John J. Milledge

Comments on 'Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable'

A recent review paper considers the potential of algal biomass as a source of liquid and gaseous biofuels, but there are a number of issues concerning the results and conclusions presented. These include the biomass energy values, which in some cases are unusually high; and the apparent production of more energy from processed biomass than is present in the original material. The main causes for these discrepancies include the choice of empirical formula for protein; confusion between values calculated on a total or volatile solids basis; and the lack of a mass balance approach. The choice of protein formula also affects predicted concentrations of ammonia in the digester. These and other minor errors contribute to some potentially misleading conclusions which could affect subsequent interpretations of the overall process feasibility.

Methods of energy extraction from microalgal biomass: a review

The potential of algal biomass as a source of liquid and gaseous biofuels is a highly topical theme, The process operations for algal biofuel production can be grouped into three areas: growth, harvesting and energy extraction, with a wide range of combinations of unit operations that can form a microalgal biofuel production system, but as yet there is no successful economically viable commercial system producing biofuel. This suggests that there are major technical and engineering difficulties to be resolved before economic algal biofuel production can be achieved. This article briefly reviews the methods by which useful energy may be extracted from microalgae biomass: (a) direct combustion, (b) pyrolysis, (c) gasification, (d) liquefaction, (e) hydrogen production by biochemical processes in certain algae, (f) fuel cells, (g) fermentation to bioethanol, (h) trans-esterification to biodiesel, (i) anaerobic digestion.

The cleanability of stainless steel used as a food contact surface: an updated short review

The objective of this study, was to examine the alterations of the fatty acids profile and the formation of trans fatty acids isomers during heating of soy oil (SO) and partially hydrogenated soy fat (PHSF) in the process of french frying. The fatty acids profile was examined by gas chromatography with a 100m capillary column. The fatty acids trans monounsaturated were the predominant among trans isomers. The isomers trans monounsaturated corresponded to 2.1% of the total, after 10 hours of frying, and 14.3% after 50h - frying. Poliunsaturated fatty acids decreased from 59.9% to 32.6% after 50h - frying. For the PHSF, the initial content of 20.2% of trans monounsaturated isomers increased to 28% after 50h - frying, and w3 and w6 - essential fatty acids decreased from 12.8 to 7.3%, in the same period. These results confirmed the formation of trans isomers in oil and hydrogenated oil in frying process, and that the trans isomers formation occurred in lower proportion in the hydrogenated oil. These results show the importance of identifying the trans fatty acids in oils and fats utilized in frying process.

High-value products from macroalgae: the potential uses of the invasive brown seaweed, Sargassum muticum.

Marine seaweeds represent an abundant source of natural products and may harbour valuable chemicals. The brown seaweed Sargassum muticum is an invasive species to the coasts of the British Isles, mainland Europe and North America. Attempts at its eradication and control have generally not been successful, although time-consuming and costly. Commercial exploration of this biomass for food, fuel and pharmaceutical products could encourage its harvesting and control. Though S. muticum might be unsuitable as a source of biofuel due to high ash and water content, this rapidly growing macroalga has a naturally high content of antioxidants, carotenoids and phenols, including the well-known anti-cancer compound fucoxanthin, making this species a potential source of a range of pharmaceutically relevant materials.

Potential process "hurdles' in the use of macroalgae as feedstock for biofuel production in the British Isles

Abstract This review examines the potential technical and energy balance hurdles in the production of seaweed biofuel, and in particular for the MacroBioCrude processing pipeline for the sustainable manufacture of liquid hydrocarbon fuels from seaweed in the UK. The production of biofuel from seaweed is economically, energetically and technically challenging at scale. Any successful process appears to require both a method of preserving the seaweed for continuous feedstock availability and a method exploiting the entire biomass. Ensiling and gasification offer a potential solution to these two requirements. However there is need for more data particularly at a commercial scale.

Energy balance of biogas production from microalgae: development of an energy and mass balance model

The paper describes the construction of a mechanistic energy balance model for the production of biogas from anaerobic digestion of micro-algal biomass grown in raceways, based on simple principles and taking into account growth, harvesting and energy extraction. The model compares operational energy inputs with the calorific value of the output biomass in terms of the energy return on operational energy invested (EROOI). Initial results indicate that production of microalgal biogas will require: a) Favourable climatic conditions. The production of microalgal biofuel in UK would be energetically challenging at best. b) Achievement of ‘reasonable yields’ equivalent to ~3% photosynthetic efficiency (25 g m-2 day-1). c) Low or no cost and embodied energy sources of CO2 and nutrients from flue gas and wastewater. d) Mesophilic rather than thermophilic digestion. e) Adequate conversion of the organic carbon to biogas (≥ 60%). The model itself provides a powerful assessment tool both for comparison of alternative options and potentially for benchmarking real schemes.

Liquid ‘hold-up’ on stainless steel surfaces: I — Effect of surface finish

Abstract The effect of stainless steel surface finish on the ‘hold-up’ of a representative food liquid (sucrose solution) was studied for drainage angles between 0·46° and 3·7°. Surface finish was found to have a significant effect on ‘hold-up’, but no correlation was found between the Ra value and ‘hold-up’. An explanation of the results is proposed based on the effect of the nature of surface roughness rather than its magnitude (as measured by the Ra value) on wetting and contact angle. The orientation of grain direction had only a small effect on ‘hold-up’ and was dependent on the surface finish. It is suggested that electro-polishing may give the most cost-effective finish for stainless steel food contact surfaces. Drainage angle was found to have a considerable effect on ‘hold-up’ and it is recommended that the minimum drainage angle for food plant should be 2°.

Liquid ‘hold-up’ on stainless steel surfaces: II — Effect of surfactant concentration

Abstract The effect of surfactant concentration on the ‘hold-up’ of a representative food liquid (sucrose solution) on various stainless steel surfaces was studied. Surface finish was found to have a significant effect on ‘hold-up’, but no correlation was found between Ra value and ‘hold-up’. Although surfactant concentration was found not to be a significant independent variable it did have a significant interaction with surface finish, the differences in ‘hold-up’ between the surface finishes being reduced by an increase in surfactant concentration. Differing surfactant concentrations in food soils is proposed as a possible explanation of the conflicting reports in the literature on the cleanability of stainless steel. An explanation of the effects of surface finish and surfactant concentration on ‘hold-up’ is proposed in terms of the effects on wetting and contact angle of surfactant and the nature of the surface.

Anaerobic digestion and gasification of seaweed

The potential of algal biomass as a source of liquid and gaseous biofuels is a highly topical theme, with over 70 years of sometimes intensive research and considerable financial investment. A wide range of unit operations can be combined to produce algal biofuel, but as yet there is no successful commercial system producing such biofuel. This suggests that there are major technical and engineering difficulties to be resolved before economically viable algal biofuel production can be achieved. Both gasification and anaerobic digestion have been suggested as promising methods for exploiting bioenergy from biomass, and two major projects have been funded in the UK on the gasification and anaerobic digestion of seaweed, MacroBioCrude and SeaGas. This chapter discusses the use of gasification and anaerobic digestion of seaweed for the production of biofuel.