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Inexpensive non-toxic flocculation of microalgae contradicts theories; overcoming a major hurdle to bulk algal production

Paper ID Volume ID Publish Year Pages File Format Full-Text
14470 1223 2012 8 PDF Available
Title
Inexpensive non-toxic flocculation of microalgae contradicts theories; overcoming a major hurdle to bulk algal production
Abstract

There are two major energy and cost constraints to bulk production of single cell microalgae for biofuels or feed: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange; and the cost of harvesting using high-speed continuous centrifugation for dewatering. This report deals with the latter; harvesting by flocculation where theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. It had been assumed that with such electrostatic flocculation, the more cells to be flocculated, the more flocculant needed, in a linear stoichiometric fashion, rendering flocculation overly expensive. Counter to theory of electrostatic flocculation, we find that the amount of alkaline flocculant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH. Various other theories abound that flocculation can be due to multi-valent cross-linking, or co-precipitation with phosphate or with magnesium and calcium, but are clearly not relevant with the flocculants we used. Monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories, leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used. The cost of flocculation of dense cultures with this procedure should be below $1.00/T algae for mixed calcium:magnesium hydroxides.

Keywords
Algae for biofuels/feed; Flocculation; Dewatering; Alkaline flocculation
First Page Preview
Inexpensive non-toxic flocculation of microalgae contradicts theories; overcoming a major hurdle to bulk algal production
Publisher
Database: Elsevier - ScienceDirect
Journal: Biotechnology Advances - Volume 30, Issue 5, September–October 2012, Pages 1023–1030
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Bioengineering