Sections
Dehydration ranges
Waste dehydration table
Scale of sludge consistency according to its water content percentage
In most cases in which sludge is generated, it has a solids content of around 40 g/L (4%). A further increase in this concentration by extracting the water retained by the sludge is impossible using gravity-based processes. As such, the sludge must be subjected to a mechanical process, normally filtration or centrifugation, to achieve dryness values of 20% or higher. Thermal processes are also effective and are used when the desired degree of dryness is much higher.
Dehydration techniques
Most efficient waste dehydration techniques are:
- Band filters
- Filter presses
- Vacuum filters
- Centrifugation
- Thermal drying
Therefore water-treatment systems generate solid waste with a high water content. Partial or complete elimination of this water is key to reducing transport and management costs. The most convenient dehydration technique in each case will depend on the characteristics of the sludge produced and external factors (workforce availability, energy prices, space available, etc.). Additionally, if a high degree of dryness is required, a thermal process must be used.
1. Waste dehydration using band filters
The operation of band filters is based on tipping the sludge onto a continuous band of filter material that passes between a series of rotating rollers. As the sludge passes between the roller it is compressed, drained and loses some of the water contained in it, resulting in a degree of dryness of 20-30% at the exit depending on the type of sludge concerned. The rollers usually have different diameters in order to slowly increase the pressure exerted and change its direction, thereby also exerting a shear effect.
The advantages of this technique include relatively low operating costs, low energy consumption and limited labor needs. The main drawbacks include the low durability of the filter material and the high sensitivity to the type of sludge.
2. Waste dehydration using filter presses
These filters comprise a frame-type structure housing a series of plates, each of which contains filter cloths. The sludge penetrates the cavities formed between two neighboring plates and high pressure (around 300 kg/cm2) is applied using a hydraulic system for up to 3 hours. The water passes through the filter cloth and is collected in a filtrate collector, whereas the sludge forms cakes with a thickness of around 2-3 cm. Finally, the plates are separated and the filter cake falls out due to gravity. After cleaning, the system is prepared to repeat the filtration cycle. Although drynesses of up to 40% can be achieved, investment costs are high and operation is discontinuous and very laborious.
3. Waste dehydration using vacuum filters
These comprise a perforated drum covered with a filter cloth in the interior of which a vacuum is applied. The drum is partitioned into various independent sections. It is partially immersed in a tank in which the liquid sludge to be dehydrated is stored. As the drum turns, the different sections each pass through a cycle involving filtration, washing and discharge of the filter cake, which can have a dryness of between 20% and 30%. The rotation rate of the drum depends on the characteristics of the sludge.
This system has a high load-carrying capacity, although investment, maintenance and operating costs are high.
4. Waste dehydration by centrifugation
Water is separated from the sludge by applying centrifugal forces, normally of around 10,000 times the force of gravity. The centrifuge comprises a conical cylindrical rotor with a helicoidal screw in its interior. These two components rotate at high speed and in the same direction, although the rotor rotates faster than the screw. The sludge is fed into the central part of the rotor and is pushed towards the periphery by the centrifugal force. As water is lighter it passes through the helicoidal screw and is collected at one end. The sludge gradually builds up on the walls of the rotor, is dragged towards the conical region and exits through an opening in the lower part of the opposite end. The dehydrated sludge typically has a dryness of between 15% and 30% depending on the type of sludge concerned and the centrifugation conditions.
This is an efficient system that works continually, is very compact and requires very little space. However, maintenance is critical and it consumes a large amount of power.
5. Waste dehydration by thermal drying
Dehydration above 35-40% is not possible using mechanical methods as the free and capillary water has already been removed and only intracellular water remains. As such, the cell structure must be broken by either biological, chemical or thermal means in order to remove this water.
Thermal drying involves directly or indirectly increasing the temperature of the sludge so that the water evaporates. It is used to reduce the volume of the sludge and therefore to reduce management costs, as well as to recover it.
Irrespective of the dehydration process, the efficiency of the process improves markedly if sludges are chemically conditioned beforehand.
The most widely used chemicals are iron(III) chloride, iron or aluminum sulfate and calcium oxide (lime). Cationic polyelectrolytes have also been used with excellent results as they are easy to dose, consumption is not particularly high and they provide high efficiency. In addition, and in contrast to inorganic salts, polyelectrolytes do not increase the quantity of sludge produced. The most effective product, and the dose to be used, will depend on the characteristics of the sludge, which is why laboratory tests must be performed in each case.
A large amount of energy is required to evaporate the water, therefore thermal drying can only be used when residual energy is available from another process.Then, the cogeneration processes are necessary
Cogeneration
Electrical power generation can be accomplished by a wide variety of processes. In the majority of these processes we find a dynamo or alternator that are driven by a heat engine or turbine. In order to move this turbine, high temperature steam is used. This steam is generated by heating ultra-pure water obtained from a water treatment plant (WTP).
When generating electrical energy not all heat from the vapor is utilized. This thermal energy “surplus” can be emitted into the atmosphere; that which is lost and not utilized to its full potential or it can be reused. This is where the different techniques of cogeneration enter the picture. These techniques allow us to take advantage of an important part of the thermal energy that is normally dissipated into the atmosphere.
Cogeneration technologies attain yields of 85% if we add the vapor that generates electricity and the residual heat that is reused, both of which help to obtain high levels of energy savings without disrupting the production process.
As we have mentioned before in previous posts, the different types of plants that exist for electrical energy generation need a water treatment plants (WTP) which can clean impurities from water that will be transformed into vapor. In addition, they require a wastewater treatment plant (WWTP) that treats the effluents that are obtained from the power generation process.
The different technologies used in a WTP and a WWTP have important thermal necessities that can be met by cogeneration plants.
The key is to use the exhaust gases and the thermal energy coming from the cooling circuits of the engines; taking advantage of them in order to provide the necessary heat energy for different kinds equipment such as vacuum evaporators, crystallizers and reverse osmosis plants.
Furthermore, efficiency is improved with heat exchangers for heating the liquid before it enters the evaporator in addition to taking advantage of the latent heat from condensation of vapours.
If a high degree of dryness is required, a thermal process must be used and one example of this requirement is heavy metal recovery from wastewater.
Heavy Metal recovery
Due to the intense industrial activity on our planet, the concentration of metals in the soil, air or water has suffered a great increase posing many environmental risks.
The high concentration of metals in nature can affect plants, animals and human beings, increasing the risk of skin and lung cancer, major diseases of the kidneys and liver, and possible affects to the nervous system; heavy metals are correlated to many development disorders like Autism, Asperger and more.
Thus, the elimination of these dissolved metals in industrial process wasters is imperative before its discharge or reuse for other uses. Recovery of heavy metals is critical.
Among the different technologies available, distillation by vacuum evaporation is the most effective choice for the recovery of water contaminated with heavy metal ions, most notably by distillation with falling film vacuum evaporators.
Vacuum evaporators make it possible to recover water free of impurities with far greater results than other effluent treatment technologies such as membranes, ion exchange, etc.
Thus, after the thermal separation process, about 90% of water is recovered, totally free of metallic ions. The remaining 10% becomes part of a liquid concentrate with a very high density of ions that can be sent to waste management or be treated again by crystallization technology. This enables the separation and recovery of a large part of the metallic ions that can be reused as raw material.
Moreover, vacuum evaporation, despite its increased short-term investment can be the cheapest option for industries with high flow rates. In the long run, it results in significant savings in waste management costs in addition to having higher energy efficiency that other treatment technologies.
