Comparative for types Sewage Treatment Plants [STP]s
Grey Water or sullage: the relatively clean waste water from baths, sinks, washing machines, and other kitchen appliances.
Black Water or sewage: waste water and sewage from toilets, flushed to evacuate human excreta.
Typically in drainage systems, above ground drainage is a two-pipe system i.e. waste pipe [for grey water] & soil pipe [for black water]. However, below ground drainage typically becomes a single pipe system connected via Inspection Chambers [ICs] and Grease Trap to intercept kitchen related waste. Hence, STPs are typically designed to treat black water.
For all developments above 20,000 sq.m. STPs are made compulsory by the Ministry of Environment & Forest [MOEF]; with consideration of a Zero discharge site i.e. treated water from STPs is to be reused for flushing, landscape, water bodies etc.
Even with developments smaller than 20,000 sq.m. STPs have become staple as costs have rationalised over the years. Also, under the law by Environmental (Protection) Act 1986 sets that by law, discharging untreated sewage into any drains other than an underground sewerage system or into open land, is an offence and invites prosecution.
Types of Sewage Treatment Plants [STP]
EA Extended Aeration Process
SBR Sequencing Batch Reactor
MBBR Moving Bed Bio Reactor
MBR Membrane Bio Reactor
ECP Electro Coaggulation Precipitation
STP Comparison Chart:
The tolerance limit for sewage discharged in India is as per BIS standards IS: 4764:1973 and defined in the comparative chart.