Urban nutrient recovery through cultivation of chlorella sorokiniana

• Main Author: Lim, Chun Yong
• Other Authors: Wang Jing-Yuan
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Environmental engineering::Waste management; DRNTU::Engineering::Environmental engineering::Water treatment; DRNTU::Engineering::Civil engineering::Water resources
• Online Access: http://hdl.handle.net/10356/63272
• Institution: Nanyang Technological University

High rate food consumption in urban cities causes vast amounts of nutrients, specifically nitrogen (N) and phosphorus (P) in this thesis, used in agriculture to end up in discarded food wastes and human urine. The latter contains 60 % of total N and 80 % of total P in all urban wastewater streams combined. Current conventional treatment processes do not recover these nutrients, resulting in more than 30 million metric tonnes (MMT) of N being released back to the atmosphere and more than 10 MMT of P not recovered for reuse, mostly buried dispersed in urban landfills. While such massive wastages are ongoing, another promising industry is yearning for cheap nutrients. Microalgae have been widely classified as the 3rd generation biofuel feedstock and nutrient resource has been identified as one of the key challenges for large-scale microalgae production. Recent years, the bulk of investigation has been on using various wastewater streams from industries, piggeries and dairies as nutrient supplies for microalgae cultivation. In this thesis, 2 waste streams have been identified for urban nutrient recovery: effluent of anaerobically digested food waste (ADE) and fresh human urine. It would be of significant environmental benefits and economic savings should the recovery of such urban waste nutrients be successfully coupled with microalgae cultivation. Chlorella sorokiniana was the microalga species used in this thesis, selected from review of available literature, and subsequently proven to be a very suitable species for the objectives. During exponential growth phase, its mixotrophic specific growth rate (0.134 h-1) was almost the addition of those of photoautotrophic culture (0.045 h-1) and heterotrophic culture (0.090 h-1), with doubling time as short as 5 h. When cultivated in semi-continuous mode, the cells could adapt with 12-hourly interchanging trophic conditions without any observed growth lag for 15 consecutive days. Such high growth capability and adaptability are advantageous for nutrient recovery from sources with fluctuating characteristics common in wastewater streams. It was also found that different trophic conditions did not have any significant impact to cell size and weight. C. sorokiniana was subsequently cultivated in 6-L semi-continuous photobioreactor using synthetic wastewater. 0.98 g L-1 d-1 of algal biomass was produced in tandem with a 94.7% removal of 254.4 mg L-1 C-acetate, a 100% removal of 84.7 mg L-1 N-NH4 and a removal of 15.0 mg L-1 P-PO4. ADE did not show signs of any microalgal growth inhibitors and the cells could grow well in medium containing high concentrations, up to 75 % (v/v), of ADE. However, other macro- and micro-nutrients were low in concentrations as compared to N and P. Magnesium has been found to be the most critical supplement required in ADE to ensure growth of the microalgal cells. However, the elemental composition of the cultivated biomasses could be different to that in the control biomasses, specifically iron and manganese. In the 6-L semi-continuous culture setup used, 6-time diluted ADE resulted in the best nutrient recovery. As high as 86.8 and 15.97 mg L-1 d-1 of N and P could be recovered, with recoveries as high as 94.6 and 100 %, respectively. Biomass productivity using ADE was consistently 20 % lower as compared to the cultivation in synthetic wastewater, but the average lipids contents were more than 35 %, higher than the 31 % in control cultures. Microalgae cultivation in concentrated urine was proven to be difficult as urea hydrolysis and precipitation occurred rapidly. The approach was then changed to use fresh urine as daily nutrient stock for N and P, microalgal biomass grew from 0.44 to 0.96 g l-1 utilising 62.64 mg l-1 of N and 10.64 mg l-1 of P, achieving 80.4 % and 96.6 % recoveries, respectively in a 1-day non-sterile cultivation cycle. Overall, microalgae grown with urine added as nutrient supplement show no signs of inferiority as compared to those grown in recipe medium BG11 in terms of mass and chlorophyll a growth rates as well as resulting lipids (36.8 %) and energy contents (21.0 kJ g-1). The use of an organic flocculant for the harvesting of the microalgal cells is important to ensure a wide range of downstream uses for the biomasses cultivated. Chitosan is one such potential flocculant and was experimented in this study. For microalgae concentrations of 0.2 to 1.0 g/L, up to 30 times concentration or more (converted from 97% algae concentration reduction in supernatant) could be achieved using a chitosan:cell ratio (w/w) of 1:160. For higher algal concentrations (2.0 to 5.0 g l-1), chitosan requirement could also be higher (up to a chitosan:cell ratio of 1:80). Multiple dosing and strict pH controls proved effective in minimising chitosan usage. In this study, chitosan flocculation could consistently harvest up to 99% of the free cells , thus providing an almost cell-free supernatant discharge that could possibly be reused, as well as producing a high-density algal broth for more effective transportation to centralised downstream green biorefining processes.