Why Does the HydroSpark+ Have a Divider in the Tank?

A small divider separates one side of the HydroSpark+ water tank. The larger side holds about 3 litres and the smaller side about 1 litre. At the 5-litre MAX line, water sits above the divider, so both sides initially appear to form one tank.

The divider becomes useful later in the filtration cycle.

A Quick Primer on How RO Filtration Works

Reverse osmosis works by pushing water through an extremely fine membrane (0.0001 µm). Pure water passes through. Dissolved solids, heavy metals, and other impurities get left behind.

RO filtration produces a reject-water stream by design. In a traditional system connected to plumbing, producing 1 litre of clean water can send 3 to 4 litres down the drain as waste. The exact amount depends on the system, the source water and the operating conditions.

How the HydroSpark+ Handles Residual Water in Tank Mode

HydroSpark+ is a countertop system, and storage-tank mode has no drain connection. The residual water therefore returns to the tank.

It goes back into the tank.

When the system filters water, two things happen at once:

1. Clean water is dispensed from the spout.
2. Residual water — what didn't pass through the membrane — is returned to the smaller side of the tank.

The residual water re-enters at the base of the smaller compartment. Since the water level starts above the divider, both sides of the tank are one connected body of water — so it simply mixes back in. The system then draws from this pool again, filters again, dispenses again, and returns residual water again.

This is called recirculation. Instead of sending residual water to a drain, the system re-treats it, so far less water is set aside than in a traditional drain-connected setup. The manufacturer's design reference for tank mode is a 4:1 ratio of clean to residual water, quoted under the manufacturer's stated conditions; the ratio in your kitchen varies with source water, temperature and the condition of the membrane.

The RO membrane works at its nominal 0.0001 µm scale on every pass, including water making another trip through the tank. The membrane still produces purified water each time; recirculation reduces the amount discarded.

So When Does the Divider Actually Matter?

As you keep dispensing water, the level in the tank drops. Eventually, it reaches the top of the divider.

At this point, the two sides of the tank separate. But the smaller side is already full to the brim of the divider — and the system keeps returning residual water to it through the valve at the base. Since it's already full, new residual water displaces the cleaner water sitting above it, making it overflow back into the main tank. Recirculation continues, but the main side keeps dropping because clean water is being dispensed out faster than residual water is being returned.

When the main side runs empty, the system stops and tells you it's time to refill.

At that point, the smaller compartment holds about 1 litre of the most concentrated residual water from the filtration cycle. The divider keeps it outside the system's intake and leaves a clear amount to pour away.

What to Do When It's Time to Refill

1. Lift out the water tank.
2. Pour away the remaining water (this is the residual water in the smaller compartment).
3. Refill the entire tank with fresh tap water to the MAX line.
4. Place it back in the unit.

No additional control or reading is needed; the tank design manages the separation.

Why a Divider Instead of a Sensor?

A water-quality sensor might seem like an alternative to a physical divider.

The HydroSpark+ does have a TDS (Total Dissolved Solids) sensor built in — you can check your water quality readings anytime from the settings screen. But the system doesn't rely on it to decide when you should flush and refill. Sensors can drift over time, need calibration, and depend on firmware logic to interpret readings correctly. The divider takes a mechanical approach instead: it is designed to cap recirculation at roughly a 4:1 clean-to-residual ratio in tank mode under the manufacturer's stated conditions. Actual recovery still varies with source water, temperature and membrane condition.

The physical divider provides a simple limit on recirculation and prevents the RO membrane from continuing through too many cycles, which supports longer membrane life.

The Short Version

The divider is a simple construction that does a surprisingly important job — it makes sure you always discard the right amount of residual water, protects your filter membrane, and keeps things simple.