Bhavnagar lab develops tech to separate potash fertilisers from spent-wash ash

The CSMCRI is one of the 37 premier national laboratories functioning under the aegis of the Council of Scientific & Industrial Research (CSIR), the apex research and development organisation headed by the Prime Minister. CSMCRI has licensed its technology to DCM Shriram Bio Enchem Limited, which is installing a plant based on it at Hariawan in Uttar Pradesh’s Hardoi district. This facility, costing Rs 57 crore, will use the spent wash ash from a 200 kilo-litre per day distillery attached to a 13,000 tonnes cane per day sugar mill. It will be operational by August, a company official said.

Spent wash is basically the wastewater or effluent that is a byproduct of alcohol production. For every one litre of alcohol that they produce from fermented cane molasses, distilleries generate 10-15 litre of spent wash. This wastewater contains about 2-3 per cent potash, but also huge organic load and salts that, if discharged without treatment, can pollute both land and water bodies.

India has some 325 molasses-based distilleries producing some 35 billion litre of alcohol and 50 billion litre of wastewater per year. Since distilleries are required to adhere to zero-discharge norms imposed by the Central Pollution Control Board, they manage the spent wash mainly by two methods.

The first is by mixing it with press-mud from sugar mills, the residual cake after clarification and filtration of cane juice. This mix is then composted and converted into manure. The second method is to concentrate the spent wash to 58-60 per cent solids and feed it into an incineration boiler. The resultant ash coming out of the boiler in dry form contains 20-21 per cent potash and in some cases, it is converted into granules for agricultural application.

However, according to Pratyush Maiti, CSMCRI’s chief scientist — the lead developer of this new technology — the ash granules may also contain sodium chloride, aluminium oxide and other substances that are harmful to soil health. Secondly, they can only be broadcast or spread on the soil like regular fertilisers. Being hard, the granules don’t break easily and the potash then may not become available to the plants as desired.

“What our process does is to separate the SOP, MOP and mixed salts. The SOP and MOP are both 100 per cent water-soluble and can be used as liquid fertiliser amenable to foliar (direct to leaf) application as well as through drip/micro-irrigation systems. The mixed salt, containing MOP (potassium chloride) and sodium chloride at low levels, is edible. We are, thus, ensuring that no undesirable material goes into the soil,” Maiti told The Indian Express.

The CSMCRI process can recover up to 90 per cent of potash from distillery boiler ash. While the ratio of SOP and MOP varies depending on the spent wash quality, the average recovery composition is about 55 per cent in the form of SOP, 10-20 per cent as MOP and 10-30 per cent as edible mixed salt. The technology also involves separation of non-salt solids from spent wash ash through leaching operations, which are converted into bricks for masonry work. “This technology is a waste-to-wealth and not a waste-management proposition.” Maiti claimed.

India currently imports its entire potassic fertiliser requirement mainly from Canada, Belarus, Israel, Jordan, Lithuania and Russia. Imports of MOP and SOP totaled 2.96 mt (valued at $ 1,023.7 million) in 2021-22 and 2.31 mt ($ 1,402.6 million) during April-February 2022-23.

Incidentally, in 2018, CSMCRI had licensed a technology for recovering potash fertilizer and other complex organics from distillery spent-wash to the Aurangabad Distillery Limited. Their plant at Walchandnagar in Maharashtra is currently producing sulphate of potash, low-chloride PDM (potash derived from molasses) and de-potash vinasse (cattle feed binder) directly from spent wash.

The CSMCRI researchers said that the existing distilleries may find attractive the technology of recovering potash from spent wash ash, as they already have some plant installed but new distilleries may find it attractive to invest in the technology of recovering potash directly from spent wash. They added that energy requirement and capital expenditure for the processes is comparable.