Investigating fouling and cleaning during the filtration of Gum Arabic to save water and reduce energy

Abstract

Gum Arabic is an exudate from Acacia trees and grows mainly in the sub Saharan region with Sudan being one of the world’s largest producers. Gum is imported as kibbled amber aggregates which undergo dissolution, filtration and pasteurisation to remove any contaminants before being spray dried to produce a fine white powder suitable for use. Gum Arabic is widely used industrially, particularly in the food industry as it is a very effective emulsification agent. During the industrial processing of Gum Arabic, large quantities of both water and energy are used. Water and energy consumption are particularly high during the dissolution and spray drying processes. Approximately 14000 tons of wastewater are produced each year from Kerry Ingredients processing plant in Cam, UK containing ca. 2.0 wt. % Gum Arabic.1 This requires expensive disposal methods.

The application of membrane technology can reduce the volume of water required during processing allowing the potential for recycling rather than requiring fresh water supplies. In addition, the method of microfiltration can separate Gum Arabic from the waste stream. This reduces the waste created during processing, resulting in a more sustainable process. The filtration of complex food products inevitably leads to fouling – the build-up of unwanted deposits on the membrane surface and in the membrane pores. This project investigates the chemistry and engineering of the Gum adhesion and removal process. This work is a study of what is occurring during fouling, methods to reduce it, and cleaning strategies to help restore the membrane following fouling. A proof of principle has been established, highlighting the potential for separating Gum Arabic from waste streams.

Presented here are the findings from the filtration of Gum Arabic through commercially available MembraloxTM alumina microfiltration tubular ceramic membranes. In addition, the use of flat sheet alumina ceramic membranes has been studied to gain an insight into the mechanisms of fouling and removal.

During microfiltration, the use of high crossflow velocities (2.3 m s-1), low transmembrane pressure (1.5 bar), and operation under dynamic conditions was found to minimise fouling. Fouling is predominantly in the form of a cake or gel layer on the membrane surface. Applying an increased crossflow velocity therefore helps to sweep foulants from the membrane surface and reduce the thickness of the foulant layer. The application of a low transmembrane pressure is expected to lead to a less compact foulant layer leading to a reduced resistance to permeate flow. Membranes with pores in the range of 0.2 – 2.0 μm were investigated, with little change in the steady state filtration flux. This suggests that the system is not limited by the pore size but rather the foulant limits mass transfer.

Analysis carried out on the foulant layer, using a variety of analytical techniques, confirmed that most of the fouling was on the surface and was caused by a mixture of organic components and calcium.

This work also investigated cleaning of the membranes following fouling. Use of sodium hydroxide, citric acid and Ultrasil 11 gave different flux recoveries and properties following cleaning. The surfaces of the membranes have been studied to gain an insight into the mechanisms of removal during cleaning. The most effective cleaning method during the first cycle is to clean with 0.5 wt. % sodium hydroxide followed by 0.1 wt. % citric acid. This lead to a flux recovery of 86 ± 5 %. It has been hypothesised in this study that the sodium hydroxide leads to swelling of the foulant layer leading to a more open structure which increases the effectiveness of cleaning with citric acid. Citric acid allows chelation of calcium present in the foulant as well, as sodium remaining on the surface following cleaning with sodium hydroxide. The addition of sodium hypochlorite to the sodium hydroxide solution aids in improving the flux recovery with an increase ca. 5 wt. % observed. The use of Ultrasil 11 contains sodium hydroxide, surfactants and EDTA. This was therefore investigated as a one-step cleaning agent, allowing swelling and chelation to occur simultaneously. The flux recovery of 50 ± 6 % was lower than that for the two stage cleaning process. This suggests that dissolution of Gum in citric acid as well as chelation improves removal.

Multiple fouling and cleaning studies were carried out to investigate the long term impact of cleaning with sodium hydroxide/sodium hypochlorite alone and a two stage clean with sodium hydroxide/sodium hypochlorite followed by citric acid. The two stage clean outperformed that of the alkali clean alone in terms of selectivity and permeate flux. While the flux recovery was greater for the two stage clean after cycle 1, little difference was observed after multiple cycles.

Pre-treating membranes allows a method to control the surface properties with the hydrophobicity, zeta potential and adhesion strength showing marked changes following simple pre-treatments with sodium hydroxide or citric acid. The effect of pre-treatment on the selectivity and throughput has been reported in this study. While pre-treatment was not shown to reduce the fouling propensity, pre-treatment with citric acid resulted in an increase in rejection of Gum Arabic from 60 % to 85 % for the first cycle using a 0.5 μm flat sheet membrane.

Date of Award	22 Jun 2016
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Bird (Supervisor), Darrell Patterson (Supervisor) & Charles Wilson (Supervisor)