Microalgal culture to treat food waste digestate

A significant proportion of all waste generated in the world is food waste, contributing up to 8% of global CO2 emissions. Conventional ways of food waste disposal including landfilling and incineration are simply inadequate and unsustainable, emphasizing the need for efficient ways recycling / valorizing food waste. Anaerobic digestion is a way to treat and valorize food waste with significantly less emission of greenhouse gases. However, anaerobic digestion itself produces a waste in the form of anaerobic digestate that is difficult to manage. Microalgae cultivation has been used for the treatment and valorization of a wide range of waste effluents and has been identified as a potentially viable option for the treatment and valorization of food waste anaerobic digestate. On the other hand, food waste digestate as a feed stock for the cultivation of microalgae has its challenges. Food waste digestate is a very high strength effluent having ammonia nitrogen concentration of up to 4000 mg L-1, which is extremely toxic to most microalgae species. Food waste digestate is also very turbid, making microalgae cultivation in this effluent extremely difficult without an unsustainably excessive amount of dilution, as light / irradiance is the most limiting factor in any microalgae cultivation system. Previous efforts to treat and valorize food waste digestate have been less than successful due to the reasons stated above. Considering these, we have identified possible solutions to mitigate the challenges involved in treatment and valorization of food waste digestate using microalgae. These are: the identification and application of microalgae species tolerant to high concentration of ammonia nitrogen as found in food waste digestion to tackle the problem of excessive dilution of the digestate before use; and the use of an inclined thin layer pond with a much shorter light path than conventional microalgae cultivation systems allowing for better utilization of incident irradiance in culture systems. Three microalgae (Scenedesmus quadricauda, MUR 268, Chlorella sp, Mur 269, and Oocystis sp, MUR 273) were capable of growth in up to 600 mg L-1 of ammonia nitrogen in food waste digestate. Further experimentation was done at indoor bench scale laboratory conditions using the most tolerant and robust of the 3 identified species, Chlorella sp, Mur 269. To gain insight into the mechanisms required for tolerance of ammonia with the identified species, especially in relation to its response to light profiles imposed by turbidity, their photosynthetic response was studied using pulse amplitude modulation (PAM) fluorometry. Indicators including electron transport rate (ETR), maximum quantum yield in actinic light (fv’/fm’) and alpha (α), alongside oxygenesis as an indicator of photosynthesis, were used to study the state of the photosystem of the organism. Maximum specific growth (μmax) rates and in-situ irradiance profiles were also studied. The effect of turbidity was accounted for by studying the above response variables in clear synthetic medium (Bolds basal medium, BBM) and food waste digestate medium. Maximum specific growth rate (μmax) and biomass productivities were 63% and 47% higher in anaerobic digestate of food waste (ADF) than in BBM, maintaining values of 0.681 ± 0.03 d-1 and 165 ± 8 mg L-1 d-1 respectively, even at high irradiance intensities of 1500 μmol m-2s-1, validating their suitability to outdoor conditions. However, Chlorella sp, Mur 269 photosystem II at optimum irradiance, as reflected in Fv’/Fm’ values, was reduced by 16% in food waste digestate in comparison to BBM. A critical look at the photosynthesis of this algae shows that adaptive tolerance methods of Chlorella sp, MUR 269 to toxicity includes adjustment of the photosynthetic unit to maximize absorption of light and compensation mechanisms for reductions in PSII activity including switching to mixotrophic growth mode. Application of Chlorella sp, Mur 269 for growth in food waste digestate was carried out using an inclined thin layer pond under outdoor conditions. Previous studies using the inclined thin layer pond had shown that though volumetric productivities and biomass densities could be improved by using the inclined thin layer pond for the treatment and valorization of digestates, areal productivities were significantly lower than paddle wheel driven raceway ponds because of the higher surface area to volume ratio in the inclined thin layer pond. To that end, depth optimisation of a 11 m2 surface area inclined thin layer pond was conducted, tailored towards its utilization for the treatment and valorization of food waste digestate using microalgae. Depth optimizations were performed by stepwise increments of the depth of the culture on the surface of the inclined thin layer pond through 0.005, 008, 0.011, 0.0145 and 0.02 m. The kinetics of electron flow around photosystem II of microalgae in-situ culture was used as descriptives for light utilization and limitations of the optimizations via variables including relative electron transfer rate, rETR, and maximum quantum yield, Fv/Fm, and derived parameters including functional relative electron transfer rate (FrETR) and functional relative electron transfer rate ratio (FrETR-ratio). Optimal culture depth determined for the inclined thin layer pond was 0.011 m, with average biomass density of 4.319 ± 0.18 g L-1 and areal productivity of 21.134 ± 1.83 g m-2 d, at an operational volume of 140 L. The most important parameters affecting growth rates and productivity were the mean irradiance inside the culture and the FrETR of photons for phytochemistry. Compared to previous study using anaerobic digestate of piggery waste effluent of similar turbidity, areal productivity was improved 9.5 times. Further, the use of food waste digestate as a source of nutrients for the cultivation of high value species of microalgae such as Dunaliella salina, that are highly sensitive to ammonia nitrogen was explored. The main aim of this study was to assess the possibility of utilizing nutrients from food waste digestate for growing D. salina. Dunaliella salina was cultivated in modified F-medium with partial to complete replacement of the nitrogen (0 – 100 % digestate nitrogen) source under high salinity (150 - 250 ‰) conditions to study both cell growth and carotenogenesis. It was found that the growth and productivity of Dunaliella salina was not inhibited by ammonia nitrogen found in the food waste digestate. Irradiance above 400 μmol photons m-2 s-1 and higher salinity had combined negative effects on growth and carotenogenesis. However, under increased irradiation and temperature, in comparison with cultures grown in synthetic medium with nitrate salts as nitrogen source, there was no significant difference in biomass productivity when D. salina was cultivated using food waste digestate as sole nitrogen source. Finally, a comparative study was performed between the depth optimized inclined thin layer pond and an open raceway pond, both occupying a surface area of 11 m2 under outdoor conditions, to ascertain and demonstrate the advantage of the inclined thin layer pond for the treatment and valorization of food waste digestate using Chlorella sp, Mur 269. The inclined thin layer pond supported a much higher average biomass density of 6.807 ± 0.15 g L-1, 7 times more in comparison to the open raceway pond, without severe photolimitation. Volumetric and areal productivities of the inclined thin layer pond were 0.563 ± 0.1 g L-1 d-1 and 31.916 ± 1.11 g m-2 d-1 respectively, 17 and 3 times higher than observed in the open raceway pond. Areal nutrient removal by the microalgae biomass were 2359.759 ± 64.75 mg m-2 d-1 and 260.815 ± 7.16 mg m-2 d-1 for nitrogen and phosphorous respectively in the inclined thin layer pond, 2.8 times higher than observed in the open raceway pond for both nutrients. The results described above show that employing tolerant species of microalgae like Chlorella sp, Mur 269 offer an advantage for the treatment of high strength effluents, reducing excessive dilution, and unequivocally shows that the inclined thin layer pond is the more performant system for the treatment of highly turbid waste effluents such as food waste digestate