Greywater Recycling Unit

Overview

Greywater, the drainage from showers, baths, bathroom basins, and laundry, makes up roughly 50–70% of household wastewater and carries a tiny fraction of the pathogens in toilet (black) water. A greywater recycling unit captures this stream, treats it to a non-potable reuse standard, and pumps it back to toilets and irrigation. Toilet flushing alone is around a quarter of domestic consumption, so a working system cuts mains demand by 30–40% in a typical household.

This unit is a packaged membrane bioreactor (MBR) plant sized for one dwelling: a 300 L Collection Tank hosting biological treatment, a submerged Ultrafiltration Module ultrafiltration stage, an inline UV Disinfection Stage reactor, and a Pressure Pump, sequenced by the System Controller.

Treatment train

Screening. Incoming greywater first passes the Prefiltration Stage, where a stainless 300 µm Screen Basket catches hair, lint, and skin squames, the materials that would otherwise mat the membrane. The Flush Valve periodically sluices the captured solids to the sewer, keeping the screen self-cleaning.

Biological stage. Soap, skin oils, and detergent residues are dissolved organics that no filter can remove; they must be eaten. The tank operates as an aerobic bioreactor: the Aeration System system's Blower Motor drives air through the Air Diffuser, keeping dissolved oxygen up so bacteria mineralize the organic load. Aeration also decides whether the system smells, greywater stored without oxygen goes septic within 24–48 hours, which is why simple storage-only "recycling" tanks fail.

Ultrafiltration. The same air stream scours the UF Membrane Bundle, a PVDF hollow-fiber bundle whose 0.04 µm pores are an absolute barrier to bacteria and protozoa and remove most viruses. The pump draws water gently through the fiber walls via the Permeate Port under a slight vacuum; clean permeate flows from the fiber cores while everything larger stays in the tank with the biomass. Membrane flux is deliberately low (around 10 L/m²·h) to stretch cleaning intervals to months.

Disinfection. Permeate then passes the UV Chamber, where a low-pressure mercury UV-C Lamp behind a Quartz Sleeve delivers at least 40 mJ/cm² at 254 nm, the dose benchmark for 4-log inactivation of bacteria and viruses. A UV Intensity Sensor watches lamp intensity; if output sags from lamp aging or sleeve fouling, the controller alarms and the Diverter Valve drops the system into bypass so untreated water is never delivered.

Delivery and safeguards

Treated water is stored briefly and delivered on demand: when a toilet valve opens, the Pressure Sensor sees pressure fall and the controller starts the Pressure Pump, whose four-stage Impeller Stack holds about 300 kPa at the fixtures, indistinguishable from mains supply at the cistern.

Reuse plumbing is governed by hard safeguards rather than good intentions:

• Backflow. The mains top-up line, used when the tank runs dry in dry weather, discharges through an open Air Gap Tundish, a Type AB air gap per EN 1717, so no failure mode can pull recycled water into the potable system. Check Valves back up each line.
• Stagnation. Treated greywater is not sterile and recontaminates during storage. If no demand occurs for 24 hours the controller opens the dump Solenoid Valve and sends the stored volume to sewer rather than let it age.
• Overflow. The Overflow Fitting passes excess inflow by gravity to the sewer, so a full tank or dead pump never backs water up into showers.
• Identification. Codes require reuse pipework to be marked (purple pipe in North America under NSF/ANSI 350 installs) and outlets labeled non-potable.

Performance and economics

Certified systems (NSF/ANSI 350, BS 8525, AS 1546.4) must hold effluent below roughly 10 mg/L BOD and 2 NTU turbidity with E. coli essentially absent, quality suitable for toilet flushing, subsurface irrigation, and cold-water laundry. Energy use of about 0.9 kWh/day, dominated by the aeration blower, works out to roughly 3 kWh per cubic metre of recycled water. The economics depend on local water and sewer tariffs; the strongest cases are water-scarce regions with mandatory reuse provisions (parts of Australia, the southwestern United States, Cyprus) and large multi-residential buildings, where one plant serves dozens of dwellings and the System Controller's fault telemetry feeds the building management system.