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Technologies for Profitable Zero Liquid Discharge

This article was authored by Vikram Dhumal, Sandra D'Sa and Dr. Sanjay Jain of Geist Research Pvt. Limited.

Generation of various waste streams has been the endemic curse of chemical manufacturing industry. The triggers include: bad chemistry, sub optimal process engineering and design or inefficient operations. The end results are same and chemical industry has “earned” a reputation for pollution. 
The focus so far has been on pollution treatment and abatement rather than prevention. Various options are available on how to “treat” various effluent streams, so as to turn them benign enough for disposal / discharge. All the options have costs, depending on the severity of pollution problem and the law of the land. Traditionally, effluent treatment plants are considered to be ‘cost centers’. In fact the conventional way of “effluent treatment” kills the value inherent in the waste stream and disposes of the very valuable commodity, water. However, profits can be earned by separating pure chemicals even from the waste water stream. Thus the overall process and hence, organizational profitability can be improved.
Typically, effluent consists of Inorganic chemicals and salts, organic chemicals and Heavy metals. While there are cost effective methods for control and treatment of heavy metals, not many solutions are present for recovery of organic and inorganic chemicals. 

With new, innovative technological solutions, various chemicals (organic as well as inorganic) can be recovered in their pure form for recycling or sale, thus leading to profitable zero liquid discharge. 

The economics can be realized in the form of savings or revenue generated. There are operational costs as well, but they are generally in the range of 10-40% of the revenue generated (or cost saved). 
Effectively, wealth can be created out of waste by enabling profit from zero liquid discharge. Using Geist’s four novel WOOW (wealth out of waste) technologies on a standalone basis or in combination with other conventional operations profitable zero discharge can be achieved. 

1. Chemical Dewatering (WOOW-ChD): 

A very dilute solution can be concentrated by reverse osmosis (RO).
Technical advances in the field have allowed almost anything (high COD, suspended particles, etc.) to be concentrated till the limits of osmotic pressure. 

In most cases, practical limit of osmotic pressure is reached at around 2-5% concentration of dissolved solids in water. In order to concentrate this further, evaporation is normally carried out, which is costly. 
Chemical dewatering can take this stream as input and concentrate it by a further 25-40% without evaporating water. In principle, ChD works like extraction, but instead of extracting the solute, it extracts the solvent (in this case, water) from the aqueous solution.           
 (Figure 1: Typical flow diagram for WOOW-ChD)              

2. Reactive Extraction (WOOW-ReX): 

High solubility of organics in water is generally a result of their functional groups.  In reactive extraction, these functional groups are targeted to form an adduct, which is preferentially soluble in the organic phase. Aqueous stream is contacted with an organic phase, which consists of extractant (to form the adduct) and solvent (carrier for the extractant as well as the adduct). The adduct formed, having affinity for the organic phase, is continually removed from aqueous phase. After completion of the process, organic phase (extract phase) is recovered by gravity separation and subjected to solvent recovery, resulting in recovery of pure organics. 
Typical operating cost of recovery (chemicals + energy) is 10-25% of the value recovered. 
                           Figure 2: Flow diagram of a WOOW-ReX process
3. Anti-Solvent Crystallization (WOOW-ASC)
Having concentrated the aqueous stream and then having recovered the organics present in one or more ReX operation, the stage is now set for ASC to recover salts in their pure, saleable form. In ASC, a suitable anti-solvent is added to the aqueous stream, which causes selective crystallization of the desired salt.  
Some of the salts obtained in their pure form from waste water by using ASC are: sodium sulfate, sodium formate, sodium acetate, potassium sulfate, potassium nitrate, potassium chloride, ammonium sulfate, ammonium salt of pTSA, sodium salt of glycolic acid, etc. The operating cost of ASC is 25-50% of the value of the salt obtained. 
Figure 3: Flow diagram of a typical WOOW-ASC process                    

4. Eutectic Freeze Crystallization (WOOW-EFC):

Many salts show very high water solubility. In such cases, conventionally water is evaporated to increase the concentration of salts beyond the saturation solubility thus causing their precipitation. In due course, a large quantity of water needs to be evaporated. The water evaporation step makes this process highly energy intensive. In almost all industrial waste water streams, many salts are present in the dissolved form. Hence the water evaporation step finally leads to precipitated salts but it contains various other impurities. 
 (Figure 4: Flow diagram of a typical WOOW-EFC process)                  

Eutectic Freed Crystallization involves increasing the concentration of salts by crystallizing the water followed by crystallization of desired salt from the mixture of salts. Many industrially important salts like Sodium Sulfate, Potassium Sulfate, Ammonium Sulfate can be recovered using EFC technology. Here again, the typical operating cost will be 30-50% of the value of the salt obtained. As the operating temperature is below room temperature, equipment can be fabricated from less exotic materials.          

The combination of WOOW-ChD, WOOW-ReX, WOOW-ASC and WOOW-EFC results in the recovery of the chemicals in pure form from waste aqueous streams. The dilute streams can either be recycled (process permitting) or alternately, since the concentrations are < 1%, they can be subjected to RO. This way profitable zero discharge can be achieved with complete recycling. 



Author: Sustainability Outlook