A Review of Conventional Waste Water Treatment Processes in the Sugar Industry
A. S. Vawda
Savola Sugar Middle East, Jeddah, Saudi Arabia, Avawda@savola.com
A review is conducted on conventional waste water treatment technologies that have proven successful over few decades. The treatment processes of beet, cane and raw sugar refineries are discussed. Since the pollutants are bio-degradable, sugar factories conventionally and universally employ biological systems.
Keywords: effluent, waste water, sugar refinery, aerobic, anaerobic, membranes, ion exchange, environment,
All sugar refineries and factories require water and consequently discharge waste water.
Refineries, cane factories and beet factories produce waste water with different organic strengths, with beet being by far the most aggressive effluents.
Waste water treatment plants are designed to comply with local environment regulations and vary from country to country. The type of by products plants, eg distilleries, board plants, animal feed plants etc also determine the quality of the waste water discharged from the plant.
While sugar, the main contaminant of sugar factory effluent, is not toxic, it readily provides a source of soluble food which is an ideal substrate for bacterial growth. The exponential growth of bacteria causes the depletion of oxygen in natural streams. Aquatic organisms that require oxygen will suffer and may die as a result. Recovery time depends on the available oxygen supply and the amount of pollution that has occurred. Waster water treatment systems utilise the very same process, but under controlled conditions.
Ideally, the minimisation of organic and hydraulic load by good design and operating discipline, cannot be over emphasized. The quest for zero effluent is a desirable journey and has economic and environmental benefits.
In order to ensure successful operation of the effluent treatment plant, it is important to have accurate information of the water streams being handles on the site. A simple water balance that is continually updated, ensures that the operations personnel keep their eyes on the ball.
Beet factories produce more waste products than cane factories or raw sugar refineries. Beet plants generate two types of waste waters, flume wastes and factory wastes. The flume waste water system is used for transporting and preliminary cleaning of beets. The sugar that is leached into this water contributes a high organic load in the flume system and can vary form a few hundred mg/L BOD to more than 20,000 mg/L. BOD. Due to the high strength of beet factory liquid wastes, anaerobic digesters are almost universal. This technology saves space and reduces sludge generation and is discussed later. Aerobics systems are also used when there is not enough land for large lagoons.
Sugar Cane factories generate two types of waste waters, spray pond/cooling tower over-flows and factory wastes. The cooling water is generated by condensing vapours in the barometric condensers and the organic content varies from 0 – 1000 mg/l depending on the extent of evaporator entrainment and blow down practice. Factory waste water has many sources: Cane yard and mill house washings containing bagacillo, tank over flows, floor washings, heat exchanger chemical cleaning, boiler ash sluicing water grease and oil spillage. Storm water run-off from cane yards are particular polluting and efforts have been made at many factories to address this4.
Raw sugar refineries generate two types of waste waters, spray pond/cooling tower over-flows and factory wastes. The cooling water is generated by condensing vapours in the barometric condensers and the organic content varies from 0 – 1000 mg/l depending on the extent of evaporator entrainment and blow down practice. Factory waste water has many sources: ion exchange effluents, waste from Phosphatation and carbonatation, tank over flows, floor washings, heat exchanger and filter chemical cleaning, grease and oil spillage. Modern refineries that have good hygiene and maintenance systems can get away with aerobic systems as the relatively low strength of the waste water is conducive to aerobic systems5.
Waste water from sugar factories have a physical, chemical and biological component with regards to its environmental impact. It is rich in carbohydrates, and while these are not toxic to aquatic life, they disturb the micro organism’s growth phase, thereby causing oxygen depletion. The principles of biological treatment are the same in nature and in a treatment plant; however, natural streams have a limited capacity to “process” high organic loads 1.
Biological wastewater plants are designed and operated on the basis of oxygen demand (BOD or COD) received and removed. For this reason, a quick and easy measurement needs to be made and COD due to its raid determination is almost universally used. The BOD test takes five days to completion and is not practical for process control.
Major treatment processes fall into two categories3. :-
a) Suspended Growth Process, for example activated sludge, aerated ponds, and anaerobic digesters etc. Here the micro-organisms are maintained as a suspension in the reactor by an appropriate mixing method.
b) Attached Growth Process, for example trickling filters, rotating biological contactors etc. In this case, the micro-organisms remain attached to some fixed object, e.g. trickling filter, biological contactor etc
The waste water treatment process can be broken down into three distinct treatment systems:
These consists of unit operations to remove suspended solids, oils and major debris. This is accomplished by screening, grit removal and oil skimming. Modern plants, like United Sugar Company of Egypt, Sokna sugar refinery, have installed a dispersed air flotation (DAF) unit to remove a large portion of suspended solids which may interfere with the secondary treatment systems.
This system undertakesmost of the work in reducing the polluting load and consist generally of the following unit operations:-
· Activated sludge process ASP (Sludge return facility)
· Waste stabilization Ponds (Oxidation ponds)
· Oxidation lagoons (Aerated Lagoons)
· Oxidation ditches (Extended Aeration Systems)
· Rotating Biological Contactor (RBC)
· Up-flow anaerobic Filter (UAF)
· Up-flow anaerobic Sludge Blanket (USAB)
These are necessary to remove or reduce the concentration of residual impurities and is applied when:-
· The quality from the secondary treatment is not suitable for disposal to a public stream.
· The concentration is too high for recycling onto a required process.
The different techniques for tertiary treatment are as follows:-
· Granular-media filtration, i.e. sand filters to remove suspended solids
· Nitrification/de-nitrification to remove nitrogen, chlorine and dissolved gases.
· Biological and chemical processes to remove nitrogen and phosphorous.
· Ion exchange, reverse osmosis, electro-dialysis, chemical precipitation, adsorption ; to dissolved inorganic and organic compounds
The anaerobic digestion steps are shown as follows:-
This process involves the conversion of impurities into a form that is readily attacked by bacteria. The high molecular substances (polymers, carbohydrates, fats), un-dissolved substances and proteins are disintegrated. These substances are transformed into fragments by means of enzymes secreted by bacteria.
The dissolved fragments are consumed by fermenting bacteria. In this stage, there is some odour created due to the fermentation process, mainly due to the formation of butric acid, propionic acid, valeric acid etc, but also alcohol if there are carbohydrates in the effluent. The byproducts of the process are hydrogen and carbon dioxide.
During this phase, the organic acids and alcohol are transformed to acetic acid. The by-product of this process is hydrogen. This reaction cannot run independently. A positive energy balance can only be reached within a certain range of hydrogen partial-pressures. Thus the acetic phase is dependent on a hydrogen consuming microbiological population.
The acetic acid is broken down into carbon dioxide and methane. The reaction is pH and temperature sensitive. The pH range for effective methane conversion is pH 6.5 – 7.6. The reaction is best managed at a mesophylic temperature range of 35 – 37 degrees C.
Advantage of the Anaerobic Processes
Figure 3 shows the relative advantage of anaerobic digestion over aerobic oxidation process, with regards to the lower sludge generation.
The author thanks the management of Savola for permission to publish this paper.
1. Asadi, M, (2007) Beet Sugar Handbook, Wiley-Interscience.
2. Govender, B. (1992) Expansion of Noodsberg Effluent Plant – Utilising a Trickling Filter. Proceedings
of the South African Sugar Technologists Association 66th p.48-51169 - 172
3. Karia,G.L., Christian, R.A.(2006), Waste Water Treatment. Concepts and Design Approach. ISBN-81-
4. Munsamy, S.S (1989). Fly Ash and Boiler Ash Handling and Disposal at Sezela. Proceedings of the South
African Sugar Technologists Association 49th p.45-47
5. Vawda, A.S. Laulloo, M.R. Banakr, A. S. (2007) Pilot Plant Evaluation of Anaerobic Digestion to extend
the capacity of United Sugar Company Waste Water Treatment Plant. Paper number 991, Proceeding
of the Sugar Industry Technologist Conference.
6. Vermeulen, P. L. M.; Vawda, A. S.(1989) Efficient and low Cost Effluent Treatment using an Ash
Disposal Dam. Proceedings of the South African Sugar Technologists Association 49th p.48-51
7. William F. DeBusk . Wastewater Treatment Wetlands: Applications and Treatment Efficiency. University
of Florida. http://edis.ifas.ufl.edu/SS294#beginning
COD Chemical Oxygen Demand
BOD Biological Oxygen Demand
MLSS Mixed Liquor Suspended Solids
F/M Ratio. Foot to Mass Loading
DO. Dissolved Oxygen
SVI Sludge Volume Index