Technical Performance Of Water Treatment Plant

High TDS in treated effluent is observed to be a widespread problem. Apart from the five CETPs that complied with TDS as well as other general parameters, only four other CETPs complied with TDS limits even though these CETPs did not comply with other generalm parameters. Thus, in all, 69 (i.e. 88.5%) out of the 78 CETPs did not complied with TDS standards.

It is also observed that salinity, primarily due to salts of sodium, is the primary contributor to the high TDS problem as high TDS is almost invariably accompanied by high Chlorides and Sodium concentration. TDS reduction is possible by softening process if TDS is mainly due to salts of divalent cations but if it is mainly due to salts of mono-valent cations then Reverse Osmosis (RO) is a technical option . However, the concentrate has to be further evaporated to get the salts in solid form that may be disposed off in secured landfill.

The TDS concentration of the wastewaters is mainly due to the inorganic ions in the water supply and those added during the use of water. In industries such as Tanneries, Pharmaceutical units, Chemicals manufacturing units and Dye & Dye Intermediates units, the high TDS in wastewaters can be primarily attributed to addition during the use of water. In all such cases the best approach for reduction of TDS is to try reduction at source by adopting cleaner technologies for reduction of net input of chemicals, practicing recovery and recycling of chemicals. The chemical treatment adopted in the treatment scheme at CETP generally does not specifically aim at removing these ions. Therefore, reduction in the TDS
concentration of the effluent during its treatment is not expected.

Dual Media Filter (DMF) unit, which has been employed in treatment schemes of CETPs inDelhi and at few other places and sand filter unit which has been employed in CETPs of textile units in Tamilnadu are normally used to improve suspended solids level from near 50 mg/L in primary settling unit’s effluent to near 10 mg/L. Incidentally, it also reduces the organic matter associated with the suspended matter so removed. It may also remove a small fraction of organic matter associated with colloidal matter that is coagulated and filtered during filtration. Filter units should not be depended upon to perform more than the expected function, as explained. If DMF unit or a rapid sand filter unit is over loaded, it will require frequent backwashing.

Activated Carbon Filter (ACF) unit, which has been employed in treatment schemes of CETPs in Delhi and at few other places is only meant for removing trace organics, such as pesticides, phenols, etc., and heavy metals, which escape the primary treatment and therefore should not be loaded with bulk organic matter. If any treatment scheme, with ACF as its terminal treatment unit, is aimed at achieving an overall efficiency so as to give the final effluent quality of BOD<30 mg/L and COD<250 mg/L, then ACF unit is most likely to be subjected to high COD, or even high BOD, thereby resulting in an early exhaustion of its activated carbon bed. Therefore, in treatment schemes aimed to achieve this quality, ACF as a terminal treatment unit can be said to be a misfit because very frequent replacement or regeneration of the bed is neither easy nor economically affordable.