Special drainage components: grease interceptor and rainwater management

One-line orientation

Two specialized drainage components appear in ARE plumbing questions: the grease interceptor (FOG capture before the sewer) and the rainwater cistern system (capture–store–treat–reuse cycle).

Key points

Grease interceptor (FOG interceptor)

Purpose: captures fats, oils, and grease (FOG) discharged from commercial kitchens before they solidify in the municipal sewer and cause blockages.
Location: installed downstream of kitchen sinks, dishwashers, floor drains in commercial / food-service buildings; typically in the floor or exterior vault.
Operation (three-layer separation):
1. Wastewater enters the interceptor and slows down.
2. Grease rises and floats at the top (lower density than water).
3. Solids settle to the bottom as sludge.
4. Relatively clear water exits in the middle zone to the sewer.
Maintenance: must be pumped out regularly to remove accumulated grease cap and sludge; a neglected interceptor passes FOG to the sewer just like having none.

Rainwater management / cistern system

Three-part memory aid:
1. Catchment — the roof or surface that collects rainwater.
2. Cistern — the tank that stores it.
3. Treatment — match treatment to the intended reuse and local requirements. These are the core ideas, not every component in a complete system.
First flush: the first volume of runoff after a dry period carries the highest pollutant load; first-flush diverters route this water to waste before filling the cistern.
Permissible reuse: typically toilet flushing, landscape irrigation, or cooling tower makeup water — not potable without additional treatment meeting drinking-water standards.
Erosion control: release of harvested or overflow rainwater must be controlled to avoid concentrated discharge that causes erosion.

Confusions / comparison

Harvested rainwater is judged against potable water on the three axes the exam cares about — whether you can drink it, how much treatment it needs, and what it may legally serve.

Water source	Potable?	Treatment required	Permitted uses
Harvested rainwater (typical reuse)	No, not at typical treatment levels	First-flush diversion, then filtration + disinfection (particulate filters, UV, or chemical)	Toilet flushing, landscape irrigation, cooling-tower makeup
Harvested rainwater upgraded to drinking standard	Yes	Higher treatment standard meeting drinking-water requirements	Same non-potable uses plus potable supply
Municipal potable supply	Yes	Treated to drinking-water standard at the source	Any use, including drinking and cooking

Exam pitfalls

A grease interceptor does not treat grease — it captures it. The interceptor removes FOG by physical separation, not chemical breakdown. The intercepted grease must be hauled away; it does not disappear.
Grease interceptors require regular maintenance. A full interceptor passes FOG straight through; pump-out frequency is code-regulated in commercial food-service projects.
Rainwater is not potable without full treatment. Cistern water is appropriate for non-potable reuse (flushing, irrigation) at typical treatment levels; drinking water requires a higher treatment standard.
First flush matters. Skipping a first-flush diverter means the most contaminated water goes into the cistern — undermining the system’s effectiveness.
Rainwater systems must manage overflow — cisterns fill during heavy rain; overflow must be directed to a safe outlet (drywell, bioswale, municipal storm drain) not to adjacent grade where it causes erosion.

→ Plumbing: DWV traps & vents (drainage system overview) · water types & treatment (greywater reuse rules) · Site module: septic system (tank + leach field + perc test — deferred) · Site: stormwater management and bioswales.

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