Sugar is one of the important agro-based products and the sugar industry hassignificant impact onIndian economy. Sugar manufacturing process requires huge amount of water and considered to be one of the water intensive industry.
Hence, there is a lot of focus on better utilization of fresh water and recycling of process effluent at various locations/ applications. This is also a key requirement to the industry to arrive at Zero Liquid Discharge (ZLD).
Wastewater/ Effluent Generation Points in Sugar Mill
There are various sources of wastewater generating in a sugar industry and the quantity of the effluent depends on the size of the factory. In general, a 5000 TCD sugar mill produces about 1500-2000 KL wastewater per day, from various unit operations. The major locations of wastewater generating points are depicted in Figure 1 and include mill house, cleaning process, cake filtration, syrup boiling and vacuum pan sections of the mill.
The combined wastewater from the mill house, cleaning process, cake filtration etc. and has COD (2000 – 4000 ppm), BOD (1000 – 2000 ppm), suspended solids (400 – 700 ppm), pH (5-6) along with inorganic nutrients such as calcium, sulphates, carbonates, nitrates, chlorides, heavy metals, oil and grease in significant concentrations. This combined wastewater is commonly treated in an effluent treatment plant (ETP).
Apart from this, another wastewater stream in sugar mill is the ‘process condensate’ which is a waste effluent from the syrup boiling and vacuum pan sections of the mill. The condensate consists of volatile organic-inorganic compounds and traces of sugar due to overloading of the evaporators, vacuum pans and extensive boiling of syrup, which further increases the pollutant ‘strength’ of the condensate.
Putrefaction of the condensate increases microbial contamination. Due to these reasons, the condensate as such is not reusable in the mill. Currently many sugar mills face challenges in managing the process condensate due to lack of dedicated treatment solutions.
Figure 1 Wastewater generating locations in a sugar mill
Characteristics of Process Condensate
The COD/ BOD is mostly due to sugars and organic acids, along with traces of other organic compounds such as phenolics. The condensate also consists of significant quantities of sulfides. Other parameters like hardness, alkalinity, chlorides, and total dissolved solids are generally found within the acceptable limits for reuse, as shown in Table 1.
Table 1 Typical characteristics of sugar mill process condensate
|Parameter||Value||Desirable value after treatment|
|COD||100 – 500 mg/L||< 25 mg/L|
|BOD||50 – 300 mg/L||< 10 mg/L|
|pH||4 – 6||7-8|
|TDS||70- 100 mg/L||<100 mg/L|
|Hardness||< 50 mg/L||< 50 mg/L|
|Alkalinity||< 50 mg/L||< 50 mg/L|
|Turbidity||< 10 NTU||< 5 NTU|
|Sulfide-S||30-50 mg/L||< 10 mg/L|
|Sulfates-S||< 50 mg/L||< 30 mg/L|
Possible end uses of condensate
After proper treatment, the condensate may be used for various unit applications and some of these applications are depicted in Figure 2. Major use of the recycled condensate can be as make-up water in cooling tower and for vapor cooling in injection channels.
In the mill house, the condensate can be used for moistening the cane fibers for sugar extraction, and to wash the crushing equipment. Additionally, in those units where the sugar mill is integrated with analcohol distillery, the condensate may also be used to dilute the molasses for fermentation. In order to make the condensate fit for these applications, proper treatment process is required.
Treatment of the Condensate
The general treatment process of sugar mill condensate is based on the following principles and the comparison between these treatments is summarized in table 2.
Physical Treatment is a process in which the pollutants are mechanically removed from the wastewater. Most common processes under physical treatment are screening, sedimentation, micro-, ultra- and nanofiltration, etc. However, dissolved solids such as sugars and miscible volatile compounds present in the condensate cannot be removed using these processes.
Some other physical processes like adsorption and reverse osmosis can be used to treat the process condensate. Activated carbon and zeolite- based adsorption is widely used in water treatment to remove trace organic –inorganic substances, chemicals, microorganisms, odor and toxins. The ability of an adsorbent to remove certain pollutants from water depends on upon several factors, so the techno-commercial feasibility of the process to treat large volume of condensate with significantly high COD (300-500 ppm) is a question. Some major disadvantages of using adsorption process are-
Reverse Osmosis (RO) is a process which uses a partially permeable membrane and is currently used by many sugar factories and other industries. RO may be used to treat the condensate as the process is effective for the removal of sugars, salts and metal ions. But organic acids present in the condensate may not be removed by RO process.
So, it is necessary to neutralize the acids by using alkali before passing though the RO process. By this process, the quality of treated water (permeate) is very good making it fit for almost any application. However, there are some disadvantages of RO process as mentioned below-
Biological wastewater treatment harnesses the action microorganisms using an effective system to clean water for proper disposal or recycle. Over several decades of history, scientists and engineers have been able to control and refine biological processes to achieve effective and economical removal of organic substances from wastewater.
Sugar mills can adopt aerobic treatment and lagoons for the treatment of process condensate. Lagoons, which use natural microbes from surroundings, typically occupy large area, have high retention time and less efficient. Furthermore, it leads to accumulation of sulfides which turn the water black. Aerobic treatment processes include aeration tanks, oxidation ditches, activated sludge, trickling filters, etc.
A good example of an aerobic biological treatment is the ‘activated sludge’ process, which may be suitable for treating low strength wastewater streams such as process condensate. Moving bed bioreactor (MBR), trickling bed bioreactors and membrane bioreactors can also be explored as other feasible options.
An advantage of using biological treatment is that apart from educing COD/BOD levels, it can simultaneously also remove other inorganic pollutants such as sulphates, nitrates, ammonia and chlorides. Using a good bioreactor design and by controlling the desired operating parameters, the quality of biologically treated water can be very good making it fit for almost any application.
However, there are some disadvantages of biological treatment as mentioned below-
Chemical methods include chemical separation such precipitation, coagulation, flocculation, solvent extraction, ion-exchange process and chemical oxidation, and other chemical reactions which destroy or remove the pollutants from wastewater. Most of the chemical separation process may not be techno-economically suitable for treatment of sugar mill process condensate due its inherent characteristics.
Possibly, ion-exchange process can be used to treat the condensate although it is more suitable for water softening and metal ions removal. However, the application of ion-exchange for such application is less studied and large scale application is limited due to high cost.
Another disadvantage is the transfer of impurities from the effluent stream to sludge that need to be disposed of because of added resin regeneration chemicals.
Hydroxyl radical-based advanced oxidation processes (AOP) efficiently removes various types of COD causing organic compounds, including difficult, non-biodegradable compounds persistent organic pollutants (POPs) and reduce COD/BOD levels. The selection of oxidizing agents depends on the chemical nature of the pollutants and other factors.
Most of the chemical oxidation methods are more suitable when the COD levels are low. Due to its inherent chemical nature, sugar mill process condensate may be directly treated using such chemical oxidation process in combination with other rather inexpensive process such as sand filtration to remove any suspended particles and colloids.
The advantages of using advanced chemical oxidation for process condensate include fast process, less retention time and capex requirements, high throughput, simultaneous COD removal and disinfection, ability to remove odor and color etc. However, like any other treatment processes, chemical oxidation also has some disadvantages as mentioned below-
Table 2 Comparison between different types of processes for treatment of sugar mill process condensate
|Issue||Reverse Osmosis||Adsorption||Aerobic Digestion||Lagoons||Ion-Exchange||Chemical Digestion|
|Retention time||Low||Low||Medium||Very high||Low||Low|
|Working Temperature||25-50 oC||25-40 oC||25-40 oC||30-50 oC||25-50 oC||25 -80 oC|
|Waste generation||Reject stream||Spent adsorbent, backwash water||High sludge||Less sludge||Spent resins, backwash water||Less sludge|
Water conservation is need of the hour and sugar mill process condensate should beconsidered as an important water resource for recycling in order to minimize the use of ground and surface water by the industry. As discussed in this article, many types of treatment processes may be used for the treatment of process condensate but the process should be selected to maximize water recovery and recyclability.
Based on the available options the Chemical digestion based treatment can be an effective method for treatment of sugar mill process condensate and can be implemented with minimal infrastructure in short time.