Application of poly(2-hydroxyethyl methacrylate) hydrogel disks for the immobilization of three different microalgal species

BACKGROUND: Algal growth on solid surfaces confers the advantage of combining the algal harvesting and bioprocessing steps at a single stage, in addition to the easier handling of the immobilized cells that occupy a reduced amount of space. The current work employed the application of macroporous poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel disks as a water-insoluble, non-toxic and recyclable immobilization matrix for different microalgal strains (Nannochloropsis sp., Dunaliella salina, and Botryococcus braunii) that offer value-added products for various commercial applications. RESULTS: The study demonstrated the effect of variations in the surface characteristics of the algal strains and hydrogel surfaces on the immobilization efficiencies. Gelatin was further used to modify PHEMA hydrogels to achieve higher bioaffinity and surface hydrophilicity. The results showed that highly salt-tolerant microalgal cells (Dunaliella salina, Nannochloropsis sp.) had significantly higher tendencies to attach on the gelatin-modified PHEMA hydrogel compared with the freshwater B. braunii colonies; embedded within an extracellular matrix mainly made of hydrophobic components, which displayed better attachment to the unmodified PHEMA hydrogels. CONCLUSION: The proposed PHEMA hydrogels are easily-manufactured and highly durable materials with the hydrogel disks still retaining their integrity after several years when in contact with a liquid. PHEMA disks also have the benefits of having adjustable porosities by changing the composition of the polymerization mixture, and modifiable surface properties by simply binding various synthetic or natural molecules on their surfaces, which can bring several new opportunities for harvesting various microalgal cells with different surface morphologies and chemical compositions

Potential for energy generation from anaerobic digestion of food waste in Australia

Published national and state reports have revealed that Australia deposits an average of 16 million Mg of solid waste into landfills yearly, of which approximately 12.6% is comprised of food. Being highly biodegradable and possessing high energy content, anaerobic digestion offers an attractive treatment option alternative to landfilling. The present study attempted to identify the theoretical maximum benefit of food waste digestion in Australia with regard to energy recovery and waste diversion from landfills. The study also assessed the scope for anaerobic process to utilize waste for energy projects through various case study scenarios. Results indicated anaerobic digestion of total food waste generated across multiple sites in Australia could generate 558 453 dam3 of methane which translated to 20.3 PJ of heating potential or 1915 GWe in electricity generation annually. This would contribute to 3.5% of total current energy supply from renewable sources. Energy contribution from anaerobic digestion of food waste to the total energy requirement in Australia remains low, partially due to the high energy consumption of the country. However its appropriateness in low density regions, which are prevalent in Australia, may allow digesters to have a niche application in the country

Effects of volumetric dilution on anaerobic digestion of food waste

Despite the increasing number of small scale digesters operating, there remains a lack of information with regards to performance optimization from an everyday user's standpoint. The objective of this study was to determine the effects of volumetric dilution and food waste composition on digester performance. Batch experiments utilizing food waste majoring in carbohydrate, protein, lipid, and cellulose, subjected to five concentrations of volumetric dilution (3.7%–17.1% total solids (TS)), were conducted. Irregardless of volumetric dilution, all assays achieved substrate degradation higher than 82.5% and did not suffer methanogenic inhibition, when provided with retention times comparable to those used in small scale digesters. Protein rich and cellulose rich waste achieved the highest methane potential varying between 0.410–0.539 m3/kg volatile solids (VS) and 0.450–0.535 m3/kg VS, respectively. Protein rich assays were also observed to be the first to achieve 50% of its Bo irregardless of concentrations, followed by carbohydrates and cellulose, and lipids having a considerably longer methanation time. Results saw an increase in total methane generated but a decrease in specific yield as % total solid increased. To successfully digest lipid rich waste a dilution no lesser than 1:4 was required

The impact of landfilling and composting on greenhouse gas emissions – A review

Municipal solid waste is a significant contributor to greenhouse gas emissions through decomposition and life-cycle activities processes. The majority of these emissions are a result of landfilling, which remains the primary waste disposal strategy internationally. As a result, countries have been incorporating alternative forms of waste management strategies such as energy recovery from landfill gas capture, aerobic landfilling (aerox landfills), pre-composting of waste prior to landfilling, landfill capping and composting of the organic fraction of municipal solid waste. As the changing global climate has been one of the major environmental challenges facing the world today, there is an increasing need to understand the impact of waste management on greenhouse gas emissions. This review paper serves to provide an overview on the impact of landfilling (and its various alternatives) and composting on greenhouse gas emissions taking into account streamlined life cycle activities and the decomposition process. The review suggests greenhouse gas emissions from waste decomposition are considerably higher for landfills than composting. However, mixed results were found for greenhouse gas emissions for landfill and composting operational activities. Nonetheless, in general, net greenhouse gas emissions for landfills tend to be higher than that for composting facilities

Field performance of small scale anaerobic digesters treating food waste

Internationally, it has been reported to have over 30 million small scale anaerobic digesters. Despite the large number of existing small scale anaerobic digesters and a growing interest in food waste digestion, there has been a paucity of research regarding low cost digester's performance. This paper aims to provide a snap shot of the performances of small scale digesters treating food waste exclusively. The performance of six small scale digesters, ranging from 1m 3 to 25m 3 were recorded and analysed. Results showed consistently high substrate degradation ranging from 92.5% to 97.5% COD with daily methane production ranging between 0.25m 3/kg VS and 0.46m 3/kg VS. Composition of biogas analyzed yielded between 63.1%-66.8% CH 4 and between 27.4%-33.3% CO 2. Results also revealed a negative linear correlation between gas production and % total solids and a positive linear correlation between gas production and the organic loading rate. While small scale digesters can effectively degrade food waste; the total dependency on digester for their gas requirements is unlikely and alternative fuel source is required

Influence of food waste composition and volumetric water dilution on methane generation kinetics

Food water composition and the amount of water addition are strong determinants of a digester’s performance. Hence, the objective of this paper is to study how variations in the majoring food groups and water additions can affect digester performance. The performance of carbohydrate, protein, lipid and cellulose rich mixed food wastes, subjected to five factors of volumetric dilution, were evaluated in a controlled laboratory scale set up at 38°C and 28°C. Substrate degradation was high for all assays with 86.6 – 100% and 87.08 – 98.04% reduction in VS and COD respectively. Maximum methane (CH4) yield varied between 362.72 (carbohydrate at 1:2 dilution) and 534.79 m3 CH4/kg VS (protein at 1:6 dilution) at 38°C and 316.83 (lipid at 1:2 dilution) and 524.72 m3 CH4/kg VS (protein at 1:6 dilution) at 28°C with the maximum rate of CH4 production varying between 0.015 (lipids at 1:2 dilution) and 0.053 m3 CH4/kg VS/day (protein at 1:6 dilution) at 38°C and between 0.006 m3 CH4/kg VS/day (lipids a 1:2 dilution) and 0.026 (protein at 1:6 dilution) m3 CH4/kg VS/day at 28°C. Lipid rich waste obtained the lowest yield while cellulose and protein showed interchangeably the highest yield. To successfully digest lipid rich waste a dilution no less than 1:4 was required to improve CH4 generation and to drastically reduce retention time. Both Bo and maximum rate of CH4 production increased as dilution factor and temperature increased while lag phase decreased. Results indicate that with sufficiently long retention time, food waste up to a dilution of 1:2 did not experience irreversible inhibition problems and achieved high substrate degradation although sufficient water additions can significantly improve a digester’s lag time and CH4 generation potential