How to Reduce Inclined Tube Settler Turbidity: 4 Hardware Upgrades + 6 Operational Methods from a 40,000 m³/d Plant

When China's new drinking water standard GB5749-2006 took effect, the turbidity limit at the consumer's tap dropped from 3 NTU to 1 NTU. For a waterworks built in the 1980s with inclined tube settlers, that meant the settled water - which feeds the filters - had to get substantially cleaner. Because settled water turbidity drives filtered water turbidity, and filtered water turbidity is essentially finished water turbidity. A 40,000 m³/d plant in Huizhou took on the challenge. This article covers exactly what they changed - the hardware upgrades and the six operational methods that together brought settled water turbidity down to where the new standard demands.

WHY SETTLED WATER TURBIDITY IS THE CONTROL POINT

In a conventional water treatment plant, turbidity reduction happens in three successive stages: the sedimentation tank removes 80–90% of the suspended solids and colloids; the filters polish what remains; and the finished water reaches the distribution system. The relationship between stages is not independent - it is cascading:

Settled water turbidity ↑ → Filtered water turbidity ↑ → Finished water turbidity ↑

If settled water turbidity is high, the filters must work harder. If it spikes, filters require forced backwashing to stay compliant. And with the new standard's 1 NTU limit at the tap - plus the reality that pipe corrosion and secondary supply contamination add turbidity in the distribution system - finished water leaving the plant must be well below 1 NTU. That puts the burden squarely on the sedimentation stage. You cannot filter your way out of poor settling.

This is not just a compliance issue. Reducing turbidity also reduces microorganisms, lowers the concentration of volatile and semi-volatile organic compounds (and their mutagenic activity), and is the primary removal mechanism for Cryptosporidium and Giardia - protozoan pathogens that chlorine disinfection alone cannot reliably inactivate.

I. THE PLANT - AND WHY INCLINED TUBES NEED HELP

The Qiaodong Waterworks of Huizhou General Water Supply Company was designed in the mid-1980s with a capacity of 40,000 m³/d per phase. The process is conventional but solid: coagulation → inclined tube settler → rapid gravity filter → disinfection. Raw water quality over the study period (January 2005–April 2007) averaged:

The inclined tube settlers, while efficient at area multiplication and laminar flow stabilisation, have an inherent structural limitation: short retention time and reduced settling distance make them less stable than conventional tanks when raw water quality or flow rate changes. The distribution zone height is 1.8 m, clear water zone 1.1 m, total retention time 33 minutes, and tube retention time just 7 minutes. When flow surges or raw water turbidity spikes, there is very little hydraulic buffer. The plant needed both hardware upgrades and operational discipline to meet the new turbidity standard.

II. FOUR HARDWARE UPGRADES

1. Pre-chlorination of raw water

Chlorine was added to the raw water before coagulation. This serves two functions: oxidation of organic pollutants (breaking down structures that would otherwise resist coagulation) and destabilisation of colloids - chlorine strips the organic coating from colloidal particle surfaces, reducing the negative charge and electrical double-layer repulsion that keeps particles dispersed. The result is that particles collide and agglomerate more readily when the coagulant is added. Pre-chlorination acts as a built-in coagulant aid.

2. In-line mixer for rapid, uniform chemical contact

Mixing is the foundation of flocculation - if coagulant and raw water are not uniformly blended in seconds, no amount of flocculation time will compensate. The plant installed an in-line mixer: a reducing tube inside the raw water pipeline that creates a sharp velocity change and local negative pressure. Head loss is approximately 0.3 m. Under design flow, the mixing time is 10–20 seconds, and the turbulence generated inside the mixer ensures the coagulant contacts every volume element of raw water. The mixer replaced less reliable mechanical mixing with a passive hydraulic device that has no moving parts.

3. Grid flocculation tank - 27 layers of controlled energy dissipation

The existing perforated vortex flocculation tank was converted to a grid flocculation tank by installing 27 layers of grids. As water flows through each grid layer, it successively contracts and expands - creating controlled velocity gradients that bring particles into contact, collision, and agglomeration. The result is flocs that are large, dense, and shear-resistant by the time they reach the tube settler inlet. A DN100 perforated sludge pipe (3.6 m long, 30 mm holes in a cross pattern) was added at the bottom, with high-pressure water connected to the outlet for backwashing when holes clog. The buffer zone uses a fixed siphon for sludge discharge, controlled by a solenoid valve from the central control room.

4. Finger-type collection troughs for uniform effluent withdrawal

Non-uniform collection from the clear water zone creates short-circuiting - some water spends less time in the tubes than the design assumes, and carries unsettled flocs into the effluent. The plant replaced the original square collection holes with ten stainless steel finger-type troughs evenly distributed across the clear water zone. The centre elevation of all collection holes was set to the same horizontal plane. This single change reduced both the turbidity difference between the front and back of the collection zone and the gap between bulk clear-water turbidity and the turbidity inside individual troughs.

III. SIX OPERATIONAL METHODS THAT LOCK IN LOW TURBIDITY

After the hardware upgrades, occasional problems still appeared: fine floc particles in the effluent, unstable settled water turbidity, and floc rising at the far end of the tank. These caused high filter loading and forced backwashes. Six operational methods were systematised to eliminate them.

Method 1 - Operate the in-line mixer at design load

The in-line mixer uses flow velocity - not mechanical energy - to generate mixing turbulence. Head loss varies with the square of the flow rate. At design flow, mixing is intense and complete. At low flow, head loss drops sharply and mixing effectiveness collapses. When the plant must reduce output, rather than throttling one mixer operating at partial load, the strategy is to run fewer treatment trains at full load. The mixer should stay in its design range whenever possible.

Method 2 - SCD-controlled coagulant dosing

The plant originally controlled coagulant dose manually - operators added chemicals based on experience and adjusted after seeing results. But a water treatment plant is continuous and dynamic; by the time the operator sees the effect of a dose change, the water that received the wrong dose has already passed through the sedimentation tank and reached the filters.

The plant adopted SCD (Streaming Current Detector) control. The SCD is an online instrument that directly measures the effect of coagulant addition - not flow rate or turbidity, but the actual streaming current that reflects colloidal charge neutralisation. It compares the detected value to a setpoint and, through a mathematical model, automatically adjusts the dosing pump. The loop is closed: measure → compare → adjust → measure. Coagulant dose tracks raw water changes in real time, not in the 30–60 minutes it takes a human operator to notice and react.

Method 3 - Low inlet velocity to the sedimentation tank

After flocculation, water enters the buffer zone at 0.096 m/s, then passes more slowly through a perforated wall into the distribution zone. The purpose of this staged velocity reduction is to preserve floc integrity. Flocs that survived the flocculation tank can still be sheared apart by an abrupt velocity gradient at the settler inlet. Keeping the inlet velocity low - and ensuring the perforated wall distributes flow evenly along the tank width - prevents floc breakage at the last possible moment before settling.

Method 4 - Timely sludge discharge, including dead zones

Inclined tube settlers have inherent sludge discharge dead zones - sludge accumulates more on the far side from the discharge standpipe than on the near side. Monitoring confirmed that clear water turbidity is lower on the side near the standpipe. When sludge builds up in the accumulation zone, the upward flow can scour the bottom sludge and lift flocs back into suspension. When sludge accumulates inside the tubes, the reduced flow cross-section increases local velocity, and the faster flow lifts flocs that were settling or already settled. Both mechanisms raise effluent turbidity.

The solution is twofold: regular sludge discharge from the accumulation zone and periodic cleaning of tube surfaces. Sludge in the flocculation tank and buffer zone must also be removed - these deposits feed a continuous supply of aged, broken floc fragments into the settler.

Method 5 - Extend siphon sludge collector dwell at tank ends

The plant uses an automatically controlled siphon-type mechanical sludge collector. During operation, the collector is intentionally slowed at both ends of its travel, and the dwell time at the ends is extended to ensure thorough sludge removal from zones where accumulation is worst. When influent turbidity is high, the tank's blowdown valve is opened simultaneously with mechanical sludge collection to drain bottom sludge water. Submersible pumps are planned for installation in key zones to provide auxiliary sludge discharge.

Method 6 - Strengthen operational discipline and system-level coordination

This plant operates as a flow and pressure regulator for the city's entire water supply system, sharing raw water intake pumps and pipelines with other plants. When the intake pump station switches between large and small pumps, or other plants open or close intake valves, the flow and pressure reaching this plant change abruptly. The hydraulic conditions in the raw water shift. Coagulant dose - even with SCD control - has a response lag. With only 33 minutes of total retention time in the settler, a sudden flow change can push unsettled water to the filters before the dosing system adapts.

The countermeasure is system-level coordination. Central dispatchers, aware of this plant's particular sensitivity, coordinate pump changes and valve operations across multiple plants and the intake station to minimise the magnitude and frequency of flow and pressure fluctuations. On the plant floor, a structured patrol system ensures that operators regularly observe floc formation before and after the perforated wall, and sludge accumulation in the settling zone. Training - mentoring, on-the-job skill development - builds the ability to judge real-time conditions and intervene before a small deviation becomes a turbidity excursion.

THE CONTROLLABLE VARIABLES - A SUMMARY

THE CORE INSIGHT

An inclined tube settler's short retention time - 33 minutes total, 7 minutes in the tubes - is both its advantage (compact, low-cost) and its vulnerability (narrow hydraulic buffer). When everything upstream is optimised - when mixing is intense and rapid, when coagulant dose tracks real-time water quality, when flocs entering the tubes are large and shear-resistant, when sludge is discharged before it accumulates, and when flow changes are dampened by system-level coordination - the settler delivers settled water turbidity low enough that filters can do their job without stress. Miss any one of these, and the turbidity number at the filter inlet tells the story.

The Huizhou plant's experience demonstrates that hitting sub-1 NTU finished water from a 1980s-vintage inclined tube settler is achievable - but it requires treating the settler not as an isolated piece of equipment but as the downstream end of a chain that starts with raw water chlorination and ends at the collection trough. Every link in that chain is a control point.