overall goal of this study was to assess the ability of treated PHW to effectively and safely grow lettuce, hydroponically. Specifically, this study measured yield and shoot content of metal(loid)s, nitrogen, and phosphorus of lettuce grown using five different source waters.
3. Results and Discussion
At the end of the 4-week growing cycle, all lettuce tissue was harvested and each individual plant weighed. The plants were then combined according to bin, dried, and weighed again. The dried plants were analyzed for nutrient content and concentrations of metal(loid)s. Table 3
summarizes the concentrations of metal(loid)s, nutrient content, and yields of lettuce for all five source water treatments.
summarizes the average concentrations across the three bins for each source water treatment of cadmium, mercury, lead, and arsenic, as well as the percentage of nitrogen and phosphorus in the dry lettuce tissue at harvest. In addition, the net fresh and total dry yields are provided. The net fresh yield is presented as the difference between the fresh yields at harvest minus the fresh mass of the plug at the start of the study. Source Water 1 (control) had the highest net fresh yield of all five source water treatments; 36.5 times higher than Source Water 2, 163.3 times higher than Source Water 3, 17.7 times higher than Source Water 4, and 1.6 times higher than Source Water 5. Source Water 1 also had the highest total dry yield of all five source water treatments; 3.06 times higher than Source Water 2, 3.5 times higher than Source Water 3, 2.6 times higher than Source Water 4, and 1.3 times higher than Source Water 5. It should be noted that the total dry yield is higher than the net fresh yield for Source Waters 2, 3, and 4, due to desiccation of above-ground biomass of many of the individual lettuce plants. Plants that had lower mass or died allowed for a decrease in net fresh yield, but still contributed to a total dry yield since some plant matter remained. Differences in metalloid concentrations thus appear to be driven by N and P contents of source water via biomass dilution.
The concentration of cadmium was the highest in the lettuce from Source Water 3, at nearly four times higher than Source Water 1 (control). Lettuce from Source Water 3 also had the highest arsenic concentration, which was nearly four times higher than Source Water 1. Lettuce from Source Water 2 had the highest mercury concentration, but it was only slightly higher than Source Water 1. The concentrations of cadmium, mercury, and lead were significantly lower for the lettuce from Source Water 5, even though it was the diluted PHW supplemented with hydroponics fertilizer, which Source Water 1 also received. Therefore, it is a possibility that a compound in the PHW prevented the lettuce from taking up cadmium, mercury, and lead, or perhaps a mechanism of the plant that prevented uptake. The concentrations of mercury and lead were lower for the lettuce from Source Water 4 than Source Water 1.
Lettuce shoot nitrogen and phosphorus contents were nearly identical for Source Water 1 and 5 even though the total dry yield was 1.3 times higher for Source Water 1. The nitrogen content for Source Water 1 was 2.8, 3.0, and 3.6 times higher than Source Waters 2, 3, and 4, respectively. The phosphorus content for Source Water 1 was 6.5, 7.4, and 4.5 times higher than Source Waters 2, 3, and 4, respectively. Figure 2
shows the net fresh yields of each bin and for each source water. Therefore, it seems that Source Waters 2, 3, and 4 could have used more nitrogen and phosphorus.
The overall trend is that Source Waters 1 and 5 had relatively lower metal(loid) concentrations, but higher nitrogen and phosphorus concentrations as well as higher fresh and dry weight. This can likely be explained by Source Waters 1 and 5 having sufficient nutrients for normal lettuce growth and thus the lettuce plants in those treatments were able to uptake more nitrogen and phosphorous and produce biomass. Meanwhile, the deficiency in nutrients for Source Waters 2, 3, and 4 likely limited plant growth and could have promoted uptake of metal(loid)s as a byproduct of plant stress. However, between Source Waters 1 and 5, Source Water 1 had a higher uptake of Cd, Hg, and Pb, and yet had higher P uptake and yields, in spite of similar concentrations of the metal(loid)s and nutrients. This finding could be a result of pH imbalance, as pH was not controlled in this study, which can impact the availability of nutrients and other compounds. Alternatively, it could be a result of an unexplored compound in PHW that impacts plant growth. Between Source Waters 2, 3, and 4, higher concentrations of contaminants and nutrients in the source water did not uniformly lead to a higher uptake of those compounds in the plant tissue. However, it is difficult to draw full conclusions from this phenomenon, as the plants were severely deficient in nutrients and already had partially desiccated, which could confound the findings for these source water treatments.
Within each source water treatment, replications had similar distributions for yields. Of the treatments, Source Water 1 had the greatest net fresh yields, followed closely by Source Water 5. Source Waters 2, 3, and 4 all had similar, but extremely poor yields. Table 4
shows the p-values for all source water comparisons of net fresh yield. The significant differences (p
< 0.05) are highlighted in red for convenience.
The statistical analysis confirmed that the Source Water 1 net fresh yield was significantly higher than all other treatments and the Source Water 5 net fresh yield was higher than Source Waters 2, 3, and 4. Figure 3
shows the total dry lettuce yield from each bin and for each source water. The average total dry yield is also displayed for all three bins for a given source water with the solid blue bar.
The total dry yield of each individual bin varied slightly. Source Waters 1, 2, and 3 had higher total dry yields from bin 1, which was the bin on the top shelf of the rack. In contrast, Source Waters 4 and 5 had higher total dry yields from bin 3, which was the shelf at the bottom of the rack. It is possible that location of each bin within the rack could have impacted the yields. However, since each treatment had one bin at each level of the rack, this potential variation is accounted for when taking the average of the three bins. Table 5
shows the p-values for all source water treatment comparisons of total dry yield with significant differences (p
< 0.05) highlighted in red.
Total dry yield from Source Water 1 and 5 was significantly higher than Source Waters 2, 3, and 4. There was not a significant difference in the total dry yield between Source Waters 1 and 5. This is in contrast to Mazur [28
] who did see a significant increase in yields and inferred that PHW can increase grain yields, whereas our lettuce yields with supplemented PHW decreased overall yields. Figure 4
shows the nitrogen and phosphorus contents in the total dry lettuce from each bin and for each treatment. Lettuce leaf nutrient concentration guidelines were provided by Midwest Laboratories as benchmarks of sufficiency for nitrogen and phosphorus.
The nitrogen and phosphorus contents for lettuce harvested from Source Waters 1 and 5 were above the norms, meaning there was sufficient nitrogen and phosphorus, while the lettuce harvested from Source Waters 2, 3, and 4 were below the norms for nitrogen and phosphorus, meaning there was a nitrogen and phosphorus limitation. Table 6
shows the p-values for all source water treatment comparisons of nitrogen and phosphorus content. The significant differences (p
< 0.05) are highlighted in red for convenience.
The statistical analysis confirmed that the nitrogen and phosphorus contents in the dry lettuce was significantly greater for Source Waters 1 and 5 than Source Waters 2, 3, and 4. There was no significant different in nitrogen or phosphorus content between Source Water 1 and Source Water 5. Figure 5
shows the concentrations of metal(loid)s in the dry lettuce from each bin and for each source water treatment.
The concentrations of cadmium, mercury, and lead were all below the maximum allowable levels for food as determined by the US Department of Agriculture Foreign Agriculture Service [30
] for all five source water treatments. The concentration of arsenic in lettuce from Source Waters 2, 3 and 4, however, were above the maximum allowable level. A study by Eregno et al. [12
] involved growing lettuce hydroponically with treated greywater with added human urine and found that lettuce leaves across all treatments were below estimated Health Risk Index and Target Hazard Quotient values and therefore implied to have low health risk for consumption. However, Eregno et al. [12
] did report that the HRI for As and THQ for As and Cr were relatively higher than the other metal(loid)s tested, which is similar to our findings for arsenic. Table 7
shows the p-values for all source water treatment comparisons of metal(loid)s content. The significant differences (p
< 0.05) are highlighted in red for convenience.
The average concentration of cadmium across the three bins was significantly lower in Source Water 1 than Source Waters 2 or 3 and significantly higher in Source Waters 2 and 3 than Source Water 5. In addition, the average concentration of arsenic across the three bins was significantly higher in Source Waters 3 and 4 than Source Waters 5. Figure 6
shows a regression analysis between the total dry lettuce yield and metal(loid) concentrations.
There is a negative correlation between lettuce leaf yield and leaf concentrations of all four metal(loid)s, meaning a higher yield led to biomass dilution, in which the load of metal(loid) elements is diluted into more tissue biomass. A strong correlation existed between total dry lettuce yield and arsenic concentration (R2
= 0.8749). A moderate correlation existed between total dry lettuce yield and cadmium concentration (R2
= 0.6607), and relatively weak correlations with lead (R2
= 0.2165) and mercury (R2
= 0.0197). Figure 7
shows a regression analysis between the total dry lettuce yield and nutrient content.
There is a positive correlation between yields and both nitrogen and phosphorus, meaning the total dry yield decreased as the concentrations of each nutrient increased. A strong correlation existed between total dry lettuce yield and both nitrogen (R2 = 0.8928) and phosphorus (R2 = 0.9373).
4. Broader Impacts
Lettuce yields and nutrient concentrations indicate that PHW source waters without supplemented hydroponic fertilizer (Source Waters 2, 3, 4) had significantly lower lettuce yields and nutrient content due to N and P limitation. Therefore, diluted PHW alone is not a sufficient sole nutrient source for meeting N and P needs of hydroponically grown lettuce. However, supplemented PHW and conventional hydroponic fertilizer were not significantly different. Thus, PHW can be used in hydroponic fertilizer formulation without a significant negative impact on yields and nutrition as a source of nitrogen and irrigation water. Still, it was seen that supplemented PHW had overall lower yields and nutrients than hydroponic fertilizer alone, indicating some inhibitory effect in PHW that warrants further investigation. Moreover, N and P supplementation promotes significant increases in the biomass of edible tissue, diluting concentrations of metal(loid)s and thus increasing the utility of PHW for hydroponic crop production from a food safety standpoint.
While the plant tissue analysis shows that arsenic exceeded the maximum allowable concentration for lettuce in Source Waters 2, 3, and 4, these treatments had vastly lower yields and showed extreme symptoms of nutrient deficiency. In other words, one potential implication of this study is that sufficient nutrient supplementation can be used to manage potential metal(loid) accumulation in edible tissues. This means that PHW needs to be supplemented with other nutrients for lettuce cultivation, but when PHW is supplemented, it can produce similar yields and nutrition to conventional hydroponic production.
As HTL is already an attractive technology of biofuel production derived from wastewater, producing hydroponic crops with the PHW byproduct only further increases the incentive to include HTL in the Food-Water-Energy Nexus approach. HTL production facilities could be coupled with hydroponic crop production facilities to address all three pillars of the Food-Water-Energy Nexus and increase the sustainability of the HTL technology.
Further research on PHW as a crop fertilizer should focus on finding methods to convert the nitrogen in PHW into plant-available inorganic forms (e.g., NO3 and NO2), identifying inhibitory compounds in PHW, finding sources of wastewater rich in phosphorous that could complement PHW’s nitrogen concentration, expanding this research to other cultivars of lettuce or other hydroponic crops, and examining the overall nutrient, water, and energy efficiency of the hydrothermal liquefaction process in conjunction with cultivation using PHW.