RODI Water FAQ: Stage Function TDS and Cost
RODI water — reverse osmosis with deionisation polish — is the foundation of every serious reef tank because Singapore PUB tap water carries chloramine, silicates and trace metals that crash corals over months. The rodi water faq below answers the eleven questions our staff field most often from new reefers comparing units and resin packs. This guide from Gensou Aquascaping at 5 Everton Park covers four-stage system function and Singapore-specific TDS targets, and this guide answers the eleven questions Singapore aquarists ask most about rodi water faq setup and operation.
What Are the Four Stages?
A standard RODI unit chains four cartridges in series. Stage 1: 5-micron sediment filter removes silt and rust. Stage 2: carbon block removes chlorine, chloramine and organics. Stage 3: thin-film composite (TFC) RO membrane rejects 95-98 per cent of dissolved solids. Stage 4: deionisation (DI) resin polishes residual TDS to zero. Some five and six-stage systems add dual carbon or post-DI for chloramine-heavy supplies like Singapore’s PUB tap.
What TDS Should I Expect?
Singapore PUB tap typically reads 80-110 TDS at the household tap, varying slightly by area. After the RO membrane, TDS drops to 4-8. After the DI resin, output should read 0-2 TDS — and 0 is the standard target for reef use. A handheld TDS meter (HM Digital TDS-EZ, SGD 25-45) is non-negotiable equipment. Test post-RO and post-DI weekly.
How Often Do I Replace Cartridges?
Sediment filter: every six months or when noticeably brown. Carbon block: every six months in Singapore (chloramine exhausts carbon faster than chlorine). RO membrane: every two to three years if pre-filters are maintained. DI resin: three to six months in Singapore’s humid climate, indicated by post-DI TDS rising above 2. Buying replacement cartridges from the water care range in bulk reduces per-unit cost.
How Much Does an RODI System Cost in Singapore?
Entry four-stage units run SGD 200-300, mid-tier five-stage units with brand-name TFC membranes (Filmtec, Dow) run SGD 350-550, and premium six-stage systems with dual DI run SGD 600-900. Replacement filter packs cost SGD 60-120 every six months. Booster pumps add SGD 150-250 if your household water pressure runs below 40 psi. HDB pressure typically suffices without a booster.
Can I Drink RODI Water?
Yes, RODI water is safe to drink, though most water purifier brands reintroduce minerals via remineralisation cartridges because pure RODI tastes flat and is mildly hypotonic. For reef-tank use, the lack of minerals is the entire point — you remineralise via salt mix. Never plumb RODI back into the household drinking line; the TDS is too low for daily human consumption without remineralisation.
How Long Does It Take to Make Water?
A typical 50 GPD (gallons per day) membrane produces around 7-8 litres per hour. Singapore tap pressure and temperature affect output significantly: cooler water (24°C) flows faster than warmer water (29°C). For a 75-litre nano reef requiring 15 litres weekly water-change volume, expect two hours of run-time. Larger 100-150 GPD systems halve the wait but cost more upfront.
What Is the Waste-Water Ratio?
Standard 4:1 waste ratio means four litres of reject water for every litre of pure RODI produced. The reject is not undrinkable — many Singapore reefers redirect it to garden watering, washing machines or floor mopping. Higher-end systems reach 2:1 or 3:1 with permeate pumps. Calculate water bill impact: producing 60 litres weekly RODI generates 240 litres of waste, which is small relative to typical household consumption.
Why Is My TDS Reading Higher Than Expected?
Three common causes. Spent DI resin (most common, fix by replacing). Failed RO membrane (test post-RO water — if TDS exceeds 10 per cent of input, the membrane has perforated). Channelling in the DI cartridge (re-pack the resin and orient correctly). Temperature compensation error on the meter is rarer but worth ruling out by testing a known reference solution.
Do I Need a Booster Pump?
Booster pumps matter if your household water pressure falls below 40 psi (2.8 bar). Most Singapore HDB and condo supplies sit at 50-65 psi, so booster pumps are usually unnecessary. Ground-floor and landed properties with private pressure systems sometimes need them. Test pressure with a SGD 30 inline pressure gauge before assuming you need a SGD 200 booster pump from the aquarium equipment range.
How Do I Store RODI Water?
Store in food-grade HDPE drums with a lid, in a cool dark spot. Pure RODI water absorbs CO2 from the air and pH drifts to 5.5-6.0 within days, which is irrelevant for salt mixing but matters for top-off. Plumb a circulation pump and small heater into the storage drum for top-off use. Many Singapore reefers use 25-50 litre kettle drums under the cabinet.
Can I Use RODI for Freshwater Tanks?
Yes — RODI is mandatory for crystal shrimp, blackwater biotopes and discus aquariums where soft, mineral-free water is required. Remineralise with Salty Shrimp GH+ for crystal shrimp or commercial discus buffer for South American biotopes. Cherry shrimp and most community fish thrive on PUB tap with conditioner alone — RODI is overkill unless you specifically target soft-water species.
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