Waste Water Treatment - (Ahmed Vawda)Por Vawada, Ahmed
Inserida em 2008-06-12 Actualizada em 2009-09-24
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 organisms 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
By far, the nature of cane sugar factory effluents is dealt with by secondary treatment. The common tertiary treatment is generally media filtration. The beet sugar industry requires some nitrification/de-nitrification process.
In this report, we shall focus on the secondary treatment, as this by far is the largest contributor to the removal of pollutants in the sugar industry.
Selection of Treatment Systems
The selection of a particular treatment plant depends on the degree of treatment required to bring the quality of effluent to a permissible level of effluent from the treatment plant. This ensures that the final effluent is either safe for disposal or acceptable for specific re-use or recycling 2.
There are basically two types of secondary treatment, i.e. aerobic and anaerobic. Aerobic treatment is selected for moderate pollution loads, while anaerobic treatment is reserved for highly contaminated waste water.
Aerobic Treatment of Waste Water
Specific bacteria operating in the presence of oxygen oxidise organic matter to release carbon dioxide and sludge. It depends on optimum temperature, pH and oxygen transfer. pH below 4 is lethal for micro organisms. There is also a minimum nutrient requirement which is differs from industry to industry. Agitation of the aerobic stage is essential to bring waste water, biomass and dissolved oxygen into contact with the micro organisms. The bacteria which clump together are referred to as activated sludge, and is eventually settled in the clarifier.
Figure 1. A typical Aerobic Flow Sheet
Anaerobic Treatment of Waste Water
Anaerobic digestion is the breakdown of organic matter by microbiological population that takes place in an oxygen free environment. Anaerobic literally means without air. Anaerobic treatment has been found to be very effective in the treatment or pre treatment of industrial waste waters containing high organic concentration. In this regard, it has become almost standard in the beet factories in Europe. It has also found favour in treating other high organic pollutants found in beet factories and distilleries. Anaerobic degradation of organic matter consists of multi-stage processes occurring simultaneously. Every stage is accomplished by a different group of bacteria. During every degradation step, the bacteria emit special by-products. The generalised anaerobic degradation model shows four stages for the transformation of carbohydrates, fats and proteins into methane and by-products.
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.
Figure 2. Typical layout of an Anaerobic Flow Sheet.
Advantage of the Anaerobic Processes
Dis-Advantage of the Anaerobic Process
- Low production of biological sludge.
- High treatment efficiency, up to 90% COD removal.
- Low capital cost, the reactor is essentially a tank.
- No oxygen requirement, hence low power consumption.
- Methane production is a potential source of fuel.
- Low nutrient requirement.
- Low operating costs
- Sensitive to temperature of influent, works best at about 35 - 40 deg C
- Sensitive to pH, works best at 6.5 7.8.
- Treatment efficiency diminishes as the influent COD reduces.
- Does not completely eliminate COD. Requires a secondary process.
- Methane gas may be a liability if no user available.
Figure 3 Carbon flow in Anaerobic and Aerobic Systems
Figure 3 shows the relative advantage of anaerobic digestion over aerobic oxidation process, with regards to the lower sludge generation.
These are tanks, usually circular, containing loose media of small rocks, plastic packing, limestone chips etc. The media has high surface areas to support the bio-films that form on its surface. The waste water is distributed through perforated rotating arms radiating from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the medias surfaces and eat or otherwise reduce the organic content. This bio-film is grazed by insect larvae and worms which help maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface7. The advantage of this system is that, unlike an activated sludge plant, trickle filters recover very quickly after temporary high strength polluting loads have gone by. These type of units are very useful to expand the capacity of existing plants2.
Membrane Bio-reactors MBR
MBRs combine activated sludge treatment with membrane filtration. The membrane component uses low pressure micro or ultra filtration and eliminates the need for clarification ad tertiary treatment. The membranes are typically immersed in the aeration tank (however, some applications utilize a separate membrane tank). One of the key benefits of a MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes. The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling8.
Rotating Biological Contactors
Rotating biological contactors (RBC) are mechanical secondary treatment systems which are robust and capable of withstanding surges in organic loads. First implemented in the 60s,they since developed into reliable operating systems based on simple construction. The rotating disk supports the growth of bacteria and micro-organisms present in the waste water, which breakdown and stabilize organic pollutants. To be successful, micro-organisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage. Effluent from the RBC is then passed through final clarifiers where the micro-organisms in suspension settle as sludge. The sludge is withdrawn from the clarifier for further treatment7.
Constructed or natural wetlands have been used to treat several types of wastewater and runoff. High levels of removal can be achieved for a number of contaminants, including COD substances, suspended solids, nutrients, metals and organic compounds, in treatment wetlands. Constructed wetlands typically incorporate a shallow layer of surface water, flowing over mineral (sandy) or organic (peat) soils. Vegetation often consists of marsh plants, such as bulrushes, reeds and other plants, but may also include floating and submerged aquatic vegetation, as well as wetland shrubs and trees.
Up to 95% removal of COD has been reported at a sugar factory where the authors claim the following benefits6:-
- Ease of operation
- Low installation and building costs
- Low operation costs
- Low energy costs
- Insensitivity to fluctuating loads
- Environmentally acceptable, offering considerable potential for wild life conservation.
The author thanks the management of Savola for permission to publish this paper.
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2. Govender, B. (1992) Expansion of Noodsberg Effluent Plant Utilising a Trickling Filter. Proceedings
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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
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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
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