Improvement of Tubular Membrane Filter efficiency in order to prevent liquid drainage and achieve Zero Liquid Discharge target.
The Central Effluent Treatment Plant (CETP) is designed to receive effluents from different units of the plant as Low Total Dissolved Solids or LTDS (TDS less than 900 ppm) & High Total Dissolved Solids or HTDS (TDS greater than 900 ppm) effluents. CETP processes and treats the effluent to make it suitable for various uses within the steel plant in the following manner:
Tubular Membrane Filtration (TMF) pretreatment is designed to remove Total Suspended Solids (TSS), Hardness, Silica, Turbidity, Microorganisms, Oil & Gas and perceptible heavy metals from the wastewater. With Lime and Dolomite dosing, hardness and silica impurities shall be filtered out using these membranes having an average pore size of 0.05 microns. The TMF works at a high crossflow velocity of 3 to 3.5 m/s pumped through 12.7mm porous tubes. The filtered water traverses across the tube wall and is collected using a permeate header while the bulk of the feed flow is returned to Concentration Tank as recycle flow. In the process, organics too are removed partially by co-precipitation. The TMF operates typically at about 3% concentration of suspended solids which scour the membrane surface continuously and prevent deposition of foulants. The back-pulse is induced by purging compressed air in a dedicated column which stores the required amount of filtrate. The operations of TMF, including back pulse, are all automated.
There should not be any other water/wastewater generated from CETP for disposal, i.e. CETP should operate on zero liquid discharge concepts. However, the TMFs are presently running with low efficiency (~ 30% of the rated capacity). Efficiency (filtration through TMF) is better at high pH but its quality deteriorates (results in high hardness) which is thus not suitable for further processing through Reverse Osmosis (RO). The issues being faced can be better understood as follows:
TMFs are based on crossflow filtration. They are provided in CETP for Turbidity & TSS reduction mainly. Lime Soda Softening process is carried out as a pre-treatment before TMFs for HTDS circuit to achieve desired hardness (less than 40ppm) for RO operation.
The designed flow is 100 m3/hr through each TMF skid. But if Lime Soda Softening Process is followed (pH=10.5-11), only 20 m3/hr of avg. flow is achieved. If excess lime dosing is done with higher pH (approx. 12), then avg. flow increases to 40 m3/hr.
2. TWW Quality issue:
a. HTDS Circuit:
By choosing excess lime dosing before TMFs to process more quantity of water, we are compromising with TMF outlet water quality. Excess lime dosing leads to high Ca hardness (~500-600 ppm), high TDS (~6000-7000 ppm), high pH (>12) in TMF outlet water. This water is not suitable to be processed through the current RO system as RO feed water requires Ca hardness ( less than 30 ppm), Mg hardness (less than 10 ppm) & TDS (less than 3352 ppm). pH can be corrected by HCL dosing, but high pH at TMF outlet requires more HCl than conventional Lime Softening Process.
b. LTDS Circuit:
Though lime softening is not needed for LTDS circuit, we are carrying out excess lime dosing in LTDS circuit also, to maintain TMF flow rates. This causes deterioration of water quality in terms of pH, TDS & Hardness. Only TSS & Turbidity are reduced.
3. Huge amounts of HCl consumption: Since RO is not in operation, both HTDS, as well as LTDS TMF outlet water, is sent to TWW network after pH correction with HCl dosing. A huge amount of HCl is consumed because of excess lime dosing. Furthermore, as this water is not as per the desired TWW water quality in terms of TDS & Hardness, it causes frequent TWW network pipeline damages.
Thus, we are seeking solutions that will enable our CETP to process LTDS effluent @330 m3/hr & HTDS effluent as per design, so there will be no drainage helping us to achieve Zero Liquid Discharge target. The solution will thus ultimately facilitate in reducing clarified water consumption in TWW network & will enable excess effluent generated by RO to be mixed with clarified water leading to a decrease in raw water intake.
The solution must enable us to achieve:
The solution should also satisfy the following conditions: