Nutrient Recycling

The recycling of nutrients from the process water from hydrothermal processing has been investigated in several studies and proposed as an advantage of hydrothermal routes over other alternative routes such as biodiesel production. Macroalgae is usually cultured in marine environments where nutrient recycling would be more of a challenge. Significant amounts of the feed nitrogen, phosphorous and potassium have been shown to concentrate in the process water after hydrothermal processing. This is the case for HTC, HTL and HTG [56-58, 88]. Minowa and Sawayama were the first to recognize this potential and attempted to cultivate microalgae in the process water from the HTG of Chlorella vulgaris [89]. It was found that all of the nitrogen in the algae was converted to ammonia which was distributed in the water phase. Cultivation trials were performed in the process water after dilution and compared to standard media, it was found that algal growth in the undiluted process water was a fraction compared to the standard media; however blending standard growth media with HTL process water showed good growth rates. It was concluded that cultivation in standard media without nitrogen plus process water was possible and therefore a saving on nutrients could be achieved. The same group later performed similar experiments but going into more detail concerning ammonia, micro-elements and nickel concentrations [98]. It has previously been shown that nickel can inhibit algae growth due to accumulation on the cell surface by adsorption and thus acting as a selective barrier for nutrient

37 uptake by the cells [99]. Chlorella vulgaris growth was shown by Spencer et al. to be inhibited by nickel levels as low as 0.85 mg/l [100]. Sawayama et al. tried to determine the optimum culture conditions using the recovered aqueous phase supplemented with necessary nutrients. The effects of nickel and ammonium ions were also investigated. The nitrogen in the algae was converted to ammonium and nickel was leached from the nickel catalyst [98]. It was shown that Chlorella vulgaris was able to grow in ammonium concentrations of 0.22 to 1.11 g/l but became toxic above 16.6 g/l. Therefore it was concluded that the recovered process water would have to be diluted at least 30 fold to avoid ammonium toxicity. The nickel concentrations found in the water phase from the nickel catalysts used were found to be very high (240 mg/l) although a 30 fold dilution would be sufficient to avoid nickel growth inhibition. Using a 75-300 fold dilution of the recovered water phase with supplementation of phosphorous and magnesium was shown to yield growth rates similar to the standard medium.

Jena et al. (2011) performed growth trials using the process water from the HTL of Spirulina to grow a strain of Chlorella [48]. It was shown that growth was possible in dilutions of the process water. When using a dilution factor of 10, no growth occurred and this was attributed to the presence of growth inhibition, possibly by nickel, phenols or fatty acids. These have all previously been shown to adversely affect algae growth [88, 100-102] and are likely constituents of the water phase. The growth in the 100, 300 and 500 fold dilutions was higher. The highest growth was observed in the 500 times dilution, reaching a maximum of around 80% compared to a standard BG11 growth medium. A mass balance on an integrated HTL system with nutrient recycling was performed and the results suggest that from 1 t of dry biomass 0.4 t of bio-crude could be produced.

The process water would contain 3.4 kg of P and 70 kg of N and significant amounts of miner nutrients. Part of this could be used to grow more algae and the rest concentrated to valuable fertilizer high in NPK, additionally the 0.18 t of CO2 produced could be used to enhance algae growth [48].

The potential issues occurring from nickel either from Ni catalyst leaching, from reactor wall corrosion or from the algae itself (minor amounts) has been highlighted in a study by Haiduc et al.[88]. They realized that when using a continuous closed loop system integrating nutrient recycling, the process water would become progressively enriched in contaminants such as nickel.

In their research paper they therefore evaluated the growth of four green microalgae strains and a cyanobacteria in standard BG11 media doped with 0, 1, 5, 10 and 25 mg/l nickel [88]. It was found that 10 mg/l had significant detrimental effects on the growth of all algae strains. Scenedesmus v.

was able to grow in BG11 doped with 1 and 5 mg/l Ni but 10 mg/l reduced the growth by around

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half while 25 mg/l inhibited growth entirely. It was concluded that if nutrient recycling is incorporated into a hydrothermal system, nickel concentrations in the effluent need to be monitored closely and if required, the stream should be diluted if the concentration approaches 25 mg/l or removed .

The potential of recycling of nutrients to cultivate algae is significant due the cost associated with supplying large amounts of nutrients. The fossil energy input for the production of growth nutrients is significant and would reduce the life cycle energy balance. Especially for phosphorous which is a finite non-renewable resource extracted from phosphate rock and requires high energy inputs.

Current estimates predict peak phosphorous reserves may be depleted in 50-100 years [103]. The limited studies on recycling of nutrients for algae cultivation are promising but additional research needs to be performed to identify problems occurring with progressive build up of growth inhibitors.

A wider range of algae strains and mixed strains should also be investigated as some species are expected to endure harsher cultivation conditions than others.

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