Recovery of Sugar
In the confectionery and many food and beverage industries, sugar (present as sucrose, fructose and glucose, etc.) is the main constituent in some of the process streams. Inevitably, it is also present in the effluent streams arising from these industries. There is, however, considerable interest among manufacturers to optimise process economics through product recovery, and to respond to environmental pressure to reduce the level of chemical oxygen demand (COD) generated by the presence of sugar within the waste stream.
These objectives can be achieved through the incorporation of membranes in the process line, thus enabling recovery of sugar for reuse in manufacture (or, if the feed stream is of poorer quality, allowing easier disposal of the concentrate following treatment) while simultaneously reducing the level of COD in the permeate discharge stream. Wastewater from sugar processors exhibits a high COD, with a 1% sugar solution typically producing a COD of about 10,000 mg/l. Water companies receiving this effluent calculate their charges on a number of factors, of which COD is a significant component. In some circumstances the consent for the COD discharged into the sewer may be capped.
- Effluent Treatment
- water recovery/reuse
- product recovery
Main benefits of reverse osmosis (RO)
Reverse osmosis offers many benefits over conventional evaporation technology:
- In many cases if the sugar is to be recycled, RO can replace the need for evaporators entirely. This depends on the concentration of sugar that is required in the retentate from the RO plant. In general, RO can concentrate sucrose to about 30%, and glucose to about 15%. Mixtures can be taken to a total concentration in proportion to the mix. For example a 50/50 mixture can be concentrated to about 23% solids.
- As RO does not use heat to effect the concentration, degradation of the sugars is avoided, thus the recovered, concentrated sugar can be reused within the process without the need for further treatment. This assumes that the feed stream in not contaminated, and that the biological quality is appropriate.
- Very often the permeate can be reused ‘as is’ or can be polished to be reused or disposed of with a reduced COD level. For example, trials with RO have observed >99.5% rejection on feed of 65,000 mg/l COD. This means that 99.5% of the COD is retained by the membranes, so the permeate only contains rejection on feed of 70,000 mg/l COD.
- In the case of a low-quality feed-stream where it may not be practical to reuse the concentrate, the volume of the waste stream is significantly reduced. Volumetric concentration factors of 5 to 10 times can be achieved from low solids feeds, giving waste volumes of 20% to 10% of the original. However, it should be noted that these streams will be higher in COD.
- RO is a relatively low energy process, using electrical power to run pumps. Typical power consumption is 4 to 5 kW/h/m3 of water removed, depending on the mode of operation. In general, the higher the concentration of the recovered sugar, the higher the energy consumption.
- Being modular, small units can be installed close to the source of the effluent stream, thus aiding effluent segregation and management.
To avoid expensive pre-filtration, open-channel tubular membranes can be employed, as in the case of National Raisin (now known as Champion Raisin), California. Here PCI’s RO membranes process 225-300 m3/d of 4° Brix wash water, concentrating it to 10-12° Brix. At this concentration it is economical to sell the concentrate to local distilleries to make grape brandy. The permeate from the RO plant is very low in dissolved solids and can be used for general wash-down water, irrigation of nearby vineyards or sent to drain. Avoiding discharge of the wash water directly to drain alone saves Champion Raisin around $300,000 per year. This is enough to keep the system return-on-investment within the original plan of three years.
Membrane technology is divided, depending upon the size of particle/molecule that can pass through the membrane, into: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). Reverse osmosis, providing the finest degree of separation at the smallest molecular level, has long been recognised in the industry as one of the most appropriate methods for separating and/or concentrating sugar solutions. In reverse osmosis, fluids are driven, under pressure, through a tight semi-permeable membrane capable of retaining most dissolved salts and molecular-size species. In order for the liquid fraction to be forced through the membrane, i.e. for the fluid to move from the concentrated side towards the dilute side, the operating pressure of the system must exceed the osmotic pressure of the sugar solution. Under crossflow conditions, a flow is maintained parallel to the membrane at the same time as the filtrate, usually referred to as permeate, passes through the the membrane, the boundary layer, which tends to oppose the permeation rate, is continually thinned by the fluid scouring action. This scouring minimises the boundary layer and thus increases the filtration rate. In general, the faster the crossflow over the membrane, the higher the rate of filtration and the smaller the plant.
Membrane filtration technology has developed both in the way membranes are packaged and in the type of material used. The result is a wide range of module configurations and membrane geometries that are suited to a variety of applications. Membranes can be configured in tubular, spiral, flat sheet or hollow fibre arrangements. Spiral membranes are those most commonly used in sugar recovery because dilute sugar solutions have a low solids. Individual spirals consist of tightly packed filter material, which increases the surface area of the membrane significantly, and are sandwiched between mesh spacers and wrapped in a small tube. Spiral membranes require careful pre-filtration to avoid blockage from large suspended solids
PCI Membranes is a specialist in the design and build of membrane filtration systems, with a proven track record in the development of hygienic systems for use in the food, beverage and confectionery industries. Based upon extensive experience and test work using one of its many pilot plants and a dedicated process development team, PCI ensures that the optimum membrane and plant design is used for each specific application.