STP design and DPR: the engineering a sewage treatment plant is actually built from.

A design basis, the process design and mass balance, unit sizing and hydraulic profile, civil and structural drawings, equipment specifications and data sheets, the electrical and instrumentation scheme, a BOQ with capital cost estimate, an implementation schedule and a projection of operating cost. It is the document on which a plant is sanctioned, tendered and later built.

Capacity follows design flow: the population or occupancy served, the per-capita water supply, the fraction returning as sewage, and a peak factor for the diurnal pattern. Building, campus and industrial-township plants are usually a few hundred to a few thousand KLD, so KLD is the natural unit; municipal plants run to several MLD and are stated in MLD. The same calculation underlies both.

The standard set in the plant's Consent to Operate, which follows the CPCB general standards as adopted or tightened by the State Pollution Control Board and by the mode of disposal: land, surface water, sewer or reuse. Reuse-linked conditions in a building Environment Clearance are frequently stricter than the general discharge norm, so the design is fixed against the consent and clearance together, not against a textbook number.

It depends almost entirely on the technology and the extent of covering. Attached-growth and membrane processes such as MBBR and MBR are compact and are what basement and podium installations are usually built on; conventional activated sludge and pond-based systems need considerably more area. Land availability is often the constraint that decides technology rather than the other way round, which is why footprint is settled in the design basis.

For most building, township and campus projects, yes in practice. Reuse of treated sewage for flushing and landscape irrigation is a routine condition of the Environment Clearance and of many municipal building bye-laws, so the design carries a tertiary polishing train, treated-water storage and a separate flushing line. Retrofitting dual plumbing after the building is up is far more expensive than designing it in.

A DPR remains worth having, because it is what lets a client compare turnkey bids on a common design basis instead of on whatever each vendor chose to assume. Without it, quotations differ in capacity assumptions, influent load and scope, and the lowest number usually turns out to be the smallest plant rather than the best value.

The served population or occupancy and water-supply figures, an influent characterisation where existing sewage can be sampled, the disposal or reuse route, the discharge standard from the consent, the available plot and its levels, and the power supply position. Where the plant serves a new development and no sewage exists to sample, design load is derived from established per-capita norms and stated explicitly as an assumption.