ETP design and engineering: matching a treatment train to the effluent a process actually produces.

An STP treats sewage, which is broadly consistent in character and predominantly biodegradable, so its design is largely a question of scale and technology. An ETP treats industrial effluent, whose contaminants, concentration and variability depend entirely on the process generating it, which is why an ETP is designed around characterisation data and frequently needs physico-chemical and tertiary stages a sewage plant never has.

Primary is physical and chemical: screening, oil and grease removal, equalisation, neutralisation, coagulation-flocculation and clarification, taking out solids and adjusting the effluent so what follows can work. Secondary is biological, reducing the biodegradable organic load. Tertiary is polishing to the discharge or reuse standard: filtration, activated carbon, disinfection, and membrane or thermal processes where dissolved solids must come out.

From measured effluent over a representative period, not from a single grab sample. Industrial flow is often batch-driven and highly variable, so the design accounts for peak flow and peak load separately, and equalisation is sized to smooth both. Where a new plant has no effluent to sample, load is derived from process water balance and comparable units, with the assumptions stated.

When dissolved solids have to be removed: because the discharge standard limits TDS, because the treated water is to be recycled to process, or because the unit is required to achieve zero liquid discharge. Reverse osmosis concentrates the dissolved load into a reject stream, and a multiple effect evaporator or similar thermal system takes that reject down to salts. Both add substantial operating cost, so they are specified where the standard requires, not by default.

The standard in the unit's Consent to Operate, which reflects the CPCB general standards, any industry-specific effluent standard notified for that category, whatever the State Board has tightened, and the disposal route: inland surface water, land for irrigation, public sewer or a common effluent treatment plant. The sewer or CETP route carries its own inlet standard, which is not the same as the surface-water norm.

The board grants Consent to Establish on the basis of the proposed treatment scheme and grants Consent to Operate against demonstrated performance, so the design is effectively examined at consent stage. It is not a design approval in an engineering sense, the obligation to meet the standard stays with the occupier regardless of what was submitted.

Because treatment cost scales with the hardest fraction. Diluting a small, concentrated, high-TDS or toxic stream into the general effluent means the entire flow now needs the expensive treatment that only the small stream required. Segregating at source, treating the difficult stream separately and concentrating it, is usually the single largest cost decision in an ETP design.

Chemical and biological sludge is thickened and dewatered, and its disposal route depends on its character, sludge from many industrial ETPs falls under the hazardous and other wastes rules and must go to an authorised treatment, storage and disposal facility under manifest. Sludge quantity and its disposal route are quantified at design stage, because they are a recurring cost and a compliance obligation, not an afterthought.