What the difference actually is
Traditional reverse osmosis systems store pre-filtered water in a pressure tank (typically 3–12 litres) under the sink. When you open the filter tap, water flows from this tank rather than directly from the membrane. Tankless RO systems — also called direct-flow or instantaneous RO — pump water on demand through the membrane with no storage tank.
| Feature | Tank RO | Tankless RO |
|---|---|---|
| Flow rate | Draws from stored tank — consistent until empty | On-demand from membrane — may slow at high draw |
| Tank under sink | ✓ Required (3–12L) | ✗ No tank — more under-sink space |
| Stagnant water risk | ⚠ Water sits in tank between uses | ✓ Fresh water on demand every time |
| Waste water ratio | 2–4:1 (waste:filtered) | 1.5–3:1 — typically more efficient |
| Pump required | Sometimes (gravity or pressure) | ✓ Always — requires electricity |
| Cost | $700–$1,200 installed | $900–$1,600 installed |
| Maintenance | Pre-filter + membrane + tank sanitise | Pre-filter + membrane (no tank to sanitise) |
| Power use | Low or none (gravity fed) | Constant low draw — pump runs on demand |
| WaterMark available | ✓ Yes (various) | ✓ Aquala P500 (confirmed WaterMark) |
The stagnant water issue with tank RO
The most genuine advantage of tankless RO is freshness. In a tank system, filtered water sits in the pressure tank between uses — potentially for hours or days in low-use households. While modern tanks have bladders that prevent direct contact between water and tank air, water quality can degrade slightly over time and bacteria can form in rarely-used systems.
For the vast majority of Australian households who use filtered water daily, this is a minor practical concern. For households who are away for extended periods or use filtered water infrequently, tank sanitisation becomes more relevant.
The waste water question — important for Perth
Standard tank RO wastes 2–4 litres of drain water per litre filtered. Tankless RO systems, with their improved pump efficiency and membrane designs, typically achieve 1.5–3:1 ratios. For a family using 3 litres of RO water per day:
| System type | Daily waste water | Annual waste water |
|---|---|---|
| Standard tank RO (3:1) | 9 litres | 3,285 litres |
| Tankless RO (2:1) | 6 litres | 2,190 litres |
| Tankless RO (1.5:1) | 4.5 litres | 1,643 litres |
Perth faces periodic water restrictions, and Water Corporation bills are volume-based. The difference between a 3:1 and 1.5:1 RO system is approximately 1,600 litres per year — not trivial when water costs are high and restrictions apply. For Perth households specifically, the better waste ratio of tankless RO has real financial value.
WaterMark and tankless RO in Australia
Any under-sink RO system in Australia requires WaterMark certification for legal installation. This is important for tankless RO specifically because many overseas tankless units are marketed in Australia without WaterMark certification — they cannot be legally connected to mains water by a licensed plumber.
The Aquala P500 holds a confirmed Australian WaterMark certification and is marketed as a tankless RO specifically designed for Australian conditions. Verify the current WaterMark number at watermark.org.au before proceeding with any tankless unit. Tankless units imported and sold online without WaterMark cannot be legally plumbed by a licensed Australian plumber.
NSF certification — performance verification
As with all RO systems, confirm NSF 58 certification for the specific model before purchasing. NSF 58 verifies contaminant removal including fluoride (90–96%), PFAS (90–99%), and other dissolved contaminants. A tankless system without NSF 58 certification has no independently verified performance data. Check nsf.org for the exact model before proceeding.
Countertop tankless RO — no plumber needed
For renters or households who want tankless RO without plumbing, countertop tankless RO systems (AquaTru Classic, Waterdrop countertop) sit on the bench with a small storage tank and require no mains connection. These are not plumbed systems — they use tap water poured in manually or connected via a diverter valve.
Countertop tankless options are more practical for renters than under-sink plumbed systems. The AquaTru Classic is NSF 42, 53, and 58 certified and available in Australia. See our countertop water filter guide for the full assessment.
Who should choose tankless under-sink RO
Frequently asked questions
- Is tankless RO better than tank RO?
- For Perth households where water waste costs money, tankless RO's better waste ratio (1.5-3:1 vs 2-4:1) has real value. For most daily-use Australian households, a quality tank RO from a WaterMark-certified supplier is functionally equivalent and costs less. The freshness advantage of tankless is meaningful mainly for infrequent users or households away for extended periods.
- Does tankless RO need WaterMark certification in Australia?
- Yes — any RO system connected to Australian mains plumbing under the sink requires WaterMark certification and installation by a licensed plumber. Countertop tankless systems (AquaTru and similar) that do not connect to mains plumbing do not require WaterMark. Confirm the specific WaterMark licence number at watermark.org.au for any plumbed tankless system.
- How much water does a tankless RO system waste?
- Tankless RO systems typically waste 1.5-3 litres per litre filtered, compared to 2-4 litres for standard tank systems. A family using 3 litres of RO water per day would produce approximately 4.5-9 litres of waste water daily. In Perth where water is billed by volume and restrictions apply, the improved ratio of tankless systems has measurable financial value over time.