STP (STP)

A Sewage Treatment Plant (STP) is a process facility that treats domestic sewage and municipal wastewater — distinct from industrial effluent — to remove organic matter, suspended solids, pathogens and nutrients before discharge to receiving water bodies, sewers or use in irrigation. STPs are mandated for residential complexes above prescribed thresholds (40-100 flats depending on state by-laws) and on industrial premises that generate sewage from canteens, washrooms and worker housing, distinct from process effluent that goes to the ETP.

The contrast with an Effluent Treatment Plant (ETP) matters operationally. Sewage is biologically well-balanced (BOD:N:P approximately 100:5:1), microbially seeded by human gut flora, and predictable in composition — biological treatment alone reliably hits 30 mg/L BOD output. Industrial effluent often lacks nutrients, contains toxics (heavy metals, solvents) that inhibit biology, varies wildly in pH and load, and needs primary chemical treatment before biology can begin. Mixing the two in a single tank is forbidden under most SPCB consents because the toxics in industrial effluent kill the biology that was treating the sewage.

STP design uses one of four standard configurations. Activated Sludge Process (ASP) is the legacy default — aerated tank + secondary clarifier + recycled sludge — robust but capex-heavy and 25-35% larger footprint. Sequencing Batch Reactor (SBR) runs fill-react-settle-decant cycles in one tank, saving footprint and giving better nutrient removal — favoured for residential complexes. Moving Bed Biofilm Reactor (MBBR) adds plastic carriers (typically K1, K3 media) inside the aeration tank that grow a fixed biofilm, doubling biomass concentration and halving footprint vs ASP. Membrane Bioreactor (MBR) replaces the secondary clarifier with ultra-filtration membranes, producing effluent clean enough for direct reuse — capex 1.8-2.5x of MBBR but reuse offsets fresh water purchase.

For recycling plants with 50-200 workers on site plus canteen, a 5-10 KLD STP is typical. The treated effluent feeds gardening, fire-fighting reserves, and toilet flushing under the dual-plumbing requirements that several states (Karnataka, Maharashtra) impose on industrial estates. Sludge from the STP is dewatered to 18-25% solids and composted on-site or sent for cement-kiln co-processing. Compliance limits for treated STP effluent: BOD ≤10 mg/L, COD ≤50 mg/L, TSS ≤20 mg/L, fecal coliform ≤230 MPN/100 mL — substantially tighter than ETP limits because the reuse case is closer-coupled to human exposure.